JPWO2007072595A1 - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

A positive electrode including a positive electrode mixture and a positive electrode current collector carrying the same, a negative electrode, a separator including a polyolefin resin, a non-aqueous electrolyte, and a non-aqueous electrolyte secondary battery including a heat-resistant insulating layer. The estimated heat generation rate at 200 ° C. is 50 W / kg or less. The positive electrode and the negative electrode are wound together with a separator and a heat-resistant insulating layer interposed therebetween.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of its safety.

  In recent years, electronic devices have become rapidly portable and cordless. Accordingly, non-aqueous electrolyte secondary batteries having a high voltage and a high energy density have been put into practical use as power sources for driving electronic devices.

The positive electrode of a nonaqueous electrolyte secondary battery generally contains a composite lithium oxide having a high redox potential. As the composite lithium oxide, for example, lithium cobaltate, lithium nickelate, lithium manganate or the like is used. On the other hand, the negative electrode of a nonaqueous electrolyte secondary battery generally contains a carbon material. The nonaqueous electrolyte secondary battery includes an electrolyte made of a nonaqueous solvent in which a lithium salt is dissolved. As the lithium salt, for example, LiClO ₄ , LiPF ₆ or the like is used. A separator is interposed between the positive electrode and the negative electrode. For the separator, for example, a microporous film made of a polyolefin-based material is used.

  When a short circuit with a relatively low resistance occurs inside the battery for some reason, a large current flows in a concentrated manner at the short circuit point. Therefore, the battery may generate heat and reach a high temperature. Various safety measures are taken for the battery so that this phenomenon does not occur.

  In the aspect of the manufacturing process, the metal powder is managed and the dust in the manufacturing atmosphere is managed to prevent foreign matter from entering the battery. Moreover, the internal short circuit is suppressed as much as possible by protecting the exposed portion of the current collector having a low resistance with, for example, an insulating tape.

  A separator having a shutdown function is also used. In the unlikely event that a short circuit with a relatively low resistance occurs inside the battery, the pores of the separator having a shutdown function are blocked at about 135 ° C. and block the ionic current. Therefore, the short circuit current is cut and heat generation stops. However, the surface temperature of the battery rises to about 120 ° C.

  In order to prevent an internal short circuit, it has also been proposed to form a layer made of inorganic fine particles and a resin binder and having a thickness of 0.1 to 200 μm on the electrode. The internal short circuit of the battery is due to the material that partially falls off the electrode during the battery manufacturing process. This proposal aims at suppressing such an internal short circuit and improving a production yield (refer patent document 1).

It has also been proposed to provide a heat resistant resin (for example, aramid) on the separator. This proposal is also intended as a safety measure for preventing an internal short circuit of the battery (see Patent Document 2).
Japanese Patent Laid-Open No. 7-220759 JP-A-9-208736

  According to the conventional proposal, it is possible to suppress heat generation to some extent when a local internal short circuit occurs. However, for example, when performing a nail penetration test, a large number of short-circuit portions are generated at the same time. The nail penetration test is a test for evaluating safety when a battery is damaged with a number of internal short circuits. In such an extreme short-circuit state, heat generation of the battery cannot always be suppressed, and the battery may reach a high temperature.

  For example, when a nail penetration test of a general lithium ion battery in which the positive electrode includes lithium cobaltate, the negative electrode includes graphite, and the separator is a polyethylene microporous film, the battery temperature is increased until the separator shutdown function is exhibited. As a result, the surface temperature of the battery reaches around 120 ° C. This temperature rise is due to Joule heat generated inside the battery due to the short-circuit current.

  Since the short-circuit current is cut by the shutdown function of the separator, heat generation to the extent that the battery temperature reaches more than that is suppressed. According to the standards for safety evaluation in the nail penetration test and the crushing test established by the Storage Battery Industry Association, it is required that there is no smoke, ignition or rupture. On the other hand, the standard regarding battery temperature is not specifically defined. Therefore, even if the surface temperature of the battery is about 120 ° C., the safety standard is satisfied if heat generation is suppressed by the shutdown function.

  However, even when the safety standard is satisfied, when the surface temperature of the battery rises to around 120 ° C., the temperature of the electronic device incorporating the battery also rises. Therefore, the deformation | transformation of the housing | casing of an electronic device, etc. occur, and the safety | security of an electronic device may reduce. Therefore, it is desired to further increase the safety or reliability of the battery. For example, even when an internal short circuit occurs, it is eager to suppress the maximum temperature reached on the battery surface to 80 ° C. or lower.

  In the proposal of Patent Document 1 (a proposal of forming a layer made of inorganic fine particles and a resin binder and having a thickness of 0.1 to 200 μm on the electrode), in the nail penetration test, the battery temperature exceeds 80 ° C. May reach.

  Even in the proposal of Patent Document 2 (a proposal of providing a heat-resistant resin on the separator), the battery surface temperature may reach a high temperature exceeding 80 ° C. in the nail penetration test.

  Therefore, in the conventional proposal, when a large number of short-circuit points occur simultaneously as in the nail penetration test, the battery surface temperature cannot always be suppressed to 80 ° C. or lower. In the nail penetration test, the following is considered as the reason why the battery surface high temperature exceeds 80 ° C.

  In the case of a single internal short circuit, the expansion of the short circuit point is prevented by the presence of a layer composed of inorganic fine particles and a resin binder and a heat resistant resin. Since the short-circuit point is instantaneously burned off by its own heat generation, the short-circuit state is completed in 0.1 to 0.5 seconds, and then electrical insulation is restored. When the short-circuit current is interrupted, the generated heat diffuses throughout the battery. Therefore, the battery temperature does not reach that high temperature. Since the low temperature region other than the short circuit point is relatively low temperature, heat diffusion occurs quickly.

  On the other hand, in the case of the nail penetration test, a number of short-circuit points are simultaneously generated in the battery. In such a severe situation, it is considered that not only heat generation due to an internal short circuit but also heat generation due to a thermal decomposition reaction of the positive electrode active material continuously occurs. Therefore, it is considered that the heat dissipation rate for diffusing heat cannot catch up with the heat generation rate, and the thermal decomposition reaction of the positive electrode active material expands in a chain. As a result, the positive electrode active material falls off or is burned out near the short circuit point. Therefore, the positive electrode current collector (for example, aluminum foil) is exposed, and a new short-circuit point is generated. As a result, the internal short-circuit state continues, and the battery surface temperature continues to rise to around 120 ° C. where the shutdown function operates. In addition, if it is a single internal short circuit, the thermal decomposition reaction of a positive electrode active material will not advance. It is considered that there is no increase in the number of short-circuit points due to falling off or burning out of the positive electrode active material.

  The present invention is intended to improve the above-described situation, and provides a nonaqueous electrolyte secondary battery having higher safety than the conventional one while maintaining a high energy density.

The present invention relates to a positive electrode mixture including a composite lithium oxide and a positive electrode including a positive electrode current collector supporting the same, a negative electrode including a material capable of occluding and releasing lithium, and a polyolefin resin interposed between the positive electrode and the negative electrode. A non-aqueous electrolyte secondary battery comprising a separator, a non-aqueous electrolyte, and a heat-resistant insulating layer interposed between the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are interposed between them and the heat resistance The non-aqueous electrolyte secondary battery is wound together with the conductive insulating layer and has an estimated heat generation rate at 200 ° C. of the positive electrode mixture of 50 W / kg or less.
The thickness of the heat resistant insulating layer is, for example, 1 μm or more and 15 μm or less, or 1 μm or more and 5 μm or less.
The estimated exothermic rate is obtained by, for example, (i) determining the relationship between the absolute temperature T and the exothermic rate V of the positive electrode mixture with an acceleration rate calorimeter or a runaway reaction measuring device (ARC), and (ii) based on the Arrhenius theorem Plot the relationship between the reciprocal (X coordinate) of absolute temperature T and the logarithm (Y coordinate) of heat generation rate V, and (iii) Approximate straight line that fits the plot existing in the heat generation region of T <200 ° C. (473 K) (Iv) The obtained approximate straight line is extrapolated to the temperature axis of T = 200 ° C. (473 K).

  Here, the heat generation region refers to a region where the absolute value of the slope of the approximate straight line having a negative slope is the largest in the plot based on the Arrhenius theorem. That is, the approximate straight line is drawn so that the absolute value of the negative slope is maximized. Further, extrapolation is a method for obtaining a numerical value expected outside the range of the data based on known numerical data, and is used in various fields.

For example, the following is preferably used for the composite lithium oxide.
(I) General formula (1): It has a composition represented by Li _{a Mb} Me _c O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and It is at least one selected from the group consisting of Mo, the element Me is at least one selected from the group consisting of Ni and Co, and the general formula (1) is 0.9 <a <1.2. , 0.02 ≦ b ≦ 0.5, 0.5 ≦ c ≦ 0.98, and 0.95 ≦ b + c ≦ 1.05.

(Ii) General formula (2): It has a composition represented by Li _{a Mb} Ni _d Co _e O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, It is at least one selected from the group consisting of Zr and Mo, and the general formula (2) is 0.9 <a <1.2, 0.02 ≦ b ≦ 0.5, 0.1 ≦ d ≦ 0. .5, 0.1 ≦ e ≦ 0.5, and 0.95 ≦ b + d + e ≦ 1.05. The general formula (2) particularly preferably satisfies 0.15 ≦ b ≦ 0.4, 0.3 ≦ d ≦ 0.5, and 0.15 ≦ e ≦ 0.4.

(Iii) It has an arbitrary composition and includes the element M, and the element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr, and Mo The element M is a composite lithium oxide distributed more in the surface layer than in the interior. Also in the composite lithium oxide represented by the general formulas (1) and (2), it is desirable that the element M is distributed more in the surface layer portion than in the composite lithium oxide.

The composite lithium oxide is preferably treated with a Si compound represented by the general formula (3): X—Si—Y ₃ . Here, X includes a functional group that reacts with the composite lithium oxide, and Y includes a functional group including C, H, O, F, or Si.

  According to the present invention, even in a harsh situation in which a number of short-circuit points are generated at the same time, heat generation due to internal short-circuiting and chained exothermic reaction are effectively suppressed. Therefore, since the short circuit is avoided, the maximum temperature of the battery can be stably suppressed to 80 ° C. or lower. ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery which improved safety | security compared with the past can be provided, maintaining a high energy density.

It is an example of the Arrhenius plot which shows the relationship between the absolute temperature T calculated | required by ARC, and the heat release rate V of various positive electrode materials. It is explanatory drawing of the measurement principle of ARC.

  The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode mixture including a composite lithium oxide and a positive electrode including a positive electrode current collector supporting the negative electrode, a negative electrode including a material capable of inserting and extracting lithium, and a separator including a polyolefin resin And a non-aqueous electrolyte and a heat-resistant insulating layer interposed between the positive electrode and the negative electrode.

  For example, the heat resistant insulating layer may be formed on one of the positive electrode and the negative electrode on the surface facing the other electrode, but the arrangement of the heat resistant insulating layer is not limited thereto. The heat-resistant insulating layer may be formed only on at least one surface of the positive electrode, may be formed only on at least one surface of the negative electrode, or may be formed only on at least one surface of the separator. Further, the heat-resistant insulating layer may be formed only on at least one surface of the positive electrode and at least one surface of the negative electrode, or may be formed only on at least one surface of the negative electrode and at least one surface of the separator, It may be formed only on at least one surface of the separator and at least one surface of the positive electrode. The heat-resistant insulating layer may be formed on at least one surface of the positive electrode, at least one surface of the negative electrode, and at least one surface of the separator. Further, the heat-resistant insulating layer may be in the form of a sheet independent from the positive electrode, the negative electrode, and the separator. However, it is desirable that the heat-resistant insulating layer is bonded to at least one surface of the positive electrode, at least one surface of the negative electrode, or at least one surface of the separator.

  The present invention has one feature in that the estimated heat generation rate at 200 ° C. of the positive electrode mixture is controlled to 50 W / kg or less. Here, the estimated exothermic rate is obtained by, for example, (i) determining the relationship between the absolute temperature T and the exothermic rate V of the positive electrode mixture using an accelerated rate calorimeter or an accelerated run calorimeter (ARC), and (ii) Based on the Arrhenius theorem, the relationship between the reciprocal of the absolute temperature T (X coordinate) and the logarithm of the heat generation rate V (Y coordinate) is plotted, and (iii) a plot existing in the heat generation region of T <200 ° C. (473 K) (Iv) is obtained by extrapolating the obtained approximate line to the temperature axis of T = 200 ° C. (473 K).

  When the estimated heat generation rate at 200 ° C. of the positive electrode mixture obtained by the above extrapolation is 50 W / kg or less, the heat-resistant insulating layer contributes to safety, particularly in a severe situation in which short-circuit points occur simultaneously and frequently. Markedly enhanced. The present invention is based on such knowledge and realizes a very high level of safety compared to the prior art.

For example, by using the following positive electrode material, the estimated heat generation rate at 200 ° C. of the positive electrode mixture can be suppressed to 50 W / kg or less.
First, it has a composition represented by the general formula (1): Li _{a Mb} Me _c O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr And Mo is at least one selected from the group consisting of Mo, the element Me is at least one selected from the group consisting of Ni and Co, and the general formula (1) is 0.9 <a <1. 2, the estimated heat generation rate of composite lithium oxide satisfying 0.02 ≦ b ≦ 0.5, 0.5 ≦ c ≦ 0.98, and 0.95 ≦ b + c ≦ 1.05 is suppressed to 50 W / kg or less It can be mentioned as an effective positive electrode material.

  From the viewpoint of reducing the estimated heat generation rate, Mn, Al and Mg are particularly preferable among the elements M, and Mn is most preferable. Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo all have the effect of reducing the estimated heat generation rate.

  Here, the a value is an initial value, and increases or decreases as the battery is charged and discharged. The initial value substantially matches the a value of the composite lithium oxide contained in the discharged battery. The standard a value of the composite lithium oxide immediately after synthesis is 1.

If the b value is less than 0.02, the effect of the element M cannot be confirmed. If the b value exceeds 0.5, the capacity reduction increases.
If the c value is less than 0.5, it is difficult to secure a certain capacity or more. If it exceeds 0.98, the effect of reducing the estimated heat generation rate cannot be obtained.

  The general formula (1) satisfies 0.95 ≦ b + c ≦ 1.05. In the initial state immediately after the synthesis (the state having no charge / discharge history), the standard value of b + c is 1, but b + c does not have to be strictly 1. In the range of 0.95 ≦ b + c ≦ 1.05, b + c = 1 can be considered substantially.

Secondly, it has a composition represented by the general formula (2): Li _{a Mb} Ni _d Co _e O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn And at least one selected from the group consisting of Zr and Mo, the general formula (2) is: 0.9 <a <1.2, 0.02 ≦ b ≦ 0.5, 0.1 ≦ d ≦ Composite lithium oxide satisfying 0.5, 0.1 ≦ e ≦ 0.5, and 0.95 ≦ b + d + e ≦ 1.05 as a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less Can be mentioned. The general formula (2) particularly preferably satisfies 0.15 ≦ b ≦ 0.4, 0.3 ≦ d ≦ 0.5, and 0.15 ≦ e ≦ 0.4.

Also in General formula (2), a value is an initial value, and it increases / decreases with charging / discharging of a battery. On the other hand, if the b value is less than 0.02, the effect of the element M cannot be confirmed, and if it exceeds 0.5, the capacity reduction increases.
If the d value is less than 0.1, the effect of adding Ni (for example, the effect of improving the theoretical capacity) is low, and if it exceeds 0.5, the voltage is lowered and the life characteristics are also deteriorated.
When the e value is less than 0.1, the effect of adding Co (for example, the effect of improving the voltage) is low, and when it exceeds 0.5, the utilization factor of the positive electrode decreases.

  The general formula (2) satisfies 0.95 ≦ b + d + e ≦ 1.05. However, the standard value of b + d + e is 1 in the initial state immediately after synthesis (the state having no charge / discharge history), but b + d + e is not necessarily exactly 1. In the range of 0.95 ≦ b + d + e ≦ 1.05, it can be considered that b + d + e = 1 substantially.

As a specific example of the positive electrode material represented by the general formula (2) and controlling the estimated heat generation rate at 200 ° C. to 50 W / kg or less, for example, LiMn _b Ni _d Co _e O ₂ (0.15 ≦ b ≦ 0.35, 0.3 ≦ d ≦ 0.5 and 0.25 ≦ e ≦ 0.35).

  Third, it has an arbitrary composition and includes an element M, and the element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr, and Mo. The element M can be cited as a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less of the composite lithium oxide that is distributed more in the surface layer than in the inside.

  Such a positive electrode material includes a compound (for example, nitrate, etc.) containing the element M on the surface of a composite lithium oxide having an arbitrary composition (for example, a composite lithium oxide represented by the general formula (1) or (2)). Sulfate) and the like, and the element M can be obtained by diffusing into the composite lithium oxide. For example, when a compound containing a composite lithium oxide and a small amount of the element M is mixed and fired at an appropriate temperature, the element M diffuses from the surface of the composite lithium oxide to the inside. As a result, it is possible to obtain a composite lithium oxide in which the element M is distributed more in the surface layer portion than in the inside. Alternatively, a composite lithium oxide carrying the element M can be obtained by mixing a liquid in which a compound containing the element M is dissolved or dispersed and the composite lithium oxide, and then removing the liquid component. When the composite lithium oxide is fired at an appropriate temperature (for example, 300 to 700 ° C.), the element M diffuses from the surface of the composite lithium oxide to the inside.

  The compound containing the element M has a large effect of suppressing the estimated heat generation rate at 200 ° C. However, when the amount of the compound containing the element M added to the composite lithium oxide increases, the utilization factor of the positive electrode decreases and the energy density of the battery decreases. Further, the exothermic reaction of the positive electrode occurs on the surface of the active material particles. Therefore, the presence of a large amount of the element M in the surface layer portion of the active material particles can efficiently suppress heat generation without greatly reducing the utilization factor of the positive electrode. That is, the estimated heat generation rate can be suppressed by a small amount of the element M by concentrating and distributing the element M on the surface layer portion of the active material particles.

  It is preferable to use a compound containing the element M in an amount such that the element M is 0.0001 to 0.05 mole per mole of the composite lithium oxide.

Fourth, a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less for the composite lithium oxide treated with the Si compound represented by the general formula (3): X—Si—Y ₃ Can be mentioned. Here, X includes a functional group that reacts with the composite lithium oxide, and Y includes a functional group including C, H, O, F, or Si. By modifying the surface of the composite lithium oxide with such a Si compound, the exothermic reaction occurring on the surface of the active material particles is suppressed, and the estimated heat generation rate is suppressed. Moreover, even if the composite lithium oxide is treated with the Si compound, the utilization factor of the positive electrode is not greatly reduced.

For example, it is desirable to treat the composite lithium oxide with a silane coupling agent represented by X—Si—Y ₃ . The method for treating the composite lithium oxide with the silane coupling agent represented by X—Si—Y ₃ is not particularly limited. For example, a silane coupling agent is mixed with water, and the resulting mixture is mixed with a composite lithium oxide and then dried. Here, in the liquid mixture of the silane coupling agent and water, the concentration of the silane coupling agent is preferably about 0.01 wt% to 5 wt%, and more preferably about 0.1 wt% to 3 wt%. Further, the amount of the silane coupling agent is preferably 0.001 to 0.5 parts by weight, more preferably 0.01 to 0.1 parts by weight, per 100 parts by weight of the composite lithium oxide.

  Examples of the silane coupling agent include vinyltriethoxysilane, vinyltrimethoxysilane, vinyltrichlorosilane, vinyltris (2-methoxyethoxy) silane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropyltriethoxy. Silane, γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltriethoxy Silane, γ-ureidopropyltriethoxysilane, γ-ureidopropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltriethoxysilane, γ-glycidoxy Propyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropyltriethoxysilane, methyltri Ethoxysilane, methyltrimethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane and the like can be used. Among these, in particular, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris (2-methoxyethoxy) silane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-aminopropyl Triethoxysilane, γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltriethoxysilane, γ-ureidopropyl Triethoxysilane, γ-ureidopropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-Grease Sidoxypropyltriethoxysilane is preferred.

The heat resistant insulating layer includes, for example, an inorganic oxide filler and a resin component. The inorganic oxide filler has high heat resistance. Therefore, even when the battery reaches a relatively high temperature, the heat-resistant insulating layer can maintain high mechanical strength. Although various resin components can be used for the heat resistant insulating layer, a particularly excellent heat resistant insulating layer can be obtained by using a resin component having high heat resistance.
The heat-resistant insulating layer may include, for example, an inorganic oxide filler and a binder (resin component) or a heat-resistant resin (resin component), but is not particularly limited. A heat-resistant insulating layer containing an inorganic oxide filler and a binder has a relatively high mechanical strength and therefore has a high durability. Here, the heat-resistant insulating layer containing the inorganic oxide filler and the binder is mainly composed of the inorganic oxide filler. For example, 80% by weight or more, preferably 90% by weight or more of the heat-resistant insulating layer is the inorganic oxide filler. The heat resistant insulating layer made of a heat resistant resin contains, for example, a heat resistant resin exceeding 20% by weight.
A heat resistant insulating layer made of a heat resistant resin has higher flexibility than a heat resistant insulating layer containing an inorganic oxide filler as a main component. This is because the heat resistant resin is more flexible than the inorganic oxide filler. Therefore, the heat-resistant insulating layer made of a heat-resistant resin can easily follow the expansion and contraction of the electrode plate accompanying charging / discharging, and can maintain high heat resistance. Therefore, the nail penetration safety is also increased.
The heat-resistant insulating layer made of a heat-resistant resin can contain, for example, less than 80% by weight of an inorganic oxide filler. By including an inorganic oxide filler, a heat-resistant insulating layer excellent in balance between flexibility and durability can be obtained. The heat-resistant resin contributes to the flexibility of the heat-resistant insulating layer, and the inorganic oxide filler having high mechanical strength contributes to durability. By including an inorganic oxide filler in the heat resistant insulating layer, the high output characteristics of the battery are improved. Although details are unknown, it is thought that this is because the void structure of the heat-resistant insulating layer is optimized by the synergistic effect of flexibility and durability. From the viewpoint of securing good high-output characteristics, the heat-resistant insulating layer made of a heat-resistant resin desirably contains 25% to 75% by weight of an insulating filler.

  The resin component (binder or heat resistant resin) of the heat resistant insulating layer desirably has a thermal decomposition starting temperature of 250 ° C. or higher. It is desirable that the resin component does not greatly deform at high temperatures. Therefore, the resin component is desirably amorphous or non-crystalline. The thermal deformation start temperature or glass transition temperature (Tg) of the resin component is desirably 250 ° C. or higher.

  The thermal decomposition initiation temperature, thermal deformation initiation temperature, and glass transition temperature of the resin component are measured by differential scanning calorimetry (DSC) or thermogravimetry-differential thermal analysis (TG-DTA). Can be measured. For example, the starting point of weight change in the TG-DTA measurement corresponds to the thermal decomposition start temperature, and the inflection point in the endothermic direction in the DSC measurement corresponds to the thermal deformation temperature or the glass transition temperature.

As specific examples of the binder constituting the heat-resistant insulating layer, for example, a fluororesin such as polyvinylidene fluoride (PVDF) or a rubbery polymer (modified acrylonitrile rubber) containing an acrylonitrile unit can be preferably used. These may be used alone or in combination of two or more. Among these, rubbery polymers containing acrylonitrile units are most suitable because they have appropriate heat resistance, elasticity and binding properties.
As specific examples of the heat-resistant resin constituting the heat-resistant insulating layer, for example, polyamide resins such as aromatic polyamide (aramid), polyimide resins, polyamide-imide resins, and the like can be preferably used. These may be used alone or in combination of two or more.
The heat-resistant insulating layer containing the inorganic oxide filler and the binder is preferably formed or bonded on at least one surface of the negative electrode, and more preferably formed or bonded on both surfaces of the negative electrode. The heat-resistant insulating layer made of a heat-resistant resin is preferably formed or bonded to at least one surface of the separator. Since the heat-resistant insulating layer is relatively brittle, it is formed or bonded only to one surface of the separator. More preferably. When the heat-resistant insulating layer made of a heat-resistant resin is formed only on one side of the separator, the ratio between the thickness A of the separator and the thickness B of the heat-resistant insulating layer: A / B is the heat-resistant insulating layer For example, 3 ≦ A / B ≦ 12 or 4 ≦ A / B ≦ 6.

As the inorganic oxide filler, for example, alumina (Al ₂ O ₃ ), titania (TiO ₂ ), silica (SiO ₂ ), zirconia, magnesia and the like can be used. These may be used alone or in combination of two or more. Of these, alumina (in particular, α-alumina) and magnesia are particularly preferable from the viewpoints of stability, cost, and ease of handling.

  Although the median diameter (D50: average particle diameter) of an inorganic oxide filler is not specifically limited, Generally it is the range of 0.1-5 micrometers, and it is desirable that it is 0.2-1.5 micrometers.

  The content of the inorganic oxide filler in the heat-resistant insulating layer containing the inorganic oxide filler and the binder is preferably 50% by weight or more and 99% by weight or less, and 90% by weight or more and 99% by weight or less. Is more preferable. When the content of the inorganic oxide filler is less than 50% by weight, the resin component becomes excessive. Therefore, it may be difficult to control the pore structure with the filler particles. On the other hand, when the content of the inorganic oxide filler exceeds 99% by weight, the resin component becomes excessive. Therefore, the strength of the heat-resistant insulating layer and the adhesion to the electrode surface or separator surface may be reduced.

  The thickness of the heat resistant insulating layer is not particularly limited. The thickness of the heat-resistant insulating layer is, for example, 1 μm or more and 15 μm or less from the viewpoint of sufficiently ensuring the short-circuit suppressing function by the heat-resistant insulating layer or insulating the short-circuit point and maintaining the design capacity. The thickness of the heat-resistant insulating layer containing the inorganic oxide filler and the binder is, for example, 3 to 15 μm or 3 to 8 μm. The thickness of the heat resistant insulating layer made of the heat resistant resin is, for example, 1.5 to 7 μm or 1.7 to 6.7 μm. If the thickness of the heat-resistant insulating layer is too large, the heat-resistant insulating layer is brittle and may be damaged when the electrode is wound. On the other hand, if the thickness is too small, the strength of the heat-resistant insulating layer is reduced and may be damaged.

  Various conventional separators can be used in the present invention. For example, a separator having a single layer structure made of a polyolefin resin such as polyethylene or polypropylene, or a separator having a multilayer structure made of a polyolefin resin can be used. Although the thickness of a separator is not specifically limited, About 15-25 micrometers is desirable.

  The positive electrode mixture includes an active material composed of a composite lithium oxide as an essential component, and includes a binder, a conductive material, and the like as optional components. As the positive electrode binder, for example, polytetrafluoroethylene (PTFE), modified acrylonitrile rubber particles, PVDF, or the like can be used. These may be used alone or in combination of two or more. PTFE and modified acrylonitrile rubber particles are preferably used in combination with carboxymethyl cellulose, polyethylene oxide, modified acrylonitrile rubber and the like. These serve as a thickener for the paste containing the positive electrode mixture and the liquid component. As the conductive material for the positive electrode, acetylene black, ketjen black, various graphites, and the like can be used. These may be used alone or in combination of two or more. The amount of the binder and the conductive material contained in the positive electrode mixture is preferably 0.1 to 5 parts by weight and 1 to 10 parts by weight, respectively, per 100 parts by weight of the active material.

  Various materials used in conventional negative electrodes can be used for the negative electrode including a carbon material or an alloy material. As the carbon material, for example, various natural graphites and various artificial graphites can be used. For example, a silicon alloy or a tin alloy can be used as the alloy material. A composite of a carbon material and an alloy material can also be used. The negative electrode can also contain a binder and a conductive material. The materials mentioned above as the binder and conductive material for the positive electrode can also be used for the negative electrode binder and conductive material.

As the non-aqueous electrolyte, a non-aqueous solvent in which a lithium salt is dissolved as a solute is preferably used. The lithium salt and the non-aqueous solvent are not particularly limited, but it is preferable to use, for example, LiPF ₆ or LiBF _{4 as the} lithium salt. As the non-aqueous solvent, it is preferable to use ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, or the like. The non-aqueous solvent is preferably used in combination of two or more, rather than using one alone. In addition, it is desirable to add vinylene carbonate, vinyl ethylene carbonate, cyclohexylbenzene, or the like as an additive to the non-aqueous electrolyte.

  Hereinafter, the present invention will be specifically described based on various experiments and examples, but the present invention is not limited to the following examples.

[Experiment 1]
Measurement of temperature near short-circuit point Ten cells of a cylindrical 18650 (diameter 18 mm, height 65 mm) non-aqueous electrolyte secondary battery were produced. Here, lithium cobaltate (LiCoO ₂ ) was used as the positive electrode active material. Moreover, the heat resistant insulating layer which consists of an inorganic oxide filler and a resin component was formed in the negative electrode surface. Using these cells, a nail penetration test was performed, and it was examined to what degree the temperature in the vicinity of the short-circuiting point rose to 0.5 seconds immediately after the nail penetration.

  Here, a thermocouple was attached to the battery surface, and a nail was stabbed near the thermocouple, and the battery surface temperature was measured. The results are shown in Table 1.

The cylindrical 18650 non-aqueous electrolyte secondary battery was manufactured in the following manner.
(I) Production of positive electrode 3 kg of lithium cobaltate, 1 kg of PVDF # 1320 (N-methyl-2-pyrrolidone (NMP) solution containing 12% by weight of PVDF) manufactured by Kureha Chemical Co., Ltd. as a binder, and acetylene 90 g of black and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a positive electrode mixture paste. This paste was applied to both sides of a 15 μm thick aluminum foil, dried and then rolled to form a positive electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of aluminum foil and a positive mix layer was controlled to 160 micrometers. The obtained electrode plate was cut into a width and a length that could be inserted into a cylindrical battery case having a diameter of 18 mm and a height of 65 mm to obtain a positive electrode.

(Ii) Production of negative electrode 3 kg of artificial graphite, 75 g of BM-400B (aqueous dispersion containing 40% by weight of styrene-butadiene copolymer) manufactured by Nippon Zeon Co., Ltd., and carboxymethyl cellulose (CMC) as a thickener 30 g and an appropriate amount of water were stirred with a double arm kneader to prepare a negative electrode mixture paste. This paste was applied to both sides of a 10 μm thick copper foil, dried and then rolled to form a negative electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of copper foil and a negative mix layer was controlled to 180 micrometers. The obtained electrode plate was cut into a width and a length that can be inserted into the battery case to obtain a negative electrode.

(Iii) Preparation of non-aqueous electrolyte Lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1: 3. A water electrolyte was prepared.

(Iv) Preparation of heat-resistant insulating layer raw material paste 950 g of alumina having a median diameter of 0.3 μm, which is an inorganic oxide filler, and BM-720H, a resin component, made by Nippon Zeon Co., Ltd. (rubber-like material containing acrylonitrile units) 625 g of an NMP solution containing 8% by weight of a polymer) and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a heat-resistant insulating layer raw material paste.

(V) Battery assembly The raw material paste for the heat-resistant insulating layer was applied to both sides of the negative electrode, and the coating film was dried to form a heat-resistant insulating layer having a thickness of 0.5 μm on each side.
A positive electrode and a negative electrode on which a heat-resistant insulating layer having a thickness of 0.5 μm was formed were wound through a separator made of a single layer of a polyethylene resin having a thickness of 20 μm to constitute an electrode plate group. This electrode plate group was inserted into the battery case, and 5.5 g of the nonaqueous electrolyte was injected into the battery case, and the opening of the case was sealed. Thus, a cylindrical non-aqueous electrolyte secondary battery (nominal capacity 2000 mAh) was completed.

The nail penetration test was performed under the following conditions.
First, each battery (cylindrical batteries 1 to 10) was charged as follows.
Constant current charging: 1400mA (end voltage 4.25V)
Constant voltage charge: 4.25V (end current 100mA)
From the side of the battery after charging, a 2.7 mm diameter iron round nail is penetrated at a speed of 5 mm / second in an environment of 20 ° C. The battery temperature in the vicinity of the puncture point was observed.

  From the results in Table 1, it was found that the temperature in the vicinity of the short-circuit point increased to 200 ° C. at the minimum in 0.5 seconds. It is generally known that lithium cobalt oxide in a charged state undergoes thermal decomposition at around 200 ° C. From this, in the situation where short-circuit points occur at the same time as in the nail penetration test, it is expected that Joule heat generation due to current occurs continuously in the vicinity of the short-circuit point and heat of decomposition reaction of the positive electrode active material is generated. This suggests that, in a conventional battery having a heat-resistant insulating layer composed of an inorganic oxide filler and a resin component, safety cannot be reliably ensured in a situation where internal short-circuits occur at the same time. .

  From the above results, it can be understood that it is very important to control the thermal stability of the positive electrode material in order to ensure safety even in a situation where internal short-circuits occur simultaneously and frequently. More specifically, it is important not only to prevent short-circuiting by the heat-resistant insulating layer but also to suppress the thermal decomposition reaction of the positive electrode active material. The positive electrode active material is required to be a material that does not easily decompose even when the vicinity of the short circuit point reaches a high temperature of 200 ° C. or higher.

[Experiment 2]
Examination of positive electrode active material Since the heat stability of the heat-resistant insulating layer and the positive electrode active material is a very important requirement, the thermal stability of the positive electrode mixture was examined next. Here, the estimated heat generation rate at 200 ° C. of the positive electrode mixture containing the positive electrode materials 1 to 3 shown in Table 2 was measured.

Using the material shown in Table 2 as the positive electrode active material, a cylindrical 18650 non-aqueous electrolyte secondary battery was prepared in the same manner as in Experiment 1, and the obtained battery was charged under the following conditions. Hereinafter, the batteries produced using the positive electrode materials 1 to 3 are referred to as batteries 1A to 3A, respectively.
Constant current charging: 1400mA (end voltage 4.25V)
Constant voltage charge: 4.25V (end current 100mA)
However, when the battery voltage is 4.25V, the positive electrode potential with respect to the metal Li corresponds to 4.35V.

  The batteries 1A to 3A in a charged state were decomposed in an atmosphere having a dew point of −40 ° C. or less, and the positive electrode was taken out. The positive electrode was cut into a 3 × 6 cm sample. Next, the positive electrode sample was enclosed in an iron cylindrical case (diameter: 8 mm, height: 65 mm) whose inner surface was plated with Ni, and the opening of the case was sealed.

  Next, using a positive electrode sample sealed in a cylindrical case, and using an accelerated rate calorimeter or an accelerated rate calorimeter (ARC), the absolute temperature T and the positive electrode mixture under the conditions shown in Table 3 The data regarding the relationship with the exothermic rate V was obtained.

  In ARC, since the sample is placed in an adiabatic environment, the temperature increase rate of the sample directly reflects the heat generation rate of the sample. Until the exothermic reaction has a heat generation rate equal to or higher than the detection sensitivity, forced heating is repeated step by step, and when the heat generation rate higher than the detection sensitivity is detected, the heat generation rate of the sample is measured in an adiabatic environment.

The meanings of terms in Table 3 are described below with reference to the conceptual diagram 2.
Temperature rising step: a temperature range (“1” in FIG. 2) of the environmental temperature that is forcibly increased in a stepwise manner in a region where no spontaneous heat generation of the sample is detected.
Exothermic detection sensitivity: Sensitivity to detect self-heating of the sample (“2” in FIG. 2). Sensitivity is arbitrarily set based on the material. When the temperature rise due to the spontaneous heat generation of the sample within the detection time (Δt) is ΔT, the sensitivity is represented by ΔT / Δt.
Stabilization time after temperature increase: After the environmental temperature is forcibly increased based on a predetermined temperature increase step, the sample temperature and the environmental temperature in the furnace are allowed to stabilize ("3" in FIG. 2) It is. This time is arbitrarily set.
Heat generation recognition temperature range: A temperature range ("4" in FIG. 2) for recognizing self-heating of the sample. When the heat generation recognition temperature range is 0.2 ° C and the heat generation detection sensitivity is 0.04 ° C / min, when the temperature rise of 0.04 ° C / min or more continues for 5 minutes (0.2 / 0.04) Is recognized as having fever.

  Conventionally, thermal analysis equipment such as differential scanning calorimetry (DSC) and thermogravimetry-differential thermal analysis (TG-DTA) is used to evaluate the thermal stability of the positive electrode active material. It's being used. However, evaluation of thermal stability by DSC or TG-DTA has the following problems. First, in the measurement by DSC, TG-DTA, etc., the heat generation rate and the heat generation peak vary depending on the measurement conditions (temperature increase rate and sample amount). Therefore, it is not suitable for accurately obtaining the heat generation rate. Next, in the case of an internal short circuit or the like, the vicinity of the short circuit point instantaneously rises to 200 ° C. or higher. Therefore, heat generation that occurs at less than 200 ° C. occurs at the same time. However, DSC, TG-DTA, etc. cannot predict the rate of heat generation in different temperature ranges. On the other hand, since ARC is an adiabatic measurement, the temperature rise rate of the sample represents the heat generation rate of the sample as it is. For this reason, unlike the thermal analysis method, ARC is very effective in measuring the reaction rate of the exothermic reaction. Therefore, in the present invention, ARC was used in evaluating the thermal stability of the positive electrode mixture at the time of an internal short circuit.

  The data obtained by ARC was plotted based on the Arrhenius theorem as shown in FIG. That is, the relationship between the reciprocal of the absolute temperature T (X coordinate) and the logarithm of the heat generation rate V (Y coordinate) was plotted. The set of plots showing the exothermic rate of the chemical reaction thus obtained can be approximated by a straight line. Therefore, by extrapolating the approximate straight line to a predetermined temperature axis, it is possible to estimate the heat generation rate in a temperature region different from the temperature region in which heat generation is actually observed. Here, as shown in FIG. 1, an approximate straight line suitable for the plot existing in the heat generation region of T <200 ° C. (473 K) is obtained, and the approximate straight line is extrapolated to the temperature axis of T = 200 ° C. (473 K). The estimated heat generation rate was obtained. The results of the estimated heat generation rate obtained are shown in Table 4.

Next, when the nail penetration test of the batteries 1A to 3A was performed under the same conditions as in Experiment 1, only in the case of the battery 1A using the positive electrode material 1 (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ ). The temperature near the short-circuit point immediately after nail penetration did not reach 200 ° C. In addition, the battery voltage after the test did not drop much compared to before the test, and the maximum temperature on the battery surface (position away from the short circuit point) did not reach 80 ° C. until the end of the test. From this, it is considered that after the occurrence of an internal short circuit, the insulation of the short circuit point worked effectively and the generation of Joule heat could be suppressed to a minimum.

  From the above results, when the thermal stability of the positive electrode material is a predetermined value, that is, when the estimated heat generation rate at 200 ° C. derived from the ARC measurement is 10 W / kg, even in a situation where internal short-circuits occur simultaneously and frequently. It can be considered that safety was ensured. This result is due to a synergistic effect between the action of the heat-resistant insulating layer and the thermal stability of the positive electrode material, and this synergistic effect has enabled the realization of a battery with high safety that has not been achieved in the past. Conceivable. This means that, in the past, it was impossible to suppress the battery temperature below 80 ° C. in a situation where short-circuit points occur simultaneously and frequently, but according to the present invention, this is possible.

Next, examples will be described.
<< Batteries X1 to X18 and Batteries Y1 to Y12 >>
The positive electrode materials 1 to 3 shown in Table 2 and the following positive electrode materials A to E obtained by mixing them were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 5. Furthermore, the thickness per adhesive surface after drying of the heat-resistant insulating layer was set as shown in Table 5. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

Positive electrode material A: a mixture of 90% by weight of positive electrode material 1 and 10% by weight of positive electrode material 2 Positive electrode material B: a mixture of 80% by weight of positive electrode material 1 and 20% by weight of positive electrode material 2 Positive electrode material C: positive electrode material 1 Mixture of 70 wt% and positive electrode material 2 30 wt% Positive electrode material D: mixture of positive electrode material 1 60 wt% and positive electrode material 2 40 wt% Positive electrode material E: 50 wt% of positive electrode material 1 and positive electrode material 2 50% by weight of mixture

  However, in the batteries X7 to X12, X16 to X18, and the batteries Y3, Y4, Y8, Y11, and Y12, only one side of the separator is made of an aramid resin and an inorganic oxide filler disclosed in the example of Patent Document 2. A film having a thickness of 0.5 to 5 μm was formed as a heat-resistant insulating layer. Specifically, a heat-resistant insulating layer was formed as follows.

  The separable flask having a stirring blade, a thermometer, a nitrogen inflow pipe and a powder addition port was sufficiently dried. In a dry separable flask, 4200 g of NMP and 270 g of calcium chloride dried at 200 ° C. for 2 hours were added, and the temperature was raised to 100 ° C. After the calcium chloride was completely dissolved, the inside of the flask was returned to 20 ± 2 ° C., and 130 g of paraphenylenediamine (PPD) was added and completely dissolved. While maintaining this solution at 20 ± 2 ° C., 24 g of terephthalic acid chloride (TPC) was dividedly added 10 times (total 240 g) every 5 minutes. Thereafter, the solution was aged for 1 hour, stirred for 30 minutes under reduced pressure, and degassed to obtain a polymerization solution of polyparaphenylene terephthalamide (PPTA: thermal decomposition start temperature of 400 ° C. or higher, amorphous).

  To this polymerization solution, an NMP solution in which 5.8% by weight of calcium chloride was dissolved was gradually added so that the PPTA finally became 2.8% by weight. Alumina particles having an average particle size of 0.5 μm were added thereto to obtain a paste having a PPTA solution: alumina weight ratio of 97: 3. This paste was applied to one side of the separator with a bar coater and dried with hot air at 80 ° C. Thereafter, the separator was washed with ion-exchanged water, calcium chloride was removed, and a separator having a heat-resistant insulating layer made of PPTA was obtained. In the electrode plate group, the separator was disposed so that the heat-resistant insulating layer was on the positive electrode side.

In batteries Y5 and 6, no heat-resistant insulating layer was formed on any of the positive electrode, the negative electrode, and the separator.
Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. The results are shown in Table 5.
A battery nail penetration test was performed under the same conditions as in Experiment 1, and the maximum temperature reached on the battery surface away from the short-circuit point was examined. Ten batteries, each of batteries X1 to X18 and batteries Y1 to Y12, were prepared, and the nail penetration test was performed with 10 batteries. For each battery, among the 10 batteries, the average value of the highest reached temperatures of the batteries that did not reach 80 ° C. and the number of batteries that reached 80 ° C. were determined. Table 10 shows “80 ° C. or higher” for all the ten samples that showed a maximum temperature of 80 ° C. or higher. The results are shown in Table 5.

Cathode material 1: LiMn _1/3 Ni _1/3 Co _1/3 O ₂
Cathode material 2: LiAl _0.05 Ni _0.8 Co _0.15 O ₂
Cathode material 3: LiCoO ₂
Cathode material A: Cathode material 1 / Cathode material 2 = 90/10 (wt%)
Cathode material B: Cathode material 1 / Cathode material 2 = 80/20 (wt%)
Cathode material C: Cathode material 1 / Cathode material 2 = 70/30 (wt%)
Cathode material D: Cathode material 1 / Cathode material 2 = 60/40 (wt%)
Cathode material E: Cathode material 1 / Cathode material 2 = 50/50 (wt%)

Hereinafter, the evaluation results will be described.
Even if a positive electrode active material having a relatively high thermal stability compared to lithium cobaltate is used from the batteries Y5 and Y6, if a simultaneous internal short circuit occurs, the battery is not present. It can be seen that the maximum surface temperature cannot always be kept below 80 ° C.

  As shown in batteries Y7 to Y12, even when the battery has a heat-resistant insulating layer, heat is generated when a positive electrode active material having an estimated heat generation rate at 200 ° C. exceeding 50 W / kg determined by the above method is used. It can be understood that the chain of reactions cannot be suppressed, and the maximum temperature reached on the battery surface cannot always be kept below 80 ° C.

  From the comparison of the batteries X13 to 18 and the batteries Y9 to 12, the safe area where the action of the heat-resistant insulating layer can be effectively utilized is the area where the estimated heat generation rate at 200 ° C. of the positive electrode active material is 50 W / kg or less. I understand. It can be understood that if the temperature is outside this region, the increase in the maximum temperature of the battery becomes significant. In the region where the estimated heat generation rate at 200 ° C. of the positive electrode active material is 50 W / kg or less, it is considered that the chain of exothermic reactions is effectively suppressed, and the heat generated at the short-circuit point is efficiently diffused.

  From the batteries X1 to 6, it can be seen that if the thickness of the heat-resistant insulating layer has a certain thickness or more, the effect of suppressing the maximum temperature of the battery when multiple shorts occur simultaneously does not greatly affect. When the thickness of the heat-resistant insulating layer is larger, the maximum temperature reached is lower. However, if the thickness is too large, it becomes difficult to maintain the energy density of the battery at a high level, and the battery tends to be damaged during winding. Since the heat-resistant insulating layer is brittle, if it is excessively thick, it partially falls off from the electrode surface or separator surface during winding. This can be confirmed from the fact that in Comparative Example 2, the number of batteries that reached 80 ° C. or higher specifically increased. Therefore, even when a positive electrode active material with high thermal stability is used, a high level of safety cannot be maintained in the nail penetration test. The thickness of the heat resistant insulating layer may be, for example, in the range of about 1 to 15 μm, or 3 to 10 μm. Similar results are obtained in the batteries X7 to 12 in which the heat-resistant insulating layer is formed on the surface of the separator. The thickness of the heat resistant insulating layer containing the aramid resin may be, for example, 1.7 to 6.7 μm.

  When the heat-resistant insulating layer has a thickness of less than 1 μm, the mechanical strength of the heat-resistant insulating layer itself decreases. Therefore, the heat-resistant insulating layer is easily destroyed due to an impact accompanying a short circuit. This can be confirmed from the fact that the number of batteries that reached 80 ° C. or higher in the battery Y1 specifically increased. Therefore, it is considered that the insulating function is reduced to some extent when the heat-resistant insulating layer has a thickness of less than 1 μm.

<< Batteries X19A to X30A and Battery Y13A >>
The following positive electrode materials F to H obtained by mixing the positive electrode materials 4 to 13 shown in Table 6-1 and the positive electrode materials 1 and 3 were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 6-1. The thickness per adhesive surface after drying of the heat resistant insulating layer was set as shown in Table 6-1. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

<< Batteries X19B to X30B and Battery Y13B >>
The following positive electrode materials F to H obtained by mixing the positive electrode materials 4 to 13 shown in Table 6-2 and the positive electrode materials 1 and 3 were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 6-2. The thickness after drying of the heat-resistant insulating layer was set as shown in Table 6-2. However, similarly to the battery X9, a film having a thickness of 5 μm made of an aramid resin and an inorganic oxide filler disclosed in Examples of Patent Document 2 was formed only on one side of the separator as a heat-resistant insulating layer. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.
The cathode material 4 to 13, using a composite lithium oxide having the composition described in Table 7 _{(LiM b Ni d Co e O} 2).

Positive electrode material F: mixture of 90% by weight of positive electrode material 1 and 10% by weight of positive electrode material 3 Positive electrode material G: mixture of 80% by weight of positive electrode material 1 and 20% by weight of positive electrode material 3 Positive electrode material H: positive electrode material 1 Mixture of 70 wt% and positive electrode material 3 30 wt%

Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. The results are shown in Table 6-1 and Table 6-2.
A battery nail penetration test was performed under the same conditions as in Experiment 1, and the maximum temperature reached on the battery surface was examined. Ten batteries X19A to X30A, X19B to X30B, and batteries Y13A and Y13B were produced, respectively, and the nail penetration test was performed with 10 batteries. For each battery, the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 6-1 and Table 6-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

Hereinafter, the evaluation results will be described.
From the comparison between the battery X21A and the batteries X22A to X30A and the comparison between the battery X21B and the batteries X22B to X30B, the estimated heat generation rate is reduced for Al, Sn, In, Fe, Cu, Mg, Ti, Zn, and Mo. It can be understood that there is an effect. Incidentally, as in the lithium complex oxide described in Table 7, when used in combination Mn and other elements ^{M 1} as the element M, the molar ratio of Mn and the element ^{M 1} is 99: 1 to 50: 50, more Is preferably 97: 3 to 90:10.

  By adding lithium cobaltate (positive electrode material 3) to the active material, it becomes possible to increase the density of the positive electrode, which is preferable from the viewpoint of increasing the battery capacity. However, in the positive electrode material H whose ratio is 30%, nail penetration safety is lowered. Therefore, when lithium cobaltate is used in combination, the amount is preferably 20% by weight or less of the entire active material.

<< Batteries X31A to X41A >>
Positive electrode materials 14 to 24 (composite lithium oxide of composition LiCo _0.98 M _0.02 O ₂ ) shown in Table 8-1 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each battery, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 8-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<< Batteries X31B to X41B >>
The positive electrode materials 14 to 24 (composite lithium oxide of composition LiCo _0.98 M _0.02 O ₂ ) shown in Table 8-2 were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each battery, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 8-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

  Mn, Al, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo were found to have an effect of reducing the estimated heat generation rate. Also in the composition based on the positive electrode material 3, by adding the element M, the estimated heat generation rate at 200 ° C. could be suppressed to 50 W / kg or less. In addition, the safety of nail penetration has been dramatically improved by the synergistic effect of the element M and the heat-resistant insulating layer.

<< Batteries X42A to X52A >>
The positive electrode materials 25 to 35 shown in Table 9-1 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 9-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<< Batteries X42B to X52B >>
The positive electrode materials 25 to 35 shown in Table 9-2 were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 9-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

The positive electrode materials 25 to 34 were prepared by mixing the positive electrode material 2 (LiAl _0.05 Ni _0.8 Co _0.15 O ₂ ) and an oxide of the element M shown in Table 9 at 1000 ° C. in an air atmosphere. Prepared by firing. The amount of the element M oxide was 0.01 mol with respect to 1 mol of the positive electrode material 2. As a result, positive electrode materials 25 to 34 made of composite lithium oxide in which the element M diffuses from the added oxide into the positive electrode material 2 and the element M is distributed more in the surface layer than in the interior were obtained.

  The positive electrode material 35 was prepared by treating the positive electrode material 2 with vinyltrimethoxysilane, which is a silane coupling agent. Here, the positive electrode material was impregnated in a mixed solution of silane coupling agent and water (the concentration of the silane coupling agent was 0.1% by weight) and then dried.

  Also in the results of Tables 9-1 and 9-2, as in Tables 8-1 and 8-2, the element M was found to have an effect of reducing the estimated heat generation rate. Since the element M is distributed at a high concentration in the surface layer portion of the active material, the effect of suppressing the estimated heat generation rate is remarkable. In addition, the safety of nail penetration has been dramatically improved by the synergistic effect of the element M and the heat-resistant insulating layer. Moreover, the same effect as the addition of the element M was obtained by the treatment with the silane coupling agent.

<Batteries X53A to X55A>
The following positive electrode materials 36 to 38 obtained by mixing the positive electrode material 1 and the positive electrode material 24 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each Example, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 10-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<Batteries X53B to X55B>
The following positive electrode materials 36 to 38 obtained by mixing the positive electrode material 1 and the positive electrode material 24 were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 10-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

Positive electrode material 36: Mixture of 10% by weight of positive electrode material 1 and 90% by weight of positive electrode material 24 Positive electrode material 37: Mixture of 50% by weight of positive electrode material 1 and 50% by weight of positive electrode material 24 Positive electrode material 38: Positive electrode material 1 90% by weight, mixture of positive electrode material 24 by 10% by weight

  From Table 10-1 and Table 10-2, it was found that even when positive electrode materials whose estimated heat generation rate was suppressed to 50 W / kg or less were mixed, nail penetration safety was significantly improved.

<< Batteries X56, 57 and 59 >>
The adhesive surface of the heat resistant insulating layer was set as shown in Table 11. When the heat-resistant insulating layer was formed on the separator, the heat-resistant insulating layer was disposed only on the positive electrode side or the negative electrode side as shown in Table 11. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

<Battery X58>
A battery was produced in the same manner as the battery X9, except that a polyamide-imide resin (thermal decomposition starting temperature 400 ° C. or higher, amorphous, glass transition point 250 ° C.) was used instead of the aramid resin.

<Battery X60>
A battery was produced in the same manner as the battery X9, except that the heat-resistant insulating layer was disposed on the negative electrode side.

<Battery X61>
A battery was produced in the same manner as the battery X58, except that the heat-resistant insulating layer was disposed on the negative electrode side.

<Battery X62>
The raw material paste for the heat-resistant insulating layer was applied onto the fluororesin sheet, dried and then peeled to produce a sheet made of a heat-resistant insulating layer having a thickness of 5 μm that was independent from the positive electrode, the negative electrode, and the separator. A battery was produced in the same manner as the battery Y6 except that a sheet composed of a heat-resistant insulating layer was inserted between the positive electrode and the separator.

<Battery X63>
A sheet made of a heat-resistant insulating layer containing a polyamide resin having the same composition as that of the battery X9 was prepared in accordance with the battery X62, and a battery was prepared in the same manner as the battery X62.

<Battery X64>
In accordance with the battery X62, a sheet made of a heat-resistant insulating layer containing a polyamideimide (PAI) resin having the same composition as the battery X58 was produced, and a battery was produced in the same manner as the battery X62.

<Battery X65>
A battery was produced in the same manner as the battery X4, except that magnesia (magnesium oxide) having a median diameter of 0.3 μm was used instead of alumina having a median diameter of 0.3 μm as the inorganic oxide filler of the heat-resistant insulating layer.

<Battery X66>
A battery was produced in the same manner as the battery X56, except that magnesia (magnesium oxide) having a median diameter of 0.3 μm was used as the inorganic oxide filler of the heat-resistant insulating layer instead of alumina having a median diameter of 0.3 μm.

  With respect to the batteries X56 to X66, a nail penetration test was performed under the same conditions as in Experiment 1, and the average value of the maximum reached temperatures of the 10 battery surfaces was determined. The results are shown in Table 11.

The evaluation results will be described below.
As Table 11 shows, the safety in the nail penetration test was improved regardless of the material of the heat-resistant insulating layer. Accordingly, it was confirmed that the same effect can be obtained if the material has heat resistance and insulation. It was also found that the heat resistant insulating layer was more effective when formed on the surface of the separator or the negative electrode. Furthermore, it has been found that similar results can be obtained by using magnesia instead of alumina.

  In the above example, a cylindrical nonaqueous electrolyte secondary battery was manufactured, but the shape of the battery of the present invention is not limited to a cylindrical shape. Moreover, although the carbon material was used for the negative electrode material and the result in the charging voltage of 4.25V was shown, also when using Si alloy and Sn alloy, the effect of a safety improvement is acquired similarly. Further, in a battery charged to a higher voltage range (4.2 to 4.6 V), the same can be obtained by using a positive electrode mixture having an estimated heat generation rate of 50 W / kg or less and a heat-resistant insulating layer in combination. The safety improvement effect can be obtained.

  The present invention is particularly suitable for non-aqueous electrolyte secondary batteries that require both high energy density and excellent safety, and can be used as a power source for portable devices such as portable information terminals and portable electronic devices. High nature. However, the lithium secondary battery of the present invention can be used for a power source of, for example, a household small-sized power storage device, a motorcycle, an electric vehicle, a hybrid electric vehicle, and the use is not particularly limited. The shape of the lithium secondary battery of the present invention is not particularly limited, but for example, a cylindrical shape or a rectangular shape is suitable. The lithium secondary battery of the present invention is a multifunctional portable device (PDA), electric tool, personal computer (PC), electric toy, electric robot, etc., large backup power supply, emergency backup power supply (USP), natural energy It is particularly suitable for a power generation leveling power source, a regenerative energy utilization system, and the like.

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of its safety.

  In recent years, electronic devices have become rapidly portable and cordless. Accordingly, non-aqueous electrolyte secondary batteries having a high voltage and a high energy density have been put into practical use as power sources for driving electronic devices.

The positive electrode of a nonaqueous electrolyte secondary battery generally contains a composite lithium oxide having a high redox potential. As the composite lithium oxide, for example, lithium cobaltate, lithium nickelate, lithium manganate or the like is used. On the other hand, the negative electrode of a nonaqueous electrolyte secondary battery generally contains a carbon material. The nonaqueous electrolyte secondary battery includes an electrolyte made of a nonaqueous solvent in which a lithium salt is dissolved. For example, LiClO ₄ , LiPF ₆ or the like is used as the lithium salt. A separator is interposed between the positive electrode and the negative electrode. For the separator, for example, a microporous film made of a polyolefin-based material is used.

  When a short circuit with a relatively low resistance occurs inside the battery for some reason, a large current flows in a concentrated manner at the short circuit point. Therefore, the battery may generate heat and reach a high temperature. Various safety measures are taken for the battery so that this phenomenon does not occur.

  In the aspect of the manufacturing process, the metal powder is managed and the dust in the manufacturing atmosphere is managed to prevent foreign matter from entering the battery. Moreover, the internal short circuit is suppressed as much as possible by protecting the exposed portion of the current collector having a low resistance with, for example, an insulating tape.

  A separator having a shutdown function is also used. In the unlikely event that a short circuit with a relatively low resistance occurs inside the battery, the pores of the separator having a shutdown function are blocked at about 135 ° C. and block the ionic current. Therefore, the short circuit current is cut and heat generation stops. However, the surface temperature of the battery rises to about 120 ° C.

  In order to prevent an internal short circuit, it has also been proposed to form a layer made of inorganic fine particles and a resin binder and having a thickness of 0.1 to 200 μm on the electrode. The internal short circuit of the battery is due to the material that partially falls off the electrode during the battery manufacturing process. This proposal aims at suppressing such an internal short circuit and improving a production yield (refer patent document 1).

It has also been proposed to provide a heat resistant resin (for example, aramid) on the separator. This proposal is also intended as a safety measure for preventing an internal short circuit of the battery (see Patent Document 2).
Japanese Patent Laid-Open No. 7-220759 JP-A-9-208736

  According to the conventional proposal, it is possible to suppress heat generation to some extent when a local internal short circuit occurs. However, for example, when performing a nail penetration test, a large number of short-circuit portions are generated at the same time. The nail penetration test is a test for evaluating safety when a battery is damaged with a number of internal short circuits. In such an extreme short-circuit state, heat generation of the battery cannot always be suppressed, and the battery may reach a high temperature.

  For example, when a nail penetration test of a general lithium ion battery in which the positive electrode includes lithium cobaltate, the negative electrode includes graphite, and the separator is a polyethylene microporous film, the battery temperature is increased until the separator shutdown function is exhibited. As a result, the surface temperature of the battery reaches around 120 ° C. This temperature rise is due to Joule heat generated inside the battery due to the short-circuit current.

  Since the short-circuit current is cut by the shutdown function of the separator, heat generation to the extent that the battery temperature reaches more than that is suppressed. According to the standards for safety evaluation in the nail penetration test and the crushing test established by the Storage Battery Industry Association, it is required that there is no smoke, ignition or rupture. On the other hand, the standard regarding battery temperature is not specifically defined. Therefore, even if the surface temperature of the battery is about 120 ° C., the safety standard is satisfied if heat generation is suppressed by the shutdown function.

  However, even when the safety standard is satisfied, when the surface temperature of the battery rises to around 120 ° C., the temperature of the electronic device incorporating the battery also rises. Therefore, the deformation | transformation of the housing | casing of an electronic device, etc. occur, and the safety | security of an electronic device may reduce. Therefore, it is desired to further increase the safety or reliability of the battery. For example, even when an internal short circuit occurs, it is eager to suppress the maximum temperature reached on the battery surface to 80 ° C. or lower.

  In the proposal of Patent Document 1 (a proposal of forming a layer made of inorganic fine particles and a resin binder and having a thickness of 0.1 to 200 μm on the electrode), in the nail penetration test, the battery temperature exceeds 80 ° C. May be reached.

  Even in the proposal of Patent Document 2 (a proposal of providing a heat-resistant resin on the separator), the battery surface temperature may reach a high temperature exceeding 80 ° C. in the nail penetration test.

  Therefore, in the conventional proposal, when a large number of short-circuit points occur simultaneously as in the nail penetration test, the battery surface temperature cannot always be suppressed to 80 ° C. or lower. In the nail penetration test, the following is considered as the reason why the battery surface high temperature exceeds 80 ° C.

  In the case of a single internal short circuit, the expansion of the short circuit point is prevented by the presence of a layer composed of inorganic fine particles and a resin binder and a heat resistant resin. Since the short-circuit point is instantaneously burned off by its own heat generation, the short-circuit state is completed in 0.1 to 0.5 seconds, and then electrical insulation is restored. When the short-circuit current is interrupted, the generated heat diffuses throughout the battery. Therefore, the battery temperature does not reach that high temperature. Since the low temperature region other than the short circuit point is relatively low temperature, heat diffusion occurs quickly.

  On the other hand, in the case of the nail penetration test, a number of short-circuit points are simultaneously generated in the battery. In such a severe situation, it is considered that not only heat generation due to an internal short circuit but also heat generation due to a thermal decomposition reaction of the positive electrode active material continuously occurs. Therefore, it is considered that the heat dissipation rate for diffusing heat cannot catch up with the heat generation rate, and the thermal decomposition reaction of the positive electrode active material expands in a chain. As a result, the positive electrode active material falls off or is burned out near the short circuit point. Therefore, the positive electrode current collector (for example, aluminum foil) is exposed, and a new short-circuit point is generated. As a result, the internal short-circuit state continues, and the battery surface temperature continues to rise to around 120 ° C. where the shutdown function operates. In addition, if it is a single internal short circuit, the thermal decomposition reaction of a positive electrode active material will not advance. It is considered that there is no increase in the number of short-circuit points due to falling off or burning out of the positive electrode active material.

  The present invention is intended to improve the above-described situation, and provides a nonaqueous electrolyte secondary battery having higher safety than the conventional one while maintaining a high energy density.

The present invention relates to a positive electrode mixture including a composite lithium oxide and a positive electrode including a positive electrode current collector supporting the same, a negative electrode including a material capable of occluding and releasing lithium, and a polyolefin resin interposed between the positive electrode and the negative electrode. A non-aqueous electrolyte secondary battery comprising a separator, a non-aqueous electrolyte, and a heat-resistant insulating layer interposed between the positive electrode and the negative electrode, wherein the positive electrode and the negative electrode are interposed between them and the heat resistance The non-aqueous electrolyte secondary battery is wound together with the conductive insulating layer and has an estimated heat generation rate at 200 ° C. of the positive electrode mixture of 50 W / kg or less.
The thickness of the heat resistant insulating layer is, for example, 1 μm or more and 15 μm or less, or 1 μm or more and 5 μm or less.
Estimated exothermic rate is obtained by, for example, (i) determining the relationship between the absolute temperature T and the exothermic rate V of the positive electrode mixture with an acceleration rate calorimeter or a runaway reaction measuring device (ARC), and (ii) based on the Arrhenius theorem Plot the relationship between the reciprocal of absolute temperature T (X coordinate) and the logarithm (Y coordinate) of heat generation rate V, and (iii) an approximate straight line that fits the plot existing in the heat generation region of T <200 ° C. (473 K) (Iv) It is obtained by extrapolating the obtained approximate straight line to the temperature axis of T = 200 ° C. (473 K).

  Here, the heat generation region refers to a region where the absolute value of the slope of the approximate straight line having a negative slope is the largest in the plot based on the Arrhenius theorem. That is, the approximate straight line is drawn so that the absolute value of the negative slope is maximized. Further, extrapolation is a method for obtaining a numerical value expected outside the range of the data based on known numerical data, and is used in various fields.

For example, the following is preferably used for the composite lithium oxide.
(I) General formula (1): It has a composition represented by Li _a M _b Me _c O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and It is at least one selected from the group consisting of Mo, the element Me is at least one selected from the group consisting of Ni and Co, and the general formula (1) is 0.9 <a <1.2. , 0.02 ≦ b ≦ 0.5, 0.5 ≦ c ≦ 0.98, and 0.95 ≦ b + c ≦ 1.05.

(Ii) General formula (2): It has a composition represented by Li _{a Mb} Ni _d Co _e O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, It is at least one selected from the group consisting of Zr and Mo, and the general formula (2) is 0.9 <a <1.2, 0.02 ≦ b ≦ 0.5, 0.1 ≦ d ≦ 0. .5, 0.1 ≦ e ≦ 0.5, and 0.95 ≦ b + d + e ≦ 1.05. The general formula (2) particularly preferably satisfies 0.15 ≦ b ≦ 0.4, 0.3 ≦ d ≦ 0.5, and 0.15 ≦ e ≦ 0.4.

(Iii) It has an arbitrary composition and includes the element M, and the element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr, and Mo. The element M is a composite lithium oxide distributed more in the surface layer than in the interior. Also in the composite lithium oxide represented by the general formulas (1) and (2), it is desirable that the element M is distributed more in the surface layer portion than in the composite lithium oxide.

The composite lithium oxide is preferably treated with a Si compound represented by the general formula (3): X—Si—Y ₃ . Here, X includes a functional group that reacts with the composite lithium oxide, and Y includes a functional group including C, H, O, F, or Si.

  According to the present invention, even in a harsh situation in which a number of short-circuit points are generated at the same time, heat generation due to internal short-circuiting and chained exothermic reaction are effectively suppressed. Therefore, since the short circuit is avoided, the maximum temperature of the battery can be stably suppressed to 80 ° C. or lower. ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery which improved safety | security compared with the past can be provided, maintaining a high energy density.

  The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode mixture including a composite lithium oxide and a positive electrode including a positive electrode current collector supporting the negative electrode, a negative electrode including a material capable of inserting and extracting lithium, and a separator including a polyolefin resin And a non-aqueous electrolyte and a heat-resistant insulating layer interposed between the positive electrode and the negative electrode.

  For example, the heat resistant insulating layer may be formed on one of the positive electrode and the negative electrode on the surface facing the other electrode, but the arrangement of the heat resistant insulating layer is not limited thereto. The heat-resistant insulating layer may be formed only on at least one surface of the positive electrode, may be formed only on at least one surface of the negative electrode, or may be formed only on at least one surface of the separator. Further, the heat-resistant insulating layer may be formed only on at least one surface of the positive electrode and at least one surface of the negative electrode, or may be formed only on at least one surface of the negative electrode and at least one surface of the separator, It may be formed only on at least one surface of the separator and at least one surface of the positive electrode. The heat-resistant insulating layer may be formed on at least one surface of the positive electrode, at least one surface of the negative electrode, and at least one surface of the separator. Further, the heat-resistant insulating layer may be in the form of a sheet independent from the positive electrode, the negative electrode, and the separator. However, it is desirable that the heat-resistant insulating layer is bonded to at least one surface of the positive electrode, at least one surface of the negative electrode, or at least one surface of the separator.

  The present invention has one feature in that the estimated heat generation rate at 200 ° C. of the positive electrode mixture is controlled to 50 W / kg or less. Here, the estimated exothermic rate is obtained by, for example, (i) determining the relationship between the absolute temperature T and the exothermic rate V of the positive electrode mixture by using an accelerated rate calorimeter (ARC) or (ii) Based on the Arrhenius theorem, the relationship between the reciprocal of the absolute temperature T (X coordinate) and the logarithm of the heat generation rate V (Y coordinate) is plotted, and (iii) a plot existing in the heat generation region of T <200 ° C. (473 K) (Iv) is obtained by extrapolating the obtained approximate line to the temperature axis of T = 200 ° C. (473 K).

  When the estimated heat generation rate at 200 ° C. of the positive electrode mixture obtained by the above extrapolation is 50 W / kg or less, the heat-resistant insulating layer contributes to safety, particularly in a severe situation in which short-circuit points occur simultaneously and frequently. Markedly enhanced. The present invention is based on such knowledge and realizes a very high level of safety compared to the prior art.

For example, by using the following positive electrode material, the estimated heat generation rate at 200 ° C. of the positive electrode mixture can be suppressed to 50 W / kg or less.
First, it has a composition represented by the general formula (1): Li _a M _b Me _c O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr. And Mo is at least one selected from the group consisting of Mo, the element Me is at least one selected from the group consisting of Ni and Co, and the general formula (1) is 0.9 <a <1. 2, the estimated heat generation rate of composite lithium oxide satisfying 0.02 ≦ b ≦ 0.5, 0.5 ≦ c ≦ 0.98, and 0.95 ≦ b + c ≦ 1.05 is suppressed to 50 W / kg or less It can be mentioned as an effective positive electrode material.

  From the viewpoint of reducing the estimated heat generation rate, Mn, Al and Mg are particularly preferable among the elements M, and Mn is most preferable. Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo all have the effect of reducing the estimated heat generation rate.

  Here, the a value is an initial value, and increases or decreases as the battery is charged and discharged. The initial value substantially matches the a value of the composite lithium oxide contained in the discharged battery. The standard a value of the composite lithium oxide immediately after synthesis is 1.

If the b value is less than 0.02, the effect of the element M cannot be confirmed. If the b value exceeds 0.5, the capacity reduction increases.
If the c value is less than 0.5, it is difficult to secure a certain capacity or more. If it exceeds 0.98, the effect of reducing the estimated heat generation rate cannot be obtained.

  The general formula (1) satisfies 0.95 ≦ b + c ≦ 1.05. In the initial state immediately after the synthesis (the state having no charge / discharge history), the standard value of b + c is 1, but b + c does not have to be strictly 1. In the range of 0.95 ≦ b + c ≦ 1.05, b + c = 1 can be considered substantially.

Second, it has a composition represented by the general formula (2): Li _{a Mb} Ni _d Co _e O ₂ , and the element M is Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn And at least one selected from the group consisting of Zr and Mo, the general formula (2) is: 0.9 <a <1.2, 0.02 ≦ b ≦ 0.5, 0.1 ≦ d ≦ Composite lithium oxide satisfying 0.5, 0.1 ≦ e ≦ 0.5, and 0.95 ≦ b + d + e ≦ 1.05 as a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less Can be mentioned. The general formula (2) particularly preferably satisfies 0.15 ≦ b ≦ 0.4, 0.3 ≦ d ≦ 0.5, and 0.15 ≦ e ≦ 0.4.

Also in General formula (2), a value is an initial value, and it increases / decreases with charging / discharging of a battery. On the other hand, if the b value is less than 0.02, the effect of the element M cannot be confirmed, and if it exceeds 0.5, the capacity reduction increases.
If the d value is less than 0.1, the effect of adding Ni (for example, the effect of improving the theoretical capacity) is low, and if it exceeds 0.5, the voltage is lowered and the life characteristics are also deteriorated.
When the e value is less than 0.1, the effect of adding Co (for example, the effect of improving the voltage) is low, and when it exceeds 0.5, the utilization factor of the positive electrode decreases.

  The general formula (2) satisfies 0.95 ≦ b + d + e ≦ 1.05. However, the standard value of b + d + e is 1 in the initial state immediately after synthesis (the state having no charge / discharge history), but b + d + e is not necessarily exactly 1. In the range of 0.95 ≦ b + d + e ≦ 1.05, it can be considered that b + d + e = 1 substantially.

As a specific example of the positive electrode material represented by the general formula (2) and controlling the estimated heat generation rate at 200 ° C. to 50 W / kg or less, for example, LiMn _b Ni _d Co _e O ₂ (0.15 ≦ b ≦ 0.35, 0.3 ≦ d ≦ 0.5 and 0.25 ≦ e ≦ 0.35).

  Third, it has an arbitrary composition and includes an element M, and the element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr, and Mo. The element M can be cited as a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less of the composite lithium oxide that is distributed more in the surface layer than in the inside.

  Such a positive electrode material includes a compound (for example, nitrate, etc.) containing the element M on the surface of a composite lithium oxide having an arbitrary composition (for example, a composite lithium oxide represented by the general formula (1) or (2)). Sulfate) and the like, and the element M can be obtained by diffusing into the composite lithium oxide. For example, when a compound containing a composite lithium oxide and a small amount of the element M is mixed and fired at an appropriate temperature, the element M diffuses from the surface of the composite lithium oxide to the inside. As a result, it is possible to obtain a composite lithium oxide in which the element M is distributed more in the surface layer portion than in the inside. Alternatively, a composite lithium oxide carrying the element M can be obtained by mixing a liquid in which a compound containing the element M is dissolved or dispersed and the composite lithium oxide, and then removing the liquid component. When the composite lithium oxide is fired at an appropriate temperature (for example, 300 to 700 ° C.), the element M diffuses from the surface of the composite lithium oxide to the inside.

  The compound containing the element M has a large effect of suppressing the estimated heat generation rate at 200 ° C. However, when the amount of the compound containing the element M added to the composite lithium oxide increases, the utilization factor of the positive electrode decreases and the energy density of the battery decreases. Further, the exothermic reaction of the positive electrode occurs on the surface of the active material particles. Therefore, the presence of a large amount of the element M in the surface layer portion of the active material particles can efficiently suppress heat generation without greatly reducing the utilization factor of the positive electrode. That is, the estimated heat generation rate can be suppressed by a small amount of the element M by concentrating and distributing the element M on the surface layer portion of the active material particles.

  It is preferable to use a compound containing the element M in an amount such that the element M is 0.0001 to 0.05 mole per mole of the composite lithium oxide.

Fourth, a positive electrode material effective for suppressing the estimated heat generation rate to 50 W / kg or less for the composite lithium oxide treated with the Si compound represented by the general formula (3): X—Si—Y ₃ Can be mentioned. Here, X includes a functional group that reacts with the composite lithium oxide, and Y includes a functional group including C, H, O, F, or Si. By modifying the surface of the composite lithium oxide with such a Si compound, the exothermic reaction occurring on the surface of the active material particles is suppressed, and the estimated heat generation rate is suppressed. Moreover, even if the composite lithium oxide is treated with the Si compound, the utilization factor of the positive electrode is not greatly reduced.

For example, it is desirable to treat the composite lithium oxide with a silane coupling agent represented by X—Si—Y ₃ . The method for treating the composite lithium oxide with the silane coupling agent represented by X—Si—Y ₃ is not particularly limited. For example, a silane coupling agent is mixed with water, and the resulting mixture is mixed with a composite lithium oxide and then dried. Here, in the liquid mixture of the silane coupling agent and water, the concentration of the silane coupling agent is preferably about 0.01 wt% to 5 wt%, and more preferably about 0.1 wt% to 3 wt%. Further, the amount of the silane coupling agent is preferably 0.001 to 0.5 parts by weight, more preferably 0.01 to 0.1 parts by weight, per 100 parts by weight of the composite lithium oxide.

  Examples of the silane coupling agent include vinyltriethoxysilane, vinyltrimethoxysilane, vinyltrichlorosilane, vinyltris (2-methoxyethoxy) silane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropyltriethoxy. Silane, γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltriethoxy Silane, γ-ureidopropyltriethoxysilane, γ-ureidopropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltriethoxysilane, γ-glycid Xylpropyltrime Xysilane, γ-glycidoxypropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, γ-chloropropyltriethoxysilane, methyltriethoxysilane, methyl Trimethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane, and the like can be used. Among these, in particular, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris (2-methoxyethoxy) silane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-aminopropyl Triethoxysilane, γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltriethoxysilane, γ-ureidopropyl Triethoxysilane, γ-ureidopropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-Glycidoxypropi Triethoxysilane are preferred.

The heat resistant insulating layer includes, for example, an inorganic oxide filler and a resin component. The inorganic oxide filler has high heat resistance. Therefore, even when the battery reaches a relatively high temperature, the heat-resistant insulating layer can maintain high mechanical strength. Although various resin components can be used for the heat resistant insulating layer, a particularly excellent heat resistant insulating layer can be obtained by using a resin component having high heat resistance.
The heat-resistant insulating layer may include, for example, an inorganic oxide filler and a binder (resin component) or a heat-resistant resin (resin component), but is not particularly limited. A heat-resistant insulating layer containing an inorganic oxide filler and a binder has a relatively high mechanical strength and therefore has a high durability. Here, the heat-resistant insulating layer containing the inorganic oxide filler and the binder is mainly composed of the inorganic oxide filler. For example, 80% by weight or more, preferably 90% by weight or more of the heat-resistant insulating layer is the inorganic oxide filler. The heat resistant insulating layer made of a heat resistant resin contains, for example, a heat resistant resin exceeding 20% by weight.
A heat resistant insulating layer made of a heat resistant resin has higher flexibility than a heat resistant insulating layer containing an inorganic oxide filler as a main component. This is because the heat resistant resin is more flexible than the inorganic oxide filler. Therefore, the heat-resistant insulating layer made of a heat-resistant resin can easily follow the expansion and contraction of the electrode plate accompanying charging / discharging, and can maintain high heat resistance. Therefore, the nail penetration safety is also increased.
The heat-resistant insulating layer made of a heat-resistant resin can contain, for example, less than 80% by weight of an inorganic oxide filler. By including an inorganic oxide filler, a heat-resistant insulating layer excellent in balance between flexibility and durability can be obtained. The heat-resistant resin contributes to the flexibility of the heat-resistant insulating layer, and the inorganic oxide filler having high mechanical strength contributes to durability. By including an inorganic oxide filler in the heat resistant insulating layer, the high output characteristics of the battery are improved. Although details are unknown, it is thought that this is because the void structure of the heat-resistant insulating layer is optimized by the synergistic effect of flexibility and durability. From the viewpoint of securing good high-output characteristics, the heat-resistant insulating layer made of a heat-resistant resin desirably contains 25% to 75% by weight of an insulating filler.

  The resin component (binder or heat resistant resin) of the heat resistant insulating layer desirably has a thermal decomposition starting temperature of 250 ° C. or higher. It is desirable that the resin component does not greatly deform at high temperatures. Therefore, the resin component is desirably amorphous or non-crystalline. The thermal deformation start temperature or glass transition temperature (Tg) of the resin component is desirably 250 ° C. or higher.

  The thermal decomposition start temperature, thermal deformation start temperature, and glass transition temperature of the resin component are determined by differential scanning calorimetry (DSC) and thermogravimetry-differential thermal analysis (TG-DTA). Can be measured. For example, the starting point of weight change in the TG-DTA measurement corresponds to the thermal decomposition start temperature, and the inflection point in the endothermic direction in the DSC measurement corresponds to the thermal deformation temperature or the glass transition temperature.

As specific examples of the binder constituting the heat-resistant insulating layer, for example, a fluororesin such as polyvinylidene fluoride (PVDF) or a rubbery polymer (modified acrylonitrile rubber) containing an acrylonitrile unit can be preferably used. These may be used alone or in combination of two or more. Among these, rubbery polymers containing acrylonitrile units are most suitable because they have appropriate heat resistance, elasticity and binding properties.
As specific examples of the heat-resistant resin constituting the heat-resistant insulating layer, for example, polyamide resins such as aromatic polyamide (aramid), polyimide resins, polyamide-imide resins, and the like can be preferably used. These may be used alone or in combination of two or more.
The heat-resistant insulating layer containing the inorganic oxide filler and the binder is preferably formed or bonded on at least one surface of the negative electrode, and more preferably formed or bonded on both surfaces of the negative electrode. The heat-resistant insulating layer made of a heat-resistant resin is preferably formed or bonded to at least one surface of the separator. Since the heat-resistant insulating layer is relatively brittle, it is formed or bonded only to one surface of the separator. More preferably. When the heat-resistant insulating layer made of a heat-resistant resin is formed only on one side of the separator, the ratio between the thickness A of the separator and the thickness B of the heat-resistant insulating layer: A / B is the heat-resistant insulating layer For example, 3 ≦ A / B ≦ 12 or 4 ≦ A / B ≦ 6.

As the inorganic oxide filler, for example, alumina (Al ₂ O ₃ ), titania (TiO ₂ ), silica (SiO ₂ ), zirconia, magnesia and the like can be used. These may be used alone or in combination of two or more. Of these, alumina (in particular, α-alumina) and magnesia are particularly preferable from the viewpoints of stability, cost, and ease of handling.

  The median diameter (D50: average particle diameter) of the inorganic oxide filler is not particularly limited, but is generally in the range of 0.1 to 5 [mu] m, and preferably 0.2 to 1.5 [mu] m.

  The content of the inorganic oxide filler in the heat-resistant insulating layer containing the inorganic oxide filler and the binder is preferably 50% by weight or more and 99% by weight or less, and 90% by weight or more and 99% by weight or less. Is more preferable. When the content of the inorganic oxide filler is less than 50% by weight, the resin component becomes excessive. Therefore, it may be difficult to control the pore structure with the filler particles. On the other hand, when the content of the inorganic oxide filler exceeds 99% by weight, the resin component becomes excessive. Therefore, the strength of the heat-resistant insulating layer and the adhesion to the electrode surface or separator surface may be reduced.

  The thickness of the heat resistant insulating layer is not particularly limited. The thickness of the heat-resistant insulating layer is, for example, 1 μm or more and 15 μm or less from the viewpoint of sufficiently ensuring the short-circuit suppressing function by the heat-resistant insulating layer or insulating the short-circuit point and maintaining the design capacity. The thickness of the heat-resistant insulating layer containing the inorganic oxide filler and the binder is, for example, 3 to 15 μm or 3 to 8 μm. The thickness of the heat resistant insulating layer made of the heat resistant resin is, for example, 1.5 to 7 μm or 1.7 to 6.7 μm. If the thickness of the heat-resistant insulating layer is too large, the heat-resistant insulating layer is brittle and may be damaged when the electrode is wound. On the other hand, if the thickness is too small, the strength of the heat-resistant insulating layer is reduced and may be damaged.

  Various conventional separators can be used in the present invention. For example, a separator having a single layer structure made of a polyolefin resin such as polyethylene or polypropylene, or a separator having a multilayer structure made of a polyolefin resin can be used. Although the thickness of a separator is not specifically limited, About 15-25 micrometers is desirable.

  The positive electrode mixture includes an active material composed of a composite lithium oxide as an essential component, and includes a binder, a conductive material, and the like as optional components. As the positive electrode binder, for example, polytetrafluoroethylene (PTFE), modified acrylonitrile rubber particles, PVDF, or the like can be used. These may be used alone or in combination of two or more. PTFE and modified acrylonitrile rubber particles are preferably used in combination with carboxymethyl cellulose, polyethylene oxide, modified acrylonitrile rubber and the like. These serve as a thickener for the paste containing the positive electrode mixture and the liquid component. As the conductive material for the positive electrode, acetylene black, ketjen black, various graphites, and the like can be used. These may be used alone or in combination of two or more. The amount of the binder and the conductive material contained in the positive electrode mixture is preferably 0.1 to 5 parts by weight and 1 to 10 parts by weight, respectively, per 100 parts by weight of the active material.

  Various materials used in conventional negative electrodes can be used for the negative electrode including a carbon material or an alloy material. As the carbon material, for example, various natural graphites and various artificial graphites can be used. For example, a silicon alloy or a tin alloy can be used as the alloy material. A composite of a carbon material and an alloy material can also be used. The negative electrode can also contain a binder and a conductive material. The materials mentioned above as the binder and conductive material for the positive electrode can also be used for the negative electrode binder and conductive material.

As the non-aqueous electrolyte, a non-aqueous solvent in which a lithium salt is dissolved as a solute is preferably used. The lithium salt and the non-aqueous solvent are not particularly limited, but it is preferable to use, for example, LiPF ₆ or LiBF _{4 as the} lithium salt. As the non-aqueous solvent, it is preferable to use ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, or the like. The non-aqueous solvent is preferably used in combination of two or more, rather than using one alone. In addition, it is desirable to add vinylene carbonate, vinyl ethylene carbonate, cyclohexylbenzene, or the like as an additive to the non-aqueous electrolyte.

  Hereinafter, the present invention will be specifically described based on various experiments and examples, but the present invention is not limited to the following examples.

[Experiment 1]
Measurement of temperature near short-circuit point Ten cells of a cylindrical 18650 (diameter 18 mm, height 65 mm) non-aqueous electrolyte secondary battery were produced. Here, lithium cobaltate (LiCoO ₂ ) was used as the positive electrode active material. Moreover, the heat resistant insulating layer which consists of an inorganic oxide filler and a resin component was formed in the negative electrode surface. Using these cells, a nail penetration test was performed, and it was examined to what degree the temperature in the vicinity of the short-circuiting point rose to 0.5 seconds immediately after the nail penetration.

  Here, a thermocouple was attached to the battery surface, and a nail was stabbed near the thermocouple, and the battery surface temperature was measured. The results are shown in Table 1.

The cylindrical 18650 non-aqueous electrolyte secondary battery was manufactured in the following manner.
(I) Preparation of positive electrode 3 kg of lithium cobaltate, 1 kg of PVDF # 1320 (N-methyl-2-pyrrolidone (NMP) solution containing 12% by weight of PVDF) manufactured by Kureha Chemical Co., Ltd. as a binder, and acetylene 90 g of black and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a positive electrode mixture paste. This paste was applied to both sides of a 15 μm thick aluminum foil, dried and then rolled to form a positive electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of aluminum foil and a positive mix layer was controlled to 160 micrometers. The obtained electrode plate was cut into a width and a length that could be inserted into a cylindrical battery case having a diameter of 18 mm and a height of 65 mm to obtain a positive electrode.

(Ii) Production of negative electrode 3 kg of artificial graphite, 75 g of BM-400B (aqueous dispersion containing 40% by weight of styrene-butadiene copolymer) manufactured by Nippon Zeon Co., Ltd., and carboxymethyl cellulose (CMC) as a thickener 30 g and an appropriate amount of water were stirred with a double arm kneader to prepare a negative electrode mixture paste. This paste was applied to both sides of a 10 μm thick copper foil, dried and then rolled to form a negative electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of copper foil and a negative mix layer was controlled to 180 micrometers. The obtained electrode plate was cut into a width and a length that can be inserted into the battery case to obtain a negative electrode.

(Iii) Preparation of nonaqueous electrolyte Lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1: 3, A water electrolyte was prepared.

(Iv) Preparation of heat-resistant insulating layer raw material paste 950 g of alumina having a median diameter of 0.3 μm, which is an inorganic oxide filler, and BM-720H, a resin component, made by Nippon Zeon Co., Ltd. (rubber properties containing acrylonitrile units) 625 g of an NMP solution containing 8% by weight of a polymer) and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a heat-resistant insulating layer raw material paste.

(V) Assembly of battery The raw material paste for the heat-resistant insulating layer was applied to both sides of the negative electrode, and the coating film was dried to form a heat-resistant insulating layer having a thickness of 0.5 μm on each side.
A positive electrode and a negative electrode on which a heat-resistant insulating layer having a thickness of 0.5 μm was formed were wound through a separator made of a single layer of a polyethylene resin having a thickness of 20 μm to constitute an electrode plate group. This electrode plate group was inserted into the battery case, and 5.5 g of the nonaqueous electrolyte was injected into the battery case, and the opening of the case was sealed. Thus, a cylindrical non-aqueous electrolyte secondary battery (nominal capacity 2000 mAh) was completed.

The nail penetration test was performed under the following conditions.
First, each battery (cylindrical batteries 1 to 10) was charged as follows.
Constant current charging: 1400mA (end voltage 4.25V)
Constant voltage charge: 4.25V (end current 100mA)
From the side of the battery after charging, a 2.7 mm diameter iron round nail is penetrated at a speed of 5 mm / second in an environment of 20 ° C. The battery temperature in the vicinity of the puncture point was observed.

  From the results in Table 1, it was found that the temperature in the vicinity of the short-circuit point increased to 200 ° C. at the minimum in 0.5 seconds. It is generally known that lithium cobalt oxide in a charged state undergoes thermal decomposition at around 200 ° C. From this, in the situation where short-circuit points occur at the same time as in the nail penetration test, it is expected that Joule heat generation due to current occurs continuously in the vicinity of the short-circuit point and heat of decomposition reaction of the positive electrode active material is generated. This suggests that, in a conventional battery having a heat-resistant insulating layer composed of an inorganic oxide filler and a resin component, safety cannot be reliably ensured in a situation where internal short-circuits occur at the same time. .

  From the above results, it can be understood that it is very important to control the thermal stability of the positive electrode material in order to ensure safety even in a situation where internal short-circuits occur simultaneously and frequently. More specifically, it is important not only to prevent short-circuiting by the heat-resistant insulating layer but also to suppress the thermal decomposition reaction of the positive electrode active material. The positive electrode active material is required to be a material that does not easily decompose even when the vicinity of the short circuit point reaches a high temperature of 200 ° C. or higher.

[Experiment 2]
Examination of positive electrode active material Since the heat stability of the heat-resistant insulating layer and the positive electrode active material is a very important requirement, the thermal stability of the positive electrode mixture was examined next. Here, the estimated heat generation rate at 200 ° C. of the positive electrode mixture containing the positive electrode materials 1 to 3 shown in Table 2 was measured.

Using the material shown in Table 2 as the positive electrode active material, a cylindrical 18650 non-aqueous electrolyte secondary battery was prepared in the same manner as in Experiment 1, and the obtained battery was charged under the following conditions. Hereinafter, the batteries produced using the positive electrode materials 1 to 3 are referred to as batteries 1A to 3A, respectively.
Constant current charging: 1400mA (end voltage 4.25V)
Constant voltage charge: 4.25V (end current 100mA)
However, when the battery voltage is 4.25V, the positive electrode potential with respect to the metal Li corresponds to 4.35V.

  The batteries 1A to 3A in a charged state were decomposed in an atmosphere having a dew point of −40 ° C. or less, and the positive electrode was taken out. The positive electrode was cut into a 3 × 6 cm sample. Next, the positive electrode sample was enclosed in an iron cylindrical case (diameter: 8 mm, height: 65 mm) whose inner surface was plated with Ni, and the opening of the case was sealed.

  Next, using a positive electrode sample sealed in a cylindrical case, using an acceleration rate calorimeter or a runaway reaction measurement device (Accelerated rate calorimeter: ARC), the absolute temperature T and the positive electrode mixture were mixed under the conditions shown in Table 3. The data regarding the relationship with the exothermic rate V was obtained.

  In ARC, since the sample is placed in an adiabatic environment, the temperature increase rate of the sample directly reflects the heat generation rate of the sample. Until the exothermic reaction has a heat generation rate equal to or higher than the detection sensitivity, forced heating is repeated step by step, and when the heat generation rate higher than the detection sensitivity is detected, the heat generation rate of the sample is measured in an adiabatic environment.

The meanings of terms in Table 3 are described below with reference to the conceptual diagram 2.
Temperature rising step: a temperature range (“1” in FIG. 2) of the environmental temperature that is forcibly increased in a stepwise manner in a region where no spontaneous heat generation of the sample is detected.
Exothermic detection sensitivity: Sensitivity to detect self-heating of the sample (“2” in FIG. 2). Sensitivity is arbitrarily set based on the material. When the temperature rise due to the spontaneous heat generation of the sample within the detection time (Δt) is ΔT, the sensitivity is represented by ΔT / Δt.
Stabilization time after temperature increase: After the environmental temperature is forcibly increased based on a predetermined temperature increase step, the sample temperature and the environmental temperature in the furnace are allowed to stabilize ("3" in FIG. 2) It is. This time is arbitrarily set.
Heat generation recognition temperature range: A temperature range ("4" in FIG. 2) for recognizing self-heating of the sample. When the heat generation recognition temperature range is 0.2 ° C and the heat generation detection sensitivity is 0.04 ° C / min, when the temperature rise of 0.04 ° C / min or more continues for 5 minutes (0.2 / 0.04) Is recognized as having fever.

  Conventionally, thermal analysis equipment such as differential scanning calorimetry (DSC) and thermogravimetry-differential thermal analysis (TG-DTA) has been used to evaluate the thermal stability of positive electrode active materials. It's being used. However, evaluation of thermal stability by DSC or TG-DTA has the following problems. First, in the measurement by DSC, TG-DTA, etc., the heat generation rate and the heat generation peak vary depending on the measurement conditions (temperature increase rate and sample amount). Therefore, it is not suitable for accurately obtaining the heat generation rate. Next, in the case of an internal short circuit or the like, the vicinity of the short circuit point instantaneously rises to 200 ° C. or higher. Therefore, heat generation that occurs at less than 200 ° C. occurs at the same time. However, DSC, TG-DTA, etc. cannot predict the rate of heat generation in different temperature ranges. On the other hand, since ARC is an adiabatic measurement, the temperature rise rate of the sample represents the heat generation rate of the sample as it is. For this reason, unlike the thermal analysis method, ARC is very effective in measuring the reaction rate of the exothermic reaction. Therefore, in the present invention, ARC was used in evaluating the thermal stability of the positive electrode mixture at the time of an internal short circuit.

  The data obtained by ARC was plotted based on the Arrhenius theorem as shown in FIG. That is, the relationship between the reciprocal of the absolute temperature T (X coordinate) and the logarithm of the heat generation rate V (Y coordinate) was plotted. The set of plots showing the exothermic rate of the chemical reaction thus obtained can be approximated by a straight line. Therefore, by extrapolating the approximate straight line to a predetermined temperature axis, it is possible to estimate the heat generation rate in a temperature region different from the temperature region in which heat generation is actually observed. Here, as shown in FIG. 1, an approximate straight line suitable for the plot existing in the heat generation region of T <200 ° C. (473 K) is obtained, and the approximate straight line is extrapolated to the temperature axis of T = 200 ° C. (473 K). The estimated heat generation rate was obtained. The results of the estimated heat generation rate obtained are shown in Table 4.

Next, the nail penetration test of the batteries 1A to 3A was performed under the same conditions as in Experiment 1. Only in the case of the battery 1A using the positive electrode material 1 (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ ). The temperature near the short-circuit point immediately after nail penetration did not reach 200 ° C. In addition, the battery voltage after the test did not drop much compared to before the test, and the maximum temperature on the battery surface (position away from the short circuit point) did not reach 80 ° C. until the end of the test. From this, it is considered that after the occurrence of an internal short circuit, the insulation of the short circuit point worked effectively and the generation of Joule heat could be suppressed to a minimum.

  From the above results, when the thermal stability of the positive electrode material is a predetermined value, that is, when the estimated heat generation rate at 200 ° C. derived from the ARC measurement is 10 W / kg, even in a situation where internal short-circuits occur simultaneously and frequently. It can be considered that safety was ensured. This result is due to a synergistic effect between the action of the heat-resistant insulating layer and the thermal stability of the positive electrode material, and this synergistic effect has enabled the realization of a battery with high safety that has not been achieved in the past. Conceivable. This means that, in the past, it was impossible to suppress the battery temperature below 80 ° C. in a situation where short-circuit points occur simultaneously and frequently, but according to the present invention, this is possible.

Next, examples will be described.
<< Batteries X1 to X18 and Batteries Y1 to Y12 >>
The positive electrode materials 1 to 3 shown in Table 2 and the following positive electrode materials A to E obtained by mixing them were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 5. Furthermore, the thickness per adhesive surface after drying of the heat-resistant insulating layer was set as shown in Table 5. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

Cathode material A: Mixture of 90 wt% of cathode material 1 and 10 wt% of cathode material 2 Cathode material B: Mixture of 80 wt% of cathode material 1 and 20 wt% of cathode material 2 Cathode material C: Cathode material 1 Mixture of 70% by weight, 30% by weight of positive electrode material 2 Positive electrode material D: 60% by weight of positive electrode material 1 and 40% by weight of positive electrode material 2 Positive electrode material E: 50% by weight of positive electrode material 1 and positive electrode material 2 50% by weight of mixture

  However, in the batteries X7 to X12, X16 to X18, and the batteries Y3, Y4, Y8, Y11, and Y12, only one side of the separator is made of an aramid resin and an inorganic oxide filler disclosed in the example of Patent Document 2. A film having a thickness of 0.5 to 5 μm was formed as a heat-resistant insulating layer. Specifically, a heat-resistant insulating layer was formed as follows.

  The separable flask having a stirring blade, a thermometer, a nitrogen inflow pipe and a powder addition port was sufficiently dried. In a dry separable flask, 4200 g of NMP and 270 g of calcium chloride dried at 200 ° C. for 2 hours were added, and the temperature was raised to 100 ° C. After the calcium chloride was completely dissolved, the inside of the flask was returned to 20 ± 2 ° C., and 130 g of paraphenylenediamine (PPD) was added and completely dissolved. While maintaining this solution at 20 ± 2 ° C., 24 g of terephthalic acid chloride (TPC) was dividedly added 10 times (total 240 g) every 5 minutes. Thereafter, the solution was aged for 1 hour, stirred for 30 minutes under reduced pressure, and degassed to obtain a polymerization solution of polyparaphenylene terephthalamide (PPTA: thermal decomposition start temperature of 400 ° C. or higher, amorphous).

  To this polymerization solution, an NMP solution in which 5.8% by weight of calcium chloride was dissolved was gradually added so that the PPTA finally became 2.8% by weight. Alumina particles having an average particle size of 0.5 μm were added thereto to obtain a paste having a PPTA solution: alumina weight ratio of 97: 3. This paste was applied to one side of the separator with a bar coater and dried with hot air at 80 ° C. Thereafter, the separator was washed with ion-exchanged water, calcium chloride was removed, and a separator having a heat-resistant insulating layer made of PPTA was obtained. In the electrode plate group, the separator was disposed so that the heat-resistant insulating layer was on the positive electrode side.

In batteries Y5 and 6, no heat-resistant insulating layer was formed on any of the positive electrode, the negative electrode, and the separator.
Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. The results are shown in Table 5.
A battery nail penetration test was performed under the same conditions as in Experiment 1, and the maximum temperature reached on the battery surface away from the short-circuit point was examined. Ten batteries, each of batteries X1 to X18 and batteries Y1 to Y12, were prepared, and the nail penetration test was performed with 10 batteries. For each battery, among the 10 batteries, the average value of the highest reached temperatures of the batteries that did not reach 80 ° C. and the number of batteries that reached 80 ° C. were determined. Table 10 shows “80 ° C. or higher” for all the ten samples that showed a maximum temperature of 80 ° C. or higher. The results are shown in Table 5.

Cathode material 1: LiMn _1/3 Ni _1/3 Co _1/3 O ₂
Cathode material 2: LiAl _0.05 Ni _0.8 Co _0.15 O ₂
Cathode material 3: LiCoO ₂
Cathode material A: Cathode material 1 / Cathode material 2 = 90/10 (wt%)
Cathode material B: Cathode material 1 / Cathode material 2 = 80/20 (wt%)
Cathode material C: Cathode material 1 / Cathode material 2 = 70/30 (wt%)
Cathode material D: Cathode material 1 / Cathode material 2 = 60/40 (wt%)
Cathode material E: Cathode material 1 / Cathode material 2 = 50/50 (wt%)

Hereinafter, the evaluation results will be described.
Even if a positive electrode active material having a relatively high thermal stability compared to lithium cobaltate is used from the batteries Y5 and Y6, if a simultaneous internal short circuit occurs, the battery is not present. It can be seen that the maximum surface temperature cannot always be kept below 80 ° C.

  As shown in batteries Y7 to Y12, even when the battery has a heat-resistant insulating layer, heat is generated when a positive electrode active material having an estimated heat generation rate at 200 ° C. exceeding 50 W / kg determined by the above method is used. It can be understood that the chain of reactions cannot be suppressed, and the maximum temperature reached on the battery surface cannot always be kept below 80 ° C.

  From the comparison of the batteries X13 to 18 and the batteries Y9 to 12, the safe area where the action of the heat-resistant insulating layer can be effectively utilized is the area where the estimated heat generation rate at 200 ° C. of the positive electrode active material is 50 W / kg or less. I understand. It can be understood that if the temperature is outside this region, the increase in the maximum temperature of the battery becomes significant. In the region where the estimated heat generation rate at 200 ° C. of the positive electrode active material is 50 W / kg or less, it is considered that the chain of exothermic reactions is effectively suppressed, and the heat generated at the short-circuit point is efficiently diffused.

  From the batteries X1 to 6, it can be seen that if the thickness of the heat-resistant insulating layer has a certain thickness or more, the effect of suppressing the maximum temperature of the battery when multiple shorts occur simultaneously does not greatly affect. When the thickness of the heat-resistant insulating layer is larger, the maximum temperature reached is lower. However, if the thickness is too large, it becomes difficult to maintain the energy density of the battery at a high level, and the battery tends to be damaged during winding. Since the heat-resistant insulating layer is brittle, if it is excessively thick, it partially falls off from the electrode surface or separator surface during winding. This can be confirmed from the fact that in Comparative Example 2, the number of batteries that reached 80 ° C. or higher specifically increased. Therefore, even when a positive electrode active material with high thermal stability is used, a high level of safety cannot be maintained in the nail penetration test. The thickness of the heat resistant insulating layer may be, for example, in the range of about 1 to 15 μm, or 3 to 10 μm. Similar results are obtained in the batteries X7 to 12 in which the heat-resistant insulating layer is formed on the surface of the separator. The thickness of the heat resistant insulating layer containing the aramid resin may be, for example, 1.7 to 6.7 μm.

  When the heat-resistant insulating layer has a thickness of less than 1 μm, the mechanical strength of the heat-resistant insulating layer itself decreases. Therefore, the heat-resistant insulating layer is easily destroyed due to an impact accompanying a short circuit. This can be confirmed from the fact that the number of batteries that reached 80 ° C. or higher in the battery Y1 specifically increased. Therefore, it is considered that the insulating function is reduced to some extent when the heat-resistant insulating layer has a thickness of less than 1 μm.

<< Batteries X19A to X30A and Battery Y13A >>
The following positive electrode materials F to H obtained by mixing the positive electrode materials 4 to 13 shown in Table 6-1 and the positive electrode materials 1 and 3 were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 6-1. The thickness per adhesive surface after drying of the heat resistant insulating layer was set as shown in Table 6-1. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

<< Batteries X19B to X30B and Battery Y13B >>
The following positive electrode materials F to H obtained by mixing the positive electrode materials 4 to 13 shown in Table 6-2 and the positive electrode materials 1 and 3 were used. The adhesive surface of the heat resistant insulating layer was set as shown in Table 6-2. The thickness after drying of the heat-resistant insulating layer was set as shown in Table 6-2. However, similarly to the battery X9, a film having a thickness of 5 μm made of an aramid resin and an inorganic oxide filler disclosed in Examples of Patent Document 2 was formed only on one side of the separator as a heat-resistant insulating layer. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.
The cathode material 4 to 13, using a composite lithium oxide having the composition described in Table 7 _{(LiM b Ni d Co e O} 2).

Positive electrode material F: mixture of 90% by weight of positive electrode material 1 and 10% by weight of positive electrode material 3 Positive electrode material G: mixture of 80% by weight of positive electrode material 1 and 20% by weight of positive electrode material 3 Positive electrode material H: positive electrode material 1 Mixture of 70 wt% and positive electrode material 3 30 wt%

Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. The results are shown in Table 6-1 and Table 6-2.
A battery nail penetration test was performed under the same conditions as in Experiment 1, and the maximum temperature reached on the battery surface was examined. Ten batteries X19A to X30A, X19B to X30B, and batteries Y13A and Y13B were produced, respectively, and the nail penetration test was performed with 10 batteries. For each battery, the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 6-1 and Table 6-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

Hereinafter, the evaluation results will be described.
From the comparison between the battery X21A and the batteries X22A to X30A and the comparison between the battery X21B and the batteries X22B to X30B, the estimated heat generation rate is reduced for Al, Sn, In, Fe, Cu, Mg, Ti, Zn, and Mo. It can be understood that there is an effect. Incidentally, as in the lithium complex oxide described in Table 7, when used in combination Mn and other elements M ¹ as the element M, the molar ratio of Mn and the element M ¹ is 99: 1 to 50: 50, more Is preferably 97: 3 to 90:10.

  By adding lithium cobaltate (positive electrode material 3) to the active material, it becomes possible to increase the density of the positive electrode, which is preferable from the viewpoint of increasing the battery capacity. However, in the positive electrode material H whose ratio is 30%, nail penetration safety is lowered. Therefore, when lithium cobaltate is used in combination, the amount is preferably 20% by weight or less of the entire active material.

<< Batteries X31A to X41A >>
Positive electrode materials 14 to 24 (composite lithium oxide of composition LiCo _0.98 M _0.02 O ₂ ) shown in Table 8-1 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each battery, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 8-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<< Batteries X31B to X41B >>
The positive electrode materials 14 to 24 shown in Table 8-2 (composite lithium oxide of composition LiCo _0.98 M _0.02 O ₂ ) were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each battery, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 8-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

  Mn, Al, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo were found to have an effect of reducing the estimated heat generation rate. Also in the composition based on the positive electrode material 3, by adding the element M, the estimated heat generation rate at 200 ° C. could be suppressed to 50 W / kg or less. In addition, the safety of nail penetration has been dramatically improved by the synergistic effect of the element M and the heat-resistant insulating layer.

<< Batteries X42A to X52A >>
The positive electrode materials 25 to 35 shown in Table 9-1 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 9-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<< Batteries X42B to X52B >>
The positive electrode materials 25 to 35 shown in Table 9-2 were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 9-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

The positive electrode materials 25 to 34 were prepared by mixing the positive electrode material 2 (LiAl _0.05 Ni _0.8 Co _0.15 O ₂ ) and the oxide of the element M shown in Table 9 and firing at 1000 ° C. in an air atmosphere. The amount of the element M oxide was 0.01 mol with respect to 1 mol of the positive electrode material 2. As a result, positive electrode materials 25 to 34 made of composite lithium oxide in which the element M diffuses from the added oxide into the positive electrode material 2 and the element M is distributed more in the surface layer than in the interior were obtained.

  The positive electrode material 35 was prepared by treating the positive electrode material 2 with vinyltrimethoxysilane, which is a silane coupling agent. Here, the positive electrode material was impregnated in a mixed solution of silane coupling agent and water (the concentration of the silane coupling agent was 0.1% by weight) and then dried.

  Also in the results of Tables 9-1 and 9-2, as in Tables 8-1 and 8-2, the element M was found to have an effect of reducing the estimated heat generation rate. Since the element M is distributed at a high concentration in the surface layer portion of the active material, the effect of suppressing the estimated heat generation rate is remarkable. In addition, the safety of nail penetration has been dramatically improved by the synergistic effect of the element M and the heat-resistant insulating layer. Moreover, the same effect as the addition of the element M was obtained by the treatment with the silane coupling agent.

<Batteries X53A to X55A>
The following positive electrode materials 36 to 38 obtained by mixing the positive electrode material 1 and the positive electrode material 24 were used. Similarly to the battery X9, a heat-resistant insulating layer made of an aramid resin and an inorganic oxide filler was formed on the separator surface so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. About each Example, the nail penetration test was done and the average value of the highest attained temperature of 10 battery surfaces was calculated | required. The results are shown in Table 10-1. The maximum temperature reached for all 10 cells was less than 80 ° C.

<Batteries X53B to X55B>
The following positive electrode materials 36 to 38 obtained by mixing the positive electrode material 1 and the positive electrode material 24 were used. Similarly to the battery X4, a heat-resistant insulating layer containing an inorganic oxide filler and BM-720H was formed on both sides of the negative electrode so that the thickness after drying was 5 μm. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery. Under the same conditions as in Experiment 2, the estimated heat generation rate at 200 ° C. of the positive electrode mixture was determined. Each battery was subjected to a nail penetration test, and the average value of the maximum temperature reached on the surface of 10 batteries was determined. The results are shown in Table 10-2. The maximum temperature reached for all 10 cells was less than 80 ° C.

Positive electrode material 36: Mixture of 10% by weight of positive electrode material 1 and 90% by weight of positive electrode material 24 Positive electrode material 37: Mixture of 50% by weight of positive electrode material 1 and 50% by weight of positive electrode material 24 Positive electrode material 38: Positive electrode material 1 90% by weight, mixture of positive electrode material 24 by 10% by weight

  From Table 10-1 and Table 10-2, it was found that even when positive electrode materials whose estimated heat generation rate was suppressed to 50 W / kg or less were mixed, nail penetration safety was significantly improved.

<< Batteries X56, 57 and 59 >>
The adhesive surface of the heat resistant insulating layer was set as shown in Table 11. When the heat-resistant insulating layer was formed on the separator, the heat-resistant insulating layer was disposed only on the positive electrode side or the negative electrode side as shown in Table 11. Other than that was carried out similarly to the experiment 1, and produced the cylindrical 18650 nonaqueous electrolyte secondary battery.

<Battery X58>
A battery was produced in the same manner as the battery X9, except that a polyamide-imide resin (thermal decomposition starting temperature 400 ° C. or higher, amorphous, glass transition point 250 ° C.) was used instead of the aramid resin.

<Battery X60>
A battery was produced in the same manner as the battery X9, except that the heat-resistant insulating layer was disposed on the negative electrode side.

<Battery X61>
A battery was produced in the same manner as the battery X58, except that the heat-resistant insulating layer was disposed on the negative electrode side.

<Battery X62>
The raw material paste for the heat-resistant insulating layer was applied onto the fluororesin sheet, dried and then peeled to produce a sheet made of a heat-resistant insulating layer having a thickness of 5 μm that was independent from the positive electrode, the negative electrode, and the separator. A battery was produced in the same manner as the battery Y6 except that a sheet composed of a heat-resistant insulating layer was inserted between the positive electrode and the separator.

<Battery X63>
A sheet made of a heat-resistant insulating layer containing a polyamide resin having the same composition as that of the battery X9 was prepared in accordance with the battery X62, and a battery was prepared in the same manner as the battery X62.

<Battery X64>
In accordance with the battery X62, a sheet made of a heat-resistant insulating layer containing a polyamideimide (PAI) resin having the same composition as the battery X58 was produced, and a battery was produced in the same manner as the battery X62.

<Battery X65>
A battery was produced in the same manner as the battery X4, except that magnesia (magnesium oxide) having a median diameter of 0.3 μm was used instead of alumina having a median diameter of 0.3 μm as the inorganic oxide filler of the heat-resistant insulating layer.

<Battery X66>
A battery was produced in the same manner as the battery X56, except that magnesia (magnesium oxide) having a median diameter of 0.3 μm was used as the inorganic oxide filler of the heat-resistant insulating layer instead of alumina having a median diameter of 0.3 μm.

  With respect to the batteries X56 to X66, a nail penetration test was performed under the same conditions as in Experiment 1, and the average value of the maximum reached temperatures of the 10 battery surfaces was determined. The results are shown in Table 11.

The evaluation results will be described below.
As Table 11 shows, the safety in the nail penetration test was improved regardless of the material of the heat-resistant insulating layer. Accordingly, it was confirmed that the same effect can be obtained if the material has heat resistance and insulation. It was also found that the heat resistant insulating layer was more effective when formed on the surface of the separator or the negative electrode. Furthermore, it has been found that similar results can be obtained by using magnesia instead of alumina.

  In the above example, a cylindrical nonaqueous electrolyte secondary battery was manufactured, but the shape of the battery of the present invention is not limited to a cylindrical shape. Moreover, although the carbon material was used for the negative electrode material and the result in the charging voltage of 4.25V was shown, also when using Si alloy and Sn alloy, the effect of a safety improvement is acquired similarly. Further, in a battery charged to a higher voltage range (4.2 to 4.6 V), the same can be obtained by using a positive electrode mixture having an estimated heat generation rate of 50 W / kg or less and a heat-resistant insulating layer in combination. The safety improvement effect can be obtained.

  The present invention is particularly suitable for non-aqueous electrolyte secondary batteries that require both high energy density and excellent safety, and can be used as a power source for portable devices such as portable information terminals and portable electronic devices. High nature. However, the lithium secondary battery of the present invention can be used for a power source of, for example, a household small-sized power storage device, a motorcycle, an electric vehicle, a hybrid electric vehicle, and the use is not particularly limited. The shape of the lithium secondary battery of the present invention is not particularly limited, but for example, a cylindrical shape or a rectangular shape is suitable. The lithium secondary battery of the present invention is a multifunctional portable device (PDA), electric tool, personal computer (PC), electric toy, electric robot, etc., large backup power supply, emergency backup power supply (USP), natural energy It is particularly suitable for a power generation leveling power source, a regenerative energy utilization system, and the like.

It is an example of the Arrhenius plot which shows the relationship between the absolute temperature T calculated | required by ARC, and the heat release rate V of various positive electrode materials. It is explanatory drawing of the measurement principle of ARC.

Claims (9)

(A) a positive electrode mixture containing a composite lithium oxide and a positive electrode containing a positive electrode current collector carrying the same;
(B) a negative electrode containing a material capable of inserting and extracting lithium;
(C) a separator containing a polyolefin resin interposed between the positive electrode and the negative electrode;
(D) a nonaqueous electrolyte, and (e) a nonaqueous electrolyte secondary battery comprising a heat-resistant insulating layer interposed between the positive electrode and the negative electrode,
The positive electrode and the negative electrode are wound together with the separator and the heat-resistant insulating layer interposed therebetween,
The non-aqueous electrolyte secondary battery in which the estimated heat generation rate at 200 ° C. of the positive electrode mixture is 50 W / kg or less.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the heat-resistant insulating layer has a thickness of 1 μm or more and 15 μm or less.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the heat-resistant insulating layer has a thickness of 1 μm or more and 5 μm or less. The estimated heat generation rate is
(I) Using an acceleration rate calorimeter or a runaway reaction measurement device (ARC), obtain the relationship between the absolute temperature T and the exothermic rate V of the positive electrode mixture,
(Ii) Based on the Arrhenius theorem, plot the relationship between the reciprocal of the absolute temperature T as the X coordinate and the logarithm of the heat generation rate V as the Y coordinate;
(Iii) Obtain an approximate straight line that fits the plot existing in the heat generation region of T <200 ° C. (473 K),
(Iv) extrapolating the obtained approximate straight line to the temperature axis of T = 200 ° C. (473 K),
The nonaqueous electrolyte secondary battery according to claim 1, which is obtained by The composite lithium oxide has a composition represented by the general formula (1): Li _{a Mb} Me _c O ₂ ,
The element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo,
The element Me is at least one selected from the group consisting of Ni and Co,
The general formula (1) is
0.9 <a <1.2,
0.02 ≦ b ≦ 0.5,
0.5 ≦ c ≦ 0.98, and 0.95 ≦ b + c ≦ 1.05
The nonaqueous electrolyte secondary battery according to claim 1, wherein The composite lithium oxide has a composition represented by the general formula (2): Li _{a Mb} Ni _d Co _e O ₂
The element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo,
The general formula (2) is
0.9 <a <1.2,
0.02 ≦ b ≦ 0.5,
0.1 ≦ d ≦ 0.5,
0.1 ≦ e ≦ 0.5, and 0.95 ≦ b + d + e ≦ 1.05
The nonaqueous electrolyte secondary battery according to claim 1, wherein   The non-aqueous electrolyte secondary according to claim 6, wherein the general formula (2) satisfies 0.15 ≦ b ≦ 0.4, 0.3 ≦ d ≦ 0.5, and 0.15 ≦ e ≦ 0.4. battery.   The composite lithium oxide includes an element M, and the element M is at least one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr, and Mo, The nonaqueous electrolyte secondary battery according to claim 1, wherein the element M is distributed more in a surface layer portion than in the composite lithium oxide. The composite lithium oxide is treated with a Si compound represented by the general formula (3): X—Si—Y ₃ , X includes a functional group that reacts with the composite lithium oxide, and Y is The nonaqueous electrolyte secondary battery according to claim 1, comprising a functional group containing C, H, O, F, or Si.

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