JP2006261059A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery having a high overcharge resistance performance and high heating resistance performance, and suppressing swelling of battery caused by oxidative degradation by an additive such as cyclohexylbenzene at continuous overcharging or high-voltage charging. <P>SOLUTION: The non-aqueous electrolyte secondary battery comprises: an electrode wound body formed by laminating and further, winding a positive electrode, a negative electrode, and two kinds of separators; and a non-aqueous electrolytic solution. Cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) are added to the non-aqueous electrolytic solution and the added quantity of these is established at CHB: 0.5-3.5 wt%, BP: 0.1-0.5 wt%, FB: 1.0-6.0 wt% respectively. In this case, it is desirable that a first separator having an air permeability of 400 sec/100 cm<SP>3</SP>or less is used at the outer periphery side of the negative electrode, and a second separator having a heat contraction coefficient of 30% in TD direction at 150°C and 3 hours maintenance is used at the inner periphery side of the negative electrode. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Currently, non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries are actively researched and developed for higher capacity, and one of the means is the lamination using different types of separators. An example in which a positive and negative electrode of a type is housed in a bag shape has been proposed (Patent Document 1), and a separator having a multilayer structure using different types of separators having different melting points has been proposed (Patent Documents 2 and 3). In addition to the original function of preventing a short circuit caused by direct contact between the laminated type positive and negative electrodes, these separators generally have a separator itself when the temperature of the battery rises due to overcharge or an external short circuit. A so-called shutdown function (fuse function) that prevents electrical contact between the bipolar plates by melting and closing the pores in the separator is provided.

  However, even in a non-aqueous electrolyte secondary battery using a separator having this type of shutdown function, a separator that prevents thermal runaway of the battery during abnormal heating may cause thermal runaway of the battery during overcharge. Further, a separator that prevents thermal runaway of a battery during overcharging may not be able to prevent the thermal runaway during abnormal heating of the battery.

Regarding such a problem, that is, the problem that the thermal runaway cannot be effectively prevented by the separator at the time of abnormal heating or overcharging of the battery, the inventors of the present application disclosed in Japanese Patent Application No. 2003-320428. Proposed. This is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and two types of separators laminated and wound to form a non-aqueous electrolyte, and a non-aqueous electrolyte secondary battery. A first separator having an air permeability of 400 sec / 100 cm ³ or less is disposed on the side, and a second separator having a heat shrinkage in the width direction (hereinafter referred to as TD direction) of 30% or less is disposed on the inner peripheral side of the negative electrode. It is to arrange. According to this, when the battery falls into an overcharged state or abnormally heated, as a result of the shutdown function of the separator, it is possible to effectively prevent thermal runaway of the battery.

  On the other hand, in this type of non-aqueous electrolyte secondary battery, a predetermined substance is also added to the non-aqueous electrolyte in order to improve battery characteristics. As such a substance (additive), for example, cyclohexylbenzene (CHB) added for improving safety during overcharge is known (Patent Document 4). When cyclohexylbenzene is added to the non-aqueous electrolyte of a non-aqueous electrolyte secondary battery using a carbon material for the negative electrode, a predetermined film is formed on the negative electrode surface due to the reaction between the negative electrode and the electrolyte. When the battery becomes hot due to heat generation at the short-circuited part, the rapid reaction between the negative electrode and the electrolyte is suppressed, and thermal runaway of the battery is avoided.

  Substances added to the electrolyte of the non-aqueous electrolyte secondary battery include cyclohexylbenzene, biphenyl (BP) added to improve deterioration of load characteristics due to charge / discharge cycles, and safety during overcharge. Fluorobenzene (FB) and the like added for improving the properties are also known (Patent Documents 5 and 6).

Japanese Patent No. 3422284 (page 2-4, FIG. 1) Japanese Patent Laid-Open No. 5-13062 (page 2-4) JP 2002-25526 A (page 2-6) Japanese Patent No. 3247103 (paragraph number 0011) Japanese Patent No. 3354057 (paragraph numbers 0008 to 0010) Japanese Patent No. 3061759 (paragraph numbers 0016 to 0017)

  However, in a non-aqueous electrolyte secondary battery in which cyclohexyl benzene is added to the non-aqueous electrolyte, cyclohexyl benzene, which is an additive in the non-aqueous electrolyte, is oxidized and decomposed when the overcharge state continues. As a result, the internal pressure of the battery rises, and as a result, a so-called battery swelling is likely to occur, in which an exterior body (metal can) containing a power generation element such as an electrode or an electrolytic solution expands. Therefore, when cyclohexylbenzene is used as a safety measure at the time of overcharge, it is necessary to suppress battery swelling due to oxidative decomposition at the same time. In this case, if the amount of cyclohexylbenzene added is too small, the original effect due to the addition of cyclohexylbenzene cannot be expected. Therefore, it is necessary to ensure a certain amount of addition while reducing the amount of cyclohexylbenzene added. It is necessary to take another means for suppressing the battery swelling due to the oxidative decomposition.

  Such a problem of battery swelling due to oxidative decomposition of the additive may also occur when an additive other than cyclohexylbenzene is added to the non-aqueous electrolyte. For example, when a required amount of biphenyl is added to improve deterioration of load characteristics due to charge / discharge cycles, or when a required amount of fluorobenzene is added to improve safety during overcharge, When the state continues, the internal pressure of the battery rises due to the oxidative decomposition thereof, and as a result, expansion of the outer package (metal can) constituting the battery, that is, battery expansion is likely to occur. The same applies when biphenyl or fluorobenzene is used together with cyclohexylbenzene and biphenyl and fluorobenzene are used simultaneously.

  In addition, in the non-aqueous electrolyte secondary battery including the two types of separators having the predetermined characteristics described above, adding an additive such as cyclohexylbenzene to the non-aqueous electrolyte further increases the safety of the battery. Can do. However, for that purpose, it is necessary to solve the above-mentioned problem of battery swelling due to the electrolyte additive during continuous overcharge.

  The present invention has been made in view of the above points. First, a non-aqueous electrolyte capable of suppressing battery swelling due to oxidative decomposition of an additive such as cyclohexylbenzene even during continuous overcharge or high-voltage charge. The purpose is to provide a secondary battery (first invention), and secondly, it has a high overcharge resistance and a high heat resistance, and an additive such as cyclohexylbenzene during continuous overcharge or high voltage charge. An object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of suppressing battery swelling due to oxidative decomposition (second invention).

  The inventors of the present invention can suppress battery swelling that is likely to occur when a required amount of an additive such as cyclohexylbenzene is added to a non-aqueous electrolyte by using a specific additive in a specific amount combination. In addition, it has been found that the original function of the additive (for example, in the case of cyclohexylbenzene, the function of suppressing the rapid reaction between the negative electrode and the electrolyte and avoiding the thermal runaway of the battery) can be exhibited effectively. It was.

  Based on such knowledge, in order to achieve the first object, the invention according to claim 1 (first invention) includes a positive electrode, a negative electrode, and two types of separators laminated and further wound. The non-aqueous electrolyte secondary battery including the formed electrode winding body and the non-aqueous electrolyte is configured as follows.

  That is, cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) are added to the non-aqueous electrolyte, and these addition amounts are CHB: 0.5 to 3.5% by weight, BP: The range is set to 0.1 to 0.5% by weight and FB: 1.0 to 6.0% by weight.

Further, in order to achieve the second object, the invention according to claim 2 (second invention) is provided on the outer peripheral side of the negative electrode as the two kinds of separators in addition to the configuration of the non-aqueous electrolyte. Is a first separator having an air permeability of 400 sec / 100 cm ³ or less, and a second separator having a heat shrinkage rate in the TD direction of 30% or less at 150 ° C. for 3 hours on the inner peripheral side of the negative electrode. , Respectively.

  According to the present invention, biphenyl and fluorobenzene are added to the non-aqueous electrolyte simultaneously with cyclohexylbenzene in a predetermined range, for example, when these additives can be added individually or of those Battery swelling due to oxidative decomposition of the additive during continuous overcharge or high voltage charge, which is likely to occur when two types are used in combination, can be effectively suppressed, and is obtained by adding the additive. The effect of preventing thermal runaway can be sufficiently secured.

In this case, a separator having an air permeability of 400 sec / 100 cm ³ or less as described above is arranged on the outer peripheral side of the negative electrode where lithium ions concentrate during charging, and the TD direction at 150 ° C. for 3 hours is held on the inner peripheral side. Adopting a configuration in which separators with a heat shrinkage rate of 30% or less are used, the effect of these separators is that when a battery falls into an overcharged state, thermal runaway does not occur and even when abnormally heated The effect of preventing thermal runaway of the battery is also obtained.

  Thus, according to the first aspect of the present invention, it is possible to realize a non-aqueous electrolyte secondary battery that can suppress battery swelling due to oxidative decomposition of cyclohexylbenzene or the like even during continuous overcharge or high voltage charge. In addition, according to the invention of claim 2, the non-aqueous electrolyte is capable of suppressing battery swelling due to oxidative decomposition of cyclohexylbenzene or the like even during continuous overcharge or high voltage charge, with high overcharge resistance and heat resistance. A secondary battery can be realized.

  In the present invention, a positive electrode and a negative electrode are wound with a predetermined separator interposed therebetween to form an electrode winding body, and this electrode winding body is formed into a cylindrical shape, a rectangular tube shape, etc. together with a non-aqueous electrolyte. The present invention is applied to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery having a hermetically sealed structure housed in a metal can.

1st invention of this application adds such cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) to the nonaqueous electrolyte solution in such a nonaqueous electrolyte secondary battery. CHB: 0.5-3.5 wt%, BP: 0.1-0.5 wt%, FB
: 1.0 to 6.0% by weight, respectively. In addition to the structure of the first invention, the second invention has an air permeability of 400 sec on the outer peripheral side of the negative electrode in addition to the configuration of the first invention for the purpose of further improving the overcharge performance and the heat resistance performance. / 100 cm ³ or less of the first separator, and the second separator having a heat shrinkage rate of 30% or less in the TD direction when held at 150 ° C. for 3 hours on the inner peripheral side of the negative electrode, It is a thing.

<< Separator >>
In the non-aqueous electrolyte secondary battery according to the present invention, it is preferable to use two types of separators having different characteristics as described above. Among these, the measurement of the thermal contraction rate about the 2nd separator was performed by the method shown in FIG. That is, a separator (TD direction: 45 mm × MD direction: 60 mm) B is sandwiched between 5 mm thick glass plates (50 × 80 mm, 47 g) G with a smooth surface, and the inside of the battery is simulated, and the inside of a thermostatic chamber at 150 ° C. And left for 3 hours. Take out the glass plate G from the thermostatic chamber with the load applied, leave it at room temperature for 1 hour, then disassemble and measure the length of the central part in both the TD and MD directions of the separator B. Shrinkage was calculated.
[(L−L ₀ ) / L ₀ ] × 100
Here, L: separator length after holding at 150 ° C., L ₀ : separator length before standing.

  The air permeability of the first separator was measured according to the air permeability test method of JIS P8117 (1998 standard).

  If the average thickness of the first separator having the predetermined air permeability and the second separator having the heat shrinkage rate in the predetermined TD direction is large, the battery capacity is decreased and the internal resistance is increased. Therefore, both are preferably 25 μm or less, more preferably 22 μm or less, and still more preferably 20 μm or less. In order to increase the battery capacity and improve the load characteristics, the thinner the separator, the better.However, in order to maintain good mechanical strength, electrolyte retention, short circuit prevention, etc., the average thickness is Both are preferably 8 μm or more.

The air permeability of the first separator is preferably 400 sec / 100 cm ³ or less, more preferably 250 sec / 100 cm ³ or less. Further, 50 sec / 100 cm ³ or more is preferable. When the air permeability is too high, the lithium ion conductivity is lowered, so that the function as a battery separator is lowered. When the air permeability is too small, the mechanical strength is lowered. Furthermore, as for the porosity of the first separator, if it is too small, the function as a battery separator is lowered, and if it is too large, the mechanical strength is lowered. Therefore, it is preferably 60% or less, more preferably 50% or less. . Moreover, 30% or more is preferable, More preferably, it is 45% or more. If it is this range, a load characteristic can be improved, suppressing an internal short circuit.

  The thermal contraction rate of the second separator is preferably 30% or less, more preferably 25% or less in the TD direction when held at 150 ° C. for 3 hours. The smaller the thermal contraction rate of the separator, the more advantageous for preventing short circuit of the battery. The porosity of the second separator is preferably 60% or less, more preferably 55% or less. Moreover, 30% or more is preferable, More preferably, it is 35% or more. If it is this range, a load characteristic can be improved, suppressing an internal short circuit.

  As said 1st and 2nd separator, a nonwoven fabric and a microporous film can be used, for example. As a material of the nonwoven fabric, for example, polypropylene, polyethylene, polyethylene terephthalate, polybutylene terephthalate and the like can be used. As a material for the microporous film, for example, polypropylene, polyethylene, ethylene-propylene copolymer, and the like can be used. Further, the separator is preferably one having sufficient strength and capable of holding a large amount of electrolyte. In order to suppress thermal shrinkage, the separator may be heat-treated at a temperature of about 100 ° C. in advance.

<Electrode wound body>
The electrode winding body constituting the non-aqueous electrolyte secondary battery of the present invention is formed in a cylindrical shape or a substantially long cylindrical shape, and is housed in an exterior body made of a metal can. After injecting the non-electrolytic solution into the exterior body that houses the electrode winding body, the opening (injection port) of the exterior body is sealed to complete the battery. The shape of the battery thus produced may be either cylindrical or rectangular. That is, either a cylindrical battery or a rectangular battery may be used. There is no problem with a rectangular battery having a part of an R shape or a cylindrical battery having a flat part.

"electrode"
The type of the positive electrode active material to be used is not particularly limited, but a lithium cobalt oxide such as LiCoO ₂ , a lithium manganese oxide such as LiMnO ₂ , and a lithium nickel such as LiNiO ₂ exhibiting an open circuit voltage of 4 V or more on the basis of Li. Lithium-containing composite oxides such as oxides, composite oxides having these as basic structures, for example, substitutes for different metal elements, alone or in a mixture of two or more, or solid solutions thereof Can do. This can increase the energy density of the battery.

  The positive electrode includes, for example, the above-described positive electrode active material, and optionally includes a conductive auxiliary such as flaky graphite and carbon black, and further, a paste containing a binder is applied on the positive electrode current collector and dried. It is produced through a step of forming a coating film containing at least a positive electrode active material and a binder on the electric body. In preparing the paste containing the positive electrode active material, the binder is preferably used as a solution previously dissolved in a solvent, and the solution is mixed with solid particles such as the positive electrode active material.

  Any material can be used for the negative electrode as long as it can dope (occlude) and dedope (release) lithium ions. In the present invention, a material capable of doping and dedoping such lithium ions is used as the negative electrode. It is called an active material. The type of the negative electrode active material is not particularly limited. For example, graphite, pyrolytic carbons, cokes, glassy carbons, sintered organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, etc. Carbonaceous materials, aluminum, silicon, tin, indium, etc. and lithium alloys, or oxides such as silicon, tin, indium, etc. that can be charged and discharged at a low voltage close to lithium can be used.

  The negative electrode is prepared by applying a paste made of the above negative electrode active material, binder, etc. on the negative electrode current collector and drying, and forming a coating film containing at least the negative electrode active material and the binder on the negative electrode current collector. Is done.

When using a carbonaceous material as a negative electrode active material, what has the following characteristic is preferable. That is, the distance (d ₀₀₂ ) between the (002) planes of the crystal of the carbonaceous material is preferably 0.350 nm or less, more preferably 0.345 nm or less, and still more preferably 0.340 nm or less. The crystallite size (Lc) in the c-axis direction is preferably 3 nm or more, more preferably 8 nm or more, and further preferably 25 nm or more. Furthermore, the average particle diameter of the carbonaceous material is preferably 10 μm to 30 μm, more preferably 15 μm to 25 μm, and the ratio of the pure carbon component to the whole carbonaceous material is preferably 99.9% by mass or more.

  Examples of the binder used for the positive electrode and the negative electrode include thermoplastic resins, polymers having rubber elasticity, polysaccharides, and the like. These can be used alone, but can also be used as a mixture of two or more. Specifically, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene propylene terpolymer, ethylene-propylene-diene copolymer, styrene butadiene rubber, polybutadiene, butyl rubber, fluoro rubber, polyethylene oxide, polyvinyl pyrrolidone, poly Epichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, cellulose resin such as carboxymethyl cellulose and hydroxypropyl cellulose, etc. It is done.

  In recent years, binders that use water as a solvent have a greater binding effect than organic solvent-based binders, and can be increased in capacity by increasing the active material ratio of the electrode. In particular, a combination of styrene-butadiene rubber and carboxymethyl cellulose is preferably used.

  As the positive electrode current collector and the negative electrode current collector, for example, a metal foil such as aluminum, copper, nickel, stainless steel, or titanium, an expanded metal, a net, or a foam metal can be used.

  As the positive electrode current collector, a foil mainly composed of aluminum is preferably used, and the purity of the aluminum is desirably 98% by mass or more and 99.9% by mass or less. The thickness of the positive electrode current collector is preferably in the range of 5 μm to 60 μm, and more preferably in the range of 8 μm to 40 μm. Further, the thickness of the positive electrode coating film (positive electrode mixture layer) is preferably in the range of 30 μm to 300 μm, more preferably in the range of 50 μm to 150 μm, on one side.

  As the negative electrode current collector, a copper foil is generally used, and among them, an electrolytic copper foil is preferably used. The thickness of the negative electrode current collector is preferably in the range of 5 μm to 60 μm, and more preferably in the range of 8 μm to 40 μm. The thickness of the negative electrode coating film (negative electrode mixture layer) is preferably in the range of 30 μm to 300 μm, more preferably in the range of 50 μm to 150 μm, per side.

  In the production of the positive electrode and the negative electrode, as an application method when applying the positive electrode active material-containing paste and the negative electrode active material-containing paste to the current collector, for example, various application methods using an extrusion coater, a reverse roll coater, a doctor blade, etc. Can be adopted.

《Electrode lead》
The negative electrode lead is welded to the exposed part of the negative electrode current collector by resistance welding, ultrasonic welding, etc. The cross-sectional area of this negative electrode lead is to reduce the resistance when a large current flows, and to generate heat. Is preferably 0.1 mm ² or more and 1.0 mm ² or less, more preferably 0.30 mm ² or more and 0.70 mm ² or less. Nickel is generally used as the negative electrode lead material, and copper, titanium, stainless steel, etc. can be used, but at least copper or a copper alloy is used to increase the adhesive strength with the copper foil as the negative electrode current collector. It is desirable to use a metal material that is included as an element. Specifically, for example, a copper alloy such as copper or a copper-nickel alloy, a composite material of copper or a copper alloy and another metal such as nickel or titanium, and the like, for example, a two-layer structure of copper and nickel The clad material is inexpensive and can be suitably used.

  As the positive electrode lead, a metal composed of a metal having low electric resistance and capable of withstanding a high potential, such as aluminum, is preferably used.

  The positive and negative electrode leads are preferably attached by methods such as spot welding and ultrasonic welding. In particular, it is desirable to attach the negative electrode lead by ultrasonic welding. In spot welding, if the applied current is increased to increase the adhesive strength, the copper foil tends to have holes, the adhesive strength is reduced, or the weld is likely to be oxidized, which may increase the impedance. Because there is.

<Non-aqueous electrolyte>
In the nonaqueous electrolyte secondary battery of the present invention, a liquid electrolyte (hereinafter referred to as “electrolytic solution”) can be used. Specifically, an organic solvent-based nonaqueous electrolytic solution in which a solute is dissolved in an organic solvent is used. The type of the organic solvent is not particularly limited, but it is particularly preferable to use a chain ester as the main solvent. Examples of such chain esters include organic compounds having a COO-bond such as diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethyl acetate (EA), and methyl propionate (MP). A solvent is mentioned. The fact that this chain ester is the main solvent of the electrolytic solution means that these chain esters occupy more than 50% by volume in the total electrolyte solution, and the chain ester is the total electrolyte solution. It is preferable to occupy 65% by volume or more in the solvent, more preferably 70% by volume or more, and still more preferably 75% by volume or more.

  However, as the solvent of the electrolytic solution, it is preferable to mix and use an ester having a high dielectric constant, for example, an ester having a dielectric constant of 30 or more, in order to improve battery capacity, rather than using only the above-mentioned chain ester. The amount of such a high dielectric constant ester in the total electrolyte solvent is preferably 10% by volume or more, and more preferably 20% by volume or more.

  Examples of the ester having a high dielectric constant include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (γ-BL), ethylene sulfite (ES), and the like. Those having a cyclic structure such as carbonate and propylene carbonate are preferred, cyclic carbonates are particularly preferred, and ethylene carbonate (EC) is most preferred.

  Examples of the solvent that can be used in addition to the ester having a high dielectric constant include 1,2-dimethoxyethane (1,2-DME), 1,3-dioxolane (1,3-DO), tetrahydrofuran (THF), 2 -Methyl-tetrahydrofuran (2-Me-THF), diethyl ether (DEE) and the like. In addition, amine-based or imide-based organic solvents, sulfur-containing or fluorine-containing organic solvents, and the like can also be used.

The solute of the electrolyte solution, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 Co 2, Li 2 C 2 F 4 (SO 3 ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2) or the like is used alone or in combination. In particular, LiPF ₆ and LiC ₄ F ₉ SO ₃ are preferable because of good charge / discharge characteristics. The concentration of the solute in the electrolytic solution is not particularly ^{limited, 0.3mol / dm 3 ~1.7mol / dm} 3, in particular 0.4mol / dm ³ ~1.5mol / dm about ³ are preferred.

  In addition to the electrolytic solution, a solid or gel electrolyte can be used. Examples of such an electrolyte include inorganic solid electrolytes and organic solid electrolytes mainly composed of polyethylene oxide, polypropylene oxide, or derivatives thereof.

"Additive"
In the non-aqueous electrolyte secondary battery of the present invention, cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) are added to the non-aqueous electrolyte in order to enhance safety during overcharge. In order to suppress battery swelling due to oxidative decomposition of these additives, the amount of these additives added is within the above-described range, that is, CHB: 0.5 to 3.5% by weight, BP: 0.00. 1 to 0.5% by weight and FB: 1.0 to 6.0% by weight, respectively. These additives cannot achieve the intended purpose (suppression of battery swelling) even if two of these additives are combined or the addition amount of one of the additives is out of the above range.

The reason why the above additives are used and the amount of these additives should be within the above range is as follows.
“Cyclohexylbenzene (CHB)”:
When the amount of CHB added is more than 3.5% by weight, the overcharge resistance is improved, but the battery swells when the overcharge state continues. When CHB is less than 0.5% by weight, the effect of mixing with FB and BP is reduced (due to the mixing effect). The improvement in overcharge resistance is diminished (the oxidative decomposition potential of CHB is about 4.6 V (vs. Li / Li +), BP is about 4.5 V, FB is about 5.1 V, and when the battery is overcharged, BP first oxidatively decomposes and acts as an overcharge inhibitor, then CHB oxidatively decomposes, Furthermore, since FB is oxidatively decomposed and works continuously as an overcharge inhibitor, by adding specific amounts of BP, CHB, and FB, respectively, overcharge resistance and overcharge performance can be improved rather than adding CHB alone. It is possible to achieve both swell reduction). A more preferable range of the addition amount of CHB is 1.0 to 3.0% by weight.

“Biphenyl (BP)”:
If the amount of BP added exceeds 0.5% by weight, the overcharge resistance is improved, but the battery swells when the overcharge state continues. When BP is less than 0.1%, the effect of mixing with CHB and FB is reduced as in CHB, and the improvement in overcharge resistance due to the mixing effect is reduced. The oxidative decomposition potential of BP is about 4.5V (vs. Li / Li +), which is about 100mV lower than that of CHB. Therefore, when the battery is overcharged to 5V at 1A, it is oxidized faster than CHB. Decomposition occurs and acts as an overcharge prevention agent earlier, and when the overcharge state continues, the oxidative decomposition potential is lower than that of CHB, and the battery swells greatly. The mixed addition with CHB and FB makes it possible to achieve both overcharge resistance and reduced swelling during overcharge as compared with the addition of BP alone. A more preferable addition amount of BP is 0.15 to 0.40% by weight.

“Fluorobenzene (FB)”:
When the amount of FB added is more than 6.0% by weight, the overcharge resistance is improved, but the battery swells when the overcharge state continues. When FB is less than 1.0% by weight, the effect of mixing with BP and CHB is reduced as in BP and CHB, and the improvement in overcharge resistance due to the mixing effect is reduced. As in the case of adding BP and CHB alone, mixed addition with BP and CHB makes it possible to achieve both overcharge resistance and reduced swelling during overcharge, compared to the addition of FB alone. CHB has the best overcharge resistance, but it is expensive compared to FB and BP. A more preferable amount of FB is 2.0 to 5.0% by weight.

<Battery structure>
Next, an embodiment when the present invention is used for a prismatic battery will be described with reference to the drawings. FIG. 2 is a cross-sectional view schematically showing the nonaqueous electrolyte secondary battery of the present embodiment. FIG. 3 is an enlarged view of part A in FIG. FIG. 2 is for explaining the positions where the positive electrode lead 1c and the negative electrode lead 2c are arranged. In an actual electrode winding body 4, as shown in FIG. In addition, the first and second separators 3a and 3b exist, but in FIG. 2, the separator is not shown and is simplified to avoid complication. Reference numeral 5 denotes an exterior body.

  2 and 3, the nonaqueous electrolyte secondary battery of the present embodiment includes a positive electrode 1, a negative electrode 2, a first separator 3a, and a second separator 3b, and the first separator 3a. The second separator 3b is impregnated with an electrolytic solution. The positive electrode 1, the first separator 3 a, the negative electrode 2, and the second separator 3 b are stacked and wound in this order to form the electrode winding body 4.

  The positive electrode 1 is formed by applying a positive electrode mixture layer 1b to both surfaces of a positive electrode current collector 1a. However, the positive electrode 1 located on the outermost surface of the electrode winding body 4 forms the positive electrode mixture layer 1b only on the inner surface of the positive electrode current collector 1a, and the outer surface of the positive electrode current collector 1a is exposed. The exposed positive electrode current collector 1 a is in electrical contact with the inner surface of the exterior body 5. Further, in the vicinity of the end portion of the positive electrode 1 located on the outermost surface of the electrode winding body 4, the positive electrode mixture layer 1 b is not formed on both surfaces of the positive electrode current collector 1 a, and the positive electrode 1 is formed in the vicinity of the end portion of the positive electrode 1. A lead 1c is attached.

  The negative electrode 2 is formed by applying a negative electrode mixture layer 2b on both surfaces of a negative electrode current collector 2a. However, the negative electrode 2 located on the innermost surface of the electrode winding body 4 forms the negative electrode mixture layer 2b only on the inner surface of the negative electrode current collector 2a, and the outer surface of the negative electrode current collector 2a is exposed. Further, in the vicinity of the end of the negative electrode 2 located on the innermost surface of the electrode winding body 4, the negative electrode mixture layer 2 b is not formed on both surfaces of the negative electrode current collector 2 a, and the negative electrode near the end of the negative electrode 2 A lead 2c is attached.

  Examples of the present invention will be described below.

(Examples 1 to 13)
A nonaqueous electrolyte secondary battery having the same structure as that shown in FIGS. 2 and 3 was produced as follows.

  92 parts by mass of lithium cobaltate, 3 parts by mass of acetylene black, and 5 parts by mass of polyvinylidene fluoride were mixed with a planetary mixer using N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode mixture-containing paint. The obtained positive electrode mixture-containing paint is intermittently applied onto a current collector made of an aluminum foil having a thickness of 20 μm by a blade coater, dried, subjected to a pressing step, cut into a predetermined size, and a sheet-like positive electrode Got. In addition, an aluminum lead was attached to the positive electrode by ultrasonic welding.

Next, as an anode, high-density artificial graphite (d ₀₀₂ : 0.336 nm, Lc: 100 nm) 97.5 parts by mass, carboxymethyl cellulose aqueous solution (concentration 1% by mass, viscosity 1500 mPa · s to 5000 mPa · s) 1.5 mass A 1 part by weight of styrene-butadiene rubber was mixed with a planetary mixer using ion-exchanged water having a specific conductivity of 2.0 × 10 ⁵ Ω / cm or more as a solvent to prepare a water-based negative electrode mixture-containing paint. The obtained water-based negative electrode mixture-containing paint was intermittently applied onto a 15 μm thick copper foil with a blade coater, dried, subjected to a pressing step, and then cut into a predetermined size to obtain a sheet-like negative electrode. Further, a lead made of a clad material of copper and nickel was attached to the negative electrode by ultrasonic welding.

Next, as a first separator, a polyethylene microporous membrane separator having an average thickness of 20 μm, an air permeability of 180 sec / 100 cm ³ , a heat shrinkage rate of 35% in the TD direction when held at 150 ° C. for 3 hours, and a porosity of 40% And a polyethylene microporous membrane separator having an average thickness of 22 μm, a permeability of 80 sec / 100 cm ³ , a heat shrinkage rate of 20% in the TD direction and a porosity of 50% when held at 150 ° C. for 3 hours, as a second separator Prepared. Further, the positive electrode, the first separator, the negative electrode, and the second separator are laminated in this order, the first separator is located on the outer peripheral side of the negative electrode, and the inner peripheral side of the negative electrode is It wound so that the 2nd separator might be located, and produced the substantially long cylindrical electrode winding object.

As the non-aqueous electrolyte, a liquid non-aqueous electrolyte was prepared by dissolving LiPF _{6 in} a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 2 to a concentration of 1 mol / dm ³ . At this time, cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) were added to the liquid nonaqueous electrolyte at the ratios shown in Table 1, respectively.

  Then, the electrode winding body is inserted into an exterior body made of a rectangular aluminum can, the end of the positive electrode lead is welded to the lid portion, the end of the negative electrode lead is welded to the output terminal of the negative electrode, and the nonaqueous electrolyte is removed. After the injection, the outer package was sealed to prepare nonaqueous electrolyte secondary batteries (square batteries) having a capacity of 1000 mAh. In these nonaqueous electrolyte secondary batteries, the inner surface of the exterior body and the current collector made of the aluminum foil on the outermost surface of the positive electrode are brought into electrical contact with each other by direct contact.

(Examples 14 to 26)
Other than using a polyethylene microporous membrane separator having an average thickness of 20 μm, air permeability of 180 sec / 100 cm ³ , heat shrinkage of 35% in TD direction at 150 ° C. for 3 hours, and porosity of 40% Produced a non-aqueous electrolyte secondary battery in the same manner as in Example 1.

(Comparative Examples 1-14)
Example 14 except that all or one or two of cyclohexylbenzene (CHB), biphenyl (BP) and fluorobenzene (FB) were added to the liquid nonaqueous electrolyte in the proportions shown in Table 2. A nonaqueous electrolyte secondary battery was produced in the same manner as.

<< Evaluation Test >>
The following measurements were performed using 10 batteries of each of the above examples and comparative examples.

"Overcharge resistance"
Using 10 batteries of each of the above examples and comparative examples, and measuring the maximum battery temperature when charging them to 5V at 1A for 3 hours, the number of batteries having a battery temperature of 135 ° C or higher. Was measured.

“Expansion amount” (battery expansion amount during continuous overcharge)
The battery was charged to 4.2 V at 1 C (2.5 hours), the maximum thickness of the battery after charging was 0%, and CCCV charging was performed at 25 ° C. to 4.3 V at 1 C for 168 hours. A battery having a swelling amount of 5% or less was determined to be OK.

"Heat resistance"
The battery was charged to 4.25 V (3 hours), then placed in an oven, heated from room temperature to 150 at a rate of 5 ° C./min, and then held at 150 ° C. for 3 hours. A battery having a battery temperature of 200 ° C. or higher was defined as NG.

"Evaluation results"
Tables 1 and 2 show the results of the above evaluation tests. In these tables, “overcharge resistance” indicates the number n of batteries having a temperature of 135 ° C. or higher and the total number of test batteries (10), and “heat resistance” indicates a battery having a temperature of 200 ° C. or higher. The number n and the total number (10) of the test batteries are shown in the form of n / 10. Furthermore, for the “expansion amount”, an average value of the expansion amount for 10 test batteries was adopted. In addition, in these tables, regarding the separator used for the electrode winding body, the two types of separators having the above-described predetermined characteristics are the “mixed” type, and the specific one type of separator is the “single” type. In Tables 1 and 2, “Decision 1” indicates a determination result for “overcharge resistance performance” and “swelling amount”, and “Decision 2” indicates a determination result for “heat resistance performance”. .

  As can be seen from Tables 1 and 2, the batteries of Examples 1 to 13 are superior to Comparative Examples 1 to 14 in terms of overcharge resistance, swelling amount during continuous overcharge, and heat resistance. Results were obtained. Further, in the batteries of Examples 14 to 26, although the good results as in the batteries of Examples 1 to 13 were not obtained for the heat resistance performance, the overcharge resistance and the swelling amount during continuous overcharge were as follows. It was also confirmed that good results were obtained compared to Comparative Examples 1-14.

  In addition, although the said Example is related with a square battery, the same effect is exhibited also about a cylindrical battery.

It is a figure which shows the measuring method of the thermal contraction rate of a separator. It is sectional drawing which shows typically the structural example of the nonaqueous electrolyte secondary battery of this invention. It is an enlarged view of the A section of FIG.

Explanation of symbols

G glass plate B separator 1 positive electrode 1a positive electrode current collector 1b positive electrode mixture layer 1c positive electrode lead 2 negative electrode 2a negative electrode current collector 2b negative electrode mixture layer 2c negative electrode lead 3a first separator 3b second separator 4 electrode roll 5 exterior body

Claims (4)

A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and an electrode winding body formed by laminating and further winding two types of separators, and a non-aqueous electrolyte solution,
To the non-aqueous electrolyte, cyclohexylbenzene (CHB), biphenyl (BP) and fluorobenzene (FB) are added,
These addition amounts are
CHB: 0.5 to 3.5% by weight,
BP: 0.1 to 0.5% by weight,
FB: 1.0 to 6.0% by weight,
A non-aqueous electrolyte secondary battery characterized by the above. A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and an electrode winding body formed by laminating and further winding two types of separators, and a non-aqueous electrolyte solution,
To the non-aqueous electrolyte, cyclohexylbenzene (CHB), biphenyl (BP), and fluorobenzene (FB) are added,
These addition amounts are
CHB: 0.5 to 3.5% by weight,
BP: 0.1 to 0.5% by weight,
FB: 1.0 to 6.0% by weight,
And
As the two types of separators, the first separator having an air permeability of 400 sec / 100 cm ³ or less is used on the outer peripheral side of the negative electrode, and the heat in the TD direction at 150 ° C. for 3 hours is used on the inner peripheral side of the negative electrode. A non-aqueous electrolyte secondary battery using a second separator having a shrinkage rate of 30% or less.   The nonaqueous electrolyte secondary battery according to claim 2, wherein the average thickness of each of the first separator and the second separator is 25 μm or less.   4. The non-aqueous electrolyte secondary battery according to claim 2, wherein the porosity of each of the first separator and the second separator is 60% or less.

Images