JP6592256B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery having excellent charge / discharge cycle characteristics and storage characteristics even in a high temperature state and excellent in overcharge characteristics.

  Lithium ion secondary batteries, which are one type of electrochemical element, are considered to be applied to portable devices, automobiles, electric tools, electric chairs, household and commercial power storage systems because of their high energy density. Yes. In particular, as a portable device, it is widely used as a power source for a mobile phone, a smartphone, or a tablet PC.

  In addition, lithium ion secondary batteries are required to improve various battery characteristics as well as to increase the capacity in accordance with the spread of applied devices. Since it is a secondary battery especially, the improvement of a charging / discharging cycling characteristic is calculated | required strongly.

  Usually, a carbon material capable of inserting and removing Li ions is used as a negative electrode active material of a lithium ion secondary battery. In particular, natural or artificial graphite is widely used because of its high capacity and excellent charge / discharge cycle characteristics.

In the case where natural or artificial graphite is used as the negative electrode active material, a method of adding an additive composed of Si or Sn or a material containing these elements to the negative electrode active material for the purpose of further improving charge / discharge cycle characteristics Has been proposed (Patent Document 1).
Further, the negative electrode mixture layer has a low density (1.4 g / cm ³ or less), and artificial graphite having a size of 15 μm or more and 20 μm or less, pitch-coated graphite having a thickness of 10 μm or less, and carbon-coated SiO are used as a negative electrode active material to increase the capacity. It has been proposed to improve the large current characteristics. (Patent Document 2)

On the other hand, Patent Document 3 has a lithium-containing transition metal oxide containing a specific metal element as a positive electrode active material, and the nonaqueous electrolyte contains a compound having two or more nitrile groups in the molecule. A non-aqueous secondary battery having a high capacity and excellent charge / discharge cycle characteristics and storage characteristics is disclosed.
Patent Document 4 discloses a non-aqueous electrolyte secondary battery that is excellent in discharge rate characteristics and high-temperature storage characteristics by using a non-aqueous electrolyte containing a specific electrolyte additive.

  However, Patent Documents 1 to 4 do not mention high-temperature cycle characteristics, and Patent Document 3 refers to the effect of a nitrile compound on the positive electrode. Thus, no mention is made of the action of the negative electrode and the nitrile. Furthermore, there is still room for improvement in each characteristic due to the higher charging upper limit voltage.

JP2012-084426A Japanese Patent No. 5302456 JP 2008-108586 A JP 2007-053083 A

  Problems with the lithium ion secondary battery using the graphite as the negative electrode active material include, for example, repeated charging / discharging or when the battery becomes overcharged in an abnormal state, Li metal is deposited as dendrite on the negative electrode surface. Can be mentioned. This Li dendrite may break through the separator and cause a short circuit, or may react with the non-aqueous electrolyte and cause gas generation. Therefore, development of the technique which suppresses generation | occurrence | production of such Li dendrite and improves the charging / discharging cycle characteristic of a battery is calculated | required.

In lithium ion secondary batteries, lithium-containing composite oxides such as LiCoO ₂ and LiMn ₂ O ₄ are generally used as the positive electrode active material. For example, when the battery is placed in a charged state at a high temperature. However, there is a problem that metals such as Co and Mn are eluted from these positive electrode active materials and deposited on the surface of the negative electrode to deteriorate the battery characteristics, and the development of a technique for avoiding this is also required. The present invention has been made in view of the above circumstances, and provides a lithium ion secondary battery excellent in charge / discharge cycle characteristics (high temperature, low temperature) and high temperature storage characteristics, and also excellent in safety during overcharge. There is to do.

The present invention relates to a lithium ion secondary battery in which an electrode body composed of a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte solution are housed in an outer package, wherein the positive electrode includes Co and / or a positive electrode active material. Or a lithium-containing oxide containing Mn, and the negative electrode has, as a negative electrode active material, artificial graphite A having an average particle diameter of more than 15 μm and 25 μm or less, an average particle diameter of 8 μm or more and 15 μm or less, and graphite particles Of graphite B whose surface is coated with amorphous carbon, and a material S containing at least one element selected from the group consisting of Si and Sn. When the total is 100 mass%, the mixed mass% (A + B) of the artificial graphite A and the graphite B contained as the negative electrode active material is 40% or more and 99% or less, and the artificial graphite A with respect to the graphite B The amount ratio (A / B) is 0.5 to 4.5, wherein the non-aqueous electrolyte, lithium tetrafluoroborate (LiBF ₄₎ and 0.05 to 2.5 mass%, more than one cyano group It is a lithium ion secondary battery containing 0.05 to 5.0% by mass of a nitrile compound containing LiPF ₆ .

  According to the present invention, it is possible to provide a lithium ion secondary battery that exhibits excellent charge / discharge cycle characteristics at high and low temperatures and is excellent in high-temperature storage characteristics and overcharge characteristics.

It is a fragmentary longitudinal cross-sectional view which represents typically an example of the lithium ion secondary battery of this invention. FIG. 2 is a perspective view of FIG. 1.

  As the negative electrode according to the lithium ion secondary battery of the present invention, one having a structure in which a negative electrode mixture layer containing a negative electrode active material, a binder or the like is provided on one side or both sides of a current collector is used.

The negative electrode active material in the present invention comprises artificial graphite A having an average particle size of more than 15 μm and 25 μm or less, mother particles having an average particle size of 8 μm or more and 15 μm or less, and graphite particles having an amorphous surface. It contains graphite B coated with carbonaceous material and material S containing at least one element selected from the group consisting of Si and Sn. Further, the nonaqueous electrolytic solution contains lithium borofluoride (LiBF ₄ ) and a nitrile compound containing two or more cyano groups. At the time of charging, Li ions are first occluded in the material S and gradually occluded to the graphite material side. After that, when excessive Li ions that could not be received on the graphite material side are generated, the material S can again accept Li ions and suppress the precipitation of Li dendrite on the negative electrode surface, so that the charge / discharge cycle characteristics and overcharge characteristics of the battery can be improved. Can be increased.

Further, LiBF ₄ forms a film on the negative electrode, but a film different from the case where only graphite is used as the negative electrode active material is formed. Thus, compared with the case where only graphite is used, storage characteristics and high temperature cycle characteristics are formed. As a result of studies by the present inventors, it has been clarified that the overcharge characteristics are improved. The reason is not clear, but is presumed as follows. If the coating on the negative electrode surface becomes non-uniform and the resistance decreases locally, excessive Li ions concentrate on that portion, so Li dendrite tends to precipitate. However, the coating on the negative electrode with LiBF ₄ is more It is considered that the interface resistance is low and uniform, and the generation of Li dendrite can be further suppressed. Furthermore, by using LiBF ₄ and a nitrile compound containing two or more cyano groups in combination, the thermal stability of the coating on the negative electrode can be improved.

Although details will be described later, in the positive electrode, a nitrile compound containing two or more of LiBF ₄ and a cyano group in the nonaqueous electrolytic solution forms a film on the positive electrode and suppresses elution of metals such as Co and Mn from the positive electrode active material. However, Co and Mn that could not be suppressed selectively move to the carbonaceous material, which eventually traps the eluted metal due to the carbonaceous material, and suppresses deterioration of the negative electrode, thereby storing the battery at a high temperature. The characteristics can be enhanced.

Artificial graphite A is artificial graphite other than graphite B. The artificial graphite referred to in the present invention is, for example, one obtained by firing coke or an organic substance at 2800 ° C. or higher, or one obtained by mixing natural graphite and the above coke or organic substance and performing a heat treatment at 2800 ° C. or higher, and further coke or The organic graphite fired at 2800 ° C. or higher is coated on the surface of the natural graphite. The peak intensity appearing at 1340 to 1370 cm ⁻¹ with respect to the peak intensity appearing at 1570 to 1590 cm ⁻¹ in the argon ion laser Raman spectrum. Artificial graphite having an R value of 0.05 to 0.2 can be used. If the average particle diameter is in the above range, the artificial graphite A may be used in combination with two or more kinds of artificial graphite.

Graphite B is composed of graphite particles serving as mother particles and amorphous carbon covering the surface. Specifically, it is graphite in which the R value, which is a peak intensity ratio appearing at 1340 to 1370 cm ⁻¹ with respect to the peak intensity appearing at 1570 to 1590 cm ⁻¹ in the argon ion laser Raman spectrum, is 0.1 to 0.7. The R value is more preferably 0.3 or more in order to ensure a sufficient coating amount of amorphous carbon. Moreover, since an irreversible capacity | capacitance will increase if there is too much coating amount of amorphous carbon, R value is 0.6 or less. Such graphite B uses, for example, natural graphite having d ₀₀₂ of 0.338 nm or less or graphite obtained by spherically shaping artificial graphite as a base material (base particle), and the surface thereof is coated with an organic compound. It can be obtained by calcining at 0 ° C., pulverizing, and sizing through a sieve. The organic compound covering the base material includes aromatic hydrocarbons; tars or pitches obtained by polycondensation of aromatic hydrocarbons under heat and pressure; tars mainly composed of a mixture of aromatic hydrocarbons. , Pitch or asphalt; In order to coat the base material with the organic compound, a method of impregnating and kneading the base material into the organic compound can be employed. Alternatively, graphite B can be produced by a vapor phase method in which a hydrocarbon gas such as propane or acetylene is carbonized by pyrolysis and deposited on the surface of graphite having d ₀₀₂ of 0.338 nm or less.

  Artificial graphite A has an average particle size of 25 μm or less, and graphite B has an average particle size of 15 μm or less. By using artificial graphite A and graphite B of such a size together, the permeability of the non-aqueous electrolyte into the negative electrode (negative electrode mixture layer) is improved. The reason for this is not clear, but when artificial graphite A and graphite B, which is relatively small and has amorphous carbon on the surface as described above, are used in combination, the negative electrode composite can be obtained by performing a press treatment during the production of the negative electrode. Since the size of the pores formed in the agent layer is made uniform, it is presumed that the nonaqueous electrolyte solution is likely to penetrate. By using the artificial graphite A and the graphite B having a relatively small particle size as described above, the permeability of the non-aqueous electrolyte is improved, and the non-aqueous electrolyte is distributed throughout the negative electrode active material. Since it becomes easier to form a stable film on the surface and the lithium ion acceptability is also increased, the charge / discharge cycle characteristics of the lithium ion secondary battery can be improved.

  In addition, since the specific surface area will increase excessively (the irreversible capacity | capacitance will increase) if the particle size of artificial graphite A is too small, it is preferable that the particle size is not so small. Therefore, in the present invention, artificial graphite A having an average particle diameter of more than 15 μm is used. In addition, if the particle size of graphite B is too small, the coating amount of amorphous carbon covering the surface varies, and there are reasons that the features of graphite B cannot be fully exhibited. It is preferable not to be too small. Therefore, in the present invention, graphite B having an average particle diameter of 8 μm or more is used.

The average particle size of graphite (artificial graphite A, graphite B, and graphite other than these) referred to in the present specification is, for example, a laser scattering particle size distribution meter (for example, Nikkiso Co., Ltd. Microtrac particle size distribution measuring device “HRA9320”). When the integrated volume is determined from particles having a small particle size distribution measured by dispersing graphite in a medium in which graphite is not dissolved or swelled, the value of 50% diameter in the volume-based integrated fraction (D _50% ) Median diameter.

The specific surface area of artificial graphite A and graphite B (according to the BET method. An example of the apparatus is “Bell Soap Mini” manufactured by Nippon Bell Co., Ltd.) is preferably 1.0 m ² / g or more, and 5.0 m ^2. / G or less is preferable.

  Further, the crystallite size in the c-axis direction in the crystal structures of artificial graphite A and graphite B: Lc is preferably 3 nm or more, more preferably 8 nm or more, and further preferably 25 nm or more. . This is because, within this range, insertion / extraction of lithium ions becomes easier. The upper limit value of Lc of graphite is not particularly limited, but is usually about 200 nm.

  The negative electrode active material includes negative electrode active materials other than artificial graphite A and graphite B (for example, graphite having the same particle size as artificial graphite A and having an average particle size of less than 15 μm or more than 25 μm, It is also possible to use artificial graphite A and graphite B) together with artificial graphite A and graphite B.

  The total content of artificial graphite A and graphite B in all negative electrode active materials contained in the negative electrode of the present invention is 40% by mass or more and 99% by mass or less. By setting it to 40% by mass or more, the above-described effects of the present invention can be ensured satisfactorily. Further, since the negative electrode active material of the present invention contains the material S containing at least one element selected from the group consisting of Si and Sn, the content is 99% by mass or less.

  From the viewpoint of further enhancing the injectability of the non-aqueous electrolyte in the battery, the charge / discharge cycle characteristics at high temperatures and the charge characteristics at low temperatures, the content ratio of artificial graphite A to graphite B in the total negative electrode active material contained in the negative electrode (A / B) is 0.5 or more and 4.5 or less. Graphite B in which amorphous carbon is coated on the surface of graphite has high charge acceptability of Li ions (particularly at low temperatures), and is 4.5 or less, so that charge / discharge cycle characteristics at sufficiently high temperatures and Charging characteristics at low temperatures can be ensured. On the other hand, if the content of graphite B is too high, the charge / discharge cycle characteristics are conversely deteriorated.

Examples of the material S include a simple substance of Si or Sn, an alloy containing Si or Sn, an oxide containing Si or Sn, and only one of these may be used, or two or more may be used in combination. . Among these, a material containing Si and O as constituent elements (however, the atomic ratio of O to Si is 0.5 ≦ x ≦ 1.5. Hereinafter, the material is referred to as “SiO _x ”). It is mentioned as preferable.
If the average particle size of the material S is too small, the dispersibility of the material S may be reduced, and the effects of the present invention may not be sufficiently obtained, and the material S has a large volume change associated with charging / discharging of the battery. If the average particle diameter is too large, the material S is likely to collapse due to expansion / contraction (this phenomenon leads to capacity degradation of the material S), and therefore it is preferably 0.1 μm or more and 10 μm or less.

SiO _x exhibits a capacity of 1000 mAh / g or more, and is characterized by significantly exceeding 370 mAh / g, which is called the theoretical capacity of graphite. On the other hand, compared with the charge / discharge efficiency (90% or more) of general graphite, SiO _x often has an initial charge / discharge efficiency of less than 80%, and the irreversible capacity increases. Therefore, a stable film is formed on the negative electrode active material by using the above-described nonaqueous electrolytic solution containing lithium borofluoride (LiBF ₄ ) and a nitrile compound containing two or more cyano groups, and the average particle diameter exceeds 15 μm. By using in combination with artificial graphite A of 25 μm or less, mother particles made of graphite having an average particle diameter of 8 μm or more and 15 μm or less, and graphite B whose surface is coated with amorphous carbon By increasing the permeability of the non-aqueous electrolyte and forming a more stable coating, high charge / discharge efficiency can be ensured even when SiO _x is added, and the charge / discharge cycle characteristics of the battery are also improved.

The SiO _x may contain Si microcrystal or amorphous phase. In this case, the atomic ratio of Si and O is a ratio including Si microcrystal or amorphous phase Si. That is, the SiO _x includes a structure in which Si (for example, microcrystalline Si) is dispersed in an amorphous SiO ₂ matrix, and is dispersed in the amorphous SiO ₂ . In combination with Si, the atomic ratio x may satisfy 0.5 ≦ x ≦ 1.5. For example, in the case of a material in which Si is dispersed in an amorphous SiO ₂ matrix and the material has a molar ratio of SiO ₂ to Si of 1: 1, since x = 1, the structural formula is represented by SiO. . In the case of a material having such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si Can be confirmed.

In addition, since SiO _x has low conductivity, for example, the surface of SiO _x may be coated with carbon, so that a conductive network in the negative electrode can be formed better.

As the carbon for covering the surface of SiO _x , for example, low crystalline carbon, carbon nanotube, vapor grown carbon fiber, or the like can be used.

Incidentally, the hydrocarbon gas is heated in the gas phase, the carbon generated by thermal decomposition of hydrocarbon gas, a method of depositing on the surface of the SiO _x particulate [vapor deposition (CVD) method], SiO _x When the surface of the carbon is coated with carbon, the hydrocarbon-based gas spreads to every corner of the SiO _x particle, and a thin and uniform film containing carbon having conductivity (carbon coating layer) on the surface of the particle and the pores of the surface. ) Can be imparted with good uniformity to the SiO _x particles with a small amount of carbon.

  As the liquid source of the hydrocarbon gas used in the CVD method, toluene, benzene, xylene, mesitylene and the like can be used, but toluene that is easy to handle is particularly preferable. A hydrocarbon-based gas can be obtained by vaporizing them (for example, bubbling with nitrogen gas). Moreover, methane gas, ethylene gas, acetylene gas, etc. can also be used.

As processing temperature of CVD method, it is preferable that it is 600-1200 degreeC, for example. Further, SiO _x subjected to CVD method is preferably granulated material was granulated by a known method (composite particles).

When the surface of SiO _x is coated with carbon, the amount of carbon is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, with respect to SiO _x : 100 parts by mass, and 95 The amount is preferably at most part by mass, more preferably at most 90 parts by mass.

  In the negative electrode mixture layer, the content ratio of the material S with respect to all the negative electrode active materials is such that the ratio of the material S is 1% by mass or more, and is preferably 2.5% by mass or more. By using the content ratio of the material S with respect to all the negative electrode active materials at such a ratio, the above-described effects due to the use of the material S can be secured satisfactorily, and the capacity of the battery can be increased.

  In addition, since the volume change accompanying the charging / discharging of the battery is large in the material S, in the battery of the present invention, when the total of the carbon material and the material S in the negative electrode mixture layer is 100% by mass, The proportion is 10% by mass or less, preferably less than 3.5% by mass. As a result, the above-described effects can be obtained while suppressing the deterioration of the battery characteristics accompanying the volume change of the material S.

  As the binder related to the negative electrode mixture layer, for example, a material that is electrochemically inactive with respect to Li in the working potential range of the negative electrode and does not affect other substances as much as possible is selected. Specifically, for example, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), carboxymethylcellulose (CMC), methylcellulose, polyimide, polyamideimide and the like are preferable. These binders may use only 1 type and may use 2 or more types together.

  Moreover, you may add various carbon blacks, such as acetylene black, a carbon nanotube, carbon fiber, etc. to a negative mix layer as a conductive support agent.

  The negative electrode is prepared, for example, by preparing a negative electrode mixture-containing composition in which a negative electrode active material and a binder and, if necessary, a conductive additive are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) or water. (However, the binder may be dissolved in a solvent), which is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the manufacturing method of the negative electrode is not limited to the above method, and may be manufactured by other manufacturing methods.

The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per side of the current collector, and the density of the negative electrode mixture layer (from the mass and thickness of the negative electrode mixture layer per unit area laminated on the current collector) (Calculated) is preferably 1.5 g / cm ³ or more, more preferably 1.6 g / cm ³ or more in order to increase the capacity of the battery. In addition, if the density of the negative electrode mixture layer is too high, adverse effects such as a decrease in the permeability of the non-aqueous electrolyte solution occur, so it is desirable that the density be 1.9 g / cm ³ or less. Moreover, as a composition of a negative mix layer, it is preferable that the quantity of a negative electrode active material is 80-99 mass%, for example, it is preferable that the quantity of a binder is 0.5-10 mass%, and a conductive support agent. When using, it is preferable that the quantity is 1-10 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is 5 μm in order to ensure mechanical strength. Is desirable.

The nonaqueous electrolytic solution of the present invention contains lithium borofluoride (LiBF ₄ ) and a nitrile compound containing two or more cyano groups.

The reason why Co and Mn in the positive electrode active material are eluted at high temperature is that LiPF ₆ in the electrolyte is decomposed to generate hydrogen fluoride (HF), and this HF destroys the crystal structure of the positive electrode active material. It is considered that Co and Mn are eluted. By containing a compound that forms a highly stable coating on the positive electrode even at high temperatures, such as LiBF ₄ and a nitrile compound, in the nonaqueous electrolyte, the reaction of HF with the positive electrode active material is suppressed, and Co and Mn Elution itself can be suppressed, and high-temperature cycle characteristics and high-temperature storage characteristics can be improved.

  In addition to the negative electrode configuration described above, this type of non-aqueous electrolyte also interacts with each other, excels in charge / discharge cycle characteristics and high-temperature storage characteristics, and is excellent in safety during overcharge. Lithium ion secondary battery.

LiBF ₄ has higher stability than LiPF _{6 at} a high temperature, and the amount of HF generated does not increase due to the decomposition of LiBF ₄ itself. In addition, since LiBF ₄ has a low molecular weight, the effect can be exhibited with a smaller amount of additive for bringing out the same effect as compared with other additives. Moreover, since LiBF ₄ forms an inorganic dense negative electrode film, the film itself has a low resistance, and an increase in load characteristics can be suppressed. Furthermore, it does not contribute to gas generation during high temperature storage.

  Examples of the nitrile compound containing two or more cyano groups include malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7. -Dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane 2,4-dimethylglutaronitrile and the like.

Among these, it is particularly desirable to use a compound represented by the general formula (1).
_{NC- (CH 2) n -CN (} 1)
[In said general formula (1), n is an integer of 2-4. ]

  These compounds can form a highly stable film on the positive electrode even at high temperature and high voltage. Thereby, it can suppress that the crystal structure of a positive electrode active material is destroyed by HF, and it becomes possible to suppress elution of Co and Mn. Of these, adiponitrile and succinonitrile have high stability at high temperatures, and are versatile and preferred.

In order to obtain the above-described effect, the content of LiBF _{4 in} the nonaqueous electrolytic solution is 0.05% by mass or more, and more preferably 0.1% by mass or more. Moreover, it is 2.5 mass% or less, and 0.5 mass% or less is more preferable.

  The content of the nitrile compound containing two or more cyano groups in the nonaqueous electrolytic solution is 0.05% by mass or more, and more preferably 0.1% by mass or more. Moreover, it is 5 mass% or less, and 2 mass% or less is more preferable.

In the present invention, as the lithium salt of the nonaqueous electrolytic solution comprises LiPF _6. LiPF ₆ is the most versatile lithium salt having a high degree of dissociation and a high Li ion transport rate. In addition to LiPF ₆ , LiClO ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO3 (n ≧ Other lithium salts such as 2) may be included to such an extent that the effects of the present invention are not impaired. The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.6 to 1.8 mol / l, and more preferably 0.9 to 1.6 mol / l.

As the nonaqueous electrolytic solution of the present invention, for example, a solution (nonaqueous electrolytic solution) prepared by dissolving the above lithium salt, LiBF ₄ and nitrile compound in the following nonaqueous solvent can be used.

Examples of the solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), γ-butyrolactone (γ-
BL), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), 1,3-dioxolane, formamide, dimethylformamide (DMF), dioxolane, acetonitrile, nitromethane , An aprotic organic solvent such as methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivative, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl ether alone, Or it can use as a mixed solvent which mixed 2 or more types.

  The non-aqueous electrolyte used in the lithium ion secondary battery of the present invention includes 1,3-propane for the purpose of further improving charge / discharge cycle characteristics and improving safety such as high-temperature storage and prevention of overcharge. Sultone, 1,3-dioxane, vinylene carbonate, vinyl ethylene carbonate, fluorinated carbonates such as 4-fluoro-1,3-dioxolan-2-one, anhydride, sulfonic acid ester, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluoro Additives (including these derivatives) such as benzene and t-butylbenzene can be appropriately added.

  Among these, it is preferable to contain 1,3-dioxane. Thereby, the charging / discharging cycle characteristic in the high temperature of a lithium ion secondary battery can further be improved.

  The content of 1,3-dioxane in the non-aqueous electrolytic solution used for the lithium ion secondary battery is preferably 0.1% by mass or more from the viewpoint of better ensuring the effect of the use. More preferably, it is 5 mass% or more. However, if the amount of 1,3-dioxane in the non-aqueous electrolyte is too large, the load characteristics of the battery may be reduced, and the effect of improving the charge / discharge cycle characteristics may be reduced. Therefore, the content of 1,3-dioxane in the non-aqueous electrolyte used for the lithium ion secondary battery is preferably 5% by mass or less, and more preferably 2% by mass or less.

  Moreover, when vinylene carbonate and 4-fluoro-1,3-dioxolan-2-one are contained, the charge / discharge cycle characteristics can be further improved. The contents in these non-aqueous electrolytes are preferably 0.1 to 5.0% by mass and 0.05 to 5.0% by mass, respectively.

Moreover, it is preferable that the non-aqueous electrolyte contains a phosphonoacetate compound represented by the following general formula (2). The phosphonoacetate compound contributes to the formation of a film on the negative electrode surface of the lithium ion secondary battery together with LiBF ₄ and produces a stronger film, thereby further deteriorating the negative electrode active material and the non-aqueous electrolyte. Can be suppressed.

［一般式（２）中、Ｒ^１、Ｒ^２およびＲ^３は、それぞれ独立して、ハロゲン原子で置換されていてもよい炭素数１〜１２のアルキル基、アルケニル基又はアルキニル基を示し、ｎは０〜６の整数を示す。］

[In General Formula (2), R ¹ , R ² and R ³ each independently represent an alkyl group, alkenyl group or alkynyl group having 1 to 12 carbon atoms which may be substituted with a halogen atom, and n Represents an integer of 0-6. ]

Specific examples of the phosphonoacetate compound represented by the general formula (2) include the following.

<Compound wherein n = 0 in the general formula (2)>
Trimethyl phosphonoformate, methyl diethyl phosphonoformate, methyl dipropyl phosphonoformate, methyl dibutyl phosphonoformate, triethyl phosphonoformate, ethyl dimethyl phosphonoformate, ethyl dipropyl phosphonoformate, ethyl Dibutylphosphonoformate, Tripropylphosphonoformate, Propyldimethylphosphonoformate, Propyldiethylphosphonoformate, Propyldibutylphosphonoformate, Tributylphosphonoformate, Butyldimethylphosphonoformate, Butyldiethylphospho Noformate, butyl dipropylphosphonoformate, methyl bis (2,2,2-trifluoroethyl) phosphonoformate, ethyl bis (2,2 2-trifluoroethyl) phosphonoacetate formate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate, etc. butyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate.

<Compound with n = 1 in the general formula (2)>
Trimethyl phosphonoacetate, methyl diethylphosphonoacetate, methyldipropylphosphonoacetate, methyldibutylphosphonoacetate, triethylphosphonoacetate, ethyldimethylphosphonoacetate, ethyldipropylphosphonoacetate, ethyldibutylphosphonoacetate, tripropyl Phosphonoacetate, propyl dimethylphosphonoacetate, propyldiethylphosphonoacetate, propyldibutylphosphonoacetate, tributylphosphonoacetate, butyldimethylphosphonoacetate, butyldiethylphosphonoacetate, butyldipropylphosphonoacetate, methylbis (2 , 2,2-trifluoroethyl) phosphonoacetate, ethyl bis (2,2,2-trifluoroethyl) phosphonoa Cetate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate, butyl bis (2,2,2-trifluoroethyl) phosphonoacetate, allyl dimethylphosphonoacetate, allyldiethylphosphonoacetate, 2- Propinyl dimethylphosphonoacetate, 2-propynyl diethylphosphonoacetate and the like.

<Compound wherein n = 2 in the general formula (2)>
Trimethyl 3-phosphonopropionate, methyl 3- (diethylphosphono) propionate, methyl 3- (dipropylphosphono) propionate, methyl 3- (dibutylphosphono) propionate, triethyl 3-phosphonopropionate, ethyl 3- (dimethylphosphono) propionate, ethyl 3- (dipropylphosphono) propionate, ethyl 3- (dibutylphosphono) propionate, tripropyl 3-phosphonopropionate, propyl 3- (dimethylphosphono) propionate, Propyl 3- (diethylphosphono) propionate, propyl 3- (dibutylphosphono) propionate, tributyl 3-phosphonopropionate, butyl 3- (dimethylphosphono) propionate, butyl 3- (diethylphosphono) propionate Onate, butyl 3- (dipropylphosphono) propionate, methyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, ethyl 3- (bis (2,2,2-trifluoroethyl) phosphono ) Propionate, propyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, butyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, and the like.

<Compound wherein n = 3 in the general formula (2)>
Trimethyl 4-phosphonobutyrate, methyl 4- (diethylphosphono) butyrate, methyl 4- (dipropylphosphono) butyrate, methyl 4- (dibutylphosphono) butyrate, triethyl 4-phosphonobutyrate, ethyl 4- (Dimethylphosphono) butyrate, ethyl 4- (dipropylphosphono) butyrate, ethyl 4- (dibutylphosphono) butyrate, tripropyl 4-phosphonobutyrate, propyl 4- (dimethylphosphono) butyrate, propyl 4- (Diethylphosphono) butyrate, propyl dibutylphosphono) butyrate, tributyl 4-phosphonobutyrate, butyl 4- (dimethylphosphono) butyrate, butyl 4- (diethylphosphono) butyrate, butyl 4- (dipropylphosphono) ) Butyrate etc.

  Among the phosphonoacetate compounds, 2-propynyldiethylphosphonoacetate (PDEA) and ethyldiethylphosphonoacetate (EDPA) are preferably used.

  The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolytic solution, and the negative electrode and the nonaqueous electrolytic solution may be those described above, and other configurations and structures are particularly limited. Rather, it is possible to apply a configuration and structure employed in a conventionally known lithium ion secondary battery.

  The positive electrode according to the lithium ion secondary battery of the present invention includes at least a positive electrode active material. For example, a positive electrode mixture layer containing a positive electrode active material is formed on one side or both sides of a current collector. . In addition to the positive electrode active material, the positive electrode mixture layer contains a binder and, if necessary, a conductive additive, and includes, for example, a positive electrode active material and a binder (and further a conductive additive). Applying a suitable solvent to the mixture (positive electrode mixture) and thoroughly kneading the resulting mixture (slurry, etc.) on the surface of the current collector, followed by drying to obtain a desired thickness Can be formed. In addition, the positive electrode after forming the positive electrode mixture layer can be subjected to press treatment as necessary to adjust the thickness and density of the positive electrode mixture layer.

In the present invention, it is assumed that a lithium-containing oxide containing Co and / or Mn is included as a positive electrode active material. Conventionally known positive electrode active materials for lithium ion secondary batteries containing these elements are used. Can be used. As a specific example of such a positive electrode active material, for example, a layered structure represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc.) Lithium-containing transition metal oxide, LiMn ₂ O ₄ and spinel-structured lithium manganese oxide obtained by substituting some of its elements with other elements, LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) Type compounds. Specific examples of the lithium-containing transition metal oxide having the layered structure include LiCoO ₂ and other oxides including at least Co, Ni, and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _{5 / 12} Ni _5/12 Co _1/6 O ₂ ) and the like.

In particular, when the lithium ion secondary battery is charged at a higher end voltage than usual before its use, in order to increase the stability of the positive electrode active material in a state charged at a high voltage, It is preferable that the various active materials exemplified above further contain a stabilizing element. Examples of such stabilizing elements include Mg, Al, Ti, Zr, Mo, and Sn.
As the positive electrode active material, only the lithium-containing oxide containing Co and / or Mn as described above can be used, but the lithium-containing oxide containing Co and / or Mn and another positive electrode active material are used in combination. You can also

Other positive electrode active materials that can be used in combination with a lithium-containing oxide containing Co and / or Mn include, for example, lithium nickel oxides such as LiNiO ₂ ; spinel-structure lithium such as Li _4/3 Ti _5/3 O ₄ Containing composite oxide; Lithium-containing metal oxide having an olivine structure such as LiFePO ₄ ; Oxides obtained by substituting the above-mentioned oxide with various elements. However, from the viewpoint of ensuring the above effect better, the content of the lithium-containing oxide containing Co and / or Mn in the total amount of the positive electrode active material contained in the positive electrode mixture layer is 50% by mass or more. Preferably there is.

  The positive electrode is a paste-like or slurry-like positive electrode mixture obtained by adding an appropriate solvent (dispersion medium) to the mixture (positive electrode mixture) containing the positive electrode active material, the conductive additive and the binder, and sufficiently kneading the mixture. The agent-containing composition can be obtained by coating the current collector and forming a positive electrode mixture layer having a predetermined thickness and density. The positive electrode is not limited to the one obtained by the above-described production method, and may be one produced by another production method.

  As the binder relating to the positive electrode, the above-described binders exemplified as those for the negative electrode can be used. Moreover, also about the conductive support agent which concerns on a positive electrode, each said conductive support agent illustrated as a thing for negative electrodes can be used.

  In the positive electrode mixture layer according to the positive electrode, the content of the positive electrode active material is, for example, 79.5 to 99% by mass, and the content of the binder is, for example, 0.5 to 20% by mass. The content of the conductive assistant is preferably, for example, 0.5 to 20% by mass.

  The separator is preferably a porous film composed of polyolefin such as polyethylene, polypropylene, and ethylene-propylene copolymer; polyester such as polyethylene terephthalate and copolymer polyester; In addition, it is preferable that a separator has the property (namely, shutdown function) which the hole obstruct | occludes in 100-140 degreeC. Therefore, the separator has a melting point, that is, a thermoplastic resin having a melting temperature measured using a differential scanning calorimeter (DSC) of 100 to 140 ° C. in accordance with JIS K 7121 as a component. Preferably, it is a single layer porous film mainly composed of polyethylene or a laminated porous film comprising a porous film such as a laminated porous film in which 2 to 5 layers of polyethylene and polypropylene are laminated. Is preferred. When a resin having a melting point higher than that of polyethylene such as polyethylene and polypropylene is used by mixing or laminating, it is desirable that polyethylene is 30% by mass or more, and 50% by mass or more as a resin constituting the porous membrane. More desirable.

  As such a resin porous membrane, for example, a porous membrane composed of the above-mentioned exemplified thermoplastic resin used in a conventionally known non-aqueous electrolyte secondary battery or the like, that is, a solvent extraction method, An ion-permeable porous membrane produced by a dry or wet stretching method can be used.

  The average pore size of the separator is preferably 0.01 μm or more, more preferably 0.05 μm or more, preferably 1 μm or less, more preferably 0.5 μm or less.

  Moreover, as a characteristic of the separator, a Gurley value represented by the number of seconds in which 100 ml of air permeates the membrane under a pressure of 0.879 g / mm 2 is 10 to 500 sec. It is desirable. If the air permeability is too high, the ion permeability is reduced, whereas if it is too low, the strength of the separator may be reduced. Further, the strength of the separator is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm.

  The lithium ion secondary battery of the present invention can be used with a charging upper limit voltage of about 4.2 V as in the case of the conventional lithium ion secondary battery. However, the charging upper limit voltage is higher than 4.4 V. It is possible to set and use as described above. With this, it is possible to stably exhibit excellent characteristics even when repeatedly used over a long period of time while increasing the capacity. In addition, it is preferable that the upper limit voltage of charge of a lithium ion secondary battery is 4.5V or less.

  The lithium ion secondary battery of the present invention can be applied to the same applications as conventionally known lithium ion secondary batteries.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention. Table 1 shows the physical properties of the composites obtained by coating the graphite and SiO surfaces used in the examples with carbon materials. Graphite A-1, A-2 and A-3 are artificial graphite A having an average particle diameter of more than 15 μm and 25 μm or less as defined in claim 1, and graphite a-1 and graphite a-2 are claims. Artificial graphite having an average particle size other than those specified. Graphite B-1 and Graphite B-2 are graphite B having an average particle diameter of 8 μm or more and 15 μm or less as defined in claim 1, and the surface of the graphite particles is coated with amorphous carbon. -1 is graphite having an average particle diameter other than those specified in the claims. Graphite C is other graphite that does not belong to artificial graphite A or graphite B.

Example 1
<Preparation of positive electrode>
A biaxial kneader comprising 100 parts by mass of LiCoO _2, 20 parts by mass of an NMP solution containing PVDF as a binder at a concentration of 10% by mass, 1 part by mass of artificial graphite and 1 part by mass of ketjen black as a conductive aid Then, NMP was added to adjust the viscosity to prepare a positive electrode mixture-containing paste. After coating the positive electrode mixture-containing paste on both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 15 μm, vacuum drying is performed at 120 ° C. for 12 hours to form a positive electrode mixture layer on both surfaces of the aluminum foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the positive electrode mixture layer, and an aluminum lead body was welded to the exposed portion of the aluminum foil to produce a strip-like positive electrode having a length of 600 mm and a width of 54 mm. . The positive electrode mixture layer in the obtained positive electrode had a thickness of 60 μm on one side.

<Production of negative electrode>
Graphite A-1 (artificial graphite whose surface is not coated with amorphous carbon) shown in Table 1: 48.5% by mass and graphite B-1 (the surface of the mother particles made of graphite, the pitch is carbon Graphite coated with amorphous carbon as a source): 48.5% by mass, and composite Si-1 having a SiO surface coated with a carbon material (average particle diameter of 8 μm, specific surface area of 7.9 m ² / g) The amount of the carbon material in the composite was 20% by mass): 3.0% by mass was mixed with a V-type blender for 12 hours to obtain a negative electrode active material. The obtained negative electrode active material mixture: 98% by mass, CMC: 1.0% by mass, and SBR: 1.0% by mass were mixed with ion-exchanged water to prepare an aqueous negative electrode mixture-containing paste.

The negative electrode mixture-containing paste is applied to both sides of a 6 μm thick copper foil (negative electrode current collector) and then vacuum-dried at 120 ° C. for 12 hours to form a negative electrode mixture layer on both sides of the copper foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the negative electrode mixture layer, and a nickel lead body was welded to the exposed portion of the copper foil to produce a strip-shaped negative electrode having a length of 620 mm and a width of 55 mm. . The density of the negative electrode mixture layer in the obtained negative electrode was 1.7 g / cm ³ .

<Adjustment of non-aqueous electrolyte>
LiPF ₆ was dissolved to a concentration of 1.1 mol / l in a solvent in which ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed at a volume ratio = 1: 1: 1, and 4-fluoro-1 was added to this solution. , 3-dioxolan-2-one 1.5% by weight, vinylene carbonate 2.0% by weight, 2-propynyl 2- (diethoxyphosphoryl) acetate 1.5% by weight, 1,3-dioxane 1. A non-aqueous electrolyte was prepared by adding 0% by mass, 0.5% by mass of adiponitrile, and 0.15% by mass of lithium borofluoride (LiBF ₄ ).

<Battery assembly>
The belt-like positive electrode is stacked on the belt-like negative electrode through a microporous polyethylene separator (porosity: 41%) having a thickness of 16 μm, wound in a spiral shape, and then pressed so as to be flat. A wound electrode body having a flat wound structure was formed, and this electrode wound body was fixed with an insulating tape made of polypropylene. Next, the wound electrode body is inserted into a prismatic battery case made of aluminum alloy having an outer dimension of thickness 5.0 mm, width 56 mm, and height 60 mm, the lead body is welded, and an aluminum alloy lid The plate was welded to the open end of the battery case. Thereafter, the non-aqueous electrolyte is injected from the inlet provided on the cover plate, and left for 1 hour, and then the inlet is sealed, and the lithium ion secondary having the structure shown in FIG. 1 and the appearance shown in FIG. A battery was obtained.

  Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1 is a partial cross-sectional view. As shown in FIG. 1, the positive electrode 1 and the negative electrode 2 are spirally wound via a separator 3. Thereafter, the flat wound electrode body 6 is pressurized so as to be flat, and is accommodated in a rectangular (square tube) battery case 4 together with a non-aqueous electrolyte. However, in FIG. 1, in order to avoid complication, the metal foil, the separator layers, the non-aqueous electrolyte, and the like used as the current collector used in the production of the positive electrode 1 and the negative electrode 2 are not illustrated.

  The battery case 4 is made of an aluminum alloy and constitutes an outer package of the battery. The battery case 4 also serves as a positive electrode terminal. And the insulator 5 which consists of PE sheets is arrange | positioned at the bottom part of the battery case 4, and it connects to each one end of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 thus drawn are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the battery case 4 via a polypropylene insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

  And this cover plate 9 is inserted in the opening part of the battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, and the inside of the battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the cover plate 9, and a sealing member is inserted into the non-aqueous electrolyte inlet 14, for example, laser welding or the like. As a result, the battery is sealed by welding. Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

  In the battery of Example 1, the battery case 4 and the cover plate 9 function as positive terminals by directly welding the positive electrode lead body 7 to the cover plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the battery case 4, the sign may be reversed. There is also.

  FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode body is not cross-sectional.

(Examples 2 to 17)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the contents of LiBF ₄ and adiponitrile were changed as shown in Table 2.

(Example 18)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that succinonitrile was used instead of adiponitrile contained in the nonaqueous electrolytic solution.

(Example 19)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that glutaronitrile was used instead of adiponitrile contained in the nonaqueous electrolytic solution.

(Example 20)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that lauronitrile was used instead of adiponitrile contained in the nonaqueous electrolytic solution.

(Example 21)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a nonaqueous electrolytic solution containing no 2-propynyl 2- (diethoxyphosphoryl) acetate was used.

(Example 22)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a nonaqueous electrolytic solution containing no 1,3-dioxane was used.

(Example 23)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a nonaqueous electrolytic solution not containing 4-fluoro-1,3-dioxolan-2-one was used.

(Examples 24-38)
Lithium ion secondary in the same manner as in Example 1 except that the negative electrode active material obtained by mixing the graphite and SiO composites shown in Table 1 coated with a carbon material at the mass ratios shown in Tables 3 and 4 was used. A battery was produced.

(Example 39)
Instead of the composite Si-1, a composite Si-2 having a SiO surface coated with a carbon material (average particle size is 5 μm, specific surface area is 5.8 m ² / g, and the amount of carbon material in the composite is 20 mass. %) Was used in the same manner as in Example 1, and a lithium ion secondary battery was prepared in the same manner as in Example 1 except that this negative electrode was also used.
(Example 40)
Example 1 except that Si-3 (average particle diameter of 0.5 μm, specific surface area of 9.5 m ² / g) whose SiO surface was not coated with a carbon material was used in place of the composite Si-1. A negative electrode was produced in the same manner as in Example 1, and a lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used.

(Example 41)
A lithium ion secondary battery was produced in the same manner as in Example 25 except that the density of the negative electrode mixture layer was changed to 1.4 g / cm ³ in the negative electrode used in Example 25.

(Example 42)
A lithium ion secondary battery was fabricated in the same manner as in Example 25 except that the density of the negative electrode mixture layer was changed to 1.6 g / cm ³ in the negative electrode used in Example 25.

(Comparative Examples 1-12)
Using the negative electrode active material obtained by mixing the graphite and SiO composites coated with the carbon materials shown in Table 1 at the mass ratios shown in Tables 5 and 6, the contents of LiBF ₄ and adiponitrile are shown in Table 6 respectively. A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the above was changed.

  The following battery characteristic evaluation was performed about each lithium ion secondary battery of the Example and the comparative example. The evaluation results of each battery are shown in Tables 7-9.

<Liquid injection test>
For each of the examples and comparative examples, the non-aqueous electrolyte was injected and sealed simply by putting the flat wound electrode body into an aluminum outer can and covering it with a lid while welding the battery. 10 pieces were prepared in a non-existing state, and these were immersed in a bath filled with a non-aqueous electrolyte having the same composition as that used for the production of each battery, and evacuated for 90 seconds (the ultimate vacuum was -98 kPa). Then, the non-aqueous electrolyte was allowed to enter the inside of the battery case from each non-aqueous electrolyte inlet. Then, after releasing the vacuum, each of the examples and comparative examples is taken out from the non-aqueous electrolyte bath at every arbitrary time, and the amount of liquid injected is obtained by measuring the mass of the battery case (the amount of liquid injected = the amount extracted). From the subsequent battery case mass-battery case mass before immersion in the non-aqueous electrolyte), the arrival time at which the predetermined injection amount was injected into the battery case was determined. The predetermined injection volume was 4.2 g.

<Initial charge / discharge test>
The lithium ion secondary batteries of Examples and Comparative Examples were left in a constant temperature bath at 25 ° C. for 5 hours, and then each battery was charged with a constant current of 0.5 C up to 4.4 V. Then, constant voltage charging was performed at 4.4 V until the current reached 0.05C. The capacity at this time was defined as a charging capacity. Thereafter, discharging was performed at a constant current of 0.2 C until the voltage reached 2.75V. The capacity at this time was defined as a discharge capacity. The obtained discharge capacity divided by the charge capacity was defined as charge / discharge efficiency (%).

<45 ° C charge / discharge cycle characteristics evaluation>
The lithium ion secondary batteries of Examples and Comparative Examples were left in a constant temperature bath at 45 ° C. for 5 hours, and then each battery was charged with a constant current to 4.4 V at a current value of 0.5 C, and subsequently 4 Charge at a constant voltage of 0.4V (total charging time of constant current charge and constant voltage charge is 2.5 hours), and then discharge at 2.75V at a constant current of 0.2C to obtain the initial discharge capacity. It was. Next, each battery was charged at a constant current of up to 4.4 V at a current value of 1 C at 45 ° C., and subsequently charged until a current value of 0.1 C was reached at a constant voltage of 4.4 V. A series of operations for discharging the value to 3.0 V was taken as one cycle, and this was repeated a plurality of times. And about each battery, the constant current-constant voltage charge and the constant current discharge were performed on the same conditions as the time of the said first time discharge capacity measurement, and the discharge capacity was calculated | required. Then, the value obtained by dividing these discharge capacities by the initial discharge capacities was expressed as a percentage to calculate a 45 ° C. cycle capacity retention rate, and the number of cycles at which the capacity retention rate decreased to 40% was measured.

<High-temperature storage test in the charged state>
About each lithium ion secondary battery of an Example and a comparative example, it carried out constant current charge to 4.4V by the electric current value of 1.0C in room temperature (23 degreeC) environment, and then carried out constant voltage charge by the voltage of 4.4V. Went. The total charging time for constant current charging and constant voltage charging was 2.5 hours. Then, it discharged until it reached 2.75V with the electric current value of 0.2C, and the capacity | capacitance before storage (initial capacity) was calculated | required. After being stored for 24 hours in an environment of 85 ° C., the battery was discharged at a current value of 0.2 C until it reached 2.75 V, and then charged at a constant current of 1.0 C to 4.4 V, followed by 4 . Constant voltage charging was performed at a voltage of 4V. The total charging time for constant current charging and constant voltage charging was 2.5 hours. Thereafter, the battery was discharged at a current value of 0.2 C until it reached 2.75 V, and the capacity after storage (recovery capacity) was determined. And according to the following formula, the capacity | capacitance recovery rate (%) after high temperature storage was calculated | required. It can be said that the higher the capacity recovery rate, the better the high-temperature storage characteristics of the battery.
Capacity recovery rate after high temperature storage = (recovery capacity after storage / standard capacity) x 100

<Overcharge test>
Five lithium ion secondary batteries of each of Examples and Comparative Examples were prepared, charged with a current value of 1A (upper limit voltage: 5.2V), and temperature changes on the battery surface during charging were measured. . A battery whose surface temperature exceeded 100 ° C. was regarded as a battery in which a significant temperature increase was observed, and the number of the batteries was examined.

<Discharge cycle test at -10 ° C charging>
3. Recharge each lithium ion secondary battery of Examples and Comparative Examples in a thermostat at 25 ° C. for 5 hours, and then charge each battery at a constant current of 0.2 C up to 4.4 V. After reaching 4V, constant voltage charging was performed at 4.4V until the current reached 0.02C. The charge capacity at this time was defined as the charge capacity at 25 ° C. Thereafter, discharging was performed at a constant current of 0.2 C until the voltage reached 2.75V. The discharge capacity at this time was defined as the discharge capacity of the first cycle.

  Each battery after the discharge was allowed to stand in a thermostatic chamber at −10 ° C. for 5 hours, and then each battery was charged with a constant current of 0.2 C up to 4.4 V, and reached 4.4 V. Conducted constant voltage charging at 4.4V until the current reached 0.02C. The charge capacity at this time was defined as a charge capacity at −10 ° C. Then, it left still in a 25 degreeC thermostat for 5 hours, and it discharged until the voltage reached 2.75V with the constant current of 0.2C.

The above series of operations (standing at low temperature to discharging at room temperature) is defined as one cycle, and the discharge capacity at the 20th cycle is divided by the discharge capacity at the 20th cycle to obtain the discharge capacity maintenance rate (%) at the 20th cycle. Asked. The results of each evaluation are shown in Tables 7-9.

1 Positive electrode 2 Negative electrode

3 Separator

Claims (6)

A lithium ion secondary battery in which an electrode body composed of a positive electrode, a negative electrode and a separator, and a non-aqueous electrolyte solution are housed in an exterior body,
The positive electrode contains a lithium-containing oxide containing Co and / or Mn as a positive electrode active material,
The negative electrode has, as a negative electrode active material, artificial graphite A having an average particle diameter of more than 15 μm and 25 μm or less, an average particle diameter of 8 μm or more and 15 μm or less, and the surface of the graphite particles are coated with amorphous carbon. Containing graphite B and a material S containing at least one element selected from the group consisting of Si and Sn;
When the total of all the negative electrode active materials contained in the negative electrode is 100 mass%, the mixed mass% (A + B) of artificial graphite A and graphite B contained as the negative electrode active material is 40% or more and 99% or less. The mass ratio (A / B) of artificial graphite A to graphite B is 0.5 or more and 4.5 or less,
The non-aqueous electrolyte contains 0.05 to 2.5% by mass of lithium borofluoride (LiBF ₄ ), 0.05 to 5.0% by mass of a nitrile compound containing two or more cyano groups, and LiPF ₆ . The lithium ion secondary battery characterized by the above-mentioned.   When the total of all the negative electrode active materials contained in the negative electrode is 100 parts by mass, the material containing at least one element selected from the group consisting of Si and Sn contained as the negative electrode active material is 1 to 10 masses. The lithium ion secondary battery according to claim 1, which is included in a part. The lithium ion secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte further contains a phosphonoacetate compound represented by the following general formula (2).

[In General Formula (2), R ¹ , R ² and R ³ each independently represent an alkyl group, alkenyl group or alkynyl group having 1 to 12 carbon atoms which may be substituted with a halogen atom, and n Represents an integer of 0-6. ]   The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte further contains 1,3-dioxane. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the nitrile compound is represented by the following general formula (1).
_{NC- (CH 2) n -CN (} 1)
[In said general formula (1), n is an integer of 2-4. ] The negative electrode has a negative electrode mixture layer containing the negative electrode active material and a binder on one side or both sides of a negative electrode current collector, and the density of the negative electrode mixture layer is 1.5 g / cm ³ or more. The lithium ion secondary battery according to any one of claims 1 to 5.

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