JP2016091632A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which exhibits a superior charging characteristic under a low-temperature condition, and excellent charge and discharge cycle characteristics under a high-temperature condition.SOLUTION: A lithium ion secondary battery comprises: a negative electrode. The negative electrode includes, as a negative electrode active material, graphite A (natural graphite of which the surface is covered with amorphous carbon and the average spherical particle diameter is 18-28 μm), graphite B (bulk mesophase graphitized material of which the average particle diameter is 10-20 μm), and graphite C (natural graphite of which the surface is covered with amorphous carbon and the average spherical particle diameter is 5-13 μm). In the negative electrode active material, the content of the graphite B and that of the graphite C are 5-20 mass% and 10-25 mass% respectively.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion secondary battery having excellent charge / discharge characteristics at low temperatures and high temperatures.

  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.

  Lithium ion secondary batteries are strongly required to have higher capacities and improved charge / discharge cycle characteristics along with the spread of applicable devices. In addition, the environment in which the applied equipment is used is expanding, and improvement of charge / discharge characteristics at low and high temperatures is also demanded.

  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 order to further increase the capacity of the battery, it is necessary to increase the density of the negative electrode. For example, Patent Document 1 proposes a negative electrode material that can be densified with a low press pressure and has good cycle characteristics by combining spherical (or elliptical) natural graphite, bulk mesophase graphitized material, and scale-like graphite. ing. However, Patent Document 1 does not mention battery characteristics at low temperatures or high temperatures.

JP 2013-2111254 A

  The present invention has been made in view of the above circumstances, and even in a high-density negative electrode, characteristics at low temperatures and high temperatures, specifically, charge acceptability at low temperatures, and charge / discharge cycles at high temperatures. An object of the present invention is to provide a lithium ion secondary battery having excellent characteristics.

The present invention is 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 is housed in an outer package, wherein the negative electrode is graphite as a negative electrode active material. A, graphite B and graphite C are contained, the content of the graphite B in the negative electrode active material is 5 to 20% by mass, and the content of the graphite C in the negative electrode active material is 10 to 25 It is a lithium ion secondary battery which is mass%.
Graphite A: Natural graphite having a spherical shape and having a surface coated with amorphous carbon, and an average particle size of 18 to 28 μm.
Graphite B: Bulk mesophase graphitized product having an average particle size of 10 to 20 μm.
Graphite C: Natural graphite having a spherical shape and having a surface coated with amorphous carbon, and an average particle size of 5 to 13 μm or less.

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

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 has a spherical shape, graphite A, which is natural graphite coated with amorphous carbon, and graphite B, which is a bulk mesophase graphitized product, and has a spherical shape and a surface that is amorphous. Graphite C which is natural graphite coated with carbon (however, the average particle size is different from that of graphite A) is used.

  Graphite A is a graphite in which the surface of natural graphite with sufficiently developed crystallites of graphite is coated with amorphous carbon, and thus has a feature of high capacity and high lithium acceptability during charging.

  The average particle diameter of graphite A is 18 to 28 μm. If it is graphite A which is this average particle diameter, the filling property (tap density) as graphite powder will become high by using together with the graphite C with a comparatively small average particle diameter mentioned later, and the negative electrode compound material layer is high. Densification is easy. Easier densification reduces the damage (crushed, cracked, etc.) of the negative electrode active material particles when the composite material layer is pressed. Even in the layer, sufficient pores can be secured. As a result, sufficient permeability of the non-aqueous electrolyte can be ensured.

  Graphite A is obtained by, for example, spheroidizing natural graphite in a conventional manner, coating the surface with an organic compound, firing at 800-1500 ° C., pulverizing, and sizing through a sieve. Can do. Examples of the organic compound to be coated include aromatic hydrocarbons; tars or pitches obtained by polycondensation of aromatic hydrocarbons under heat and pressure; tars, pitches or the like mainly composed of a mixture of aromatic hydrocarbons. Asphalts; and the like. In order to coat the natural graphite with the organic compound, a method of impregnating and kneading the natural graphite with the organic compound can be employed. Moreover, the graphite A can also 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 natural graphite.

Graphite A is a natural graphite that is spherical and has a surface coated with amorphous carbon. If the average particle size is 18 to 28 μm, there is no particular limitation, but the (002) interplanar distance in X-ray diffraction is there _{d 002} is 0.338nm or less and a specific surface area in BET method of 3 to 5 m ² / g, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum _{(I 1360} / I ₁₅₈₀ ) is more preferably 0.15 to 0.50.

Here, the average particle diameter is an average particle diameter (D ₅₀₎ measured by dispersing these fine particles in a medium in which particles are not dissolved using a laser scattering particle size distribution analyzer (for example, “LA-920” manufactured by HORIBA). _% ). The specific surface area is determined by the BET method, and an example of a measuring apparatus is “Bell Soap Mini” manufactured by Bell Japan. Also, R value refers to a R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum _(I 1360 _/ I _1580), argon laser having a wavelength of 514.5nm [ For example, it can be obtained from a Raman spectrum obtained using “T-5400” (Laser power: 1 mW) manufactured by Ramanaor.
The subsequent “average particle diameter”, “specific surface area”, and “R value” are all determined by the method described above.

  The content of graphite A in the negative electrode active material is preferably 20 to 85% by mass. Within this range, graphite B and graphite C can contain an appropriate amount described later in the negative electrode active material, and the effects of the present invention can be exhibited.

  Graphite B is artificial graphite of bulk mesophase and has a low specific surface area. Therefore, excessive side reaction between the graphite surface and the non-aqueous electrolyte does not occur, the charge / discharge efficiency is high, and the charge / discharge characteristics at high temperature. It has excellent features.

  By using graphite B having high charge / discharge efficiency and low reactivity (particularly at high temperatures), a lithium ion secondary battery having excellent charge / discharge cycle characteristics at high temperatures can be provided. In addition, the bulk mesophase artificial graphite has an effect of suppressing excessive crushing of the negative electrode active material particles because of low orientation, and, as with the above-described graphite A, ensures sufficient permeability of the non-aqueous electrolyte. Can do.

  The average particle diameter of the graphite B is 10 to 20 μm, and the content in the negative electrode active material is 5 to 20 mass%. Although details are unknown, it has been found that high charge / discharge cycle characteristics and nonaqueous electrolyte permeability can be ensured by setting the graphite B having the average particle diameter to a predetermined content.

Graphite B is a synthetic graphite bulk mesophase, if the average particle diameter of 10 to 20 [mu] m, is not particularly limited, _{d 002} a (002) interplanar distance in X-ray diffraction is 0.338nm or less, BET the specific surface area of law 1~3m ² / g, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum _(I 1360 _/ I ₁₅₈₀₎ is from 0.15 to 0. 50 is more preferable.

  Like graphite A, graphite C is a graphite in which the surface of natural graphite with sufficiently developed crystallites of graphite is coated with amorphous carbon, and thus has a high capacity and high lithium acceptability during charging. .

  The average particle diameter of graphite C is 5 to 13 μm, and the content in the negative electrode active material is 10 to 15% by mass. If the graphite C is spherical with this average particle size, the fillability (tap density) as graphite powder is increased by using it together with the above-described spherical graphite A having a relatively large average particle size. By increasing the filling property, sufficient contact between the negative electrode active material particles can be secured, so that the peel strength of the negative electrode mixture layer can be increased, and the negative electrode mixture layer adheres to the press roll during press processing. Production trouble can be suppressed. Furthermore, since the average particle diameter is smaller than that of graphite A, the lithium acceptability during charging is superior to that of graphite A, and the charge / discharge characteristics at a lower temperature can be improved.

  Particularly preferably, tar, pitch or asphalt is deposited on the surface of natural graphite and heat-treated at a high temperature.

Graphite C can be obtained in the same manner as graphite A. Graphite C is natural graphite having a spherical shape and coated with amorphous carbon on the surface. If the average particle size is 5 to 13 μm, there is no particular limitation, but the (002) plane in X-ray diffraction is not limited. the distance a is _{d 002} is 0.338nm or less and a specific surface area in BET method of 3 to 5 m ² / g, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum (I ₁₃₆₀ / I ₁₅₈₀ ) is more preferably 0.40 to 0.75.

  As long as the above-mentioned graphite A, graphite B, and graphite C are contained in the negative electrode active material in the present invention, other natural graphite and artificial graphite may be included within a range not departing from the object of the present invention. In addition, carbonaceous materials other than graphite (pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, etc.), Si or Sn alone, Si or Sn alloys, Si or Sn are included. Oxides and the like may also be included without departing from the object of the present invention.

  It is preferable that the total amount of graphite A, B, and C is 50% by mass or more based on the total amount of the negative electrode active material because the above-described effects of the present application can be particularly exhibited.

  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 one side of the current collector, and the density of the negative electrode mixture layer (the negative electrode mixture per unit area laminated on the current collector) (Calculated from the mass and thickness of the layer) is preferably 1.0 to 1.9 g / cm ³ . In order to increase the capacity of the battery, it is more preferably 1.58 to 1.8 g / cm ³ .

Conventionally, when the density of the negative electrode mixture layer is increased to 1.58 g / cm ³ or more, the negative electrode mixture layer of the non-aqueous electrolyte described later, for example, because the voids of the mixture layer are reduced and the pores are reduced. The penetration into the inside deteriorates, and sufficient charge / discharge characteristics of the battery cannot be secured. However, by using a mixture of the above-described graphite A, graphite B and graphite C of the present invention in a predetermined ratio for the negative electrode mixture layer, the permeability of the non-aqueous electrolyte can be increased even if the density of the mixture layer is increased. And charge / discharge characteristics at an excellent low temperature or high temperature can be ensured.

Moreover, as a composition of a negative mix layer, it is preferable that the quantity of a negative electrode active material is 80-95 mass%, for example, it is preferable that the quantity of a binder is 1-20 mass%, and uses a conductive support agent. When it does, 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 lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and the negative electrode active material used for the negative electrode may be any of those described above. There is no restriction | limiting, The structure and structure employ | adopted by the lithium ion secondary battery known conventionally can be applied.

  As a positive electrode which concerns on the lithium ion secondary battery of this invention, what formed the positive mix layer containing a positive electrode active material in the single side | surface or both surfaces of a collector is mentioned, for example. 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, conventionally known positive electrode active materials for lithium ion secondary batteries can be used as the positive electrode active material. 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.

  As the non-aqueous electrolyte used in the lithium ion secondary battery of the present invention, a non-aqueous electrolyte used in a conventional lithium ion secondary battery is preferably employed. For example, a solution prepared by dissolving a lithium salt in a non-aqueous solvent can be used.

Examples of the lithium salt related to the non-aqueous electrolyte include 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 And other lithium salts such as LiC _n F _{2n + 1} SO3 (n ≧ 2). In particular, LiPF ₆ is a lithium salt that has a high degree of dissociation, a high transport rate of Li ions, and the highest versatility. 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.

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 Aprotic organic solvents such as methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether alone, Or it can use as a mixed solvent which mixed 2 or more types.

As a non-aqueous electrolyte, for the purpose of further improving charge / discharge characteristics at high temperatures, for example, the above-described lithium salt, LiBF ₄ , and a nitrile compound containing one or more cyano groups are dissolved in a non-aqueous solvent. The solution prepared in (1) may be used.

As a cause of the elution of Co and Mn in the positive electrode active material at a high temperature, LiPF ₆ in the electrolytic solution decomposes to generate hydrogen fluoride (HF), and this HF destroys the crystal structure of the positive electrode active material. Thus, Co and Mn may be eluted. LiBF ₄ and the nitrile compound contain a compound that forms a highly stable coating on the positive electrode even at high temperatures in the non-aqueous electrolyte, thereby suppressing the reaction of HF with the positive electrode active material, and Co and Mn. Elution itself can be suppressed, and high-temperature cycle characteristics and high-temperature storage characteristics can be improved.

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.

The nitrile compound containing one or more cyano groups is particularly preferably 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. ]

  The compound of the general formula (1) is, for example, 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.

  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 one or more cyano groups in the non-aqueous electrolyte 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.

  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, propyl dimethylphosphonoformate, propyldiethylphosphonoformate, propyl dibutylphosphonoformate, 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 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 to be. 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.

(Examples 1 to 13)
<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 (natural graphite with a spherical surface coated with amorphous carbon), graphite B (bulk mesophase graphitized material), and graphite C (spherical with an amorphous surface), which have the average particle sizes shown in Table 1. The natural graphite coated with carbon) was mixed at a mass ratio shown in Table 1 for 12 hours using a V-type blender to obtain a negative electrode active material. The tap density of this negative electrode active material was measured with a powder tester (manufactured by Hosokawa Micron Corporation, PT-R). The number of taps was 180. The results are shown in Table 2.

  98 parts by mass of the negative electrode active material, 1.0 part by mass of CMC, and 1.0 part by mass of SBR were mixed with ion-exchanged water to prepare an aqueous negative electrode mixture-containing paste.

  The negative electrode mixture-containing paste was applied to both sides of a copper foil (negative electrode current collector) having a thickness of 8 μm (there was a one-side applied part in which only one side was applied), and then vacuum-dried at 120 ° C. for 12 hours. Then, a negative electrode mixture layer was formed on both sides of the copper foil. Thereafter, the peel strength of the negative electrode was measured with a 90 ° peel tester (tester industry). The peel strength was measured according to the following procedure. A sample is prepared by cutting out a single-side coated part in which a negative electrode mixture layer is formed only on one side of the negative electrode to a size of 10 cm in the longitudinal direction and 1 cm in the width direction. One side of the double-sided tape is attached to the entire region in the width direction from one end of the sample to 7 cm in the longitudinal direction, and the other side of the double-sided tape is attached to the phenolic resin plate. Of the sample, a part up to 3 cm long in the longitudinal direction from the other end where the double-sided tape is not attached is vertically folded, and this part is sandwiched between chucks of a testing machine, at an angle of 90 ° to the phenol resin plate The sample is pulled in the longitudinal direction at a speed of 5 cm / min, and the load when the negative electrode mixture layer is peeled from the current collector is measured. Then, the obtained load (gf) is divided by the width (cm) of the negative electrode to calculate the peel strength (gf / cm). The results of peel strength are shown in Table 2.

As described above, after forming the negative electrode mixture layer on both sides of the copper foil, press treatment is performed, and the thickness is adjusted so that the density of the negative electrode mixture layer is 1.7 g / cm ^3. A nickel lead body was welded to produce a strip-shaped negative electrode having a length of 620 mm and a width of 55 mm. The negative electrode mixture layer in the obtained negative electrode had a thickness of 70 μm per one side.

(The description was moved from the bottom)
About each negative electrode of the Example which performed the press process, the osmosis | permeation time of electrolyte solution was measured. 2 μL of the above-described electrolyte solution was dropped on the surface of the negative electrode mixture layer of each negative electrode, and the time required for the electrolyte solution to completely penetrate into the negative electrode mixture layer was defined as the electrolyte solution penetration time. Each measurement was carried out 5 times, and the average is shown in Table 2.

<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.

(Example 14)
A negative electrode was prepared in the same manner as in Example 1 except that the density of the negative electrode mixture layer was changed to 1.8 g / cm ³ by changing the pressing pressure. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used.
As in Example 1, the tap density was measured at the time of the negative electrode active material, the peel strength was measured at the negative electrode before pressing, and the infiltration time of the electrolyte was measured at the negative electrode after pressing.

(Examples 15 and 16)
Example 1 except that graphite D (artificial graphite) and graphite E (natural graphite) having the average particle diameter shown in Table 1 were added in addition to graphite A, graphite B, and graphite C in the contents shown in Table 1. In the same manner, a lithium ion secondary battery was produced.
As in Example 1, the tap density was measured at the time of the negative electrode active material, the peel strength was measured at the negative electrode before pressing, and the infiltration time of the electrolyte was measured at the negative electrode after pressing.

(Comparative Examples 1-12)
Graphite A (natural graphite with a spherical surface coated with amorphous carbon), graphite B (bulk mesophase graphitized material) and graphite C (spherical with an amorphous surface as shown in Table 1 having an average particle size D50% A lithium ion secondary battery was produced in the same manner as in Example 1 except that a negative electrode active material in which natural graphite coated with carbonaceous material was mixed in the content shown in Table 1 was used.
As in Example 1, the tap density was measured at the time of the negative electrode active material, the peel strength was measured at the negative electrode before pressing, and the infiltration time of the electrolyte was measured at the negative electrode after pressing.

(Comparative Example 13)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that scaly graphite (graphite F) having an average particle diameter of 8 μm was used in the content shown in Table 1 instead of graphite C.
As in Example 1, the tap density was measured at the time of the negative electrode active material, the peel strength was measured at the negative electrode before pressing, and the infiltration time of the electrolyte was measured at the negative electrode after pressing.

(Comparative Example 14)
A lithium ion secondary battery was produced in the same manner as in Comparative Example 9 except that the density of the negative electrode mixture layer was changed to 1.8 g / cm ³ by changing the pressing pressure.
As in Example 1, the tap density was measured at the time of the negative electrode active material, the peel strength was measured at the negative electrode before pressing, and the infiltration time of the electrolyte was measured at the negative electrode after pressing.

  The following battery characteristic evaluation was performed about each lithium ion secondary battery of the Example and the comparative example.

<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 (%). The results are shown in Table 3.

<Charge acceptance test at -5 ° C>
The lithium ion secondary batteries of Examples and Comparative Examples were allowed to stand in a thermostat at 25 ° C. for 5 hours, and then each battery was charged with a constant current of 1 C up to 4.4 V, reaching 4.4 V. After that, constant voltage charging was performed at 4.4 V until the current reached 0.1 C. 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.

  Each battery after the discharge was allowed to stand in a thermostatic bath at −5 ° C. for 5 hours. After that, each battery was charged with a constant current of 1 C up to 4.4 V, and after reaching 4.4 V, Constant voltage charging was performed at 4.4 V until the current reached 0.1 C. The charge capacity at this time was defined as a charge capacity at −5 ° 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 series of operations described above is one cycle, and among the charge capacities at -5 ° C in the fifth cycle, the charge capacities in the constant current charge region and the charge capacities in the constant current charge region among the charge capacities at 25 ° C The ratio was determined and evaluated as charge acceptability at −5 ° C. (charge characteristics at low temperature). These results are shown in Table 3.

<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, the 45 ° C. cycle capacity retention rate was calculated, and the capacity retention rate at 500 cycles was examined. These results are shown in Table 3.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (3)

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 negative electrode contains the following graphite A, graphite B, and graphite C as a negative electrode active material,
The content of the graphite B in the negative electrode active material is 5 to 20% by mass,
The lithium ion secondary battery in which the content of the graphite C in the negative electrode active material is 10 to 25% by mass.
Graphite A: Natural graphite having a spherical shape and having a surface coated with amorphous carbon, and an average particle size of 18 to 28 μm.
Graphite B: Bulk mesophase graphitized product having an average particle size of 10 to 20 μm.
Graphite C: Natural graphite having a spherical shape and having a surface coated with amorphous carbon, and an average particle size of 5 to 13 μm or less.   2. The lithium ion secondary battery according to claim 1, wherein a content of the graphite A in the negative electrode active material is 20 to 85 mass%.   The lithium ion secondary battery according to claim 1 or 2, wherein the negative electrode composite material layer density of the negative electrode is 1.58 to 1.8 g / cc.

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