JP5167713B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery using a mixture of graphite and amorphous carbon as a negative electrode active material.

  Non-aqueous electrolyte secondary batteries are widely used as power sources for various electronic devices such as mobile phones, personal computers, and video cameras, taking advantage of their high energy density. Recently, its application has been extended to large-capacity power sources such as electric vehicles.

In a nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as lithium cobaltate (LiCoO ₂ ) is used as a positive electrode active material, a carbonaceous material is used as a negative electrode active material, and phosphorus hexafluoride is used as an organic solvent as an electrolyte. An organic electrolytic solution in which a lithium salt such as lithium acid (LiPF ₆ ) is dissolved has been used.

  Conventionally, graphite has been mainly used as the negative electrode active material. Graphite has a redox potential that is close to the redox potential exhibited by metallic lithium, provides a high discharge voltage as a battery, high true density, high crystallinity, soft particles, and relatively low initial irreversible capacity. In addition, it has features such as large charge / discharge capacity per unit volume, flat discharge curve, that is, small change in discharge voltage, and for consumer mobile phones, portable devices and portable information terminals that require high energy density. It has been put into practical use as a negative electrode active material for batteries.

  However, as described in Patent Document 1, when graphite is used for the negative electrode active material, the expansion / contraction is repeated in the charge / discharge reaction, and the volume change is large, so that the capacity drop due to the charge / discharge cycle is large. there were.

  On the other hand, amorphous carbon typified by coke and hard carbon has a lower true density than graphite, and is harder and more agglomerated, and has a relatively large initial irreversible capacity. Although it is not suitable for batteries with high demands, the charge / discharge curve of these amorphous carbons is gentle and the reactivity with the electrolyte is relatively low, so the input / output characteristics, cycle life, and high temperature discharge It is very promising for batteries with high demands on characteristics.

  Therefore, Patent Documents 2 and 3 disclose techniques for improving charge / discharge cycle characteristics by mixing graphite and amorphous carbon as a negative electrode active material of a nonaqueous electrolyte secondary battery. As the amorphous carbon, non-graphitizable carbon such as low crystalline coke or glassy carbon, which has a relatively small volume change during charge and discharge, is used.

  Patent Document 4 discloses a technique in which the surface of carbon particles, which are negative electrode active materials, has a coating layer having a carbonate structure containing sulfur in a lithium ion secondary battery, and is excellent in high rate charge / discharge characteristics. It is described that a battery having small self-discharge and excellent charge / discharge cycle characteristics can be obtained.

  Furthermore, the expansion / contraction of the negative electrode mixture layer in charge / discharge is disclosed in Patent Document 5, and it is described that the volume expansion of the negative electrode pellet is in the range of 10 to 35%.

Patent Documents 6 and 7 disclose a technique using an electrolyte solution in which a compound having a sulfonyl group such as an imide salt and 1,3-propene sultone is added to an aprotic solvent in a non-aqueous electrolyte secondary battery. Yes.
Japanese Patent Laid-Open No. 11-073959 Japanese Patent Application Laid-Open No. 07-192724 JP-A-11-250909 Japanese Patent No. 3327468 Japanese Patent No. 3458389 JP 2003-203673 A Japanese Patent Application Laid-Open No. 2004-014459

  In a large nonaqueous electrolyte secondary battery, a power generating element in which a positive electrode plate, a separator, and a negative electrode plate are laminated or wound in order is usually housed in a metal battery container. As the shape of the battery container, a rectangular shape or a long cylindrical shape is used.

  In the case of a rectangular or long cylindrical large battery, swelling of the maximum area of the battery container due to expansion of the negative electrode active material during charge / discharge is a serious problem. That is, when a large number of single cells are connected in series or in parallel to form an assembled battery, it is necessary to provide a gap corresponding to the swelling between the assembled batteries in advance, which increases the volume of the assembled battery. There was a problem that the volumetric energy density of the sample became small.

  Even when the negative electrode active material is mixed with graphite and amorphous carbon, as in the non-aqueous electrolyte secondary batteries disclosed in Patent Documents 2 and 3, swelling of large-sized batteries of rectangular and long cylindrical types It was difficult to suppress.

  In Patent Document 4, a film containing sulfur is provided on the carbon particles, but artificial graphite is used as the carbon material, and the negative electrode mixture layer and battery swelling associated with charge / discharge are not studied. Moreover, although patent document 5 has description about the volume expansion of a negative mix layer, it was unknown whether the battery swelling could be suppressed.

  Further, Patent Documents 6 and 7 describe that an imide salt and a compound having a sulfonyl group are added to the electrolytic solution, but graphite and amorphous carbon are used alone as the negative electrode active material. The effect of mixing the material with graphite and amorphous carbon was unclear.

  Conventionally, in a non-aqueous electrolyte secondary battery using a carbon material as a negative electrode active material, decomposition of the electrolyte solution in the negative electrode is prevented by the SEI film on the negative electrode surface. When this battery is charged and discharged at a high temperature of about 60 ° C., even when the volume expansion of the negative electrode active material exceeds 10%, the SEI film on the negative electrode surface becomes soft and can follow the volume change of the negative electrode active material. The SEI coating did not collapse.

  However, when the non-aqueous electrolyte secondary battery is charged and discharged at a low temperature around 0 ° C., the SEI film becomes hard, and when an active material having a large volume change such as graphite is used for the negative electrode, the SEI film is destroyed and electrolysis is performed. There was a problem that the decomposition of the liquid progressed.

  Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery that is less swollen at both high and low temperatures by optimizing the composition of the negative electrode active material and the coating on the negative electrode surface.

The present invention provides a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes graphite and amorphous carbon as active materials, and the ratio of graphite to the active material in the negative electrode is 50. The coating film containing sulfur and nitrogen is present on the surface of the negative electrode at a mass% or less, and the sulfur content in the coating film is 0.012 to 0.83 mass% .

  With the configuration of the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery with a small battery swelling accompanying the progress of the charge / discharge cycle.

  The present invention provides a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes graphite and amorphous carbon as active materials, and the ratio of graphite to the active material in the negative electrode is 50. It is characterized in that a coating containing sulfur and nitrogen is present on the surface of the negative electrode at a mass% or less, and the present invention further comprises a sulfur content in the coating of 0.012 to 0.83 mass%. Thus, a non-aqueous electrolyte secondary battery with small swelling is provided.

  In a rectangular or long cylindrical non-aqueous electrolyte secondary battery, when the charge / discharge cycle is repeated, the maximum area of the battery container expands. In particular, this tendency becomes remarkable when the capacity of the nonaqueous electrolyte secondary battery is 3 Ah or more. Here, the “maximum area portion of the battery container” refers to the A surface in the rectangular battery shown in FIG. 1 and the surface opposite thereto, and in the long cylindrical battery shown in FIG. It is the opposite surface.

  One of the causes of the swelling of the battery is expansion of the negative electrode mixture layer accompanying the volume change at the time of charging / discharging of the carbon material which is the negative electrode active material. When the negative electrode active material is graphite, the ratio of the negative electrode mixture layer thickness during charging to that during discharging is as large as about 1.15.

  On the other hand, when hard carbon, which is a kind of amorphous carbon, is used for the negative electrode active material, the ratio of the negative electrode mixture layer thickness at the time of charging to the time of discharging becomes as small as about 1.01. However, since the true density of amorphous carbon is smaller than that of graphite and the initial irreversible capacity is relatively large, a high energy density battery cannot be obtained.

  Therefore, the present invention uses a mixture of graphite and amorphous carbon in the negative electrode active material, and makes the ratio of graphite in the active material 50% by mass or less, whereby the negative electrode mixture layer during charging with respect to discharging The thickness ratio can be in the range of about 1.02 to 1.10, and as a result, battery swelling associated with the progress of the charge / discharge cycle is suppressed.

  When the proportion of graphite in the negative electrode active material is larger than 50% by mass, the ratio of the negative electrode mixture layer thickness during charging to discharging exceeds 1.10. As a result, the battery swells as the charge / discharge cycle progresses. It gets bigger.

  Although it does not specifically limit as graphite used for this invention, Preferably natural graphite and artificial graphite (Mesophase type graphite, such as MCMB or MCF), are mentioned. Examples of the amorphous carbon used in the present invention include soft carbons (cokes such as petroleum coke, pitch coke, and coal coke), hard carbons, pyrolytic carbons, carbon fibers, and glassy carbons. .

  Another cause of battery swelling is an increase in battery internal pressure due to gas generation due to decomposition of the electrolyte solvent. In a non-aqueous electrolyte secondary battery, an SEI film is formed on the surface of the negative electrode active material at the initial stage of charge and discharge, and this SEI film serves as a lithium ion conductive protective film. The reaction is suppressed.

  Here, SEI (Solid Electrolyte Interface) coating is formed on the surface of metallic lithium or carbon material when the metal lithium or carbon material is charged for the first time in a non-aqueous electrolyte and the solvent in the electrolyte is reduced. Refers to the passivation film (edited by Masayuki Yoshio, edited by Akiya Ozawa, “Lithium ion secondary batteries-materials and applications”, Nikkan Kogyo Shimbun (1996)).

  When the negative electrode active material has a large volume change due to charge / discharge, such as graphite, the SEI coating formed on the negative electrode surface is destroyed by expansion during charging, and a new SEI coating is formed. It is difficult to suppress gas generation.

  In the present invention, the presence of a stable SEI film containing sulfur and nitrogen on the negative electrode surface suppresses the decomposition of the electrolytic solution accompanying charge / discharge, suppresses the increase in the internal pressure of the battery, and prevents the battery from swelling. It is. As a method of forming an SEI film containing sulfur and nitrogen on the negative electrode surface, an unsaturated sultone compound such as 1,3-propene sultone and an imide salt such as lithium bistrifluoromethanesulfonate are simultaneously added to the electrolytic solution. There is a way to do it.

  However, since the SEI film containing sulfur formed only from the unsaturated sultone compound is hard, it cannot follow the expansion / contraction of the negative electrode active material, particularly in a low temperature environment, and is easily destroyed by charge / discharge. On the other hand, since the SEI film containing nitrogen formed only from an imide salt is similarly hard, there is a problem that the negative electrode active material cannot follow the expansion / contraction of the negative electrode active material, particularly under a low temperature environment, and is easily destroyed by charge / discharge. is there.

  Therefore, in the present invention, by adding an unsaturated sultone compound and an imide salt to the electrolyte at the same time, an SEI coating that is flexible enough to follow the expansion and contraction of the negative electrode active material even in a low-temperature environment is formed in the negative electrode. It is formed on the surface.

  The concentration of the unsaturated sultone compound in the electrolyte is 0.06 to 4.0% by mass, and the concentration of the imide salt is 0.02 to 0.3 mol / L, thereby including the sulfur and nitrogen of the present application. A film can be formed. Further, the sulfur content in the coating can be controlled by the amount of the unsaturated sultone compound or imide salt added to the electrolyte.

  In the present invention, the presence of a film containing sulfur and nitrogen can be confirmed by EPMA or ICP, and the concentration of sulfur in the film containing sulfur and nitrogen can be quantitatively analyzed by ICP.

  In the present invention, 1,3-propene sultone, 1,4-butene sultone, 1,3-propen-2-ene sultone, and the like can be used as the unsaturated sultone compound.

In the present invention, as the imide salt, an imide lithium salt represented by the general formulas LiNR ₁ R ₂ , LiN (SO ₂ R ₁ ) (SO ₂ R ₂ ), Li (COR ₁ ) (COR ₂ ) is used. be able to. In these general formulas, R ₁ and R ₂ represent an alkyl group, and R ₁ and R ₂ may be the same or different. Specifically, lithium bistrifluoromethanesulfonate imide (LiN (SO ₂ CF ₃ ) ₂ ), lithium bispentafluoroethanesulfonate imide (LiN (SO ₂ CF ₂ CF ₃ ) ₂ ), LiN (COCF ₃ ) ₂ LiN (COCF ₂ CF ₃ ) ₂ or the like can be used.

  And the film formed on the negative electrode surface contains sulfur and nitrogen, the sulfur content in the film is in the range of 0.012 to 0.83 mass%, and in the graphite and amorphous carbon in the negative electrode active material When the proportion of graphite is 50% by mass or less, a non-aqueous electrolyte secondary battery with small battery swelling can be obtained even when used repeatedly for a long time in a high temperature environment and a low temperature environment.

  This is because the SEI coating containing each of sulfur and nitrogen alone is relatively harder than those formed by ordinary electrolyte solvents (cyclic carbonates, chain carbonates, etc.), and therefore, particularly in low temperature environments. In this case, it becomes impossible to follow the expansion / contraction at the time of charge / discharge, and peels off from the active material surface. For this reason, the new surface is exposed on the active material surface. The reaction between the nascent surface and the electrolyte causes gas generation and the battery swells.

  However, as in the present invention, an SEI film containing sulfur and nitrogen at the same time is formed on the negative electrode surface, and the proportion of graphite in the negative electrode active material is 50% by mass or less, so that the expansion / contraction during charge / discharge can be reduced. It is presumed that since the rate is reduced and the above-described deterioration mode does not proceed, battery swelling can be suppressed.

  As a solvent for the nonaqueous electrolyte of the nonaqueous electrolyte secondary battery of the present invention, in addition to the unsaturated sultone compound, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, γ-valerolactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, halogenated dioxolane, trifluoroethyl methyl ether, ethylene glycol diacetate, propylene glycol di- Acetate, ethylene glycol dipropionate, propylene glycol dipropionate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propionate Propyl onate, methyl fluoroacetate, ethyl fluoroacetate, propyl fluoroacetate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, methyl isopropyl carbonate, ethyl isopropyl carbonate, diisopropyl carbonate, dibutyl Alkoxy and halogen-substituted cyclic phosphazenes such as carbonate, acetonitrile, fluoroacetonitrile, ethoxypentafluorocyclotriphosphazene, diethoxytetrafluorocyclotriphosphazene, phenoxypentafluorocyclotriphosphazene and chain phosphazenes, triethyl phosphate, phosphoric acid Phosphorus such as trimethyl, trioctyl phosphate Alkyl esters, triethyl borate, boric acid esters such as tributyl borate, N- methyl oxazolidinone, a non-aqueous solvent such as N- ethyl-oxazolidinones, or in itself can be used a mixture of these solvents.

The nonaqueous electrolyte is used by dissolving the supporting salt in these nonaqueous solvents. As the supporting salt, in addition to the lithium imide salt, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiBF ₂ C ₂ O ₄ , LiBC ₄ O ₈ , LiPF ₂ (C ₂ O ₄ ) ₂ And salts such as LiPF ₃ (CF ₂ CF ₃ ) ₃ or mixtures thereof can be used.

  In order to improve battery characteristics, a small amount of additives may be mixed in the non-aqueous electrolyte. Vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, phenyl vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene Carbonates such as carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, fluoroethylene carbonate, vinyl esters such as vinyl acetate and vinyl propionate, diallyl sulfide, allyl phenyl sulfide, allyl vinyl sulfide, allyl ethyl sulfide, propyl sulfide, diallyl disulfide , Sulfides such as allyl ethyl disulfide, allyl propyl disulfide, allyl phenyl disulfide, Methyl tansulfonate, ethyl methanesulfonate, propyl methanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, propyl ethanesulfonate, methyl benzenesulfonate, ethyl benzenesulfonate, propyl benzenesulfonate, phenyl methanesulfonate, Chain sulfonate esters such as phenyl ethanesulfonate, phenyl propanesulfonate, methyl benzylsulfonate, ethyl benzylsulfonate, propyl benzylsulfonate, benzyl methanesulfonate, benzyl ethanesulfonate, benzyl propanesulfonate, dimethyl Sulfite, diethyl sulfite, ethyl methyl sulfite, methyl propyl sulfite, ethyl propyl sulfite, diphenyl sulfite, methyl phenyl sulfite Ite, ethyl methyl sulfite, ethylene sulfite, vinyl ethylene sulfite, divinyl ethylene sulfite, propylene sulfite, vinyl propylene sulfite, butylene sulfite, vinyl butylene sulfite, vinylene sulfite, phenyl ethylene sulfite, etc. Sulfites, benzene, toluene, xylene, biphenyl, cyclohexylbenzene, 2-fluorobiphenyl, 4-fluorobiphenyl, diphenyl ether, tert-butylbenzene, orthoterphenyl, metaterphenyl, naphthalene, fluoronaphthalene, cumene, fluorobenzene, Aromatic compounds such as 2,4-difluoroanisole, halogen-substituted alkanes such as perfluorooctane, and trimethylsilyl borate Such as triethylsilyl borate, etc. may be appropriately added depending on the purpose.

There is no restriction | limiting in particular as a positive electrode active material of the nonaqueous electrolyte secondary battery of this invention, A various material can be used suitably. For example, transition metal compounds such as manganese dioxide and vanadium pentoxide, transition metal chalcogen compounds such as iron sulfide and titanium sulfide, Li _m MPO ₄ having an olivine structure (M is one or more transition metals, and 0 < m <2.1), and these transition metal and lithium complex oxides Li _x MO _2-δ (where M represents Co, Ni or Mn, 0.4 ≦ x ≦ 1.2, 0 ≦ δ ≦ 0.5), or at least one element selected from Al, Mn, Fe, Ni, Co, Cr, Ti, Zn, or P, B, etc. A compound containing a nonmetallic element can be used. Further, preferably a composite oxide of lithium, manganese, cobalt, and nickel, that is, the general formula Li _a Mn _b Co _c Ni _d O ₂ (where 0 <a ≦ 1.2, 0 ≦ b ≦ 1, 0 ≦ c ≦ A positive electrode active material represented by 1, 0 ≦ d ≦ 1) can be used.

  Moreover, as a separator of the nonaqueous electrolyte secondary battery of this invention, a woven fabric, a nonwoven fabric, a synthetic resin microporous membrane, etc. can be used, and especially a synthetic resin microporous membrane can be used suitably. Among them, polyolefin microporous membranes such as polyethylene and polypropylene microporous membranes, polyethylene and polypropylene microporous membranes combined with aramid and polyimide, or microporous membranes composited with these, thickness, membrane strength, membrane resistance, etc. It is suitably used in terms of

[Examples 1 to 6 and Comparative Examples 1 to 4]
[Example 1]
The positive electrode was produced as follows. 86% by mass of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as a positive electrode active material, 6% by mass of acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder 8% by mass was mixed, and N-methyl-2-pyrrolidone (NMP) was added to the mixture to prepare a positive electrode mixture paste. This positive electrode mixture paste was applied to both sides of an aluminum current collector having a thickness of 20 μm and dried, followed by vacuum drying at 150 ° C. for 5 hours and compression molding with a roll press. The dimensions of the positive electrode obtained were 30 mm and 640 mm in width, and the thickness including the current collector was 140 μm.

  The negative electrode was produced as follows. As the negative electrode active material, graphite and hard carbon were mixed and used. As graphite, natural graphite having an interlayer distance d002 = 0.336 nm and an average particle diameter D50 = 7 μm was used, and as hard carbon, hard carbon having an interlayer distance d002 = 0.379 nm and an average particle diameter D50 = 9 μm was used.

  Here, D50 represents the particle size when the sum of the volume fractions below a certain particle size is 50% with respect to the total volume fraction of the measured total particle system, and means the average particle size. The particle size distribution was measured using a laser diffraction type particle size distribution measuring device (SALD2200, manufactured by Shimadzu Corporation).

  The above-mentioned natural graphite and hard carbon are mixed at a mass ratio of 50:50 to make a negative electrode active material, 95% by mass of this negative electrode active material and 5% by mass of PVdF are mixed, NMP is added to this mixture, and a negative electrode mixture paste is prepared. Prepared. This negative electrode mixture paste was applied to both sides of a 10 μm thick copper current collector and dried, followed by vacuum drying at 100 ° C. for 5 hours and compression molding with a roll press. The dimensions of the obtained negative electrode were 31 mm in width, 600 mm in length, and the thickness including the current collector was 130 μm.

For the separator, a microporous polyethylene film with a thickness of 25 μm was used, and for the electrolyte, ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 (volume ratio) ) In a mixed solvent of 0.8 mol / L of LiPF ₆ and 0.2 mol / L of lithium bistrifluoromethanesulfonate imide (LiN (CF ₃ SO ₂ ) ₃ ) as a base electrolyte, A solution obtained by adding 0.06% by mass of 1,3-propene sultone to this base electrolyte was used.

  Then, the positive electrode and the negative electrode are wound in a flat shape through a separator to form a power generation element. The power generation element is housed in an aluminum battery case, and after injecting an electrolyte, the battery case and the battery lid are laser welded. As a result, a nonaqueous electrolyte secondary battery A of Example 1 having a design capacity of 600 mAh (hereinafter simply referred to as “battery A”. The same applies to nonaqueous electrolyte secondary batteries other than Example 1).

  FIG. 1 shows the appearance of a non-aqueous electrolyte secondary battery according to the present invention. In FIG. 1, 1 is a non-aqueous electrolyte secondary battery, 2 is a rectangular battery case, 3 is a battery lid, and 4 is a negative electrode terminal. 5 is a safety valve, and 6 is a liquid injection port. The battery case 2 also serves as a positive electrode terminal, and the positive electrode of the power generation element is connected to the inner surface of the battery case 2, while the negative electrode plate of the power generation element is connected to the negative electrode terminal 4 via a lead. The outer dimensions of the obtained battery were a width of 34.0 mm, a height of 50.0 mm, and a thickness of 5.15 mm.

[Example 2]
A battery B of Example 2 was produced in the same manner as in Example 1 except that the electrolyte used was a base electrolyte with 0.6% by mass of 1,3-propene sultone added.

[Example 3]
A battery C of Example 3 was produced in the same manner as in Example 1 except that the electrolyte solution used was 4.0% by mass of 1,3-propene sultone added to the base electrolyte solution.

[Comparative Example 1]
A battery D of Comparative Example 1 was produced in the same manner as in Example 1 except that 0.05% by mass of 1,3-propene sultone was added to the base electrolyte as the electrolytic solution.

[Comparative Example 2]
A battery E of Comparative Example 2 was produced in the same manner as in Example 1, except that the electrolyte used was 5.0% by mass of 1,3-propene sultone added to the base electrolyte.

[Example 4]
A battery F of Example 4 was produced in the same manner as in Example 1 except that a 5:95 mixture of natural graphite and hard carbon was used as the negative electrode active material.

[Example 5]
A battery G of Example 5 was produced in the same manner as in Example 4 except that an electrolytic solution obtained by adding 0.6% by mass of 1,3-propene sultone to the base electrolytic solution was used.

[Example 6]
A battery H of Example 6 was produced in the same manner as in Example 4 except that the electrolyte used was a base electrolyte with 4.0 mass% of 1,3-propene sultone added.

[Comparative Example 3]
A battery I of Comparative Example 3 was produced in the same manner as in Example 4 except that 0.05% by mass of 1,3-propene sultone was added to the base electrolyte as the electrolytic solution.

[Comparative Example 4]
A battery J of Comparative Example 4 was produced in the same manner as in Example 4, except that 5.0% by mass of 1,3-propene sultone was added to the base electrolyte as the electrolyte.

  Table 1 summarizes the contents of the negative electrode active material composition and the concentration of 1,3-propene sultone in the electrolyte of the batteries of Examples 1 to 6 and Comparative Examples 1 to 4.

[Characteristic measurement]
About the battery of Examples 1-6 and Comparative Examples 1-4, the charge / discharge cycle test was done and the capacity | capacitance retention and the swelling of the battery were measured. The test temperature is 25 ° C., charging is performed at a constant current of 600 mA up to 4.2 V, and further at a constant voltage of 4.2 V for a total of 3 hours at a constant current and a constant voltage, and then discharged at a constant current of 500 mA to 2.5 V. The initial discharge capacity was measured. Thereafter, charge / discharge cycles under the same conditions were repeated for 40 cycles under a test temperature of 60 ° C. and 10 cycles under a 0 ° C. environment, for a total of 50 cycles. The rate (%) was measured. Here, the “capacity maintenance ratio” indicates the ratio (%) of the discharge capacity at 25 ° C. after 300 cycles to the initial discharge capacity. The data was an average of 3 cells.

  Further, the battery thickness was measured before the charge / discharge cycle test and after the end of 300 cycles of discharge. FIG. 3 shows a method for measuring the battery thickness. 3 is an X-X ′ cross-sectional view of the prismatic battery shown in FIG. 1. In FIG. 3, symbol 2 is a battery case, and 7 is a power generation element. And the thickness (Y-Y 'of FIG. 3) of the center part of the largest area part of the battery case of the square battery shown in FIG. 3 was measured with calipers.

  The cycle test results and battery thickness measurement results for the batteries A to J obtained in Examples 1 to 6 and Comparative Examples 1 to 4 are summarized in Table 2. The thickness increase rate indicates the ratio (%) of the battery thickness increase after the cycle test to the battery thickness before the cycle test.

  Table 2 shows the capacity retention rate and thickness increase rate after 300 cycles of the batteries of Examples 1 to 6 and Comparative Examples 1 to 4.

  Further, after confirming the initial discharge capacity, each battery was disassembled in a dry room, and the negative electrode plate was taken out. The removed negative electrode plate was immersed in dimethyl carbonate for cleaning, and then vacuum dried at 25 ° C. for 2 hours. Thereafter, the surface of the electrode plate cut out to a predetermined size was analyzed by EPMA, and it was confirmed that all coatings contained sulfur (S) and nitrogen (N). Furthermore, about the negative electrode plate after vacuum drying, the compound material was shaved off in the dry room, and it was set as the sample for ICP analysis. In the ICP analysis method, 50 cc of 0.1 M HCl aqueous solution is weighed, and 500 mg of powder is put into this aqueous HCl solution at room temperature and immersed for 5 minutes at 25 ° C. Thereby, the surface of the negative electrode active material is dissolved by the acid. Next, the residual powder remaining undissolved from the aqueous HCl solution is removed. Only the aqueous HCl solution remaining as the supernatant was subjected to ICP analysis.

  Thereby, the quantitative analysis of sulfur (S) contained in the coating formed on the negative electrode surface was performed. The results are summarized in Table 3. Although the content of nitrogen (N) contained in the coating is unknown from ICP analysis, the presence of nitrogen (N) in the coating was confirmed by EPMA analysis as described above.

  From the results of Tables 1 to 3, when the content of sulfur (S) in the coating formed on the negative electrode surface is in the range of 0.012 to 0.83% by mass, the capacity retention rate is large, and swelling occurs. It was found that a small battery could be obtained.

[Examples 7 to 12 and Comparative Example 5]
[Example 7]
As the negative electrode active material, a mixture of natural graphite and hard carbon in a mass ratio of 30:70 was used. As the electrolyte, ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 ( In a mixed solvent having a volume ratio of LiPF ₆ of 0.8 mol / L and bistrifluoromethanesulfonic acid imide lithium (LiN (CF ₃ SO ₂ ) ₃ ) of 0.2 mol / L are dissolved, and 1,3- A battery K of Example 7 was made in the same manner as Example 2 except that one containing 0.6% by mass of propene sultone was used. [Example 8]
A battery L of Example 8 was produced in the same manner as Example 2 except that a 10:90 mixture of natural graphite and hard carbon was used as the negative electrode active material.

[Comparative Example 5]
A battery M of Comparative Example 5 was produced in the same manner as in Example 2 except that a mixture of natural graphite and hard carbon having a mass ratio of 70:30 was used as the negative electrode active material.

[Example 9]
A battery N of Example 9 was produced in the same manner as in Example 2 except that a 5:75:20 mixture of natural graphite, hard carbon, and coke was used as the negative electrode active material.

[Example 10]
A battery O of Example 10 was produced in the same manner as in Example 2 except that a 5:50:45 mixture of natural graphite, hard carbon, and coke was used as the negative electrode active material.

[Example 11]
A battery P of Example 11 was produced in the same manner as in Example 2 except that a 5:25:70 mixture of natural graphite, hard carbon, and coke was used as the negative electrode active material.

[Example 12]
A battery Q of Example 12 was produced in the same manner as Example 2 except that a 5:95 mixture of natural graphite and coke was used as the negative electrode active material.

  The negative electrode active material compositions of the batteries of Examples 7 to 12 and Comparative Example 5 are summarized in Table 4. Table 4 also shows the results of Example 2 and Example 5 for comparison.

[Characteristic measurement]
For the batteries K to Q of Examples 7 to 12 and Comparative Example 5, the measurement of the initial discharge capacity and the charge / discharge test of 300 cycles were performed under the same conditions as in Example 1, the capacity retention rate, the thickness increase rate, and Quantitative analysis of sulfur (S) contained in the coating formed on the negative electrode surface was performed. The results are summarized in Table 5. The presence of nitrogen (N) in the coating was confirmed by EPMA analysis.

  From the results shown in Tables 4 and 5, the content of graphite in the negative electrode active material is determined whether the negative electrode active material is a mixture of graphite and hard carbon, a mixture of graphite, hard carbon and coke, or a mixture of graphite and coke. Is 50% by mass or less, and a film containing sulfur and nitrogen is formed on the surface of the negative electrode, and when the content of sulfur is 0.11 to 0.13% by mass, the battery has a large capacity retention and a small swelling. It turns out that it is obtained.

  As in Comparative Example 5, when the negative electrode active material is a mixture of 70% by mass of graphite and 30% by mass of hard carbon, the negative electrode active material is greatly expanded / contracted by charge / discharge, and thus once formed on the negative electrode surface. It is presumed that the applied film is collapsed by the subsequent charge / discharge, the electrolytic solution is subsequently decomposed, and gas is generated inside the battery, so that the swelling of the battery cannot be suppressed.

[Examples 13 to 16]
[Example 13]
As an electrolyte, a mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 (volume ratio) and LiPF ₆ as an electrolyte with 0.98 mol / L and bistrifluoro Example except that 0.02 mol / L of lithium methanesulfonate imidolithium (LiN (CF ₃ SO ₂ ) ₃ ) was dissolved and 0.6 mass% of 1,3-propene sultone was added. In the same manner as in Example 7, a battery R of Example 13 was produced.

[Example 14]
A battery S of Example 14 was prepared in the same manner as in Example 13, except that LiPF ₆ was 0.95 mol / L and LiN (CF ₃ SO ₂ ) ₃ ) was 0.05 mol / L as the electrolyte salt. Produced.

[Example 15]
The battery T of Example 15 was made in the same manner as Example 13, except that LiPF ₆ was 0.90 mol / L and LiN (CF ₃ SO ₂ ) ₃ ) was 0.10 mol / L as the electrolyte salt. Produced.

[Example 16]
The battery U of Example 16 was made in the same manner as in Example 13, except that LiPF ₆ was 0.70 mol / L and LiN (CF ₃ SO ₂ ) ₃ ) was 0.30 mol / L as the electrolyte salt. Produced.

  Table 6 summarizes the electrolyte salt compositions of the batteries of Examples 13-16. Table 6 also shows the results of Example 7 for comparison.

[Characteristic measurement]
For the batteries R to U of Examples 13 to 16, the measurement of the initial discharge capacity and the charge / discharge test of 300 cycles were performed under the same conditions as in Example 1, and the capacity retention rate and the thickness increase rate were obtained. Are summarized in Table 7. Further, the presence of nitrogen (N) contained in the coating formed on the negative electrode surface was confirmed by EPMA analysis, and the content of sulfur (S) contained in the coating determined by ICP analysis was all 0.11 to 0. It was almost constant between 13% by mass.

From the results of Table 6 and Table 7, it was found that when a mixture of LiPF ₆ and an imide salt was used as the electrolyte salt, a battery having a large capacity retention and a small swelling was obtained regardless of the concentration of the imide salt. .

The figure which shows the external appearance of a square battery. The figure which shows the external appearance of a long cylindrical battery. X-X 'sectional drawing of the square battery of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 2 Battery case 3 Battery cover 4 Negative electrode terminal 5 Safety valve 6 Injection port 7 Power generation element

Claims (1)

In a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the negative electrode includes graphite and amorphous carbon as active materials, and the ratio of graphite to the active material in the negative electrode is 50% by mass or less. A non-aqueous electrolyte secondary battery , wherein a film containing sulfur and nitrogen is present on the negative electrode surface, and the sulfur content in the film is 0.012 to 0.83 mass% .