JP5192703B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery having a high capacity and an improved charge / discharge cycle life.

  With the widespread use of mobile devices such as mobile phones and laptop computers, the role of secondary batteries as power sources is gaining importance. Since these secondary batteries are small, light and have a high capacity and are required to have a performance that does not easily deteriorate even after repeated charge and discharge, lithium ion secondary batteries are most frequently used.

  Carbon such as graphite and hard carbon is mainly used for the negative electrode of the lithium ion secondary battery. Although carbon can repeat charge / discharge cycles satisfactorily, the capacity has been improved to near the theoretical capacity, so a significant increase in capacity cannot be expected in the future. On the other hand, since there is a strong demand for improving the capacity of lithium ion secondary batteries, negative electrode materials having a higher capacity, that is, a higher energy density than carbon, have been studied.

  In the negative electrode of lithium ion secondary batteries, metal lithium has been studied from the viewpoint of high energy density and light weight, but as the charge / discharge cycle progresses, dendrites (dendrites) are formed on the surface of the metal lithium during charging. There is a problem that the crystals are deposited and the crystal penetrates the separator, causing an internal short circuit and a short life.

As a material for increasing the energy density, it has been studied to use, as a negative electrode active material, a Li storage material that forms an alloy with lithium represented by the composition formula Li _x A (A is made of an element such as aluminum). This negative electrode has a large amount of occlusion and release of lithium ions per unit volume, and has a high capacity. Recently, the use of silicon as a negative electrode active material has been described in Non-Patent Document 1. It is said that a high capacity negative electrode can be obtained by using such a negative electrode material.

  Although this type of silicon-based negative electrode has a large amount of lithium ion storage and release per unit volume and high capacity, the electrode active material itself expands and contracts when lithium ion is stored and released. Progressed, the irreversible capacity in the first charge / discharge was large, and the charge / discharge cycle life was short.

  Patent Document 1 proposes a method of using silicon oxide as an active material as a measure for reducing irreversible capacity using silicon and improving charge / discharge cycle life. In Patent Document 1, since the volume expansion / shrinkage per unit weight of the active material can be reduced by using silicon oxide as the active material, improvement in cycle characteristics has been confirmed. On the other hand, since the conductivity of the oxide is low, there is a problem that the current collecting property is lowered and the irreversible capacity is large. In addition, Patent Document 2 proposes that iron or titanium is added to silicon oxide in order to improve current collecting performance when silicon oxide is used as an active material. However, since these metals are weak in corrosion resistance and oxidation resistance to the electrolytic solution, there is a problem that the conductivity decreases when the cycle is repeated only by adding the metal. Further, Patent Document 3 proposes a method of using, as an active material, particles obtained by combining a carbon material with silicon or silicon oxide as a measure for reducing irreversible capacity and improving the charge / discharge cycle life. Although this improved the cycle characteristics, it was still insufficient.

  On the other hand, it has been conventionally reported that a resin material having thermosetting properties is used as a binder (binder) for the purpose of improving cycle characteristics. Specifically, a method of mixing and sintering tin oxide, silicon oxide, and carbon with a polyimide binder is proposed in Patent Document 4, and a mixture of active material particles containing silicon and / or silicon alloy and conductive metal powder is prepared. Patent Document 5 proposes a method of sintering a mixture with a polyimide binder in a non-oxidizing atmosphere on the surface of the current collector. However, these have not led to the realization of cycle characteristics comparable to those of a carbon negative electrode, which is a judgment in actual use.

Japanese Patent No. 2999741 Japanese Patent No. 3010226 JP 2004-139886 A JP 2002-117835 A Japanese Patent Laid-Open No. 2002-260637 Hong Li, Xuejie Huang, Liquan Chen, Zhengang Wu, and Yong Liang, Electrochem.Solid-State Lett., Volume 2, Issue 11, p.547-549 (November 1999)

  The present invention solves the problem that although the silicon or silicon oxide negative electrode has a high capacity, the charge / discharge cycle life is insufficient and the charge / discharge efficiency in the first charge / discharge is low. An object of the present invention is to provide a non-aqueous electrolyte secondary battery that improves current collecting performance, has high charge / discharge efficiency at the first charge / discharge, has high energy density, and has good cycle characteristics even at high temperatures. It is in.

  In order to solve the above problems, a non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery having a negative electrode, a positive electrode, and a lithium ion conductive non-aqueous electrolyte. A mixture of active material particles coated with carbon having a mixed composition of amorphous carbon and graphite carbon, and a thermosetting resin that causes a dehydration condensation reaction by heating, and the active resin is heated by the thermosetting resin. The active material particles and the current collector are bound between the material particles, and the non-aqueous electrolyte includes a non-aqueous solvent and a sulfonate ester. The non-aqueous solvent preferably contains at least a chain carbonate or a cyclic carbonate, and the sulfonate ester is composed of at least one of a cyclic sulfonate ester, a cyclic disulfonate ester, or a chain disulfonate ester. Preferably, the cyclic sulfonic acid ester is preferably composed of at least one of 1,3-propane sultone or 1,4-butane sultone, and the cyclic disulfonic acid ester is methylene methane disulfonate, ethylene methane disulfonate. And at least one selected from propylene methane disulfonate.

  Providing a non-aqueous electrolyte secondary battery that suppresses gas generation during initial charge, has high charge / discharge efficiency in the first charge / discharge, and has good cycle characteristics by containing a sulfonate ester in the non-aqueous electrolyte. be able to. The organic sulfur compound reacts by the initial charge, and a film generally called a SEI (Solid Electrolyte Interface) film is formed on the negative electrode surface. By forming the film, the transfer between the negative electrode active material and the electrons becomes smooth.

  According to the present invention, the expansion and contraction of the electrode active material itself is alleviated by mixing silicon oxide with simple silicon that exhibits high capacity and large volume expansion due to charging, and further coating the surrounding area with amorphous carbon. Therefore, the charge / discharge cycle life is improved. On the other hand, a mixture of simple silicon and silicon oxide is coated with graphite-based carbon to improve conductivity and lead to an improvement in initial charge / discharge capacity, as well as corrosion resistance and oxidation resistance to silicon electrolyte as an active material. You can keep the sex. Furthermore, by including a sulfonic acid ester as an electrolyte component, the initial charge / discharge capacity and the cycle life can be improved. In particular, when a cyclic diester sulfonic acid is used, cycle life at a high temperature can be improved. In addition, since the thermosetting resin that functions as a binder causes a dehydration condensation reaction by heating, it exhibits an action of firmly binding between the active material particles and between the active material particles and the current collector. The initial charge / discharge capacity can be improved by improving the current collecting property.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic cross-sectional view of the active material particles of the negative electrode of the nonaqueous electrolyte secondary battery of the present invention.

As shown in FIG. 1, the active material particle 5 of the negative electrode has a particle size D in which the elemental silicon 1 and the silicon oxide 2 are used as nuclei and the periphery thereof is coated with carbon which is a mixture of amorphous carbon 3 and graphite carbon 4. ₉₅ is a particle of 50 μm or less, desirably 20 μm or less. D ₉₅ indicates the particle size when the sum of the volume ratios below a certain particle size is 95%.

  The simple silicon 1 occludes or releases Li during charge / discharge. The silicon oxide 2 has a role of relaxing expansion and contraction of the active material itself. The carbon coating layer on the outside is a mixture of amorphous carbon 3 and graphite carbon 4. It is preferable that the carbon covered in the negative electrode active material is 5% or more by weight, desirably 10% or more and less than 50%.

  As a method for producing the negative electrode active material particles, first, silicon and silicon oxide as nuclei are mixed and sintered under high temperature and reduced pressure. Next, a mixed sintered product of silicon and silicon oxide is introduced into a gaseous atmosphere of an organic compound in a high temperature non-oxygen atmosphere, or a mixed sintered product of silicon and silicon oxide and a precursor of carbon in a high temperature non-oxygen atmosphere. By mixing the body resin, a carbon coating layer is formed around the core of silicon and silicon oxide. Here, since the ratio of the graphite-based carbon in the carbon coating layer depends on the temperature at which the coating layer is formed, it is desirable that both amorphous carbon and graphite-based carbon be supported in the coating layer if it is 600 ° C. or higher and lower than 1200 ° C. I can do it.

  FIG. 2 shows a cross-sectional view of the nonaqueous electrolyte secondary battery of the present invention. As shown in FIG. 2, the negative electrode 8 includes an active material layer 6 and a current collector 7. The positive electrode 11 includes an active material layer 9 and a current collector 10. The negative electrode current collector 7 is usually made of copper foil, and the positive electrode current collector 10 is usually made of aluminum foil. A separator 12 is disposed between the negative electrode 8 and the positive electrode 11. As the separator 12, a polyolefin film such as polypropylene or polyethylene, or a porous film such as a fluororesin can be used. A laminate film can be used as the exterior film 13 and includes a fusion layer and an insulating layer for fusing the films together. Further, a negative electrode lead tab 14 and a positive electrode lead tab 15 for taking out electricity are fixed from the negative electrode 8 and the positive electrode 11, respectively, by a fusion part of the exterior film.

  The negative electrode active material layer 6 includes negative electrode active material particles generated by the above method and a thermosetting binder represented by polyimide, polyamide, polyamideimide, polyacrylic acid resin, and polymethacrylic acid resin. It is formed by dispersing and kneading in a solvent such as N-methyl-2-pyrrolidone (NMP), coating on the current collector 7, and drying in a high temperature atmosphere. In the active material layer 6 of the negative electrode, carbon black, acetylene black, or the like may be mixed in order to impart conductivity as necessary. The thickness of the current collector 7 is preferably 4 to 100 μm because it is preferable to maintain the strength, and more preferably 5 to 30 μm in order to increase the energy density.

  Nonaqueous solvents contained in the electrolyte include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Chain carbonates such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, and γ-lactones such as γ-butyrolactone, , 2-Ethoxyethane (DEE), chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3 dioxolane, formamide, acetamide , Dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl- Mix one or more aprotic organic solvents such as 2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid ester Although it can be used, it is not limited to these.

  As the sulfonic acid ester contained in the electrolyte, a cyclic monosulfonic acid ester, a cyclic disulfonic acid ester, a chain sulfonic acid ester, and the like can be appropriately used. Examples of cyclic monosulfonic acid esters include 1,3-propane sultone (1,3-PS), α-trifluoromethyl-γ-sultone, β-trifluoromethyl-γ-sultone, and γ-trifluoromethyl-γ-sultone. , Α-methyl-γ-sultone, α, β-di (trifluoromethyl) -γ-sultone, α, α-di (trifluoromethyl) -γ-sultone, α-undecafluoropentyl-γ-sultone, Examples include α-heptafluoropropyl-γ-sultone and 1,4-butane sultone (1,4-BS). Examples of the cyclic disulfonate include methylene methane disulfonate (MMDS), ethylene methane disulfonate (EMDS), and propylene methane disulfonate (PMDS). Examples of chain sulfonic acid esters include methanesulfonic acid methyl ester, methanesulfonic acid ethyl ester, busulfan, and dimethyl-methane disulfonate.

  Among these sulfonic acid esters, it is particularly preferable to use 1,3-PS or MMDS. 1,3-PS and MMDS are considered to form a decomposition film on the electrode of the lithium ion secondary battery. For example, the lowest unoccupied orbital energy (LUMO) of 1,3-PS is 0.07 eV, which is higher than EC (LUMO: 1.18 eV) or DEC (LUMO: 1.26 eV) where 1,3-PS is a solvent molecule. It is conceivable that the film is first decomposed to form a film. As a result, decomposition of solvent molecules is suppressed, and suppression of battery swelling due to gas generation and improvement of rate characteristics can be expected. Further, for example, when lithium manganate is included in the positive electrode, it is considered that Mn eluted at a high temperature by addition of MMDS is prevented from adsorbing on the negative electrode surface, and as a result, it is effective for improving cycle characteristics by suppressing resistance increase.

  The concentration of the sulfonate ester contained in these electrolytes is not particularly limited, but in the case of a secondary battery using a positive electrode containing lithium manganate as the positive electrode active material layer, 0.1% by mass The content is preferably 5.0% by mass or less, more preferably 0.5% by mass or more and 3.0% by mass or less. If it is less than 0.1% by mass, a sufficient film is not formed on the electrode surface, and the effect of improving the cycle characteristics is small. If it exceeds 5.0% by mass, the resistance increases and the cycle characteristics deteriorate.

The supporting salt contained in the electrolyte is not particularly limited, but electrolytes generally used for nonaqueous electrolyte secondary batteries such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ are used. Can be used.

Examples of the positive electrode active material include LiCoO ₂ , LiNi _1-x Co _x O ₂ , LiNi _x Mn _{2 -x} O ₄ (0 <x <1), LiFePO ₄ , LiMn ₂ O ₄ composite oxide positive electrode material. Although it can be used, it is not limited to these.

  Examples of the separator include polyolefin such as polypropylene and polyethylene, and porous films such as fluororesin, but are not limited thereto.

  Examples of the present invention will be described below.

Example 1
Single silicon and silicon oxide were mixed at a molar ratio of 1: 1, melted at 1400 ° C. and 13.3 Pa, and rapidly cooled to form a silicon-silicon oxide mixed powder. The particle size D _{95 of the} mixture was 20 μm or less. The maximum particle size was pulverized to 50 μm or less with a ball mill in an ethanol solution. The powder and the phenol resin were mixed and baked at 900 ° C. in a nitrogen atmosphere, and then pulverized so that the maximum particle size D ₉₅ was 30 μm or less. Thus, composite active material particles having a carbon ratio in the active material of 20% were produced.

Using the composite active material particles thus produced, an active material layer was produced as follows. For the negative electrode active material layer, the electrode material mixed with the above active material particles, polyimide, and NMP was applied onto a 10 μm copper foil, dried at 125 ° C. for 5 minutes, then subjected to compression molding with a roll press and dried again. Drying was performed in an oven at 300 ° C. for 10 minutes. This active material layer / copper foil sheet is punched out to 30 × 28 mm, a nickel tab for taking out electric charges is fused by ultrasonic waves, and for the positive electrode active material, active material particles made of lithium manganate, polyvinylidene fluoride as a binder, An electrode material mixed with NMP as a solvent is applied onto an aluminum foil of 20 μm, dried at 125 ° C. for 5 minutes, the active material layer / aluminum foil sheet is punched out to 30 × 28 mm, and an aluminum tab for taking out electric charges is used. Fusion was performed by ultrasonic waves. After laminating the negative electrode active material layer / copper foil sheet, the separator, and the positive electrode active material layer / aluminum foil sheet in this order so that the active material layer faces the separator, the laminate film is sandwiched, and the electrolyte solution is injected under vacuum. The laminate type battery was produced by sealing with. The electrolytic solution is 30% by mass of ethylene carbonate (EC) and 58% by mass of diethyl carbonate (DEC), and 1% by mass of 1,3-PS with respect to the electrolytic solution consisting of 12% by mass of LiPF ₆ as a lithium salt. What was added was used.

(Example 2)
A battery was fabricated in the same manner as in Example 1 except that the electrolyte was adjusted so that the concentration of 1,3-PS was 0.5% by mass.

(Example 3)
A battery was produced in the same manner as in Example 1 except that the electrolyte was adjusted so that the concentration of 1,3-PS was 0.1% by mass.

Example 4
A battery was produced in the same manner as in Example 1 except that the electrolyte was adjusted so that the concentration of 1,3-PS was 3.0% by mass.

(Example 5)
A battery was fabricated in the same manner as in Example 1 except that the electrolyte was adjusted so that the concentration of 1,3-PS was 5.0% by mass.

(Comparative Example 1)
A nonaqueous electrolyte battery was produced in the same manner as in Example 1 except that the 1,3-PS content was 0% by mass.

  The nonaqueous electrolyte secondary batteries obtained in Examples 1 to 5 and Comparative Example 1 were charged to a battery voltage of 4.2 V (charging conditions: current: 0.2 C, time: 6.5 hours, temperature 20 ° C.). The battery was discharged at 0.2 C to a battery voltage of 2.5 V, and the discharge capacity for charging at that time was defined as the charge / discharge efficiency. The capacity retention rate of the obtained nonaqueous electrolyte secondary battery was shown as the ratio of the discharge capacity (1.0 C) at the 300th cycle to the discharge capacity (1.0 C) at the first cycle. The cycle conditions were as follows: charge: upper limit voltage 4.2 V, current 1.0 C, time 2.5 hours, discharge: lower limit voltage 2.5 V, current 1.0 C, both at 20 ° C. and 60 ° C. did. Table 1 shows the charge / discharge efficiency and capacity retention rate.

  In the nonaqueous electrolyte secondary battery of the present invention, as shown in Examples 1 to 5 of Table 1, when the concentration of the sulfonic acid ester is 0.1 or more and 5.0% by mass or less, the sulfonic acid ester in the cycle characteristics and the like It was found that the characteristics were clearly better than Comparative Example 1 in which no was added.

(Example 6)
A battery was prepared and evaluated in the same manner as in Example 1 except that MMDS was used instead of 1,3-PS.

(Example 7)
A battery was prepared and evaluated in the same manner as in Example 2 except that MMDS was used instead of 1,3-PS.

(Example 8)
A battery was prepared and evaluated in the same manner as in Example 3 except that MMDS was used instead of 1,3-PS.

Example 9
A battery was prepared and evaluated in the same manner as in Example 4 except that MMDS was used instead of 1,3-PS.

(Example 10)
A battery was prepared and evaluated in the same manner as in Example 5 except that MMDS was used instead of 1,3-PS.

(Comparative Example 2)
A battery was prepared and evaluated in the same manner as in Comparative Example 1 except that MMDS was used instead of 1,3-PS.

  Table 2 shows the initial charge / discharge efficiency and capacity retention rate for Examples 6 to 10 and Comparative Example 2.

  In the nonaqueous electrolyte secondary battery of the present invention, as shown in Examples 6 to 10 in Table 2, when the concentration of the sulfonic acid ester is 0.1 or more and 5.0% by mass or less, the sulfonic acid ester in cycle characteristics and the like It was found that the characteristics were clearly better than those of Comparative Example 2 in which no was added. Even in a high-temperature cycle of 60 degrees, the cycle characteristics are good when the concentration of the sulfonic acid ester is 0.1 to 5.0% by mass.

The schematic cross section of the active material particle of the negative electrode of the nonaqueous electrolyte secondary battery of this invention. Sectional drawing of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

1 elemental silicon 2 silicon oxide 3 amorphous carbon 4 graphite carbon 5 (negative electrode) active material particles
6 Active material layer 7 (for negative electrode) Current collector 8 (for negative electrode) Negative electrode 9 Active material layer 10 (for positive electrode) Current collector 11 (for positive electrode) Positive electrode 12 Separator
13 Exterior film
14 Negative lead tab
15 Positive lead tab

Claims (3)

In a non-aqueous electrolyte secondary battery having a negative electrode, a positive electrode, and a lithium ion conductive non-aqueous electrolyte,
The negative electrode is a thermosetting resin that generates a dehydration-condensation reaction by heating with active material particles in which the periphery of a mixed sintered product of simple silicon and silicon oxide is coated with carbon having a mixed composition of amorphous carbon and graphite carbon A mixture of
Between the active material particles and the active material particles and the current collector are bound by the thermosetting resin,
The non-aqueous electrolyte includes a non-aqueous solvent and at least one selected from methylene methane disulfonate, ethylene methane disulfonate, and propylene methane disulfonate .   The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous solvent contains at least a chain carbonate or a cyclic carbonate. In a non-aqueous electrolyte secondary battery having a negative electrode, a positive electrode, and a lithium ion conductive non-aqueous electrolyte,
The negative electrode comprises a mixture of active material particles in which the periphery of a mixture of simple silicon and silicon oxide is coated with carbon having a mixed composition of amorphous carbon and graphite carbon, and a thermosetting resin that causes a dehydration condensation reaction by heating. Including
Between the active material particles and the active material particles and the current collector are bound by the thermosetting resin,
A nonaqueous electrolyte secondary battery, wherein the nonaqueous electrolyte includes a nonaqueous solvent and at least one selected from methylenemethane disulfonate, ethylenemethane disulfonate, and propylene methane disulfonate.

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