JP5016276B2 - Negative electrode for non-aqueous electrolyte secondary battery, method for producing the same, and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of a negative electrode of a non-aqueous electrolyte secondary battery.

Nonaqueous electrolyte batteries are small and light, have high energy density, and are used as main power sources for various electronic devices and power sources for memory backup. In recent years, with the remarkable development of portable electronic devices and the like, as devices are further reduced in size, performance, and maintenance-free, there is a strong demand for higher energy density of nonaqueous electrolyte batteries.
Since battery characteristics largely depend on the characteristics of the positive electrode active material and the negative electrode active material, many studies on the positive electrode active material and the negative electrode active material have been conducted.

  For example, Si can generate Li and an intermetallic compound, and can absorb and release Li reversibly. When this Si is used for the negative electrode active material, the theoretical capacity of Si is about 4200 mAh / g, which is extremely large compared to the theoretical capacity of about 370 mAh / g of the conventionally used carbon material. For this reason, many improvement studies using Si as a negative electrode active material have been conducted for the purpose of reducing the size and increasing the capacity of the battery.

However, Si particles are easily cracked due to changes in volume accompanying the insertion and release of Li. For this reason, although the negative electrode active material containing Si has a high capacity, it has a disadvantage in that the capacity reduction accompanying the charge / discharge cycle is large and the cycle life is shortened.
On the other hand, for example, in Patent Document 1, it consists of an A phase mainly composed of Si and a B phase containing a silicide of a transition metal, and at least one of the A phase and the B phase is in an amorphous state or a low crystalline state. It has been proposed to use a negative electrode active material in at least one state. Thereby, the volume change accompanying absorption and discharge | release of Li is reduced, and cycle life improves.

By the way, a positive electrode and a negative electrode consist of a mixture containing the active material, conductive material, binder, etc. which contribute to charging / discharging reaction. The conductive material is used to improve electronic conductivity between the active material particles. The binder is used for adhesion of the electrode material in the mixture such as active material particles and conductive material, and adhesion between the mixture and the current collector.
As the binder, a fluorine-based resin such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) is used. This fluororesin is stable with respect to non-aqueous electrolytes and has excellent binding properties with active materials and conductive materials.

However, when Si, Sn or the like is used as an active material, the volume of the active material changes greatly due to insertion and extraction of Li during charge / discharge, so even if the fluorine resin is used as a binder, Is difficult to maintain well. Moreover, the adhesiveness between the mixture and the current collector tends to be lowered. Therefore, the current collecting property of the mixture decreases with charge / discharge, the utilization factor of the active material decreases, and deterioration due to the charge / discharge cycle becomes very large.
In addition, when polyimide is used as the binder, the binding property of the electrode material in the mixture and the binding property between the mixture and the current collector are improved, and an active material having a large volume change during charge / discharge is used. However, it is known that the charge / discharge cycle characteristics can be obtained without the mixture being detached from the current collector.

  For example, Patent Document 2 proposes the following for the purpose of improving cycle characteristics. In the negative electrode for a secondary battery, a conductive intermediate layer containing polyimide as a binder is disposed in a mixture layer containing an active material composed of at least one of silicon and a silicon alloy, or between the mixture layer and the metal foil current collector. To do. Sintering is performed in a non-oxidizing atmosphere with the conductive intermediate layer disposed on the metal foil current collector. The conductive intermediate layer suppresses separation of the mixture layer from the current collector due to the expansion and contraction of the negative electrode active material accompanying the charge / discharge reaction, and this intermediate layer adheres between the mixture layer and the current collector. Increase sex.

By the way, regarding the manufacture of mobile devices, as a method of attaching electronic components on a printed circuit board, reflow soldering capable of soldering electronic components at a high density and in a lump is often employed.
The reflow soldering is performed as follows. A solder cream or the like is applied to the part to be soldered on the printed circuit board. Thereafter, the printed circuit board on which electronic components are mounted is passed through a furnace in a high-temperature atmosphere set so that the soldered portion becomes 200 to 260 ° C. At this time, the solder is melted and soldered.

  For this reason, when a non-aqueous electrolyte secondary battery is installed on a printed circuit board as a memory backup application and the above reflow soldering is adopted, the battery itself is also required to have heat resistance. In response to this, it has been studied to employ heat-resistant materials for battery constituent members such as an electrolytic solution, a separator, and a gasket.

As a binder for a non-aqueous electrolyte secondary battery excellent in heat resistance, for example, polyimide (melting point: about 500 ° C.) can be mentioned. Polyimide has extremely high thermal stability and excellent heat resistance compared to other organic polymer materials.
However, when polyimide is used as a negative electrode binder in a non-aqueous electrolyte secondary battery, the low temperature characteristics of the battery are likely to deteriorate.

Patent Document 3 proposes the following. In the nonaqueous electrolyte battery, a carbon material is used as the negative electrode active material. A polyimide resin as a binder and an acrylic acid polymer, methacrylic acid polymer and urethane polymer as a binder aid are mixed, and then the binder aid is decomposed and removed by heat treatment. This improves the cycle characteristics.
However, since the binding aid is decomposed and removed by heat treatment, and only polyimide functions as a binder, the low-temperature characteristics are reduced as described above.

  Furthermore, Patent Document 4 proposes that a mixture of polyimide and fluorine-based polymer, which is soluble in an organic solvent in a state where imidization is completed, is used as a binder of an electrode mixture. Thereby, imidation by the heat processing of an electrode mixture becomes unnecessary, and productivity improves.

However, the above-mentioned binder, which is soluble in organic solvents, elutes into the organic electrolyte in the non-aqueous electrolyte secondary battery, so it is difficult to maintain the function as a binder, and cycle characteristics and storage characteristics are degraded. To do. Moreover, since it does not heat-process at high temperature, the water which generate | occur | produces in the case of the dehydration condensation by imidation remains, and this may have a bad influence on a positive electrode active material.
JP 2004-335272 A JP 2004-288520 A JP-A-9-265990 Japanese Patent Laid-Open No. 10-188992

  An object of the present invention is to provide a negative electrode excellent in binding property even when the active material contains Si and excellent in electronic conductivity even when polyimide is used as a binder, and a method for producing the same. Another object of the present invention is to provide a high energy density non-aqueous electrolyte battery having excellent charge / discharge cycle characteristics, low temperature characteristics, and heat resistance using the negative electrode.

The present invention includes an active material containing Si, a binder, and a conductive material, and the binder is obtained by heating a mixture containing polyamic acid and polyacrylic acid at 150 to 300 ° C. The polyamic acid is imidized, and the conductive material relates to a negative electrode for a nonaqueous electrolyte secondary battery, which is a carbon material.
The present invention also relates to a nonaqueous electrolyte secondary battery comprising the above negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.

Furthermore, the present invention includes a step of obtaining a negative electrode mixture by mixing an active material containing Si, a binder raw material solution containing polyamic acid and polyacrylic acid, and a carbon material as a conductive material, followed by heating and drying (1) ) And the negative electrode mixture are pressure-molded to obtain pellets, and then the pellets are heated at 150 to 300 ° C. to imidize the polyamic acid to obtain polyimide, and polyimide and polyacrylic acid are used as binders. The present invention relates to a method for manufacturing a negative electrode, including a step (2) of obtaining a negative electrode including the negative electrode.

  According to the present invention, polyacrylic acid is preferentially bonded to the Si-containing negative electrode active material, and it is possible to prevent polyimide from strongly covering the negative electrode active material. Electronic conductivity can be obtained. Further, according to the present invention, by using the negative electrode, a high energy density non-aqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics, low temperature characteristics, and heat resistance can be obtained.

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery including a negative electrode active material containing Si, a binder, and a conductive material, wherein the binder is polyimide and polyacrylic acid, and the conductive material is a carbon material.

  Conventionally, when polyimide is used alone as a binder, since polyimide has excellent heat resistance and binding properties, the cycle characteristics of the battery are improved, but the low temperature characteristics of the battery are lowered. This is because when the negative electrode active material particles containing Si are widely covered with polyimide, the contact between the negative electrode active material particles and the carbon material as the conductive material is hindered, and the electronic conductivity of the negative electrode is reduced. Guessed.

  In addition, when polyacrylic acid is used alone as the binder, polyacrylic acid has a lower binding property than polyimide and has low heat resistance, so the low temperature characteristics of the battery do not deteriorate as in the case of polyimide. Cycle characteristics and heat resistance are reduced.

  On the other hand, when a mixture of polyimide and polyacrylic acid is used as a negative electrode binder as in the present invention, polyacrylic acid is preferentially bonded to negative electrode active material particles containing Si, and the negative electrode active material particles are polyimide. It becomes difficult to be covered with. Thereby, the electronic conductivity of a negative electrode is improved and the fall of the low temperature characteristic of the battery which arises when a polyimide is used alone as a binder can be suppressed. In addition, when both polyimide and polyacrylic acid are used for the binder, since the polyimide is excellent in binding properties, cycle characteristics equivalent to those when the polyimide is used alone for the binder can be obtained.

Thus, by using the negative electrode, a high energy density non-aqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics, low temperature characteristics, and heat resistance can be obtained.
The polyacrylic acid content in the negative electrode is preferably 0.5 to 30 parts by weight per 100 parts by weight of the negative electrode active material.
The polyimide content in the negative electrode is preferably 6.5 to 40 parts by weight per 100 parts by weight of the negative electrode active material.
The weight ratio of polyacrylic acid and polyimide contained in the negative electrode is preferably 5 to 90: 9 to 95.

Examples of the negative electrode active material containing Si that can be alloyed with lithium include silicon simple substance, silicon oxide, and silicon alloy. As the silicon oxide, for example, SiO _x (0 <x <2, preferably 0.1 ≦ x ≦ 1) can be used. As the silicon alloy, for example, an alloy containing Si and a transition metal M (M-Si alloy) can be used. For example, it is preferable to use a Ni—Si alloy, a Ti—Si alloy, or the like. Further, the negative electrode active material containing Si may be any of single crystal, polycrystal, and amorphous.

  The negative electrode active material mainly comprises a first phase containing Si (A phase) and a second phase containing a transition metal silicide (B phase), and at least one of the first phase and the second phase One is preferably in at least one of an amorphous state and a low crystalline state. A nonaqueous electrolyte secondary battery having a high capacity and excellent cycle life can be obtained. The B phase is preferably a phase containing a transition metal and a silicide.

The A phase is a phase that contributes to the occlusion and release of Li, that is, a phase that can electrochemically react with Li. The A phase is preferably a single Si phase in that the amount of Li occlusion / release per unit weight or volume of the A phase is large. However, since Si has poor electronic conductivity, elements such as phosphorus, boron, or transition metals may be added to the A phase in order to improve the electronic conductivity of the A phase.
The B phase containing silicide has a high affinity with the A phase, and cracking at the crystal interface is unlikely to occur even when the volume of the B phase expands during charging. The B phase has higher electron conductivity and higher hardness than the A phase mainly composed of Si. Therefore, when the active material contains the B phase, low electronic conductivity in the A phase is improved, and stress during expansion is relaxed, and cracking of the active material particles is suppressed.

The B phase may be composed of a plurality of phases. For example, the B phase is composed of two phases having different composition ratios of the transition metal M and silicon, such as MSi ₂ and MSi (M is a transition metal). Further, for example, it may be composed of three or more phases including the above two phases and a silicide phase of different transition metal M. The transition metal M is preferably at least one selected from the group consisting of Ti, Zr, Ni, Cu, Fe, and Mo. The transition metal M silicide has high electronic conductivity and high strength. Among these, the transition metal M is more preferably Ti. B phase preferably contains TiSi _2.

  When the negative electrode active material particle containing Si contains a transition metal, the transition metal present on the surface of the negative electrode active material particle is oxidized, and an oxide of the transition metal is formed on the surface of the negative electrode active material particle. Since there is a hydroxyl group (—OH group) on the surface of the transition metal oxide, the bond between the negative electrode active material and polyacrylic acid becomes stronger, and polyacrylic acid preferentially binds to the negative electrode active material. Even when is used, the low temperature characteristics of the battery do not deteriorate.

For example, graphite or carbon black is used as the carbon material in the negative electrode. The content of the carbon material in the negative electrode is not particularly limited, but is preferably 1.0 to 50 parts by weight per 100 parts by weight of the negative electrode active material, and more preferably 1.0 to 40 parts by weight per 100 parts by weight of the negative electrode active material. .
In the method for producing a negative electrode of the present invention, an active material containing Si, a binder raw material solution containing polyamic acid and polyacrylic acid, and a carbon material as a conductive material are mixed, and then heated and dried to obtain a negative electrode mixture. After the step (1) and the negative electrode mixture are pressure-molded to obtain pellets, the pellets are heated to imidize polyamic acid to obtain polyimide, and the negative electrode containing polyimide and polyacrylic acid as a binder (2).

  As the binder raw material solution, for example, an N-methyl-2-pyrrolidone (NMP) solution containing polyamic acid and polyacrylic acid is used. In the binder raw material solution, polyimide may be used directly instead of polyamic acid. However, polyimide is hardly dissolved in a solvent such as NMP and is not easily dispersed uniformly in the negative electrode mixture. On the other hand, in the binder raw material solution, polyamic acid which is a polyimide precursor is easily dissolved in a solvent such as NMP. For this reason, a polyamic acid can be uniformly disperse | distributed in a negative electrode mixture, and a polyimide can be disperse | distributed uniformly in a negative electrode by imidating this polyamic acid. In step (1), for example, the negative electrode mixture is heated and dried at 60 ° C. for 12 hours under vacuum. Since the heating temperature in the step (1) is sufficiently lower than the heating temperature for causing the imidation reaction described later, the imidization reaction does not occur in the step (1).

  By the heat treatment in the step (2), the polyamic acid is imidized (dehydration polymerization) to obtain a polyimide. This polyimide and polyacrylic acid function as a binder for the negative electrode. As the heat treatment, hot air, infrared rays, far infrared rays, and electron beams are used alone or in combination.

  The heating temperature of the pellet is preferably 200 to 300 ° C, more preferably 200 to 250 ° C. When the pellet is heat-treated at 200 to 300 ° C., the imidization of the polyamic acid proceeds sufficiently and the amount of polyacrylic acid added during the preparation of the negative electrode can be left in the negative electrode without decomposing the polyacrylic acid. it can. The imidization reaction in the step (2) tends to proceed at 200 ° C. or higher. In addition, when the heating temperature exceeds 300 ° C., polyacrylic acid is easily decomposed, and when the remaining amount of polyacrylic acid in the negative electrode decreases, polyacrylic acid is preferentially bonded to the negative electrode active material containing Si, and the negative electrode The effect of suppressing the coating of the active material surface with polyimide is reduced, the electron conductivity of the negative electrode is lowered, and the effect of improving the low temperature characteristics of the battery cannot be sufficiently obtained. Although moisture is generated by dehydration polymerization by imidization, the moisture is removed because the pellets are heated at 200 to 300 ° C. For this reason, moisture does not enter the battery system.

  The imidation ratio of the polyamic acid is preferably 80% or more. When the imidization reaction of the polyamic acid is less than 80%, the polyimide does not function sufficiently as a binder, so that the cycle characteristics are likely to deteriorate. The imidation ratio of the polyamic acid can be controlled, for example, by adjusting the heating temperature and heating time of the pellet in the step (2). The imidization rate can be determined using infrared spectroscopy (IR).

  The minimum content with which the binding property between the negative electrode active material particles is sufficiently maintained is appropriate as the content of the binder in the negative electrode mixture from the viewpoint of battery characteristics. From such a viewpoint, the total content of polyamic acid and polyacrylic acid in the negative electrode mixture is preferably 0.5 to 30 parts by weight per 100 parts by weight of the negative electrode active material. When the total content of polyamic acid and polyacrylic acid in the negative electrode mixture is less than 0.5 parts by weight per 100 parts by weight of the negative electrode active material, the effect as a binder becomes insufficient. On the other hand, if the total content of polyamic acid and polyacrylic acid in the negative electrode mixture exceeds 30.0 parts by weight per 100 parts by weight of the negative electrode active material, the amount of binder becomes excessive and the amount of active material is relatively reduced. , Battery capacity decreases.

  In view of obtaining excellent cycle characteristics and low temperature characteristics, the polyamic acid content in the negative electrode mixture is preferably 10 to 95 parts by weight per 100 parts by weight of the total of polyamic acid and polyacrylic acid. When the polyamic acid content in the negative electrode mixture is less than 10.0 parts by weight per 100 parts by weight of the total of polyamic acid and polyacrylic acid, the amount of polyimide obtained is reduced, and the cycle characteristics are deteriorated. If the polyacrylic acid content in the negative electrode mixture exceeds 95 parts by weight per 100 parts by weight of the total of polyamic acid and polyacrylic acid, the amount of polyacrylic acid that can be preferentially bonded to the negative electrode active material is insufficient, Since the negative electrode active material is firmly covered, the low-temperature characteristics of the battery are likely to deteriorate.

  The nonaqueous electrolyte secondary battery of the present invention includes the above negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. By using the negative electrode, a high energy density non-aqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics, low temperature characteristics, and heat resistance can be obtained. The shape and size of the nonaqueous electrolyte secondary battery are not particularly limited. The negative electrode of the present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a cylindrical shape and a rectangular shape. Further, in the nonaqueous electrolyte secondary battery of the present invention, as described above, since a material containing fluorine is not used in the binder, hydrogen fluoride generated by thermal decomposition of the binder reacts with the negative electrode active material and the battery deteriorates. There is nothing.

A positive electrode consists of a positive electrode mixture containing a positive electrode active material, a binder, and a electrically conductive material, for example.
As the positive electrode active material, a lithium-containing or non-containing compound capable of inserting and extracting lithium ions is used. For _{example, Li x CoO 2, Li x} NiO 2, Li x MnO 2, Li x Mn 1 + y O 4, Li x Co y Ni 1-y O 2, Li x Co y M 1-y O z, Li x _{Ni 1-y M y O z} , Li x Mn 2 O 4, Li x Mn 2-y M y O 4 (M = Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al , Cr, Pb, Sb, and B). In the above, x is 0 to 1.2, y is 0 to 0.9, and z is 2.0 to 2.3. The x value changes due to charge / discharge. Further, transition metal chalcogenides, vanadium oxides and lithium compounds thereof, niobium oxides and lithium compounds thereof, conjugated polymers using organic conductive substances, chevrel phase compounds, and the like are used. The said compound may be used independently and may be used in combination of 2 or more type.

The binder and the conductive material of the positive electrode are not particularly limited as long as they can be used in the nonaqueous electrolyte secondary battery.
For the separator, for example, a microporous film having excellent ion permeability is used. For example, a glass fiber sheet, a nonwoven fabric, a woven fabric, etc. are used.

Also, from the viewpoint of organic solvent resistance and hydrophobicity, polypropylene, polyethylene, polyphenylene sulfide, polyethylene terephthalate, polyamide, polyimide, and the like are used as the separator material. These may be used alone or in combination of two or more. In general, inexpensive polypropylene is used. However, in order to impart reflow resistance to the battery, it is preferable to use polypropylene sulfide, polyethylene terephthalate, polyamide, polyimide, or the like having a heat distortion temperature of 230 ° C. or higher.
The thickness of the separator is, for example, 10 to 300 μm. Further, the porosity of the separator may be appropriately determined according to the permeability of electrons and ions, the material of the separator, and the like, but generally 30 to 80% is desirable.

For the nonaqueous electrolyte, for example, a nonaqueous solvent in which a lithium salt is dissolved is used.
Nonaqueous solvents include, for example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl Chain carbonates such as methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate and ethyl propionate, and γ-lactones such as γ-butyrolactone Chain ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethylsulfoxide Sid, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl -2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, dimethyl sulfoxide, N-methylpyrrolidone, butyl diglyme, methyl tetraglyme Aprotic organic solvents such as these may be used, and these may be used alone or in combination of two or more.

  Among these, from the viewpoint of reflow resistance, it is preferable to use ethylene carbonate, propylene carbonate, sulfolane, butyl diglyme, methyl tetraglyme, or γ-butyrolactone having a boiling point of 200 ° C. or higher at normal pressure.

Examples of the lithium salt include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , LiB ₁₀ Cl _10. , Lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, lithium chloroborane, lithium tetraphenylborate, LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) _{2 and the} like. Or may be used in combination of two or more. Further, a solid electrolyte such as a gel may be used. The concentration of the lithium salt in the nonaqueous electrolyte is not particularly limited, but is preferably 0.2 to 2.0 mol / L, and more preferably 0.5 to 1.5 mol / L.

EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited to the following examples.
Example 1
(1) Production of negative electrode active material Ti powder (manufactured by High-Purity Chemical Co., Ltd., purity 99.99%, particle size so that the proportion of the Si phase as the A phase in the negative electrode active material particles is 30% by weight. Less than 20 μm) and Si powder (manufactured by Kanto Chemical Co., Inc., purity 99.999%, particle size less than 20 μm) were mixed at a weight ratio of 32.2: 67.8.

Then, the mixed powder was put into a vibration mill container, and further a stainless ball (diameter 2 cm) was added so as to occupy 70% by volume of the capacity of the mill container. After evacuating the inside of the container, the inside of the container was replaced with Ar (Nihon Oxygen Co., Ltd., purity 99.999%) until the inside of the container reached 1 atm. Thereafter, mechanical alloying was performed for 60 hours while applying a vibration of 60 Hz to obtain a Ti—Si alloy.
As a result of X-ray diffraction measurement of the obtained Ti—Si alloy powder, it was confirmed that Si single phase and TiSi ₂ phase existed in the alloy particles. Further, as a result of observing this alloy material with a transmission electron microscope (TEM), it was confirmed that an Si phase having an amorphous or about 10 nm crystal and a TiSi ₂ phase having about 15 to 20 nm crystal were present.

(2) Preparation of binder raw material solution Polyacrylic acid was added to polyamic acid solution (Ube Industries, U-Varnish A, 20 wt% NMP (N-methyl-2-pyrrolidone) solution), which is a polyimide precursor. 10% by weight of powder (manufactured by Nippon Pure Chemical Co., Ltd., Jurimer AC-10LHP) was dissolved to obtain a binder raw material solution.

(3) Production of negative electrode Next, the negative electrode active material and the binder raw material solution obtained above, and graphite powder (SP-5030, manufactured by Nippon Graphite Co., Ltd.) were mixed as a conductive material. This mixture was dried under vacuum at 60 ° C. for 12 hours to obtain a negative electrode mixture. At this time, the weight ratio of the Ti—Si alloy, graphite powder, polyamic acid, and polyacrylic acid in the negative electrode mixture was 100: 20: 5: 5.

  Thereafter, the negative electrode mixture was pressure-molded to obtain a disc-shaped negative electrode pellet having a diameter of 4.0 mm and a thickness of 0.3 mm. This negative electrode pellet was heated at 250 ° C. for 12 hours to imidize the polyamic acid present inside the pellet to obtain a negative electrode. At this time, the imidation ratio was 98%. In addition, the imidation ratio was calculated | required using the infrared spectroscopy (IR). In addition, it was confirmed by infrared spectroscopy (IR) that the amount of polyacrylic acid added during the preparation of the negative electrode remained in the negative electrode even after heating.

(4) Production of positive electrode Manganese dioxide and lithium hydroxide were mixed at a molar ratio of 2: 1 and then fired at 400 ° C. for 12 hours in air to obtain lithium manganate. 88 parts by weight of the lithium manganate powder obtained above as a positive electrode active material, 6 parts by weight of carbon black as a conductive material, and an aqueous dispersion containing 6 parts by weight of a fluororesin as a binder were mixed. This mixture was dried under vacuum at 60 ° C. for 12 hours to obtain a positive electrode mixture. This positive electrode mixture was pressure-molded to obtain a disc-shaped positive electrode pellet having a diameter of 4.0 mm and a thickness of 1.1 mm. This positive electrode pellet was heated at 250 ° C. for 12 hours to obtain a positive electrode.

(5) Production of coin-type battery The coin-type battery shown in FIG. 1 was produced by the following procedure. FIG. 1 is a longitudinal sectional view of a coin-type battery of the present invention.
First, the positive electrode 12 obtained above was disposed on the inner surface of a stainless steel positive electrode can 11, and a separator 13 made of a porous polyethylene sheet was disposed on the positive electrode 12. Next, the electrolytic solution was poured into the positive electrode can 11. For the electrolytic solution, an organic solvent containing 1 mol / L of LiN (CF ₃ SO ₂ ) ₂ as a lithium salt was used. As the organic solvent, a mixed solvent of PC, EC, and DME (volume ratio PC: EC: DME = 1: 1: 1) was used.

On the separator 13 in the positive electrode can 11, the negative electrode 14 obtained above was disposed. A stainless steel negative electrode can 16 with a polypropylene gasket 15 attached to the peripheral edge thereof was placed in the opening of the positive electrode can 11. The open end of the positive electrode can 11 was caulked to the peripheral edge of the negative electrode can 16 via the gasket 15 to seal the open portion of the positive electrode can 11. At this time, a pitch was applied to a contact portion between the positive electrode can 11 and the negative electrode can 16 and the gasket 15. Thus, a coin-type battery having a diameter of 6.8 mm and a thickness of 2.1 mm was produced.
In addition, said negative electrode 14 was used in the state which made the negative electrode active material occlude lithium in the presence of electrolyte solution, and was electrochemically alloyed with the negative electrode active material.

  In this embodiment, polypropylene was used as the gasket material. In addition, polyphenylene sulfide, polyether ketone, polyamide, polyimide, liquid crystal polymer, and the like are used from the viewpoints of stability to the electrolytic solution and heat resistance. Moreover, these may be used independently and may be used in combination of 2 or more type. Moreover, you may add fillers, such as an inorganic fiber, to the said polymer. In general, inexpensive polypropylene is used, but in order to impart reflow resistance to the battery, it is preferable to use polyphenylene sulfide, polyether ketone, polyimide, or liquid crystal polymer having a heat distortion temperature of 230 ° C. or higher.

  In this example, in order to improve the hermeticity of the battery, a pitch was applied as a sealing material to the close contact portion of the gasket with the positive electrode can and the negative electrode can, but in addition to this, asphalt pitch, butyl rubber, fluorine System oil or the like may be used. When the sealing material is transparent, it is preferable to color it to clarify the presence or absence of application. Further, instead of applying the sealing material to the gasket, the sealing material may be applied in advance to the contact portion between the positive electrode can and the negative electrode can.

<< Comparative Example 1 >>
Instead of the binder raw material solution of Example 1, a polyamic acid solution (Ube Industries, U-Varnish A, 20 wt% NMP solution) was used, and the Ti—Si alloy, graphite, and polyamide in the negative electrode mixture The weight ratio of the acid was 100: 20: 10. Except for this, a coin-type battery was produced in the same manner as in Example 1.

<< Comparative Example 2 >>
Instead of the binder raw material solution of Example 1, an NMP solution in which 10% by weight of polyacrylic acid powder (manufactured by Nippon Pure Chemical Co., Ltd., Jurimer AC-10LHP) was used, a Ti—Si alloy in the negative electrode mixture, The weight ratio of graphite and polyacrylic acid was 100: 20: 10. Except for this, a coin-type battery was produced in the same manner as in Example 1.

<< Comparative Example 3 >>
As the negative electrode active material, graphite (SP-5030, manufactured by Nippon Graphite Co., Ltd.) was used instead of the Ti—Si alloy, and graphite, polyamic acid, and polyacrylic acid were used 100: 5: A coin-type battery was produced in the same manner as in Example 1 except that the negative electrode mixture contained at a ratio of 5 was used.

The batteries of Example 1 and Comparative Examples 1 to 3 were evaluated as follows.
(6) Battery charge / discharge test The charge / discharge cycle test of the coin-type battery obtained above was performed as follows in a thermostatic bath set at 20 ° C.
In the range of battery voltage 2.0-3.3V, charging / discharging was performed 50 cycles with the constant current of 0.02CA. The ratio of the discharge capacity at the 50th cycle to the discharge capacity at the second cycle (hereinafter referred to as initial capacity) was defined as the cycle capacity maintenance rate. The closer the cycle capacity retention rate is to 100, the better the cycle characteristics.

  Moreover, the said charge / discharge cycle test was done in the -20 degreeC thermostat as a low temperature characteristic of a battery. The ratio of the initial capacity at −20 ° C. to the initial capacity at 20 ° C. was determined and used as the low temperature capacity retention rate. The closer this low temperature capacity retention rate is to 100, the better the low temperature characteristics.

(7) Heat resistance test of negative electrode After each battery is charged, the battery is disassembled and the negative electrode in which lithium is occluded is taken out. Using this negative electrode, a differential scanning calorimeter (Thermo Electric DSC8230 manufactured by Rigaku Corporation) is used. Thermal analysis measurement (DSC measurement) was performed. In the DSC measurement, about 5 mg of the taken-out negative electrode was placed in a stainless steel sample container (withstand pressure: 50 atm) and heated from room temperature to 400 ° C. at a temperature rising rate of 10 ° C./min in a still air atmosphere.
At this time, the temperature at which the exothermic peak attributed to the negative electrode appears was defined as the exothermic peak temperature. In addition, it shows that it is excellent in heat resistance, so that this peak temperature is high.
The evaluation results are shown in Table 1.

  In the battery of Example 1 using a mixture of polyimide and polyacrylic acid as the negative electrode binder, the low temperature characteristics were significantly improved as compared with the battery of Comparative Example 1 using polyimide alone as the negative electrode binder. This is considered to be because polyacrylic acid preferentially bonds to the negative electrode active material, thereby suppressing the polyimide from strongly bonding to the negative electrode active material and lowering the low temperature characteristics. Further, the cycle characteristics were improved to the same level as in Comparative Example 1 in which polyimide was used alone.

  In the battery of Example 1 using the Ti—Si alloy as the negative electrode active material, the initial capacity increased compared to the battery of Comparative Example 3 using graphite as the negative electrode active material. Moreover, the negative electrode used for the battery of Example 1 showed superior heat resistance as compared with the negative electrode used for the battery of Comparative Example 3. This is thought to be because the reactivity is greater when lithium is inserted into the graphite than when lithium is inserted into the Ti—Si alloy. When a Ti—Si alloy is used as the negative electrode active material, lithium is inserted into and desorbed from the Ti—Si alloy preferentially over graphite as a conductive material. Thus, lithium does not insert into and desorb from graphite, and only the Ti—Si alloy is involved in the battery reaction as an active material. Therefore, it is considered that the heat resistance of the negative electrode is better when a Ti—Si alloy is used as the negative electrode active material than graphite.

From Table 1, the temperature (exothermic peak temperature in Table 1) at which the exothermic peak attributed to the thermal decomposition of the negative electrode varies depending on the type of binder and the blending ratio of the binder, and when using a binder containing polyimide, it has excellent heat resistance. It was found that a negative electrode was obtained.
From the above, in the negative electrode, by using a Ti—Si alloy as the active material, polyimide and polyacrylic acid as the binder, and a carbon material as the conductive material, it has excellent low temperature characteristics, charge / discharge cycle characteristics, and heat resistance. It was found that a non-aqueous electrolyte secondary battery with a capacity was obtained.

<< Examples 2 to 4 and Reference Example 1 >>
In this example and reference example , when polyimide and polyacrylic acid were used for the negative electrode binder, the heating temperature of the negative electrode pellet containing polyamic acid which is a precursor of polyimide was examined.
A coin-type battery was produced and evaluated in the same manner as in Example 1 except that the heating temperature of the negative electrode pellet was changed to the temperature shown in Table 2. The evaluation results are shown in Table 2 together with the results of the battery of Example 1.

Since the negative electrode of Example 2 in which the heating temperature of the negative electrode pellet was 150 ° C. had a low imidization rate and most of the polyamic acid was not changed to polyimide, the cycle characteristics were deteriorated in the battery using this negative electrode.
In the batteries of Examples 1 to 4, the amount of polyacrylic acid added during the production of the negative electrode remained almost, and excellent low-temperature characteristics were obtained.

On the other hand, in the battery of Reference Example 1 , the low-temperature capacity maintenance rate decreased. This is presumably because, in the negative electrode of Reference Example 1 having a heat treatment temperature of 400 ° C., most of the polyacrylic acid was decomposed, and the effect of improving the low-temperature characteristics due to the negative electrode containing polyacrylic acid was reduced. The amount of polyacrylic acid in the negative electrode after heating was examined by infrared spectroscopy (IR).
In particular, in Examples 1, 3 and 4, since a high capacity non-aqueous electrolyte secondary battery excellent in low temperature characteristics, cycle characteristics and heat resistance was obtained, the imidization ratio of the polyamic acid was 80% or more. Yes, the heating temperature of the negative electrode pellet is preferably 200 to 300 ° C.

<< Examples 6 to 10 >>
In this embodiment, the examined content of the binder material in the negative electrode mixture during preparation of the anode using the polyimide and polyacrylic acid binder (polyamide acids and poly A acrylic acid).
The content ratio of the binder raw material per 100 parts by weight of the negative electrode active material in the negative electrode mixture was variously changed to the amount shown in Table 3 without changing the blending ratio of the polyamic acid and the polyacrylic acid in the binder raw material. A coin-type battery was produced and evaluated under the same conditions as in Example 1.
The evaluation results are shown in Table 3 together with the evaluation results of Example 1.

In the battery of Example 6 in which the content of the binder raw material in the negative electrode mixture was 0.2 parts by weight per 100 parts by weight of the negative electrode active material, the cycle characteristics deteriorated. This is presumably because the amount of binder in the negative electrode was small and the effect of the binder was reduced.
On the other hand, in the battery of Example 10 in which the content of the binder raw material in the negative electrode mixture was 40 parts by weight per 100 parts by weight of the negative electrode active material, the initial capacity was lowered. This is presumably because the amount of the binder in the obtained negative electrode was excessive and the amount of the negative electrode active material was relatively reduced.
In Examples 1 and 7 to 9, a high-capacity non-aqueous electrolyte secondary battery having excellent cycle characteristics was obtained. Therefore, the content of the binder raw material in the negative electrode mixture was 0.00 by weight per 100 parts by weight of the negative electrode active material. 5 to 30 parts by weight is preferred.

<< Examples 11 to 14 and Comparative Example 4 >>
Table 4 shows the content per 100 parts by weight of the binder material (polyamic acid and polyacrylic acid) of the polyamic acid in the negative electrode mixture without changing the content of the binder raw material in the negative electrode mixture at the time of preparing the negative electrode. The value was changed variously. Except for this, a coin-type battery was prepared and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 4 together with the results of Example 1.

  In the battery of Example 11 in which the polyacrylic acid content in the binder raw material was 5.0 parts by weight per 100 parts by weight of the binder raw material, the cycle characteristics and the low temperature characteristics were deteriorated. This is presumably because the content of polyamic acid, which is a polyimide precursor, is small, and the effect of polyimide is reduced.

On the other hand, in the battery of Comparative Example 4 in which the polyamic acid content in the binder raw material was 100 parts by weight per 100 parts by weight of the binder raw material, the low-temperature characteristics were significantly lowered. This is presumably because there was no polyacrylic acid amount preferentially bonded to the Ti—Si alloy over the polyimide, and the polyimide was strongly bonded to the Ti—Si alloy.
In Examples 1 and 12 to 14, a non-aqueous electrolyte secondary battery excellent in low temperature characteristics and cycle characteristics was obtained. Therefore, the polyamic acid content in the negative electrode mixture was 10 to 95 weights per 100 parts by weight of the binder raw material. Part is preferred.

<< Examples 15 to 22 >>
Powder (high purity) of transition metal M (M is Zr, Ni, Cu, Fe, Mo, Co, or Mn) so that the proportion of the Si phase that is the A phase in the negative electrode active material particles is 30% by weight. Chemical Co., Ltd., purity 99.99%, particle size less than 20 μm) and Si powder (Kanto Chemical Co., Ltd., purity 99.999%, particle size less than 20 μm) were mixed. The mixing weight ratio of the transition metal M and Si is Zr: Si = 43.3: 56.7, Ni: Si = 35.8: 64.2, Cu: Si = 37.2: 62.8, Fe, respectively. : Si = 34.9: 65.1, Mo: Si = 44.2: 55.8, Co: Si = 35.8: 64.2, and Mn: Si = 34.6: 65.4.

Then, the mixed powder was put into a vibration mill container, and further a stainless ball (diameter 2 cm) was added so as to occupy 70% by volume of the capacity of the mill container. After evacuating the inside of the container, the inside of the container was replaced with Ar (Nihon Oxygen Co., Ltd., purity 99.999%) until the inside of the container reached 1 atm. Thereafter, mechanical alloying was performed for 60 hours while applying 60 Hz vibration to obtain an M-Si alloy.
As a result of X-ray diffraction measurement of the obtained M-Si alloy powder, it was confirmed that Si single phase and MSi ₂ phase exist in the alloy particles. Further, as a result of observing this alloy material with a transmission electron microscope (TEM), it was confirmed that an amorphous or Si phase having a crystal of about 10 nm and an MSi ₂ phase having a crystal of about 15 to 20 nm were present.

A negative electrode mixture was obtained in the same manner as in Example 1 except that M-Si alloy powder or the Si powder was used instead of Ti-Si alloy powder. At this time, the weight ratio of the M-Si alloy powder or the above-mentioned Si powder, graphite powder, polyamic acid, and polyacrylic acid in the negative electrode mixture was 100: 20: 5.0: 5.0.
Except for the above, a coin-type battery was produced and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 5 together with the results of Example 1.

In the batteries of Example 1 and 15 to 21, excellent low temperature characteristics were obtained. A transition metal oxide is formed on the surface of the negative electrode active material. Since a hydroxyl group (—OH group) exists on the surface of the transition metal oxide, a hydrogen bond is formed with polyacrylic acid having a carboxyl group (—COOH group). Thereby, polyacrylic acid can couple | bond with an M-Si alloy preferentially rather than a polyimide.
In addition, in the batteries of Examples 1 and 15 to 21 using the Si alloy containing a transition metal as the negative electrode active material, excellent cycle characteristics and low temperature characteristics were obtained as compared with the battery of Example 22 using Si alone. It was.

  The cause of the above result is estimated as follows. The main factor of cycle deterioration when the negative electrode active material contains Si is a decrease in the current collecting property of the negative electrode accompanying charge / discharge. In other words, the expansion / contraction of the active material particles generated when lithium is inserted / extracted reduces the contact between the active material particles and the current collector and between the active material particles, thereby destroying the electron conduction network of the negative electrode. As a result, the resistance of the negative electrode increases. However, in the case of using the Si alloy, it is considered that such a decrease in the current collecting property of the negative electrode is suppressed as compared with the case of using Si alone.

  The non-aqueous electrolyte secondary battery of the present invention has a high capacity and is excellent in cycle characteristics and low-temperature characteristics. Therefore, the non-aqueous electrolyte secondary battery is suitably used as a main power source and a memory backup power source for various electronic devices such as mobile phones and digital cameras. Used.

It is a longitudinal cross-sectional view of an example of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

11 Positive electrode can 12 Positive electrode 13 Separator 14 Negative electrode 15 Gasket 16 Negative electrode can

Claims (12)

Including an active material containing Si, a binder, and a conductive material;
The binder is obtained by heating a mixture containing polyamic acid and polyacrylic acid at 150 to 300 ° C., by the heating, the polyamic acid, Ri Contact is imidized,
The conductive material is a negative electrode for a non-aqueous electrolyte secondary battery, which is a carbon material.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the heating is performed at 200 to 300 ° C.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 2, wherein the imidization ratio of the polyamic acid is 80% or more. The negative electrode active material comprises a first phase containing Si and a second phase containing a transition metal silicide,
2. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein at least one of the first phase and the second phase is at least one of an amorphous state and a low crystalline state.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the transition metal is at least one selected from the group consisting of Ti, Zr, Ni, Cu, Fe, and Mo. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the silicide of the transition metal is TiSi ₂ .   A non-aqueous electrolyte secondary battery comprising the negative electrode according to claim 1, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. (1) a step of obtaining a negative electrode mixture by heating and drying after mixing an active material containing Si, a binder raw material solution containing polyamic acid and polyacrylic acid, and a carbon material as a conductive material;
After the negative electrode mixture is pressure-molded to obtain a pellet, the pellet is heated at 150 to 300 ° C. to imidize the polyamic acid to obtain a polyimide, and a negative electrode containing polyimide and polyacrylic acid as a binder. A method for producing a negative electrode for a non-aqueous electrolyte secondary battery, comprising the step (2).   The method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 8, wherein the heating temperature of the pellet in the step (2) is 200 to 300 ° C.   The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to claim 8, wherein the imidization ratio of the polyamic acid in the step (2) is 80% or more.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 8, wherein the total content of the polyamic acid and the polyacrylic acid in the negative electrode mixture is 0.5 to 30 parts by weight per 100 parts by weight of the active material. Manufacturing method.   The production of a negative electrode for a nonaqueous electrolyte secondary battery according to claim 8, wherein the content of the polyamic acid in the negative electrode mixture is 10 to 95 parts by weight per 100 parts by weight of the total of the polyamic acid and the polyacrylic acid. Method.

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