WO2014171053A1

WO2014171053A1 - Binder for negative electrodes of secondary batteries, negative electrode of secondary battery, and lithium ion secondary battery

Info

Publication number: WO2014171053A1
Application number: PCT/JP2014/001210
Authority: WO
Inventors: 佑介杉山; 加代子湯川; 敬史毛利; 合田　信弘; 村瀬　正和
Original assignee: 株式会社豊田自動織機
Priority date: 2013-04-17
Filing date: 2014-03-05
Publication date: 2014-10-23
Also published as: JP5725075B2; JP2014211977A

Abstract

A binder which is composed of a reaction product between a polymer that has a carboxyl group in at least some side chains and a compound of a metal element that is selected from among copper (Cu), nickel (Ni) and cobalt (Co) is used together with a negative electrode active material for the formation of a negative electrode active material layer. By using this binder, the initial efficiency and the capacity retention rate after a cycle test of a secondary battery are improved.

Description

Binder for Secondary Battery Anode, Secondary Battery Anode, and Lithium Ion Secondary Battery

The present invention relates to a binder for a negative electrode used for a secondary battery such as a lithium ion secondary battery, a secondary battery negative electrode, and a lithium ion secondary battery.

The lithium ion secondary battery is a secondary battery that has a high charge / discharge capacity and can achieve high output. Currently, it is mainly used as a power source for portable electronic devices, and is further expected as a power source for electric vehicles expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and removing lithium (Li) on a positive electrode and a negative electrode, respectively. Then, it operates by moving lithium ions in the electrolytic solution provided between both electrodes.

In lithium ion secondary batteries, lithium-containing metal complex oxides such as lithium cobalt complex oxide are mainly used as the active material of the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material of the negative electrode There is. The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode and the electrolyte that constitute the secondary battery. Above all, research and development of active material materials that form active materials are actively conducted. For example, silicon or silicon oxide having a higher capacity than carbon is being studied as a negative electrode active material.

By using silicon as the negative electrode active material, a battery with a higher capacity than using a carbon material can be obtained. However, silicon has a large volume change associated with absorption and release of Li during charge and discharge. In addition, it is known that silicon itself is pulverized by repeating occluding and releasing. Therefore, there is a problem that silicon is pulverized and is separated or separated from the current collector, and the charge and discharge cycle life of the battery is short. In order to address this problem, it has been studied to suppress the volume change associated with the storage and release of Li during charge and discharge by using silicon oxide as the negative electrode active material.

For example, use of silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) is being studied as a negative electrode active material. It is known that, when heat treated, SiO _x segregates Si in SiO ₂ in particles. This is called disproportionation reaction, and it is separated into two phases of Si phase and SiO ₂ phase by internal reaction of solid. The Si phase obtained by separation is very fine. In addition, the SiO ₂ phase covering the Si phase has the function of suppressing the decomposition of the electrolytic solution. Therefore, a secondary battery using a negative electrode active material composed of SiO _x decomposed into Si and SiO ₂ is excellent in cycle characteristics.

As the silicon particles forming the Si phase of SiO _x are finer, the cycle characteristics of the secondary battery using it as the negative electrode active material are improved. Thus, Japanese Patent No. 3865033 (Patent Document 1) describes a method of heating and subliming metal silicon and SiO ₂ into silicon oxide gas and cooling it to produce SiO _x . According to this method, the particle size of silicon particles constituting the Si phase can be made into nanosize of 1 to 5 nm.

JP-A 2009-102219 (patent document 2) decomposes a silicon raw material into an elemental state in high temperature plasma, and rapidly cools it to liquid nitrogen temperature to obtain silicon nanoparticles, and then this silicon nanoparticles are A manufacturing method of fixing in a SiO ₂ -TiO ₂ matrix by a sol-gel method or the like is described.

However, in the manufacturing method described in Patent Document 1, the matrix is limited to a sublimable material. Therefore, since the matrix is limited to SiO ₂ , the matrix and lithium react irreversibly at the time of lithium storage. Such a reaction is a major cause of cell capacity reduction. Further, the manufacturing method described in Patent Document 2 requires high energy for plasma discharge. Furthermore, in the silicon composite obtained by these manufacturing methods, there is a problem that the dispersibility of silicon particles in the Si phase is low and aggregation is easy. When the Si particles aggregate and the particle size becomes large, the secondary battery using it as the negative electrode active material has a low initial capacity, and the cycle characteristics also deteriorate.

By the way, in recent years, nano silicon materials which are expected to be used in various fields such as semiconductors, electricity and electrons have been developed. For example, Physical Review B (1993), vol 48, 8172-8189 (Non-patent Document 1) describes a method of synthesizing layered polysilane by reacting hydrogen chloride (HCl) and calcium disilicide (CaSi ₂ ). The layered polysilane thus obtained is described to be applicable to light emitting devices and the like. Since this material can be manufactured by a method other than sublimation method or plasma discharge, it is possible to solve problems such as irreversible capacity and initial efficiency decrease due to aggregation.

And JP-A-2011-090806 (Patent Document 3) describes a lithium ion secondary battery using layered polysilane as a negative electrode active material.

Patent No. 3865033 JP, 2009-102219, A JP, 2011-090806, A

However, the negative electrode active material composed of layered polysilane described in Patent Document 3 has a problem that it is not preferable as a negative electrode active material for a secondary battery because the BET specific surface area is large. For example, in the negative electrode of a lithium ion secondary battery, when the BET specific surface area is large, the irreversible capacity consumed by the negative electrode is increased to accelerate the decomposition of the electrolytic solution, and it is difficult to achieve high capacity. There is also a problem that SEI is likely to occur and the cycle characteristics are low.

Therefore, it was recalled that the layered polysilane described in Patent Document 3 is fired in a non-oxidizing atmosphere. According to this method, nanosilicon having a crystallite size of several nm can be obtained, so it is suitable as a negative electrode active material. However, a lithium ion secondary battery using nanosilicon manufactured by this method as a negative electrode active material has a problem that the initial efficiency is low and the capacity retention rate after the cycle test is low.

The present invention has been made in view of such circumstances, and it is an object of the present invention to solve this problem by improving the binder of the negative electrode.

The feature of the binder for a secondary battery negative electrode according to the present invention for solving the above problems is a polymer having a carboxyl group in at least a part of side chains, and at least one selected from copper (Cu), nickel (Ni) and cobalt (Co) It consists of a reaction product of a compound of one kind of metal element.

And, the feature of the secondary battery negative electrode of the present invention comprises a current collector and a negative electrode active material layer formed on the surface of the current collector, and the negative electrode active material layer comprises a negative electrode active material and a binder of the present invention And to include.

The binder for a secondary battery negative electrode according to the present invention comprises a polymer having a carboxyl group at least in part of a side chain, and a compound of at least one metal element selected from copper (Cu), nickel (Ni) and cobalt (Co) It consists of the reaction product of. By containing this metal element, the conductivity is improved as compared with a binder consisting only of a normal polymer, and hence the initial efficiency of the secondary battery is improved. Furthermore, since this metal element can change its valence, transfer of electrons occurs following the potential change in charge and discharge, and the secondary battery using the negative electrode of the present invention has a further improved initial efficiency and a post cycle operation. Efficiency also improves.

3 is a SEM image of the gray powder prepared in Example 1.

The binder for a secondary battery negative electrode according to the present invention comprises a polymer having a carboxyl group at least in part of a side chain, and a compound of at least one metal element selected from copper (Cu), nickel (Ni) and cobalt (Co) It consists of the reaction product of. As a polymer which has a carboxyl group in at least one part side chain, polyacrylic acid, polymethacrylic acid, polyaspartic acid, polyglutamic acid etc. are illustrated. Further, examples of compounds of metal elements selected from copper (Cu), nickel (Ni) and cobalt (Co) include acetates, nitrates, chlorides, fluorides, hydroxides and the like of the respective metal elements.

The compound of at least one metal element selected from copper (Cu), nickel (Ni) and cobalt (Co) is contained in an amount of 0.01 to 10 parts by mass with respect to 100 parts by mass of the polymer having a carboxyl group in at least a part of side chains. Is desirable. When the compound of the metal element is less than 0.01 parts by mass, it is difficult to exhibit the effect of containing the metal element, and when the compound of the metal element exceeds 10 parts by mass, the viscosity of the reactant of the polymer and the metal element becomes too high. It becomes difficult to prepare a slurry for forming the active material layer.

A salt compound, a complex compound, etc. are illustrated as a reaction product of the said polymer and the compound of the said metallic element.

In order to react the above-mentioned polymer and the compound of the above-mentioned metal element to obtain a reaction product, a method of mixing both and heating, a method of dissolving both the polymer and the compound of the metal element in a soluble solvent and heating Can be used. In order to obtain a uniform reactant at the molecular level, it is preferable to use a method of heating in a state in which both are mixed in a liquid state as in the latter method.

The binder for a secondary battery negative electrode of the present invention can be used together with known negative electrode active materials such as graphite, hard carbon, silicon, carbon fiber, tin (Sn), silicon oxide and the like. Among the negative electrode active materials, silicon oxides represented by SiO _x (0.3 ≦ x ≦ 1.6) or nanosilicon obtained by firing the layered polysilane described in Non-Patent Document 1 and Patent Document 3 is particularly preferable preferable.

The present inventors conducted intensive studies on the layered polysilanes described in Non-Patent Document 1 and Patent Document 3. Layered polysilanes can be obtained by reacting calcium disilicide (CaSi ₂ ) with an acid, such as an aqueous solution of hydrogen chloride (HCl). Calcium disilicide (CaSi ₂ ) forms a layered crystal in which a Ca atomic layer is inserted between the (111) faces of diamond-shaped Si, and layered polysilane by extracting calcium (Ca) in reaction with an acid. Is obtained.

Alternatively, a mixture of hydrogen fluoride (HF) aqueous solution and hydrogen chloride (HCl) aqueous solution can be used as the acid. When the molar ratio of hydrogen chloride (HCl) is 100, the molar ratio of hydrogen fluoride (HF) is preferably 100 or less. When the amount of hydrogen fluoride (HF) exceeds this ratio, impurities such as CaF ₂ and CaSiO are generated, and it is difficult to separate the impurities from the layered polysilane, which is not preferable.

The reaction ratio between the acid and calcium disilicide (CaSi ₂ ) is preferably in excess of the acid. Moreover, it is preferable to carry out reaction atmosphere under inert gas atmosphere. The reaction time and reaction temperature are not particularly limited, but the reaction temperature is usually 0 ° C. to 100 ° C., and the reaction time is 0.25 to 24 hours.

The nanosilicon obtained by the above reaction is aggregated to form aggregated particles. Therefore, pulverization of aggregated particles occurs due to repeated expansion and contraction at the time of charge and discharge as a power storage device, and there is a problem that the specific surface area is increased and the cycle characteristics are deteriorated due to the generation of SEI.

Therefore, it is particularly preferable to use a negative electrode active material which is composed of agglomerated particles made of nanosilicon and a carbon layer composed of amorphous carbon and covering at least a part of the agglomerated particles to form a composite. The carbon layer made of amorphous carbon covers at least a part of the aggregated particles. This carbon layer exerts an effect of reinforcing the agglomerated particles. Although a conductive support agent such as graphite, acetylene black and ketjen black may be used for the negative electrode, these carbons are crystalline and not amorphous. The thickness of the composite carbon layer covering the nanosilicon aggregate is preferably in the range of 1 to 100 nm, and more preferably in the range of 5 to 50 nm. When the thickness of the carbon layer is too thin, it is difficult to express the effect, and when the carbon layer is too thick, the initial capacity decreases.

In the case of forming a carbon layer, mixing amorphous carbon produced separately by some method with the agglomerated particles of nanosilicon is not only inhomogeneous, but also carbon covers at least a part of the agglomerated particles. Have difficulty. Therefore, a method has been developed for producing a homogeneous negative electrode active material by ensuring that amorphous carbon covers at least a part of aggregated particles. The manufacturing method includes an aggregation particle forming step of forming a structure in which a plurality of six-membered rings composed of silicon atoms are continuous and heat treating a layered polysilane represented by a composition formula (SiH) _n to obtain aggregated particles of nanosilicon; It is desirable to carry out in this order the polymerization step of polymerizing the aromatic heterocyclic compound in the state where the particles and the aromatic heterocyclic compound are mixed, and the carbonization step of carbonizing the polymer of the aromatic heterocyclic compound .

The process of forming the agglomerated particles of nanosilicon is as described above.

In the polymerization step, the aromatic heterocyclic compound is polymerized in a state in which the nanosilicon aggregate particles and the aromatic heterocyclic compound are mixed. As a result, a polymer of the aromatic heterocyclic compound in a state of being attached to the aggregated particles of nanosilicon is obtained. Here, aromatic heterocyclic compounds include five-membered aromatic heterocyclic compounds such as furan, pyrrole, thiophene, imidazole, pyrazole, oxazole, isoxazole, thiazole, and isothiazole, and polycyclic compounds such as indole, benzimidazole and benzofuran Polymerizable compounds such as aromatic heterocyclic compounds can be used.

Although various polymerization methods can be employed to polymerize these compounds, in the case of pyrrole and the like, the method of heating in the presence of a polymerization catalyst such as concentrated hydrochloric acid or iron trichloride is convenient. In particular, if iron trichloride is used, polymerization can be performed in a non-water atmosphere and oxidation of Si can be suppressed, so that an effect of increasing the initial capacity when used as a power storage device is obtained.

In the carbonization step, the polymer of the aromatic heterocyclic compound is carbonized in a state of being mixed with the agglomerated particles of nanosilicon. In this step, as in the production of nanosilicon, heat treatment may be performed at a temperature of 100 ° C. or more in an inert atmosphere, preferably heat treatment at 400 ° C. or more. Since the aromatic heterocyclic compound is a polymer, carbonization proceeds without being evaporated even when heated, and a complex in which a carbon layer made of amorphous carbon is bonded to the surface of the agglomerated particle of nanosilicon Is obtained. When the heat treatment is performed in a state where the aggregated particles of nanosilicon and the aromatic heterocyclic compound are mixed without performing the polymerization step, the aromatic heterocyclic compound is evaporated and carbonization is difficult.

In the negative electrode active material in which amorphous carbon covers at least a part of the aggregated particles, the composition ratio of silicon to carbon is desirably Si / C = 3/1 to 20/1 by weight. When this ratio exceeds 20/1, the effect of forming a carbon layer is not exhibited, and when it is less than 3/1, the capacity of the secondary battery decreases.
<Secondary battery negative electrode>

The secondary battery negative electrode of the present invention comprises a current collector and a negative electrode active material layer formed on the surface of the current collector, and the negative electrode active material layer comprises a negative electrode active material and a binder of the present invention. Including.

A current collector is a chemically inert electron conductor for keeping current flowing to an electrode during discharge or charge. The current collector may be in the form of a foil, a plate or the like, but is not particularly limited as long as it has a shape according to the purpose. For example, copper foil or aluminum foil can be suitably used as the current collector.

The negative electrode active material is as described above. In order to produce a negative electrode of a non-aqueous secondary battery using a negative electrode active material, a negative electrode active material powder, a conductive additive such as carbon powder, the binder of the present invention, and an appropriate amount of organic solvent are added and mixed The slurry can be prepared by applying the slurry onto the current collector by a method such as roll coating, dip coating, doctor blade method, spray coating, curtain coating or the like, and drying or curing the binder. .

The binder is required to bind the active material and the like in a small amount as much as possible, but the addition amount thereof is preferably 0.5 wt% to 50 wt% of the total of the active material, the conductive additive and the binder. If the binder is less than 0.5 wt%, the formability of the electrode is reduced, and if it exceeds 50 wt%, the energy density of the electrode is reduced.

As the binder, in addition to the binder of the present invention, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide imide (PAI), Carboxymethylcellulose (CMC), polyvinyl chloride (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), polyacrylic An acid (PAA) etc. can also be mixed.

A conductive aid is added to enhance the conductivity of the electrode. As conductive additives, carbon black fine particles such as carbon black, graphite, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), etc. may be used alone or in combination. Can be added. The use amount of the conductive aid is not particularly limited, but can be, for example, about 20 to 100 parts by mass with respect to 100 parts by mass of the negative electrode active material. If the amount of the conductive additive is less than 20 parts by mass, efficient conductive paths can not be formed, and if it exceeds 100 parts by mass, the formability of the electrode is deteriorated and the energy density is lowered.

There is no particular limitation on the organic solvent, and a mixture of plural solvents may be used. N-methyl-2-pyrrolidone and N-methyl-2-pyrrolidone and ester solvents (ethyl acetate, n-butyl acetate, butyl cellosolve acetate, butyl carbitol acetate etc.) or glyme solvents (diglyme, triglyme, tetraglyme etc.) Mixed solvents of are particularly preferred.

When the secondary battery is a lithium ion secondary battery, the negative electrode can also be pre-doped with lithium. In order to dope the negative electrode with lithium, for example, an electrode forming method in which a half cell is assembled using metallic lithium as a counter electrode and electrochemically dope lithium can be used. The doping amount of lithium is not particularly limited.

When the secondary battery is a lithium ion secondary battery, known positive electrodes, electrolytes and separators which are not particularly limited can be used. The positive electrode may be one that can be used in a lithium ion secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer contains a positive electrode active material and a binder, and may further contain a conductive aid. The positive electrode active material, the conductive additive and the binder are not particularly limited as long as they can be used in a lithium ion secondary battery.

Examples of the positive electrode active material include metal lithium, LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₃ , sulfur and the like. The current collector may be any one commonly used for a positive electrode of a lithium ion secondary battery, such as aluminum, nickel, stainless steel and the like. As the conductive additive, the same one as described in the above-mentioned negative electrode can be used.

The electrolytic solution is one in which a lithium metal salt which is an electrolyte is dissolved in an organic solvent. The electrolyte is not particularly limited. As an organic solvent, use is made of one or more selected from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Can. Further, as an electrolyte to be dissolved, a lithium metal salt soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ and LiCF ₃ SO ₃ can be used.

For example, lithium metal salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 and the} like in an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate or the like at a concentration of about 0.5 mol / L to 1.7 mol / L A dissolved solution can be used.

The separator is not particularly limited as long as it can be used for a lithium ion secondary battery. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

When the secondary battery is a lithium ion secondary battery, the shape thereof is not particularly limited, and various shapes such as a cylindrical shape, a laminated shape, and a coin shape can be adopted. In any of the shapes, the separator is interposed between the positive electrode and the negative electrode to form an electrode body, and the distance from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside is for current collection After connection using a lead or the like, the electrode body is sealed in a battery case together with an electrolytic solution to form a battery.

Hereinafter, embodiments of the present invention will be specifically described by examples and comparative examples.

<Preparation of Binder>
2 g of copper acetate was dissolved in 20 ml of pure water, and 10 ml of 0.5 N aqueous NaOH solution was added dropwise while stirring. After the entire amount has been added, the mixture is stirred for 2 hours, and the deposited precipitate is separated by filtration, washed twice with 10 ml of pure water, rinsed with 10 ml of acetone and then dried under vacuum for 3 hours to obtain blue basic copper acetate (see Formula 1) 1 g of powder was obtained.

Next, 0.8 g of a polyacrylic acid (see Formula 2) having a number average molecular weight of 500,000 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above-mentioned basic copper acetate powder is further dissolved in this solution, and kept at 60 ° C. for 3 hours while stirring for reaction to prepare an NMP solution of polyacrylic acid-basic copper acetate reactant shown in Formula 3. did. In the polyacrylic acid-basic copper acetate reactant, it is considered that basic copper acetate is bound to the carboxyl group of a part of the side chain of polyacrylic acid and has an acetyl group at its terminal.

20 ml of a mixed solution of 3 ml of a 46% strength by weight aqueous HF solution and 300 ml of a 36% strength by weight aqueous HCl solution at 0 ° C. in an ice bath and 5 g calcium disilicide (CaSi ₂ ) in an argon gas stream Was added and stirred. After confirming that foaming was completed, the temperature was raised to room temperature, and after stirring for another 2 hours at room temperature, 20 ml of distilled water was added and the mixture was further stirred for 5 minutes. At this time, yellow powder floated.

The resulting mixed solution was filtered and the residue was washed with 10 ml of distilled water and then with 10 ml of ethanol and dried under vacuum to obtain 5.5 g of layered polysilane. The layered polysilane was heat-treated at 500 ° C. for 1 hour in an argon gas with 1% by volume or less of O ₂ to obtain a powder of nanosilicon agglomerated particles. The powder was subjected to X-ray diffraction measurement (XRD measurement) using a CuKα ray. According to XRD measurement, a halo considered to be derived from Si fine particles was observed. The Si fine particles had a crystal grain size of about 7 nm calculated from Scherrer's equation from the half value width of the diffraction peak on the (111) plane as a result of X-ray diffraction measurement.

A 0.5 g of furan was vacuum impregnated for 3 hours to 1 g of this nanosilicon powder, and concentrated hydrochloric acid was added. After the addition of concentrated hydrochloric acid, the reaction mixture was treated at 60 ° C. for 3 hours to polymerize furan, filtered and washed to remove concentrated hydrochloric acid. The resulting powder was dried under vacuum for 3 hours and then calcined at 500 ° C. in argon gas to carbonize the furan polymer to obtain a gray powder. The yield of gray powder was 1.22 g per 1 g of nanosilicon powder. In the present example, the composition ratio of silicon to carbon was Si / C = 82/18 in weight ratio.

The SEM photograph of the obtained gray powder is shown in FIG. From FIG. 1, a composite structure is confirmed in which nanosilicon aggregate particles in the μm order are wrapped in a carbon layer with a maximum thickness of about 200 nm. Table 1 also shows the results of measurement of the specific surface areas of the gray powder and the nanosilicon powder used for producing the gray powder by the BET method.

It can be seen that the specific surface area is reduced by coating the nanosilicon agglomerated particles with a carbon layer.

The gray powder was subjected to X-ray diffraction measurement (XRD measurement) using a CuKα ray. As a result, the gray powder did not show a peak at 2θ = 26 ° (crystalline carbon peak) present in acetylene black, and it was found that carbon contained in the gray powder was amorphous. Further, it was also found from the half value width that the particle size of Si in the gray powder is 10 nm or less.
Preparation of Lithium Ion Secondary Battery

45 parts by mass of the obtained gray powder, 40 parts by mass of natural graphite powder, 5 parts by mass of acetylene black, and 10 parts by mass of an NMP solution of the polyacrylic acid-basic copper acetate compound obtained above are mixed. A slurry was prepared. The slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of about 20 μm using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely bonded by a roll press. This was vacuum dried at 100 ° C. for 2 hours to form a negative electrode having a thickness of 16 μm of the negative electrode active material layer.

A lithium ion secondary battery (half cell) was produced using the negative electrode produced according to the above procedure as an evaluation electrode. The counter electrode was a metal lithium foil (thickness 500 μm).

The counter electrode was cut into a diameter of 13 mm, and the evaluation electrode was cut into a diameter of 11 mm, and a separator (a glass filter made by Hoechst Celanese and "Celgard 2400" made by Celgard) was interposed therebetween to obtain an electrode body battery. The electrode battery was housed in a battery case (CR2032 type coin battery member, manufactured by Takasen Co., Ltd.). In the battery case, a non-aqueous electrolytic solution in which LiPF ₆ is dissolved at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate mixed at 1: 1 (volume ratio) is injected, and the battery case is sealed to make lithium ion. I got a secondary battery.

<Preparation of Binder>
2 g of copper acetate was dissolved in 20 ml of pure water, and while stirring, 20 ml of 0.5 N aqueous NaOH solution was dropped. After the entire amount is added, the solution is stirred for 2 hours, and the deposited precipitate is separated by filtration, washed twice with 10 ml of pure water, rinsed with 10 ml of acetone, and dried under vacuum for 3 hours to obtain a powder of copper hydroxide (see Formula 4). I got 1.5 g. The color tone of this powder was more greenish blue than the basic copper acetate powder of Example 1.

Next, 0.8 g of polyacrylic acid as in Example 1 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above-mentioned copper hydroxide powder was further dissolved in this solution and reacted at 60 ° C. with stirring to prepare an NMP solution of polyacrylic acid-copper reactant represented by Chemical Formula 5. The polyacrylic acid-copper reactant is considered to have a structure in which two molecules of polyacrylic acid are crosslinked by copper (Cu).

Preparation of Lithium Ion Secondary Battery

A lithium ion secondary battery was obtained in the same manner as Example 1, except that the NMP solution of the polyacrylic acid-basic copper acetate reactant was replaced with the same amount of the NMP solution of the polyacrylic acid-copper reactant. .

<Preparation of Binder>
2.1 g of nickel acetate was dissolved in 20 ml of pure water, and 10 ml of 0.5 N aqueous NaOH solution was added dropwise while stirring. After the entire amount is added, the solution is stirred for 2 hours, and the deposited precipitate is separated by filtration, washed twice with 10 ml of pure water, rinsed with 10 ml of acetone, and dried under vacuum for 12 hours to obtain basic nickel acetate [Ni (OH) (OOCCH ₃ ) Obtained 1.2 g of a powder of].

Next, 0.8 g of polyacrylic acid as in Example 1 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above-mentioned basic nickel acetate powder is further dissolved in this solution, and the mixture is reacted while maintaining it at 60 ° C. for 3 hours while stirring to prepare an NMP solution of polyacrylic acid-basic nickel acetate reactant shown in Formula 6 did.

Preparation of Lithium Ion Secondary Battery

A lithium ion secondary battery is prepared in the same manner as in Example 1, except that the NMP solution of the polyacrylic acid-basic copper acetate reactant is replaced with the same amount of the NMP solution of the polyacrylic acid-basic nickel acetate reactant. I got
Comparative Example 1

NMP of polyacrylic acid in which 0.8 g of polyacrylic acid similar to Example 1 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP) instead of NMP solution of polyacrylic acid-basic copper acetate compound A lithium ion secondary battery was obtained in the same manner as in Example 1 except that the same amount of solution was used.
<Battery characteristic test 1>

With respect to the lithium ion secondary batteries of Examples 1 to 3 and Comparative Example 1, the initial charge capacity when charged under the conditions of a temperature of 25 ° C. and a current of 0.2 mA was measured, and the results are shown in Table 2. In addition, the initial discharge capacity at the time of discharging at a current of 0.2 mA was measured, and the results are shown in Table 2. Furthermore, the initial efficiency (initial charge capacity / initial discharge capacity) was calculated, and the results are shown in Table 2.

In addition, the lithium ion secondary batteries of Examples 1 to 3 and Comparative Example 1 are charged to 1 V under conditions of a temperature of 25 ° C. and a current of 0.2 mA, and after rest for 10 minutes, 0.01 V under the conditions of 0.2 mA of current A cycle test was performed by repeating the cycle of 10 cycles of discharging to 10 minutes and stopping for 10 minutes. Then, the capacity maintenance ratio, which is the ratio of the charge capacity at the 10th cycle to the charge capacity at the first cycle, and the coulombic efficiency, which is the ratio of the discharge capacity after 10 cycles to the charge capacity after 10 cycles, are measured. Shown in.

The lithium ion secondary batteries of Examples 1 to 3 all have an improved initial efficiency compared to Comparative Example 1. This is considered to be due to the efficient transfer of electrons due to the change in valence of copper or nickel. Moreover, although the lithium ion secondary battery of Example 2 has the highest initial efficiency, other battery characteristics are low. This is considered to be due to the occurrence of aggregation and the uneven distribution of the binder because the main chains of the polyacrylic acid are cross-linked by copper as shown in Formula 5. Therefore, when importance is placed on the cycle characteristics, as the compound of the metal element used for producing the binder of the present invention, basic acetate is more preferable than hydroxide, and one having an acyl group at the end of the side chain is preferable. .

The secondary battery negative electrode of the present invention can be used for a secondary battery, an electric double layer capacitor, a lithium ion capacitor, and the like. In addition, the lithium ion secondary battery of the present invention is useful as a non-aqueous secondary battery used for driving motors of electric vehicles and hybrid vehicles, personal computers, portable communication devices, home appliances, office devices, industrial devices, etc. In particular, it can be suitably used for driving a motor of an electric car or a hybrid car that requires a large capacity and a large output.

Claims

It is characterized by comprising a reactant of a polymer having a carboxyl group in at least a part of side chain and a compound of at least one metal element selected from copper (Cu), nickel (Ni) and cobalt (Co). Binder for secondary battery negative electrode.
The binder for a secondary battery negative electrode according to claim 1, wherein the polymer is polyacrylic acid.
The binder for a secondary battery negative electrode according to claim 1, wherein the reactant has an acyl group at an end of the side chain.
The binder for a secondary battery negative electrode according to any one of claims 1 to 3, wherein the metal element is contained in an amount of 0.01 to 10 parts by mass with respect to 100 parts by mass of the polymer.
A current collector and a negative electrode active material layer formed on the surface of the current collector, the negative electrode active material layer comprising a negative electrode active material, and the binder according to any one of claims 1 to 4. A secondary battery negative electrode characterized by including.
The negative electrode active material is an agglomerated particle made of nanosilicon produced by heat treating a layered polysilane represented by a composition formula (SiH) n without forming a structure in which a plurality of six-membered rings composed of silicon atoms are linked. 6. The secondary battery negative electrode according to item 5.
The secondary battery negative electrode according to claim 6, further comprising a carbon layer composed of amorphous carbon and covering at least a part of the aggregated particles.
8. The secondary battery negative electrode according to claim 7, wherein a composition ratio of silicon to carbon is Si / C = 3/1 to 20/1 by weight ratio.
A lithium ion secondary battery comprising the secondary battery negative electrode according to any one of claims 5 to 8.