JP2009176433A - Method of manufacturing negative active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery using a negative active material making containing of an additive in an electrolyte unnecessary, preventing decomposition of an electrolyte solvent and insertion of an electrolyte solvent molecule into between negative active material layers in the initial charge, minimizing loss in charge discharge (insertion and releasing of lithium) reaction, and having high charge discharge efficiency. <P>SOLUTION: The method of manufacturing the negative active material for the nonaqueous electrolyte secondary battery is that the negative active material made of a material capable of absorbing and releasing lithium is mixed with an aqueous solution containing an alkali metal ion, a reducing agent, and a reducing polymerization material, and a passive layer is formed on the surface of the negative active material. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a negative electrode for a non-aqueous electrolyte secondary battery in which a high charge / discharge rate is improved by forming a passivation layer on the surface of a negative electrode active material.

Since the nonaqueous electrolyte secondary battery has a high energy density, it can be reduced in size and weight as compared with a lead storage battery, a nickel cadmium battery, a nickel hydrogen battery, and the like. For these reasons, non-aqueous electrolyte secondary batteries have been mainly used in portable devices such as mobile phones and notebook computers.

A typical configuration of a non-aqueous electrolyte secondary battery includes a carbon material as a negative electrode active material, a lithium transition metal oxide such as lithium cobalt oxide as a positive electrode active material, and an organic solvent such as ethylene carbonate and diethyl carbonate as an electrolytic solution. include those with a Li salt such as lithium hexafluorophosphate (LiPF _6). The material of each of the negative electrode, the positive electrode, and the electrolyte of the battery is a battery that can take a very large number of configurations as long as it can move lithium ions and can be charged and discharged by charge transfer.

In addition to LiPF ₆ , a fluorine-based complex salt such as LiBF ₄ or a salt such as LiN (SO ₂ Rf) ₂ .LiC (SO ₂ Rf) ₃ (where Rf = CF ₃ , C ₂ F ₅ ) is also used as the lithium salt. . Also, in order to ensure high electrical conductivity and safety, the electrolytic solution is usually a cyclic carbonate ester high dielectric constant / high boiling point solvent such as ethylene carbonate, propylene carbonate, and low viscosity solvent dimethyl carbonate, ethyl methyl carbonate, A lower chain carbonate such as diethyl carbonate may be used, and a lower fatty acid ester may be partially used.

When graphite or carbon material is used as the negative electrode active material, the charging / discharging mechanism is lithium insertion / extraction reaction between layers. During the first charge, an electrolyte decomposition reaction occurs on the surface of the negative electrode, and a passive film having no electron conductivity called a solid electrolyte interface (hereinafter referred to as “SEI”) film is formed on the surface. The negative electrode active material on which the SEI film is formed undergoes lithium ion insertion / desorption reaction through the SEI film.

In addition, when the SEI film is formed, the non-aqueous electrolyte secondary battery has electrolyte solvent molecules (for example, ethylene carbonate) interposed between layers of the negative electrode active material (graphite or carbon material) simultaneously with the insertion of lithium. It is generally known that since the insertion may cause the negative electrode active material to collapse, the initial charge / discharge efficiency is reduced, that is, the battery performance is reduced.

  Therefore, as a method for improving the battery performance of the nonaqueous electrolyte secondary battery, the cathode, the anode containing a carbon material having a crystallinity> 0.8, the first solvent having a high dielectric constant, and the low viscosity are used. A lithium storage battery comprising a mixture of at least two aprotic organic solvents containing a second solvent and an electrolyte comprising a lithium salt, wherein the electrolyte contains at least one unsaturated bond and is a passivating layer In order to form a lithium storage battery (Patent Document 1) further containing a soluble compound of the same type as at least one of the solvents, which can be reduced at the anode at a potential higher by 1 V than lithium.

JP-A-8-45545

In the method described in Patent Document 1, a mixture of at least two aprotic organic solvents including a first solvent having a high dielectric constant and a second solvent having a low viscosity and an electrolyte solution composed of a lithium salt are obtained. The soluble compound added to the electrolyte at the initial charge of the battery has a role of reducing at a potential higher than the insertion potential of solvated lithium ions. The soluble compound is a method of forming an SEI film on the negative electrode active material (carbon material) before insertion of lithium by reduction, and the physical property of preventing the insertion of solvent molecules around lithium ions by forming the SEI film. It is possible to improve the battery performance by constructing a barrier and invading carbon with lithium ions alone.

However, in the method described in Patent Document 1, vinylene carbonate (referred to as a reduced polysynthetic material in the present invention) is decomposed as an additive prior to insertion of solvent molecules (for example, ethylene carbonate), and the negative electrode active A SEI film is formed on the surface of the material to inhibit the insertion of solvent molecules, and the capacity consumed for the decomposition of the additive causes irreversible capacity, making it difficult to obtain sufficient charge / discharge efficiency.

  In recent years, as the application field of non-aqueous electrolyte secondary batteries has expanded to automobiles and industrial fields, the demand for batteries has increased, and high-energy density, high-power batteries are expected to be developed while maintaining high safety. It does not lead to meeting current demands.

  Therefore, as a result of various studies, the inventors mixed the negative electrode active material, the alkali metal ion, the reducing agent, and the aqueous solution containing the reductive polymerizable material to form an SEI film on the surface of the negative electrode active material. Knowing that it is possible to form an SEI film at a stage prior to the charging of the battery, and that it is possible to improve the battery performance such as the initial charge and discharge efficiency of the non-aqueous electrolyte secondary battery It was.

The method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery according to the present invention includes a negative electrode active material made of a material capable of occluding and releasing lithium, an aqueous solution containing an alkali metal ion, a reducing agent, and a reductive polymerizable material. And a passivation layer is formed on the surface of the negative electrode active material.

The method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is characterized in that the negative electrode active material is graphite or a carbon material.

The non-aqueous electrolyte secondary battery according to the present invention has a positive electrode capable of occluding and releasing lithium ions, a negative electrode, and a non-aqueous electrolyte, and the negative electrode active material obtained by the above-described manufacturing method. Features.

  According to the present invention, it is not necessary to contain an additive in the electrolytic solution, and decomposition of the electrolytic solution solvent and insertion of electrolytic solution solvent molecules between the negative electrode active material layers can be suppressed in the first charge. As a result, the loss due to the charge / discharge (lithium insertion / desorption) reaction is minute, and it is possible to obtain a non-aqueous electrolyte secondary battery using a negative electrode active material having high charge / discharge efficiency.

As the negative electrode active material used in the present invention, naturally produced graphite is processed, amorphous carbon obtained by artificially firing organic raw materials at 2000 ° C. or lower, and organic raw materials are artificially formed at 2000 ° C. or higher. It is possible to use an artificial graphite-based carbon material having a flat potential characteristic that is baked at a high temperature and has a developed graphite structure.

Alkali metal (Li, Na, K, Rb, Cs) halides, carbonates, oxalates, nitrates, sulfates, and the like can be used as the alkali metal ion supply material used in the present invention.
In addition, by including an alkali metal halide in the negative electrode active material, it is possible to expand the interlayer of graphite or carbon material amount, and no extra resistance is generated in and out of lithium ions, leading to an improvement in output. It is possible.

As the reducing agent used in the present invention, ascorbic acid, sodium ascorbate, erythorbic acid, sodium erythorbate, oxalic acid, sulfite, hydrazine sulfate, hydrazine and the like are used.

As the reductive polymerizable material used in the present invention, vinylene carbonate (VC), derivatives thereof, acrylic acid esters, and the like are used.
Here, the reduction polymerizable material referred to in the present invention may be any material that can form a passivation layer on the surface of a carbon material or the like by a reducing action.

In the present invention, as a method of forming a passive film called SEI film on the surface of the negative electrode active material, for example, carbon powder is dispersed in alkali metal ions, and then reduced to passivate on the surface of the active material. An aqueous solution in which a reducing polymerizable material capable of forming a chemical layer is dissolved and a reducing agent are added and mixed and stirred. Then, the method of removing water | moisture content by filtration etc. and drying is mentioned.

The binder used for the production of the negative electrode includes natural rubber (NR), styrene / butadiene rubber (SBR), butadiene rubber (BR), acrylonitrile / butadiene copolymer rubber (NBR), and methyl methacrylate / butadiene copolymer. Rubber (MBR), chloroprene rubber (CR), acrylic rubber (ABR), styrene butadiene / styrene copolymer (SBS), butyl rubber (IIR), thiocol, urethane rubber, silicon rubber, fluoro rubber, acrylic ester resin emulsion, etc. A water-dispersed emulsion resin may be mentioned.
When using a binder selected from these, water-soluble methylcellulose, carboxymethylcellulose (CMC), carboxymethylcellulose sodium salt, carboxymethylcellulose lithium salt, polyvinyl alcohol, polyvinylpyrrolidone, sodium polyacrylate as a thickener , Polyacrylic acid, polyethylene glycol, polyethylene oxide, or a combination thereof may be used.
As the binder, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or the like can be used.

As an electrolytic solution used in the present invention, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCl, LiBr, LiB [C ₆ H ₅ ] ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , At least one of LiOSO ₂ CF _{3 and the} like as an organic solvent, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 3 methyl-γ-butyrolactone, acetyl-γ-butyrolactone, γ-valerolactone, tetrahydrofuran, alkyl Tetrahydrofuran, dialkyltetrahydrofuran, alkoxytetrahydrofuran, dialkoxytetrahydrofuran, 1,3-dioxolane, alkyl-1,3-dioxolane, 1,4-dioxolane, 1, 2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, ethylene glycol dialkyl ether, diethylene glycol dialkyl ether, triethylene glycol dialkyl ether, tetraethylene glycol dialkyl ether, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, alkyl propionate It is possible to use those dissolved in at least one kind of solvent such as ester, dialkyl malonate, alkyl acetate and the like.

An embodiment of the present invention will be described below. In addition, this invention is not limited only to a following example.

  First, 10 g of graphite powder as a negative electrode active material was dispersed in 20 g of a 0.5 mass% sodium chloride (NaCl) aqueous solution as alkali metal ions, and then a 0.5 mass% vinylene carbonate (VC) aqueous solution as a reductive polymerizable material. 150 g and 300 g of a 0.5% by mass ascorbic acid aqueous solution as a reducing agent are added and stirred for 30 minutes to act. Thereafter, suction filtration was performed, and vacuum drying was performed at 100 ° C. for 3 hours to obtain Sample A.

  Next, styrene-butadiene rubber (SBR) as a binder and carboxymethylcellulose sodium (CMC) (negative electrode active material: SBR: CMC = 98: 1: 1) as a binder to Sample A obtained by the above method. The negative electrode active material mixture made into a slurry by mixing and stirring is applied to a copper foil current collector having a thickness of 10 μm, dried, and then rolled by a roller press to homogenize the negative electrode active material mixture. The negative electrode was obtained.

The negative electrode and lithium metal are used as a counter electrode and a reference electrode, and a mixed solvent of ethylene carbonate + diethyl carbonate (ethylene carbonate: diethyl carbonate = 1: 1) in which 1 mol of LiPF ₆ is dissolved is used as the electrolyte. Then, a beaker cell was produced (Invention 1).

In addition, after preparing Sample A in the same manner as in Example 1, a negative electrode active material mixture was prepared to prepare a negative electrode, and then a mixed solvent of ethylene carbonate + diethyl carbonate in which 1 mol of LiPF ₆ was dissolved in an electrolytic solution (ethylene carbonate: 1% by weight of vinylene carbonate was further added to diethyl carbonate = 1: 1) to prepare a beaker cell (Invention 2).

(Comparative Example 1)
A negative electrode active material mixture was prepared in the same manner as in Example 1 except that 10 g of graphite powder was used as the negative electrode active material without using an alkali metal ion, a reductive polymerizable material, and a reducing agent, and then a negative electrode was prepared. A beaker cell was prepared in the same manner as in Example 1 (Comparative Example 1).
(Conventional example)
A negative electrode active material mixture was prepared in the same manner as in Example 1 except that 10 g of graphite powder was used as the negative electrode active material without using an alkali metal ion, a reductive polymerizable material, and a reducing agent. 1% by weight of vinylene carbonate was further added to a mixed solvent of ethylene carbonate + diethyl carbonate (ethylene carbonate: diethyl carbonate = 1: 1) in which 1 mol of LiPF ₆ was dissolved in a beaker cell (conventional example 1).

In order to evaluate the characteristics of various negative electrodes produced, tests on the initial charge / discharge efficiency and the capacity retention rate at the 30th cycle were performed.
As test conditions, charging (Li insertion) and discharging (Li desorption) were performed at a constant current of 0.5 C rate within a range of 0.0 to 1.5 V on the basis of Li / Li ⁺ .
The initial charge / discharge efficiency is the ratio of the initial discharge capacity to the initial charge capacity, and the capacity maintenance ratio is the ratio of the discharge capacity at the 30th cycle when the initial discharge capacity is 100%.
Table 1 shows the initial charge and discharge efficiency and the capacity retention rate at the 30th cycle when charging and discharging at the 0.5 C rate.

As shown in Table 1, the present invention 1 and 2 using the negative electrode active material in which the negative electrode active material was mixed with an aqueous solution containing an alkali metal ion, a reducing agent, and a reduction polymerizable material, and the initial charge and discharge efficiency and capacity were The maintenance rate was excellent. This is because the VC added to the negative electrode active material itself before the charge / discharge test suppresses the collapse of the graphite structure due to the electrolyte solvent molecules, the decomposition of the electrolyte during the first charge, and the VC added to the electrolyte This is considered to suppress decomposition.
However, Comparative Example 1 using the negative electrode active material not subjected to these treatments was inferior in both initial charge / discharge efficiency and capacity retention rate. In addition, in Conventional Example 1 in which a reduction polymerizable material is added to the electrolytic solution, it is considered that the initial charge / discharge efficiency is reduced because the capacity consumed for the decomposition of the additive causes the irreversible capacity.
Even when this negative electrode is combined with a positive electrode using a lithium transition metal oxide such as lithium cobalt oxide as an active material to form a non-aqueous electrolyte secondary battery, the initial charge / discharge efficiency and capacity retention rate are also excellent. Is obtained.

Claims (3)

  Mixing a negative electrode active material made of a material capable of occluding and releasing lithium with an aqueous solution containing an alkali metal ion, a reducing agent, and a reductive polymerizable material to form a passivation layer on the surface of the negative electrode active material; A method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery.   The method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is graphite or a carbon material.   A nonaqueous electrolytic solution comprising a positive electrode capable of inserting and extracting lithium ions, a negative electrode, and a nonaqueous electrolytic solution, wherein the negative electrode has a negative electrode active material obtained by the method according to claim 2. Secondary battery.