JPH09161848A - Carbon electrode material for lithium secondary battery, method for producing the same, and lithium secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve ion conductivity and charging/discharging efficiency by coating carbon electrode material for a lithium secondary battery with a coat, insoluble in an aprotic organic solvent and having lithium ion conductivity. SOLUTION: Carbon electrode material for a lithium secondary battery, having a graphitization degree of 0.4 or more, is dipped into a solution, containing a compound, functioning as the matrix of one kind of more macromolecule solid electrolyte, at 0-60 deg.C, to adsorb the compound onto the surface of the carbon electrode material. Next, a crosslinking compound, such as a silane coupling agent, of 0.5-5wt.%, is added to the solution to crosslink the matrix at 0-150 deg.C to coat the surface of the carbon electrode material by a coat insoluble in an aprotic organic solvent and having a lithium ion conductivity of 0.5-10wt.%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a carbon electrode material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same.

[0002]

2. Description of the Related Art The need for batteries with high energy density is increasing more and more due to portable electronic devices and higher performance thereof. Among them is a lithium secondary battery. This uses two types of intercalation compounds having different redox potentials, each of which functions as a lithium ion carrier, for the positive and negative electrode materials.

For the negative electrode, graphite or a carbon material having a turbostratic structure is used. Among them, graphite materials are regarded as promising because of their particularly high discharge capacity and their potential flatness. (Japanese Patent Publication No. 62-23433, etc.) [Detailed Description of the Invention]

[0004]

However, although graphite has excellent charge / discharge characteristics, there is a significant reduction in irreversible capacity in the first cycle. Although the cause of this is not entirely clear, one of the causes has been reported to be the decomposition of the solvent or supporting electrolyte on the graphite surface. This decomposition reaction continues until the decomposition products are deposited on the carbon surface and grow to such a thickness that electrons cannot move directly from the surface to the solvent or the like. In the case of graphite, solvent molecules and lithium ions may be co-intercalated, the graphite layer may peel off and a new graphite surface may appear, which may react with the electrolytic solution, resulting in a large loss of irreversible capacity (Journal of Electrochemical Society, vol. 137, 2009 (1990)).

The decrease in irreversible capacity causes a decrease in energy density due to an excessive addition of a positive electrode agent for compensating for it, and an increase in internal resistance due to the coating film, so it is desirable to reduce it, but until now, it has been possible to reduce it. No effective means for it was found.

The present inventor has found that one method of solving this problem is to modify the carbon surface with a silane coupling agent, and as a result, the decomposition reaction on the carbon surface can be significantly suppressed ( Japanese Patent Application No. 7-19994
0).

However, it has been found that the ionic conductivity of the coating film formed at this time is not necessarily high, which may cause a large decrease in discharge capacity.

Under the above circumstances, the present invention further modifies the surface of the carbon electrode material to obtain a carbon electrode material for a lithium secondary battery, which is excellent in ionic conductivity and charge / discharge efficiency, a method for producing the same, and the same. It is intended to provide a lithium secondary battery.

[0009]

Means for Solving the Problems In order to solve the above problems, the present inventor has conducted diligent research on the surface modification of a carbon material for a negative electrode. As a result, the carbon electrode material is insoluble in an aprotic organic solvent, Moreover, they have found that coating with a lithium ion conductive film is effective and completed the present invention.

That is, the present invention provides (1) a carbon electrode material for a lithium secondary battery, which is coated with a film that is insoluble in an aprotic organic solvent and has a lithium ion conductivity, and (2) a carbon electrode. It is characterized in that at least one compound that functions as a matrix of a polymer solid electrolyte is adsorbed on the material, and then at least one crosslinkable compound is reacted therewith to coat the carbon electrode material with a film. A method for producing a carbon electrode material for a lithium battery according to claim 1, and (3) a lithium secondary battery using the carbon electrode material according to (1) as a negative electrode.

Since the film of the carbon electrode material of the present invention is insoluble in an aprotic solvent, it is stably retained on the electrode surface without being detached from the carbon electrode material even when it comes into contact with the electrolyte containing the solvent. . The film can suppress the decomposition reaction on the surface of the carbon electrode while maintaining the lithium ion conductivity, so that the charging / discharging efficiency of the lithium secondary battery using this electrode can be improved. .

The above-mentioned film may be made of a material that is insoluble in an aprotic solvent, is reliably held on the carbon electrode material, and has a function of conducting lithium ions. However, in the present invention, the coating material is preferably composed of a reaction product of a compound that functions as a matrix of the polymer solid electrolyte and a compound that crosslinks the compound.
The compound functioning as a matrix of the above-mentioned solid polymer electrolyte has a hydrophobic group in the molecule as a skeleton, and at least one end has a crosslinkable functional group,
For example, a polyether having a crosslinkable group at the terminal as shown by the following formula and polyacrylonitrile are preferable.

R ¹ O (EO) _m R ² , R ¹ O (PO) _m R ² ,
R ¹ O [(EO) _m (PO) _n ] R ² , R ¹ O [(EO)
_m (TFPO) _n ] R ² , R ¹ O [(TFPO) _m (EO) _n
(TFPO) _m ] R ² , R ³ [(OE) _m OR ² ] _p , R
³ [(OP) _n OR ² ] _p , R ³ [(OE) _m (OP) _n O
R ² ] _p , R ³ [(OE) _m (OTFPO) _n OR ² ] _p , R ¹
(CH ₂ CH (CN)) _m R ² [wherein R ¹ and R ² are hydrogen, an alkyl group having 1 to 16 carbon atoms (wherein R ¹ and R ² are not simultaneously an alkyl group), or an alkenyl Group, an acryloyl group, a methacryloyl group, a functional group that imparts crosslinkability to a polyether such as an epoxy-containing group, R ³ is an aliphatic hydrocarbon residue having 1 to 16 carbon atoms, E is an ethylene unit, P is a propylene unit,
TFPO is a trifluoropropylene oxide unit, m ≧
1, n ≧ 1, P = integer of 2 to 8] When R ¹ and R ² are alkenyl groups, allyl, crotyl, vinyl and the like are preferable. The alkenyl group may be substituted with an alkyl group, halogen, especially fluorine, and examples thereof include 2,2,3,4,4-perfluoro-3-butenyl and 3-perfluorooctyl-2-propenyl. preferable. Further, the aliphatic hydrocarbon residue of R ³ is a residue of a polyhydric alcohol, and preferable examples thereof include carbon hydrogen such as glycerin, pentaerythritol, trimethylolethane, dipentaerythritol, mannitol, D-sorbitol and tripentaerythritol. Residues can be mentioned. Further, R ³ may be substituted with an alkyl group or the like.

Specific preferred examples of the matrix compound for the polymer solid electrolyte include, for example, polyethylene glycol, polypropylene glycol, copolymers of polyethylene glycol / polypropylene glycol,
Examples thereof include polymers such as polyacrylonitrile, and surfactants such as polyethylene glycol monocetyl ether, polyethylene glycol monooleyl ether, polyethylene glycol monododecyl ether, polyethylene glycol monohexadecyl ether, and polyethylene glycol monostearic acid.

Among them, a copolymer of polypropylene glycol and polyethylene glycol / polypropylene glycol is preferable because of its adsorptivity to carbon. Here, the polypropylene glycol may be either a diol type or a triol type. The proportion of polyethylene glycol in the polyethylene glycol / polypropylene glycol copolymer is preferably 50% or less. The average molecular weight of both is preferably 1,000 or more, more preferably 2,000 or more.

The crosslinkable compound which reacts with the functional group of the compound functioning as the matrix of the polymer solid electrolyte to insolubilize the compound in the aprotic organic solvent is the functional group of R ¹ and R ² . Those having a functional group reactive with, for example, a hydroxyl group, a vinyl group, an acryloyl group, a methacryloyl group, an epoxy group, an amino group, an isocyanate group or an alkoxy group can be used.

As such a crosslinkable compound, a silane coupling agent represented by the following formula is particularly preferable.

R ⁴ Si (R ⁵ ) _n X _3-n [wherein R ⁴ and R ⁵ are alkyl groups which may be substituted with an amino group having 1 to 16 carbon atoms, an epoxy group or a mercapto group, vinyl Group, halogen, X is a hydrolyzable group such as a lower alkylalkoxy group] Examples of the silane coupling agent usable in the present invention include methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane and hexyl. Examples include trimethoxysilane, decyltrimethoxysilane, dodecyltrimethoxysilane, and hexadecyltrimethoxysilane. Further, those having an ethoxy group instead of the methoxy group and those having two alkyl groups substituted such as dimethyldimethoxysilane can be used.

Those having an amino group at the terminal include 3-
Examples include aminopropylmethoxysilane, 3-aminopropyltriethoxysilane and N-3 (aminoethyl) 3-aminopropyltrimethoxysilane.

As those having an epoxy group at the terminal,
Examples include (3-glycidoxypropyl) methyldiethoxysilane and 3-glycidoxypropyltrimethoxysilane. These may be used alone or in combination.

Next, the method for producing the carbon electrode material of the present invention will be described.

The carbon electrode material of the present invention is used as a negative electrode of a lithium secondary battery and is used to occlude lithium as a negative electrode active material. Raw carbon materials for such carbon electrode materials include natural graphite, organic resins,
Further, carbonization of petroleum, coal-based pitch or the like, or graphitization can be used. Among them, those having advanced graphitization are preferable because they have a large discharge capacity. Specifically, powder X
From the peak of line diffraction, Franklin's formula (Japan Society for the Promotion of Science 117th Committee, Carbon 36, 25 (196
The degree of graphitization obtained using 3)) is preferably 0.4 or more. If it is less than 0.4, the flatness of the potential during charge / discharge may be deteriorated.

The form of the carbon material is not particularly limited, and a pulverized product or a fibrous product obtained by using the above material as a raw material,
Moreover, the thing etc. which were made into a film can be mentioned.

The surface of the carbon material is insoluble in an aprotic organic solvent and coated with a lithium ion conductive film.

This treatment is carried out by first adsorbing the compound functioning as the matrix of the polymer solid electrolyte on the surface of the carbon material, and then by crosslinking the matrix compound with a crosslinking compound such as a silane coupling agent. . The adsorption of the matrix of the polymer solid electrolyte in the first stage is preferably performed by a method of immersing the carbon material in an aqueous solution of the matrix or an organic solvent solution at 0 ° C to 60 ° C. In the latter reaction, preferably, after the adsorption in the first step, a crosslinking compound such as a silane coupling agent is added to the solution, and the reaction temperature is 0 to 15
It is preferred to crosslink the matrix at 0 ° C. In addition, the carbon material having the matrix adsorbed is heated to 50 to 200 ° C.
It can also be carried out by a method such as heating to, and spraying a solvent solution of the crosslinkable compound on this. The concentration of the crosslinkable compound in the treatment solution is preferably 0.5 to 5 wt%.

On the surface of the carbon material which has been subjected to such treatment, a film of a polymer which becomes a solid electrolyte matrix and a crosslinkable compound is formed and functions as a protective film. Therefore, when used as a negative electrode, the ionic conductivity of the film is increased. While the above is maintained, the decomposition reaction on the surface is suppressed, the irreversible capacity decrease is reduced, and the charge / discharge efficiency is increased.

The amount of the solid electrolyte matrix polymer and the crosslinkable compound attached to the negative electrode made of a carbon material cannot be uniquely determined because it changes depending on the specific surface area of the carbon material. Respectively, usually 0.5 to 10 wt%, preferably 1.5 to 7.5.
wt%. If it is less than 0.5% by weight, the coating amount is too small to exert an effect, and if it exceeds 10% by weight, electron transfer and / or ionic conduction between carbon particles or with a current collector is hindered, and charge / discharge characteristics are deteriorated. Not preferable. The carbon electrode material thus obtained is suitable as a negative electrode for a lithium secondary battery.

The lithium secondary battery of the present invention basically comprises a positive electrode, a negative electrode and an electrolyte.

For the negative electrode, the coated carbon electrode material of the present invention is used.

The positive electrode used in the present invention is not particularly limited, and known ones can be used. For example, a lithiated metal-chalcogen bond, particularly a transition metal-chalcogen bond, a metal halide and the like can be mentioned.
Chalcogens include oxygen, sulfur, selenium, tellurium and polonium belonging to the 6th group of the periodic table. Preferred transition metals include cobalt, nickel, manganese, iron, chromium, titanium, vanadium and molybdenum. Among them, the compound is LiCoO ₂ ,
LiNiO ₂ , LiMnO ₄ , LiCo _0.92 Sn _0.08 O ₂
And LiCo _1-x Ni _x O _{2 and the} like are preferable.

As the electrolyte used in the present invention, a known liquid electrolyte or solid electrolyte can be used. For example, as the liquid electrolyte, LiClO ₄ , LiP
_{F 6, LiBF 4, LiCF 3} SO 3, ethylene carbonate containing lithium salt LiAsF _6, etc., propylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, gamma
An aprotic organic solvent such as butyrolactone or a mixture thereof may be mentioned.

As the solid electrolyte, the above-mentioned lithium salt and polyethylene oxide, polypropylene oxide,
Examples thereof include polyacrylonitrile, a (meth) acrylate complex of a polyether having a polyethylene oxide chain, or those obtained by adding the above-mentioned aprotic organic solvent as a plasticizer.

The method for producing such a solid electrolyte is not particularly limited. For example, (1) a method in which a polymer compound, a lithium salt and a plasticizer are mixed and heated and melted,
(2) A method of dissolving the polymer compound, the lithium salt and the plasticizer in an appropriate organic solvent for mixing, and then evaporating the organic solvent for mixing, and (3) mixing the monomer, the lithium salt and the plasticizer, Examples thereof include a method of irradiating with ultraviolet rays, electron beams or molecular beams to form a polymer.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

Example 1 Polypropylene glycol (average molecular weight 4000) 25
0.5 g of artificial graphite (LX-012J-10 manufactured by Toyo Tanso Co., Ltd.) is added to 5 ml of water containing mg, and the mixture is stirred overnight.
Hexyltrimethoxysilane (Toray Dow Corning) 0.05g and 3-aminopropyltriethoxysilane 0.05g (Shin-Etsu Silicone) were dissolved in this, and the mixture was further stirred overnight. This was filtered, washed, and at 100 ° C. It was left for 2 hours and finally vacuum dried at 80 ° C. At this time, the amount of polypropylene glycol attached was 5 wt% and the amount of silane coupling agent attached was about 4 wt%.

200 mg of the prepared powder, 8.3 mg of polyvinylidene fluoride as a binder, and N-methylpyrrolidone as a solvent were mixed. After the solvent was evaporated at 80 ° C., it was pressed and punched into a predetermined size to prepare an electrode.

Using this carbon electrode as a test electrode, lithium metal as a counter electrode, and a propylene carbonate / ethylene carbonate mixed solution containing 1 M lithium perchlorate as an electrolytic solution, a charge / discharge test was conducted in the range of 0 to 1.5 V. It was Table 1 shows the results of the obtained charge / discharge efficiency and discharge capacity in the first cycle.
Shown in

Example 2 The artificial graphite powder was surface-modified in the same manner as in Example 1 except that only 0.1 g of 3-aminopropyltriethoxysilane was used as the silane coupling agent.

Using the prepared carbon powder, a charge / discharge test was conducted under the same conditions as in Example 1. Table 1 shows the obtained results.

Example 3 Polypropylene glycol having an average molecular weight of 3000 or 4000 was adsorbed on artificial graphite in an amount of 10 or 15 mg in advance, and then hexyltrimethoxysilane 0.05 g or 3-aminopropyltriethoxysilane 0 was added as a silane coupling agent thereto. The artificial graphite powder was surface-modified in the same manner as in Example 1 by reacting 0.05 g.

A charging / discharging test was conducted under the same conditions as in Example 1 using the prepared carbon powder. Table 1 shows the obtained results.

Example 4 0.5 ml of a polyethylene glycol / polypropylene glycol copolymer (average molecular weight: 1750, polyethylene glycol ratio 10%) was added in advance and adsorbed thereto, and hexyltrimethoxysilane 0.05 g, 3
A surface modification of artificial graphite was carried out in the same manner as in Example 1 by reacting 0.05 g of aminotriethoxysilane.

Using the prepared carbon powder, a charge / discharge test was conducted under the same conditions as in Example 1. Table 1 shows the obtained results.

COMPARATIVE EXAMPLE Hexyltrimethoxysilane 0.05 was prepared in the same manner as in Example 1 without adsorbing the polyether compound in advance.
g, 3-aminopropyltriethoxysilane 0.05 g
The artificial graphite powder obtained by treatment in Example 1 was subjected to a charge / discharge test under the same conditions as in Example 1. Table 1 shows the obtained results.

[0045]

[Table 1]

As shown in Table 1, a lithium secondary battery using a carbon material, which has been surface-modified by adsorbing polypropylene glycol, which serves as a matrix of a solid electrolyte in advance, and reacting it with a silane coupling agent, has a solid electrolyte matrix. It can be seen that the discharge capacity is increased and the charge / discharge efficiency is also increased as compared with the case where the compound that becomes

[0047]

EFFECT OF THE INVENTION By previously adsorbing a compound serving as a matrix of a solid electrolyte and then using a carbon material surface-modified by reacting a crosslinkable compound as a negative electrode,
It has become possible to provide a lithium secondary battery having excellent charge / discharge efficiency and discharge capacity in the first cycle.

Claims (6)

[Claims] 1. A carbon electrode material for a lithium secondary battery, which is coated with a film that is insoluble in an aprotic organic solvent and has a lithium ion conductivity. 2. The film is prepared by previously adsorbing at least one compound that functions as a matrix of a polymer solid electrolyte onto a carbon electrode material, and at least one compound is adsorbed on the carbon electrode material.
The carbon electrode material for a lithium secondary battery according to claim 1, which is formed by reacting one or more crosslinkable compounds. 3. The compound which functions as a matrix of the polymer solid electrolyte is a polyether compound.
A carbon electrode material for a lithium secondary battery as described above. 4. The carbon electrode material for a lithium secondary battery according to claim 2, wherein the crosslinkable compound is a silane coupling agent. 5. A carbon electrode material is formed into a film by adsorbing at least one or more kinds of compounds functioning as a matrix of a polymer solid electrolyte on the carbon electrode material, and then reacting it with at least one or more kinds of crosslinkable compounds. The method for producing a carbon electrode material for a lithium battery according to claim 1, characterized by coating. 6. A lithium secondary battery, wherein the carbon electrode material according to claim 1 is used as a negative electrode.