Non-aqueous electrolyte battery and non-aqueous electrolyte

1. Field of the Invention

The present invention relates to a non-aqueous electrolyte battery, and, particularly, to an improvement of a non-aqueous electrolyte.

2. Description of Related Art

At present, as organic solvents which constitute non-aqueous electrolytes for non-aqueous electrolyte batteries, there are used, for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, dipropyl carbonate, methyl propionate, tetrahydrofuran, 1,3-dioxolan, and 1,2-dimethoxyethane each alone or in admixture. As electrolyte salts, there are used, for example, LiClO 4 , LiBF 4 , LiPF 6 , LiCF 3 SO 3 , and (CF 3 SO 2 ) 2 NLi each alone or in admixture. Especially, carbonic acid esters are mainly used as organic solvents and LiPF 6 is mainly used as electrolyte salts because they are superior in conductivity and are safe for environments.

However, when batteries made using non-aqueous electrolytes comprising the above organic solvents and electrolyte salts are stored in charged state, electrode materials react with the organic solvents and the electrolyte salts to cause decomposition of the non-aqueous electrolytes, and, as a result, capacity of the batteries after stored tends to decrease. Especially, in secondary batteries which are made using carbon materials as negative electrode materials, occurrence of reduction reaction at negative electrodes is accelerated and said tendency becomes more conspicuous.

The object of the present invention is to provide a non-aqueous electrolyte battery which is inhibited from the deterioration of non-aqueous electrolyte when stored in charged state, especially, the occurrence of reaction between the negative electrode materials and the non-aqueous electrolyte and, thus, which has excellent storage characteristics.

For attaining the above object, the present invention provides a non-aqueous electrolyte battery having a positive electrode, a negative electrode and a non-aqueous electrolyte prepared by dissolving an electrolyte salt in an organic solvent, wherein the non-aqueous electrolyte contains a compound represented by the following formula (1):

(in the above formula (1), R 1 , R 2 and R 3 independently represent a hydrogen atom, a halogen atom or a straight chain or branched chain alkyl group). This compound when added to the electrolyte forms a film on the surface of the negative electrode to prevent the electrolyte and the negative electrode from contacting with each other. As a result, occurrence of decomposition reaction of the electrolyte on the negative electrode can be diminished.

FIG. 1 is a longitudinal sectional view of a cylindrical battery in the examples of the present invention and the comparative examples.

FIG. 2 is a graph which shows relations between amounts of the additives and capacity recovery rate.

In the present invention, an isocyanuric acid derivative represented by the following formula (1) is added to the non-aqueous electrolyte.

In the above formula (1), R 1 , R 2 and R 3 independently represent a hydrogen atom, a halogen atom or a straight chain or branched chain alkyl group, and the alkyl group is preferred.

Unless otherwise notified, the term “alkyl” in this specification preferably means a straight chain or branched chain alkyl of 1-30 carbon atoms which is substituted or unsubstituted or a cycloalkyl of 3-30 carbon atoms which is substituted or unsubstituted, and the straight chain or branched chain alkyl is preferred. These groups can be saturated or unsaturated, and are preferably saturated. Preferred alkyl groups are straight chain or branched chain alkyl groups of 1-10 carbon atoms which are substituted or unsubstituted, and specifically are methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group and decyl group. More preferred are straight chain or branched chain alkyl groups of 1-6 carbon atoms which are substituted or unsubstituted, and specifically are methyl group, ethyl group, propyl group, butyl group, pentyl group and hexyl group.

The alkyl groups may contain hydroxyl group, carboxyl group, aldehyde group, carbonyl group, ether linkage or ester linkage, namely, may be substituted with hydroxyl group, carboxyl group, aldehyde group, —COR′ group, —OR′ group, —O—CO—R′ group or —CO—OR′ group where R′ represents the above-mentioned alkyl groups, and more preferably unsubstituted alkyl groups of 1-6 carbon atoms and further preferably unsubstituted alkyl groups of 1-3 carbon atoms.

Examples of the compounds represented by the formula (1) are isocyanuric acid, isocyanuric acid monochloride, isocyanuric acid dichloride, isocyanuric acid trischloride, mono(carboxymethyl) isocyanurate, di(carboxymethyl) isocyanurate, tris(carboxymethyl) isocyanurate, mono(2-carboxyethyl) isocyanurate, di(2-carboxyethyl) isocyanurate, tris(2-carboxyethyl) isocyanurate, mono(3-carboxypropyl) isocyanurate, di(3-carboxypropyl) isocyanurate, tris(3-carboxypropyl) isocyanurate, mono(2-carboxypropyl) isocyanurate, di(2-carboxypropyl) isocyanurate, tris(2-carboxypropyl) isocyanurate, mono(1-carboxypropyl) isocyanurate, di(1-carboxypropyl) isocyanurate, tris(1-carboxypropyl) isocyanurate, and derivatives of them, for example, alkoxy derivatives such as mono(methoxycarboxymethyl) isocyanurate, di(methoxycarboxymethyl) isocyanurate, tris(methoxycarboxymethyl) isocyanurate, mono(2-methoxycarboxyethyl) isocyanurate, di(2-methoxycarboxyethyl) isocyanurate, tris(2-methoxycarboxyethyl) isocyanurate, mono(3-methoxycarboxypropyl) isocyanurate, di(3-methoxycarboxypropyl) isocyanurate, tris(3-methoxycarboxypropyl) isocyanurate, mono(2-methoxycarboxypropyl) isocyanurate, di(2-methoxycarboxypropyl) isocyanurate, tris(2-methoxycarboxypropyl) isocyanurate, mono(1-methoxycarboxypropyl) isocyanurate, di(1-methoxycarboxypropyl) isocyanurate, tris(1-methoxycarboxypropyl) isocyanurate, mono(ethoxycarboxymethyl) isocyanurate, di(ethoxycarboxymethyl) isocyanurate, tris(ethoxycarboxymethyl) isocyanurate, mono(2-ethoxycarboxyethyl) isocyanurate, di(2-ethoxycarboxyethyl) isocyanurate, tris(2-ethoxycarboxyethyl) isocyanurate, mono(3-ethoxycarboxypropyl) isocyanurate, di(3-ethoxycarboxypropyl) isocyanurate, tris(3-ethoxycarboxypropyl) isocyanurate, mono(2-ethoxycarboxypropyl) isocyanurate, di(2-ethoxycarboxypropyl) isocyanurate, tris(2-ethoxycarboxypropyl) isocyanurate, mono(1-ethoxycarboxypropyl) isocyanurate, di(1-ethoxycarboxypropyl) isocyanurate, and tris(1-ethoxycarboxypropyl) isocyanurate, and the like.

A preferred embodiment of the present invention is that the compound is represented by the formula (1) where R 1 , R 2 and R 3 independently of one another are each an alkyl group having a carboxyl group or an alkyl group containing an ester linkage. A more preferred embodiment is that the compound represented by the formula (1) is tris(2-carboxyethyl) isocyanurate or a derivative thereof, preferably an alkoxy derivative, more preferably tris(2-methoxycarboxyethyl) isocyanurate. This is because when one of the compounds represented by the formula (1) forms a film on the negative electrode, if the organic compound has a carboxyl group, it positively reacts with active sites on the negative electrode and, hence, reactivity of the active sites on the negative electrode lowers to make it possible to further inhibit the decomposition of the electrolyte on the negative electrode. In the case of having an ester linkage, the same effect can also be obtained by carboxyl group or hydroxyl group produced upon decomposition of the ester linkage.

Amount of the compound added is preferably not less than 0.01% by weight and less than 20% by weight based on the weight of the non-aqueous electrolyte.

The non-aqueous electrolyte of the present invention contains an organic solvent and an electrolyte salt which is soluble in the solvent. As the organic solvent, mention may be made of non-protonic organic solvents, for example, cyclic carbonic acid esters such as ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC), non-cyclic carbonic acid esters such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate and ethyl propionate, cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone, non-cyclic ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolan, formamide, acetamide, dimethylformamide, dioxolan, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid esters such as trimethyl phosphate and thiethyl phosphate, trimethoxymethane, dioxolan derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propanesultone, anisole, and N-methylpyrrolidone. These are used each alone or in admixture of two or more. Preferably, at least one organic compound selected from the group consisting of the carbonic acid esters, the cyclic carboxylic acid esters and the phosphoric acid esters is contained in the non-aqueous electrolyte. Especially preferably, at least one organic compound selected from the group consisting of the cyclic carboxylic acid esters and the phosphoric acid esters is contained in the non-aqueous electrolyte. This is because flash point and ignition point of these compounds are very high and, hence, batteries higher in safety can be obtained.

As the electrolyte salts dissolved in these organic solvents, mention may be made of, for example, LiClO 4 , LiBF 4 , LiPF 6 , LiAlCl 4 , LiSbF 6 , LiSCN, LiCl, LiCF 3 SO 3 , LiCF 3 CO 2 , LiAsF 6 , LiB 10 Cl 10 , lithium salts of lower aliphatic carboxylic acids, LiBr, LiI, chloroborane lithium, lithium tetraphenylborate, salts having an imide skeleton ((C 2 F 5 SO 2 ) 2 NLi, (CF 3 SO 2 ) 2 NLi, (CF 3 SO 2 ) (C 4 F 9 SO 2 )NLi, etc.), and salts having a methide skeleton ((CF 3 SO 2 ) 3 CLi, etc.). These may be used each alone or in combination of two or more in the electrolyte and the like, and LiPF 6 is especially preferred. Amount of the lithium salts dissolved in the non-aqueous solvent is not especially limited, but preferably is 0.2-2 mol/l, especially preferably 0.5-1.5 mol/l.

Furthermore, in order to make the electrolyte nonflammable, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride chloride can be contained in the electrolyte. Moreover, carbon dioxide can be contained in the electrolyte for giving suitability for high-temperature storage.

Furthermore, there may be used a gel electrolyte comprising an organic solid electrolyte which contains the above non-aqueous electrolyte. As the organic solid electrolyte, effective are polymeric matrix materials such as polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride and polyhexafluoropropylene, and derivatives, mixtures and composite thereof. Especially preferred are copolymers of vinylidene fluoride and hexafluoropropylene and mixtures of polyvinylidene fluoride and polyethylene oxide.

The negative electrode materials used in the present invention may be compounds capable of occluding or discharging lithium ions, such as lithium, lithium alloys, alloys, intermetallic compounds, carbon materials, organic compounds, inorganic compounds, metal complexes, and organic high molecular compounds. These may be used each alone or in combination.

The effect of the present invention is especially high when the negative electrode materials are carbon materials. Examples of the carbonaceous materials are cokes, pyrolytic carbons, natural graphite, artificial graphite, mesocarbon microbeads, graphitized mesophase spherules, vapor deposited carbons, glassy carbons, carbon fibers (polyacrylonitrile fibers, pitch fibers, cellulose fibers and vapor deposited carbon fibers), amorphous carbons, and carbons prepared by firing organic materials. These may be used each alone or in combination. Among them, preferred are graphite materials such as those obtained by graphitizing mesophase spherules at high temperatures, natural graphite and artificial graphite, and especially preferred are graphitized mesophase spherules.

As the positive electrode active materials, there may be used those which can be generally used for non-aqueous electrolyte batteries. Examples thereof are Li x CoO 2 , Li x NiO 2 , Li x MnO 2 , and Li x Mn 2 O 4 (0<x≦1.2).

Having thus generally described the present invention, the following specific examples are provided to illustrate the invention. The examples are not intended to limit the scope of the invention in any way.

Examples of the present invention will be explained referring to the drawings.