JPH11120992A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the initial charging/discharging efficiency and cycle characteristic of a nonaqueous electrolyte secondary battery having a carbon negative electrode capable of storing/discharging lithium. SOLUTION: A coating layer made of an ionic conductive polymer, a water- soluble polymer, or alkaline metal salt is formed on the surface of a carbon negative electrode, the wettability between the carbon negative electrode and a nonaqueous electrolyte layer is improved, the decomposition of the nonaqueous electrolyte layer is suppressed, and the deposition of the decomposition product of the nonaqueous electrolyte layer on the surface of the negative electrode is suppressed. Even if the carbon negative electrode is made of a graphite carbon material and propylene carbonate is contained in the nonaqueous electrolyte layer in particular, the decomposition of this solvent on the surface of graphite is suppressed, and a good cycle characteristic is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to an improvement in initial charge / discharge efficiency and cycle characteristics by improving the performance of a carbon anode.

[0002]

2. Description of the Related Art In recent years, there has been remarkable progress in miniaturization, thinning, and weight reduction of electronic devices. In particular, in the OA field, miniaturization has been progressing from desktop type to laptop type and notebook type. ing. In addition, the field of new small electronic devices such as electronic notebooks and electronic still cameras has emerged, and furthermore, in addition to the miniaturization of conventional hard disks and floppy disks, the development of memory cards, which are new memory media, has been promoted. In the wave of downsizing, thinning, and weight reduction of such electronic devices, high performance is also required for secondary batteries that support such electric power, and high energy related to lead-acid batteries and nickel-cadmium batteries is required. The development of lithium secondary batteries as density batteries has been rapidly advanced.

As a positive electrode active material of a lithium secondary battery,
Until now, TiS ₂ , MoS ₂ , C
transition metal chalcogen compounds such as o ₂ S ₆ , FeS ₂ , NbS ₂ , ZrS ₂ , VSe ₂ or V ₂ O ₅ , Mn
Many transition metal oxides such as O _{2 and} CoO ₂ have been studied. More recently, as a positive electrode active material capable of obtaining a high discharge voltage of about 4 V and achieving high energy, a layered composite oxide represented by a general formula LiMO ₂ (M is a metal atom), a general formula LiM ₂ A composite oxide having a spinel structure represented by O ₄ has been proposed. The former includes lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), and the latter includes lithium manganese oxide (LiMn ₂ O ₄ ).

[0004] On the other hand, as the negative electrode active material, many proposals have been made on lithium metal and lithium alloy at the beginning of development. However, high electromotive force can be obtained from lithium metal, and it is lightweight and easy to increase the density, but discharge (lithium deposition)
While the charging and recharging (lithium elution) are repeated, dendrites of lithium are formed at specific sites on the surface of the negative electrode, and short-circuiting with the positive electrode due to the penetration of the dendrites easily occurs. As for the lithium alloy, the problem of short circuit as described above has been alleviated, but it has not been possible to achieve a practically satisfactory capacity as a secondary battery.

[0005] Instead of these lithium metal-based negative electrodes,
A carbon negative electrode using a highly safe carbon material capable of reversibly occluding / releasing lithium ions has been proposed, and has become a mainstream of the negative electrode of a lithium secondary battery that is currently in practical use.
The carbon anode has a highly reversible host function of absorbing / releasing lithium as a guest between layers of a mesh-like molecular layer in which 6-membered rings composed of sp ² carbon atoms are connected. Some carbon materials having low crystallinity as compared to a perfect graphite structure have sites capable of accommodating lithium in addition to between layers, and exhibit a capacity density higher than the theoretical capacity density of graphite.

[0006] The capacity of lithium and the reversibility of occlusion / release vary depending on characteristics such as crystallinity, orientation, pore structure, particle size, and specific surface area.
It is greatly affected by manufacturing conditions such as pulverization. For this reason, carbon materials using various raw materials and manufacturing methods have been conventionally proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 2-66656 discloses a conductive carbon material obtained by burning a furfuryl alcohol resin at 1100 ° C., and Japanese Unexamined Patent Publication (Kokai) No. 61-277515 discloses an aromatic polyimide in an inert atmosphere at a temperature of 2000 ° C. Conductive carbon material obtained by heat treatment in
No. 457 discloses carbon materials obtained by graphitizing graphitizable spherical carbon. Further, JP-A-61-77
Japanese Patent Publication No. 275 discloses a secondary battery in which an insulating or semiconductive carbon material having a polyacene structure obtained by heat-treating a phenolic polymer is used for an electrode.

[0007]

However, a general problem of the carbon anode made of the carbon material as described above is that a solvent is decomposed on the surface of the anode at the time of initial charging, or lithium is trapped inside the anode. This leads to a reduction in initial charge / discharge efficiency and a deterioration in cycle characteristics. In particular, although propylene carbonate is a useful solvent, it is reductively decomposed on the surface of the carbon negative electrode, so that it is difficult to smoothly proceed with the occlusion / release of lithium. This has been a cause of preventing the combined use with a graphite-based carbon anode. In addition, a high lithium insertion / desorption rate with respect to the carbon material is a preferable requirement for increasing the battery capacity under a large current. However, unless the occlusion rate is sufficiently high, metallic lithium may be lost during rapid charging. It is likely to be deposited on the surface of the carbon negative electrode and impair the cycle life and safety. Therefore, an object of the present invention is to provide a nonaqueous electrolyte secondary battery using a carbon anode having high initial charge / discharge efficiency and excellent cycle characteristics.

[0008]

Means for Solving the Problems The present inventor has conducted studies to achieve the above-mentioned object, and as a result, has found that lithium has been absorbed / removed.
By providing a certain coating layer on the surface of the releasable carbon negative electrode, it was found that a non-aqueous electrolyte secondary battery having high initial charge / discharge efficiency and excellent cycle characteristics was obtained.
The present invention has been proposed. The coating layer,
By performing functions such as improving the wettability between the carbon anode and the nonaqueous electrolyte layer, suppressing the decomposition of the nonaqueous electrolyte layer, or suppressing the deposition of decomposition products of the components of the nonaqueous electrolyte layer on the negative electrode surface, It contributes to improvement of charge / discharge efficiency and cycle characteristics.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a coating layer provided on the surface of a carbon negative electrode is required not to hinder the movement of lithium and to be stable and insoluble in an electrolytic solution. The constituent material and the manufacturing method are not particularly limited as long as the requirements are satisfied.
However, particularly suitable materials for practical use include ion-conductive polymers, water-soluble polymers, and at least one of alkali metal salts, alkali metal oxides, and alkali metal hydroxides.

Examples of the ion-conductive polymer include polymer matrices such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, and polyacrylonitrile, which are commonly used in the production of solid polymer electrolytes. Accordingly, a coating layer can be formed by, for example, applying a composite in which an electrolyte salt is dissolved to the surface of a carbon negative electrode. Alternatively, a coating layer may be formed by forming a film of vinyl fluoride, octamethyltetrasiloxane, hexamethyldisiloxane, hexamethylcyclosiloxane, or the like by plasma polymerization.

Examples of the water-soluble polymer include polyvinyl alcohol, polyethylene oxide, polypropylene oxide, polyvinylpyrrolidone, a hydrolyzate of a styrene-maleic anhydride copolymer or a water-soluble salt thereof, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose or these. And polyacrylic acid or a water-soluble salt thereof. Particularly preferred in practical use is polyvinyl alcohol, and the higher the degree of polymerization and the higher the degree of saponification, the better. These water-soluble polymers are used alone or in combination of two or more. Examples of the water-soluble salt include a lithium salt, a sodium salt, an ammonium salt, and an amine salt, and a lithium salt is particularly preferable. In order to form the coating layer, a coating solution in which these water-soluble polymers are dissolved in a polar solvent such as water or alcohol is applied to the surface of the carbon negative electrode. Further, when using an alkali metal salt, an alkali metal oxide, or an alkali metal hydroxide, a method of applying an aqueous solution in which these are dissolved to the surface of the carbon negative electrode, or a method of sputtering these targets, is used to form the coating layer. Form.

[0012] The thickness of the coating layer formed in the present invention is preferably 0.1 to 50 µm when any of the above materials is used. When it is thinner than 0.1 μm, it functions as a coating layer, that is, improves wettability between the carbon anode and the electrolyte layer, suppresses decomposition of the electrolyte layer, or deposits decomposition products of components of the electrolyte layer on the surface of the negative electrode. In the case where functions such as suppression cannot be sufficiently performed, and when the thickness is larger than 50 μm, movement of lithium is hindered and charge / discharge density is likely to be reduced. A more preferable range of the film thickness is:
0.5 to 10 μm.

The carbon material contained as the negative electrode active material of the carbon negative electrode according to the present invention is a conventionally known carbon material obtained by firing materials such as coke, pitch, synthetic polymer, and natural polymer at 500 to 3000 ° C. in a reducing atmosphere. Carbon material and natural graphite. Among them, graphitic carbon materials such as natural graphite, artificial graphite, graphitized mesocarbon microbeads (MCMB), graphitized mesophase pitch-based carbon fibers, graphite whiskers, and carbon materials containing boron are particularly preferable. C of these carbon materials
The axial distance d ₀₀₂ is 3.35 to 3.40 °.
The lower limit of the numerical 3.35Å is complete corresponds to the plane spacing d ₀₀₂ of the c-axis direction of the c-axis direction of the graphite structure, the theoretical capacity density of intercalation compound LiC ₆ lithium in the graphite structure is doped maximum , 372 mAh / g. A carbon material having a surface distance d ₀₀₂ of 3.40 ° or less in the c-axis direction can be said to be a carbon material having a developed graphite structure.
It has been difficult to use propylene carbonate, which is frequently used as a solvent for the non-aqueous electrolyte, in combination. However, in the present invention, since the coating layer is provided on the surface of the carbon negative electrode, no problem occurs even when used in combination with propylene carbonate.

In the present invention, it is also effective to form a carbon anode by mixing two or more kinds of carbon materials. This allows
Adhesion on the current collector, battery capacity, charge / discharge characteristics, and the like can be optimized according to desired characteristics. The plurality of carbon materials may have different average particle sizes. From the perspective of increasing the initial battery capacity due to the increase in surface area,
Generally, the smaller the average particle size of the active material, the better. However,
If it is too small, the battery capacity will decrease during the charge / discharge cycle, or the dispersibility of the negative electrode paint for preparing the carbon negative electrode will decrease, and the contact area with the non-aqueous electrolyte layer will increase. The disadvantage is that the non-aqueous solvent is easily decomposed. Therefore, the lower limit of the average particle size is about 1 μm. On the other hand, if the average particle size is too large, problems in film forming characteristics such as a decrease in dispersibility in the negative electrode paint and a brittle active material layer to be formed arise. Therefore, the upper limit of the average particle size is approximately 50 μm. A more preferable range of the average particle size is 3 to 20 μm.

The carbon anode can be formed into a sheet. Examples of a method for producing the negative electrode at this time include a method in which a carbon material sheet is directly formed on a current collector by a wet papermaking method using a dispersion liquid in which a carbon material is dispersed together with a suitable binder, followed by drying. There is a method of pressing a carbon material sheet formed on a support by a wet papermaking method onto a current collector, or a method of applying a negative electrode coating material containing a carbon material and a binder on the current collector and drying it. The binder is required to have excellent resistance to an electrolytic solution and strong mechanical strength for maintaining an electrode film on the premise that the characteristics of the negative electrode active material are not impaired. For example, Teflon, polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene-butadiene rubber, nitrocellulose, cyanoethylcellulose, polyacrylonitrile,
Polyvinyl fluoride, polyvinyl acetate, polyvinylidene fluoride, polyethylene, polystyrene, polypropylene,
Examples include polytetrafluoroethylene, polymethyl methacrylate, polychloroprene, polyvinyl pyridine and the like.

As the current collector, for example, a metal such as copper, stainless steel, gold, platinum, nickel, aluminum, molybdenum, titanium or the like, formed into a sheet, foil, mesh, punched metal, expanded metal or the like, Alternatively, a mesh or nonwoven fabric made of a fiber obtained by compounding these metals by plating, vapor deposition, kneading, or the like may be used. These can be similarly used as a positive electrode current collector described later.
Among them, copper, aluminum, and stainless steel are advantageous from the viewpoint of electrical conductivity, chemical stability, electrochemical stability, economy, workability, etc., and further, from the viewpoint of lightness and electrochemical stability. Copper and aluminum are particularly preferred.

More preferably, the surface of the current collector is roughened. By performing the surface roughening, the contact area with the active material layer is increased, the adhesion is improved, and an effect of lowering the impedance as a battery can be expected. In the preparation of an electrode using an active material paint, the adhesion between the active material and the current collector can be significantly improved by performing a surface roughening treatment. Examples of the surface roughening method include polishing using emery paper, blasting, and chemical or electrochemical etching. Blasting is suitable for roughening the surface of the stainless steel current collector, but etching is most suitable for the aluminum current collector. This is because, because aluminum is a soft metal, the current collector itself is deformed before roughening is performed in blasting. If it is an etching treatment, the μ can be obtained without causing deformation or a significant decrease in strength of the aluminum current collector.
The surface can be effectively roughened on the order of m.

On the other hand, as the positive electrode active material constituting the positive electrode of the non-aqueous electrolyte secondary battery of the present invention, TiS ₂ , MoS ₂ ,
Co ₂ S ₆ , FeS ₂ , NbS ₂ , ZrS ₂ , VSe ₂
Transition metal chalcogen compounds such as V ₂ O ₅ , MnO ₂ ,
Transition metal oxides such as CoO ₂ , LiCoO ₂ , LiNi
LiCoXO ₂ , LiNiXO in which a lithium-containing composite oxide such as O ₂ , LiFeO ₂ , LiMn ₂ O ₄ , or Co, Ni, Fe, Mn in these lithium-containing composite oxides is partially replaced by another element X ₂ , LiFe
XO ₂ , LiMn ₂ XO _{4 and the} like can be mentioned. Among these, lithium-containing composite oxides have become the mainstream of recent positive electrode active materials because of their high potential and high energy, and are synthesized by firing high-temperature starting materials such as carbonates, hydroxides, and nitrates. Is used. Further, these active materials may be used in combination of several types.

In order to prepare a positive electrode using these positive electrode active materials, the positive electrode active material is added to a solvent such as dimethylformamide, N-methylpyrrolidone, tetrahydrofuran or the like, if necessary, with a binder, a conductive agent and the like. It is most common to mix and disperse with the additives to prepare a high-concentration positive electrode paint, apply the positive-electrode paint on the current collector, and then dry it. In this case, the dispersion of the positive electrode active material is performed using a device such as a roll mill, a ball mill, and a valence mill. The application of the positive electrode paint is performed using a wire bar or a blade coater, or by a spray method.

As the above-mentioned binder, the same binder as that used at the time of producing the carbon anode can be appropriately selected and used. Further, as the conductive agent, any material may be used as long as it is an electron conductive material that does not cause a chemical change in the configured battery system, and a commonly used material such as natural graphite and artificial graphite can be used. .

The non-aqueous electrolyte layer used in the non-aqueous electrolyte secondary battery of the present invention may contain an electrolyte solution obtained by dissolving an electrolyte salt in an organic solvent, or may comprise a solid electrolyte. Is also good. The electrolyte salt used for preparing the electrolyte solution is not particularly limited as long as it is a commonly used electrolyte salt.
_{4, LiAsF 6, LiPF 6,} LiBF 4, LiB
r, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , L
Examples thereof include lithium salts such as iC (CF ₃ SO ₂ ) ₃ . The concentration of the electrolyte is about 0.1 to 10 mol / l, depending on the electrode and the organic solvent used.

Examples of the organic solvent include furan solvents (tetrahydrofuran, 2-methyltetrahydrofuran, etc.); ether solvents (methylal, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-2-dimethoxy) Amide solvents (N-methylformamide, N-ethylformamide, N, N-dimethylformamide, N-methylacetamide, N-ethylacetamide, N-methylpyrrolidinone, etc.); nitrile solvents (Acetonitrile, benzonitrile, 3-methoxypropionitrile, etc.); sulfolane solvents (sulfolane,
Chain carbonate-based solvents (dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl isopropyl carbonate, etc.); Cyclic carbonate-based solvents (ethylene carbonate, propylene carbonate, butylene) Lactone solvents (γ-butyrolactone, γ-valerolactone, δ)
-Valerolactone, 3-methyl-1,3-oxazolidine-2-one, etc.); alcohol solvents (ethylene glycol, propylene glycol, glycerin, methyl cellosolve, 1,2-butanediol, 1,3-butanediol, 1 , 4-butanediol, diglycerin, polyoxyalkylene glycol, cyclohexanediol, xylene glycol, etc.) Phosphoric acid and phosphate ester solvents (normal phosphoric acid, metaphosphoric acid, pyrophosphoric acid, polyphosphoric acid, phosphorous acid, trimethylphosphorus) 2-imidazolidinone-based solvent (1,3-dimethyl-2-imidazolidinone, etc.); pyrrolidone-based solvent; 1,4-dioxane; dioxolane; dichloroethane alone or as a mixture. Among them, propylene carbonate is a useful solvent because it has a high relative dielectric constant, is liquid at room temperature, and can improve the low-temperature characteristics of batteries, but is reductively decomposed by contact with a graphite-based carbon anode, and the surface of the anode is reduced. However, there has been a problem that a solid such as lithium carbonate is precipitated, and it has been conventionally difficult to use it together with a graphite-based carbon anode. In the present invention, since the surface of the carbon negative electrode is covered with the coating layer, such a combination is possible, and a high-performance lithium secondary battery can be provided.

In the present invention, when the non-aqueous electrolyte layer is constituted by using a solid polymer electrolyte layer, the non-aqueous electrolyte secondary layer which is resistant to deformation and free from deformation, leakage or gas generation of the electrolyte, and which is highly reliable. A battery can be manufactured. As a material of the solid electrolyte, a polymer material such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, and polyacrylonitrile is used as a matrix, and a complex in which the above-described electrolyte salt is dissolved in the matrix, or a gel cross-linked body thereof, Examples thereof include those obtained by grafting an ion-dissociating group such as low-molecular-weight polyethylene oxide and crown ether onto the polymer main chain, and gel-like bodies obtained by adding the above-mentioned electrolyte solution to a high-molecular-weight polymer.

In the battery of the present invention, a separator may be used. As the separator, a separator that has low resistance to ion migration of the electrolyte layer and has excellent electrolyte retention is used. For example, nonwoven fabrics and woven fabrics made of fibers such as glass, polyester, Teflon, polyflon, and polypropylene alone or in combination are exemplified. This separator, using a wound body and a laminate sandwiched between the counter electrode and the negative electrode, cylindrical, coin type,
Various types of nonaqueous electrolyte secondary batteries such as a gum type and a flat type can be manufactured, but this type is not particularly limited. When a solid electrolyte is used, the separator need not be particularly used as long as the solid electrolyte itself can also serve as the separator.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

Example 1 In this example, the surface of a carbon negative electrode using natural graphite as a negative electrode active material was covered with a coating layer made of a coating film of polyvinylidene fluoride as an ion-conductive polymer, and a Li plate was used as a counter electrode. For the non-aqueous electrolyte layer, LiPF ₆ is made of ethylene carbonate /
A button type secondary battery for evaluation was constructed using each of the solution layers dissolved in a propylene carbonate / dimethyl carbonate mixed solvent.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 4-vinylpyridine and 2
-Hydroxyethyl methacrylate in a molar ratio of 1: 3, 100 parts by weight of a copolymer, and hexamethylene diisocyanate-methyl ethyl ketone (MEK) oxime block (Sumitomo Bayer Urethane Co .; trade name BL)
3175) was mixed with 5 parts by weight to prepare a binder resin composition. 3 parts by weight of this binder resin composition was dissolved in 67 parts by weight of N-methylpyrrolidone, and natural graphite ( _d002
= 3.355%), and 30 parts by weight were added and mixed and dispersed under an inert atmosphere using a roll mill to prepare a negative electrode coating material. The negative electrode paint is applied on a 20 μm-thick Cu foil using a wire bar in the air, dried at 130 ° C. for 20 minutes, and roll-pressed to a total thickness of 80 μm having a 60 μm-thick negative electrode active material layer. Was prepared. Subsequently, a coating layer was formed on the surface of the carbon negative electrode. In other words, N-vinylidene fluoride was added to N-
The solution dissolved in methylpyrrolidone was applied to the surface of the carbon negative electrode using a doctor blade, and the coating film was dried to form a coating layer having a thickness of 3.5 μm.

Next, a button-type secondary battery for evaluation was assembled using the carbon anode. A Li plate was used as a counter electrode. The non-aqueous electrolyte layer has a ratio of 2: 5: 3 of ethylene carbonate / propylene carbonate / dimethyl carbonate.
(Volume ratio) An electrolyte solution obtained by dissolving LiPF ₆ at a concentration of 2.0 mol / l in a mixed solvent was used by impregnating a separator made of porous polypropylene with a separator. The above counter electrode and the negative electrode were laminated with the separator interposed therebetween to obtain a button-type secondary battery for evaluation.

Example 2 In this example, the surface of a carbon negative electrode using a fired product of petroleum pitch coke containing boron (B) as a negative electrode active material was coated with a plasma polymerized film of vinyl fluoride, an ion conductive polymer. A Li plate as a counter electrode and LiN (CF ₃ SO ₂ ) ₂ as propylene carbonate /
A button type secondary battery for evaluation was constructed using each of the solution layers dissolved in a dimethyl carbonate mixed solvent.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 3 parts by weight of polyvinylidene fluoride as a binder was added to N-methylpyrrolidone (NM
P) Dissolved in 62 parts by weight and calcined at 2500 ° C. of petroleum pitch coke containing 4.5% boron (d ₀₀₂ =
3.356 °) 35 parts by weight were added and mixed and dispersed in a roll mill under an inert atmosphere to obtain a negative electrode paint. The above negative electrode paint is coated with a wire bar in the atmosphere to a thickness of 20 mm.
It was applied on a μm Cu foil, dried at 120 ° C. for 20 minutes, and roll-pressed to produce a carbon negative electrode having a total thickness of 80 μm and a negative electrode active material layer having a thickness of 60 μm. Subsequently, a plasma polymerized film of vinyl fluoride was formed as a coating layer on the surface of the carbon negative electrode. That is, the flow rate of vinyl fluoride = 10 cm ³ /
Min, pressure = 0.5 Torr, RF frequency = 10 MHz,
Under the condition of RF discharge output = 20 W, a coating layer having a thickness of 0.7 μm was formed.

Next, a button-type secondary battery for evaluation was assembled using the carbon anode. A Li plate was used as a counter electrode. For the non-aqueous electrolyte layer, a propylene carbonate / dimethyl carbonate (5: 5 (volume ratio)) mixed solvent of LiN was used.
An electrolyte solution in which (CF ₃ SO ₂ ) ₂ was dissolved at a concentration of 2.0 mol / l was used by impregnating a separator made of porous polypropylene with a separator. Assembling of the button type secondary battery for evaluation was performed in the same manner as in Example 1.

Example 3 In this example, the surface of a carbon negative electrode using a calcined petroleum pitch coke containing boron (B) as a negative electrode active material was coated with a coating film of polyvinyl alcohol (PVA), a water-soluble polymer. A button-type secondary battery for evaluation is used, using a Li plate as a counter electrode, and a solution layer in which LiN (CF ₃ SO ₂ ) ₂ is dissolved in a propylene carbonate / dimethyl carbonate mixed solvent as a nonaqueous electrolyte layer. Was configured.

First, the negative electrode coating material described in Example 2 was applied on a Cu foil as a current collector to produce a carbon negative electrode. Subsequently, a coating layer was formed on the surface of the carbon negative electrode. That is, PVA (degree of polymerization 1700, degree of saponification 99.3 mol)
% Solution) A solution obtained by heating and dissolving 2.5 parts by weight in 80 parts by weight of pure water is applied to the surface of a carbon negative electrode using a doctor blade, and the coating film is dried to form a coating layer having a thickness of 2.5 μm. did. The assembly of the counter electrode, the nonaqueous electrolyte layer, and the button-type secondary battery for evaluation was the same as in Example 2.

Example 4 In this example, the surface of a carbon negative electrode using a baked product of mesocarbon microbeads as a negative electrode active material was covered with a coating layer made of a coating film of polyvinyl alcohol (PVA) as a water-soluble polymer. Li plate for counter electrode, LiP for non-aqueous electrolyte layer
A button type secondary battery for evaluation was constructed using each solution layer in which F ₆ was dissolved in a mixed solvent of ethylene carbonate / propylene carbonate / dimethyl carbonate.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 3 parts by weight of polyvinylidene fluoride as a binder is dissolved in 67 parts by weight of N-methylpyrrolidone, and 28 parts of mesocarbon microbeads are further dissolved.
30 parts by weight of a product baked at 00 ° C. (d ₀₀₂ = 3.380 °) was added and mixed and dispersed under an inert atmosphere using a roll mill to obtain a coating for a negative electrode. The above negative electrode paint was applied on a Cu foil having a thickness of 20 μm using a wire bar in the air.
After drying at 20 ° C. for 20 minutes, a film thickness of 6
A carbon negative electrode having a total thickness of 80 μm and a negative electrode active material layer of 0 μm was prepared.

Subsequently, a coating layer was formed on the surface of the carbon negative electrode. That is, a solution obtained by heating and dissolving 2.5 parts by weight of PVA (polymerization degree: 2,000, saponification degree: 98 to 99 mol% or more) in 80 parts by weight of pure water is applied to the surface of the carbon anode using a doctor blade, and the coating film is formed. Dry to thickness 2.5μ
m of coating layers were formed. The assembly of the counter electrode, the non-aqueous electrolyte layer, and the button type secondary battery for evaluation was the same as in Example 1.

Example 5 In this example, the surface of a carbon negative electrode using a mixture of natural graphite and calcined petroleum pitch coke containing boron (B) as a negative electrode active material was coated with a water-soluble polymer polyvinyl alcohol coating film. Button layer type secondary battery for evaluation using a Li plate as a counter electrode and a solution layer in which LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate / propylene carbonate / dimethyl carbonate as a non-aqueous electrolyte layer. Was configured.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 3 parts by weight of polyvinylidene fluoride as a binder is dissolved in 67 parts by weight of N-methylpyrrolidone, and 2500 parts of petroleum pitch coke containing 15 parts by weight of natural graphite (d ₀₀₂ = 3.355 °) and 4.5% of boron. C. and 15 parts by weight of a calcined product (d ₀₀₂ = 3.356 °) were added and mixed and dispersed under an inert atmosphere using a roll mill to obtain a paint for a negative electrode. The above negative electrode paint is applied on a 20 μm thick Cu foil using a wire bar in the air, dried at 120 ° C. for 10 minutes, and passed through a roll press to form a 60 μm thick negative electrode active material layer having a total thickness of 80 μm.
Was prepared. The assembly of the counter electrode, the non-aqueous electrolyte layer, and the button type secondary battery for evaluation was the same as in Example 1.

Example 6 In this example, the surface of a carbon negative electrode using a polyimide fired product as a negative electrode active material was covered with a coating layer composed of a coating film of hydroxyethyl cellulose, which is a water-soluble polymer.
As the plate and the nonaqueous electrolyte layer, a solution layer in which LiN (CF ₃ SO ₂ ) ₂ was dissolved in a mixed solvent of propylene carbonate / diethyl carbonate was used, respectively, to construct a button type secondary battery for evaluation.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 3 parts by weight of polyvinylidene fluoride as a binder is dissolved in 62 parts by weight of N-methylpyrrolidone, and a polyimide baked product at 1000 ° C. (d ₀₀₂ =
3.750 °) 35 parts by weight were added and mixed and dispersed under an inert atmosphere using a roll mill to obtain a negative electrode paint. The above negative electrode paint is coated with a wire bar in the atmosphere to a thickness of 20 mm.
It was applied on a μm Cu foil, dried at 120 ° C. for 20 minutes, and roll-pressed to produce a carbon negative electrode having a total thickness of 80 μm and a negative electrode active material layer having a thickness of 60 μm. Subsequently, a coating layer was formed on the surface of the carbon negative electrode. That is, 2.0 parts by weight of hydroxyethyl cellulose was added to pure water 8
The solution heated and dissolved in 0 parts by weight was applied to the surface of the carbon negative electrode using a doctor blade, and the coating film was dried to form a coating layer having a thickness of 1.5 μm.

Next, a button-type secondary battery for evaluation was assembled using the above carbon negative electrode. A Li plate was used as a counter electrode. In the non-aqueous electrolyte layer, LiN in a 4: 6 (volume ratio) mixed solvent of propylene carbonate / diethyl carbonate was used.
An electrolyte solution in which (CF ₃ SO ₂ ) ₂ was dissolved at a concentration of 2.0 mol / l was used by impregnating a separator made of porous polypropylene with a separator. Assembling of the button type secondary battery for evaluation was performed in the same manner as in Example 1.

Example 7 In this example, the surface of a carbon anode using a calcined fluid coke as an anode active material was treated with lithium carbonate (Li ₂ C).
O ₃ ) was covered with a coating layer composed of a sputtering film, a Li plate was used as a counter electrode, and a solution layer in which LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate / propylene carbonate / dimethyl carbonate was used as a nonaqueous electrolyte layer. Button type secondary battery was constructed.

First, a negative electrode paint was applied on a Cu foil as a current collector to prepare a carbon negative electrode. The coating material for the negative electrode was prepared as follows. That is, 2 parts by weight of polyvinylidene fluoride as a binder was dissolved in 58 parts by weight of N-methylpyrrolidone, and 40 parts by weight of a calcined product of Fluid Coke at 2500 ° C. (d ₀₀₂ = 3.370 °) was added. The mixture was mixed and dispersed under an atmosphere to obtain a negative electrode paint. The negative electrode paint was applied on a 20 μm thick Cu foil using a wire bar in the air, dried at 120 ° C. for 20 minutes, and roll-pressed to a total thickness of 80 μm having a 60 μm-thick negative electrode active material layer. Was prepared. Subsequently, a coating layer was formed on the surface of the carbon negative electrode.
That is, using an RF sputtering apparatus and a LiCO ₃ target, Ar gas flow rate = 20 cm ³ / min, pressure =
A coating layer having a thickness of 0.5 μm was formed under the conditions of 0.02 Torr, RF frequency = 10 MHz, and RF discharge output = 50 W. The assembly of the counter electrode, the non-aqueous electrolyte layer, and the button type secondary battery for evaluation was the same as in Example 1.

Example 8 In this example, the surface of a carbon negative electrode using a calcined product of petroleum pitch coke containing boron (B) as a negative electrode active material was covered with a coating layer made of a coating film of polyvinyl alcohol (PVA). Materials include lithium-cobalt composite oxide (LiCo
O ₂ ) and a polymer solid electrolyte layer containing LiPF ₆ as the non-aqueous electrolyte layer were used to form a lithium ion secondary battery.

First, the negative electrode coating material described in Example 2 was applied on a Cu foil as a current collector to produce a carbon negative electrode. Subsequently, the PVA solution described in Example 3 was applied to the surface of the carbon negative electrode. Was applied to form a coating layer. Next, when producing a positive electrode, LiCoO ₂ as a positive electrode active material was synthesized. That is, lithium carbonate (Li ₂ CO ₃ ) and cobalt carbonate (CoCO ₃ ) are mixed at a molar ratio of 1: 2, and calcined in air at 900 ° C. for 5 hours to form LiCo.
To give the O _2. The paint for the positive electrode was prepared as follows.
That is, 3 parts by weight of polyvinylidene fluoride as a binder is dissolved in 80 parts by weight of N-methylpyrrolidone, 91 parts by weight of the above LiCoO ₂ and 6 parts by weight of conductive graphite are added, and the mixture is rolled under an inert atmosphere using a roll mill. Was mixed and dispersed. The obtained coating material for a positive electrode was applied on an Al foil having a thickness of 20 μm using a doctor blade in the air to obtain 12 μm.
After drying at 0 ° C. for 20 minutes, a film thickness of 60
A positive electrode having an overall thickness of 80 μm and a positive electrode active material layer of μm was produced.

Next, a polymer solid electrolyte layer was formed by a photopolymerization method. First, 20 parts by weight of LiPF ₆ and 70 parts by weight of a mixed solvent of ethylene carbonate / propylene carbonate / dimethyl carbonate (2: 5: 3 (volume ratio)) were mixed to prepare an electrolyte solution. 12.8 parts by weight of polyoxyethylene acrylate, 0.2 parts by weight of trimethylpropane acrylate, and 0.02 parts by weight of benzoin isopropyl ether were added to this electrolyte to prepare a photopolymerizable polymer solution. This photopolymerizable polymer solution was impregnated into the above-described positive electrode and negative electrode, and irradiated with light from a high-pressure mercury lamp to solidify the electrolyte solution. Three sides were heat-sealed while uniformly applying pressure to the thus-obtained solid-state power generating element part, and the remaining one side was sealed under reduced pressure to produce a lithium ion secondary battery.

COMPARATIVE EXAMPLE 1 Except that a coating layer made of polyvinylidene fluoride was not formed on the surface of a carbon negative electrode using natural graphite as a negative electrode active material,
A button type secondary battery for evaluation was produced in the same manner as in Example 1.

[0048] Except for not formed Comparative Example 2 boron (B) added coating layer made of vinyl fluoride on the surface of the carbon anode to petroleum pitch coke fired product active material, for evaluation in the same manner as in Example 2 Was manufactured.

Comparative Example 3 A button-type secondary battery for evaluation was prepared in the same manner as in Example 4 except that a coating layer made of PVA was not formed on the surface of a carbon anode using mesocarbon microbeads as an anode active material. did.

Comparative Example 4 The same as Example 5 except that a coating layer made of PVA was not formed on the surface of a carbon negative electrode using a mixture of natural graphite and calcined petroleum pitch coke containing boron (B) as a negative electrode active material. A button-type secondary battery for evaluation was prepared.

Comparative Example 5 A button-type secondary battery for evaluation was prepared in the same manner as in Example 6, except that a coating layer made of hydroxyethylcellulose was not formed on the surface of a carbon negative electrode using a polyimide fired product as a negative electrode active material. did.

Comparative Example 6 A button-type secondary battery for evaluation was prepared in the same manner as in Example 7, except that a coating layer made of lithium carbonate was not formed on the surface of a carbon anode using a calcined product of fluid coke as an anode active material. Produced.

Comparative Example 7 Lithium ion was obtained in the same manner as in Example 8 except that a coating layer made of vinyl fluoride was not formed on the surface of a carbon anode using a fired product of petroleum pitch coke containing boron (B) as an anode active material. A secondary battery was manufactured.

Here, Examples 1 to 8 and Comparative Examples 1 to 7
A charge / discharge cycle test was performed on each of the batteries prepared in the above. For the test, a charge / discharge measurement device (Toyo System Co., Ltd .; T
OSCAT3000U type) was used. The following conditions A to C were used as measurement conditions depending on the type of battery. [Condition B] Constant current constant voltage charging at a current density of 1.0 mA / cm ² to a battery voltage of 0 V for 10 minutes Pause at a current density of 1.0 mA / cm ² at a constant voltage discharge of a battery voltage of 1.0 V for 10 minutes [Condition B] current density 1.5 mA / cm ^{2 and} until the battery voltage 0V constant current constant voltage charge for 10 minutes quiescent current density 1.5 mA / cm ² at a constant current discharge quiescent for 10 minutes until the battery voltage 1.0V [Requirement C] current density 1. Constant current constant voltage charging to battery voltage 4.2 V at 0 mA / cm ² Pause for 10 minutes Constant current discharge to battery voltage 3.0 V at current density of 1.0 mA / cm ² Pause for 10 minutes Pausing the results of Examples 1 to 7 [Table 1], the results of Comparative Examples 1 to 6 are summarized in [Table 2], and the results of Example 8 and Comparative Example 7 are summarized in [Table 3].

In Examples 1 to 7 and Comparative Examples 1 to 6, a Li plate was used as a counter electrode, and excessive Li was present in the battery system. Therefore, the discharge capacity densities (mAh / g) shown in [Table 1] and [Table 2] are indicators of the performance of the negative electrode. [Table 1] and [Table 2] show the initial charge / discharge efficiency (%) and the discharge capacity density at the initial stage and after 200 cycles in the form of a list. On the other hand, in Example 8 and Comparative Example 7, a carbon anode, a solid polymer electrolyte, and a cathode using a lithium-containing composite oxide as a cathode active material were used.
Performance evaluation is performed in a form close to an actual battery. Therefore, the discharge capacity (mAh) shown in [Table 3] is an index of the performance of the whole battery. [Table 3] shows the initial and 2
The discharge capacity after 00 cycles is shown in a table format.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

Looking at the results of Examples 1 to 7 and Comparative Examples 1 to 6, the batteries of the comparative example having no coating layer on the surface of the carbon negative electrode were different from the batteries of the example in the initial discharge capacity density. Almost the same. However, looking at the discharge capacity density after 200 cycles, each of the batteries of the example had an initial value of 80%.
While maintaining the value in the high 90% range to the low 90% range,
The values of the batteries of the comparative examples were in the range of 70% to the early half of the 80% range. The initial charge / discharge efficiency of each battery of the comparative example was 5 to 12% lower than that of each battery of the example. Therefore, it was found that the batteries of the present invention in which the coating layer was provided on the surface of the carbon negative electrode had improved initial charge / discharge efficiency and cycle characteristics even though propylene carbonate was contained in the electrolyte layer. Was. This is because the coating layer improves the wettability between the carbon anode and the electrolyte layer, suppresses decomposition of the electrolyte layer,
This is because it contributed to the suppression of the deposition of decomposition products of the electrolyte layer on the negative electrode surface.

Next, when comparing Example 8 with Comparative Example 7,
In Comparative Example 7, the discharge capacity after 200 cycles was reduced to about 72% of the initial value, whereas in Example 8, about 85% of the initial value was maintained. Therefore, even in an evaluation close to an actual battery, it was demonstrated that the battery of the present invention had excellent cycle characteristics.

As described above, the present invention has been described based on the eight embodiments, but the present invention is not limited to these embodiments. For example, in the above-described embodiments, a button type secondary battery and a lithium ion secondary battery for evaluation were manufactured, but the present invention can be widely applied to other non-aqueous electrolyte secondary batteries. Other details such as the type and formation method of the coating layer to be formed, selection of materials for the negative electrode active material, the counter electrode active material, and the non-aqueous electrolyte, the composition and preparation conditions of the negative electrode paint, and the form of the battery to be produced are described in this book. Selection, modification, and combination can be appropriately made without departing from the spirit of the invention.

[0062]

As is clear from the above description, in the present invention, by providing a coating layer on the surface of the carbon negative electrode, the merits of the carbon negative electrode such as excellent reversibility of lithium insertion / removal and excellent safety can be maintained. In addition, it is possible to provide a nonaqueous electrolyte secondary battery having improved initial charge / discharge efficiency and cycle characteristics. The coating layer contributes to improvement of battery characteristics even when formed using any of an ion-conductive polymer, a water-soluble polymer, and an alkali metal compound. Specifying the surface distance d ₀₀₂ in the c-axis direction of the carbon material contained in the carbon negative electrode as a negative electrode active material in the c-axis direction at 3.40 ° and combining two or more types of carbon materials are both initial charging and discharging efficiencies and cycle characteristics. It is effective for improvement. In addition, the fact that propylene carbonate and a graphite-based carbon anode which can easily decompose it can be used in combination according to the present invention has industrial significance because the degree of freedom of material selection for achieving excellent battery performance is expanded. large. The carbon negative electrode is also excellent in matching with the polymer solid electrolyte, and can provide a non-aqueous electrolyte secondary battery having a high discharge capacity without causing any capacity reduction by using the polymer solid electrolyte. It becomes possible.

Claims (8)

[Claims] 1. A non-aqueous electrolyte secondary battery including at least a positive electrode, a non-aqueous electrolyte layer, and a carbon negative electrode capable of inserting and extracting lithium, wherein a coating layer is provided on a surface of the carbon negative electrode. Non-aqueous electrolyte secondary battery characterized by the above-mentioned. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the coating layer is made of an ion conductive polymer. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein said coating layer is made of a water-soluble polymer. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein said coating layer is made of at least one of an alkali metal salt, an alkali metal oxide and an alkali metal hydroxide. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon material contained as the negative electrode active material in the carbon negative electrode has a surface distance d _{002 in} the c-axis direction of 3.40 ° or less. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein said carbon anode is composed of two or more carbon materials. 7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte layer contains propylene carbonate. 8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte layer is a solid polymer electrolyte layer.