Anode for lithium ion secondary battery, and lithium ion secondary battery

The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode and having a large discharge capacity.

Lithium-ion secondary batteries have excellent features such as high operating voltage, large battery capacity, and long cycle life, and because they have less environmental pollution, nickel-cadmium batteries and nickel, which have been the mainstream in the past, are used. Widely used in place of hydrogen batteries. Lithium-ion secondary batteries became practical because it was discovered that a carbon material with lithium ions intercalated could be a stable active material instead of lithium metal, which had a safety problem as a negative electrode material. This is due to the recognition of the role of carbon materials in the practical application and performance improvement of ion secondary batteries.

With the recent increase in performance and functionality of portable electronic devices such as mobile phones and notebook computers, power consumption has increased, and there is a demand for even higher capacity lithium-ion secondary batteries. The capacity of the lithium-ion secondary battery is largely dominated by the discharge capacity per mass of the carbon material for the negative electrode, but the discharge capacity per mass is the theoretical capacity of 372 mAh / g of high-purity natural graphite among the carbon materials. Is the limit. Therefore, instead of carbon, it is considered to use silicon, tin, etc., which have a high discharge capacity per unit mass. For example, Patent Document 1 proposes an electrode in which a primary active material layer such as silicon or germanium is formed on a current collector, and a carbon secondary active material layer is further formed on the primary active material layer. However, the first active material layer covers the entire current collector, and since these materials have a large expansion coefficient during charging, carbon is separated from the second active material layer, and a sufficient current cannot be supplied. There was a problem. For example, since silicon expands in volume by as much as 300%, there is a problem that the negative electrode collapses due to repeated charging and discharging, and the discharge capacity deteriorates. Further, an electrode for a secondary battery in which a thin film such as a tin-nickel alloy or a lithium-aluminum alloy is formed on a current collector and a layer of a carbon material is formed on the thin film has been proposed (Patent Document 2). .. This has the problem that the current supply to the layer of carbon material is insufficient.<patcit num="1"><text>Japanese Unexamined Patent Publication No. 2001-283834</text></patcit><patcit num="2"><text>Japanese Unexamined Patent Publication No. 2002-15729</text></patcit>

<p> The present invention uses a metal or alloy such as silicon or tin, which has a larger discharge capacity per unit mass than carbon as a negative electrode active material but has a large expansion rate, but has a predetermined area ratio on the surface of the current collector. Since a thin film of the alloy of the above is formed and the current is directly supplied to the layer of the carbon material, it is possible to absorb the expansion of metals such as silicon and tin in the negative electrode mixture layer and the alloy during charging of lithium ions. It is an object of the present invention to provide a lithium ion secondary battery having a high discharge capacity and excellent cycle characteristics, and to provide a negative electrode for that purpose.</p>

<p> The present invention is characterized in that a thin layer of metal that occludes and desorbs lithium ions is provided on the surface of the current collector at an area ratio of 5 to 90%, and a layer of carbon material is further provided on the thin layer. It is a negative electrode for a lithium ion secondary battery.</p><p> In the negative electrode for a lithium ion secondary battery of the present invention, the metal forming the thin layer of the metal is preferably a simple substance or an alloy of silicon, tin, antimony, niobium or aluminum.</p><p> In the negative electrode for a lithium ion secondary battery of the present invention, the layer thickness of the thin layer of the metal is preferably 10 μm or less.</p><p> In the negative electrode for a lithium ion secondary battery of the present invention, the current collector is preferably copper, nickel, stainless steel or iron.</p><p> The present invention is a lithium ion secondary battery using any of the above-mentioned negative electrodes for a lithium ion secondary battery.</p>

<p> The negative electrode for a lithium ion secondary battery of the present invention can absorb this expansion because a metal such as silicon or tin, which expands greatly during lithium ion storage, is in contact with the current collector. Moreover, since the metal is dispersed in the form of particles, the conductivity of the carbon material to the layer is also good. As a result, when a lithium ion secondary battery is manufactured using the negative electrode for a lithium ion secondary battery of the present invention, a high discharge capacity can be obtained and the cycle characteristics are also excellent.</p>

Hereinafter, the present invention will be specifically described. The present invention has a thin layer of a metal (including an alloy, which will be described later) that occludes and desorbs lithium ions on the surface of the current collector at an area ratio of 5 to 90%, and carbon on the thin layer. A negative electrode for a lithium ion secondary battery having a layer of material.

(Current collector) The material of the current collector used for the negative electrode is a metal such as copper, stainless steel, nickel, or iron, and copper is particularly preferable. The shape of the current collector is not particularly limited, but a foil-like shape, a mesh-like shape, a mesh-like shape such as expanded metal, or the like is used. In the case of a foil, the thickness of the current collector is preferably 5 to 20 μm, particularly preferably 5 to 15 μm. The size of the current collector is determined by the size of the lithium ion secondary battery.

(Thin layer of metal) The thin layer of metal existing on the surface of the current collector used for the negative electrode effectively acts to increase the discharge capacity of the negative electrode. The thin layer of the metal is formed on a part of the surface of the current collector, not on the entire surface. It has an area ratio of 5 to 90%, preferably 10 to 80%, and more preferably 20 to 70%. If it is less than 5%, the expansion of the metal as a negative electrode due to the lithium ions in the thin layer of the metal cannot be absorbed. If it exceeds 90%, the conductivity as a negative electrode is inferior. The area ratio of the thin layer of the metal is an average value obtained by observing the mapping image of the characteristic X-ray of the metal used in 10 fields of view. Further, since the metal is dispersed in the form of particles or the particles are fused with each other, the metal has excellent conductivity as a negative electrode and a high discharge capacity can be obtained. The thickness of the thin layer of the metal is preferably 10 μm or less, more preferably 0.5 to 8 μm. If it exceeds 10 μm, collapse due to expansion and contraction of the thin layer of metal may occur. The layer thickness is a value obtained by measuring the layer thickness at 10 points with a micrometer and arithmetically averaging the layer thickness.

The thin layer of the metal is partially thinned by etching after forming a thin layer of the metal or alloy once by a method of applying a melt of the metal or alloy to the current collector, a method of plating the metal or alloy, or the like. It is formed by a method of removing a layer, a method of adhering a metal to a current collector by sputtering or vacuum deposition, and a method of masking a part of the surface of the current collector when forming a thin layer. In order to smooth the thin layer of metal once formed, treatments such as gas blowing, heat treatment, and rolling may be performed.

The metal or alloy is not particularly limited as long as it is a metal or alloy capable of occluding and desorbing lithium ions, but is preferably a simple substance or alloy of a metal such as silicon, tin, antimony, niobium, or aluminum. Particularly preferred are elemental silicon or tin or alloys containing silicon or tin. The shape of the thin layer of the metal or alloy is not particularly limited, but it is preferably distributed in the form of particles when observed from the direction perpendicular to the surface of the current collector. The particle size of the metal or alloy distributed in the form of particles is 1 nm to 100 μm, preferably 10 nm to 50 μm, and more preferably 0.1 to 10 μm. The particle size is a value obtained by measuring the maximum diameter of 100 particles with a scanning electron microscope and arithmetically averaging them.

(Carbon Material Layer) The negative electrode of the present invention has a carbon material layer on the thin layer of the metal. The carbon material layer acts to prevent the collapse of a thin layer of metal that expands by absorbing lithium ions. As a result, the cycle characteristics of the lithium ion secondary battery can be improved. The carbon material layer is formed on a thin layer of metal on the surface of the current collector and on a portion without the thin layer of metal, that is, on the surface of the current collector. So to speak, since the carbon material has a structure in which the carbon material bites into the surface of the current collector like a wedge between the metals, the prevention of the collapse of the thin layer of the metal is further strengthened. Here, the amount of adhesion of the carbon material layer is 5 to 40 mg / cm.², Especially 10 ~ 20mg / cm²Is preferable. Within this range, the expansion of the thin layer of metal can be absorbed and the cycle characteristics can be improved. Further, the amount of adhesion can be determined by measuring the mass before and after forming the carbon material layer and determining the difference in mass before and after the formation.

The formation of the carbon material layer is not particularly limited as long as it is a method that can sufficiently bring out the performance of the carbon material, has high moldability with respect to the carbon material powder, and can obtain a chemically and electrochemically stable negative electrode. For example, a carbon material such as natural graphite composite, carbon fiber graphitized product, or mesocarbon microspherical graphitized product is adjusted to an appropriate particle size by classification, etc., and mixed with a binder in the presence of an organic solvent to prepare a negative mixture. A paste is prepared and the paste is applied onto a thin layer of the metal on one or both sides of the current collector to form a carbon material layer.

As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferably used, and for example, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and carboxy are used. Methyl cellulose, styrene butadiene rubber, etc. are used. These can also be used together. It is preferable to use an organic solvent-based binder such as polyvinylidene fluoride. The binder is usually preferably used in an amount of 0.5 to 20% by mass based on the total amount of the negative electrode mixture paste. In the present invention, a negative electrode having excellent charge / discharge characteristics can be obtained by using a water-soluble and / or water-dispersible water-based binder as well as an organic solvent-based binder that dissolves or disperses in an organic solvent. be able to.

(Negative electrode) To prepare a more specific negative electrode, for example, a carbon material and a fluorine-based resin powder such as polytetrafluoroethylene are mixed and kneaded in a solvent such as isopropyl alcohol to obtain a paste, which is then used. This is a method of applying to a current collector. Further, after mixing a highly crystalline carbon material, a fluororesin such as polyvinylidene fluoride and / or carboxymethyl cellulose, styrene butadiene rubber, etc. with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol, etc. to form a slurry. , Can also be applied. The paste can be prepared by stirring using a known stirrer, mixer, kneader, kneader or the like. When the negative electrode mixture paste is applied onto the thin layer of the metal and dried, the carbon material layer is uniformly and firmly adhered to the current collector having the thin layer of the metal. A carbon material layer can also be formed by dry-mixing a carbon material and a resin powder such as polyethylene or polyvinyl alcohol and hot-press molding in a mold. After forming the carbon material layer, crimping such as press pressure can be performed to produce a negative electrode in which the adhesive strength between the carbon material layer and the current collector having a thin metal layer is further increased.

(Lithium Ion Secondary Battery) A lithium ion secondary battery is essentially a battery mechanism in which lithium ions are stored in a negative electrode during charging and discharging and desorbed from the negative electrode during discharging. Lithium-ion secondary batteries usually have a negative electrode, a positive electrode and a non-aqueous electrolyte as main battery components. The lithium ion secondary battery of the present invention is not particularly limited except that it has the above-mentioned thin metal layer and carbon material layer on the current collector as a negative electrode, and general lithium ions for other battery components. It conforms to the elements of the secondary battery.

As the material for the positive electrode (positive electrode active material), it is preferable to select a material capable of occluding and desorbing a sufficient amount of lithium. Such positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, and vanadium oxides (V).₂O₅, V₆O₁₃, V₂O₄, V₃O₈Etc.) and lithium-containing compounds such as Li compounds, general formula M_XMo₆S_8-Y(In the formula, X is a numerical value in the range of 0 X 4, Y is a numerical value in the range of 0 Y 1, and M represents a metal such as a transition metal). Can be used.

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more kinds of transition metals. Specifically, the lithium-containing transition metal oxide is LiM (1)._1-XM (2)_XO₂(In the equation, X is a numerical value in the range of 0 X 1, and M (1) and M (2) represent at least one type of transition metal) or LIM (1)._2-YM (2)_YO₄(Y in the formula is a numerical value in the range of 0 Y 1, and M (1) and M (2) represent at least one kind of transition metal element). In the above, examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In and Sn, and preferably Co, Fe, Mn, Ti and Cr. , V, Al. More specifically, the lithium-containing transition metal oxide is LiCoO.₂, Li_XNi_YM_1-YO₂(M is a transition metal element other than Ni, preferably at least one selected from Co, Fe, Mn, Ti, Cr, V, Al, 0.05 X 1.10, 0.5 Y 1.0) Oxide, LiNiO₂, LiMnO₂, LiMn₂O₄And so on.

The lithium-containing transition metal oxide as described above uses, for example, Li, an oxide or salt of the transition metal as a starting material, and these starting materials are mixed according to the composition, and the temperature is 600 to 1000 ° C in an oxygen presence atmosphere. It can be obtained by firing in a range. The starting material is not limited to oxides or salts, and can also be synthesized from hydroxides and the like. In the present invention, the compound may be used alone or in combination of two or more as the positive electrode active material. Further, a carbon salt such as lithium carbonate and various additives such as a conventionally known conductive agent and binder can be appropriately added. In order to form a positive electrode with such a positive electrode material, for example, a positive electrode mixture composed of a positive electrode material, a binder, and a conductive agent for imparting conductivity to the electrodes is applied to both sides of a current collector to form a positive electrode. Form a drug layer. As the binder, any of those exemplified for the negative electrode can be used. As the conductive agent, for example, graphite particles are used.

The shape of the current collector is not particularly limited, and a foil shape, a mesh shape, a mesh shape such as an expanded metal, or the like is used. For example, examples of the current collector include aluminum, stainless steel, and nickel. The thickness is preferably 10 to 40 μm. In the case of the positive electrode as well as the negative electrode, the positive electrode mixture is dispersed in a solvent to form a paste, and the paste-like positive electrode mixture is applied to the current collector and dried to form a positive electrode mixture layer. Alternatively, after forming the positive electrode mixture layer, crimping such as press pressurization may be further performed. As a result, the positive electrode mixture layer is uniformly and firmly adhered to the current collector.

As the electrolyte used in the present invention, an electrolyte salt used in a normal non-aqueous electrolyte solution can be used, for example, LiPF.₆, LiBF₄, LiAsF₆, LiClO₄, LiB (C₆H₅), LiCl, LiBr, LiCF₃SO₃, LiCH₃SO₃, LiN (CF)₃SO₂), LiC (CF)₃SO₂)₃, LiN (CF)₃CH₂OSO₂)₂, LiN (HCF)₂CF₂CH₂OSO₂)₂, LiN [(CF₃)₂CHOSO₂]₂, LiB [C₆H₃(CF₃)₂]₄, LiAlCl₄, LiSiF₆Lithium salts such as can be used. In particular, LiPF6 and LiBrF4 are preferably used from the viewpoint of oxidative stability. The electrolyte concentration in the electrolytic solution is preferably 0.1 to 5 mol / L, more preferably 0.5 to 3.0 mol / L. The non-aqueous electrolyte may be a liquid-based non-aqueous electrolyte solution, or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery and a polymer gel electrolyte battery.

When a liquid non-aqueous electrolyte solution is used, the solvent used is ethyle carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, etc. 2-Methyltatrahydrofuran, γ-butyllactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethylborate , Tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, dimethylsulfite, etc. it can.

When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer electrolyte, it contains a matrix polymer compound gelled with a plastic agent (non-aqueous electrolyte solution). Independently uses ether-based polymer compounds such as polyethylene oxide and its crosslinked compounds, polymethacrylate-based compounds, polyacrylate-based compounds, and fluorine-based polymers such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. , Or can be mixed and used. Among these, from the viewpoint of redox stability and the like, it is preferable to use a fluorine-based polymer compound such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer. As the electrolyte salt and the non-aqueous solvent constituting the plasticizer contained in the polymer solid electrolyte and the polymer gel electrolyte, any of the above can be used. In the case of a gel electrolyte, the electrolyte salt concentration in the non-aqueous electrolyte solution as a plasticizer is preferably 0.1 to 5 mol / L, more preferably 0.5 to 2.0 mol / L.

The method for producing such a solid electrolyte is not particularly limited, but for example, a method in which a polymer compound forming a matrix, a lithium salt and a solvent are mixed and heated to be melted, or an appropriate organic solvent for mixing is used. A method in which a molecular compound, a lithium salt and a solvent are dissolved and then an organic solvent for mixing is evaporated, and a monomer, a lithium salt and a solvent are mixed and irradiated with ultraviolet rays, an electron beam or a molecular beam to form a polymer. You can give a method to make it. The addition ratio of the solvent in the solid electrolyte is preferably 10 to 90% by mass, more preferably 30 to 80% by mass. When it is 10 to 90% by mass, the conductivity is high, the mechanical strength is high, and it is easy to form a film.

In the lithium ion secondary battery of the present invention, a separator can also be used. The separator is not particularly limited, and examples thereof include woven fabrics, non-woven fabrics, and microporous membranes made of synthetic resin. In particular, a porous membrane made of synthetic resin is preferably used, and among them, a polyolefin-based microporous membrane is preferable in terms of thickness, membrane strength, and membrane resistance. Specifically, it is a microporous membrane made of polyethylene and polypropylene, or a microporous membrane made of a composite thereof.

In the lithium ion secondary battery of the present invention, since the initial charge / discharge efficiency is high, it is also possible to use a gel electrolyte. The gel electrolyte secondary battery is configured by laminating the negative electrode, the positive electrode, and the gel electrolyte in the order of, for example, the negative electrode, the gel electrolyte, and the positive electrode, and accommodating them in the battery exterior material. In addition to this, a gel electrolyte may be further arranged on the outside of the negative electrode and the positive electrode. In a gel electrolyte secondary battery using such a negative electrode, the irreversible capacity is small even when the gel electrolyte contains propylene carbonate and the particle size is small enough to sufficiently lower the impedance as the carbon material powder. It can be suppressed. Therefore, a large discharge capacity can be obtained and a high initial charge / discharge efficiency can be obtained.

Further, the structure of the lithium ion secondary battery according to the present invention is arbitrary, and its shape and form are not particularly limited, and can be arbitrarily selected from cylindrical type, square type, coin type, and button type. be able to. In order to obtain a safer sealed non-aqueous electrolyte battery, it is preferable to provide a means for detecting an increase in the internal pressure of the battery and shutting off the current in the event of an abnormality such as overcharging. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, the structure may be enclosed in a laminated film.

Next, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. Further, in the following Examples and Comparative Examples, a button-type secondary battery for evaluation having a configuration as shown in FIG. 1 was produced and evaluated, but the actual battery was evaluated according to a known method based on the concept of the present invention. Can be made.

(Example 1) A copper foil having a thickness of 20 μm is immersed in a tin chloride aqueous solution, tin-plated, and then the tin-plated copper foil is heated to 300 ° C. for 2 seconds, and at the same time, nitrogen gas is sprayed on the surface of the tin-plated layer. The amount of plating per unit area on copper foil is 1.0 mg / cm²Formed a thin layer of tin. The thickness of the thin layer was 2.8 μm, and the thin layer of tin was distributed in the form of particles having an average particle size of 3.0 μm, and the area ratio was 50%. The mesocarbon globules obtained by heat-treating the coal tar pitch to generate mesocarbon globules, and the mesocarbon globules obtained by extraction and filtration are calcined and graphitized to obtain 90% by mass of the mesocarbon globules graphitized, and polyfluor as a binder. 10% by mass of vinylidene conjugation was mixed with N-methylpyrrolidone as a solvent and stirred at 500 rpm for 5 minutes using a homomixer to prepare an organic solvent-based negative electrode mixture paste. The amount of the negative electrode mixture paste applied onto the copper foil (current collector 7b) having a thin layer of tin is 11 mg / cm.²Then, the solvent was volatilized at 90 ° C in vacuum and dried. Next, the negative electrode mixture applied on the copper foil is pressed by a roller press and further punched into a circular shape with a diameter of 15.5 mm to form a carbon material layer (adhesion amount: 10 mg / cm).²) Made a working electrode 2 in close contact with a copper foil (thickness: 20 μm) through a thin layer of tin (thickness: 2.8 μm).

The counter electrode 4 was produced by pressing a lithium metal foil against a nickel net and punching it into a circular shape of 15.5 mm in a direct line to form a current collector (7a) made of a nickel net, and the lithium metal foil was brought into close contact with the current collector. .. LiPF in a solvent mixed at a ratio of 33% by mass of ethylene carbonate and 67% by mass of methyl ethyl carbonate.₆Was dissolved at a concentration of 1 mol / L to prepare a non-aqueous electrolytic solution. The obtained non-aqueous electrolyte solution was impregnated into a polypropylene porous body to prepare a separator 5 impregnated with the electrolyte solution.

A button-type secondary battery shown in FIG. 1 was manufactured as an evaluation battery. The evaluation battery has a hermetically sealed structure in which the outer cup 1 and the outer can 3 are crimped via an insulating gasket 6 in the peripheral portion thereof, and a collection of nickel nets is formed inside the outer cup 3 in order from the inner surface of the outer can 3. It is a battery system in which an electric body 7a, a disk-shaped working electrode 2 made of lithium foil, and a current collector 7b made of copper foil are laminated. In the evaluation battery, the separator 5 impregnated with the electrolyte solution is sandwiched between the working electrode 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a, and then laminated, and then the working electrode 2 is attached to the exterior. In the cup 1, the counter electrode 4 is housed in the outer can 3, the outer cup 1 and the outer can 3 are combined, and the peripheral portion between the outer cup 1 and the outer can 3 is caulked and sealed via the insulating gasket 6. Made.

The evaluation battery was subjected to the following charge / discharge test at a temperature of 25 ° C. Constant current charging was performed with a current value of 0.9 mA until the circuit voltage reached 0 mV, and when the circuit voltage reached 0 mV, constant voltage charging was switched to, and charging was continued until the current value reached 20 μA. The charge capacity was calculated from the amount of electricity supplied during that period. After that, it rested for 120 minutes. Next, constant current charging was performed with a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity was calculated from the amount of energization during that period. In this test, the process of occluding lithium ions in the carbon material was defined as charging, and the process of desorbing lithium ions from the carbon material was defined as discharging. The discharge capacity was 402 mAh / g, which was a high capacity exceeding the theoretical value of 372 mAh / g of natural graphite. Further, the negative electrode after charging / discharging at this time was taken out, the thickness of the working electrode after charging / discharging was measured, and compared with the thickness of the working electrode before charging / discharging. The thickness of the working electrode was measured with a micrometer. In addition, the expansion coefficient between before and after charging and discharging the working electrode was calculated from the following equation. The expansion rate was 18%. Expansion rate (%) = [(Thickness after charging / discharging-Thickness before charging) / Thickness before charging] x 100

(Example 2) In Example 1, the basis weight of the thin layer of tin was 2.0 mg / cm.²A working electrode and an evaluation battery were prepared under the same methods and conditions as in Example 1 except that the layer thickness was 4.5 μm and the area ratio was 60%, and a charge / discharge test was performed in the same manner as in Example 1. It was. The discharge capacity was 453 mAh / g and the expansion coefficient was 23%.

(Comparative Example 1) In Example 1, the basis weight of the thin layer of tin was 5.0 mg / cm.²The working electrode and the evaluation battery were produced under the same methods and conditions as in Example 1 except that the layer thickness was 14.0 μm, the area ratio was 50%, and the carbon material layer was omitted, and the same as in Example 1. A charge / discharge test was conducted. The discharge capacity was 991 mAh / g and the expansion coefficient was 53%.

(Comparative Example 2) In Example 1, a working electrode and an evaluation battery were produced under the same method and conditions as in Example 1 except that a copper foil having no thin layer of tin was used, and in the same manner as in Example 1. A charge / discharge test was performed. The discharge capacity was 345 mAh / g and the expansion coefficient was 14%.

(Comparative Example 3) In Example 2, the basis weight of the thin layer of tin was 3.3 mg / cm.²A working electrode and an evaluation battery were prepared under the same methods and conditions as in Example 2 except that the layer thickness was 4.5 μm and the area ratio was 100%, and a charge / discharge test was performed in the same manner as in Example 1. It was. The discharge capacity was 563 mAh / g and the expansion coefficient was 35%.

<tables num="1"><img file="JP2005293960A_D0001.tif" /></tables> As shown in Table 1, the examples are excellent in that the discharge capacity is high and the expansion coefficient is small.

Next, using the working electrodes prepared in Examples 1 and 2 and Comparative Examples 1 to 3, LiPF was used in a solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 2.₆A non-aqueous electrolyte solution having a concentration of 1 mol / L was used, the counter electrode was lithium cobalt oxide, and an evaluation battery was separately prepared with a porous separator between the working electrode and the counter electrode. Using the working electrode, the discharge capacity retention rate was determined. The results are shown in Fig. 2. The discharge capacity retention rate refers to the change in the discharge capacity of each cycle when the initial discharge capacity is 100%.

The discharge capacity retention rate in the 100th cycle was best at 93% when the current collector of Comparative Example 2 did not have a thin tin layer, and the current collectors of Examples 1 and 2 had a thin tin layer. 91% and 87% had the second best. On the other hand, 78% of the current collectors of Comparative Example 3 had a thin layer of tin with an area ratio of 100%, and 71% of the current collectors of Comparative Example 1 did not have a carbon material layer. From the above, when a negative electrode having a metal thin layer having a predetermined area ratio is used for the current collector as in Examples 1 and 2, lithium ions having a high discharge capacity and excellent cycle characteristics (discharge capacity retention rate). Secondary batteries can be manufactured.

The lithium ion secondary battery using the negative electrode of the present invention is mounted on a small electronic device and can contribute to its high performance and high functionality.

<figref num="1">It is sectional drawing of the evaluation battery for evaluating charge / discharge characteristics.</figref><figref num="2">It is a graph which shows the discharge capacity maintenance rate of the evaluation battery.</figref>

Code description

1 Exterior cup 2 Working electrode 3 Exterior can 4 Counter electrode 5 Electrolyte solution impregnated separator 6 Insulation gasket 7a, 7b Current collector