JP2007213875A - Lithium secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having high energy density and excelling in a charge-discharge cycle characteristic; and to provide its manufacturing method. <P>SOLUTION: This lithium secondary battery is provided with: a positive electrode 2 composed by arranging a positive electrode active material layer 23 on a surface of a positive electrode collector 20 made of conductive metal foil; a negative electrode composed by arranging a negative electrode active material layer containing silicon on a negative electrode collector made of conductive metal foil; a separator arranged between the positive electrode and the negative electrode; a battery case for housing a cylindrical electrode body formed by facing the positive electrode and the negative electrode to each other through the separator and by spirally rolling them; a positive electrode collector tab 7 connected to the positive electrode collector 20 and extracted from the electrode body; a negative electrode collector tab connected to the negative electrode collector and extracted from the electrode body; and a nonaqueous electrolyte. The lithium secondary battery is characterized in that the positive electrode collector tab 7 is formed out of the positive electrode collector 20, and the negative electrode collector tab is formed out of the negative electrode collector. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery and a method for manufacturing the same.

  In recent years, as one of new secondary batteries with high output and high energy density, there has been a lithium secondary battery that uses a non-aqueous electrolyte and moves lithium ions between a positive electrode and a negative electrode for charging and discharging. It's being used.

  Lithium secondary batteries have been put into practical use as power sources for information technology-related portable electronic devices such as mobile phones and laptop computers, and are widely used. In the future, with the further miniaturization and higher functionality of these portable devices, it is expected that the load on the lithium secondary battery, which is the power source, will increase, and the demand for higher energy density of the lithium secondary battery is extremely high It is very expensive.

  An effective means for increasing the energy density of a battery is to use a material having a large energy density as an active material. Therefore, recently, in a lithium secondary battery, as a negative electrode active material having a high energy density, instead of graphite which has been put into practical use, an element such as Al, Sn, or Si that occludes lithium by an alloying reaction with lithium is used. Alloy materials have been proposed and studied.

  However, in an electrode using a material that is alloyed with lithium as an active material, the volume of the active material expands / contracts when lithium is occluded / released. There is a problem in that peeling occurs, current collection into the electrode is reduced, and charge / discharge cycle characteristics are deteriorated.

  In the negative electrode using a material containing silicon as a negative electrode active material to be alloyed with lithium, the present applicant has an active material thin film made of a material containing silicon on a current collector of a conductive metal foil having irregularities on the surface. It has been found that the electrode in which the electrode is formed exhibits a high current collecting property in the electrode and good charge / discharge cycle characteristics can be obtained (Patent Document 1, etc.). In addition, an electrode in which a mixture layer containing an active material composed of a silicon-containing material and a binder is sintered in a non-oxidizing atmosphere is disposed in the electrode due to high adhesion between the mixture layer and the current collector. It has been found that high current collecting properties are exhibited and good charge / discharge cycle characteristics can be obtained (Patent Document 2, etc.).

  Further, in order to achieve a higher energy density of the battery, not only the active material having a high energy density as described above is used, but also as many positive electrode actives as possible in a battery container of a predetermined size. It is also necessary to pack the material and the negative electrode active material. In a commonly used battery, a positive electrode and a negative electrode are opposed to each other via a resin porous separator, and an electrode body wound in a flat shape or a cylindrical shape is stored in a rectangular or cylindrical container. As a result, high energy density is achieved.

  However, in a cylindrical battery using a material containing silicon as a negative electrode active material, stress is generated due to a large increase in the volume of the negative electrode active material during occlusion of lithium, and this stress is deformed by external force compared to the positive electrode and the negative electrode. Since many separators are easily added to the porous separator made of resin, the separator is compressed and clogged, the lithium ion conductivity between the positive electrode and the negative electrode is greatly reduced, and the charge / discharge characteristics are deteriorated.

  In the cylindrical battery using the mixture layer containing the silicon negative electrode active material as described above and the negative electrode having a current collector with high adhesion, a portion where the positive electrode and the negative electrode face each other with a separator interposed therebetween. It has been found that cycle characteristics are improved by using a spiral cylindrical electrode body having a curvature radius of 1.5 mm or more (Patent Document 3).

Although such a method can improve the cycle characteristics to some extent, further improvement of the cycle characteristics is required.
JP 2002-83594 A Japanese Patent Laid-Open No. 2002-260637 JP 2005-166530 A

  It is an object of the present invention to provide a lithium secondary battery having a high energy density and excellent charge / discharge cycle characteristics and a method for producing the same.

  In the present invention, a positive electrode in which a positive electrode active material layer is arranged on the surface of a positive electrode current collector made of a conductive metal foil, and a negative electrode active material layer containing silicon is arranged on a negative electrode current collector made of a conductive metal foil. A battery container containing a negative electrode, a separator disposed between the positive electrode and the negative electrode, a cylindrical electrode body in which the positive electrode and the negative electrode are opposed to each other with the separator interposed between them, and a positive electrode current collector A lithium secondary battery comprising: a positive current collector tab connected to the electrode body and drawn from the electrode body; a negative electrode current collector tab connected to the negative electrode current collector and drawn from the electrode body; and a non-aqueous electrolyte. Is formed from a positive electrode current collector, and the negative electrode current collector tab is formed from a negative electrode current collector.

  In the present invention, since the positive electrode current collector tab and the negative electrode current collector tab are formed from the positive electrode current collector and the negative electrode current collector, respectively, the current collector tab is made of a metal such as an Al plate or Ni plate as in the prior art. Unlike the case of forming from a plate, the electrode body can be smoothly deformed with deformation of the silicon active material during charging. For this reason, clogging due to the compression of the separator can be suppressed, and excellent charge / discharge cycle characteristics can be obtained.

  Moreover, in this invention, it is preferable that the positive electrode current collection tab and the negative electrode current collection tab are formed in each edge part of a positive electrode and a negative electrode. When the shape of the electrode is a rectangle generally used in a cylindrical battery, a current collecting tab is preferably formed at the end in the longitudinal direction. By forming the current collecting tab at the end in the longitudinal direction, the winding process of the electrode body can be facilitated.

  Moreover, in this invention, it is preferable that a positive electrode current collection tab and a negative electrode current collection tab are the area of 5% or less of each electrode area of a positive electrode and a negative electrode. When the area of the positive electrode current collector tab and the negative electrode current collector tab exceeds 5% of the electrode area of each of the positive electrode and the negative electrode, smooth deformation of the electrode body during charge / discharge is less likely to occur, and the separator is clogged. It tends to occur and the charge / discharge characteristics may deteriorate.

  The negative electrode of the present invention is preferably one in which a negative electrode mixture layer containing negative electrode active material particles containing silicon and a negative electrode binder is sintered on the surface of a conductive metal foil negative electrode current collector. By using such a negative electrode, even when a material containing silicon with large volume expansion during occlusion of lithium is used as the negative electrode active material, the adhesion between the negative electrode active material particles and the negative electrode can be reduced due to the effect of sintering. Adhesion between the mixture layer and the negative electrode current collector is greatly improved, and high current collection is maintained in the negative electrode, so that a battery having a high energy density and excellent charge / discharge cycle characteristics can be obtained. it can.

  In this case, the negative electrode binder preferably has high mechanical strength and is excellent in elasticity. Due to the excellent mechanical strength of the binder, the destruction of the binder did not occur even when the volume of the silicon negative electrode active material changed during the insertion and release of lithium, and the volume change of the silicon active material followed. Since the mixture layer can be deformed, the current collecting property in the electrode is maintained, and excellent charge / discharge cycle characteristics can be obtained. Thus, a polyimide resin can be preferably used as the binder having high mechanical strength. Moreover, fluorine-type resins, such as polyvinylidene fluoride and polytetrafluoroethylene, can also be used preferably.

  The negative electrode binder is particularly preferably thermoplastic. For example, when the negative electrode binder has a glass transition temperature or a melting point, the binder is obtained by performing a heat treatment to sinter and dispose the negative electrode mixture layer on the negative electrode current collector surface at a temperature higher than the glass transition temperature or the melting point. It can be heat-sealed with the active material particles and the current collector, the adhesion between the active material particles and between the mixture layer and the current collector can be further improved, and the current collection in the electrode can be greatly improved. Excellent charge / discharge cycle characteristics can be obtained.

  In this case, it is preferable that the negative electrode binder remains without being completely decomposed after the heat treatment for disposing the negative electrode mixture layer on the surface of the negative electrode current collector. When the binder is completely decomposed after the heat treatment, the binding effect of the binder is lost, so that the current collecting property in the electrode is greatly lowered, resulting in poor charge / discharge cycle characteristics.

  The amount of the negative electrode binder in the lithium secondary battery of the present invention is preferably 5% by weight or more of the total weight of the negative electrode mixture layer, and the volume occupied by the binder is preferably 5% or more of the total volume of the negative electrode mixture layer. Here, the total volume of the negative electrode mixture layer is the sum of the volumes of materials such as the active material and the binder contained in the mixture layer, and when there are voids in the mixture layer, The volume occupied by this void is not included. When the binder amount is less than 5% by weight of the total weight of the mixture layer and the volume of the binder is less than 5% of the total volume of the mixture layer, the binder amount is too small with respect to the negative electrode active material particles. Due to this, the adhesion in the electrode becomes insufficient. On the other hand, if the amount of the binder is excessively increased, the resistance in the electrode increases, so that initial charging becomes difficult. Therefore, the amount of the negative electrode binder is preferably 50% by weight or less of the total weight of the negative electrode mixture layer, and the volume occupied by the binder is preferably 50% or less of the total volume of the negative electrode mixture layer.

  As the negative electrode active material in the lithium secondary battery of the present invention, the negative electrode is sintered on the surface of the conductive metal foil negative electrode current collector as described above, with the negative electrode mixture layer containing the negative electrode active material particles and the negative electrode binder. The particles are preferably particles containing silicon and / or a silicon alloy. In this case, the silicon alloy includes a solid solution of silicon and one or more other elements, an intermetallic compound of silicon and one or more other elements, and a co-crystal alloy of silicon and one or more other elements. Etc. Examples of the method for producing the alloy include an arc melting method, a liquid quenching method, a mechanical alloying method, a sputtering method, a chemical vapor deposition method, and a firing method. In particular, examples of the liquid quenching method include a single roll quenching method, a twin roll quenching method, and various atomizing methods such as a gas atomizing method, a water atomizing method, and a disk atomizing method.

  Moreover, as a negative electrode active material in the lithium secondary battery of this invention, you may use what coat | covered the surface of the particle | grains containing a silicon and / or a silicon alloy with a metal. Examples of the coating method include an electroless plating method, an electrolytic plating method, a chemical reduction method, a vapor deposition method, a sputtering method, and a chemical vapor deposition method.

  Moreover, as a negative electrode active material in the lithium secondary battery of the present invention, particles of silicon alone can be preferably used.

  The average particle diameter of the negative electrode active material particles in the lithium secondary battery of the present invention is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 15 μm or less. When using active material particles with a small particle size, the absolute amount of volume expansion / contraction of the active material particles associated with insertion / extraction of lithium during charging / discharging is reduced, so that the negative electrode binder is less likely to be destroyed, and the electrode It is possible to suppress a decrease in the current collecting ability, and to obtain excellent charge / discharge cycle characteristics.

  The particle size distribution of the negative electrode active material particles in the lithium secondary battery of the present invention is preferably as narrow as possible. In the case of a wide particle size distribution, there is a large difference in the absolute volume expansion and contraction associated with the insertion and extraction of lithium between active material particles with greatly different particle sizes, so that distortion occurs in the mixture layer. And the binder breaks down. Therefore, the current collecting property in the electrode is lowered, and the charge / discharge cycle characteristics are lowered.

  The conductive metal foil as the negative electrode current collector in the lithium secondary battery of the present invention preferably has a surface roughness Ra of 0.2 μm or more on the surface on which the negative electrode mixture layer is disposed. Even when the conductive metal foil having such a surface roughness Ra is used as the negative electrode current collector, the binder enters the surface irregularities of the current collector and the anchor effect is exhibited between the binder and the current collector. Since adhesiveness is obtained, even if the volume change of the active material particles due to insertion and extraction of lithium occurs, peeling of the mixture layer from the current collector is suppressed. When the negative electrode mixture layers are disposed on both sides of the current collector, the surface roughness Ra on both sides of the current collector is preferably 0.2 μm or more.

  The surface roughness Ra and the average distance S between the local peaks are preferably 100Ra ≧ S. The surface roughness Ra and the average interval S between the local peaks are defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured, for example, with a surface roughness meter.

  In order to set the surface roughness Ra of the conductive metal foil to 0.2 μm or more, the conductive metal foil may be roughened. Examples of such roughening treatment include a plating method, a vapor phase growth method, an etching method, and a polishing method. The plating method and the vapor phase growth method are methods for roughening the surface by forming a thin film layer having irregularities on the surface of the metal foil. Examples of the plating method include an electrolytic plating method and an electroless plating method. Examples of the vapor deposition method include sputtering, chemical vapor deposition, and vapor deposition. Examples of the etching method include a physical etching method and a chemical etching method. Examples of the polishing method include sandpaper polishing and blasting.

  Examples of the conductive metal foil negative electrode current collector in the present invention include foils of metals made of metals such as copper, nickel, iron, titanium, cobalt, or combinations thereof.

  Moreover, it is preferable that the electroconductive metal foil negative electrode collector in this invention has high mechanical strength. Due to the high mechanical strength of the current collector, the current collector is destroyed even when stress generated by the volume change of the silicon negative electrode active material is applied to the current collector during insertion and extraction of lithium. Since this can be alleviated without causing plastic deformation, separation of the mixture layer from the current collector is suppressed, current collection in the electrode is maintained, and excellent charge / discharge cycle characteristics can be obtained.

  Although the thickness of the electroconductive metal foil negative electrode collector in this invention is not specifically limited, It is preferable that it is the range of 10 micrometers-100 micrometers.

  The upper limit of the surface roughness Ra of the conductive metal foil negative electrode current collector in the present invention is not particularly limited, but the thickness of the conductive metal foil is preferably in the range of 10 μm to 100 μm as described above. The upper limit of the surface roughness Ra is substantially 10 μm or less.

  In the negative electrode of the present invention, the thickness X of the negative electrode mixture layer preferably has a relationship of 5Y ≧ X, 250Ra ≧ X with the thickness Y and surface roughness Ra of the negative electrode conductive metal foil. When the thickness X of the mixture layer is 5Y or 250 Ra or more, the volume of the mixture layer during expansion / contraction during charging / discharging is large, so depending on the irregularities on the surface of the metal foil collector, the mixture layer and the collector The adhesive layer cannot be maintained and peeling of the mixture layer from the current collector occurs.

  Although the thickness X of the negative mix layer in this invention is not specifically limited, 1000 micrometers or less are preferable, More preferably, they are 10 micrometers-100 micrometers.

  In the negative electrode of the present invention, conductive powder may be mixed in the mixture layer. By mixing the conductive powder, a conductive network of the conductive powder is formed around the active material particles, so that the current collecting property in the electrode can be further improved. As the conductive powder, a material similar to that of the conductive metal foil can be preferably used. Specifically, it is an alloy or a mixture made of a metal such as copper, nickel, iron, titanium, cobalt, or a combination thereof. In particular, copper powder is preferably used as the metal powder. Also, conductive carbon powder can be preferably used.

  The mixing amount of the conductive powder in the negative electrode mixture layer is 50% by weight or less of the total weight with the negative electrode active material, and the volume occupied by the conductive powder is 20% or less of the total volume of the negative electrode mixture layer. preferable. When there is too much mixing amount of electroconductive powder, since the ratio of the negative electrode active material in a negative mix layer becomes relatively small, the charge / discharge capacity of a negative electrode becomes small. Moreover, in this case, since the ratio of the binder amount compared to the total amount of the active material and the conductive agent in the mixture layer is reduced, the strength of the mixture layer is reduced and charge / discharge cycle characteristics are reduced.

  The average particle size of the conductive powder is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less.

The positive electrode active material in the lithium secondary battery of the present invention is preferably a lithium transition metal composite oxide. Examples of the lithium transition metal composite oxide include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.33 Co _0.33 Mn _0.34 O _{2 and the} like. In particular, a lithium transition metal composite oxide containing Li, Ni, Mn and Co having a layered structure and LiCoO ₂ can be preferably used.

The BET specific surface area of the lithium transition metal composite oxide is preferably 3 m ² / g or less. When exceeding 3 m < ² > / g, since a contact area with a nonaqueous electrolyte becomes large, a side reaction will be produced at the time of charging / discharging, and it is unpreferable since a charging / discharging characteristic falls.

  Moreover, it is preferable that the average particle diameter (average particle diameter of a secondary particle) of lithium transition metal complex oxide is 20 micrometers or less. When the average particle diameter exceeds 20 μm, the movement distance of lithium in the lithium transition metal composite oxide particles is increased, so that the charge / discharge cycle characteristics are deteriorated.

  In the positive electrode of the lithium secondary battery of the present invention, it is preferable that a positive electrode conductive agent is contained in the positive electrode mixture layer. As the positive electrode conductive agent, various known conductive agents can be used. For example, a conductive carbon material can be preferably used, and in particular, acetylene black or ketjen black can be preferably used.

  The amount of the positive electrode conductive agent is preferably 1% by weight or more and 5% by weight or less of the positive electrode mixture layer. When the amount of the positive electrode conductive agent is less than 1% by weight of the positive electrode mixture layer, the amount of the conductive agent is too small, so that a sufficient conductive network is not formed around the positive electrode active material, The current collecting property is reduced and the charge / discharge characteristics are deteriorated. In addition, when the amount of the positive electrode conductive agent exceeds 5% by weight of the positive electrode mixture layer, the amount of the conductive agent is too large, so that the binder is consumed for adhesion of the conductive agent, and between the positive electrode active material particles and the positive electrode current collector. The adhesion of the positive electrode active material to the body is reduced, the positive electrode active material is easily detached, and the charge / discharge characteristics are deteriorated.

  As the positive electrode binder in the lithium secondary battery of the present invention, various known binders can be used without limitation as long as they do not dissolve in the non-aqueous electrolyte solvent of the present invention. For example, fluorine such as polyvinylidene fluoride can be used. A resin, a polyimide resin, polyacrylonitrile, or the like can be preferably used.

  The amount of the positive electrode binder is preferably 1% by weight or more and 5% by weight or less of the positive electrode mixture layer. The amount of the positive electrode binder is preferably 1% by weight or more and 5% by weight or less of the positive electrode mixture layer. When the amount of the positive electrode binder is less than 1% by weight of the positive electrode mixture layer, the contact area between the positive electrode active material particles increases and the contact resistance decreases, but the amount of the binder is too small, so Further, the adhesion of the positive electrode active material to the positive electrode current collector is reduced, the positive electrode active material is easily detached, and the charge / discharge characteristics are deteriorated. Further, when the amount of the positive electrode binder exceeds 5% by weight of the positive electrode mixture layer, the adhesion of the positive electrode active material between the positive electrode active material particles and the positive electrode current collector is improved, but the amount of the binder is too large. The contact area between the positive electrode active material particles decreases, the contact resistance increases, and the charge / discharge characteristics deteriorate.

  The conductive metal foil as the positive electrode current collector in the lithium secondary battery of the present invention can be used without limitation as long as it is stably present without being dissolved in the nonaqueous electrolyte at the potential applied to the positive electrode during charging and discharging. For example, an Al foil can be preferably used.

The density of the positive electrode mixture layer in the present invention is preferably 3.0 g / cm ³ or more. When the density of the positive electrode mixture layer is 3.0 g / cm ³ or more, the contact area between the positive electrode active materials is increased, and the current collecting property in the positive electrode mixture layer is improved. Obtainable.

  The non-aqueous electrolyte solvent in the lithium secondary battery of the present invention is not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, etc. Chain carbonates and esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1, Ethers such as 2-dioxane and 2-methyltetrahydrofuran, nitriles such as acetonitrile, amides such as dimethylformamide, and the like can be used alone or in combination. It is possible to use. In particular, a mixed solvent of a cyclic carbonate and a chain carbonate can be preferably used.

Further, the nonaqueous electrolyte solvent in the present invention is not particularly limited, but is a chemical formula LiXF _y such as LiPF ₆ , LiBF ₄ , LiAsF ₆ (wherein X is P, As, Sb, B, Bi, Y is 6 when X is P, As, or Sb, and y is 4 when X is B, Bi, Al, Ga, or In. , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl _12, and other lithium compounds can be used. Among these, LiPF ₆ can be particularly preferably used.

  Moreover, it is preferable that the nonaqueous electrolyte in the present invention dissolves carbon dioxide. Since carbon dioxide is dissolved in the non-aqueous electrolyte, lithium occlusion / release reactions occur smoothly on the surface of the positive and negative active materials, and more excellent charge / discharge cycle characteristics can be obtained.

Furthermore, examples of the non-aqueous electrolyte in the present invention include gel polymer electrolytes in which a polymer electrolyte such as polyethylene oxide and polyacrylonitrile is impregnated with an electrolytic solution, and inorganic solid electrolytes such as LiI and Li ₃ N. The non-aqueous electrolyte in the present invention can be used without limitation as long as the lithium compound as a solute that develops lithium ion conductivity and the solvent that dissolves and retains the lithium compound do not decompose during charge / discharge or storage of the battery.

  The method for producing a lithium secondary battery of the present invention is a method for producing the lithium secondary battery of the present invention, wherein the positive electrode current collector tab and the negative electrode current collector tab are respectively connected to the positive electrode current collector and the negative electrode current collector. A step of forming the electrode body, a step of producing a cylindrical electrode body by causing the positive electrode and the negative electrode to face each other through a separator and spirally winding the electrode body, and a step of storing the electrode body in a battery container. It is characterized by that.

  In the lithium secondary battery of the present invention, as described above, the negative electrode mixture layer including the negative electrode active material particles containing silicon and the negative electrode binder is sintered and disposed on the surface of the negative electrode current collector. Preferably, the negative electrode in this case is produced by applying a slurry in which negative electrode active material particles are uniformly mixed and dispersed in a negative electrode binder solution onto the surface of the negative electrode current collector. It is preferable to provide a step of sintering in a non-oxidizing atmosphere in a state where the mixture layer is disposed and the negative electrode mixture layer is disposed on the surface of the negative electrode current collector.

  In this case, the sintering in the production of the negative electrode is preferably performed in, for example, a vacuum, a nitrogen atmosphere, or an inert gas atmosphere such as argon. Further, it may be performed in a reducing atmosphere such as a hydrogen atmosphere. The temperature of the heat treatment at the time of sintering is preferably a temperature equal to or lower than the melting point of the conductive agent metal foil current collector and the active material particles. For example, when a copper foil is used as the conductive metal foil, the melting point is preferably 1083 ° C. or lower. In addition, as described above, it is also preferable from the viewpoint of improving the current collecting property of the negative electrode that the heat treatment for sintering is performed at a temperature at which the negative electrode binder is not completely decomposed. For this reason, More preferably, it is 200-500 degreeC, More preferably, it is 300-450 degreeC. Moreover, although sintering of a negative electrode may be performed in oxidizing atmospheres, such as air | atmosphere, it is preferable that the temperature of the heat processing for sintering is 300 degrees C or less in this case. Further, as a sintering method, a discharge plasma sintering method or a hot press method may be used.

  Moreover, in the manufacturing method of this invention, after forming a mixture layer on a collector, it is preferable to roll this mixture layer with a collector before sintering. By this rolling, the packing density of the mixture layer can be increased, the thickness of the mixture layer can be reduced, the adhesion between the active material particles, and the adhesion between the mixture layer and the current collector can be enhanced. For this reason, it can be set as the battery which is high energy density and was excellent in charging / discharging cycling characteristics.

  According to the present invention, a lithium secondary battery having a high energy density and excellent charge / discharge cycle characteristics can be obtained.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be appropriately modified and implemented without departing from the scope of the present invention. It is.

(Experiment 1)
[Production of positive electrode]
Li ₂ CO ₃ and CoCO ₃ were mixed in a mortar so that the molar ratio of Li and Co was 1: 1, and then heat-treated at 800 ° C. for 24 hours in an air atmosphere. A lithium cobalt composite oxide represented by LiCoO ₂ having a particle size of about 7 μm was obtained. The BET specific surface area of the obtained lithium cobalt composite oxide was 0.49 m ² / g.

N-methyl-2-pyrrolidone as a dispersion medium, LiCoO ₂ powder as a positive electrode active material, carbon material powder as a positive electrode conductive agent, and polyvinylidene fluoride as a positive electrode binder, an active material, a conductive agent, and a binder Was added so that the weight ratio was 94: 3: 3, and then kneaded to obtain a positive electrode mixture slurry.

  This positive electrode mixture slurry was applied to both sides of an aluminum foil having a thickness of 15 μm, a length of 460 mm, and a width of 40 mm as a positive electrode current collector so that the coated portion had a length of 410 mm and a width of 40 mm on both the front and back surfaces. And then rolled.

The amount of the mixture layer on the current collector was 53 mg / cm ² and the thickness was 160 μm.

  FIG. 1 is a plan view showing a state in which the positive electrode current collecting tab 7 is formed at the longitudinal end portion 21 of the current collector 20 of the positive electrode 2 obtained as described above. A positive electrode mixture layer 23 is formed on the positive electrode current collector 20, and the positive electrode mixture layer 23 is formed in a portion from the end 21 in the longitudinal direction of the positive electrode current collector 20 to a distance A = 26 mm. It is not formed and is an uncoated portion 22. A positive electrode current collecting tab 7 is formed at the end 21 of the positive electrode current collector 20. As shown in FIG. 2, the positive electrode current collector tab 7 is formed by forming a notch 24 at an end 21 of the positive electrode current collector 20 and bending it at a bent portion 24a. The distance C of the portion where the positive electrode current collecting tab 7 protrudes from the positive electrode current collector 20 is 8 mm, and the width D of the positive electrode current collecting tab 7 is 4 mm. The length B of the portion where the positive electrode current collector tab 7 and the positive electrode current collector 20 overlap is 5 mm.

(Production of negative electrode)
N-methyl-2-pyrrolidone as a dispersion medium, silicon powder having an average particle size of 5.5 μm as a negative electrode active material (purity 99.9%), glass transition temperature 190 ° C. as a negative electrode binder, density 1.1 g / Cm ³ of thermoplastic polyimide was mixed so that the weight ratio of the active material, the conductive agent and the binder was 90:10 to prepare a negative electrode mixture slurry.

  This negative electrode mixture slurry was applied on both sides of a Cu-0.03% wt Zr alloy foil having a thickness of 25 μm, a length of 460 mm, and a width of 42 mm with a surface roughness Ra of 1.0 μm, which is a negative electrode current collector, After drying, it was rolled.

Next, it was heat-treated at 400 ° C. for 10 hours in an argon atmosphere and sintered. The amount of the mixture layer on the current collector was 5.6 mg / cm ² and the thickness was 60 μm.

  FIG. 3 is a plan view showing the negative electrode produced as described above. A negative electrode current collecting tab 8 is formed at the end 31 of the negative electrode current collector 30 of the negative electrode 3. Similarly to the positive electrode current collector tab 7 shown in FIG. 1, the negative electrode current collector tab 8 is formed by forming a cut at the end 31 of the negative electrode current collector 30 and bending the cut. The length F of the portion of the negative electrode current collector tab 8 protruding from the negative electrode current collector 30 is 10 mm, and the width G is 4 mm. Moreover, the length E of the overlapping part of the negative electrode current collection tab 8 and the negative electrode current collector 30 is 5 mm.

(Preparation of electrolyte)
Into a solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7, LiPF ₆ is dissolved at 1 mol / liter, and carbon dioxide is blown to a saturated state to dissolve the carbon dioxide. did.

(Production of electrode body)
An electrode body was prepared using one positive electrode, one negative electrode, two polyethylene porous separators having a thickness of 22 μm, a length of 500 mm, and a width of 44 mm.

  As shown in FIG. 4, one separator 4 is sandwiched between the positive electrode 2 and the negative electrode 3, and another separator 4 is placed on the negative electrode 3 so that the positive electrode current collecting tab 7 becomes the innermost circumference. A cylindrical electrode body having a diameter of 12.8 mm and a height of 4 mm was produced by spirally winding in the winding direction indicated by X. In FIG. 4, the distance H is 21 mm.

  FIG. 5 is a perspective view showing the produced cylindrical electrode body. As shown in FIG. 5, a positive electrode current collecting tab 7 is provided so as to protrude upward from the electrode body 5, and a negative electrode current collecting tab 8 is provided so as to protrude downward.

[Production of battery]
The above electrode body is inserted into a stainless steel (SUS) cylindrical outer casing under a carbon dioxide atmosphere at normal temperature and normal pressure, and an electrolytic solution is injected into the outer casing to obtain a lithium secondary battery A1 shown in FIG. Produced. The diameter of the lithium secondary battery is 14 mm, and the height is 50 mm.

  FIG. 6 shows a schematic cross-sectional view of the cylindrical battery A1. As shown in FIG. 6, the cylindrical battery A <b> 1 is formed by winding a cylindrical metal outer can 1 having an opening on the upper side, a positive electrode 2, and a negative electrode 3 through a separator 4 and winding them in a spiral shape. The electrode body 5 includes a nonaqueous electrolyte impregnated in the electrode body 5, a sealing lid 6 that seals the opening of the metal outer can 1, and the like. The sealing lid 6 is a positive electrode terminal, the metal outer can 1 is a negative electrode terminal, and a positive electrode current collecting tab 7 attached to the upper surface side of the electrode body 5 is connected to the sealing lid 6 and a negative electrode collector attached to the lower surface side. An electric tab 8 is connected to the metal outer can 1. The battery container includes a metal outer can 1 and a sealing lid 6. The upper and lower surfaces of the electrode body 5 are covered with an upper insulating plate 9 and a lower insulating plate 10 for insulating the electrode body 5 from the metal outer can 1. The sealing lid 6 is caulked and fixed to the opening of the metal outer can 1 via an insulating packing 11.

(Experiment 2)
In Experiment 1, a comparative battery B1 was manufactured using a conventionally used aluminum flat plate and nickel flat plate as the positive electrode current collecting tab and the negative electrode current collecting tab, respectively. As shown in FIG. 7, an aluminum flat plate having a thickness of 70 μm, a length of 38 mm, and a width of 4 mm was attached near the end 21 of the positive electrode current collector 20 of the positive electrode 2 by an ultrasonic welding method. The distance A is 30 mm, the distance B is 30 mm, the distance C is 8 mm, and the width D is 4 mm.

  Further, a conventionally used nickel flat plate was used as the negative electrode current collecting tab, and the negative electrode current collecting tab 8 was attached to the negative electrode current collector 30 as shown in FIG. Referring to FIG. 8, a nickel flat plate having a thickness of 70 μm, a length of 40 mm, and a width of 4 mm was attached to the end portion 31 of the negative electrode current collector 30 of the negative electrode 3 by a spotting method. The distance E is 30 mm, the distance F is 10 mm, and the width G is 4 mm.

  Using the positive electrode 2 and the negative electrode 3 produced as described above, as shown in FIG. 9, an electrode body was produced in the same manner as in Experiment 1, and a lithium secondary battery B1 was produced using this electrode. In FIG. 9, the distance H is 25 mm.

[Evaluation of charge / discharge cycle characteristics]
Charge / discharge cycle characteristics of the batteries A1 and B1 were evaluated. Each battery was charged to 4.2 V at a current value of 1000 mA at 25 ° C., and continuously charged to a current value of 50 mA while being held at 4.2 V, and then discharged to 2.75 V at a current value of 1000 mA. Was one cycle of charge and discharge. The number of cycles to reach 80% of the discharge capacity at the first cycle was measured and defined as the cycle life. The cycle life of each battery is an index with the cycle life of the battery A1 as 100.

  Table 1 shows the cycle life of the batteries A1 and B1.

  As is apparent from the results shown in Table 1, according to the present invention, the battery A1 in which the positive electrode current collector tab and the negative electrode current collector tab were formed from the positive electrode current collector and the negative electrode current collector was obtained by using the conventional metal plate as the current collector tab. Compared to the battery B1, the cycle life is excellent. This is because, in the conventional battery B1 in which a metal plate is attached as a current collecting tab on the current collector, the electrode body accompanying the volume expansion of the silicon active material during charging due to the mechanical strength of the tab of the metal plate itself Smooth deformation is suppressed, and the separator is subjected to a lot of stress associated with the expansion of the silicon active material, resulting in clogging due to the compression of the separator, greatly reducing the lithium ion conductivity between the positive electrode and the negative electrode. This is probably because the discharge characteristics are greatly reduced. On the other hand, when the positive electrode current collector tab and the negative electrode current collector tab are formed from the positive electrode current collector and the negative electrode current collector according to the present invention, expansion of the silicon active material occurs during charging, and the electrode body tends to be deformed accordingly. In this case, it is considered that the electrode body is smoothly deformed, the occurrence of clogging of the separator can be suppressed, and excellent charge / discharge characteristics can be obtained.

The top view which shows the positive electrode in one Example according to this invention. The top view which shows the notch for forming the positive electrode current collection tab in the positive electrode current collector in one Example according to this invention. The top view which shows the negative electrode in one Example according to this invention. The top view for demonstrating the state when winding in the spiral shape which makes a positive electrode and a negative electrode oppose through a separator in one Example according to this invention. The perspective view which shows the electrode body in one Example according to this invention. The typical sectional view showing the lithium secondary battery produced in one example according to the present invention. The top view which shows the positive electrode of a comparative example. The top view which shows the negative electrode of a comparative example. The top view which shows a state when a positive electrode and a negative electrode are made to oppose through a separator in a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Metal exterior can 2 ... Positive electrode 3 ... Negative electrode 4 ... Separator 5 ... Electrode body 6 ... Sealing lid 7 ... Positive electrode current collection tab 8 ... Negative electrode current collection tab 9 ... Upper insulating plate 10 ... Lower insulating plate 11 ... Insulating packing 20 ... Positive electrode current collector 21 ... End of positive electrode current collector 30 ... Negative electrode current collector 31 ... End of negative electrode current collector

Claims (4)

A positive electrode in which a positive electrode active material layer is disposed on a surface of a positive electrode current collector made of a conductive metal foil; a negative electrode in which a negative electrode active material layer containing silicon is disposed on a negative electrode current collector made of a conductive metal foil; and A separator disposed between a positive electrode and the negative electrode; a battery container containing a cylindrical electrode body in which the positive electrode and the negative electrode are opposed to each other with the separator interposed therebetween; and the positive current collector A lithium secondary battery comprising a positive electrode current collector tab connected to a body and drawn from the electrode body, a negative electrode current collector tab connected to the negative electrode current collector and drawn from the electrode body, and a nonaqueous electrolyte,
The lithium secondary battery, wherein the positive electrode current collector tab is formed from the positive electrode current collector, and the negative electrode current collector tab is formed from the negative electrode current collector.   The lithium secondary battery according to claim 1, wherein the positive electrode current collecting tab and the negative electrode current collecting tab are formed at respective end portions of the positive electrode and the negative electrode.   3. The lithium according to claim 1, wherein the negative electrode is formed by sintering a negative electrode mixture layer containing negative electrode active material particles containing silicon and a negative electrode binder on the surface of the negative electrode current collector. Secondary battery. A method for producing the lithium secondary battery according to any one of claims 1 to 3,
Forming the positive electrode current collector tab and the negative electrode current collector tab from the positive electrode current collector and the negative electrode current collector, respectively;
Producing the cylindrical electrode body by causing the positive electrode and the negative electrode to face each other through the separator and spirally wound;
And a step of housing the electrode body in the battery container.