JP2005174653A - Lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having excellent cycle characteristics and a high energy density which uses a negative electrode in which a mix layer including active material particles containing silicon and/or a silicon alloy and a binder is provided by sintering on the surface of a collector made of a conductive metal foil. <P>SOLUTION: The lithium secondary battery comprises: a positive electrode 5 in which a positive electrode mix layer including a positive electrode active material and a positive electrode binder is provided on the surface of a positive electrode collector; a negative electrode 4 in which a negative electrode mix layer including a negative electrode active material containing silicon and/or a silicon alloy and a negative electrode binder is provided by sintering on the surface of a negative electrode collector; a separator 3 provided between the positive electrode 5 and the negative electrode 4; and a nonaqueous electrolyte. The lithium secondary battery is configured such that, for example, no opposing positive electrode mix layer is present at a portion 4a of the negative electrode 4 where the negative electrode mix layer is curved, so that no charge-discharge reaction occurs at the portion 4a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

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, a lithium secondary battery using a non-aqueous electrolyte and charging and discharging by moving lithium ions between a positive electrode and a negative electrode has been developed. It's being used.

  Since the lithium secondary battery has a high energy density, it has been put into practical use as a power source for electronic mobile devices related to information technology of mobile phones and notebook computers, and is widely spread. In the future, with further miniaturization and higher functionality of these portable devices, it is expected that the load on the lithium secondary battery as a power source will increase. For this reason, the request | requirement for the high energy density of a lithium secondary battery is very high.

  In order to increase the energy density of a battery, it is an effective means to use a material having a larger energy density as an active material. Therefore, recently, in a lithium secondary battery, as a negative electrode active material having a higher energy density instead of graphite that is currently in practical use, Al, Sn, Si, etc. that occlude lithium by an alloying reaction with lithium are used. Elemental alloy materials have been proposed and studied.

  However, in an electrode using a material alloyed with lithium as an active material, the volume of the active material expands and contracts when lithium is occluded / released, so that the active material is pulverized or peeled off from the current collector. Occurs, the current collecting property in the electrode is lowered, and the charge / discharge cycle characteristics are deteriorated.

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

However, it has not been studied in detail what battery structure is excellent in cycle characteristics when the negative electrode is used.
Japanese Patent Laid-Open No. 2002-260637

  An object of the present invention is to provide a lithium secondary battery using a negative electrode in which active material particles containing silicon and / or a silicon alloy and a mixture layer containing a binder are sintered and arranged on the surface of a current collector made of a conductive metal foil. An object of the present invention is to provide a lithium secondary battery excellent in cycle characteristics and having a high energy density, and a method for manufacturing the same.

  The present invention relates to a positive electrode in which a positive electrode mixture layer including a positive electrode active material and a positive electrode binder is disposed on a surface of a positive electrode current collector, a negative electrode active material including silicon and / or a silicon alloy, and a negative electrode mixture layer including a negative electrode binder In a lithium secondary battery comprising a negative electrode that is sintered and disposed on the surface of a negative electrode current collector, a separator that is disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, the positive electrode and the negative electrode are interposed via the separator. It arrange | positions so that it may oppose and it is comprised so that a charging / discharging reaction may not arise in the part in which the negative mix layer was bent.

  The lithium secondary battery of the present invention is configured such that no charge / discharge reaction occurs in the bent portion of the negative electrode mixture layer. For this reason, in the part by which the negative mix layer was bent, the expansion / contraction of the volume of the negative mix layer accompanying charging / discharging does not arise. For this reason, destruction of the negative electrode mixture layer and peeling from the current collector can be suppressed, and current collection in the electrode can be maintained. When the charge / discharge reaction occurs in the bent portion of the negative electrode mixture layer, stress generated in the mixture layer is not appropriately relaxed, and the mixture layer is destroyed. The destruction of the mixture layer occurs not only in the portion where the negative electrode mixture layer is bent but also in the portion connected to this portion. For this reason, in accordance with the present invention, by configuring so that the charge / discharge reaction does not occur in the bent portion of the negative electrode mixture layer, the decrease in the current collecting property in the bent portion of the negative electrode mixture layer and other portions is suppressed. As a result, excellent charge / discharge cycle characteristics can be obtained.

  In the present invention, as a specific method for preventing the charge / discharge reaction from occurring in the bent portion of the negative electrode mixture layer, the opposing positive electrode mixture layer does not exist in the bent portion of the negative electrode mixture layer. And a method of providing a lithium ion non-conductive substance on the surface of the positive electrode mixture layer, a method of providing a lithium ion non-conductive substance on the surface of the negative electrode mixture layer, and the like. Examples of the method for providing the lithium ion non-conductive substance include a method of attaching a plastic film or the like to the surface of the positive electrode mixture layer or the negative electrode mixture layer.

  Moreover, the method etc. which block | close the porous part in the separator which touches the part into which the negative mix layer was bent, and block | interruption of the lithium ion conduction between a positive electrode and a negative electrode are mentioned. Specifically, a method of heating the separator portion in the bent portion of the negative electrode mixture layer and closing the void portion can be mentioned. Moreover, when using a solid electrolyte, the method of comprising so that a solid electrolyte may not exist in the part into which the negative mix layer was bent is also mentioned.

  The production method of the present invention is a method by which the lithium secondary battery of the present invention can be produced, and is characterized in that the charge / discharge reaction does not occur in the bent portion of the negative electrode mixture layer.

  According to the production method of the present invention, a lithium secondary battery excellent in charge / discharge cycle characteristics can be produced for the above-described reason.

  Examples of a method for preventing the charge / discharge reaction from occurring in the bent part of the negative electrode mixture layer include, for example, alternately folding the negative electrode provided with the negative electrode mixture layer on both sides and providing the positive electrode mixture layer on both sides. The method of arrange | positioning the positive electrode between the folding surfaces of a negative electrode is mentioned. Further, a method of sticking a plastic film on the surface portion of the positive electrode mixture layer facing the bent portion of the negative electrode mixture layer, a method of sticking a plastic film on the surface of the bent portion of the negative electrode mixture layer, or the like may be used. .

  In the present invention, the negative electrode is preferably sintered in a non-oxidizing atmosphere.

  In the present invention, the negative electrode mixture layer may be disposed on the surface of the negative electrode current collector by applying a slurry in which the negative electrode active material particles are dispersed in a binder solution onto the surface of the negative electrode current collector. it can. In this case, the negative electrode active material particles are preferably mixed and dispersed uniformly in the binder solution.

  Moreover, in the manufacturing method of this invention, after forming a mixture layer on the surface of a negative electrode collector, it is preferable to roll a negative mix layer with a negative electrode collector before sintering. By such rolling, the packing density in the negative electrode mixture layer is increased, and the adhesion between the active material particles and the adhesion between the mixture layer and the current collector can be enhanced. For this reason, more favorable charge / discharge cycle characteristics can be obtained.

  Sintering in a non-oxidizing atmosphere in the present invention can be performed, for example, in 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 heat treatment temperature at the time of sintering is preferably a temperature not higher than the melting points of the metal foil current collector and the active material particles. For example, when copper foil is used as the metal foil current collector, it is preferably 1083 ° C. or lower, which is the melting point of copper, more preferably in the range of 200 to 500 ° C., more preferably 300 to 450. Within the range of ° C. As a sintering method, a discharge plasma sintering method or a hot press method may be used.

  Hereinafter, the negative electrode, the positive electrode, and the nonaqueous electrolyte in the lithium secondary battery of the present invention will be described.

<Negative electrode>
Examples of the negative electrode active material used in the present invention include silicon and / or silicon alloy particles. Examples of silicon alloys include solid solutions of silicon and one or more other elements, intermetallic compounds of silicon and one or more other elements, and eutectic alloys of silicon and one or more other elements. It is done. 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.

  In addition, as the negative electrode active material particles used in the present invention, particles obtained by coating the surfaces of silicon and / or silicon alloy particles with metal or the like may be used. 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. The metal covering the particle surface is preferably the same metal as the metal foil used as the current collector. By coating the same metal as the metal foil, the bonding property with the current collector during sintering is greatly improved, and further excellent charge / discharge cycle characteristics can be obtained.

  The negative electrode active material particles used in the present invention may contain particles made of a material alloyed with lithium. Examples of materials for alloying lithium include germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, indium, and alloys thereof.

  The average particle diameter of the negative electrode active material particles used in the present invention is not particularly limited, but for effective sintering, it is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. is there. As the particle diameter of the active material particles is smaller, good cycle characteristics tend to be obtained. The average particle size of the conductive powder used by adding to the mixture layer is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. is there.

  By using active material particles with a small average particle diameter, the absolute amount of volume expansion / contraction of the active material particles associated with insertion / extraction of lithium in the charge / discharge reaction is reduced. Since the absolute amount of strain between the active material particles is also reduced, the binder is not broken, the decrease in the current collecting property in the electrode can be suppressed, and good charge / discharge characteristics can be obtained.

  The particle size distribution of the active material particles is preferably as narrow as possible. If the particle size distribution is wide, there will be a large difference in the absolute volume expansion / contraction due to the insertion / desorption of lithium between active material particles with greatly different particle sizes. Resulting in binder breakage. For this reason, the current collection property in an electrode falls and charging / discharging cycling characteristics fall.

  The negative electrode current collector in the present invention preferably has an arithmetic average roughness Ra of the surface of 0.2 μm or more. By using a current collector having a surface with such an arithmetic average roughness Ra, the contact area between the mixture layer and the current collector can be increased, and the adhesion between the mixture layer and the current collector can be increased. Can be improved. For this reason, the current collection property in an electrode can further be improved. When the mixture layer is disposed on both sides of the current collector, the arithmetic average roughness Ra on both sides of the current collector is preferably 0.2 μm or more.

  The arithmetic average roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994). The arithmetic average roughness Ra can be measured by, for example, a surface roughness meter.

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

  In the present invention, the thickness of the negative electrode current collector is not particularly limited, but is preferably in the range of 10 to 100 μm.

  In the present invention, the upper limit of the arithmetic mean roughness Ra of the negative electrode current collector surface is not particularly limited, but the current collector surface is preferably in the range of 10 to 100 μm. It is preferable that the upper limit of the arithmetic average roughness Ra is substantially 10 μm or less.

  The negative electrode current collector in the present invention is made of a conductive metal foil. As such a conductive metal foil, for example, a metal such as copper, nickel, iron, titanium, cobalt, or an alloy made of a combination thereof can be exemplified. In particular, those containing a metal element that easily diffuses into the active material are preferable. As such a thing, the metal foil containing a copper element, especially copper foil or copper alloy foil is mentioned. Since copper easily diffuses into the silicon material as the active material by heat treatment, it can be expected that the adhesion between the current collector and the active material is improved by sintering. In addition, when the purpose of improving the adhesion between the current collector and the active material is through such sintering, a metal foil having a layer containing a copper element on the surface of the current collector in contact with the active material is used as the current collector. Use it. Accordingly, when a metal foil made of a metal element other than copper is used, it is preferable to form a copper or copper alloy layer on the surface thereof.

  It is preferable to use a heat resistant copper alloy foil as the copper alloy foil. Here, the heat resistant copper alloy means a copper alloy having a tensile strength of 300 MPa or more after annealing at 200 ° C. for 1 hour.

  As described above, the negative electrode current collector used in the present invention preferably has large irregularities on the surface thereof. For this reason, when the arithmetic average roughness Ra of the heat-resistant copper alloy foil surface is not sufficiently large, large irregularities may be provided on the surface by providing electrolytic copper or an electrolytic copper alloy on the foil surface. The electrolytic copper layer and the electrolytic copper alloy layer can be formed by an electrolytic method.

  In the present invention, a roughening treatment may be performed to form large irregularities on the surface of the negative electrode current collector. Examples of such roughening treatment include a vapor phase growth method, an etching method, and a polishing method. Examples of the vapor phase growth method include a sputtering method, a CVD method, and a vapor deposition method. Examples of the etching method include a physical etching method and a chemical etching method. Examples of the polishing method include polishing by sandpaper and polishing by a blast method.

  In the present invention, the thickness X of the negative electrode mixture layer preferably has a relationship of 5Y ≧ X and 250Ra ≧ X with the thickness Y of the negative electrode current collector and the arithmetic average roughness Ra of the surface thereof. When the thickness X of the mixture layer exceeds 5Y or 250Ra, the mixture layer may peel from the current collector.

  The thickness X of the negative electrode mixture layer is not particularly limited, but is preferably 1000 μm or less, and more preferably 10 μm to 100 μm.

  In the present invention, conductive powder can be mixed in the negative electrode mixture layer. By adding 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 current collector 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 amount of the conductive powder added to the negative electrode mixture layer is preferably 50% by weight or less of the total weight with the active material particles. If the amount of the conductive powder added is too large, the mixing ratio of the active material particles becomes relatively small, so that the charge / discharge capacity of the electrode becomes small.

  The negative electrode binder used in the present invention is preferably one that remains without being completely decomposed after the heat treatment for sintering. Since the binder remains without being decomposed even after the heat treatment, in addition to improving the adhesion between the active material particles and the current collector and between the active material particles by sintering, the binding force by the binder is also added, thereby improving the adhesion. It can be further increased. In addition, when a metal foil having an arithmetic average roughness Ra of 0.2 μm or more is used as a current collector, an anchor effect is manifested between the binder and the current collector due to the binder entering the irregularities on the surface of the current collector. In addition, the adhesion is further improved. For this reason, detachment | desorption of the active material layer from the electrical power collector by expansion / contraction of the volume of the active material at the time of occlusion / release of lithium can be suppressed, and favorable charge / discharge cycle characteristics can be obtained.

  As the negative electrode binder in the present invention, polyimide is preferably used. Examples of the polyimide include thermoplastic polyimide and thermosetting polyimide. As the polyimide, thermoplastic polyimide is particularly preferably used. When a thermoplastic polyimide having a glass transition temperature is used as the negative electrode binder, heat treatment of the negative electrode at a temperature higher than the glass transition temperature results in the binder being thermally fused to the active material particles and the current collector, resulting in high adhesion. It is possible to greatly improve the current collecting property in the electrode. That is, if the temperature of the heat treatment for sintering the negative electrode mixture layer and the conductive metal foil negative electrode current collector is higher than the glass transition temperature of the negative electrode binder, in addition to the effect of improving adhesion by sintering, Since the effect of improving the adhesion by heat fusion is also obtained, the current collecting property in the electrode can be greatly improved.

  Polyimide can also be obtained by heat treatment of polyamic acid. The polyimide obtained by the heat treatment of the polyamic acid is one in which the polyamic acid is dehydrated and condensed by the heat treatment to become a polyimide. The imidation ratio of polyimide is preferably 80% or more. The imidation ratio is a mol% of the generated polyimide with respect to the polyimide precursor (polyamic acid). Those having an imidization rate of 80% or more can be obtained, for example, by heat-treating a polyamic acid N-methyl-2-pyrrolidone (NMP) solution at a temperature of 100 to 400 ° C. for 1 hour or more. For example, when heat treatment is performed at 350 ° C., the imidization rate becomes 80% after about 1 hour, and the imidation rate becomes 100% after about 3 hours.

  In the present invention, it is preferable that the binder remains without being completely decomposed even after the heat treatment for sintering. Therefore, when polyimide is used as the binder, sintering is performed at 600 ° C. or lower where the polyimide is not completely decomposed. It is preferable to carry out the treatment.

  In the present invention, the amount of the binder in the negative electrode mixture layer is preferably 5% by weight or more of the total weight of the mixture layer. The volume occupied by the binder is preferably 5% or more of the total volume of the mixture layer. If the amount of the binder in the mixture layer is too small, the adhesiveness within the electrode due to the binder may be insufficient. Further, if the amount of the binder in the mixture layer is too large, the resistance in the electrode increases, so that the initial charge may be difficult. Therefore, the amount of binder in the mixture layer is preferably 50% by weight or less of the total weight, and the volume occupied by the binder is preferably 50% or less of the total volume of the mixture layer.

<Positive electrode>
Examples of the positive electrode active material used in the present invention include lithium-containing transition metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ , MnO Examples include metal oxides such as ₂ that do not contain lithium. In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  The positive electrode binder used in the present invention can be used without limitation as long as it can be used as a binder for an electrode of a lithium secondary battery. For example, a fluorine resin such as polyvinylidene fluoride, a polyimide resin preferably used as a negative electrode binder, or the like can be used.

<Nonaqueous electrolyte>
The solvent of the nonaqueous electrolyte used 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 Examples include chain carbonates such as carbonate. When a cyclic carbonate is present in the nonaqueous electrolyte solvent, a cyclic carbonate is preferably used because a high-quality film excellent in lithium ion conductivity is particularly easily formed on the surface of the active material particles. In particular, ethylene carbonate is preferably used. In addition, a mixed solvent of a cyclic carbonate and a chain carbonate can be preferably used. Such a mixed solvent particularly preferably contains ethylene carbonate and diethyl carbonate. Further, mixed solvents of the above cyclic carbonate and ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane, and chain esters such as γ-butyrolactone, sulfolane, and methyl acetate are also exemplified. .

The solutes of the nonaqueous electrolyte include LiPF ₆ , LiBF ₄ , 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, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and the like and their Mixtures are exemplified. In particular, LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, and when X is P, As, or Sb, y is 6, and X is B, Bi, al, Ga, or a y is 4) when an in,, lithium perfluoroalkyl sulfonic acid imide _{LiN (C m F 2m + 1} SO 2) (C n F 2n + 1 SO 2) ( wherein, m and n Are each independently an integer of 1 to 4) or lithium perfluoroalkylsulfonic acid methide LiC (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂₎ ) (Wherein, p, q and r are each independently an integer of 1 to 4). Among these, a mixed solute of LiPF ₆ and LiN (C ₂ F ₅ SO ₂ ) ₂ is particularly preferably used.

Further, examples of the electrolyte 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 electrolyte of the lithium secondary battery of the present invention is not limited as long as the lithium compound as a solute that exhibits ionic conductivity and the solvent that dissolves and retains the lithium compound do not decompose at the time of battery charging, discharging, or storage. Can be used.

  The separator used in the lithium secondary battery of the present invention is not particularly limited, and any separator can be used as long as it can be used as a separator for a lithium secondary battery. For example, a microporous film made of polyethylene or polypropylene can be used.

  According to the present invention, in a lithium secondary battery using a negative electrode in which a mixture layer containing active material particles containing silicon and / or a silicon alloy and a binder is sintered on the surface of a current collector and disposed. A lithium secondary battery having excellent characteristics and a high energy density 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 implemented with appropriate modifications within a range not changing the gist thereof. Is.

(Experiment 1)
(Production of negative electrode)
8.6% by weight containing 81.8 parts by weight of silicon powder (purity 99.9%) having an average particle diameter of 3 μm as an active material material and 18.2 parts by weight of thermoplastic polyimide having a glass transition temperature of 190 ° C. as a binder The N-methyl-2-pyrrolidone solution was uniformly mixed and dispersed to prepare a negative electrode mixture slurry.

  As the current collector, a Cu-0.03% by weight Zr alloy foil having an arithmetic average surface roughness Ra of 0.7 μm and a thickness of 35 μm, whose surfaces were roughened by electrolytic copper plating, was used. The negative electrode mixture slurry was applied to both sides of the current collector and then dried. The obtained product was cut into a rectangular shape of 42 × 183 mm, rolled, then heat-treated in an argon atmosphere at 400 ° C. for 1 hour, and sintered to obtain a negative electrode.

  The thickness of the negative electrode (including the current collector) was 65 μm. Therefore, the thickness of the mixture layer arranged on one side of the current collector was 15 μm. The thickness of the mixture layer / the arithmetic average surface roughness of the current collector was 30, and the thickness of the mixture layer / the thickness of the current collector was 0.43.

In this negative electrode, the density of polyimide was 1.1 g / cm ³ , and the volume occupied by polyimide was 31.8% of the total volume of the mixture layer.

[Production of positive electrode]
Using Li ₂ CO ₃ and CoCO ₃ as starting materials, weigh them so that the atomic ratio of Li: Co is 1: 1, mix them in a mortar, press this with a 17 mm diameter mold, pressurize After the molding, it was fired in air at 800 ° C. for 24 hours to obtain a LiCoO ₂ fired body. This was pulverized in a mortar to prepare an average particle size of 20 μm.

94 parts by weight of the obtained LiCoO ₂ powder and 3 parts by weight of artificial graphite powder as a conductive agent were mixed in a 6% by weight N-methyl-2-pyrrolidone solution containing 3 parts by weight of polyvinylidene fluoride as a binder. Then, a positive electrode mixture slurry was prepared.

  This positive electrode mixture slurry was applied onto both sides of an aluminum foil (thickness 15 μm) as a current collector, dried, and then rolled. The electrode thickness (including the current collector) was 155 μm. The obtained product was cut into a 40 mm × 30 mm rectangular shape to obtain a positive electrode.

(Preparation of electrolyte)
As an electrolytic solution, 1 mol / liter of LiPF ₆ was dissolved in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7, and vinylene carbonate was mixed with this solution so as to be 5% by weight. Thus, an electrolytic solution was prepared.

[Production of battery]
A lithium secondary battery was fabricated using one negative electrode, five positive electrodes, and one separator made of a polyethylene porous body having a thickness of 27 μm.

  As illustrated in FIGS. 2 and 3, the negative electrodes 4 are alternately folded, and the electrode body 6 is formed so that the positive electrodes 5 are arranged between the folded surfaces of the negative electrodes 4. In addition, the separator 3 was arrange | positioned between the negative electrode 4 and the positive electrode 5, and the electrode body 6 was formed so that the negative electrode 4 and the positive electrode 5 might oppose through the separator 3. FIG. 2 is a perspective view, and FIG. 3 is a cross-sectional view taken along the line AA ′ of an electrode body in which a positive electrode and a negative electrode are combined in the state shown in FIG.

  In the electrode body shown in FIG. 3, the negative electrode 4 is bent at the end of the electrode body as shown in FIG. 1, and a bent portion 4a is formed. As shown in FIG. 1, the positive electrode 5 is not present in the bent portion 4 a of the negative electrode 4. Therefore, the bent portion 4a of the negative electrode is configured so that no charge / discharge reaction occurs.

  As shown in FIGS. 4 and 5, the electrode body produced in this way is inserted together with the above electrolyte into the exterior body 7 made of an aluminum laminate in an argon atmosphere at normal temperature and normal pressure, and the lithium secondary battery A1. Was made. A closed portion 8 is formed around the exterior body 7 by heat sealing. 4 is a front view of the lithium secondary battery, and FIG. 5 is a cross-sectional view taken along the line AA ′ shown in FIG.

(Experiment 2)
In Experiment 1, the negative electrode was formed into a rectangular shape with a size of 42 × 32 mm, and six negative electrodes, five positive electrodes, and 12 separators were used to form the electrode body shown in FIGS. 6 and 7. 6 is a perspective view, and FIG. 7 is a cross-sectional view taken along the line AA ′ of FIG. As shown in FIG. 6 and FIG. 7, five positive electrodes 5 were laminated between six negative electrodes 4 with a separator 3 therebetween to form an electrode body 6. A lithium secondary battery B1 was produced in the same manner as in Experiment 1 except that this electrode body 6 was used.

(Experiment 3)
In Experiment 1, the size of the positive electrode was a rectangular shape of 40 × 150 mm, and the size of the negative electrode was a rectangular shape of 42 × 182 mm. Using one positive electrode and one negative electrode, as shown in FIGS. 8 and 9, the separator 3 is disposed between the negative electrode 4 and the positive electrode 5, and the separator 3 is disposed outside the negative electrode 4. The electrode body 6 was formed by winding in a spiral shape, flattening and then flattening. The size of the electrode body 6 was 1.5 × 30.6 × 44 mm. A lithium secondary battery B2 was produced in the same manner as in Experiment 1 except that this electrode body 6 was used.

  Further, in the production of the battery B2, a positive electrode 5 in which a polypropylene film having a thickness of 30 μm was stuck on the surface portion of the positive electrode 5 facing the portion where the negative electrode 4 bends was produced. A lithium secondary battery A2 was produced in the same manner as B2. By sticking the polypropylene film to the surface of the positive electrode mixture layer of the positive electrode, the charge / discharge reaction in this portion does not occur.

(Experiment 4)
As a comparison, the effect of the structure of the electrode body on the charge / discharge cycle characteristics was examined in a battery using natural graphite as the negative electrode active material.

(Production of negative electrode)
98% by weight of natural graphite powder having an average particle diameter of 18 μm as an active material, 48% by weight of an aqueous dispersion containing 1 part by weight of styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener ) 3% by weight of an aqueous solution containing 1 part by weight was mixed to prepare a negative electrode mixture slurry.

  This negative electrode mixture slurry was applied on both sides of a rolled copper foil having a thickness of 35 μm as a current collector and dried. The obtained product was cut out to a predetermined size and rolled into a negative electrode. The thickness of the negative electrode (including the current collector) was 161 μm.

[Production of battery]
A lithium secondary battery C1 was produced in the same manner as in the production of the battery A1 in Experiment 1, except that the negative electrode using natural graphite was used as the negative electrode.

  A lithium secondary battery C2 was produced in the same manner as in the production of the battery A2 in Experiment 3, except that the negative electrode using natural graphite was used as the negative electrode.

  A lithium secondary battery C3 was produced in the same manner as in the production of the battery B1 of Experiment 2, except that the negative electrode using the natural graphite was used as the negative electrode.

  A lithium secondary battery C4 was produced in the same manner as in the production of the battery B2 in Experiment 3, except that the negative electrode using natural graphite was used as the negative electrode.

[Evaluation of charge / discharge cycle characteristics]
Charge / discharge cycle characteristics were evaluated for the batteries A1 to A2, B1 to B2, and C1 to C4. Each battery was charged to 4.2 V at a current value of 180 mA at 25 ° C., and then discharged to 2.75 V at a current value of 180 mA. The number of cycles to reach 80% of charge / discharge in the first cycle was measured and defined as the cycle life. Table 1 shows the measurement results for batteries A1 to A2 and B1 to B2. Table 2 shows the measurement results for C1 to C4.

  The cycle life of the batteries A1 to A2 and B1 to B2 is an index with the cycle life of the battery A1 as 100. The cycle life of C1 to C4 is an index with the cycle life of the battery C1 as 100.

  As can be seen from Table 1, the battery A1 in which the opposing positive electrode mixture layer does not exist in the bent portion of the negative electrode exhibits excellent cycle characteristics as compared with the comparative battery B2. In addition, in the bent portion of the negative electrode, the battery A2 in which a polypropylene film is attached to the surface of the opposing positive electrode mixture layer shows better cycle characteristics than the comparative battery B2. In addition, the comparative battery B1 using the laminated electrode body in which the bent portion of the negative electrode does not exist also shows good cycle characteristics.

  On the other hand, in the batteries C1 to C4 using natural graphite as the negative electrode active material, there is no significant difference in cycle characteristics.

  From the above, in a battery using active material particles containing silicon and / or silicon alloy as a negative electrode active material, the negative electrode mixture layer is bent due to expansion / contraction of the negative electrode active material volume due to charge / discharge reaction. It can be seen that a large stress is generated at, and thus the mixture layer is destroyed, the current collecting property in the electrode is deteriorated, and the cycle characteristics are deteriorated. According to the present invention, in the bent portion of the negative electrode mixture layer, by constituting so as not to cause a charge / discharge reaction, it is possible to suppress the breakdown of the mixture layer in the bent portion of the negative electrode mixture layer, and charge / discharge A decrease in cycle characteristics can be suppressed. On the other hand, when natural graphite is used as the negative electrode active material, no significant change occurs in the volume of the negative electrode active material due to charge / discharge, and therefore, such a difference due to the electrode structure is hardly recognized.

(Experiment 5)
Here, the influence of the average particle size of the negative electrode active material on the charge / discharge cycle characteristics of the battery was examined.

  Batteries A3 and A4 were produced in the same manner as in Experiment 1, except that in Experiment 1, silicon powder having an average particle diameter of 8 μm or 29 μm was used as the negative electrode active material.

  The cycle characteristics of this battery were evaluated in the same manner as described above. The cycle life is an index with the cycle life of the battery A1 as 100. Table 3 also shows the cycle life of the battery A1.

  As is clear from Table 3, it can be seen that the battery using the negative electrode active material having a smaller particle diameter exhibits better cycle characteristics. This is because the use of active material particles having a small particle size can reduce the absolute amount of volume expansion / contraction of the active material particles associated with insertion / extraction of lithium in the charge / discharge reaction. This is probably because the absolute amount of distortion between the active material particles in the electrode at the time was reduced, the binder was not broken, and the decrease in the current collecting property in the electrode could be suppressed.

(Experiment 6)
Here, the influence of the surface roughness Ra of the negative electrode current collector on the charge / discharge cycle characteristics of the battery was examined.

  In Experiment 1, as a negative electrode current collector, instead of a Cu-0.03% by weight Zr alloy foil having a surface roughness Ra of 0.7 μm roughened by electrolytic copper plating, the surface roughness roughened by electrolytic copper plating Except for using a Cu-0.03% by weight Zr alloy foil with a Ra of 0.2 μm, or a Cu-0.03% by weight Zr alloy foil with a surface roughness Ra of 0.18 μm by electrolytic copper plating roughening. Batteries A5 and A6 were produced in the same manner as in Experiment 1.

  For these batteries, the cycle characteristics were evaluated in the same manner as in Experiment 1. The cycle life is an index with the cycle life of the battery A1 as 100. Table 4 also shows the cycle life of the battery A1.

  As is clear from Table 4, the batteries A1 and A5 using the current collector having a surface roughness Ra of 0.2 μm or more are the batteries A6 using the current collector having a surface roughness Ra of less than 0.2 μm. It can be seen that excellent cycle characteristics are exhibited as compared with FIG. This is because by using a metal foil having a surface roughness Ra of 0.2 μm or more, an anchor effect by the binder between the binder and the current collector is greatly obtained, and the adhesion between the mixture layer and the current collector is greatly improved. However, it is considered that separation of the mixture layer from the current collector due to expansion and contraction of the active material particles accompanying insertion and extraction of lithium was suppressed, and the current collecting property in the electrode was improved.

(Experiment 7)
Here, the effect of the conductive powder added to the negative electrode mixture layer on the charge / discharge cycle characteristics of the battery was examined.

  In Experiment 1, a battery A7 was produced in the same manner as in Experiment 1, except that 20% by weight of Ni powder having an average particle size of 3 μm was added to the negative electrode mixture layer with respect to the silicon powder.

  With respect to this battery A7, the cycle characteristics were evaluated in the same manner as described above. The cycle life is an index with the cycle life of the battery A1 as 100. Table 5 also shows the cycle life of the battery A1.

  As can be seen from Table 5, the battery A7 with Ni powder added to the mixture layer has improved cycle characteristics compared to the battery A1 without conductive powder added to the mixture layer. . This is considered because the current collection in the mixture layer was improved by forming a conductive network around the active material particles.

(Experiment 8)
Here, the influence of the sintering conditions in the production of the negative electrode on the cycle characteristics was examined.

  In Experiment 1, a battery A8 was produced in the same manner as in Experiment 1, except that heat treatment for sintering the negative electrode was performed at 700 ° C. for 1 hour. Further, in Experiment 1, a battery B3 was produced in the same manner except that the negative electrode was not heat-treated.

  For these batteries, the cycle characteristics were evaluated in the same manner as described above. The cycle life is an index with the cycle life of the battery A1 as 100. Table 6 also shows the cycle life of the battery A1.

  As is clear from Table 6, the battery A1 that had been heat-treated for the negative electrode showed much better cycle characteristics than the battery B3 that had not been heat-treated. This is presumably because the heat treatment of the negative electrode improved the adhesion within the mixture layer and between the mixture layer and the current collector due to the sintering effect, and improved the current collection within the negative electrode.

  In addition, it can be seen that the battery A8 subjected to heat treatment at 700 ° C. for 1 hour has greatly reduced cycle characteristics as compared with the battery A1 subjected to heat treatment at 400 ° C. for 1 hour. This is presumably because the heat treatment at 700 ° C. decomposes the binder, so that the adhesion within the negative electrode due to the binder is greatly reduced, and the current collection within the electrode is reduced.

(Experiment 9)
Here, the influence which the difference in the preparation methods of a negative electrode has on the charging / discharging cycling characteristics of a battery was examined.

(Production of negative electrode)
The negative electrode mixture slurry in Experiment 1 was applied on the surface of a glass plate and dried. The mixture layer on the sheet formed on the glass plate was peeled from the glass plate and cut into a rectangular shape of 42 × 32 mm. This was arrange | positioned on both surfaces of the negative electrode electrical power collector in Experiment 1, heat-processed at 400 degreeC for 1 hour in argon atmosphere, crimping | bonding, and it sintered and it was set as the negative electrode. The thickness of the negative electrode (including the current collector) was 65 μm. Therefore, the thickness of the mixture layer disposed on one side of the current collector is estimated to be 15 μm, the mixture layer thickness / copper foil surface roughness is 30, and the mixture layer thickness / copper foil thickness is 0.43. Met.

Moreover, in this negative electrode, the density of the polyimide was 1.1 g / cm ³ , and the area occupied by the polyimide was 31.8% of the total volume of the mixture layer.

[Production of battery]
In Experiment 1, a lithium secondary battery A9 was produced in the same manner except that the negative electrode produced as described above was used as the negative electrode.

[Evaluation of charge / discharge cycle characteristics]
The cycle characteristics of this battery were evaluated in the same manner as in Experiment 1. The cycle life is an index with the cycle life of the battery A1 as 100. Table 7 also shows the cycle life of the battery A1.

  As is apparent from Table 7, a sheet-like mixture layer was first prepared, placed on a current collector, and subjected to a heat treatment for sintering while being subjected to pressure bonding. The charge / discharge cycle characteristics are lower than that of the battery A1 using a negative electrode produced by applying a mixture slurry on a current collector to form a mixture layer and then performing a heat treatment for sintering. I understand that.

  In the negative electrode of the battery A9, it is difficult for the mixture layer to enter the irregularities on the surface of the current collector. Therefore, like the negative electrode of the battery A1, the binder appears in the irregularities on the surface of the current collector. This is probably because the effect of greatly improving the adhesion between the mixture layer and the current collector due to the anchor effect was not obtained.

The expanded sectional view which shows the electrode body in one Example of this invention. The perspective view which shows the structure of the electrode body in one Example of this invention. Sectional drawing which shows the electrode body in one Example of this invention. The front view which shows the lithium secondary battery in one Example of this invention. Sectional drawing which follows the AA 'line shown in FIG. The perspective view which shows the structure of the electrode body of a comparative example. Sectional drawing which shows the structure of the electrode body of a comparative example. The perspective view which shows the structure of the electrode body of a comparative example. Sectional drawing which shows the structure of the electrode body of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode tab 2 ... Negative electrode tab 3 ... Separator 4 ... Negative electrode 5 ... Positive electrode 6 ... Electrode body 7 ... Exterior body 8 ... Closure part

Claims (19)

A positive electrode in which a positive electrode mixture layer including a positive electrode active material and a positive electrode binder is disposed on the surface of the positive electrode current collector, and a negative electrode mixture layer including a negative electrode active material including silicon and / or a silicon alloy and a negative electrode binder are collected in the negative electrode In a lithium secondary battery comprising a negative electrode sintered and disposed on the surface of a body, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte,
The lithium secondary battery is configured such that the positive electrode and the negative electrode are arranged to face each other with the separator interposed therebetween, and a charge / discharge reaction does not occur in a portion where the negative electrode mixture layer is bent.  2. The lithium secondary battery according to claim 1, wherein a charging / discharging reaction is not caused by the absence of an opposing positive electrode mixture layer in a portion where the negative electrode mixture layer is bent. 3. .   In the bent portion of the negative electrode mixture layer, a lithium ion non-conducting substance is provided on the surface of the opposing positive electrode mixture layer, so that no charge / discharge reaction occurs. The lithium secondary battery according to claim 1.   The structure in which charging / discharging reaction does not occur by providing a lithium ion non-conducting substance on the surface of the negative electrode mixture layer in the bent portion of the negative electrode mixture layer. 2. The lithium secondary battery according to 1.   The lithium secondary battery according to claim 3 or 4, wherein the lithium ion non-conductive substance is a plastic film.   The lithium secondary battery according to claim 1, wherein an arithmetic average roughness Ra of the surface of the negative electrode current collector is 0.2 μm or more.   The lithium anode according to any one of claims 1 to 6, wherein the negative electrode current collector is a copper foil or a copper alloy foil, or a metal foil having a copper layer or a copper alloy layer provided on a surface thereof. Next battery.   The negative electrode current collector is an electrolytic copper foil or an electrolytic copper alloy foil, or a metal foil provided with an electrolytic copper layer or an electrolytic copper alloy layer on a surface thereof. The lithium secondary battery as described.   The lithium secondary battery according to claim 1, wherein the negative electrode binder remains after heat treatment for sintering.   The lithium secondary battery according to claim 1, wherein the negative electrode binder is polyimide.   11. The lithium secondary battery according to claim 1, wherein an average particle diameter of the negative electrode active material is 10 μm or less.   The lithium secondary battery according to claim 1, wherein the negative electrode active material is silicon particles.   The lithium secondary battery according to claim 1, wherein conductive powder is mixed in the negative electrode mixture layer.   The lithium secondary battery according to claim 1, wherein the negative electrode is sintered in a non-oxidizing atmosphere. A positive electrode in which a positive electrode mixture layer including a positive electrode active material and a positive electrode binder is disposed on the surface of the positive electrode current collector, and a negative electrode mixture layer including a negative electrode active material including silicon and / or a silicon alloy and a negative electrode binder are collected in the negative electrode A method for producing a lithium secondary battery comprising a negative electrode sintered and disposed on a surface of a body, a separator disposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte,
A method for producing a lithium secondary battery, characterized in that a charge / discharge reaction does not occur in a portion where the negative electrode mixture layer is bent.   The lithium according to claim 15, wherein the negative electrodes having the negative electrode mixture layer provided on both sides are alternately folded, and the positive electrode having the positive electrode mixture layer provided on both sides is disposed between the folded surfaces of the negative electrode. A method for manufacturing a secondary battery.   The method for producing a lithium secondary battery according to claim 15, wherein a plastic film is attached to a surface portion of the positive electrode mixture layer facing a bent portion of the negative electrode mixture layer.   The method for producing a lithium secondary battery according to claim 15, wherein a plastic film is attached to a surface of the bent portion of the negative electrode mixture layer. The method for producing a lithium secondary battery according to claim 15, wherein the negative electrode is sintered in a non-oxidizing atmosphere.

Images