JP4744244B2 - Lithium secondary battery and method for producing negative electrode for lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery and a method for producing a negative electrode for a lithium secondary battery.

  In recent years, as a new secondary battery with high output and high energy density, a lithium secondary battery using a non-aqueous electrolyte and moving lithium ions between a positive electrode and a negative electrode to perform charging and discharging has been used. Has been.

  In such a lithium secondary battery, a negative electrode using a material containing silicon as a negative electrode active material has been studied. However, an electrode using a silicon-containing material as an active material can be used to pulverize the active material due to expansion / contraction of the active material volume during lithium insertion / release, or to peel the active material from the current collector. Therefore, there is a problem that the current collecting property in the electrode is lowered and the charge / discharge cycle characteristics are deteriorated.

  In addition, since the volume of the active material expands and contracts when lithium is occluded / released, cracks occur on the surface of the active material particles, resulting in an active new surface. And a non-aqueous electrolyte reaction occurs, and a new film is formed.

  The present applicant has found that a negative electrode for a lithium secondary battery in which a mixture layer containing a silicon material and a binder is sintered on a surface of a current collector of conductive metal foil having irregularities is arranged with a mixture layer and a collector layer. It has been found that a high current collecting property is exhibited in the electrode due to the high adhesion with the electric body, and that good charge / discharge cycle characteristics are exhibited (Patent Document 1).

  However, when such a lithium secondary battery is stored in a charged state at a high temperature, there is a problem that gas is generated in the battery due to a decomposition reaction of the nonaqueous electrolyte, and the battery container swells. In particular, when an active material containing silicon is used as the negative electrode active material, the amount of gas generated during high-temperature storage is greater than when a carbon active material is used.

  This is because, when silicon is used as the negative electrode active material, the active material volume expands and contracts when lithium is occluded / released, so that the active material particle surface is cracked and an active new surface is generated. As a result, the surface area of the negative electrode active material increases with each charge and discharge, and the area in contact with the nonaqueous electrolyte increases, so the amount of decomposition of the nonaqueous electrolyte on the negative electrode surface increases during high-temperature storage and gas generation also increases. On the other hand, when a carbon material is used as the negative electrode active material, the carbon negative electrode generates less carbon dioxide than when a silicon negative electrode is used because the active material does not crack due to insertion and extraction of lithium. For this reason, in the case of using a silicon negative electrode, it is desired to suppress the gas generated during high-temperature storage.

  As a method for suppressing gas generation during high-temperature storage in a lithium ion battery, it has been proposed to add lithium oxide in the lithium secondary battery (Patent Document 2 and Patent Document 3).

  In each of the above publications, in any lithium secondary battery using a carbon negative electrode, lithium oxide is added to the positive electrode, the negative electrode, the battery can wall surface, etc. inside the lithium secondary battery, thereby generating during high-temperature storage. It is described that carbon dioxide can be absorbed and an increase in battery swelling can be suppressed. It is described that lithium oxide effectively absorbs carbon dioxide generated by decomposition of an electrolytic solution or the like during high-temperature storage, and as a result, an increase in battery swelling can be suppressed.

  When an active material containing silicon is used for the negative electrode, the gas generated from the cracked surface of the active material is significant, so it seems effective to add lithium oxide into the negative electrode.

  However, when an active material containing silicon is used for the negative electrode, if lithium oxide is added to the negative electrode, there arises a problem that good charge / discharge cycle characteristics cannot be obtained. This is because, when an active material containing silicon is used for the negative electrode, the volume of the active material expands and contracts when lithium is occluded / released. Therefore, when lithium oxide is added to the negative electrode, stress relaxation is inhibited, This is because the electrode plate strength decreases. As a result, the active material is peeled off and the current collecting structure cannot be maintained, so that good charge / discharge cycle characteristics cannot be obtained.

  When a carbon negative electrode is used, even if lithium oxide is added to the negative electrode, the volume change of the carbon active material during insertion and extraction of lithium is smaller than that of the silicon active material. The problem of inhibiting the stress relaxation of the change and lowering the electrode plate strength hardly occurs. Therefore, when lithium oxide is added to the carbon negative electrode, unlike the case of using the silicon negative electrode, good cycle characteristics can be obtained.

For this reason, when a silicon negative electrode is used, even if lithium oxide in the negative electrode effectively absorbs carbon dioxide and suppresses battery swelling during high-temperature storage, it is difficult to achieve both charge / discharge cycle characteristics.
Japanese Patent Laid-Open No. 2002-260637 JP-A-6-140077 JP 2003-208923 A

  An object of the present invention is to provide a lithium secondary battery that can suppress gas generation during high-temperature storage and improve charge / discharge cycle characteristics in a lithium secondary battery using an active material containing silicon as a negative electrode. It is to provide.

  The lithium secondary battery of the present invention includes a negative electrode obtained by disposing an active material particle containing silicon and / or a silicon alloy, a binder layer containing a binder and lithium oxide on a current collector, and firing it, a positive electrode, and non-aqueous And an electrolyte.

  In accordance with the present invention, the mixture layer containing lithium oxide is disposed on the current collector and then baked, whereby the charge / discharge cycle characteristics are excellent and the increase in battery thickness after high-temperature storage can be suppressed.

  The negative electrode obtained by firing a mixture layer containing active material particles, lithium oxide, and a binder on the surface of the current collector has good adhesion between the active material particles and the current collector, and is capable of absorbing and releasing lithium. Even when the volume of the active material particles is expanded or contracted, the active material particles are hardly peeled off from the current collector, and a high current collecting property can be obtained. When firing is not performed, the lithium oxide added to the negative electrode results in insufficient stress relaxation due to expansion and contraction of the volume of the active material during insertion and extraction of lithium, resulting in a decrease in electrode plate strength and good cycle characteristics. I can't get it.

  The lithium oxide added to the negative electrode mixture layer is preferably uniformly dispersed in the mixture layer. Since the lithium oxide is uniformly dispersed in the negative electrode mixture layer, the stress relaxation of the change in the volume of the active material during insertion and extraction of lithium is improved. In addition, carbon dioxide generated on the negative electrode surface can be immediately absorbed near the negative electrode during high-temperature storage.

  In the present invention, the particle diameter of lithium oxide added to the mixture layer is not particularly limited, but the particle diameter is such that stress relaxation due to volume change of the active material at the time of occlusion / release of lithium is not hindered. It is preferable that it is small. The average particle diameter of lithium oxide is preferably 100 μm or less, more preferably 50 μm or less, and further preferably in the range of 0.01 to 10 μm.

  Examples of the apparatus for uniformly mixing the active material particles, the lithium oxide and the binder include a roll mill, an attritor, a bar mill, a pebble mill, a sand mill, a kedy mill, a mechanofusion, and a raking machine.

  Lithium oxide is preferably added to the negative electrode, and even when added to a place other than the negative electrode, the carbon dioxide absorption effect is small. Rather than absorbing carbon dioxide once generated as a gas, it is better to absorb immediately when carbon dioxide is generated at the nascent interface of the active material where the active material breaks and is in contact with the non-aqueous electrolyte. This is because carbon dioxide can be absorbed effectively.

  The lithium oxide used in the present invention is preferably in the range of 0.01 to 5% by weight with respect to the active material particles as the content in the negative electrode mixture layer. That is, it is preferably within a range of 0.01 to 5 parts by weight with respect to 100 parts by weight of the negative electrode active material particles.

  When the content of lithium oxide exceeds 5% by weight with respect to the negative electrode active material, stress relaxation due to volume change of the active material containing silicon cannot be achieved, and the amount of negative electrode active material in the negative electrode mixture layer decreases. Therefore, a sufficient capacity may not be obtained. On the other hand, when the content of lithium oxide is less than 0.01% by weight with respect to the negative electrode active material, there is a limit to the amount of carbon dioxide absorbed by lithium oxide. In some cases, the effect of the present invention to suppress the effect cannot be sufficiently obtained.

  The solvent of the non-aqueous 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. 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. 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 Bi, Al, Ga or a y is 4) when the 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 perfluoroalkyl sulfonic acid methide _{LiC (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) ( In the formula, p, q and r are each independently an integer of 1 to 4, and a mixed solute may be used. Among these, LiPF ₆ is particularly preferably used.

  The electrolyte of the lithium secondary battery of the present invention is limited as long as the lithium compound as a solute that develops lithium ion conductivity and the solvent that dissolves and retains the lithium compound are not decomposed by the voltage at the time of charging, discharging or storing the battery. Can be used without any problem.

  In the present invention, it is preferable that carbon dioxide is dissolved in the nonaqueous electrolyte. This is because when the active material containing silicon is used for the negative electrode and carbon dioxide is dissolved in the nonaqueous electrolyte, excellent charge / discharge cycle characteristics can be obtained.

  However, when carbon dioxide is dissolved in the non-aqueous electrolyte, good charge / discharge cycle characteristics can be obtained. On the other hand, compared with the case where carbon dioxide is not dissolved, the amount of carbon dioxide generated during high-temperature storage is increased. . However, in accordance with the present invention, by containing lithium oxide in the negative electrode, the generated carbon dioxide is effectively absorbed by the lithium oxide contained in the negative electrode, so that battery swelling can be suppressed. As a result, even better charge / discharge cycle characteristics can be obtained, and both battery swelling reduction during high-temperature storage can be achieved.

  The amount of carbon dioxide dissolved in the nonaqueous electrolyte is preferably 0.01% by weight or more, more preferably 0.05% by weight or more, and further preferably 0.1% by weight or more. Usually, it is preferable to dissolve carbon dioxide in the nonaqueous electrolyte until it is saturated.

  The amount of carbon dioxide dissolved can be determined, for example, by measuring the weight of the non-aqueous electrolyte after dissolving carbon dioxide and the weight of the non-aqueous electrolyte before dissolving carbon dioxide.

  Examples of the negative electrode active material particles 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.

  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 on which the mixture layer is disposed is 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. In particular, in the present invention, since the mixture layer is disposed on the negative electrode current collector and then fired to sinter the mixture layer, the negative electrode collector having the arithmetic average roughness Ra of the surface is thus obtained. By using the current collector, the contact area between the active material particles and the current collector is increased, and the adhesion between the active material particles and the current collector can be effectively improved by sintering. Further, when the binder enters the uneven portion on the surface of the current collector, an anchor effect is exhibited between the binder and the current collector, so that higher adhesion can be obtained. For this reason, peeling of the mixture layer from the current collector due to expansion / contraction of the volume of the active material particles accompanying insertion / release of lithium can be effectively suppressed. 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.

  As the negative electrode current collector in the present invention, a conductive metal foil is preferably used. 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.

  As described above, the negative electrode current collector used in the present invention preferably has large irregularities on the surface thereof as described above. 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.

  The negative electrode binder used in the present invention is preferably one that remains without being completely decomposed even after heat treatment for sintering (firing). 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. In addition, if the amount of the binder in the mixture layer is too large, the resistance in the electrode increases, so that initial charging 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.

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.

  In the method for producing a negative electrode for a lithium secondary battery according to the present invention, a slurry in which active material particles containing silicon and / or a silicon alloy and lithium oxide are mixed and dispersed in a binder solution is applied on the surface of the current collector. A step of disposing a mixture layer containing active material particles, a binder and lithium oxide on the current collector, and a step of firing the mixture layer in a state where the mixture layer is disposed on the current collector. It is a feature.

  In the production method of the present invention, the mixture layer is fired in a state where the mixture layer is disposed on the current collector, and the mixture layer is sintered. Such firing is preferably performed in a vacuum, a nitrogen atmosphere, or a non-oxidizing atmosphere such as argon. It may be performed in a reducing atmosphere such as a hydrogen atmosphere. The temperature of the heat treatment at the time of firing is preferably a temperature not higher than the melting points of the current collector and the active material particles. For example, when copper foil is used as the current collector, the melting point is preferably 1080 ° C. or lower. A more preferable firing temperature is in the range of 200 to 500 ° C, and more preferably in the range of 300 to 450 ° C. As a firing method, a discharge plasma sintering method or a hot press method may be used.

  In addition, since lithium oxide reacts with moisture and carbon dioxide in the atmosphere and changes to lithium hydroxide and lithium carbonate, part of the lithium oxide added to the negative electrode mixture layer is also used in the negative electrode manufacturing process. It is thought that it has changed to lithium hydroxide or lithium carbonate. Since lithium oxide is obtained by heating lithium carbonate under reduced pressure, lithium hydroxide and lithium carbonate in the negative electrode mixture layer can be changed again to lithium oxide by performing a firing step according to the present invention. In addition, the effect of lithium oxide as a gas absorbent can be further enhanced.

  Moreover, in the manufacturing method of this invention, after arrange | positioning a mixture layer on a collector, it is preferable to roll a mixture layer with a collector before baking. By such rolling, the packing density in the mixture layer can be increased, the adhesion between the active material particles and the adhesion between the active material particles and the current collector can be improved, and further favorable charge / discharge cycle characteristics Can be obtained. In addition, by rolling, the lithium oxide added to the negative electrode mixture is more closely attached to the active material particles, so that carbon dioxide generated on the negative electrode surface during high-temperature storage can be immediately absorbed, and battery swelling is more effective. Can be suppressed.

  According to the present invention, in a lithium secondary battery using an active material containing silicon as a negative electrode active material, the generation of gas during high-temperature storage can be suppressed, and swelling of the battery after high-temperature storage can be suppressed. Good 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 implemented with appropriate modifications within a range not changing the gist thereof. Is.

Example 1
(Production of negative electrode)
To N-methyl-2-pyrrolidone as a solvent, silicon powder having an average particle size of 5.5 μm as an active material (purity 99.9%), lithium oxide (average particle size 5 μm), and polyimide as a binder, It mixed and added so that a weight ratio might be set to 9: 0.18 (2 weight% with respect to silicon powder): 1, and the negative mix slurry was produced.

  This negative electrode mixture slurry was applied to one side (rough surface) of an electrolytic copper foil (thickness 35 μm) having a surface roughness (arithmetic average roughness) Ra as a current collector of 0.5 μm and dried. The obtained product was cut into a rectangular shape of 380 × 52 mm, rolled, and then heat-treated (fired) at 400 ° C. for 1 hour in an argon atmosphere, and sintered to prepare a negative electrode.

[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 the 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 5% by weight N-methylpyrrolidone solution containing 3 parts by weight of polyvinylidene fluoride as a binder. An agent slurry was obtained.

  This positive electrode mixture slurry was applied onto an aluminum foil as a current collector, dried and then rolled. The obtained product was cut out to 402 mm × 50 mm to obtain a positive electrode.

[Production of non-aqueous electrolyte]
Carbon dioxide is blown into a non-aqueous electrolyte prepared by dissolving 1 mol / liter of LiPF ₆ in a solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7 at 25 ° C. for 10 minutes. A non-aqueous electrolyte dissolved until was obtained. The amount of carbon dioxide dissolved was 0.37% by weight.

[Production of battery]
A lithium secondary battery A1 in which the negative electrode, the positive electrode, and the nonaqueous electrolyte were inserted into an aluminum laminate outer package was produced.

  FIG. 1 is a front view showing the produced lithium secondary battery. 2 is a cross-sectional view taken along line AA in FIG. The positive electrode and the negative electrode are wound so as to face each other through a separator made of a polyethylene porous body, and are inserted as an electrode body 15 into an exterior body 11 made of an aluminum laminate as shown in FIG. A positive electrode current collecting tab 13 made of aluminum is attached to the positive electrode, and a negative electrode current collecting tab 14 made of nickel is attached to the negative electrode, and these are drawn out from the exterior body 11 to the outside. As shown in FIG.1 and FIG.2, the closing part 12 is formed in the periphery of the exterior body 11 by heat sealing.

(Comparative Example 1)
In the production of the negative electrode of Example 1, a battery B1 having the same configuration as that of Example 1 was produced, except that heat treatment was not performed after rolling.

(Comparative Example 2)
A battery B2 having the same configuration as that of Example 1 was prepared except that lithium oxide was not added and heat treatment was not performed in the production of the negative electrode of Example 1.

(Comparative Example 3)
In the production of the negative electrode of Example 1, a battery B3 having the same configuration as that of Example 1 was produced, except that lithium oxide was not added.

[Evaluation of charge / discharge cycle characteristics]
The lithium secondary batteries A1 and B1 to B3 produced as described above were subjected to a charge / discharge cycle test. Each battery was charged to 4.2 V at a current value of 600 mA at 25 ° C., and then discharged to 2.75 V at a current value of 600 mA. 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 B3 as 100.

[Evaluation of battery swelling during high temperature storage]
About lithium secondary battery A1 produced as mentioned above and B1-B3, the battery thickness evaluation at the time of high temperature storage was performed in the charging state. Each battery was charged to 4.2 V at a current value of 600 mA at 25 ° C., and then discharged to 2.75 V at a current value of 600 mA. After performing this two cycles, high temperature storage was performed at 60 ° C. for 20 days in a state of being charged to 4.2 V at a current value of 600 mA. The battery thickness before and after high temperature storage was measured, and the increase in battery thickness was determined.

  Table 1 shows the cycle life of the charge / discharge cycle test and the increase in battery thickness after high-temperature storage. Here, the battery thickness increase amount indicates a relative value when the battery thickness increase amount before and after high temperature storage in the battery B3 is 100.

  As apparent from Table 1, a cycle is obtained by sintering a mixture layer containing active material particles containing silicon and / or a silicon alloy, lithium oxide and a binder on the surface of the conductive metal foil current collector. It can be seen that the lifetime is dramatically improved. This is because the Ra of the current collector surface is roughened to 0.2 μm or more, so that the adhesion between the active material particles and the current collector is improved and the surface of the current collector is improved by sintering. It is because the anchor effect was expressed between the binder and the current collector by the binder entering the uneven portion.

  Moreover, it turns out that battery B1 which added lithium oxide to the negative electrode has a low charging / discharging cycle characteristic compared with battery B2 which does not add lithium oxide. This is because, since lithium oxide is added to the negative electrode, stress relaxation due to volume expansion of the silicon active material during charge / discharge cannot be performed, and the electrode plate strength decreases. The decrease in the electrode plate strength causes the active material or the like to be pulverized and the current collecting structure is broken, so that good charge / discharge cycle characteristics cannot be obtained.

  However, it can be seen that the battery A1 in which the negative electrode with lithium oxide added in the negative electrode is heat-treated can obtain better charge / discharge cycle characteristics than the battery B1 in which the heat treatment is not performed. By performing the heat treatment, the adhesion between the active material, the binder and the current collector is improved as described above. Therefore, even if lithium oxide is added, the electrode plate strength does not decrease, and lithium oxide is not added to the negative electrode. It turned out that the favorable cycling characteristics equivalent to battery B3 are acquired.

  Moreover, it turns out that the battery thickness increase amount at the time of high temperature storage has reduced the battery thickness increase amount, when lithium oxide is added. This is thought to be because lithium oxide added to the negative electrode absorbed carbon dioxide generated on the negative electrode surface and the positive electrode. When silicon is used as the negative electrode active material, cracks occur on the surface of the active material particles during charging and discharging, the negative electrode surface area increases, and gas generation from the negative electrode during high-temperature storage is greater than that of the carbon negative electrode, but oxidation occurs in the negative electrode. It seems that by adding lithium, carbon dioxide generated from the negative electrode is immediately absorbed, and the increase in battery swelling is suppressed.

  From the above results, it can be seen that by using lithium anode added to the negative electrode and using the fired negative electrode, good cycle characteristics can be obtained and the increase in battery thickness during high-temperature storage can be reduced.

(Examples 2 and 3)
Batteries A2 and A3 having the same configuration as in Example 1 were respectively added except that 0.01 wt% and 5 wt% of lithium oxide were added to the negative electrode mixture slurry at the time of preparing the negative electrode, respectively. Produced.

  About the lithium secondary battery A2 and A3 produced as mentioned above, the above-mentioned charging / discharging cycle test and the battery swelling evaluation at the time of high temperature storage were performed. The results are shown in Table 2. The battery thickness increase amount indicates a relative value when the battery thickness increase amount before and after high temperature storage in the battery B3 is 100. Table 2 also shows the results of batteries A1 and B3.

  From the results of Table 2, the batteries A1 to A3 in which lithium oxide was added to the negative electrode mixture layer had a difference in addition concentration compared to the battery B3 to which lithium oxide was not added, but the battery thickness increased after high-temperature storage. You can see that the amount is small.

  In addition, when the concentration of lithium oxide added to the negative electrode mixture layer is changed, the amount of increase in thickness is the lowest when 2% by weight of the active material is added, and the generation of gas can be suppressed. I understand. When 5 wt% is added, the battery thickness increase amount is almost the same as when 2 wt% is added.

(Comparative Example 4)
Here, in order to examine the addition location of lithium oxide, a separator was processed into a bag shape, and lithium oxide was put in a bag-shaped separator, which was attached to the outermost periphery of the electrode body. The amount of lithium oxide added was 2% by weight based on the weight of the silicon active material, and the battery C1 was constructed in the same manner as in Example 1 except that a separator bag having lithium oxide in the outermost periphery was used as the separator. Produced.

(Comparative Example 5)
In order to examine the influence of the heat treatment temperature of lithium oxide, a battery C2 was produced with the same configuration as in Comparative Example 4 except that lithium oxide heat treated at 400 ° C. for 1 hour was placed in a bag-like separator.

  The lithium secondary batteries C1 and C2 produced as described above were subjected to battery swelling evaluation during high-temperature storage by the same method as described above, and the amount of increase in battery thickness during high-temperature storage was measured. The results are shown in Table 3. Here, the battery thickness increase amount indicates a relative value when the battery thickness increase amount before and after high-temperature storage of the battery B3 in which lithium oxide is not added to the inside of the battery is 100.

  As is apparent from Table 3, the batteries C1 and C2 to which lithium oxide was added to the outermost periphery showed an increase in the battery thickness that was almost equal to or greater than the battery thickness increase of the battery B3 to which lithium oxide was not added. It can be seen that when lithium is added to other than the negative electrode mixture layer, there is no effect of suppressing gas generation. From this, it is preferable that lithium oxide immediately absorbs carbon dioxide in the vicinity of the generation of carbon dioxide in the negative electrode, and that the effect of the present invention can be obtained by adding lithium oxide to the negative electrode. By adding lithium oxide into the negative electrode, the generated gas can be effectively absorbed, and battery swelling can be reduced.

  Moreover, it can be seen from the comparison between the battery C1 and the battery C2 that even when lithium oxide is disposed on the outermost periphery of the electrode body, the increase in battery thickness can be reduced by heat treating the lithium oxide. Lithium oxide absorbs moisture and carbon dioxide in the atmosphere, and part of it easily changes to lithium hydroxide or lithium carbonate. By heat treating lithium oxide, lithium hydroxide and lithium carbonate are converted again to lithium oxide. This is considered to be because the carbon dioxide absorption function of the original lithium oxide was able to be exhibited.

  In the present invention, lithium oxide is contained in the mixture layer, and the mixture layer is fired in a state of being disposed on the negative electrode, and the lithium oxide is heat-treated during the firing. For this reason, the carbon dioxide absorption function inherent to lithium oxide can be exhibited, and the amount of carbon dioxide absorbed can be increased. Therefore, it is possible to more effectively suppress an increase in battery swelling during high-temperature storage, and good charge / discharge cycle characteristics can be obtained.

The front view which shows the lithium secondary battery produced in the Example according to this invention. Sectional drawing in alignment with the AA shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Exterior body 12 ... Closure part 13 ... Positive electrode current collection tab 14 ... Negative electrode current collection tab 15 ... Electrode body

Claims (4)

  A negative electrode obtained by arranging and firing a mixture layer containing active material particles containing silicon and / or a silicon alloy, a binder and lithium oxide on a current collector, a positive electrode, and a non-aqueous electrolyte. Lithium secondary battery.   2. The lithium secondary battery according to claim 1, wherein a content of the lithium oxide in the mixture layer is 0.01 to 5 wt% with respect to the active material particles.   The lithium secondary battery according to claim 1 or 2, wherein carbon dioxide is dissolved in the non-aqueous electrolyte. The active material particles containing silicon and / or silicon alloy and a slurry in which lithium oxide is mixed and dispersed in a binder solution are applied onto the surface of the current collector, whereby a mixture containing the active material particles, the binder and lithium oxide is applied. Placing a layer on the current collector;
And a step of firing the mixture layer in a state where the mixture layer is disposed on the current collector. A method for producing a negative electrode for a lithium secondary battery, comprising:

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