JP5127888B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

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

  In recent years, a lithium secondary battery that uses a non-aqueous electrolyte and charges and discharges by moving lithium ions between a positive electrode and a negative electrode has been used as one of the new secondary batteries with high output and high energy density. Yes.

  As such a negative electrode for a lithium secondary battery, a material using a material alloyed with lithium as a negative electrode active material has been studied. As a material alloyed with lithium, for example, a silicon material has been studied. However, materials that alloy with lithium, such as silicon materials, expand and contract the volume of the active material when occluding and releasing lithium. Detach from the body. For this reason, there existed a problem that the current collection property in an electrode fell and charging / discharging cycling characteristics worsened.

  The present applicant, after disposing a mixture layer containing active material particles containing silicon and / or a silicon alloy and a binder on the surface of the current collector as a negative electrode for a lithium secondary battery that solves such problems, Have proposed a negative electrode for a lithium secondary battery obtained by sintering (Patent Document 1).

  On the other hand, in a lithium secondary battery using a carbon material or metallic lithium as a negative electrode active material, it has been proposed to dissolve carbon dioxide in a non-aqueous electrolyte or to enclose carbon dioxide in a battery can (patent) Literature 2-12).

International Publication No. 02/21616 Pamphlet U.S. Pat. No. 4,853,304 JP-A-6-150975 JP-A-6-124700 JP 7-176323 A Japanese Patent Laid-Open No. 7-249431 JP-A-8-64246 Japanese Patent Laid-Open No. 9-63649 Japanese Patent Laid-Open No. 10-40958 JP 2001-307771 A JP 2002-329502 A JP 2003-86243 A

  The lithium secondary battery proposed by the present applicant is a battery having a large charge / discharge capacity and excellent cycle characteristics, but the active material particles of the negative electrode become porous due to repeated charge / discharge, and the thickness of the negative electrode increases. There was a problem to do.

  An object of the present invention is a lithium secondary battery using a negative electrode including active material particles containing silicon and / or a silicon alloy, having a large charge / discharge capacity, excellent cycle characteristics, and porosity of the active material particles by charge / discharge. An object of the present invention is to provide a lithium secondary battery and a method for producing the same that can suppress the quality improvement and can reduce the increase in the thickness of the electrode after charge and discharge.

  The lithium secondary battery of the present invention includes 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 made of a conductive metal foil. A lithium secondary battery comprising a positive electrode and a non-aqueous electrolyte, characterized in that carbon dioxide is dissolved in the non-aqueous electrolyte.

  The dissolution of carbon dioxide in the non-aqueous electrolyte in the present invention means that carbon dioxide is intentionally dissolved in the non-aqueous electrolyte. That is, in the manufacturing process of a normal lithium secondary battery, dissolution of carbon dioxide that is inevitably dissolved in the nonaqueous electrolyte is not included. Carbon dioxide is generally dissolved in a non-aqueous electrolyte solvent. Accordingly, carbon dioxide may be dissolved in the nonaqueous electrolyte in which the solute is dissolved, or the nonaqueous electrolyte may be prepared by dissolving the solute in a solvent in which carbon dioxide is dissolved in advance.

  By dissolving carbon dioxide in the non-aqueous electrolyte, it is possible to prevent the active material particles from being made porous due to the charge / discharge reaction. Therefore, the increase in the thickness of the layer of active material particles due to charge / discharge can be reduced, and the volume energy density of the lithium secondary battery can be increased.

  A negative electrode obtained by sintering a mixture layer containing active material particles containing silicon and / or silicon alloy and a binder on the surface of a current collector made of conductive metal foil has a large charge / discharge capacity, Excellent cycle characteristics. The present inventors have found that in such a negative electrode, when the charge / discharge reaction is repeated, the porous material gradually becomes porous from the surface of the active material particles toward the inside thereof. Such porosity increases the thickness of the electrode and decreases the volume energy density. Such porous formation in the active material is considered to be caused by the alteration of silicon due to the irreversible reaction of silicon as the active material.

  By making carbon dioxide dissolved in the non-aqueous electrolyte according to the present invention, it is possible to prevent the active material particles from becoming porous. For this reason, the increase in the thickness of an electrode can be decreased and a volume energy density can be raised. Although the details of why the porous material of the active material particles can be suppressed by dissolving carbon dioxide in the nonaqueous electrolyte are not clear, a stable film with high lithium ion conductivity is formed on the surface of the particles. This is probably because of this.

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

  In the present invention, the amount of carbon dioxide dissolved in the nonaqueous electrolyte is preferably 0.001% by weight or more, more preferably 0.01% by weight or more, and further preferably 0.05% by weight or more. More preferably 0.1% by weight or more. Usually, it is preferable to dissolve carbon dioxide in the nonaqueous electrolyte until it is saturated. Here, the amount of carbon dioxide dissolved does not include carbon dioxide inevitably dissolved in the nonaqueous electrolyte. That is, carbon dioxide that dissolves in the non-aqueous electrolyte in a normal manufacturing process is not included. Therefore, 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. Specifically, it can be obtained by the following equation.

Dissolved amount of carbon dioxide in non-aqueous electrolyte (% by weight) = [(weight of non-aqueous electrolyte after dissolving carbon dioxide) − (weight of non-aqueous electrolyte before dissolving carbon dioxide)] / (dioxide Weight of non-aqueous electrolyte after dissolving carbon) × 100
In the present invention, it is preferable that carbon dioxide is contained in the voids inside the battery. The void inside the battery is formed between an electrode body formed by, for example, facing a positive electrode and a negative electrode with a separator interposed therebetween and a battery outer package. The voids can contain carbon dioxide, for example, by performing in a carbon dioxide atmosphere when assembling the battery, or by releasing carbon dioxide dissolved in the electrolyte from the electrolyte. When carbon dioxide in the electrolyte is consumed with charging / discharging, the carbon dioxide in the gap is dissolved in the electrolyte, and the carbon dioxide in the electrolyte can be replenished.

  In the present invention, it is preferable that the nonaqueous electrolyte contains a compound containing fluorine. By including such a compound in the non-aqueous electrolyte, cycle characteristics can be further improved.

  Examples of the fluorine-containing compound include a fluorine-containing lithium salt and a fluorine-containing solvent.

Examples of the fluorine-containing lithium salt 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, LiXF y ( wherein, X is P, As, Sb, B, Bi, Al, Ga , or, Y is 6 when X is P, As or Sb, and y is 4 when X is B, Bi, Al, Ga, or In), lithium perfluoroalkylsulfonic 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), and 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 ) , Q and r are each independently an integer of 1 to 4.

Examples of the fluorine-containing solvent include compounds obtained by substituting hydrogen atoms of cyclic carbonates such as butylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate and diethyl carbonate with fluorine atoms. For example, trifluoromethylated propylene carbonate in which hydrogen atoms of propylene carbonate are substituted with fluorine atoms, 1,1,1-trifluorodiethyl carbonate (CF ₃ CH ₂ OCOOCH ₂ CH ₃ ), trifluoroethyl methyl carbonate (CF ₃ CH ₂ OCOOCH ₃ ). Moreover, you may use the compound which substituted hydrogen atoms, such as ether solvents, such as 1, 2- dimethoxyethane, 1, 2- diethoxyethane, and cyclic esters, such as (gamma) -butyrolactone, with the fluorine atom. Examples of such include bis 1,2- (2,2,2-trifluoroethoxy) ethane (CF ₃ CH ₂ OCH ₂ CH ₂ OCH ₂ CF ₃ ).

  When a fluorine-containing lithium salt is used as the solute of the nonaqueous electrolyte, the fluorine-containing lithium salt is preferably added so as to have a concentration of 0.1 to 2 mol / liter with respect to the nonaqueous electrolyte. Moreover, it is preferable that the total amount of lithium salt is 0.5-2 mol / liter. When the concentration is less than 0.1 mol / liter, the effect of containing fluorine cannot be obtained sufficiently, which is not preferable. Moreover, when the total amount of the lithium salt is less than 0.5 mol / liter, the lithium ion conductivity of the nonaqueous electrolyte cannot be sufficiently obtained, which is not preferable. On the other hand, when the concentration exceeds 2 mol / liter, the viscosity of the non-aqueous electrolyte increases, the ion conductivity of the non-aqueous electrolyte decreases, and the salt precipitates at low temperatures, which is not preferable.

  When a fluorine-containing compound is used as the non-aqueous electrolyte solvent, it is preferably used in an amount of 1% by volume or more in all the solvents. When the concentration is less than 1% by volume, the effect of containing fluorine may not be sufficiently obtained.

  In the present invention, a fluorine-containing compound that is hardly soluble in the electrolytic solution may be previously contained in the separator. Further, a fluorine-containing compound may be added in advance to the negative electrode mixture layer. Examples of such a fluorine-containing compound include lithium fluoride.

  When a fluorine-containing compound is added to the negative electrode mixture layer, it is preferable to add 0.05 to 5% by weight of the fluorine-containing compound with respect to the total amount of the mixture. If it is less than 0.05% by weight, the effect of containing fluorine may not be sufficiently obtained. On the other hand, if it is more than 5% by weight, the resistance in the active material layer increases, which is not preferable.

  The 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.

  The active material particles used in the present invention may be those obtained by coating the surface of silicon and / or silicon alloy particles with a metal or the like. 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 active material particles used in the present invention may include 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 active material particles used in the present invention is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less for effective sintering. . 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.

  In addition, as described above, in the present invention, it is considered that a stable film having high lithium ion conductivity is formed on the surface of the active material particles by dissolving carbon dioxide in the nonaqueous electrolyte. By using the active material particles having a small average particle diameter, the lithium ion conductive film is present densely in the mixture layer. Accordingly, since a dense lithium ion conduction path is formed in the mixture layer, it is considered that the uniformity of the charge / discharge reaction distribution in the electrode is further improved. For this reason, cracking of the active material due to strain generated by the bias in volume change accompanying the insertion and extraction of lithium in the active material is suppressed, so that generation of new surfaces in the active material particles is also suppressed, and charge / discharge characteristics are improved. Further improvement can be achieved.

  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 current collector of 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.

  In the present invention, the thickness of the 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 current collector surface is not particularly limited, but since the current collector thickness 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 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 such a heat-resistant copper alloy, for example, those listed in Table 1 can be used.

上述のように、本発明において用いる集電体は、その表面に大きな凹凸を有することが好ましい。このため、耐熱性銅合金箔表面の算術平均粗さＲａが十分に大きくない場合には、その箔表面に電解銅または電解銅合金を設けることにより、その表面に大きな凹凸を設けてもよい。電解銅層及び電解銅合金層は、電解法により形成することができる。

As described above, the 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 applied to form large irregularities on the surface of the 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 mixture layer preferably has a relationship of 5Y ≧ X and 250Ra ≧ X with the thickness Y of the 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.

  Although the thickness X of a mixture layer is not specifically limited, 1000 micrometers or less are preferable, More preferably, they are 10 micrometers-100 micrometers.

  In the present invention, conductive powder can be mixed in the 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 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 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 binder in the present invention, polyimide is preferably used. Examples of the polyimide include thermoplastic polyimide and thermosetting polyimide. 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 this invention, it is preferable that the quantity of the binder in a mixture layer is 5 weight% or more of the total weight of a 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.

  A lithium secondary battery according to another aspect of the present invention is a current collector made of a conductive metal foil and including a mixture layer containing active material particles and a binder that are made porous from the surface toward the inside by charging and discharging. A lithium secondary battery including a negative electrode disposed on a surface, a positive electrode, and a nonaqueous electrolyte, characterized in that carbon dioxide is dissolved in the nonaqueous electrolyte.

  Examples of the active material particles that become porous from the surface toward the inside by charging / discharging include silicon particles and silicon alloy particles. By dissolving carbon dioxide in the non-aqueous electrolyte, the porous active material particles due to charge / discharge can be suppressed, and an increase in the thickness of the electrode due to charge / discharge can be suppressed. For this reason, the volume energy density of a battery can be raised.

  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. 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 and propylene carbonate are 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 or propylene 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, LiPF ₆ 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.

Examples of the positive electrode material of the lithium secondary battery of the present invention include 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 and other lithium-containing transition metal oxides. Examples thereof include metal oxides that do not contain lithium, such as MnO ₂ . In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  The production method of the present invention is a method by which the lithium secondary battery of the present invention can be produced, and a mixture layer containing active material particles containing silicon and / or silicon alloy and a binder is formed from a conductive metal foil. A lithium secondary battery using a negative electrode, a positive electrode, and a non-aqueous electrolyte. And an assembling process.

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

  Examples of the method for dissolving carbon dioxide in the non-aqueous electrolyte include a method for dissolving carbon dioxide by bringing carbon dioxide into contact with the non-aqueous electrolyte. As such a method, a method of blowing gaseous carbon dioxide into the non-aqueous electrolyte can be mentioned. By this method, a non-aqueous electrolyte in which carbon dioxide is dissolved can be obtained efficiently and easily. Other methods include a method of stirring the non-aqueous electrolyte in carbon dioxide and a method of bringing high-pressure carbon dioxide into contact with the non-aqueous electrolyte. Further, carbon dioxide may be dissolved in the non-aqueous electrolyte by adding a substance that generates carbon dioxide to the non-aqueous electrolyte. Examples of the substance that generates carbon dioxide include bicarbonate and carbonate. Also, dry ice or the like may be used.

  Moreover, when manufacturing a lithium secondary battery using the non-aqueous electrolyte in which carbon dioxide is dissolved, it is preferable that the amount of carbon dioxide dissolved in the non-aqueous electrolyte is stably controlled. For this reason, it is preferable to assemble the lithium secondary battery in an atmosphere containing carbon dioxide. For example, it is preferable to perform the step of injecting a nonaqueous electrolyte in which carbon dioxide is dissolved into the battery and the subsequent steps in an atmosphere containing carbon dioxide. Moreover, it is preferable to stabilize the amount of carbon dioxide dissolved by injecting a non-aqueous electrolyte in which carbon dioxide is dissolved into the battery and then exposing it to a high-pressure carbon dioxide atmosphere. Since the saturated dissolution amount of carbon dioxide varies depending on the temperature of the nonaqueous electrolyte, it is preferable that the temperature of the lithium secondary battery is controlled so as not to vary as much as possible in the manufacturing process.

  Moreover, you may dissolve a carbon dioxide in a nonaqueous electrolyte by performing manufacture of the lithium secondary battery of this invention in the atmosphere containing a carbon dioxide. For example, the carbon dioxide may be dissolved in the nonaqueous electrolyte by leaving the battery before sealing in an atmosphere containing carbon dioxide and performing sealing after a predetermined time.

  In the present invention, the mixture layer may be disposed on the surface of the metal foil current collector by applying a slurry in which active material particles are dispersed in a binder solution onto the surface of the metal foil current collector. it can.

  Moreover, in the manufacturing method of this invention, after forming a mixture layer on the surface of a metal foil collector, it is preferable to roll a mixture layer with a metal foil collector before sintering. By such rolling, the packing density in the 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.

  The sintering in the present invention is preferably performed in a non-oxidizing atmosphere, and 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.

  According to the present invention, lithium that has a large charge / discharge capacity, excellent cycle characteristics, can suppress the formation of porous active material particles due to charge / discharge, and can reduce an increase in the thickness of the electrode after charge / discharge. It can be set as a secondary battery.

The figure which shows the FIB-SIM image of the cross section of the negative electrode of lithium secondary battery A1 according to this invention. The figure which shows the FIB-SIM image of the cross section of the negative electrode of lithium secondary battery A1 according to this invention. The figure which shows the FIB-SIM image of the cross section of the negative electrode of comparative battery B1. The figure which shows the FIB-SIM image of the cross section of the negative electrode of comparative battery B1. The figure which shows the surface-analysis result (positive ion) by TOF-SIMS of a negative electrode. The figure which shows the surface analysis result (negative ion) by TOF-SIMS of a negative electrode. The top view which shows the lithium secondary battery produced in the Example according to this invention. Sectional drawing which shows the cross section of the lithium secondary battery shown in FIG. The figure which shows the relationship between the dissolution amount of the carbon dioxide in electrolyte solution, and cycle life.

  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)
81.8 parts by weight of silicon powder (purity 99.9%) having an average particle diameter of 3 μm as active material particles was mixed with an 8.6% by weight N-methylpyrrolidone solution containing 18.2 parts by weight of polyimide as a binder. Thus, a negative electrode mixture slurry was obtained.

  This negative electrode mixture slurry was applied to one side (rough surface) of an electrolytic copper foil (thickness 35 μm) (current collector a1) having a surface arithmetic average roughness Ra of 0.5 μm and dried. The obtained product was cut out into a 25 mm × 30 mm rectangular shape and rolled, and then heat-treated in an argon atmosphere at 400 ° C. for 30 hours and sintered to obtain a negative electrode. The thickness of the sintered body (including the current collector) was 50 μm. Therefore, the thickness of the mixture layer was 15 μm, the thickness of the mixture layer / the arithmetic average roughness of the copper foil surface was 30, and the thickness of the mixture layer / the thickness of the copper foil was 0.43.

Moreover, in this negative electrode, the density of the polyimide was 1.1 g / cm ³ , and the volume occupied by the 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.

90 parts by weight of the obtained LiCoO ₂ powder and 5 parts by weight of artificial graphite powder as a conductive agent were mixed in a 5% by weight N-methylpyrrolidone (NMP) solution containing 5 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 an aluminum foil as a current collector, dried and then rolled. What was obtained was cut into a 20 mm × 20 mm square to form 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 to prepare an electrolytic solution x. The electrolytic solution x was cooled to 5 ° C., and carbon dioxide gas was blown into the electrolytic solution x at a flow rate of 300 ml / min for about 30 minutes until the weight of the electrolytic solution did not change. This was heated up to 25 degreeC and was set as the electrolyte solution a1.

  The weight of the electrolytic solution before blowing carbon dioxide gas and the weight of the electrolytic solution after blowing carbon dioxide gas were measured, and the amount of carbon dioxide dissolved in the electrolytic solution a1 was measured and found to be 0.37% by weight. In addition, the weight of the electrolyte solution after blowing carbon dioxide was measured under a carbon dioxide gas atmosphere.

[Production of battery]
The above positive electrode, negative electrode, and electrolytic solution were inserted into an aluminum laminate outer package to produce a lithium secondary battery A1. Note that the lithium secondary battery was produced in a carbon dioxide gas atmosphere at normal temperature and normal pressure.

  FIG. 7 is a front view showing the manufactured lithium secondary battery. 8 is a cross-sectional view taken along line AA in FIG. The positive electrode and the negative electrode are disposed so as to face each other with a separator made of a polyethylene porous body, and are inserted as an electrode body 5 into an exterior body 1 made of an aluminum laminate as shown in FIG. A positive electrode current collecting tab 3 made of aluminum is attached to the positive electrode, and a negative electrode current collecting tab 4 made of nickel is attached to the negative electrode, and these are drawn out from the exterior body 1 to the outside. As shown in FIG.7 and FIG.8, the closing part 2 is formed in the periphery of the exterior body 1 by heat sealing.

(Experiment 2)
Electrolytic solution b1 was prepared by adding 5 wt% vinylene carbonate to electrolytic solution x in Experiment 1 without blowing carbon dioxide gas. Using this electrolytic solution b1, a battery B1 was manufactured in the same manner as in Experiment 1 except that the battery was manufactured under an argon atmosphere.

[Evaluation of charge / discharge cycle characteristics]
Charge / discharge cycle characteristics of the batteries A1 and B1 were evaluated. Each battery was charged at a constant current of up to 4.2 V at a current value of 14 mA at 25 ° C., then charged at a voltage of 4.2 V to a current value of 0.7 mA, and then discharged to 2.75 V at a current value of 14 mA. This was defined as one cycle of charge / discharge. The number of cycles to reach 80% of the discharge capacity at the first cycle was measured and defined as the cycle life. The results are shown in Table 2. The cycle life of each battery is an index with the cycle life of the battery A1 as 100.

表２から明らかなように、二酸化炭素を溶解させた電解液ａ１を用いた電池Ａ１は、二酸化炭素を溶解させていない電解液ｂ１を用いた電池Ｂ１に比べ、サイクル寿命が長くなっていることがわかる。なお、電解液ｂ１において、ビニレンカーボネートを添加しているのは、二酸化炭素ガスを吹き込まず、かつビニレンカーボネートを添加していない電解液を用いると、サイクル寿命が非常に短くなるため、活物質の多孔質化が観察しにくくなることを考慮したものである。

As is clear from Table 2, the battery life A1 using the electrolytic solution a1 in which carbon dioxide is dissolved has a longer cycle life than the battery B1 using the electrolytic solution b1 in which carbon dioxide is not dissolved. I understand. In addition, in the electrolytic solution b1, vinylene carbonate is added because carbon dioxide gas is not blown, and if an electrolytic solution to which vinylene carbonate is not added is used, the cycle life becomes very short. This is because it becomes difficult to observe the porous structure.

[FIB-SIM observation]
After the above charge / discharge cycle test, the negative electrodes were respectively taken out from the batteries A1 and B1, and the cross section of each negative electrode was observed by FIB-SIM. FIB-SIM observation means processing with a focused ion beam (FIB) so that the cross section is exposed, and observing the cross section with a scanning ion microscope (SIM).

  1 and 2 are SIM images of the negative electrode of the battery A1. FIG. 2 is an enlarged view of FIG. 3 and 4 are SIM images of the negative electrode of the battery B1. FIG. 4 is an enlarged view of FIG. Since the cross section is observed from an angle of 45 degrees, the actual length in each figure is the scale of each figure (FIGS. 1 and 3 are 10 μm, and FIGS. 2 and 4 are 1 μm). It becomes √2 times the length measured in. Therefore, it can be seen that the thickness of the mixture layer is about 25 μm in the negative electrode of the battery A1 (FIG. 1), and the thickness of the mixture layer is about 42 μm in the negative electrode of the battery B1 (FIG. 3).

  In FIG. 1, black portions in the mixture layer are portions where the active material particles are not made porous, and white portions are portions where the porous portion is made porous. Therefore, it can be seen that in the battery A1, only the surface portion of the active material particles is made porous.

  On the other hand, as shown in FIG. 3, in the negative electrode of battery B1, the black part has decreased and the white part has increased. Therefore, it can be seen that many portions are made porous in the negative electrode of the battery B1.

  Further, as described above, the thickness of the mixture layer of the battery B1 is thicker than the thickness of the mixture layer of the battery A1, and in the negative electrode of the battery B1, the active material particles become porous, It can be seen that the thickness of the mixture layer is increased.

  From the above, it can be seen that by using an electrolytic solution in which carbon dioxide is dissolved in accordance with the present invention, it is possible to suppress the porosity of the active material particles and to suppress an increase in the thickness of the electrode. Therefore, according to the present invention, an increase in the thickness of the battery after the charge / discharge cycle can be suppressed, and a battery having a high volumetric energy density can be obtained.

  Although it is not clear about the detailed reason why the porous material of the active material particles can be suppressed by using a non-aqueous electrolyte in which carbon dioxide is dissolved according to the present invention, the active material is obtained by using a non-aqueous electrolyte in which carbon dioxide is dissolved. A coating having excellent lithium ion conductivity is formed on the surface of the particles, which suppresses irreversible alteration of the active material particles associated with the charge / discharge reaction, thereby suppressing the formation of a porous material.

(Experiment 3)
Here, the influence of the average particle size of the silicon powder on the cycle characteristics was examined.

  In Experiment 1, a battery A2 was produced in the same manner as in Experiment 1, except that silicon powder having an average particle diameter of 20 μm was used. Further, in Experiment 2, a battery B2 was produced in the same manner as in Experiment 2 except that silicon powder having an average particle diameter of 20 μm was used.

  For these batteries, the cycle characteristics were evaluated in the same manner as in Experiment 2. 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 batteries A1 and B1.

表３から明らかなように、平均粒子径１０μｍ以下であるケイ素粉末を活物質として用いた電池Ａ１は、電池Ａ２に比べ、優れたサイクル特性を示していることがわかる。従って、平均粒子径１０μｍ以下の活物質粉末を用いた場合に、二酸化炭素を溶解した非水電解質を用いることによる充放電特性向上の効果が、より顕著に得られることがわかる。

As is clear from Table 3, the battery A1 using silicon powder having an average particle diameter of 10 μm or less as an active material shows excellent cycle characteristics as compared with the battery A2. Therefore, it can be seen that, when an active material powder having an average particle diameter of 10 μm or less is used, the effect of improving the charge / discharge characteristics by using a non-aqueous electrolyte in which carbon dioxide is dissolved is obtained more remarkably.

(Experiment 4)
Here, the influence of the arithmetic average roughness Ra of the current collector surface on the cycle characteristics was examined.

  In Experiment 1, instead of the current collector a1, an electrolytic copper foil having a different arithmetic average roughness Ra was used as the current collector. Specifically, an electrolytic copper foil having an arithmetic average roughness Ra of 0.2 μm and an electrolytic copper foil having an arithmetic average roughness Ra of 0.17 μm were used. Using these current collectors, batteries A3 and A4 were produced in the same manner as in Experiment 1.

  For each of the above 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 4 also shows the cycle life of the battery A1.

表４から明らかなように、算術平均粗さＲａが０．２μｍ以上である集電体を用いた電池Ａ１及びＡ３は、算術平均粗さＲａが０．２μｍ未満である集電体を用いた電池Ａ４に比べ、優れたサイクル特性を示している。これは、算術平均粗さＲａが０．２μｍ以上の集電体を用いることにより、活物質粒子と集電体表面との接触面積が大きくなり、焼結が効果的に起こり、活物質粒子と集電体との密着性が向上したことによるためと考えられる。また、集電体表面に対するバインダーによるアンカー効果がより大きく得られるため、合剤層と集電体間の密着性がさらに向上し、電極内の集電性が向上したためであると考えられる。

As is clear from Table 4, the batteries A1 and A3 using the current collector having an arithmetic average roughness Ra of 0.2 μm or more used the current collector having an arithmetic average roughness Ra of less than 0.2 μm. Compared to the battery A4, excellent cycle characteristics are shown. This is because, by using a current collector having an arithmetic average roughness Ra of 0.2 μm or more, the contact area between the active material particles and the current collector surface is increased, sintering occurs effectively, and the active material particles This is thought to be due to the improved adhesion to the current collector. Moreover, since the anchor effect by the binder with respect to the electrical power collector surface is acquired more, it is thought that it is because the adhesiveness between a mixture layer and an electrical power collector further improved, and the electrical power collection property in an electrode improved.

(Experiment 5)
Here, the influence of electrode sintering conditions on cycle characteristics was examined.

  In Experiment 1, a battery A5 was produced in the same manner as in Experiment 1, except that the electrode was heat-treated at 600 ° C. for 10 hours.

  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 5 also shows the cycle life of the battery A1.

表５から明らかなように、６００℃１００時間で熱処理を行った電池Ａ５は、４００℃３０時間で熱処理を行った電池Ａ１に比べ、サイクル特性が大きく低下していることがわかる。これは、６００℃の熱処理では、バインダーが分解されるためバインダーによる電極内の密着性が大きく低下し、集電性が低下したためであると考えられる。

As is clear from Table 5, the battery A5 that was heat-treated at 600 ° C. for 100 hours has a greatly reduced cycle characteristic as compared with the battery A1 that was heat-treated at 400 ° C. for 30 hours. This is probably because the heat treatment at 600 ° C. decomposes the binder, so that the adhesion within the electrode due to the binder is greatly reduced, and the current collecting property is reduced.

(Experiment 6)
Here, the effect of the conductive powder added to the mixture layer on the cycle characteristics was examined.

  A battery A6 was produced in the same manner as in Experiment 1, except that nickel powder having an average particle diameter of 3 μm was added to the mixture layer so as to be 20% by weight with respect to the total of the silicon powder.

  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 6 also shows the cycle life of the battery A1.

表６から明らかなように、合剤層にニッケル粉末を添加した電池Ａ６の方が、合剤層に導電性粉末を添加していない電池Ａ１に比べ、サイクル特性が向上していることがわかる。これは、導電性粉末が、活物質粒子の周りで導電性ネットワークを形成することにより、合剤層内の集電性が向上したためと考えられる。

As can be seen from Table 6, the battery A6 in which nickel powder is added to the mixture layer has improved cycle characteristics compared to the battery A1 in which no conductive powder is added to the mixture layer. . This is considered because the current collection property in the mixture layer was improved by the conductive powder forming a conductive network around the active material particles.

  In the above embodiment, the mixture layer is provided only on one side of the negative electrode current collector, but the mixture layer may be provided on both sides of the current collector. In this case, it is preferable that convex portions according to the present invention are formed on both surfaces of the current collector.

[TOF-SIMS observation]
In the negative electrode obtained by depositing an amorphous silicon thin film on a current collector made of a conductive metal foil by a sputtering method, the applicant of the present invention can make the active material porous due to the charge / discharge cycle, and It has been found that such porosification is suppressed by using a non-aqueous electrolyte in which carbon dioxide is dissolved. Using such a silicon thin film electrode as a negative electrode, a battery X1, a battery Y1, and a battery Y2 were produced. The battery X1 uses a non-aqueous electrolyte in which carbon dioxide is dissolved, the battery Y1 uses a non-aqueous electrolyte in which carbon dioxide is not dissolved, and the battery Y2 does not dissolve carbon dioxide, and vinylene carbonate. A non-aqueous electrolyte added with 20% by weight of (VC) is used. The batteries X1, Y1, and Y2 after the initial charge were subjected to surface analysis of the negative electrode by TOF-SIMS (time-of-flight secondary ion mass spectrometry).

  FIG. 5 is a TOF-SIMS spectrum of positive ions, and FIG. 6 is a TOF-SIMS spectrum of negative ions. 5 and 6, “LiPF6 + CO2” indicates the spectrum of the battery X1, “LiPF6” indicates the spectrum of the battery Y1, and “LiPF6 + VC20 wt%” indicates the spectrum of the battery Y2.

As is apparent from FIGS. 5 and 6, the surface of the negative electrode in the battery X1 has a significant decrease in Si ions and ions containing Si and an increase in Li ₂ F ⁺ ions compared to those of the batteries Y1 and Y2. You can see that From this, it can be seen that the concentration of Si on the surface of silicon is greatly reduced by using a non-aqueous electrolyte in which carbon dioxide is dissolved. This is presumably due to the formation of a film containing no Si on the surface of silicon as the active material. This coating is a stable coating with excellent lithium ion conductivity. When such a coating is formed on the silicon surface, the alteration of silicon occurs during the charge / discharge process in which lithium ions are absorbed and released by silicon. It is considered that the silicon particles can be prevented from becoming porous and the silicon particles can be prevented from becoming porous.

  In addition, it is considered that a film containing Si as an active material is formed on the negative electrodes of the battery Y1 and the battery Y2. Moreover, it is estimated that the production | generation of such a film is causing the porosity in the active material surface. When a non-aqueous electrolyte in which carbon dioxide is dissolved is used, such a film is not formed, so that it is considered that the porous formation of the active material is suppressed.

  Also in the present invention, in the same manner as described above, a stable coating having excellent lithium ion conductivity is formed on the surface of the active material particles, thereby charging and discharging the lithium ions into and out of the active material particles. It is considered that the active material particles can be prevented from being altered and the active material particles from being made porous.

(Experiment 7)
Here, the effect of the amount of carbon dioxide dissolved in the electrolyte on the cycle characteristics was examined.

[Preparation of positive electrode and negative electrode]
A positive electrode and a negative electrode were produced in the same manner as in Experiment 1.

(Preparation of electrolyte)
An electrolyte solution x was prepared in the same manner as in Experiment 1, and carbon dioxide gas was blown into the electrolyte solution x in the same manner as in Experiment 1 to prepare an electrolyte solution a1.

  In an argon gas atmosphere, the electrolytic solution x and the electrolytic solution a1 were mixed at a volume ratio shown in Table 7 to prepare electrolytic solutions a2, a3, and a4.

〔電池の作製〕
実験１と同様にして、電解液ａ１を用いたリチウム二次電池Ａ１を作製した。

[Production of battery]
In the same manner as in Experiment 1, a lithium secondary battery A1 using the electrolytic solution a1 was produced.

  The battery A7 using the electrolytic solution a2, the battery A8 using the electrolytic solution a3, the battery A9 using the electrolytic solution a4, and the battery B3 using the electrolytic solution x are manufactured under an atmospheric pressure argon gas atmosphere. Was prepared in the same manner as in Experiment 1.

[Evaluation of charge / discharge cycle characteristics]
About each said battery, the cycle characteristic was evaluated similarly to the above. The evaluation results are shown in Table 8.

  The cycle life is an index with the cycle life of the battery A1 as 100. Table 8 also shows the cycle life of the battery A1.

  FIG. 9 shows the relationship between the amount of carbon dioxide dissolved in the electrolyte using each battery and the cycle life.

表８及び図９から明らかなように、二酸化炭素が０．０１重量％以上溶解している電解液を用いた電池Ａ１及びＡ７〜Ａ９は、二酸化炭素が溶解していない電解液ｘを用いた電池Ｂ３に比べ、サイクル寿命が長くなっていることがわかる。また、電解液中の二酸化炭素の溶解量が０．０５重量％以上になると、サイクル寿命がその飽和値のほぼ８０％を超えることがわかる。さらに、電解液中の二酸化炭素の溶解量が０．１重量％になると、サイクル寿命がその飽和値にほぼ近づくことがわかる。

As apparent from Table 8 and FIG. 9, the batteries A1 and A7 to A9 using the electrolytic solution in which carbon dioxide was dissolved by 0.01% by weight or more used the electrolytic solution x in which carbon dioxide was not dissolved. It can be seen that the cycle life is longer than that of the battery B3. It can also be seen that when the amount of carbon dioxide dissolved in the electrolytic solution is 0.05% by weight or more, the cycle life exceeds approximately 80% of its saturation value. Furthermore, it can be seen that when the amount of carbon dioxide dissolved in the electrolyte solution is 0.1% by weight, the cycle life almost approaches its saturation value.

  From the above, the amount of carbon dioxide dissolved in the electrolytic solution is preferably 0.01% by weight or more, more preferably 0.05% by weight or more, and 0.1% by weight or more. Is more preferable.

(Experiment 8)
Here, the effect of the inclusion of fluorine in the nonaqueous electrolyte on the cycle characteristics was examined.

(Preparation of electrolyte)
A solution in which 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 was prepared. This is designated as electrolyte P0. The electrolytic solution P0 was cooled to 5 ° C., and carbon dioxide gas was blown into the electrolytic solution at a rate of 300 ml / min in a carbon dioxide atmosphere. Blowing was performed until the weight of the electrolyte did not change (about 30 minutes). This was heated up to 25 degreeC and was set as the electrolyte solution P1. The weight of the electrolytic solution after the carbon dioxide gas was blown was measured in a carbon dioxide gas atmosphere, and the amount of carbon dioxide gas dissolved in the electrolytic solution was measured by examining the change in the weight of the electrolytic solution. %Met.

An electrolyte solution was prepared in the same manner as the electrolyte solution P0 except that LiBF ₄ was used instead of the lithium salt LiPF ₆ of the electrolyte solution P0, and was designated as electrolyte solution B0. Carbon dioxide gas was blown into the electrolytic solution B0 in the same manner as the electrolytic solution P1 to obtain an electrolytic solution B1.

An electrolyte solution was prepared in the same manner as the electrolyte solution P0 except that LiN (C ₂ F ₅ SO ₂ ) ₂ was used instead of the lithium salt LiPF ₆ of the electrolyte solution P0, and this was designated as the electrolyte solution N0. Carbon dioxide gas was blown into the electrolytic solution N0 in the same manner as the electrolytic solution P1 to obtain an electrolytic solution N1.

An electrolyte solution was prepared in the same manner as the electrolyte solution P0 except that LiClO ₄ was used instead of the lithium salt LiPF ₆ of the electrolyte solution P0, and was designated as electrolyte solution C0. Carbon dioxide gas was blown into the electrolytic solution C0 in the same manner as the electrolytic solution P1 to obtain an electrolytic solution C1.

[Production of battery]
Using the same positive electrode, negative electrode, and electrolytic solutions P1, B1, N1, and C1 as in Experiment 1, lithium secondary batteries AP1, AB1, AN1, and AC1 were produced in a carbon dioxide gas atmosphere at normal temperature and normal pressure. .

  Lithium secondary batteries AP0, AB0, AN0, and AC0 were prepared using the same positive electrode, negative electrode, and electrolytic solutions P0, B0, N0, and C0 as in Experiment 1 in an argon gas atmosphere at normal temperature and normal pressure.

[Evaluation of charge / discharge cycle characteristics]
About each said battery, the charge / discharge cycle characteristic was evaluated. The batteries AP1, AB1, AC1, AP0, AB0, and AC0 were charged at constant current to 4.2V at a current value of 14 mA at 25 ° C., and then charged at a voltage of 4.2 V to a current value of 0.7 mA. Then, it discharged to 2.75V with the electric current value of 14 mA, and this was made into 1 cycle charging / discharging.

  The batteries AN1 and AN0 were charged / discharged in the same manner as the above charge / discharge except that the constant current charge was performed up to 4.0V.

  The number of cycles to reach 80% of the discharge capacity at the first cycle was measured and defined as the cycle life. The results are shown in Table 9.

  In Table 9, the cycle life A is an index with the cycle life of the battery AP1 as 100. The cycle life B is an index with the cycle life of a battery in which carbon dioxide is contained in the electrolyte in each battery as 100.

表９から明らかなように、二酸化炭素が溶解している電解液を用いた電池ＡＰ１、ＡＢ１、ＡＮ１、及びＡＣ１は、二酸化炭素が溶解していない電解液を用いた電池ＡＰ０、ＡＢ０、ＡＮ０、及びＡＣ０に比べ、サイクル寿命が長いことがわかる。特に、フッ素含有リチウム塩を用いた電池ＡＰ１、ＡＢ１、及びＡＮ１は、フッ素を含有していないリチウム塩を用いた電池ＡＣ１に比べ、サイクル寿命向上の割合が大きくなっていることがわかる。このことから、フッ素含有リチウム塩を含むことにより、二酸化炭素による良質な被膜の形成を促進することができるか、あるいは二酸化炭素による被膜をさらに良質な被膜にすることができるものと思われる。これは、充放電に伴うフッ素含有リチウム塩の分解により、フッ化水素などが生成され、これが二酸化炭素に影響を及ぼして良質な被膜が形成されるものと考えられる。このような被膜により、充放電反応に伴う活物質の割合により生じる新生面での被膜の形成に消費されるリチウムイオンの量が減るため、充放電効率の低下が抑制されると考えられる。また、活物質粒子表面で形成される被膜は、リチウムイオン伝導性に優れた被膜であるため、活物質内の充放電反応分布の均一性が向上されると考えられる。このため、活物質内でのリチウムの吸蔵・放出に伴う体積変化の偏りにより発生する歪みが抑制され、充放電効率が向上すると考えられる。

As is apparent from Table 9, the batteries AP1, AB1, AN1, and AC1 using the electrolyte solution in which carbon dioxide is dissolved are the batteries AP0, AB0, AN0, in which the electrolyte solution in which carbon dioxide is not dissolved. It can be seen that the cycle life is longer than that of AC0. In particular, it can be seen that the batteries AP1, AB1, and AN1 using a fluorine-containing lithium salt have a higher cycle life ratio than the battery AC1 using a lithium salt that does not contain fluorine. From this, it is considered that the formation of a high-quality coating film by carbon dioxide can be promoted by including the fluorine-containing lithium salt, or the coating film by carbon dioxide can be made a higher-quality coating film. This is probably because hydrogen fluoride and the like are generated by the decomposition of the fluorine-containing lithium salt accompanying charge / discharge, which affects carbon dioxide and forms a high-quality film. Such a coating is considered to reduce the amount of lithium ions consumed for forming the coating on the new surface caused by the ratio of the active material accompanying the charge / discharge reaction, thereby suppressing the decrease in charge / discharge efficiency. Moreover, since the film formed on the surface of the active material particles is a film excellent in lithium ion conductivity, it is considered that the uniformity of the charge / discharge reaction distribution in the active material is improved. For this reason, it is thought that the distortion which generate | occur | produces by the bias | inclination of the volume change accompanying insertion / release of lithium in an active material is suppressed, and charge / discharge efficiency improves.

  In each of the above examples, the negative electrode current collector having an uneven surface is used only on one surface, and the active material layer is disposed on the uneven surface. However, the present invention is not limited to this. Instead, a negative electrode in which a current collector having irregularities on both sides is used and an active material layer is disposed on both sides may be used.

(Experiment 9)
(Preparation of electrolyte)
A solution in which LiPF ₆ was dissolved at 1 mol / liter in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 was prepared. This electrolytic solution is referred to as ED0.

  The electrolyte solution ED0 was cooled to 5 ° C., and carbon dioxide gas was blown into the electrolyte solution at a flow rate of 300 ml / min until the weight of the electrolyte solution did not change (30 minutes). This was heated up to 25 degreeC and was set as electrolyte solution ED1. Calculate the amount of carbon dioxide dissolved in the electrolyte by measuring the weight of the electrolyte after carbon dioxide gas injection in a carbon dioxide gas atmosphere and examining the change in the weight of the electrolyte before and after carbon dioxide gas injection. As a result, it was 0.37% by weight.

  Propylene carbonate (PC) was used as the cyclic carbonate in the electrolytic solution, diethyl carbonate (DEC) was used as the chain carbonate, and carbon dioxide was dissolved in the same manner as the electrolytic solution ED1 to prepare electrolytic solution PD1.

  Propylene carbonate (PC) was used as the cyclic carbonate in the electrolytic solution, methyl ethyl carbonate (MEC) was used as the chain carbonate, and carbon dioxide was dissolved in the same manner as in the electrolytic solution ED1 to prepare an electrolytic solution PM1. .

  Ethylene carbonate (EC) was used as the cyclic carbonate in the electrolytic solution, methyl ethyl carbonate (MEC) was used as the chain carbonate, and carbon dioxide was dissolved in the same manner as in the electrolytic solution ED1 to prepare an electrolytic solution EM1. .

LiPF ₆ was used in the same manner as in the electrolyte ED1 in a solvent in which ethylene carbonate (EC) was used as the cyclic carbonate in the electrolyte, dimethyl carbonate (DMC) was used as the chain carbonate, and these were mixed at a volume ratio of 1: 1. And the carbon dioxide was dissolved and electrolyte solution EDM1 was produced.

  The gas amounts of carbon dioxide dissolved in the electrolytic solutions PD1, PM1, EM1, and EDM1 were 0.36 wt%, 0.64 wt%, 0.54 wt%, and 0.46 wt%, respectively.

[Production of battery]
Using the same positive electrode and negative electrode as in Experiment 1 and using the above-mentioned ED0, ED1, PD1, PM1, EM1, and EDM1 as the electrolyte, a lithium secondary battery was produced in the same manner as in Experiment 1. When manufacturing a battery using the electrolytic solutions ED1, PD1, PM1, EM1 and EDM1 in which carbon dioxide is dissolved, the positive electrode, the negative electrode and the electrolytic solution are coated with an aluminum laminate in a carbon dioxide gas atmosphere at normal temperature and normal pressure. Made by inserting into the body.

  Further, in the case of using the electrolytic solution ED0 in which carbon dioxide was not dissolved, the positive electrode, the negative electrode, and the electrolytic solution were inserted into the aluminum laminate outer package under an argon gas atmosphere at normal temperature and normal pressure.

  AED0 is a battery using the electrolytic solution ED0, AED1 is a battery using the electrolytic solution ED1, APD1 is a battery using the electrolytic solution PD1, APM1 is a battery using the electrolytic solution PM1, and AEM1 is a battery using the electrolytic solution EM1. A battery using the electrolytic solution EDM1 was designated as AEDM1.

  The following table summarizes the presence and absence of the electrolytic solution and carbon dioxide dissolution of each battery produced as described above.

〔充放電サイクル特性の評価〕
実験１と同様にして、上記の各電池について、充放電サイクル特性を評価した。なお、各電池のサイクル寿命は、電池ＡＥＤ１のサイクル寿命を１００とした指数である。結果を表１１に示す。

[Evaluation of charge / discharge cycle characteristics]
In the same manner as in Experiment 1, the charge / discharge cycle characteristics of each battery were evaluated. The cycle life of each battery is an index with the cycle life of the battery AED1 as 100. The results are shown in Table 11.

表１１に示す結果から明らかなように、環状カーボネートとしてプロピレンカーボネートを用い、鎖状カーボネートとしてジエチルカーボネートを用いた電池ＡＰＤ１が、環状カーボネートとしてエチレンカーボネートを用い、鎖状カーボネートとしてジエチルカーボネートを用いた電池ＡＥＤ１よりも優れたサイクル特性を示している。プロピレンカーボネートは、黒鉛負極の場合には一般に用いられないが、上記のようにケイ素負極を用いる場合には良好な結果を示すことがわかった。これは、プロピレンカーボネートを用いた場合、電解液の粘度が低くなり、電極内に含浸されやすくなるため、ケイ素表面に均質な被膜が形成され、サイクル初期の容量の低下を抑制することができるためであると考えられる。

As is clear from the results shown in Table 11, battery APD1 using propylene carbonate as the cyclic carbonate and diethyl carbonate as the chain carbonate is a battery using ethylene carbonate as the cyclic carbonate and diethyl carbonate as the chain carbonate. Cycle characteristics superior to AED1 are shown. Propylene carbonate is not generally used in the case of a graphite negative electrode, but it has been found that good results are obtained when a silicon negative electrode is used as described above. This is because when propylene carbonate is used, the viscosity of the electrolyte solution becomes low and the electrode is easily impregnated, so that a uniform film is formed on the silicon surface and the reduction in capacity at the beginning of the cycle can be suppressed. It is thought that.

  In other batteries APM1, AEM1, and AEDM1, the effect of improving the cycle life by dissolving carbon dioxide in the electrolytic solution is recognized.

(Reference experiment)
[Production of carbon anode]
In an aqueous solution in which carboxymethyl cellulose, a thickener, is dissolved in water, artificial graphite as a negative electrode active material and styrene-butadiene rubber as a binder, the weight ratio of the active material, the binder, and the thickener is The mixture was added in a ratio of 95: 3: 2, and then kneaded to prepare a negative electrode slurry. After apply | coating the produced slurry on the copper foil as a collector, it dried and then rolled using the rolling roller, and the negative electrode was produced by attaching a current collection tab.

[Production of positive electrode]
90 parts by weight of LiCoO ₂ powder and 5 parts by weight of artificial graphite powder as a conductive agent are mixed in a 5% by weight N-methylpyrrolidone aqueous solution containing 5 parts by weight of polytetrafluoroethylene as a binder, and a positive electrode mixture A slurry was obtained. This slurry was applied to an aluminum foil as a positive electrode current collector by a doctor blade method and then dried to form a positive electrode active material layer. A positive electrode tab was attached on the region of the aluminum foil where the positive electrode active material was not applied, and a positive electrode was produced.

[Production of non-aqueous electrolyte]
A solution was prepared by dissolving LiPF _{6 so as} to be 1 mol / liter in a solution in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7.

  2% by weight of vinylene carbonate was added to this solution to obtain a nonaqueous electrolyte c2.

  Carbon dioxide was blown into the nonaqueous electrolyte c2 at a temperature of 25 ° C. for 30 minutes and dissolved until the saturation amount of carbon dioxide was reached. This was designated as the nonaqueous electrolyte c1. The amount of carbon dioxide dissolved was 0.37% by weight.

  The nonaqueous electrolytes c1 and c2 are as follows.

Nonaqueous electrolyte c1: Nonaqueous electrolyte in which CO ₂ is dissolved Nonaqueous electrolyte c2: Nonaqueous electrolyte in which CO ₂ is not dissolved [Preparation of battery]
A lithium secondary battery was produced using the negative electrode, the positive electrode, and the nonaqueous electrolyte.

  An electrode group wound with a separator made of porous polyethylene sandwiched between the positive electrode and the negative electrode, and the non-aqueous electrolyte are inserted into an outer package made of aluminum laminate, and a positive electrode current collecting tab and a negative electrode current collecting tab The battery was completed by heat-sealing the periphery of the outer package so that the battery goes out.

  The specifications of the manufactured battery are as shown in Table 12.

非水電解質ｃ１を用いて作製した電池を電池Ｃ１とし、非水電解質ｃ２を用いて作製した電池を電池Ｃ２とした。なお、電池Ｃ１は、高純度の二酸化炭素ガスの雰囲気中で作製した。

The battery produced using the nonaqueous electrolyte c1 was designated as battery C1, and the battery produced using the nonaqueous electrolyte c2 was designated as battery C2. The battery C1 was manufactured in an atmosphere of high-purity carbon dioxide gas.

[Charge / discharge cycle test]
The lithium secondary batteries C1 and C2 produced as described above were subjected to a charge / discharge cycle test. The charging / discharging conditions are as follows: at 25 ° C., constant current charging to 4.2 V at a current value of 600 mA is performed, then constant voltage charging is performed at 4.2 V up to 30 mA, discharging to 2.75 V at a current value of 600 mA. Cycle charge / discharge. Table 13 shows capacity retention ratios obtained by dividing the discharge capacity at the 500th cycle by the discharge capacity at the first cycle. Table 13 shows the amount of increase in the thickness of the battery after 500 cycles and the amount of increase in the thickness of the active material per electrode layer determined from this amount.

表１３に示す結果から明らかなように、炭素材料を負極活物質とする場合には、非水電解質に二酸化炭素を溶解させることによるサイクル劣化の抑制及び電池厚み増加の抑制の効果は、ほとんど得られないことがわかる。

As is apparent from the results shown in Table 13, when the carbon material is used as the negative electrode active material, the effects of suppressing cycle deterioration and suppressing battery thickness increase by dissolving carbon dioxide in the nonaqueous electrolyte are almost obtained. I can't understand.

DESCRIPTION OF SYMBOLS 1 ... Exterior body 2 ... Closure part 3 ... Positive electrode current collection tab 4 ... Negative electrode current collection tab 5 ... Electrode body

Claims (18)

After disposing a mixture layer containing active material particles containing silicon and / or silicon alloy and a binder, which is made porous from the surface toward the inside by charging and discharging, on the surface of the current collector made of conductive metal foil , In a lithium secondary battery comprising a sintered negative electrode, a positive electrode, and a non-aqueous electrolyte,
A lithium secondary battery comprising 0.037% by weight or more of carbon dioxide dissolved in the non-aqueous electrolyte.   The lithium secondary battery according to claim 1, wherein the negative electrode is sintered in a non-oxidizing atmosphere. The lithium secondary battery according to claim 1 or 2, characterized in that it contains carbon dioxide inside the battery voids. The nonaqueous electrolyte, a lithium secondary battery according to any one of claims 1 to 3, characterized in that it comprises a cyclic carbonate. The lithium secondary battery according to any one of claims 1 to 4, characterized in that it contains the compound containing fluorine in the non-aqueous electrolyte. The lithium secondary battery according to claim 5 , wherein the fluorine-containing compound is a fluorine-containing lithium salt. The fluorine-containing lithium salt is LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, and y is 6 when X is P, As, or Sb, X but B, Bi, Al, Ga or y when in, is 4), or _{LiN (C m F 2m + 1} SO 2) (C n F 2n + 1 SO 2) ( wherein, m and n are each independently It is an integer of 1-4), The lithium secondary battery of Claim 6 characterized by the above-mentioned. Average particle diameter, the lithium secondary battery according to any one of claims 1 to 7, wherein the at 10μm or less of the active material particles. Arithmetic average roughness Ra of the surface of the current collector, the lithium secondary battery according to any one of claims 1-8, characterized in that at 0.2μm or more. The lithium secondary according to any one of claims 1 to 9 , wherein the 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. battery. The lithium secondary battery according to any one of claims 1 to 10 , wherein the binder remains even after heat treatment for sintering. The binder, the lithium secondary battery according to any one of claims 1 to 11, characterized in that a polyimide. The active material particles, lithium secondary battery according to any one of claims 1 to 12, characterized in that a silicon particles. The lithium secondary battery according to any one of claims 1 to 13, wherein the conductive powder to the mixture layer is mixed. A method for producing a lithium secondary battery according to any one of claims 1 to 14 ,
Disposing a mixture layer containing active material particles containing silicon and / or a silicon alloy and a binder on the surface of a current collector made of a conductive metal foil, and sintering to produce the negative electrode;
Dissolving carbon dioxide in the non-aqueous electrolyte;
And a step of assembling a lithium secondary battery using the negative electrode, the positive electrode, and the non-aqueous electrolyte. The method for producing a lithium secondary battery according to claim 15 , wherein the negative electrode is sintered in a non-oxidizing atmosphere. The method for producing a lithium secondary battery according to claim 15 or 16 , wherein the step of dissolving carbon dioxide in the non-aqueous electrolyte includes a step of blowing gaseous carbon dioxide into the non-aqueous electrolyte. The method for producing a lithium secondary battery according to any one of claims 15 to 17 , wherein the step of assembling the lithium secondary battery includes a step of assembling the lithium secondary battery in a carbon dioxide atmosphere. .

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