JP6494194B2 - Coated positive electrode active material for lithium secondary battery, method for producing the same, and lithium secondary battery using the same - Google Patents

Description

  The present invention relates to a coated positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same.

  Lithium secondary batteries have high energy (high capacity) and are widely used in mobile phones and portable personal computers. In recent years, interest in electric vehicles has increased from the viewpoint of preventing global warming accompanying the increase in carbon dioxide, and lithium secondary batteries have been studied as power sources. Although it is a lithium secondary battery having such characteristics, when the lithium secondary battery is stored and used at a high temperature, the electrolytic solution in the battery is decomposed and the battery performance is deteriorated. Accordingly, there is a demand for a lithium secondary battery that has a small deterioration in battery performance during storage and use at high temperatures.

For example, Patent Document 1 includes an active material carrier capable of inserting and extracting lithium ions and a low melting glass portion containing a low melting glass and supported on the surface of the active material carrier by heat fusion. An electrode active material characterized by is shown. Patent Document 2 describes a method for producing a positive electrode active material having a positive electrode active material and a coating layer formed to cover the surface of the positive electrode active material and made of Li ₃ BO ₃ . .

JP 2009-64732 A JP 2012-89406 A

In Patent Document 1, the low melting point glass portion is supported so as to cover a part of the surface of the active material carrier, and the low melting point glass does not cover the entire surface of the active material carrier. The portion of the active material carrier that is not coated with the low-melting glass cannot suppress the decomposition of the electrolytic solution at a high temperature, which may cause deterioration of battery performance. In Patent Document 2, it has been to form a coating layer made of Li ₃ BO ₃ to the surface of the positive electrode active material, Li ₃ BO ₃ to be coated on the positive electrode active material has a lower Li ion conductivity. Therefore, in the prior art, it has not been sufficient to suppress deterioration of battery performance during high-temperature storage and use of a high-capacity lithium secondary battery.

  In view of the above-described conventional situation, an object of the present invention is to provide a high-capacity lithium secondary battery in which deterioration of battery performance during high-temperature storage / use is small, a coated positive electrode active material used therefor, and a method for producing the same.

  As a result of extensive research, the present inventors have reduced direct contact between the electrolyte and the positive electrode active material to the details of the positive electrode active material to improve the heat resistance of the battery. In order not to impair the conductivity, the inventors have found that the above problems can be solved by coating the positive electrode active material with a solid electrolyte having high ion conductivity, and have completed the invention.

That is, the coated positive electrode active material for a lithium secondary battery according to the present invention is formed so as to cover the positive electrode active material and the surface of the positive electrode active material, La _0.51 Li _0.34 TiO _2.94 , _{Li 7 La 3 Zr 2 O 12} , 50Li 4 SiO 4 · 50Li 3 BO 3, Li x Al y Ge z (PO 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) and _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 < least one solid selected from the group consisting of z <2) And a coating layer made of an electrolyte.

  By this invention, decomposition | disassembly of the electrolyte solution at the time of high temperature preservation | save and use is suppressed, and deterioration of the battery performance at the time of high temperature preservation | save and use of a high capacity | capacitance lithium secondary battery is suppressed. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is a figure which represents typically the internal structure of the lithium secondary battery which concerns on one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications can be made. In the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

<Battery structure>
FIG. 1 is a diagram schematically showing the internal structure of a lithium secondary battery according to an embodiment of the present invention. A lithium secondary battery 1 according to an embodiment of the present invention shown in FIG. 1 includes a positive electrode 10, a separator 11, a negative electrode 12, a battery container 13, a positive electrode current collecting tab 14, a negative electrode current collecting tab 15, an inner lid 16, and release of internal pressure. The valve 17, the gasket 18, a positive temperature coefficient (PTC) resistance element 19, a battery lid 20, and an axis 21 are roughly configured. The battery lid 20 is an integrated part including the inner lid 16, the internal pressure release valve 17, the gasket 18, and the positive temperature coefficient resistance element 19. A positive electrode 10, a separator 11, and a negative electrode 12 are wound around the shaft center 21.

  In the electrode group in which the separator 11 is inserted between the positive electrode 10 and the negative electrode 12 and wound around the shaft center 21, the shaft center 21 can be any known shaft as long as it can carry the positive electrode 10, the separator 11, and the negative electrode 12. The heart can be used. In this embodiment, the electrode group is formed in a cylindrical shape. The shape of the battery container 13 is formed in a cylindrical shape in accordance with the shape of the electrode group.

  The material of the battery container 13 is selected from materials that are corrosion resistant to the electrolyte, such as aluminum, stainless steel, nickel-plated steel, and the like. When the battery container 13 is electrically connected to the positive electrode 10 or the negative electrode 12, in a portion in contact with the electrolytic solution, the material of the battery container 13 is not affected by corrosion or alloying with lithium ions. The material of the battery container 13 is selected.

  The electrode group is housed in the battery container 13, the negative electrode current collecting tab 15 is connected to the inner wall of the battery container 13, and the positive electrode current collecting tab 14 is connected to the bottom surface of the battery lid 20. The electrolytic solution is injected into the battery container 13 before the battery is sealed. As a method for injecting the electrolyte, there are a method of adding directly to the electrode group with the battery cover 20 opened, or a method of adding from an injection port installed in the battery cover 20.

  Thereafter, the battery lid 20 is brought into close contact with the battery container 13 to seal the entire battery. If there is an electrolyte inlet, seal it as well. The battery can be sealed using a known technique such as welding or caulking.

<Positive electrode>
The positive electrode 10 is roughly composed of a coated positive electrode active material, a conductive agent, a binder, and a current collector. The coated positive electrode active material includes a positive electrode active material and a coating layer made of a solid electrolyte formed so as to cover the surface of the positive electrode active material. Below, the manufacturing method of a covering positive electrode active material is shown.

  The method for producing a coated positive electrode active material of the present invention is a method for producing a coated positive electrode active material comprising a positive electrode active material and a coating layer comprising a solid electrolyte formed so as to coat the surface of the positive electrode active material. , A step of preparing a homogeneous organic solvent / water mixed solution containing all of the solid electrolyte raw material by dissolving all of the solid electrolyte raw material in a mixed solvent of an organic solvent and water having a hydroxyl group, the organic solvent / water mixed solution and the positive electrode It includes a step of preparing a sol by mixing an active material, a step of preparing a gel by removing a solvent from the sol, and a step of baking the gel.

  According to the present invention, a homogeneous organic solvent / water mixed solution containing all solid electrolyte raw materials is prepared by completely dissolving all solid electrolyte raw materials in a mixed solvent of an organic solvent having hydroxyl group and water. Can do. A homogeneous sol is prepared by mixing a homogeneous organic solvent / water mixed solution containing all the solid electrolyte raw material and the positive electrode active material, and the solid electrolyte raw material is placed on the positive electrode active material surface by removing the solvent from the sol. It is possible to prepare a gel in a state of being uniformly coated. Furthermore, by baking the gel, a coated positive electrode active material in which a coating layer made of a solid electrolyte is uniformly coated on the surface of the positive electrode active material can be prepared.

  Hereinafter, the manufacturing method of the covering positive electrode active material of this invention is demonstrated.

  In the step of preparing a homogeneous organic solvent / water mixed solution containing all solid electrolyte raw materials by dissolving all solid electrolyte raw materials in a mixed solvent of organic solvent and water having a hydroxyl group, all solid electrolyte raw materials have a hydroxyl group. Dissolves completely in a mixed solvent of organic solvent and water. In the method of dissolving all of the solid electrolyte raw material in a mixed solvent of an organic solvent having hydroxyl group and water, a solution A is prepared by dissolving in advance a water-insoluble one of the solid electrolyte raw material in an organic solvent having a hydroxyl group. Then, a water-soluble material is dissolved in water to prepare solution B, and solutions A and B are mixed to prepare a homogeneous organic solvent / water mixed solution containing all the solid electrolyte raw materials (method 1). ), A method of preparing a homogeneous organic solvent / water mixed solution containing all the solid electrolyte raw materials by mixing and dissolving all the solid electrolyte raw materials with a mixed solvent of an organic solvent having hydroxyl group and water (Method 2), etc. However, after mixing all of the solid electrolyte raw material and the organic solvent having hydroxyl group and water mixed solvent, the organic solvent / water mixed solution containing all of the solid electrolyte raw material does not gel but remains undissolved. There is no solid electrolysis To take place is a raw material for re-precipitation, it may be used any method.

In the case of preparing a homogeneous organic solvent / water mixed solution containing all of the solid electrolyte raw material in Method 1, among the solid electrolyte raw materials that are hardly soluble in water, there is a metal alkoxide compound constituting the solid electrolyte, It depends on the solid electrolyte used. For example, in the case of using _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) as a solid electrolyte, the inner water of the raw material of the solid electrolyte The metal alkoxide compounds constituting the hardly soluble solid electrolyte are Al (C ₃ H ₇ O) ₃ and Ti (C ₃ H ₇ O) ₄ . For example, when La _0.51 Li _0.34 TiO _2.94 is used as the solid electrolyte, the metal alkoxide compound constituting the solid electrolyte that is hardly soluble in water among the raw materials of the solid electrolyte is La (C ₃ H ₇ O ) ₃ and _{Ti (C 3 H 7 O)} 4. For example, when Li ₇ La ₃ Zr ₂ O ₁₂ is used as the solid electrolyte, the metal alkoxide compound constituting the solid electrolyte that is hardly soluble in water among the raw materials of the solid electrolyte is La (C ₃ H ₇ O) ₃ and Zr. a _{(C 3 H 7 O) 4} . For example, when 50Li ₄ SiO ₄ .50Li ₃ BO ₃ is used as the solid electrolyte, the metal alkoxide compound constituting the solid electrolyte that is hardly soluble in water among the raw materials of the solid electrolyte is Si (C ₃ H ₇ O) ₄ . is there. For example, in the case of using _{Li x Al y Ge z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) as a solid electrolyte, the inner water of the raw material of the solid electrolyte Al (C ₃ H ₇ O) ₃ and Ge (C ₃ H ₇ O) ₄ are the metal alkoxide compounds constituting the hardly soluble solid electrolyte. Among the solid electrolyte raw materials, those readily soluble in water include metal salts constituting the solid electrolyte and / or inorganic salts of metal counter anions in the solid electrolyte, which differ depending on the solid electrolyte used. For example _{Li x Al y Ti z (PO} 4) as a solid electrolyte 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) When using the out of the material of the solid electrolyte, water The metal salt that constitutes the solid electrolyte that is readily soluble in water and the inorganic salt of the counter anion of the metal in the solid electrolyte are CH ₃ COOLi and (NH ₄ ) ₂ HPO ₄ , respectively. For example, when La _0.51 Li _0.34 TiO _2.94 is used as the solid electrolyte, the metal salt constituting the solid electrolyte that is easily soluble in water among the raw materials of the solid electrolyte is CH ₃ COOLi. For example, when Li ₇ La ₃ Zr ₂ O ₁₂ is used as the solid electrolyte, the metal salt constituting the solid electrolyte that is readily soluble in water among the raw materials of the solid electrolyte is CH ₃ COOLi. For example, when 50Li ₄ SiO ₄ .50Li ₃ BO ₃ is used as the solid electrolyte, among the raw materials of the solid electrolyte, the metal salt constituting the solid electrolyte that is easily soluble in water and the inorganic salt of the counter anion of the metal in the solid electrolyte are: , CH ₃ COOLi and B (OH) ₃ . For example _{Li x Al y Ge z (PO} 4) as a solid electrolyte 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) When using the out of the material of the solid electrolyte, water The metal salt that constitutes the solid electrolyte that is readily soluble in water and the inorganic salt of the counter anion of the metal in the solid electrolyte are CH ₃ COOLi and (NH ₄ ) ₂ HPO ₄ , respectively. When preparing a homogeneous organic solvent / water mixed solution containing all the raw materials for the solid electrolyte in Method 1, the solution A may be prepared first, or the solution B may be prepared first. Further, when mixing the solution A and the solution B, the solution A may be added to the solution B, or the solution B may be added to the solution A.

  In the step of preparing a homogeneous organic solvent / water mixed solution containing all the solid electrolyte raw materials by dissolving all the solid electrolyte raw materials in a mixed solvent of water and an organic solvent having a hydroxyl group, the organic solvent having a hydroxyl group includes carbon. The alcohol of number 1-4 is preferable. Examples of the alcohol having 1 to 4 carbon atoms include methanol, ethanol, 2-methoxyethanol, 1-propanol, 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-butanol, and 2 -Alcohols such as butanol or mixed solvents thereof, but are not limited to these. As the organic solvent having a hydroxyl group, an alcohol having a small molecular weight is preferable.

  In the step of preparing a homogeneous organic solvent / water mixed solution containing all solid electrolyte raw materials by dissolving all the solid electrolyte raw materials in a mixed solvent of hydroxyl-containing organic solvent and water, the ratio of the organic solvent having hydroxyl groups to water D ([Hydroxyl group-containing organic solvent / (Hydroxyl group-containing organic solvent + water)] × 100) (mol%) is preferably 1 mol% to 20 mol%, more preferably 2 mol% to 10 mol%. .

  After the step of preparing a homogeneous organic solvent / water mixed solution containing all of the solid electrolyte raw material by dissolving all of the solid electrolyte raw material in a mixed solvent of an organic solvent having hydroxyl group and water, the above organic solvent / water mixed solution and A positive electrode active material is mixed to prepare a sol. A positive electrode active material is mixed so that it may become homogeneous in the solution containing all the raw materials of a solid electrolyte.

  After the step of preparing the sol by mixing the organic solvent / water mixed solution and the positive electrode active material, the solvent is removed from the sol to prepare a gel. When removing the solvent from the sol, an evaporator, a vacuum dryer or the like is used.

After the gel is prepared, the gel is baked. By firing, the raw material of the solid electrolyte is crystallized to become a solid electrolyte. The firing temperature of the gel can be appropriately changed within a temperature range in which the positive electrode active material does not deteriorate based on the crystallization temperature of the solid electrolyte used. For example, _La 0.51 _Li 0.34 _TiO 2.94 as a solid _{electrolyte, Li 7 La 3 Zr 2 O} 12, 50Li 4 SiO 4 · 50Li 3 BO 3, Li x Al y Ge z (PO 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) and _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1, When 1 <z <2) is used, the firing temperature (internal temperature) is preferably 500 ° C to 900 ° C. Other firing conditions such as firing time, temperature raising time, and cooling time can be appropriately changed depending on the firing temperature to be achieved, the amount of raw materials to be fired, and the like.

Examples of the solid electrolyte used for the coated positive electrode active material include La _0.51 Li _0.34 TiO _2.94 , Li ₇ La ₃ Zr ₂ O ₁₂ , 50Li ₄ SiO ₄ .50Li ₃ BO ₃ , Li _x Al _y Ge _z ( _{PO 4) 3 (x + 3y} + 4z = 9,0 <x <2,0 <y <1,1 <z <2), Li x Al y Ti z (PO 4) 3 (x + 3y + 4z = 9,0 <x <2, 0 <y <1, 1 <z <2) or mixtures thereof. _{Li x Al y Ge z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) and _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9 , 0 <x <2, 0 <y <1, 1 <z <2) are, for example, Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ , Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ etc. Is mentioned. Coating The solid electrolyte used for the positive electrode active _material, represented by _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) oxide A physical solid electrolyte is preferred.

Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like. In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2−x M _x O ₂ (where M is selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ti) At least one selected from the group consisting of Fe, Co, Ni, Cu and Zn, wherein x is 0.01 to 0.2), Li ₂ Mn ₃ MO ₈ (wherein M is Fe, Co, Ni, Cu and Zn). Li _1-x A _x Mn ₂ O ₄ (where A is at least one selected from the group consisting of Mg, B, Al, Fe, Co, Ni, Cr, Zn and Ca) x is 0.01 to 0.1), LiNi _1-x M _x O ₂ (where M is at least one selected from the group consisting of Co, Fe and Ga, and x is 0.01 to 0.1). 0.2 and is), LiFeO _{, Fe 2 (SO 4) 3} , LiCo 1-x M x O 2 ( however, M is at least one selected Ni, from the group consisting of Fe and Mn, x is 0.01 to 0.2 LiNi _1-x M _x O ₂ (wherein M is at least one selected from the group consisting of Mn, Fe, Co, Al, Ga, Ca and Mg, and x is 0.01-0. 2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO ₄ , Li (Ni _x Co _y Mn _1-xy ) O ₂ (where 0 <x <1, 0 <y <1 And the like can be used. Among these, _{LiCoO 2, Li (Ni x Co} y Mn 1-x-y) O 2 ( however, 0 <x <a 1,0 <y <1) is more preferable.

  The particle size of the positive electrode active material is usually defined so as to be equal to or less than the thickness of the mixture layer formed from the coated positive electrode active material, the conductive agent, and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the positive electrode active material powder, the coarse particles can be removed in advance by sieving classification or wind classification to produce particles having a thickness of the mixture layer thickness or less. desirable. The particle size of the positive electrode active material is preferably 5 μm to 20 μm.

  In the coated positive electrode active material, the coating amount of the solid electrolyte can be appropriately changed within a range that sufficiently covers the surface of the positive electrode active material and does not impair the ionic conductivity of the positive electrode active material itself. The amount of solid electrolyte coating in the coated positive electrode active material is the same as the total amount of the positive electrode active material in that it prevents decomposition of the electrolyte during the operation and suppresses deterioration of battery performance during high-temperature storage and use of high-capacity lithium secondary batteries. On the other hand, it is usually 0.1 wt% to 7.0 wt%, preferably 0.2 wt% to 3.0 wt%, more preferably 0.3 wt% to 1.0 wt%.

  In the coated positive electrode active material produced by the above method, the cumulative pore volume in the range of pore diameters of 100 nm to 10 μm measured using a mercury intrusion pore distribution measuring device (BELSORP-mini, manufactured by Nippon Bell Co., Ltd.) 0.148 (mL / g) or less. This is presumably because the coated positive electrode active material produced by the method of the present invention is coated so as to fill the voids until the solid electrolyte reaches 100 nm to 10 μm voids present in the positive electrode active material. In the coated positive electrode active material in which the cumulative pore volume in the pore diameter range of 100 nm to 10 μm is larger than 0.148 (mL / g), the solid electrolyte covers 100 nm to 10 μm voids present in the positive electrode active material. In many cases, the electrolytic solution and the positive electrode active material are in direct contact with each other. Accumulated pore volume is 0.148 (mL / g) in terms of suppressing degradation of the electrolyte during high-temperature storage and use, and suppressing deterioration of battery performance during high-temperature storage and use of high-capacity lithium secondary batteries. The following is preferable. The cumulative pore volume of the coated positive electrode active material having a pore diameter in the range of 100 nm to 10 μm may be measured for the coated positive electrode active material itself, or a positive electrode composite containing a binder, a conductive agent, etc. in addition to the coated positive electrode active material. You may obtain | require from the pore distribution obtained by measuring in the state of an agent layer.

  A positive electrode is manufactured using the coated positive electrode active material manufactured as described above. Specifically, a coated positive electrode active material is mixed with a binder, a conductive agent, and an organic solvent to prepare a positive electrode mixture, which is then applied to a current collector by, for example, a doctor blade method, a dipping method, a spray method, or the like. After the application, the organic solvent can be dried and can be produced by pressure molding with a roll press. Moreover, a plurality of mixture layers can be laminated on the current collector by performing a plurality of times from application to drying.

  In addition, since the coated positive electrode active material is oxide-based and generally has high electric resistance, a conductive agent made of carbon powder or the like for supplementing electric conductivity is used. Since both the coated positive electrode active material and the conductive agent are usually powders, a binder can be mixed with the powders, and the powders can be bonded together and simultaneously bonded to the current collector.

  As the current collector of the positive electrode 10, for example, an aluminum foil having a thickness of 10 μm to 100 μm, an aluminum perforated foil having a thickness of 10 μm to 100 μm and a pore diameter of 0.1 mm to 10 mm, an expanded metal, a foam metal plate, or the like is used. It is done. In addition to aluminum, materials such as stainless steel and titanium can also be applied, and any current collector can be used without being limited by the material, shape, manufacturing method, and the like.

  Although the thickness of a positive mix layer can be 10 micrometers-100 micrometers, for example, it is not limited to this.

  For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like is preferably used for the binder related to the positive electrode mixture layer.

<Negative electrode>
The negative electrode 12 is roughly composed of a negative electrode active material, a binder, and a current collector, and the negative electrode active material is mixed with a binder and an organic solvent to prepare a negative electrode mixture, which is, for example, a doctor blade method, a dipping method, It can be produced by applying and pressing the current collector by a spray method or the like. Moreover, a plurality of mixture layers can be laminated on the current collector by applying and drying the negative electrode mixture slurry a plurality of times.

  As the negative electrode active material, for example, a graphitized material obtained from natural graphite, petroleum coke or coal pitch coke, etc., treated at a high temperature of 2500 ° C. or higher, mesophase carbon, amorphous carbon, carbon fiber, lithium and alloy Or a metal supporting material on the surface of carbon particles. Examples of the metal to be supported include metals selected from lithium, silver, aluminum, tin, silicon, indium, gallium, and magnesium, or alloys thereof. Alternatively, the metal or the metal oxide may be used as the negative electrode active material. Furthermore, lithium titanate can also be used.

  The particle size of the negative electrode active material is usually defined so as to be equal to or less than the thickness of the mixture layer formed from the negative electrode active material and the binder. When the negative electrode active material powder has coarse particles having a size equal to or greater than the thickness of the mixture layer, the coarse particles can be removed in advance by sieving classification or airflow classification to produce particles having a thickness of the mixture layer or less. desirable.

  A copper foil or the like can be used for the current collector of the negative electrode 12. For example, a copper foil having a thickness of 5 μm to 30 μm is used.

  Although the thickness of a negative mix layer can be 10 micrometers-100 micrometers, for example, it is not limited to this.

  As the binder for the negative electrode mixture layer, the same binders as those exemplified above as the binder for the positive electrode mixture layer can be used.

<Electrolyte>
The electrolytic solution can be prepared by dissolving an electrolyte in a solvent. The electrolytic solution used in one embodiment of the present invention is a solvent in which dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like is mixed with ethylene carbonate, and lithium hexafluorophosphate (LiPF ₆ ) or lithium tetrafluoroborate as an electrolyte. There is a solution in which (LiBF ₄ ) is dissolved. The present invention is not limited to the type of solvent and electrolyte, and the mixing ratio of the solvents, and other electrolytic solutions can be used.

  Examples of the solvent used for the electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, fluoroethylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, and 2-methyl. Tetrahydrofuran, dimethyl sulfoxide, for example, dioxolane such as 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphate triester, trimethoxymethane, diethyl ether, sulfolane, 3-methyl-2-oxazolidinone, Tetrahydrofuran, 1,2-diethoxyethane, chloroethylene carbonate or chloropropylene carbonate, or these Nonaqueous solvent mixture such solvents. Other solvents may be used as long as they do not decompose on the positive electrode 10 or the negative electrode 12 incorporated in the battery of the present invention.

The electrolyte used in the electrolytic solution, for _{example, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6 or imide salts such as lithium represented by lithium trifluoromethane sulfonimide, There are many types of lithium salts. An electrolyte other than this may be used as long as it does not decompose on the positive electrode 10 and the negative electrode 12 incorporated in the battery of the present invention.

  Instead of the electrolytic solution, a solid polymer electrolyte (polymer electrolyte) can be used. As the solid polymer electrolyte, for example, an ion conductive polymer such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyhexafluoropropylene, or polyethylene oxide can be used. When these solid polymer electrolytes are used, the separator 11 can be omitted.

Furthermore, an ionic liquid can be included as an electrolytic solution. Examples of the ionic liquid include 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), a mixed salt of lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), triglyme and tetraglyme, On the positive electrode 10 and the negative electrode 12 from the group consisting of a cyclic quaternary ammonium cation (including N-methyl-N-propylpyrrolidinium) and an imide anion (including bis (fluorosulfonyl) imide). A combination that does not decompose can be selected and used for the battery of the present invention.

<Separator>
A separator 11 is inserted between the positive electrode 10 and the negative electrode 12 for the purpose of preventing a short circuit due to direct contact between the positive electrode 10 and the negative electrode 12. The separator 11 can be a polyolefin polymer sheet made of polyethylene, polypropylene, or the like, or a two-layer structure in which a polyolefin polymer, tetrafluoropolyethylene or the like is welded to a fluorine polymer sheet. A mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 11 so that the separator 11 does not shrink when the battery temperature increases. Further, a glass fiber nonwoven fabric or the like may be used. Since these separators 11 need to permeate lithium ions during charging and discharging of the battery, it is preferable that the pore diameter is 0.01 to 10 μm and the porosity is 20 to 90%.

  In the above embodiment, the electrode group is formed in a cylindrical shape, but various types such as a laminate of strip-shaped electrodes or a structure in which the positive electrode 10 and the negative electrode 12 are wound into an arbitrary shape such as a flat shape, etc. It may be in shape. The shape of the battery case 13 may be, for example, a flat oval shape, a flat oval shape, a square shape, or the like other than the cylindrical shape as described above. Furthermore, the axis 21 can be omitted depending on the battery shape or for the purpose of improving the volume occupancy of the electrode inside the battery.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited to these.

Example 1
<Positive electrode>
First, a coated positive electrode active material was manufactured. As the positive electrode active material, LiCoO ₂ having an average particle diameter of 10 μm was used. Since the solid electrolyte is Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ , the raw materials for the solid electrolyte are CH ₃ COOLi, Al (C ₃ H ₇ O) ₃ , Ti (C ₃ H ₇ O ₃ ) and (NH ₄ ) ₂ HPO ₄ were used, and the amounts of the respective raw materials were adjusted so that the respective molar ratios were 1.5: 0.5: 1.5: 3. Moreover, the amount of each raw material was adjusted so that the weight ratio of LiCoO ₂ and Li _1.5 Al _0.5 Ti _1.5 (PO ₄ ) ₃ was 95.5: 0.5. As a solvent for dissolving Al (C ₃ H ₇ O) ₃ and Ti (C ₃ H ₇ O) ₃ , 1-propanol which is a hydrophilic solvent is used, and CH ₃ COOLi and (NH ₄ ) ₂ HPO ₄ are dissolved. Water was used as the solvent. The ratio D of 1-propanol and water was adjusted to 5 mol%.

Al (C ₃ H ₇ O) ₃ and Ti (C ₃ H ₇ O) ₃ were added to 1-propanol and dissolved by sonication for 30 minutes to obtain a solution A. Next, CH ₃ COOLi and (NH ₄ ) ₂ HPO ₄ were added to water to prepare a solution B, and the solution B was added dropwise to the solution A to obtain a solution C. Thereafter, LiCoO ₂ was added to the solution C. The mixture was stirred for 1 hour with a three-one motor, the solvent was blown off with an evaporator, and then vacuum-dried at 60 ° C. for 24 hours. Then, it baked at 700 degreeC for 1 hour, and obtained the covering positive electrode active material.

The coated positive electrode active material obtained by the above method, a conductive agent (SP270: graphite manufactured by Nippon Graphite Co., Ltd.), and a polyvinylidene fluoride binder were mixed at a ratio of 85: 7.5: 7.5% by weight, and N-methyl-2 -A mixture of pyrrolidone was added and mixed to prepare a positive electrode mixture slurry. The slurry was applied to a 20 μm thick aluminum foil by a doctor blade method and dried. The coating amount of the positive electrode mixture slurry was 200 g / m ² . Then, it pressed and the positive electrode was prepared.

  With respect to the positive electrode thus obtained, the mixture layer was peeled off from the positive electrode, and the cumulative pore volume V (mL / g) of the coated positive electrode active material at 100 nm to 10 μm was measured using a mercury intrusion pore distribution measuring device (BELSORP- mini, manufactured by Nippon Bell Co., Ltd.).

<Negative electrode>
Graphite was used as the negative electrode active material. Polyvinylidene fluoride was mixed with graphite at a weight ratio of 95: 5, and further mixed with N-methyl-2-pyrrolidone to prepare a negative electrode mixture slurry. The slurry was applied to a copper foil having a thickness of 10 μm by a doctor blade method and dried. The mixture was pressed so that the bulk density was 1.5 g / cm ³ to prepare a negative electrode.

<Electrolyte>
As the electrolytic solution, 1M LiPF ₆ [EC / EMC / DMC = 1/1/1] was used.

<Battery>
A polyolefin separator was inserted between the positive electrode and the negative electrode to form an electrode group. Thereto, an electrolytic solution was injected. Thereafter, the battery was wound to produce an 18650 type battery. Then, the battery was initialized by repeating charging / discharging 3 cycles.

The lithium secondary battery thus obtained was charged and discharged with an upper limit voltage of 4.2 V and a lower limit voltage of 2.5 V, and a current density of 0.1 mA / cm ² . The discharge capacity was taken as the capacity of the battery, and the capacity at 85 ° C. after 10 days storage test / capacity before storage test was calculated as the capacity maintenance rate.
As a result, the solution C obtained by the above method did not gel, and V of the coated positive electrode active material was 0.148 mL / g. The initial battery capacity was 996 mAh, and the capacity maintenance rate after the storage test was 77%.

(Example 2)
Example 1 was repeated except that the ratio D of 1-propanol to water in Solution C was changed from 5 mol% to 3 mol%. V of the coated positive electrode active material was 0.144 mL / g. The initial battery capacity was 990 mAh, and the capacity retention rate after the storage test was 75%.

(Example 3)
The same procedure as in Example 1 was performed except that the ratio D of 1-propanol to water in Solution C was changed from 5 mol% to 10 mol%. V of the coated positive electrode active material was 0.145 mL / g. The initial battery capacity was 994 mAh, and the capacity maintenance rate after the storage test was 75%.

Example 4
Example 1 was repeated except that the firing temperature was changed from 700 ° C to 900 ° C. V of the coated positive electrode active material was 0.110 mL / g. The initial battery capacity was 988 mAh, and the capacity retention rate after the storage test was 76%.

(Example 5)
The molar ratio of CH ₃ COOLi: Al (C ₃ H ₇ O) ₃ : Ti (C ₃ H ₇ O) ₃ : (NH ₄ ) ₂ HPO ₄ is changed from 1.5: 0.5: 1.5: 3 to 1. .3: Same as Example 1 except for 0.3: 1.7: 3. V of the coated positive electrode active material was 0.142 mL / g. The initial battery capacity was 995 mAh, and the capacity retention rate after the storage test was 76%.

(Comparative Example 1)
The same procedure as in Example 1 was performed except that the coated positive electrode active material was changed to a positive electrode active material not coated with a solid electrolyte. V of the positive electrode active material was 0.150 mL / g. The initial battery capacity was 1001 mAh, and the capacity retention rate after the storage test was 68%.

(Comparative Example 2)
The same procedure as in Example 1 was performed except that the ratio D of 1-propanol to water in Solution C was changed from 5 mol% to 0 mol%. Solution C gelled and a homogeneous solution C could not be obtained. V of the positive electrode active material was 0.152 mL / g. The initial battery capacity was 995 mAh, and the capacity retention rate after the storage test was 67%.

(Comparative Example 3)
The same procedure as in Example 1 was performed except that the ratio D of 1-propanol to water in Solution C was changed from 5 mol% to 30 mol%. Solution C gelled and a homogeneous precursor could not be obtained. V of the positive electrode active material was 0.155 mL / g. The initial battery capacity was 990 mAh, and the capacity retention rate after the storage test was 67%.

  In addition, this invention is not limited to above-described embodiment, Various modifications are included. For example, with respect to a part of the configuration of the embodiment, it is possible to add, delete, or replace another configuration.

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery container 14 Positive electrode current collection tab 15 Negative electrode current collection tab 16 Inner cover 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery cover 21 Axis center

Claims (4)

La _0.51 Li _0.34 TiO _2.94 , Li ₇ La ₃ Zr ₂ O ₁₂ , 50Li ₄ SiO ₄ .50Li ₃ BO formed so as to cover the surface of the positive electrode active material and the positive electrode active material. _{3, Li x Al y Ge z} (PO 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2) and _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9, 0 <x <2, 0 <y <1, 1 <z <2), and a covering layer made of at least one solid electrolyte selected from the group consisting of: A method for producing a substance , comprising:
The cumulative pore volume V at 100 nm to 10 μm of the coated positive electrode active material is 0.148 (mL / g) or less,
A step of preparing a homogeneous organic solvent / water mixed solution containing all the solid electrolyte raw materials by dissolving all of the raw materials of the solid electrolyte in a mixed solvent of an organic solvent having hydroxyl group and water, and the above organic solvent / water mixed solution and positive electrode active A step of preparing a sol by mixing substances, a step of preparing a gel by removing a solvent from the sol, and a step of baking the gel,
The production method, wherein the ratio D ([organic solvent having a hydroxyl group / (organic solvent having a hydroxyl group + water)] × 100) (mol%) of the organic solvent having a hydroxyl group is 1 mol% to 20 mol%. The solid electrolyte is _{Li x Al y Ti z (PO} 4) 3 (x + 3y + 4z = 9,0 <x <2,0 <y <1,1 <z <2), a lithium secondary battery according to claim 1 manufacturing method of use coated positive electrode active material. The manufacturing method of the covering positive electrode active material for lithium secondary batteries of Claim 1 or 2 whose organic solvent which has a hydroxyl group is at least 1 sort (s) of alcohol selected from the group which consists of C1-C4 alcohol. The lithium secondary battery which has a positive electrode containing the covering positive electrode active material for lithium secondary batteries obtained by the manufacturing method of the covering positive electrode active material for lithium secondary batteries of Claim 1.

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