JP2013164942A - Coated cathode active material and all-solid lithium secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cathode active material capable of remarkably improving battery characteristics of an all-solid lithium secondary battery.SOLUTION: The coated cathode active material of the present invention is characterized in that the surface of a cathode active material is coated with a coating agent layer containing, as a main component, a compound represented by general formula (I): LiS. In the formula (I), n represents a number of 1 or greater.

Description

  The present invention relates to a coated positive electrode active material having excellent battery characteristics, and an all-solid lithium secondary battery using the coated positive electrode active material.

  2. Description of the Related Art In recent years, lithium secondary batteries exhibiting battery characteristics based on lithium ion conductivity are often used as power sources for information-related devices and communication devices such as mobile phones, digital cameras, and notebook computers. As a lithium secondary battery, a battery in which a solution obtained by dissolving an electrolytic salt in a non-aqueous solvent is contained as an electrolyte is widely known. However, since the non-aqueous solvent is flammable, such a lithium secondary battery may not be preferable from the viewpoint of safety.

  For the purpose of ensuring safety against the above problems, all-solid lithium secondary batteries using an electrolyte that does not contain a non-aqueous solvent, that is, an electrolyte made of a solid material (solid electrolyte), are widely used. It has been. However, since the lithium ion conductivity in the solid is inferior to the lithium ion conductivity in the liquid, the battery characteristic of the solid lithium secondary battery is that of a lithium secondary battery using an electrolyte that is a non-aqueous solvent. There was a problem that it was inferior to the characteristics.

In order to improve the battery characteristics of the solid lithium secondary battery, it has been studied to coat the surface of the positive electrode active material with a substance having lithium ion conductivity. For example, Patent Document 1 includes a film of a metal sulfide selected from nickel, manganese, iron, and cobalt on the surface of raw material particles selected from LiCoO ₂ , LiMn ₂ O ₄ , and Fe ₂ O ₃ . An all-solid lithium secondary battery using a positive electrode active material is described. Patent Document 2 describes that the surface of a positive electrode active material is coated with LiNbO ₃ in an all-solid lithium secondary battery.

  Patent Document 3 describes forming a resistance layer formation suppression coat layer on the surface of the positive electrode active material. Furthermore, Patent Document 3 describes that the coating layer uses a material having no reactivity with respect to the positive electrode active material and the solid electrolyte material and having lithium ion conductivity.

Non-Patent Document 1 describes a technique for coating the surface of a positive electrode active material with an oxide having lithium ion conductivity (for example, Li ₄ Ti ₅ O ₁₂ , LiNbO ₃ , LiTaO _3, etc.). .

JP 2008-251520 A International publication 2011/040118 pamphlet JP 2009-266728 A

Advanced materials 2006.18, (pages 2226-2229)

  However, even when the positive electrode active material described in Patent Documents 1 to 3 and Non-Patent Document 1 is used, the battery characteristics when it is an all-solid lithium secondary battery are not yet sufficient. There is.

  An object of this invention is to provide the positive electrode active material which can improve the battery characteristic of an all-solid-state lithium secondary battery notably in view of the above problems.

  As a result of intensive studies to solve such problems, the present inventors have found that the positive electrode active material (coated positive electrode active material) whose surface is coated with a coating layer mainly composed of lithium sulfide is all-solid lithium. It has been found that when used in a secondary battery, the resistance at the contact interface between the positive electrode active material and the solid electrolyte can be sufficiently reduced. The secondary battery was found to be remarkably excellent in battery characteristics (discharge capacity and high rate characteristics), and the present invention was completed.

That is, the present invention has the following contents.
(1) A coated positive electrode active material, wherein the surface of the positive electrode active material is coated with a coating layer containing a compound represented by the following general formula (I) as a main component.
Li _{2 Sn} (I)
In the above formula (I), n represents a number of 1 or more.
(2) The coated positive electrode active material according to (1), wherein the coating amount of the coating layer is ² to 50 mg / m ² with respect to the surface of the positive electrode active material.

(3) The coating according to (1) or (2), wherein the coating layer contains a conductive additive and the content ratio is 0.5 to 8 parts by mass with respect to 100 parts by mass of the positive electrode active material. Positive electrode active material.
(4) A lithium secondary battery using the coated positive electrode active material according to any one of (1) to (3) and an electrolyte that is an inorganic solid sulfide composite.

(5) A method for producing a coated positive electrode active material according to any one of (1) to (3), comprising the following steps (i) and (ii) in this order: Production method.
(I) A step of dissolving the compound represented by the general formula (I) in an organic solvent to obtain a precursor solution of the coating layer.
(Ii) A step of applying the precursor solution obtained in (i) above to the surface of the positive electrode active material and then drying the positive electrode active material with the precursor solution attached to the surface.
(6) A method for producing a coated positive electrode active material according to any one of (1) to (3), wherein a compound represented by the following general formula (II) is used as a positive electrode active material, and the surface of the positive electrode active material is sulfided The manufacturing method characterized by including the process of exposing hydrogen.
_{LiQ a R b S c O 2} (II)
In the formula (II), Q, R, and S are different from each other, and are selected from nickel, cobalt, aluminum, magnesium, vanadium, chromium, calcium, zirconium, niobium, molybdenum, tungsten, and manganese. Indicates. A, b and c each independently represent a numerical value of 0 to 1 and satisfy a + b + c = 1.

  According to the present invention, the battery characteristics are excellent despite the use of lithium sulfide, which is considered to be ineffective as a material for covering the positive electrode active material for lithium ion secondary batteries because it does not have lithium ion conductivity. A coated positive electrode active material can be obtained. More specifically, since the positive electrode active material is coated with a coating layer mainly composed of lithium sulfide, when used in an all-solid lithium secondary battery, the positive electrode active material and the solid electrolyte are in contact with each other. The resistance at the interface can be significantly reduced. Therefore, the all-solid lithium secondary battery of the present invention using this coated positive electrode active material has an effect that the battery characteristics are remarkably excellent.

It is the schematic which shows an example of the chemical reaction at the time of charging the all-solid-state lithium secondary battery using the covering positive electrode active material of this invention, and a movement of lithium ion and an electron. In the Example and comparative example of this invention, it is the schematic of the all-solid-state lithium secondary battery for performing evaluation. It is a figure which shows the charging / discharging curve of the all-solid-state lithium secondary battery in which the covering positive electrode active material obtained by Example 1 of this invention and the comparative example 1 was used. It is a figure which shows the charging / discharging curve of the all-solid-state lithium secondary battery in which the covering positive electrode active material obtained in Example 8 of this invention and the comparative example 3 was used. Measurement in which the surface of the coated positive electrode active material obtained in Example 8 and Comparative Example 3 of the present invention was confirmed by X-ray photoelectron spectroscopy (XPS), and sulfide ions (S ²⁻ ) were detected on the surface. It is a figure which shows a result.

Hereinafter, the present invention will be described in detail.
The coated positive electrode active material of the present invention is obtained by coating the surface of the positive electrode active material with a coating layer containing a compound represented by the following general formula (I) as a main component.
Li _{2 Sn} (I)
In the above formula (I), n represents a number of 1 or more. n may be a natural number or a decimal number. For example, Li ₂ S and Li ₂ S ₂ are shown as Li ₂ S _1.5 when they are present in equal amounts.

  In the present invention, coating means that a coating layer is formed on part or all of the surface of the positive electrode active material.

The positive electrode active material is described below.
The positive electrode active material is not particularly limited, and specifically, lithium cobaltate, lithium nickelate, lithium manganate, lithium iron phosphate, LiNi _a Co _b Al _c O ₂ (where a + b + c = 1). (Hereinafter, in the present invention, it may be referred to as “NCA”). Among these, lithium cobaltate (LiCoO ₂ ) and NCA are preferable from the viewpoint of particularly excellent battery characteristics when an all-solid lithium secondary battery is used.

  The positive electrode active material may be used by being mixed with an additive such as a conductive agent, if necessary, as long as the effects of the present invention are not impaired.

Examples of the compound represented by the above formula (I) include Li ₂ S (lithium monosulfide or lithium sulfide), Li ₂ S ₂ (lithium disulfide) which is lithium polysulfide, Li ₂ S ₃ (lithium trisulfide), and the like. Can be mentioned. In the present invention, the compounds represented by the above formula (I) may be collectively referred to as “lithium sulfide”.

  The compound (lithium sulfide) represented by the above formula (I) does not have lithium ion conductivity. This can be confirmed by measuring the impedance of the compound represented by the above formula (I).

  Regarding the effect of the present invention, it is possible to obtain a coated positive electrode active material capable of exhibiting excellent battery characteristics by coating the positive electrode active material with a coating layer mainly composed of lithium sulfide as described above. Along with the problems of the prior art in the positive electrode active material in which no layer is provided and the positive electrode active material coated with a coating layer containing a substance other than lithium sulfide as a main component, this will be described in detail below.

When a positive electrode active material not provided with a coating layer is used, the following mechanism is used at the contact interface between the positive electrode active material and a solid electrolyte such as a sulfide-based solid electrolyte when the battery is used. A resistive layer is created.
When an oxide positive electrode active material, which is a conductor having ionic conductivity and electron conductivity, and a solid electrolyte come into contact with each other, a lithium ion attractive property is obtained from the difference in electronegativity between oxygen atoms and sulfur atoms at the contact interface. There will be a difference. As a result, lithium ions are attracted to the positive electrode active material side having oxygen atoms with higher electron-withdrawing properties. As a result, an ion-deficient layer (portion where ions are deficient) is formed on the solid electrolyte side, and the positive electrode active material is A charge layer containing excessive lithium ions is formed on the material side. That is, the lithium ion concentration in the solid electrolyte is lowered, and the lithium ion concentration in the positive electrode active material is increased, so that a lithium ion concentration gradient is generated at the contact interface. Here, since the positive electrode active material has electronic conductivity, lithium ions tend to move in a direction to eliminate the concentration gradient. Therefore, in the portion where the lithium ions migrated from the solid electrolyte exist excessively on the positive electrode active material side, the lithium ions diffuse throughout the positive electrode active material.

  However, since the solid electrolyte does not have electronic conductivity, the ion-deficient layer is maintained as it is. And the movement of the lithium ion to the positive electrode active material side continues, and only the ion-deficient layer on the solid electrolyte side develops greatly. Since ion conductivity is reduced in the ion-deficient layer, it is considered that the output performance in the case of a battery is significantly reduced. That is, when a positive electrode active material in which a coating layer is not formed is used, it is considered that the lithium ion deficient layer in the solid electrolyte acts as a resistance layer, which deteriorates battery characteristics.

  In order to eliminate such deterioration of battery characteristics, as described in Non-Patent Document 1 above, an oxide having lithium ion conductivity but not electron conductivity is used as a coating agent. A technique for coating an active material is known. A lithium ion concentration gradient does not appear between the coating layer made of oxide and the positive electrode active material. On the other hand, the lithium ion concentration gradient as described above appears at the contact interface between the coating layer and the solid electrolyte. Here, the main difference from the case where no coating layer is provided is that the coating layer made of oxide and the solid electrolyte are both materials that do not have electron conductivity, so that lithium ion diffusion occurs. Without, the lithium ion concentration gradient is maintained. In such a state, the deterioration of the battery characteristics is suppressed to some extent as compared with the state of only the positive electrode active material in which the coating layer is not provided and only the ion-deficient layer is greatly developed. However, there is a problem that suppression of deterioration of battery characteristics due to the presence of an ion-deficient layer is still insufficient.

  On the other hand, in the present invention, lithium sulfide is used instead of oxide as the coating agent used in the coating layer, and therefore both the coating layer and the solid electrolyte have the same sulfur atom as the anion constituent element. Have. As a result, there is no difference in electron withdrawing property between the coating layer and the solid electrolyte, and lithium ions do not move. Further, since the coating layer itself does not have lithium ion conductivity even at the contact interface between the coating layer and the positive electrode active material, lithium ions do not move and the lithium ion concentration gradient itself does not occur in the first place. As a result, the deterioration of battery characteristics is sufficiently suppressed.

The technical idea, configuration and operational effects of the present invention will be further described.
In order for the lithium secondary battery to operate, movement of lithium ions is essential between the positive electrode active material and the solid electrolyte. Therefore, in the conventional technical common sense, it has been said that the material for coating the positive electrode active material must be a substance having lithium ion conductivity. On the other hand, the present invention has been made based on a completely different idea from the prior art. “Even if a substance having no lithium ion conductivity is used, if a lithium ion moves by another mechanism, it can be used as a battery. It was achieved from a new point of view.

  More specifically, the coated positive electrode active material of the present invention exhibits battery characteristics when it is made a lithium secondary battery by transferring lithium ions in a chemical reaction system involving redox. Thereby, even if it is a case where the coating material layer which does not have lithium ion conductivity is formed, the movement of lithium ion generate | occur | produces. That is, in the present invention, a redox reaction that occurs due to a potential change associated with charge / discharge is utilized, and the function as a battery is expressed by giving directionality to the movement of lithium ions from the potential gradient.

This principle will be described in detail with reference to FIG.
FIG. 1 shows the movement of lithium ions and electrons when charging an all-solid lithium secondary battery using the coated positive electrode active material of the present invention.
When charging is performed, a driving force (propulsive force) for moving lithium ions from the positive electrode to the negative electrode is applied. As shown in FIG. 1, a lithium ion extraction reaction from the coating layer 2 to the solid electrolyte 3 occurs at the contact interface between the coating layer 2 and the solid electrolyte layer 3. At that time, electrons are emitted, but the electrons remain in the coating layer 2 because the solid electrolyte 3 has no electron conductivity. Then, since the coating layer 2 becomes unstable in terms of charge, it is considered that the electron orbit extracts lithium ions from adjacent molecules (lithium sulfide) in an attempt to stabilize the charge. It is considered that such reactions are chained and eventually lithium ions are extracted at the contact interface between the positive electrode active material 1 and the coating layer 2. That is, in the present invention, when an electrochemical reaction occurs, the lithium ions are successively transferred to adjacent molecules, so that even if a substance having no lithium ion conductivity is used, Ions move. In the coating layer 2, the traveling direction of lithium ions is determined by the electric field gradient (potential gradient) generated by the applied voltage for charging.

  In other words, in the present invention, a specific coating layer having no lithium ion conductivity is provided on the surface of the positive electrode active material, rather than using lithium ion conductivity to develop battery characteristics as in the prior art. Thus, as shown in FIG. 1, in each layer of the positive electrode active material, the coating layer, and the solid electrolyte, lithium ions are transferred stepwise with an electrochemical reaction, thereby expressing battery characteristics. ing. Therefore, the lithium ion concentration at the contact interface between the solid electrolyte layer and the coating layer is always constant, and a portion with excessive lithium ions or a lithium ion deficient layer does not appear, that is, no lithium ion concentration gradient appears. When the coated positive electrode active material of the present invention is used in an all-solid lithium secondary battery, the resistance at the contact interface is reduced as compared with a conventional all-solid lithium secondary battery using lithium ion conductivity. Thus, the battery characteristics are remarkably excellent.

  That is, in the present invention, the characteristics of an all-solid lithium secondary battery can be expressed even when a substance having no lithium ion conductivity is used by being coated with a specific coating layer. Furthermore, the resistance at the contact interface between the positive electrode active material and the solid electrolyte can be reduced, and as a result, the battery characteristics can be improved.

  Theoretically, it is considered that some electric resistance is generated even when lithium ions are transferred by the redox reaction. However, since this electric resistance is lower than the electric resistance caused by the lithium ion-deficient layer, the battery characteristics can be improved. In FIG. 1, d represents a number of about several tens, but the smaller the value of d, the smaller the electric resistance due to the delivery of lithium ions. That is, from the viewpoint of further reducing the electric resistance, it is preferable that the number of lithium sulfide molecules in the coating layer is small.

  The coating layer is mainly composed of lithium sulfide as described above. That is, the coating layer preferably contains 80% by mass or more of lithium sulfide as described above, and 90% by mass or more. Is more preferable.

  The coating agent layer may contain a conductive additive for the purpose of further improving the conductivity of the positive electrode active material and thus improving the battery characteristics. It is preferable that content of a conductive support agent is 0.5-8 mass parts with respect to 100 mass parts of positive electrode active materials. When the content of the conductive auxiliary is less than 0.5 parts by mass, good battery characteristics may not be exhibited depending on the type of the positive electrode active material used. On the other hand, when the content of the conductive auxiliary exceeds 8 parts by mass, the binding property of the positive electrode active material may be lowered.

  Examples of the conductive assistant include carbon nanotubes, acetylene black, ketjen black, and carbon fiber. Among these, carbon fiber is preferable from the viewpoint of excellent conductivity.

Coverage of the coating layer for the positive electrode active material is preferably 2 to 50 mg / ^{m 2,} more preferably 4 to 30 mg / ^{m 2.} If it is less than 4 mg / m ² , the resistance at the contact interface between the positive electrode active material and the solid electrolyte may not be sufficiently reduced, and good battery characteristics may not be exhibited. On the other hand, if it exceeds 50 mg / m ² , the number of molecules in the coating increases, and the electrical resistance for causing the above-described continuous redox reaction may become so large that it cannot be ignored. Therefore, it is not preferable. More specifically, if the coating amount becomes too large, the number of lithium sulfide molecules in the coating layer increases too much, so that the number of times lithium ions are transferred in the coating layer in FIG. In some cases, the electrical resistance increases and the battery characteristics deteriorate. In the present invention, a coating layer may be formed on a part of the surface of the positive electrode active material, or a coating layer may be formed on the entire surface.

  In addition, when said coating amount is converted into the thickness of the coating layer, the thickness is preferably about 1 to 30 nm, and preferably about 2 to 20 nm.

  The average particle diameter of the coated positive electrode active material is not particularly limited, and is usually in the range of about 0.1 to 50 μm.

  A method for producing the coated positive electrode active material of the present invention by coating the coating material layer on the positive electrode active material will be described below. That is, as the production method, (a) a method of immersing the positive electrode active material in the precursor solution of the coating layer and then drying, (b) spraying the precursor solution of the coating layer on the positive electrode active material, Next, a method of drying in a fluidized bed, (c) a method of mixing a precursor solution of a coating agent layer and a positive electrode active material and stirring to form a mixture, and drying the mixture while continuing stirring can be mentioned. In particular, (b) and (c) are preferable. Drying includes natural drying, vacuum room temperature drying, heating atmospheric drying, and heating vacuum drying, and the method and conditions for dipping, spraying, stirring and drying are not particularly limited.

Of the methods using the precursor solution, the wet method (c) will be described in detail below.
The wet method includes the following steps (i) and (ii) in this order.
(I) A step of dissolving the compound represented by the general formula (I) in an organic solvent to obtain a precursor solution of the coating layer.
(Ii) A step of drying the positive electrode active material in a state where the precursor solution adheres to the surface after the precursor solution obtained in (i) above is applied to the surface of the positive electrode active material.

  More specifically, first, a precursor solution is prepared by dissolving lithium sulfide and, if necessary, a conductive additive and other additives in an organic solvent such as ethanol. In the preparation, the dissolution method and the dissolution conditions are not particularly limited.

  Next, an appropriate positive electrode active material and the precursor solution are mixed and stirred, and the organic solvent is naturally evaporated while stirring until the obtained wet powder exhibits a certain degree of fluidity. Then, when dried in vacuum with heating, the coated positive electrode of the present invention in a state where the coating layer is attached to the surface of the positive electrode active material (that is, the surface of the positive electrode active material is coated with the coating layer) An active material can be obtained. Stirring conditions and drying conditions are not particularly limited and can be appropriately selected. If stirring is continued until the organic solvent is completely removed, the generation of secondary agglomerated particles can be suppressed, but even if secondary agglomerated particles are generated depending on the conditions, use a screen or mortar as necessary. The coated positive electrode active material without secondary agglomerated particles can also be obtained by crushing.

As a production method other than the method using the precursor solution, a gas phase method may be mentioned.
The gas phase method is a production method including a step of exposing a surface of the positive electrode active material to hydrogen sulfide using a compound represented by the following general formula (II) as the positive electrode active material.
_{LiQ a R b S c O 2} (II)
In the above formula (II), Q, R, and S are different from each other and represent an element selected from nickel, cobalt aluminum, magnesium, vanadium, chromium, calcium, zirconium, niobium, molybdenum, tungsten, and manganese. Of these, nickel, cobalt, and aluminum are preferable. A, b and c each independently represent a numerical value of 0 to 1 and satisfy a + b + c = 1.

  In addition, the exposure method and conditions in a vapor phase method are not specifically limited, It can adjust suitably.

In the present invention, it is preferable to form a coating layer on the surface of the positive electrode active material by a vapor phase method. The mechanism of the gas phase method and the effects achieved by employing the gas phase method will be described below.
In general, on the surface of the positive electrode active material, which is the compound represented by (III) above, a reaction represented by the following formula (1) occurs due to moisture present in the atmosphere, and hydroxylation occurs on the surface of the positive electrode active material. Lithium is deposited. As a result, there is a problem that only a battery having significantly deteriorated battery characteristics can be obtained.

  In particular, when nickel is contained in the positive electrode active material, heat treatment is performed in an oxygen atmosphere as a pretreatment for the purpose of avoiding precipitation of lithium hydroxide and removing moisture. However, if moisture is not completely removed or if it reacts with moisture in the atmosphere again after heat treatment, lithium hydroxide is not completely removed and still remains on the surface of the positive electrode active material. As a result, only a battery having a large variation in battery characteristics may be obtained for each pretreatment. That is, in the prior art, a complicated operation of controlling the moisture content on the surface of the positive electrode active material is separately required.

  Further, when the raw material lithium hydroxide is used in an excessive amount during the synthesis of the positive electrode active material, unreacted lithium hydroxide remains on the surface of the positive electrode active material, leading to deterioration of battery characteristics. Therefore, even in this case, a separate operation for removing unreacted lithium hydroxide is required.

However, when the vapor phase method is employed, the following formula (by the simple operation of reacting hydrogen sulfide with lithium hydroxide as an impurity that naturally precipitates on the surface of the positive electrode active material due to moisture in the atmosphere or the like, Reaction like 2) can be advanced. As a result, lithium sulfide is generated on the surface of the positive electrode active material.

  In other words, when the vapor phase method is used, hydrogen sulfide is formed on the surface of the positive electrode active material without using a complicated operation of controlling the amount of water on the surface of the positive electrode active material or removing unreacted lithium hydroxide. The effect that the coating layer which has lithium sulfide as a main component can be formed in the simple process of exposing is produced. In particular, even in the atmosphere where the low dew point is controlled, the generated water is sequentially removed even if it is generated, so that the reaction like the above formula (2) easily proceeds.

  Furthermore, when the vapor phase method is used, compared to the case where the wet method is used, when the obtained coated positive electrode active material is an all-solid lithium secondary battery, the battery characteristics are excellent even at a high current density. An effect is produced.

The effect in the manufacturing method of the covering positive electrode active material of this invention show | played by using lithium sulfide is described below.
In the prior art, a positive electrode active material is coated with a material that is poorly soluble in a solvent such as an oxide such as titanium or niobium. Therefore, the oxide cannot be directly coated on the positive electrode active material. First, an alkoxide such as ethyl alkoxide is generated from a metal such as titanium or niobium, and then an alcohol solution containing the alkoxide is used as the positive electrode active material. After coating, a process of drying and heat treatment was necessary. In other words, the process of coating the positive electrode active material with the coating layer is very complicated and may not be practical. On the other hand, in the present invention, since lithium sulfide, which is a substance soluble in a volatile organic solvent, is used, the positive electrode active material can be coated with a simple process.

  The all solid lithium secondary battery of the present invention uses a positive electrode layer containing the above-described coated positive electrode active material of the present invention. Furthermore, the all solid lithium secondary battery of the present invention uses a solid electrolyte layer made of an inorganic solid sulfide composite. As the solid electrolyte layer, one represented by the following general formula (III) is preferably used.

_{Li 2 S-M x S y} (III)
In the above formula (III), M represents phosphorus, silicon, germanium, gallium, boron or aluminum, and x and y are arbitrary integers that give a stoichiometric ratio depending on the type of M. It is. Of these, M is preferably phosphorus from the viewpoint of simplicity of synthesis and cost. Note that x and y are arbitrary integers that give a stoichiometric ratio depending on the type of M.

Such an inorganic solid sulfide composite is not particularly limited as long as it exhibits the effects of the present invention, but Li ₂ S—P ₂ S _{5 and the} like are particularly preferably used from the viewpoint of being excellent in the simplicity of synthesis. It is done.

  A pellet-shaped solid electrolyte layer can be obtained by pressing the inorganic solid sulfide composite as described above.

  In the all solid lithium secondary battery of the present invention, in addition to the positive electrode layer obtained from the above positive electrode active material and the solid electrolyte layer composed of the above inorganic solid sulfide composite, the negative electrode layer obtained from a known negative electrode active material , A positive electrode current collector, a negative electrode current collector, and the like are used. The positive electrode layer and the negative electrode layer may be a mixture layer containing the inorganic solid sulfide composite.

The manufacturing method of the all-solid-state lithium secondary battery of this invention is not specifically limited, For example, the pellet battery using a thin film battery, a metal mold | die, etc. are mentioned.
The shape of the all solid lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin shape, a laminate shape, a square shape, and a cylindrical shape.

  The use of the all solid lithium secondary battery of the present invention is not particularly limited, and examples thereof include power supplies for mobile phones, digital cameras, video cameras, notebook computers, electric vehicles, hybrid vehicles, and the like.

The technical idea, configuration and effect of the present invention will be described again.
In the present invention, it is essential to coat the positive electrode active material with a coating layer mainly composed of lithium sulfide. Thereby, it is possible to reduce the resistance generated at the contact interface between the positive electrode active material and the solid electrolyte, and it is possible to obtain a coated positive electrode active material and an all solid lithium secondary battery excellent in battery characteristics. Many studies have been made so far regarding the improvement of battery characteristics by reducing the interfacial resistance by covering with such a positive electrode active material. However, the lithium ion transfer mechanism based on the redox reaction has been achieved for the first time in the present invention. Is done. As a result, it is possible to effectively suppress the deterioration of the battery characteristics caused by the generation of the lithium ion deficient layer as described above, and as a result, the battery characteristics can be further improved as compared with the conventional technique. .

  Next, the present invention will be described more specifically with reference to examples. The present invention is not limited by these.

The coated positive electrode active material and the all-solid lithium secondary battery of the present invention were evaluated by the following methods.
(1) Battery characteristics evaluation (discharge capacity, discharge energy density)
An all-solid lithium secondary battery having a structure as shown in FIG. 2 was produced as follows and used as a battery characteristic evaluation battery.
That is, an inorganic solid sulfide complex represented by Li ₂ S—P ₂ S ₅ was used as the solid electrolyte. As the negative electrode active material, graphite (capacity: 360 mAhr / g) was used. In a glove box having a nitrogen atmosphere (dew point: −80 ° C. or lower), a positive electrode active material and a negative electrode active material are mixed with a solid electrolyte at a predetermined ratio using a mortar, respectively, did. The mixing ratio was (positive electrode active material) :( solid electrolyte) = 7: 3, (negative electrode active material) :( solid electrolyte) = 6: 4 in mass ratio.

Next, 50 mg of solid electrolyte was put into a mold having a diameter of 10 mm and pressed to form pellets. Further, 15 mg of the negative electrode mixture was placed on one side of the solid electrolyte pellet and pressed, and 20 mg of the positive electrode mixture was placed on the other side and pressed (maximum pressure: 10 t / cm ² ), and then from the mold. As shown in FIG. 2, a pellet was obtained in which the positive electrode mixture layer 4 and the negative electrode mixture layer 5 were disposed on both sides of the solid electrolyte layer 6. Then, as shown in FIG. 2, an aluminum foil as the positive electrode current collector 7 and a copper foil as the negative electrode current collector 8 were arranged so as to contact the positive electrode mixture layer 4 and the negative electrode mixture layer 5, respectively. . Further, an insulating film 10 was disposed on the positive electrode current collector 7, and stainless steel support plates 9 and 9 were disposed on both sides of the positive electrode current collector 7 and the negative electrode current collector 8. Next, the laminate film 11 was used, and the both ends were laminated in a state where the electric lead-out tag 12 was sandwiched between them, so that vacuum sealing was performed to obtain an all-solid lithium secondary battery for evaluation.

Using the pellets for evaluation (trade name “TOSCAT-3000” manufactured by Toyo System Co., Ltd.), the discharge capacity and the discharge energy density were measured while applying a pressure of 600 kgf / cm ^{2 in} the atmosphere. The measurement conditions were a cut-off voltage of 2-4.3 V and a current density of 0.1 mA / cm ² .

Example 1
A coated positive electrode active material provided with a coating layer by a wet method was obtained as follows.
That is, in a glove box whose dew point was adjusted to −80 ° C. or less, 5 g of ethanol was added to 135 mg of lithium sulfide (Li ₂ S) and dissolved by stirring for 10 minutes to obtain a precursor solution of a coating layer. . Next, 20 g of granular lithium cobaltate (0.23 m ² / g) (LiCoO ₂ ) as a positive electrode active material dried at 200 ° C. for 24 hours under vacuum was added to a round bottom stainless steel container (capacity: 200 ml). And 2.9 g of the above precursor solution was added and stirred with a spatula. When the stirring was continued, ethanol spontaneously evaporated, and a wet powder having a certain degree of fluidity was obtained with the precursor solution adhering to the surface of lithium cobaltate, which was the positive electrode active material.

This wet powder is transferred to an eggplant flask with a cock (capacity: 200 ml), discharged in a sealed state to the outside of the glove box, heated in a 70 ° C. water bath, shaken and dried in a vacuum for 40 minutes, and fluidized. A high dry powder was obtained. This dry powder was put into the glove box again, removed from the eggplant flask, and spread on an alumina dish. Then, the dry powder was transferred to a drying furnace connected to a glove box so as not to come into contact with the atmosphere, and dried under vacuum at 200 ° C. for 12 hours to obtain a powdered coated positive electrode active material. Thereafter, the coated positive electrode active material was applied to a screen (aperture: 45 μm). Those that were oversized were crushed in a mortar and screened again. In addition, the coating amount was 0.38 mass% with respect to the positive electrode active material, and was 16.6 mg / m ² . Table 1 shows the evaluation results of the battery characteristics of the all-solid lithium secondary battery using the coated positive electrode active material of Example 1.

(Example 2)
In a glove box with a dew point adjusted to −80 ° C. or lower, 5 g of ethanol was added to 79 mg of lithium sulfide and 56 mg of sulfur, and the mixture was stirred for 10 minutes to dissolve, thereby preparing a lithium disulfide solution. Further, using this solution, a coated positive electrode active material was obtained in the same manner as in Example 1. The coating amount was 0.38 mass% with respect to the positive electrode active material, and was 16.6 mg / m ² . Table 1 shows the evaluation results of the battery characteristics of the all-solid lithium secondary battery using the coated positive electrode active material of Example 2.

(Example 3)
A coated positive electrode active material and an all-solid lithium secondary battery were produced in the same manner as in Example 2 except that the composition shown in Table 1 was used, and the battery characteristics were evaluated. The evaluation results are shown in Table 1.

Example 4
A coated positive electrode active material and an all-solid lithium secondary battery were produced in the same manner as in Example 2 except that the composition shown in Table 1 was used, and the battery characteristics were evaluated. The coating amount was 1.09 mass% with respect to the positive electrode active material, and was 47.2 mg / m ² . The evaluation results are shown in Table 1.

(Example 5)
A coated positive electrode active material and an all-solid lithium secondary battery were produced in the same manner as in Example 2 except that the composition shown in Table 1 was used, and the battery characteristics were evaluated. In addition, the coating material layer was formed in a part of positive electrode active material surface, and this coating amount was 0.12 mass% with respect to the positive electrode active material, and was 5.0 mg / m < ² >. The evaluation results are shown in Table 1.

(Example 6)
Vapor-grown carbon fiber (trade name “VGCF”, manufactured by Showa Denko KK) as a conductive additive is coated in an amount of 0.8 g so as to have a ratio of 4 parts by mass with respect to 100 parts by mass of the positive electrode active material. A coated positive electrode active material and an all-solid lithium secondary battery were produced in the same manner as in Example 1 except that they were blended in the agent layer, and the battery characteristics were evaluated. The evaluation results are shown in Table 1.

(Example 7)
As the positive electrode active material, LiNi _0.8 Co _0.15 Al _0.05 O ₂ (NCA) (specific surface area: 0.44 m ² / g) was used instead of lithium cobalt oxide, and the amount of lithium sulfide charged was shown in Table 1. A coated positive electrode active material and an all-solid lithium secondary battery were prepared in the same manner as in Example 2 except that the changes were made as shown in FIG. The coating amount was 0.38 mass% with respect to the positive electrode active material, and was 16.2 mg / m ² . The evaluation results are shown in Table 1.

(Example 8)
Using the coated positive electrode active material obtained by the vapor phase method, an all-solid lithium secondary battery was produced.
That is, 10 g of NCA similar to the positive electrode active material used in Example 7 was spread on an alumina dish, and exposed for 4 months at room temperature in a nitrogen atmosphere with a dew point adjusted to −80 ° C. or lower and a hydrogen sulfide concentration adjusted to 200 ppm. A coated positive electrode active material was obtained.

Using this coated positive electrode active material, an all-solid lithium secondary battery was produced in the same manner as in Example 1, and the battery characteristics were evaluated. In the evaluation, the cutoff voltage was 2-4 V, and the current density was 10 mA / cm ² .

(Comparative Example 1)
A secondary battery was produced in the same manner as in Example 1 except that 20 g of lithium cobaltate (0.23 m ² / g) dried at 200 ° C. under vacuum for 24 hours was used as the positive electrode active material. The battery characteristics were evaluated. The evaluation results are shown in Table 1.

(Comparative Example 2)
Instead of the solution obtained by adding 5 g of ethanol to lithium sulfide, a positive electrode active material treated with 2.9 g of ethanol was used (that is, lithium sulfide was not used). Similarly, secondary batteries were produced and the battery characteristics were evaluated. The evaluation results are shown in Table 1.

(Comparative Example 3)
A secondary battery was fabricated in the same manner as in Example 1 except that the NCA used in Example 7 as the positive electrode active material was used as the positive electrode active material without treatment (that is, lithium sulfide was not used). Was evaluated. The evaluation results are shown in Table 1.

  As is clear from Table 1, the all solid lithium secondary battery using the coated positive electrode active material obtained in Examples 1 to 7 was excellent in battery characteristics.

  The coated positive electrode active material obtained in Example 7 was remarkably excellent in battery characteristics because NCA was used as the positive electrode active material.

The charge / discharge curves of the batteries of Example 1 and Comparative Example 1 are shown in FIG. 3, and the charge / discharge curves of Example 8 and Comparative Example 3 are shown in FIG. From FIG. 3 and FIG. 4, when a positive electrode active material coated with a coating layer mainly composed of lithium sulfide is used, the discharge capacity at the time of charging is low and the electric capacity at the time of discharging is high. It is clear that an all-solid lithium secondary battery capable of exhibiting the battery characteristics can be obtained. As is apparent from FIG. 4, in Example 8 in which the coated positive electrode active material obtained by the vapor phase method was used, the battery characteristics were obtained even when discharged at a high current density of 10 mA / cm ^2. It is clear that this is superior.

  Further, from the comparison between Comparative Example 1 and Comparative Example 2, in the all solid lithium secondary battery including the positive electrode active material using alcohol as in Comparative Example 2, it is caused by the moisture contained in the alcohol. Thus, it is clear that the battery characteristics of the obtained all solid lithium secondary battery are further deteriorated.

Furthermore, the surface of the positive electrode active material obtained in Example 8 and Comparative Example 3 was confirmed by X-ray photoelectron spectroscopy (XPS), and sulfide ions (S ²⁻ ) were detected. The XPS measurement results are shown in FIG. As is clear from FIG. 5, a peak of sulfide ions is obtained in Example 8. That is, in Example 8, it is clear that sulfide is generated on the surface of the positive electrode active material and a coating layer is formed only by a simple operation of exposing hydrogen sulfide.

  In addition, according to XPS, an elemental composition analysis result at a depth of several nanometers from the surface can be obtained, but in Example 8, the analysis results of Ni, Co, and Al that are constituent elements of the positive electrode active material are bulky. Obtained by composition ratio. From this result, it is clear that the thickness of the formed coating layer mainly composed of lithium sulfide is smaller than several nm, that is, a positive electrode active material having excellent battery characteristics can be obtained even when the coating layer is thin. It is.

DESCRIPTION OF SYMBOLS 1 Positive electrode active material 2 Coating agent layer 3 Solid electrolyte layer 4 Positive electrode compound material layer 5 Negative electrode compound material layer 6 Solid electrolyte layer 7 Positive electrode collector 8 Negative electrode collector 9 Stainless support plate 10 Insulating film 11 Laminate film 12 Electricity derivation tag

Claims (6)

A coated positive electrode active material, wherein the surface of the positive electrode active material is coated with a coating layer containing a compound represented by the following general formula (I) as a main component.
Li _{2 Sn} (I)
In the above formula (I), n represents a number of 1 or more. The coated positive electrode active material according to claim 1, wherein the coating amount of the coating layer is ² to 50 mg / m ² with respect to the surface of the positive electrode active material.   The coated positive electrode active material according to claim 1 or 2, wherein the coating layer contains a conductive additive and the content ratio is 0.5 to 8 parts by mass with respect to 100 parts by mass of the positive electrode active material. .   A lithium secondary battery comprising the coated positive electrode active material according to any one of claims 1 to 3 and an electrolyte that is an inorganic solid sulfide composite. It is a method of manufacturing the covering positive electrode active material of any one of Claims 1-3, Comprising: The manufacturing of the covering positive electrode active material characterized by including the following process (i) and (ii) in this order. Method.
(I) A step of dissolving the compound represented by the general formula (I) in an organic solvent to obtain a precursor solution of the coating layer.
(Ii) A step of applying the precursor solution obtained in (i) above to the surface of the positive electrode active material and then drying the positive electrode active material with the precursor solution attached to the surface. It is a method of manufacturing the covering positive electrode active material of any one of Claims 1-3, Comprising: The compound shown by the following general formula (II) is made into a positive electrode active material, Hydrogen sulfide is formed on the surface of this positive electrode active material The manufacturing method characterized by including the process of exposing.
_{LiQ a R b S c O 2} (II)
In the formula (II), Q, R, and S are different from each other, and are selected from nickel, cobalt, aluminum, magnesium, vanadium, chromium, calcium, zirconium, niobium, molybdenum, tungsten, and manganese. Indicates. A, b and c each independently represent a numerical value of 0 to 1 and satisfy a + b + c = 1.

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