JP2013089321A - Lithium ion secondary battery and method for producing positive electrode active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a lithium ion secondary battery in which a reaction at an interface between a positive electrode active material and a solid electrolyte is suppressed furthermore, so that rate characteristics and cycle characteristics thereof can be further improved; and a method for producing a positive electrode active material for the lithium ion secondary battery.SOLUTION: A lithium ion secondary battery 100 includes: a positive electrode layer 110 including a positive electrode active material 111 particle; a negative electrode layer 120; and a sulfur-based solid electrolyte layer 130 held between the positive electrode layer and the negative electrode layer and including at least LiS and PS. In the lithium ion secondary battery, a surface of the positive electrode active material 111 particle is coated with aLiO-ZrO.

Description

  The present invention relates to a lithium ion secondary battery excellent in rate characteristics and cycle characteristics, and a method for producing a positive electrode active material for the lithium ion secondary battery.

  Lithium ion secondary batteries have large charge / discharge capacity, high operating potential, and excellent charge / discharge cycle characteristics, so portable information terminals, portable electronic devices, small household electric power storage devices, motorcycles powered by motors, There is an increasing demand for applications such as electric vehicles and hybrid electric vehicles. In a lithium ion secondary battery, a non-aqueous electrolyte solution in which a lithium salt is dissolved in an organic solvent is used as an electrolyte. Such a non-aqueous electrolyte solution is easy to ignite, leak of the electrolyte solution, etc. Because of this, safety is a concern. Therefore, in recent years, for the purpose of improving the safety of lithium ion secondary batteries, all solid-state lithium ion secondary batteries (hereinafter referred to as “all solid state secondary batteries”) using solid electrolytes made of inorganic materials that are non-combustible materials have been developed. Research) has been actively conducted.

  Although sulfides, oxides, and the like can be used as the solid electrolyte of the all-solid-state secondary battery, a sulfide-based solid electrolyte is the most promising material from the viewpoint of lithium ion conductivity. However, when a sulfide-based solid electrolyte is used, a reaction occurs at the interface between the positive electrode active material and the solid electrolyte during charging, and a resistance component is generated at this interface, so that the positive electrode active material and the solid electrolyte are Resistance (hereinafter also referred to as “interface resistance”) tends to increase when lithium ions move on the interface. Due to the increase in the interfacial resistance, the lithium ion conductivity is lowered, which causes a problem that the output of the lithium ion secondary battery is lowered.

In order to solve such a problem, it has been studied to reduce the interface resistance by coating the surface of a positive electrode active material such as LiCoO ₂ (hereinafter also referred to as “LCO”) with another material.

For example, Non-Patent Document 1 discloses a technique for coating LCO with SiO ₂ or Li ₂ SiO ₃ , and Non-Patent Document 2 discloses a technique for coating LCO with Li ₂ Ti ₂ O ₅ . Patent Documents 1 and 2 disclose a technique for coating a positive electrode active material such as LCO with ZrO ₂ . Furthermore, Patent Document 3 discloses a technique for coating the surface of a positive electrode active material with an oxide such as aluminum oxide, zirconium oxide, titanium oxide, boron oxide, or silicon oxide.

  In Patent Document 4 and Patent Document 5, instead of coating the particle surface of the positive electrode active material, the bias of lithium ions in the vicinity of the interface between these two layers between the positive electrode layer and the sulfide-based solid electrolyte layer is detected. A technique for providing a buffer layer for buffering and an intermediate layer for suppressing mutual diffusion between both layers is disclosed.

JP-T 2009-541938 Special table 2011-519139 gazette JP 2008-103204 A JP 2010-40439 A JP 2011-44368 A

J. et al. Power sources, 189, pp. 527-530, 2009 J. et al. Power sources, 195, pp. 599-603, 2010

However, like the techniques disclosed in Non-Patent Documents 1 and 2 and Patent Documents 1 to 5, the surface of the positive electrode active material is coated with an oxide such as SiO ₂ or the positive electrode layer and the solid electrolyte layer are formed. It is not sufficient to suppress the reaction at the interface between the positive electrode active material and the solid electrolyte simply by providing a buffer layer or an intermediate layer between them, and it is possible to further reduce the resistance component, and thus the rate as a battery. Improvement of characteristics and cycle characteristics is desired.

  Therefore, the present invention has been made in view of the above-described situation, and lithium ions capable of further suppressing the reaction at the interface between the positive electrode active material and the solid electrolyte and further improving rate characteristics and cycle characteristics. It aims at providing the manufacturing method of a secondary battery and the positive electrode active material for this lithium ion secondary battery.

As a result of intensive studies to solve the above problems, the present inventor has made the reaction at the interface between the positive electrode active material and the solid electrolyte by covering the surface of the positive electrode active material with aLi ₂ O—ZrO _2. It was found that the rate characteristics and cycle characteristics of the lithium ion secondary battery can be remarkably improved by using the positive electrode active material whose surface is coated in this way. The invention has been completed.

That is, according to an aspect of the present invention, a positive electrode layer including positive electrode active material particles coated with aLi ₂ O—ZrO ₂ (0.1 ≦ a ≦ 2.0), a negative electrode layer, and the positive electrode layer, A sulfur-based solid electrolyte layer sandwiched between the negative electrode layer and containing at least Li ₂ S and P ₂ S _5, and aLi with respect to the total amount of the positive electrode active material particles and the aLi ₂ O—ZrO _{2 A} lithium ion secondary battery in which the ratio of the coating amount of ₂ O—ZrO ₂ is 0.1 mol% or more and 2.0 mol% or less is provided.

  In the lithium ion secondary battery, the positive electrode active material is preferably a lithium salt of a transition metal oxide having a layered rock salt structure.

_Examples of the lithium salt of the transition metal oxide having the layered rock salt type structure include Li _1-x-yz Ni _x Co _y Al _z O ₂ or Li _1-x-yz Ni _x Co _y Mn _z O _2. A lithium salt of a ternary transition metal oxide represented by (0 <x <1, 0 <y <1, 0 <z <1, and x + y + z <1).

According to another aspect of the present invention, after mixing the positive electrode active material particles, lithium alkoxide and zirconium alkoxide in an alcohol solution, and evaporating and drying the alcohol while irradiating the obtained mixed solution with ultrasonic waves. The positive electrode active material for lithium ion secondary batteries is obtained by firing at a temperature of 750 ° C. or less to obtain positive electrode active material particles coated with aLi ₂ O—ZrO ₂ (0.1 ≦ a ≦ 2.0). A method is provided.

According to the present invention, by covering the surface of the positive electrode active material with aLi ₂ O—ZrO ₂ , the reaction at the interface between the positive electrode active material and the solid electrolyte can be further suppressed, and thereby rate characteristics In addition, it is possible to provide a lithium ion secondary battery that can further improve cycle characteristics, and a method for producing a positive electrode active material for the lithium ion secondary battery.

It is explanatory drawing which shows the mode of the increase in interface resistance in an all-solid-state secondary battery. It is explanatory drawing which shows typically the structure of the lithium ion secondary battery which concerns on suitable embodiment of this invention. aLi is a flowchart showing a flow of a method for manufacturing ₂ O-ZrO ₂ coated with the cathode active material. 6 is a graph showing evaluation results of charge / discharge characteristics of Example 2. 6 is a graph showing the evaluation results of charge / discharge characteristics of Comparative Example 1. 5 is a graph showing the evaluation results of impedance in Example 2. 10 is a graph showing the evaluation results of impedance in Comparative Example 1.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

[1. Problems when using solid electrolyte]
First, before describing a lithium ion secondary battery according to a preferred embodiment of the present invention, a problem when a solid electrolyte is used will be described with reference to FIG. FIG. 1 is an explanatory diagram showing an increase in interface resistance in an all-solid secondary battery.

  In an all-solid-state secondary battery using a solid electrolyte, since the positive electrode active material and the electrolyte are solid, the electrolyte is less likely to penetrate into the positive electrode active material than when an organic electrolyte is used as the electrolyte. Since the area of the interface with the electrolyte tends to decrease, it is difficult to secure a sufficient movement path for lithium ions and electrons. Therefore, as shown in FIG. 1, in the all-solid-state secondary battery 1, a positive electrode mixture containing mixed particles obtained by mixing particles of the positive electrode active material 11 and particles of the solid electrolyte 13 is used as a positive electrode material. By using a negative electrode mixture containing mixed particles obtained by mixing 12 particles and solid electrolyte 13 particles as the negative electrode material, the area of the interface between the active material and the solid electrolyte is increased.

  However, as described above, when a sulfide-based solid electrolyte is used as the solid electrolyte 13 of the all-solid secondary battery 1, a reaction occurs at the interface between the positive electrode active material 11 and the solid electrolyte 13 during charging. Since the resistance component is generated at the interface, the high resistance layer 15 is formed on the surface of the positive electrode active material 11, so that the interface resistance between the positive electrode active material 11 and the solid electrolyte 13 is likely to increase. Here, the “high resistance layer 15” is a layer made of a resistance component formed on the surface of the positive electrode active material 11 when the positive electrode active material 11 and the solid electrolyte 13 react with each other, and It means a layer having higher resistance when lithium ions move than inside the active material 11 or the solid electrolyte 13. As described above, when the area of the interface between the positive electrode active material 11 and the solid electrolyte 13 is increased, a movement path of lithium ions and electrons can be secured, but the high resistance layer 15 is easily formed. Then, the movement of lithium ions from the positive electrode active material 11 to the solid electrolyte 13 is hindered by the high resistance layer 15, and the lithium ion conductivity is lowered, so that the output of the all-solid secondary battery 1 is lowered. It was.

Therefore, in the lithium ion secondary battery according to a preferred embodiment of the present invention described below, the surface of the positive electrode active material is coated with aLi ₂ O—ZrO ₂ , so that the interface between the positive electrode active material and the solid electrolyte is obtained. As a result, the rate characteristics and cycle characteristics of the lithium ion secondary battery are remarkably improved.

[2. Configuration of lithium ion secondary battery]
Next, the configuration of the lithium ion secondary battery according to a preferred embodiment of the present invention will be described in detail with reference to FIG. FIG. 2 is an explanatory diagram schematically showing the configuration of the lithium ion secondary battery according to the present embodiment.

  As shown in FIG. 2, the lithium ion secondary battery 100 according to this embodiment includes a positive electrode layer 110, a negative electrode layer 120, a solid electrolyte layer 130 sandwiched between the positive electrode layer 110 and the negative electrode layer 120, Have a laminated structure.

[2.1. Positive electrode layer 110]
The positive electrode layer 110 includes particles of the positive electrode active material 111, and the surface of the positive electrode active material 111 is covered with a coating layer 113 made of aLi ₂ O—ZrO ₂ . An all-solid-state lithium ion secondary battery using a sulfur-based solid electrolyte has a problem that the interface resistance increases due to a reaction at the interface between the positive electrode active material and the solid electrolyte, and the output of the battery decreases. However, according to the lithium ion secondary battery 100 according to the present embodiment, the surface of the positive electrode active material 111 is covered with the coating layer 113 made of aLi ₂ O—ZrO ₂ , so that the coating layer 113 is solid. Since direct contact between the solid electrolyte particles 131 included in the electrolyte layer 130 and the positive electrode active material 111 can be prevented, a resistance component is hardly generated at the interface between the positive electrode active material 111 and the solid electrolyte 131. Further, when the surface of the positive electrode active material 111 is coated with aLi ₂ O—ZrO ₂ , a decrease in the lithium-in concentration at the interface between the positive electrode active material 111 and the solid electrolyte 131 is suppressed, and further, lithium ions are Since a movable path can be formed, it is also possible to suppress an increase in resistance at the interface between the positive electrode active material 111 and the solid electrolyte 131. For this reason, the lithium ion secondary battery 100 according to the present embodiment is excellent in rate characteristics and cycle characteristics.

Since aLi ₂ O—ZrO ₂ is chemically stable, if the surface of the positive electrode active material 111 is coated with aLi ₂ O—ZrO ₂ , the positive electrode active material 111 and the solid electrolyte 131 are in direct contact with each other. Since it can prevent, reaction in the interface of the positive electrode active material 111 and the solid electrolyte 131 is suppressed, and the production | generation of a resistance component can be suppressed.

  In addition, the positive electrode active material 111 should just be coat | covered with the coating layer 113 at least one part of the surface, and when the whole surface of the positive electrode active material 111 is coat | covered with the coating layer 113, the surface of the positive electrode active material 111 May be partially covered with the covering layer 113.

In addition, the “coating” in the present embodiment means that a state where aLi ₂ O—ZrO ₂ is arranged in a form that does not flow is maintained on the surface of the particles of the positive electrode active material 111. Further, in the present embodiment, the coating layer 113 covering the particle surface of the positive electrode active material 111 has a lithium ion conductivity and does not flow even when in contact with the positive electrode active material 111 or the solid electrolyte 131. Can be maintained.

In addition, the formation of the coating layer 113 made of aLi ₂ O—ZrO _{2 on} the particle surface of the positive electrode active material 111 means that, for example, the contrast due to the structural difference between the positive electrode active material 111 and the coating layer 113 is reduced. The difference can be confirmed by a method such as a microscopic image analysis (scanning electron microscope (SEM) or transmission electron microscope (TEM) image) analysis.

  Hereinafter, the positive electrode active material 111 and the coating layer 113 included in the positive electrode layer 110 will be described in detail.

(Positive electrode active material 111)
The positive electrode active material 111 included in the positive electrode layer 110 according to the present embodiment is not particularly limited as long as it is a material capable of reversibly occluding and releasing lithium ions. For example, lithium cobalt oxide (LCO), Lithium nickelate, lithium nickelcobaltate, lithium nickelcobaltaluminate (hereinafter also referred to as “NCA”), nickelcobalt lithium manganate (hereinafter also referred to as “NCM”), lithiummanganate , Lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, vanadium oxide and the like. These positive electrode active materials 111 may be used independently and 2 or more types may be used together.

The positive electrode active material 111 is particularly preferably a lithium salt of a transition metal oxide having a layered rock salt structure among the examples of the positive electrode active materials listed above. “Layered” as used herein means a thin sheet-like shape, and “rock salt structure” refers to a sodium chloride structure, which is a kind of crystal structure, and includes cations and anions. Each of the face-centered cubic lattices formed by each indicates a structure that is shifted from each other by a half of the edge of the unit lattice. As a lithium salt of a transition metal oxide having such a layered rock salt structure, for example, Li _1-x-yz Ni _x Co _y Al _z O ₂ (NCA) or Li _1-x-yz Ni _x Co _y Mn _z O ₂ (NCM) (0 <x <1, 0 <y <1, 0 <z <1, and x + y + z <1) is represented by a lithium salt of a ternary transition metal oxide. Can be mentioned.

  Thus, by using the lithium salt of the ternary transition metal oxide as the positive electrode active material 111, an all solid-state lithium ion battery excellent in energy density and thermal stability can be obtained. In addition, lithium salt particles (present as aggregates of primary particles) of ternary transition metal oxides such as NCA and NCM are smaller in particle size and larger in specific surface area than particles such as LCO, for example. (About 10 times). Therefore, the contact area between the positive electrode active material 111 and the solid electrolyte 131 is increased and the lithium ion conductivity is improved, so that the output of the battery is increased. Further, by including Ni as a constituent element of the positive electrode active material 111, the capacity density of the lithium ion secondary battery 100 is increased, and since the metal elution in the charged state is small, the lithium ion secondary battery 100 in the charged state is used. Can improve long-term reliability.

(Coating layer 113)
As described above, the coating layer 113 is a layer made of aLi ₂ O—ZrO ₂ and is coated on the particle surface of the positive electrode active material 111. By using aLi ₂ O—ZrO ₂ as a component of the coating layer 113 that covers the positive electrode active material 111, the effect of suppressing the reaction at the interface between the positive electrode active material 111 and the solid electrolyte 131 is dramatically improved. In particular, when aLi ₂ O—ZrO ₂ is used as a component of the coating layer 113, in addition to the effect of improving the cycle characteristics and impedance of the lithium ion secondary battery 100, other materials are used as the component of the coating layer 113. Rather, the initial discharge capacity can be dramatically increased. _{Further, aLi 2} O-ZrO 2 is excellent in lithium ion conductivity.

Here, aLi ₂ O—ZrO ₂ which is a component of the coating layer 113 is a composite oxide of Li ₂ O and ZrO ₂ , and a mixing ratio of these oxides, that is, a of a in the aLi ₂ O—ZrO ₂ . The range is preferably 0.1 ≦ a ≦ 2.0. By setting 0.1 ≦ a ≦ 2.0, not only the cycle characteristics and impedance improvement effect of the lithium ion secondary battery 100 but also the initial discharge capacity increase effect can be made more remarkable. .

aLi ₂ O—ZrO ₂ can be obtained by heating Li ₂ O and ZrO ₂ to a temperature at which both of them are melted, melting and mixing them at a predetermined ratio, holding them for a predetermined time, and then rapidly cooling them.

As the coating amount of the coating layer 113, the ratio of the coating amount of _{aLi 2} O-ZrO 2 to the total amount of the positive electrode active material 111 and _{aLi 2} O-ZrO 2 is more than 0.1 mol% 2.0 mol% or less Preferably there is. By setting the coating amount of the coating layer 113 within the above range, both a high initial discharge capacity and excellent cycle characteristics can be achieved. On the other hand, when the coating amount of the coating layer 113 is less than 0.1 mol%, the cycle characteristics tend to be inferior. When the coating amount of the coating layer 113 exceeds 2.0 mol%, the initial discharge capacity tends to decrease. is there.

(Other additives)
In addition to the particles of the positive electrode active material 111 whose surface is coated with the coating layer 113, for example, an additive such as a conductive agent, a binder, an electrolyte, a filler, a dispersant, and an ionic conductive agent is appropriately added to the positive electrode layer 110. It may be selected and blended.

  Examples of the conductive agent include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Is mentioned. As said electrolyte, the sulfur type solid electrolyte etc. which are mentioned later are mentioned. Moreover, as said filler, a dispersing agent, an ionic conductive agent, etc., the well-known substance normally used for the electrode of a lithium ion secondary battery can be used.

[2.2. Negative electrode layer 120]
(Negative electrode active material 121)
The negative electrode active material 121 included in the negative electrode layer 120 according to the present embodiment is not particularly limited as long as it is a material capable of alloying with lithium or reversibly occluding and releasing lithium, for example, lithium, Metals such as indium, tin, aluminum, silicon and alloys thereof, transition metal oxides such as Li _4/3 Ti _5/3 O ₄ , SnO, artificial graphite, graphite carbon fiber, resin-fired carbon, pyrolysis gas Examples include carbon materials such as phase-grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin calcined carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, and non-graphitizable carbon. . These negative electrode active materials 121 may be used independently and 2 or more types may be used together.

(Other additives)
In addition to the particles of the negative electrode active material 121, for example, additives such as a conductive agent, a binder, an electrolyte, a filler, a dispersant, and an ionic conductive agent may be appropriately selected and mixed in the negative electrode layer 120. Good. Specific examples thereof include the same substances as those of the positive electrode layer 110 described above.

[2.3. Solid electrolyte layer 130]
The solid electrolyte layer 130 according to the present embodiment contains a sulfur-based solid electrolyte containing at least Li ₂ S and P ₂ S ₅ as the solid electrolyte 131. This sulfur-based solid electrolyte is known to have higher lithium ion conductivity than other inorganic compounds. In addition to Li ₂ S and P ₂ S ₅ , sulfur such as SiS ₂ , GeS ₂ , B ₂ S _3, etc. It may contain things. The solid electrolyte layer 130 may be an inorganic solid electrolyte obtained by appropriately adding Li ₃ PO ₄ , halogen, a halogen compound, or the like to the sulfur-based solid electrolyte as the solid electrolyte 131.

In addition, the sulfur-based solid electrolyte is heated to a temperature higher than the temperature at which both of Li ₂ S and P ₂ S ₅ are melted, melted and mixed at a predetermined ratio, held for a predetermined time, and then rapidly cooled. Can be obtained (melt quenching method). Further, a sulfide containing Li ₂ S and _P 2 _{S 5} can be obtained by treating the mechanical milling (MM) method. The mixing ratio of the Li ₂ S and the sulfide containing P ₂ S ₅ is usually 50:50 to 80:20, preferably 60:40 to 75:25 in terms of molar ratio.

[3. Manufacturing method of lithium ion secondary battery]
The configuration of the lithium ion secondary battery 100 according to the preferred embodiment of the present invention has been described in detail above. Next, a method for manufacturing the lithium ion secondary battery 100 having the above-described configuration will be described. The lithium ion secondary battery 100 can be manufactured by preparing the positive electrode layer 110, the negative electrode layer 120, and the solid electrolyte layer 130, and then laminating these layers. Hereinafter, each process is explained in full detail.

[3.1. Preparation of positive electrode layer 110]
The manufacturing method of the positive electrode layer 110 is as follows. For example, a mixture of the positive electrode active material 111 whose surface is coated with aLi ₂ O—ZrO ₂ and various additives is added to a solvent such as water or an organic solvent to form a slurry or paste, and the resulting slurry or paste Is applied to a current collector using a doctor blade or the like, dried, and then consolidated with a rolling roll or the like, whereby the positive electrode layer 110 can be obtained.

The current collector that can be used at this time is, for example, a plate-like body made of indium, copper, magnesium, stainless steel, titanium, iron, cobalt, nickel, zinc, aluminum, germanium, lithium, or an alloy thereof. And foil. Note that the positive electrode layer 110 may be formed by compacting a mixture of the positive electrode active material 111 whose surface is coated with aLi ₂ O—ZrO ₂ and various additives into a pellet shape without using a current collector.

Here, a manufacturing method of the positive electrode active material 111 whose surface is coated with aLi ₂ O—ZrO ₂ will be described with reference to FIG. FIG. 3 is a flowchart showing a flow of a manufacturing method of the positive electrode active material 111 coated with aLi ₂ O—ZrO ₂ .

As shown in FIG. 3, first, lithium alkoxide and zirconium alkoxide are stirred and mixed in a solvent composed of an organic solvent such as alcohol and ethyl acetoacetate and water, and an alcohol solution of aLi ₂ O—ZrO ₂ (aLi ₂ O— A coating solution for ZrO ₂ coating is prepared (step S101). The lithium alkoxide can be obtained, for example, by reacting organic lithium with an alcohol. The time for stirring and mixing is not particularly limited, but may be, for example, about 30 minutes. Note that in a compound having a structure of CH ₃ —CO—CH ₂ —CO—O—R such as ethyl acetoacetate, two carbonyl groups in the structure act as a chelating agent to stabilize unstable metals. Since it is effective, it works here as a stabilizer for zirconium alkoxide.

Next, the coating liquid for coating aLi ₂ O—ZrO ₂ prepared in step S101 is mixed with the positive electrode active material 111 described above, and this mixed solution is heated to about 40 ° C. while stirring to evaporate a solvent such as alcohol. Dry (step S103). At this time, the mixed solution is irradiated with ultrasonic waves. Thereby, the precursor of aLi ₂ O—ZrO ₂ can be supported on the particle surface of the positive electrode active material 111.

Further, the precursor of aLi ₂ O—ZrO ₂ supported on the particle surface of the positive electrode active material 111 is fired (step S105). At this time, the firing temperature is set to 750 ° C. or lower. Moreover, although baking time is not specifically limited, What is necessary is just to be about 2 hours, for example. The firing is performed while blowing oxygen gas. By blowing oxygen gas, reduction of nickel in the positive electrode material containing nickel can be suppressed and capacity can be maintained.

As described above, the positive electrode active material 111 whose surface is coated with aLi ₂ O—ZrO ₂ can be obtained through the steps S101 to S105 (step S107).

[3.2. Preparation of negative electrode layer 120]
The manufacturing method of the negative electrode layer 120 is as follows. For example, a mixture of the negative electrode active material 121 and various additives is added to a solvent such as water or an organic solvent to form a slurry or paste, and the obtained slurry or paste is used as a current collector using a doctor blade or the like. After apply | coating and drying, the negative electrode layer 120 can be obtained by compacting with a rolling roll etc. FIG.

  The current collector that can be used at this time is, for example, a plate-like body made of indium, copper, magnesium, stainless steel, titanium, iron, cobalt, nickel, zinc, aluminum, germanium, lithium, or an alloy thereof. And foil. Note that the negative electrode layer 120 may be formed by compacting a mixture of the negative electrode active material 121 and various additives into a pellet shape without using a current collector. Moreover, when using a metal or its alloy as the negative electrode active material 121, you may use a metal sheet (foil) as it is.

[3.3. Preparation of solid electrolyte layer 130]
The method for producing the solid electrolyte layer 130 is as follows. As a method for producing a sulfur-based solid electrolyte used as the solid electrolyte 131, there are the above-described melt quenching method and mechanical milling method (MM method).

In the case of the melt quenching method, a mixture of a predetermined amount of Li ₂ S and P ₂ S ₅ and pelletized is reacted at a predetermined reaction temperature in a vacuum and then rapidly cooled to obtain a sulfur-based solid. An electrolyte can be obtained. The reaction temperature at this time is preferably 400 ° C to 1000 ° C, more preferably 800 ° C to 900 ° C. Moreover, reaction time becomes like this. Preferably it is 0.1 to 12 hours, More preferably, it is 1 to 12 hours. Furthermore, the quenching temperature of the reaction product is usually 10 ° C. or less, preferably 0 ° C. or less, and the cooling rate is usually about 1 to 10,000 K / sec, preferably 1 to 1000 K / sec.

In the case of the MM method, a predetermined amount of Li ₂ S and P ₂ S ₅ are mixed and reacted for a predetermined time by a mechanical milling method, whereby a sulfur-based solid electrolyte can be obtained. The mechanical milling method using the above raw materials has an advantage that the reaction can be performed at room temperature. According to the MM method, since a solid electrolyte can be produced at room temperature, the raw material is not thermally decomposed, and a solid electrolyte having a charged composition can be obtained.

  Although the rotation speed and rotation time of the MM method are not particularly limited, the higher the rotation speed, the higher the production rate of the solid electrolyte, and the longer the rotation time, the higher the conversion rate of the raw material to the solid electrolyte.

  Thereafter, the obtained solid electrolyte is heat-treated at a predetermined temperature and then pulverized to form a particulate solid electrolyte 131.

  The particulate solid electrolyte 131 thus obtained is subjected to a known film forming method such as a blast method, an aerosol deposition method, a cold spray method, a sputtering method, a vapor deposition method (CVD), or a thermal spraying method. The solid electrolyte layer 130 can be produced by forming a film using the film. Alternatively, a method may be used in which a solution in which the solid electrolyte 131 is mixed with a solvent or a binder (such as a binder or a polymer compound) is applied, and then the solvent is removed to form a film. In addition, the solid electrolyte 131 itself, the solid electrolyte 131 and a binder (binder, polymer compound, etc.) and a support (strengthening the strength of the solid electrolyte layer 130, or a material or compound for preventing a short circuit of the solid electrolyte 131 itself) Etc.) can also be formed by pressing an electrolyte.

[3.4. Lamination of each layer]
The lithium ion secondary battery 100 according to the present embodiment can be manufactured by laminating the positive electrode layer 110, the solid electrolyte layer 130, and the negative electrode layer 120 obtained as described above in this order, and pressing or the like. .

  Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

Example 1
First, lithium methoxide and zirconium (IV) propoxide were mixed in a mixed solution of ethanol, ethyl acetoacetate and water for 30 minutes. Next, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (hereinafter referred to as “NCM333”) manufactured by Nippon Kagaku Co., Ltd. as a positive electrode active material was added to this mixed solution as aLi ₂ O to NCM333. -ZrO ₂ (a = 1) coverage of is added to a 0.1 mol%, the solvent was evaporated to dryness with stirring and heating the mixture to 40 ° C.. At this time, ultrasonic waves were applied to the mixed solution. Furthermore, the precursor of Li ₂ O—ZrO ₂ supported on the surface of NCM333 was baked at 300 ° C. for 2 hours while blowing oxygen, and NCM333 with 0.1 mol% of Li ₂ O—ZrO ₂ coated on the surface. (Hereinafter referred to as “surface-coated NCM333”).

Moreover, In foil (thickness 0.05mm) used as a negative electrode was punched out with φ13 (mm) and set in a cell container. On top of that, 80 mg of a solid electrolyte Li2S-P2S5 (80-20 mol%) (SE) mechanically milled (MM treated) was laminated, and the surface was lightly adjusted with a molding machine. Further, a mixture of the surface-coated NCM333 obtained as described above, SE, and vapor-grown carbon fiber (VGCF) as a conductive agent at a ratio of 60/35/5% by mass is used as a positive electrode mixture. Was laminated on top of SE. In that state, a pressure was applied at a pressure of 3 t / cm ² to produce a pellet to obtain a test cell.

  The obtained test cell was charged at 25 ° C. with a constant current of 0.02 C to an upper limit voltage of 4.3 V, and after measuring the initial discharge capacity, 0.1 C was discharged to a discharge end voltage of 2.5 V, The charge / discharge was repeated in the same manner. The capacity retention rate relative to the initial capacity after 30 cycles was measured, and the cycle characteristics of the test cell were evaluated.

(Example 2)
A test cell was prepared in the same manner as in Example 1 except that the coating amount of aLi ₂ O—ZrO ₂ (a = 1) was 0.5 mol%, and the battery characteristics were evaluated.

(Example 3)
A test cell was prepared in the same manner as in Example 1 except that the coating amount of aLi ₂ O—ZrO ₂ (a = 1) was 2 mol%, and the battery characteristics were evaluated.

Example 4
A test cell was prepared in the same manner as in Example 2 except that Li _1/3 Ni _1/3 Co _1/3 Al _1/3 O ₂ (NCA) was used as the positive electrode active material, and the battery characteristics were evaluated. did.

(Example 5)
A test cell was prepared in the same manner as in Example 2 except that LiCoO ₂ (LCO) was used as the positive electrode active material, and the battery characteristics were evaluated.

(Example 6)
A test cell was prepared in the same manner as in Example 2 except that a in aLi ₂ O—ZrO ₂ was set to 0.1, and battery characteristics were evaluated.

(Example 6)
A test cell was produced in the same manner as in Example 2 except that a in aLi ₂ O—ZrO ₂ was set to 2, and battery characteristics were evaluated.

(Comparative Example 1)
A test cell was prepared in the same manner as in Example 1 except that the positive electrode active material (NCM333) was not surface-coated, and the battery characteristics were evaluated.

(Comparative Example 2)
A test cell was prepared in the same manner as in Example 1 except that the coating amount of aLi ₂ O—ZrO ₂ (a = 1) was 0.05 mol%, and the battery characteristics were evaluated.

(Comparative Example 3)
A test cell was prepared in the same manner as in Example 1 except that the coating amount of aLi ₂ O—ZrO ₂ (a = 1) was 3 mol%, and battery characteristics were evaluated.

(Comparative Example 4)
A test cell was prepared in the same manner as in Example 2 except that the material for coating the surface of the positive electrode active material (NCM333) was Li ₂ SiO _3, and the battery characteristics were evaluated.

(Comparative Example 5)
A test cell was prepared in the same manner as in Example 2 except that the material to be coated on the surface of the positive electrode active material (NCM333) was Li ₂ Ti ₂ O _5, and the battery characteristics were evaluated.

(Comparative Example 6)
A test cell was prepared in the same manner as in Example 2 except that a in aLi ₂ O—ZrO ₂ was set to 0, that is, the material coated on the surface of the positive electrode active material (NCM333) was ZrO _2. The battery characteristics were evaluated.

(Comparative Example 7)
A test cell was prepared in the same manner as in Example 2 except that a in aLi ₂ O—ZrO ₂ was set to 2.5, and battery characteristics were evaluated.

  The positive electrode active materials used in Examples 1 to 7 and Comparative Examples 1 to 7, the material of the coating layer, the coating amount, and the evaluation results of the obtained battery characteristics are shown in Table 1 below.

As shown in Table 1, the surface of the positive electrode active material was coated with aLi ₂ O—ZrO ₂ (0.1 ≦ a ≦ 2), and the coating amount was 0.1 to ₂ mol%. In all cases, a high initial discharge capacity was obtained and cycle characteristics were excellent.

On the other hand, when the surface of the positive electrode active material was not coated or the coating amount was small, the initial discharge capacity was low and the cycle characteristics were inferior. Moreover, when the coating amount of the coating layer on the positive electrode active material was too large, or when the material of the coating layer was a material other than aLi ₂ O—ZrO ₂ , the initial discharge capacity was low. Moreover, when the material of the coating layer was ZrO ₂ , the cycle characteristics were inferior. _Furthermore, the amount of Li ₂ O in aLi ₂ O-ZrO 2 is also too large to become a initial discharge capacity is low.

(Evaluation of charge / discharge characteristics and impedance)
Next, with reference to FIGS. 4A, 4B, 5A, and 5B, the results of evaluating the charge / discharge characteristics, impedance, and rate characteristics of the test cells of Example 2 and Comparative Example 1 will be described. 4A and 4B are graphs showing the evaluation results of the charge / discharge characteristics of Example 2 and Comparative Example 1, respectively. 5A and FIG. 5B are graphs showing the impedance evaluation results of Example 2 and Comparative Example 1, respectively.

  Regarding the charge / discharge characteristics, the cut-off voltage was set in the range of 4.3 V during charge and 2.5 V during discharge, and measured by a constant current method to evaluate the charge / discharge characteristics during initial charge and after 30 cycles. Moreover, the impedance evaluated the impedance at the time of initial charge and 30 cycles after by the alternating current impedance method.

As shown in FIGS. 4A, 4B, 5A, and 5B, the test cell of Example 2 in which the surface of the positive electrode active material was coated with aLi ₂ O—ZrO ₂ was subjected to surface coating treatment of the positive electrode active material. It can be seen that the increase in impedance is clearly suppressed as compared with Comparative Example 2 that is not known, and as a result, the charge / discharge characteristics are improved.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 100 Lithium ion secondary battery 110 Positive electrode layer 111 Positive electrode active material 113 Cover layer 120 Negative electrode layer 121 Negative electrode active material 130 Solid electrolyte layer 131 (Sulfur system) Solid electrolyte

Claims (4)

a positive electrode layer comprising positive electrode active material particles coated with aLi ₂ O—ZrO ₂ (0.1 ≦ a ≦ 2.0);
A negative electrode layer;
A sulfur-based solid electrolyte layer sandwiched between the positive electrode layer and the negative electrode layer and containing at least Li ₂ S and P ₂ S ₅ ;
With
The positive active ratio of material particles and the _{aLi 2} O-ZrO 2 of _ALI to the total amount of 2 O-ZrO ₂ coating amount is less 0.1 mol% or more 2.0 mol%, a lithium ion secondary battery.   The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is a lithium salt of a transition metal oxide having a layered rock salt structure. The lithium salt is Li _1-x-yz Ni _x Co _y Al _z O ₂ or Li _1-x-yz Ni _x Co _y Mn _z O ₂ (0 <x <1, 0 <y <1 The lithium ion secondary battery according to claim 2, which is a lithium salt of a ternary transition metal oxide represented by 0 <z <1 and x + y + z <1). The positive electrode active material particles, lithium alkoxide, and zirconium alkoxide are mixed in an alcohol solution, and the alcohol is evaporated and dried while irradiating the obtained mixed solution with ultrasonic waves, followed by firing at a temperature of 750 ° C. or less, and aLi _{2 O-ZrO} 2 to obtain a coated positive electrode active material particles (0.1 ≦ a ≦ 2.0), the method for producing a positive electrode active material for a lithium ion secondary battery.

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