JP2024118761A - Lithium secondary battery - Google Patents

Abstract

To provide a lithium secondary battery having an excellent cycle characteristic.SOLUTION: A lithium secondary battery comprises: a positive electrode 20; a negative electrode 30; a separator 10 that separates the positive electrode and the negative electrode; a solid electrolyte layer 40 that is arranged to at least a part between the separator and the negative electrode; and a nonaqueous electrolyte that carries a conduction of a lithium ion between the negative electrode and the positive electrode. The solid electrolyte layer 40 contains a garnet type structure solid electrolyte. A average density of the solid electrolyte layer 40 is 0.5 g/cm3 or more and 3.6 g/cm3 or less. A lithium metal is deposited to between the negative electrode 30 and the solid electrolyte at a charging, and the lithium metal is melted at the discharging.SELECTED DRAWING: Figure 1

Description

The present invention relates to a lithium secondary battery.

Lithium secondary batteries can achieve high capacities and are widely used in a variety of applications, from mobile batteries for mobile phones and laptops to automobile batteries and large-scale power storage batteries.

Unlike lithium ion secondary batteries, which charge and discharge by inserting and extracting lithium ions into and from the materials that make up the electrodes, lithium secondary batteries charge and discharge by depositing and dissolving lithium metal (see, for example, Patent Document 1). Because lithium metal has an extremely base potential, it is expected that lithium secondary batteries will be able to achieve a high theoretical energy density.

In lithium secondary batteries, metallic lithium precipitates during charging. Metallic lithium may precipitate in a tree-like shape (forming a dendrite) with the starting point of precipitation as the root. Metallic lithium precipitated in a tree-like shape dissolves when the lithium secondary battery is discharged. There is no problem if the metallic lithium precipitated in a tree-like shape dissolves from the branches, but the root part may dissolve first. In this case, the metallic lithium that has lost its root floats in the electrolyte and loses conductivity. Metallic lithium floating in the electrolyte cannot be conductive and cannot contribute to subsequent charging and discharging. As a result, the cycle characteristics of the lithium secondary battery are reduced.

In addition, when the negative electrode of a lithium secondary battery comes into contact with the electrolyte, a side reaction occurs in which a coating is formed on the surface of the negative electrode. This side reaction may cause the electrolyte to decompose, and this side reaction also causes a decrease in the cycle characteristics of the lithium secondary battery.

Japanese Patent Application Publication No. 8-321310

The cycle characteristics of a battery are an important parameter, and there is a need to develop methods for improving the cycle characteristics of lithium secondary batteries, in addition to the method described in Patent Document 1.

This disclosure was made in consideration of the above problems, and aims to provide a lithium secondary battery with excellent cycle characteristics.

(1) A lithium secondary battery according to a first aspect includes a positive electrode, a negative electrode, a separator separating the positive electrode and the negative electrode, a solid electrolyte layer disposed at least partially between the separator and the negative electrode, and a non-aqueous electrolyte responsible for conducting lithium ions between the negative electrode and the positive electrode. The solid electrolyte layer includes a solid electrolyte having a garnet structure. The average density of the solid electrolyte layer is 0.5 g/ ^cm3 or more and 3.6 g/ ^cm3 or less. In the lithium secondary battery, lithium metal precipitates between the negative electrode and the solid electrolyte during charging, and the lithium metal dissolves during discharging.

(2) In the lithium secondary battery according to the above aspect, the solid electrolyte layer may have an average density of 0.7 g/cm ³ or more and 3.0 g/cm ³ or less.

(3) In the lithium secondary battery according to the above aspect, the average thickness of the solid electrolyte layer may be 5 μm or more and 30 μm or less.

(4) In the lithium secondary battery according to the above aspect, the solid electrolyte having a garnet structure may account for 90% by mass or more and 98% by mass or less of the solid electrolyte layer.

(5) In the lithium secondary battery according to the above aspect, the average particle size of the solid electrolyte having a garnet structure may be 0.1 μm or more and 10 μm or less.

(6) In the lithium secondary battery according to the above aspect, the area of a first surface of the separator facing the solid electrolyte layer may be larger than the area of a second surface of the solid electrolyte layer facing the separator.

The lithium secondary battery current collector according to the above embodiment can provide a lithium secondary battery with excellent cycle characteristics.

1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention. 2A to 2C are conceptual diagrams of the initial state, the charged state, and the discharged state of the lithium secondary battery according to the present embodiment.

The present embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show enlarged characteristic parts for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the present invention.

"First embodiment"
[Lithium secondary battery]
Fig. 1 is a cross-sectional schematic diagram of a lithium secondary battery according to a first embodiment. The lithium secondary battery 100 shown in Fig. 1 includes a power generating element 50, an exterior body 60, terminals 70, 72, and a non-aqueous electrolyte. The exterior body 60 covers the periphery of the power generating element 50 and houses the power generating element 50. The power generating element 50 is connected to the outside by a pair of terminals 70, 72 connected to the power generating element 50. The non-aqueous electrolyte is housed in the exterior body 60. Fig. 1 illustrates a case where one power generating element 50 is provided in the exterior body 60, but a plurality of power generating elements 50 may be stacked.

(Power generation element)
The power generating element 50 includes a separator 10, a positive electrode 20, a negative electrode 30, and a solid electrolyte layer 40. The separator 10 is between the positive electrode 20 and the negative electrode 30. The solid electrolyte layer 40 is at least partially between the separator 10 and the negative electrode 30. The power generating element 50 may be a laminate in which the separator 10, the positive electrode 20, the negative electrode 30, and the solid electrolyte layer 40 are laminated, or a wound body in which these laminates are wound.

<Positive electrode>
The positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22. The positive electrode active material layer 24 may be formed on both surfaces of the positive electrode current collector 22.

[Positive electrode current collector]
The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[Positive electrode active material layer]
The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive assistant and a binder as necessary.

The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, remove and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a compound of the general formula: LiNi _x Co _y Mn _z M _a O ₂ (wherein x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and a lithium titanate (Li _{4 Ti5O12} ), _{LiNixCoyAlzO2} (0.9<x+y+z< _1.1 ). The positive _electrode active material _may be an organic material. For example, the positive electrode active material _may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

The positive electrode active material may include, for example, any one selected from the group consisting of nickel, cobalt, manganese, and aluminum. The positive electrode active material may be, for example, a ternary compound including any one selected from the group consisting of nickel, cobalt, manganese, and aluminum. Nickel-cobalt-manganese lithium oxide (NCM) and nickel-cobalt-aluminum lithium oxide (NCA) are examples of ternary compounds. Ternary compounds can also be used at high potentials.

The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF ₃ , conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The lithium-free material may be any one of the materials or a combination of a plurality of materials. When the positive electrode active material is a lithium-free material, for example, discharge is performed first. Lithium is inserted into the positive electrode active material by discharging. In addition, lithium may be pre-doped chemically or electrochemically into the lithium-free positive electrode active material.

The conductive additive enhances the electronic conductivity between the positive electrode active materials. The conductive additive is, for example, carbon powder, carbon nanotubes, carbon material, metal powder, a mixture of carbon material and metal powder, or conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal powder is, for example, copper, nickel, stainless steel, iron, etc. The conductive oxide is, for example, indium tin oxide (ITO).

The content of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive assistant relative to the total mass of the positive electrode active material, the conductive assistant, and the binder is 0.5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 5% by mass or less.

The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Any known binder can be used. The binder is preferably one that does not dissolve in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. The binder is, for example, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, or a mixture thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so the energy density of the lithium secondary battery 100 will be low.

<Negative Electrode>
The negative electrode 30 has a negative electrode current collector. The negative electrode current collector contains a metal having excellent electrical conductivity. Examples of the metal having excellent electrical conductivity include copper, nickel, aluminum, iron, platinum, and gold. In addition, it is preferable that the metal used for the negative electrode current collector is a material that is difficult to form an alloy with lithium. For example, copper, nickel, titanium, stainless steel, platinum, and alloys thereof are difficult to form an alloy with lithium. The negative electrode current collector is, for example, copper.

During the charging process of the lithium secondary battery 100, lithium metal is deposited on the surface of the negative electrode current collector. During the discharging process of the lithium secondary battery 100, the lithium metal deposited on the surface of the negative electrode current collector dissolves in the non-aqueous electrolyte. That is, during charging of the lithium secondary battery, the negative electrode 30 includes the negative electrode current collector and lithium metal. In addition, in preparation for a shortage of the amount of lithium contributing to charging and discharging, a lithium foil may be provided on one side of the negative electrode current collector on the separator 10 side from the initial state before charging and discharging.

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30. The separator 10 prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 spreads in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched film of a mixture of polyethylene, polypropylene, etc. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be an inorganic-coated separator. The inorganic-coated separator is formed by applying a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica to the surface of the above-mentioned film. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

<Solid electrolyte layer>
The solid electrolyte layer 40 is disposed at least partially between the separator 10 and the negative electrode 30. The solid electrolyte layer 40 is in contact with the negative electrode 30, for example, with lithium metal sandwiched therebetween. The solid electrolyte layer 40 may also be in contact with the negative electrode 30, for example.

The solid electrolyte layer 40 is formed, for example, on one side of the negative electrode 30 on the separator 10 side. The solid electrolyte layer 40 prevents direct contact between the negative electrode 30 and the nonaqueous electrolyte.

The solid electrolyte layer 40 contains a solid electrolyte. The solid electrolyte layer does not allow the electrolyte solution to pass through, but allows only lithium ions to pass through. The solid electrolyte is, for example, an oxide-based solid electrolyte.

The solid electrolyte has a crystal structure of _a garnet type structure. Examples of the solid electrolyte of the garnet type structure _{are Li7La3Zr2O12} , Li _{(7-3x)AlxLa3Zr2O12 (} 0<x≦0.5), Li _{(7-x) La3Zr (2-x)TaxO12 ( 0} <x≦ _{1 )} , _{Li7La ( 3-x) YxZr2O12} (0<x≦1.5), Li _{(7-x) La3Zr ( 2 - x) NbxO12} ( ₀ <x≦ ₁ ), and Li _{(7-2x) La3Zr (2-x) WxO12} (0<x≦1). _The solid electrolyte having _a garnet type structure _is preferably _{Li6.25Al0.25La3Zr2O12 , Li6.75La3Zr1.75Ta0.25O12, or Li6.75La3Zr1.75Ta0.25O12 . The solid} electrolyte layer _{40 may} include _a solid electrolyte other _{than a} solid electrolyte having a garnet _{type crystal} structure.

The average particle size of the solid electrolyte with a garnet structure is, for example, 0.1 μm or more and 10 μm or less. The smaller the average particle size, the more grain boundaries there are between the solid electrolytes. The grain boundaries may inhibit the ion conduction of lithium. The larger the average particle size, the more the contact area between the solid electrolytes may decrease, which may inhibit the lithium ion conduction.

The average particle size of the solid electrolyte is determined by measuring the particle size of at least 100 solid electrolyte particles observed in cross-sectional images taken with a scanning electron microscope or a transmission electron microscope. Specifically, the particle size of each of the 100 solid electrolyte particles is measured, the frequency of particle sizes is graphed, and the most frequent value is taken as the average particle size. If the particles are irregular in shape, the diameter of the major axis is used to calculate the average particle size. The particle size determined from the cross-sectional image roughly coincides with the particle size measured using a particle size distribution measuring device.

The solid electrolyte layer 40 contains, for example, a binder in addition to the solid electrolyte. The binder may be the same as that described above. The solid electrolyte with a garnet structure preferably occupies, for example, 90% by mass or more and 98% by mass or less of the solid electrolyte layer 40. If the ratio of the solid electrolyte in the solid electrolyte layer 40 is low, the conductivity of lithium ions decreases. Also, if the ratio of the solid electrolyte in the solid electrolyte layer 40 is high, cracks and the like are likely to occur in the solid electrolyte layer 40. If the nonaqueous electrolyte reaches the negative electrode 30 through the cracks, a side reaction occurs between the negative electrode 30 and the electrolyte, and the cycle characteristics of the lithium secondary battery 100 decrease.

The average density of the solid electrolyte layer 40 is 0.5 g/cm ³ or more and 3.6 g/cm ³ or less. The average density of the solid electrolyte layer 40 is preferably 0.7 g/cm ³ or more and 3.0 g/cm ³ or less. If the average density of the solid electrolyte layer 40 is low, the nonaqueous electrolyte reaches the negative electrode 30, causing a side reaction between the negative electrode 30 and the electrolyte, and the cycle characteristics of the lithium secondary battery 100 deteriorate. In contrast, if the average density of the solid electrolyte layer 40 is high, the function of the nonaqueous electrolyte present in the pores of the solid electrolyte layer 40 to assist ion conduction is lost.

The area of the first surface S1 of the separator 10 facing the solid electrolyte layer 40 is, for example, larger than the area of the second surface S2 of the solid electrolyte layer 40 facing the separator 10. When viewed from the stacking direction of the power generating element 50, the second surface S1 is included in the first surface S1. By making the area S1 of the separator larger than the area S2 of the solid electrolyte layer, the separator 10 can reliably cover the solid electrolyte layer 40 and the positive electrode 20, improving the safety of the power generating element 50. In addition, the area of the negative electrode 30 when viewed from the stacking direction is preferably larger than, for example, the second surface S2 of the solid electrolyte layer 40, and is preferably 100% to 105% of the second surface S2 of the solid electrolyte layer 40.

The average thickness of the solid electrolyte layer 40 is, for example, 5 μm or more and 30 μm or less. The average thickness of the solid electrolyte layer 40 is obtained by measuring the film thickness at 10 different points in the plane in which the solid electrolyte layer 40 spreads from a cross-sectional image of a scanning electron microscope or a transmission electron microscope and averaging the measured film thicknesses. If the thickness of the solid electrolyte layer 40 is thin, the possibility that the non-aqueous electrolyte will reach the negative electrode 30 side when a crack or the like occurs in the solid electrolyte layer 40 increases. Also, if the thickness of the solid electrolyte layer 40 is too thick, the lithium ion conductivity of the solid electrolyte layer 40 is inferior to the lithium ion conductivity of the non-aqueous electrolyte, and the element resistance of the lithium secondary battery 100 increases.

<Non-aqueous electrolyte>
The non-aqueous electrolyte is enclosed in the exterior body 60 and is impregnated in the power generating element 50. The non-aqueous electrolyte is, for example, a liquid-based electrolytic solution. The electrolytic solution includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

The solvent is not particularly limited as long as it is a solvent generally used in lithium ion secondary batteries. The solvent includes, for example, any of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, a chain ester compound, and a chain ether compound. The solvent may include a mixture of these in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate, and the like. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Examples of the cyclic ester compound include γ-butyrolactone, and the like. Examples of the chain ester compound include propyl propionate, ethyl propionate, ethyl acetate, and the like. Examples of the chain ether compound include ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and the like.

The electrolytic salt is, for example, a lithium salt. The electrolyte is, for example, _LiPF6 , _{LiClO4 , LiBF4 , LiCF3SO3 , LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ) , LiN} ( _CF3CF2CO ) ₂ , LiBOB, LiN( _FSO2 ) ₂ , _etc. The lithium salt may _be used alone or in combination of _two or more. From the viewpoint of the degree _{of ionization} , it is preferable that the electrolyte contains _LiPF6 . The dissociation rate of the electrolytic salt in a carbonate solvent at room temperature is preferably 10% or more.

The electrolyte is preferably, for example, LiPF ₆ dissolved in a carbonate solvent. The concentration of LiPF ₆ is, for example, 1 mol/L. When the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit a charging behavior similar to that of soft carbon. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, lithium can be reacted with the polyimide uniformly. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate, or vinylene carbonate.

<Exterior body>
The power generating element 50 and the non-aqueous electrolyte are sealed inside the exterior body 60. The exterior body 60 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium secondary battery 100 from the outside.

As shown in FIG. 1, the exterior body 60 has a metal foil 62 and a resin layer 64 laminated on each side of the metal foil 62. The exterior body 60 is a metal laminate film in which the metal foil 62 is coated on both sides with a polymer film (resin layer 64).

The metal foil 62 may be, for example, aluminum foil. The resin layer 64 may be a polymer film such as polypropylene. The materials constituting the resin layer 64 may be different on the inside and outside. For example, the material on the outside may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the material for the polymer film on the inside may be polyethylene (PE), polypropylene (PP), etc.

<Terminals>
The terminals 72 and 70 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 72 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 70 connected to the negative electrode 30 is a negative electrode terminal. The terminals 70 and 72 are responsible for electrical connection between the power generating element 50 and the outside. The terminals 70 and 72 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 70 and 72 are preferably protected with insulating tape to prevent short circuits.

[Method of manufacturing lithium secondary battery]
A method for manufacturing the lithium secondary battery 100 according to this embodiment will be described below. First, the positive electrode 20 and the negative electrode 30 are fabricated.

The positive electrode 20 is prepared by applying paint containing a positive electrode active material onto the positive electrode current collector 22 and drying it. The paint containing the positive electrode active material contains the positive electrode active material, a binder, and a solvent, and is mixed with a conductive assistant as necessary. The solvent may be, for example, water, N-methyl-2-pyrrolidone, etc.

The composition ratio of the positive electrode active material, conductive material, and binder in the paint is preferably 80 wt% to 98 wt%: 0.1 wt% to 10 wt%: 0.1 wt% to 10 wt% by mass. These mass ratios are adjusted so that the total is 100 wt%. There are no particular restrictions on the method of mixing the components that make up the paint, and there are no particular restrictions on the mixing order either.

The prepared paint is applied to the positive electrode current collector 22. There are no particular limitations on the application method, and any method normally used for preparing electrodes can be used. Examples include the slit die coating method and the doctor blade method.

Then, the solvent in the paint applied to the positive electrode collector 22 is removed. There is no particular limitation on the method of removal. For example, the positive electrode collector 22 to which the paint has been applied may be dried in an atmosphere of 80°C or higher and 150°C or lower. Then, a positive electrode 20 is obtained in which a positive electrode active material layer 24 is formed on the positive electrode collector 22.

A commercially available negative electrode current collector can be used for the negative electrode 30. Lithium metal foil can be attached to one side of the negative electrode current collector. A publicly known separator 10 can be used. The solid electrolyte layer 40 can be produced by applying a paste made by mixing a powder of a solid electrolyte with a garnet structure and a binder to one side of the negative electrode 30, removing the solvent, and applying pressure. The average density of the solid electrolyte layer 40 can be adjusted by changing the pressure conditions.

Then, the separator is sandwiched between the positive electrode 20 and the negative electrode 30 so that the surface of the negative electrode 30 on which the solid electrolyte layer 40 is formed faces the separator 10. If there are multiple layers of positive electrodes 20 and negative electrodes 30, they are stacked so that the solid electrolyte layer 40 and the separator 10 are disposed between the positive electrode 20 and the negative electrode 30.

Next, the produced power generating element 50 is enclosed in an exterior body 60. A non-aqueous electrolyte is injected into the exterior body 60. After the non-aqueous electrolyte is injected, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 50. The lithium secondary battery 100 is obtained by sealing the exterior body 60 by applying heat, etc. Note that instead of injecting the electrolyte into the exterior body 60, the power generating element 50 may be impregnated with the electrolyte. After the electrolyte is injected into the power generating element, it is preferable to leave it for 24 hours.

The lithium secondary battery 100 according to this embodiment has excellent cycle characteristics. FIG. 2 shows the state near the negative electrode 30 during charging and discharging of the lithium secondary battery 100. FIG. 2 shows an image of the initial state 1, the charging state 2, and the discharging state 3 of the lithium secondary battery 100.

In the initial state 1 of the lithium secondary battery 100, the negative electrode 30 and the solid electrolyte layer 40 are in contact. When the lithium secondary battery 100 is charged, it goes from the initial state 1 to a charging state 2. In the charging state 2, lithium metal 80 precipitates between the negative electrode 30 and the solid electrolyte layer 40. The presence of the solid electrolyte layer 40 makes it difficult for the lithium metal 80 to precipitate locally, and dendrites are less likely to form.

Next, when the lithium secondary battery 100 is subjected to a discharge process, it goes from the charge state 2 to the discharge state 3. In the discharge state 3, the lithium metal 80 that was precipitated during charging dissolves. Not all of the lithium metal 80 dissolves, and some remains. In subsequent charge and discharge, the charge state 2 and the discharge state 3 are repeated.

Lithium ions can pass through the solid electrolyte layer 40, but non-aqueous electrolytes cannot pass through the solid electrolyte layer 40. Therefore, direct contact between the electrolyte and the negative electrode 30 is suppressed during the charge and discharge process of the lithium secondary battery 100. If the electrolyte comes into direct contact with the negative electrode 30, a side reaction occurs that decomposes the electrolyte.

As described above, the lithium secondary battery 100 according to this embodiment has excellent cycle characteristics because localized precipitation of lithium metal during charging is suppressed and direct contact between the electrolyte and the negative electrode 30 is suppressed.

The above describes the embodiments of the present invention in detail with reference to the drawings, but each configuration and their combinations in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention.

Example 1
First, a positive _electrode was prepared. NCM (composition formula: _{LiNi0.8Co0.1Mn0.1O2} ) was prepared as a positive _electrode active material, carbon black as a conductive material, and _PVDF as a binder. These were mixed in a solvent to prepare a paint, which was then applied onto a positive electrode current collector made of aluminum foil. The mass ratio of the positive electrode active material, conductive material, and binder was 95:2:3. After application, the solvent was removed.

Next _, a negative electrode was prepared. The negative electrode used a copper foil as a current collector. _A paste in which a garnet-type solid electrolyte _{Li6.25Al0.25La3Zr2O12} and a _binder were mixed in a mass ratio of 95 mass%:5 mass% was applied to the copper foil surface. The average particle size of the solid electrolyte was set to ₁ μm. The average particle size of the solid electrolyte was measured using a particle size distribution measuring device. The applied paste was then dried and pressed to form a solid electrolyte layer with an average density of 1.1 g/ ^cm3 and an average thickness of 15 μm.

The prepared positive and negative electrodes were then stacked with a separator between them to prepare a power generating element. The solid electrolyte layer was stacked so that it was located between the negative electrode and the separator. The number of stacked positive and negative electrodes was one. A laminate of polyethylene and polypropylene was used for the separator. The obtained power generating element was impregnated with an electrolyte solution and then enclosed in an exterior body. The electrolyte solution used was a mixture of ethylene carbonate (EC):diethyl carbonate (DEC) = 3:7 (volume ratio) in which lithium hexafluorophosphate (LiPF6) was dissolved to a concentration of 1 mol/L. The obtained lithium secondary battery was then charged and discharged to measure the cycle characteristics.

The cycle characteristics were measured using a secondary battery charge/discharge battery tester (manufactured by Hokuto Denko Corporation). The cycle characteristics were evaluated in an environment of 25°C. The cycle characteristics were evaluated by determining the number of cycles at which the discharge capacity became 70% or less of the initial discharge capacity, with one charge/discharge cycle being a constant current/constant voltage charge at 1/3C to 4.3V, and a constant current discharge at 1C to 3.0V.

(Examples 2 and 3)
Examples 2 and 3 differ from Example 1 in that the average density of the solid electrolyte layer was changed. The evaluation results of each configuration are shown in Table 1. In Examples 2 and 3, the cycle characteristics were obtained in the same manner as in Example 1 except for the conditions shown in Table 1.

(Examples 4 and 5)
Examples 4 and 5 differ from Example 1 in that the average thickness of the solid electrolyte layer was changed. The evaluation results of each configuration are shown in Table 1. In Examples 4 and 5, the cycle characteristics were determined in the same manner as in Example 1 except for the conditions shown in Table 1.

(Examples 6 to 8)
Examples 6 to 8 differ from Example 1 in that the weight ratio of the solid electrolyte to the binder in the solid electrolyte layer was changed. The evaluation results of each configuration are shown in Table 1. In Examples 6 to 8, the cycle characteristics were determined in the same manner as in Example 1, except for the conditions shown in Table 1.

Example 9
Example 9 differs from Example 1 in that the average density of the solid electrolyte layer was changed. The evaluation results of each configuration are shown in Table 1. In Example 9, the cycle characteristics were obtained in the same manner as in Example 1 except for the conditions shown in Table 1.

(Comparative Example 1)
Comparative Example 1 differs from Example 1 in that no solid electrolyte layer was formed on one side of the negative electrode. The evaluation results of each configuration are shown in Table 1. In Comparative Example 1, the cycle characteristics were determined in the same manner as in Example 1 except for the conditions shown in Table 1.

(Comparative Example 2)
Comparative Example 2 differs from Example 1 in that no separator was provided. The evaluation results of each configuration are shown in Table 1. In Comparative Example 2, the cycle characteristics were determined in the same manner as in Example 1 except for the conditions shown in Table 1.

(Comparative Example 3)
Comparative Example 3 differs from Example 1 in that the solid electrolyte constituting the solid electrolyte layer _was changed to NASICON type _{Li1.3Al0.3Ti1.7} ( _PO4 ) ₃ . The evaluation results of each configuration are shown in Table 1. In Comparative Example 3, _the cycle characteristics were obtained in the same manner as in Example 1 except for the conditions shown in Table 1.

(Comparative Examples 4 and 5)
Comparative Examples 4 and 5 differ from Example 1 in that the average density of the solid electrolyte layer was changed. The evaluation results of each configuration are shown in Table 1. In Examples 4 and 5, the cycle characteristics were obtained in the same manner as in Example 1 except for the conditions shown in Table 1.

All of Examples 1 to 9 had better cycle characteristics than Comparative Examples 1 to 5. Comparative Examples 2 and 4 did not function properly as batteries, and the cycle characteristics could not be measured.

10...separator, 20...positive electrode, 22...positive electrode current collector, 24...positive electrode active material layer, 30...negative electrode, 40...solid electrolyte layer, 50...power generating element, 60...exterior body, 70, 72...terminal, 80...lithium metal, 100...lithium secondary battery, 1...initial state, 2...charging state, 3...discharging state

Claims (6)

A positive electrode and
A negative electrode;
a separator separating the positive electrode and the negative electrode;
a solid electrolyte layer disposed at least partially between the separator and the negative electrode;
A non-aqueous electrolyte that is responsible for conducting lithium ions between the negative electrode and the positive electrode,
the solid electrolyte layer includes a solid electrolyte having a garnet structure,
The average density of the solid electrolyte layer is 0.5 g/ ^cm3 or more and 3.6 g/ ^cm3 or less,
A lithium secondary battery, wherein lithium metal is precipitated between the negative electrode and the solid electrolyte during charging, and the lithium metal dissolves during discharging. 2. The lithium secondary battery according to claim 1, wherein the solid electrolyte layer has an average density of 0.7 g/cm ^<3> or more and 3.0 g/cm ^<3> or less. The lithium secondary battery according to claim 1, wherein the average thickness of the solid electrolyte layer is 5 μm or more and 30 μm or less. The lithium secondary battery according to claim 1, wherein the solid electrolyte having a garnet structure occupies 90% by mass or more and 98% by mass or less of the solid electrolyte layer. The lithium secondary battery according to claim 1, wherein the average particle size of the solid electrolyte having a garnet structure is 0.1 μm or more and 10 μm or less. The lithium secondary battery according to claim 1, wherein the area of the first surface of the separator facing the solid electrolyte layer is greater than the area of the second surface of the solid electrolyte layer facing the separator.

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