JP2022148439A - solid secondary battery - Google Patents

Abstract

To provide a solid secondary battery including a solid electrolyte layer containing only an oxide solid electrolyte as the solid electrolyte, in which short-circuiting or the decrease in battery capacity due to separation of lithium can be suppressed sufficiently.SOLUTION: A solid secondary battery includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The solid electrolyte layer contains only an oxide solid electrolyte as the solid electrolyte. The binding strength between the negative electrode layer and the solid electrolyte layer is 14 mN/mm or more and 100 mN/mm or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to solid secondary batteries.

Examples of solid secondary batteries using lithium as a negative electrode active material include those using lithium deposited on the negative electrode layer by charging as an active material. In such a solid secondary battery, if the lithium deposited on the negative electrode layer side grows in the form of branches that weave through the gaps in the solid electrolyte layer, it not only causes a short circuit in the battery, but also reduces the battery capacity. cause a decline.

Therefore, as an all-solid secondary battery capable of suppressing the generation and growth of lithium dendrites in the solid electrolyte layer, the one disclosed in Patent Document 1 has been considered. In the all-solid secondary battery described in Patent Document 1, by using an alloying element that forms an alloy or compound with lithium as the negative electrode active material, lithium is occluded in the negative electrode active material layer at the initial stage of charging. After the charge capacity of the negative electrode active material layer is exceeded, lithium can be deposited inside the negative electrode active material layer or on the back side (current collector side) of the negative electrode active material layer. As a result, generation and growth of lithium dendrites in the solid electrolyte layer can be suppressed, and short circuits and reduction in battery capacity can be suppressed.

JP 2019-096610 A

However, when the negative electrode layer described in Patent Document 1 was used in combination with a solid electrolyte layer containing only an oxide-based solid electrolyte as the solid electrolyte, the expected short-circuit suppression effect could not be obtained. It is thought that there is room for improvement in suppressing lithium dendrites when used in combination with a solid electrolyte layer containing only an oxide-based solid electrolyte as the solid electrolyte.
Therefore, the present invention sufficiently suppresses the generation and growth of lithium dendrites even in a solid secondary battery having a solid electrolyte layer containing only an oxide-based solid electrolyte as a solid electrolyte, and suppresses short circuit and decrease in battery capacity. An object of the present invention is to provide a solid secondary battery capable of

As a result of intensive studies by the present inventors to solve the above-described problems, the present invention has been made by controlling the binding force between the negative electrode layer and the solid electrolyte layer to contain only an oxide-based solid electrolyte as the solid electrolyte. It was not completed until it was discovered that even a solid secondary battery having a solid electrolyte layer that has a solid electrolyte layer that is capable of exhibiting a sufficient short-circuit suppressing effect can be exhibited.

That is, a solid secondary battery according to the present invention includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, wherein the solid electrolyte layer is a solid electrolyte. It contains only an oxide-based solid electrolyte as a solid electrolyte, and is characterized in that the binding force between the negative electrode layer and the solid electrolyte layer is 14 mN/mm or more and 100 mN/mm or less.

According to the solid secondary battery configured in this way, since the binding force between the negative electrode layer and the solid electrolyte layer is set to 14 mN/mm or more and 100 mN/mm or less, the solid electrolyte layer containing the sulfide-based solid electrolyte Even when a solid electrolyte layer made of an oxide-based solid electrolyte harder than the solid electrolyte is used, the deposition of lithium between the negative electrode layer and the solid electrolyte layer is suppressed, and the generation and growth of lithium dendrites are sufficiently prevented. can be suppressed. It is possible to suppress the short circuit and capacity reduction of the solid secondary battery due to the generation and growth of lithium dendrites.

The negative electrode layer includes a plate-shaped or foil-shaped negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the film strength of the negative electrode active material layer is 16 MPa or more and 85 MPa. If the ratio is below, it is possible to suppress the occurrence of cracks in the negative electrode active material layer due to deposition of lithium in the negative electrode active material layer, which is preferable.

As a specific embodiment of the present invention, the negative electrode active material layer may contain a negative electrode active material that forms an alloy or compound with lithium.

According to the solid secondary battery according to the present invention, in the solid secondary battery in which lithium is deposited on the negative electrode layer by charging exceeding the initial charge capacity of the negative electrode active material layer, oxidation as a solid electrolyte forming the solid electrolyte layer Even when a solid electrolyte is used, it is possible to suppress short-circuiting and reduction in battery capacity due to deposition of lithium.

1 is a cross-sectional view showing a schematic configuration of a solid secondary battery according to one embodiment of the present invention; FIG. FIG. 4 is a schematic cross-sectional view showing how lithium is deposited when the solid secondary battery according to the present embodiment is charged.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant description.

<1. Basic Configuration of Solid Secondary Battery According to Present Embodiment>
The solid secondary battery according to this embodiment is an all-solid secondary battery 1 including a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30, as shown in FIG.

(1-1. Positive electrode layer)
The cathode layer 10 includes a cathode current collector 11 and a cathode active material layer 12 . Examples of the positive electrode current collector 11 include indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc ( Zn), aluminum (Al), germanium (Ge), lithium (Li), or a plate or foil made of alloys thereof. The positive electrode current collector 11 may be omitted.

The positive electrode active material layer 12 contains a positive electrode active material and a solid electrolyte. The solid electrolyte contained in the positive electrode layer 10 may be an oxide-based solid electrolyte described in the section of the solid electrolyte layer 30, or may be a sulfide-based solid electrolyte.

The positive electrode active material may be any positive electrode active material capable of reversibly intercalating and deintercalating lithium ions.

For example, the positive electrode active material includes lithium cobalt oxide (hereinafter referred to as LCO), lithium nickel oxide, lithium nickel cobalt oxide, and lithium nickel cobalt aluminum oxide (hereinafter referred to as NCA). , lithium salts such as nickel cobalt lithium manganate (hereinafter referred to as NCM), lithium manganate, lithium iron phosphate, nickel sulfide, copper sulfide, lithium sulfide, sulfur, iron oxide, Alternatively, it can be formed using vanadium oxide or the like. These positive electrode active materials may be used alone, or two or more of them may be used in combination.

Moreover, it is preferable that the positive electrode active material is formed by including a lithium salt of a transition metal oxide having a layered rock salt structure among the lithium salts described above. Here, the “layered rocksalt structure” means that oxygen atomic layers and metal atomic layers are alternately and regularly arranged in the <111> direction of the cubic rocksalt structure, and as a result, each atomic layer forms a two-dimensional plane. It is a structure that The term "cubic rock salt structure" refers to a sodium chloride structure, which is a type of crystal structure. represents a structure that is offset by 1/2 of the edge of .

Lithium salts of transition metal oxides having such a layered _rocksalt structure include, for example, _{LiNixCoyAlzO2 ( NCA} ) or _{LiNixCoyMnzO2 ( NCM} ) (where ₀ <x<1,0<y<1,0<z<1, and x+y+z=1), and lithium salts of ternary transition metal oxides.

When the positive electrode active material contains the lithium salt of the ternary transition metal oxide having the layered rock salt structure, the energy density and thermal stability of the all-solid secondary battery 1 can be improved.

The positive electrode active material may be covered with a coating layer. Here, the coating layer of the present embodiment may be of any type as long as it is known as a coating layer of a positive electrode active material for an all-solid secondary battery. Examples of coating layers include Li ₂ O—ZrO ₂ and the like.

Further, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity density of the all-solid secondary battery 1 can be increased, and metal elution from the positive electrode active material in a charged state can be reduced. Thereby, the all-solid secondary battery 1 according to the present embodiment can improve long-term reliability and cycle characteristics in a charged state.

Here, examples of the shape of the positive electrode active material include particle shapes such as spherical and elliptical. Moreover, the particle size of the positive electrode active material is not particularly limited as long as it is within a range applicable to the positive electrode active material of conventional all-solid secondary batteries. The content of the positive electrode active material in the positive electrode layer 10 is not particularly limited as long as it is within the range applicable to the positive electrode layer of the conventional all-solid secondary battery.

In addition to the above-described positive electrode active material and solid electrolyte, the positive electrode layer 10 also contains additives such as conductive aids, binders, fillers, dispersants, and ion conductive aids. may be

Examples of conductive aids that can be blended in the positive electrode layer 10 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of binders that can be blended in the positive electrode layer 10 include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Further, as fillers, dispersants, ion-conducting aids, etc. that can be blended in the positive electrode layer 10, known materials that are generally used for electrodes of all-solid secondary batteries can be used.

(1-2. Negative electrode layer)
The negative electrode layer 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22 laminated on the negative electrode current collector 21 . Negative electrode current collector 21 is preferably made of a material that does not react with lithium, that is, does not form either an alloy or a compound. Examples of materials forming the negative electrode current collector 21 include copper (Cu), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), and nickel (Ni). The negative electrode current collector 21 may be composed of any one of these metals, or may be composed of an alloy or clad material of two or more metals. The negative electrode current collector 21 is, for example, plate-like or foil-like.

The negative electrode active material layer 22 contains a negative electrode active material.
Examples of the negative electrode active material include those containing amorphous carbon and an alloying element that forms an alloy or compound with lithium through an electrochemical reaction during charging. Examples of the alloying element include at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc. Here, examples of amorphous carbon include carbon black and graphene. Examples of carbon black include acetylene black, furnace black, ketjen black, and the like. In order to improve electronic conductivity, the surface of silicon may be coated with a carbon layer having a thickness of about 1 nm or more and 10 nm or less.

Here, when any one or more of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, and zinc is used as the alloying element, these negative electrode active materials are, for example, in the form of particles. and the particle size thereof is preferably 4 μm or less, more preferably 300 nm or less. In this case, the characteristics of the all-solid secondary battery 1 are further improved. Here, as the particle diameter of the negative electrode active material, for example, a median diameter (so-called D ₅₀ ) measured using a laser particle size distribution system can be used.

In addition to the above, the negative electrode active material layer 22 contains additives used in conventional all-solid secondary batteries, such as binders, fillers, dispersants, ion conductors, solid electrolytes, etc., as appropriate. good too.

(1-3. Solid electrolyte layer)
The solid electrolyte layer 30 is formed between the positive electrode layer 10 and the negative electrode layer 20 and contains a solid electrolyte.

As the solid electrolyte, an oxide-based solid electrolyte is used in this embodiment.
Examples of oxide-based solid electrolytes include Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li _0.34 La _0.51 TiO _2.94 , Li _1.07 Al _0.69 Ti _{1.46 ( PO4 ) 3 , 50Li4SiO4-50Li2BO3 , 90Li3BO3-10Li2SO4 , Li2.9PO3.3N0.46 , Li7La3Zr2O12 _ _ _ _ _} etc. can be mentioned.

Solid electrolyte layer 30 may further contain a binder. Examples of binders include styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. The binder in the solid electrolyte layer 30 may be the same as or different from the binder in the positive electrode active material layer 12 and the negative electrode active material layer 22 .

In addition, in this embodiment, the solid electrolyte layer 30 uses the one containing only the above-described oxide-based solid electrolyte as the solid electrolyte. Furthermore, the solid electrolyte layer 30 of this embodiment consists of only an oxide-based solid electrolyte.

(1-4. Relation between charge capacity of positive electrode layer and negative electrode layer)
In the all-solid secondary battery 1 according to the present embodiment, the ratio between the charge capacity of the positive electrode active material layer 12 and the charge capacity of the negative electrode active material layer 22, that is, the capacity ratio, satisfies the requirements of the following formula (1). is configured to
0.01<b/a<0.5 (1)
a: charge capacity (mAh) of positive electrode active material layer 12
b: charge capacity (mAh) of negative electrode active material layer 22

Here, the charge capacity of the positive electrode active material layer 12 is obtained by multiplying the charge capacity density (mAh/g) of the positive electrode active material by the mass of the positive electrode active material in the positive electrode active material layer 12 . When a plurality of types of positive electrode active materials are used, the value of charge capacity density×mass is calculated for each positive electrode active material, and the sum of these values is taken as the charge capacity of the positive electrode active material layer 12 . The charge capacity of the negative electrode active material layer 22 is also calculated in a similar manner. That is, the charge capacity of the negative electrode active material layer 22 is obtained by multiplying the charge capacity density (mAh/g) of the negative electrode active material by the mass of the negative electrode active material in the negative electrode active material layer 22 . When multiple kinds of negative electrode active materials are used, the value of charge capacity density×mass is calculated for each negative electrode active material, and the sum of these values is used as the capacity of the negative electrode active material layer 22 . Here, the charge capacity densities of the positive electrode and negative electrode active materials are capacities estimated using an all-solid-state half-cell using lithium metal as the counter electrode. In practice, the charge capacities of the positive electrode active material layer 12 and the negative electrode active material layer 22 are directly measured by measurement using an all-solid half-cell.

Specific methods for directly measuring the charge capacity include the following methods. First, the charge capacity of the positive electrode active material layer 12 was measured by preparing a test cell using the positive electrode active material layer 12 as a working electrode and Li as a counter electrode, and performing CC-CV charging from OCV (open circuit voltage) to the upper limit charging voltage. Measure. The upper limit charging voltage is defined in the standard of JIS C 8712:2015. Refers to the voltage obtained by applying the provisions of 3.2.3 (Safety requirements when applying different upper limit charging voltages). Regarding the charge capacity of the negative electrode active material layer 22, a test cell was prepared using the negative electrode active material layer 22 as a working electrode and Li as a counter electrode, and CC-CV charging was performed from OCV (open circuit voltage) to 0.01 V. Measure.

The test cell described above can be produced, for example, by the following method. The positive electrode active material layer 12 or the negative electrode active material layer 22 whose charge capacity is to be measured is punched into a disk shape with a diameter of 13 mm. 200 mg of the same solid electrolyte powder as used in the all-solid secondary battery 1 is hardened at 40 MPa to form pellets having a diameter of 13 mm and a thickness of about 1 mm. This pellet is placed in a cylinder with an inner diameter of 13 mm, the positive electrode active material layer 12 or the negative electrode active material layer 22 punched out into a disk shape is placed from one side, and a lithium foil having a diameter of 13 mm and a thickness of 0.03 mm is placed from the other side. put in. Furthermore, one stainless steel disk is inserted from both sides, and the whole is pressurized in the axial direction of the cylinder at 300 MPa for 1 minute to integrate the contents. The integrated contents are taken out from the cylinder and enclosed in a case so that a pressure of 22 MPa is constantly applied to form a test cell. The charge capacity of the positive electrode active material layer 12 can be measured by, for example, CC-charging the test cell produced as described above at a current density of 0.1 mA and then CV-charging it to 0.02 mA. .

By dividing this charge capacity by the mass of each active material, the charge capacity density is calculated. The initial charge capacities of the positive electrode active material layer 12 and the negative electrode active material layer 22 may be initial charge capacities measured during the first cycle of charging. This value was used in the examples described later.

As described above, the charge capacity of the positive electrode active material layer 12 becomes excessively large with respect to the charge capacity of the negative electrode active material layer 22 . As will be described later, in the present embodiment, the all-solid secondary battery 1 is charged beyond the charge capacity of the negative electrode active material layer 22 . That is, the negative electrode active material layer 22 is overcharged. At the beginning of charging, lithium is occluded in the negative electrode active material layer 22 . That is, the negative electrode active material forms an alloy with lithium ions that have migrated from the positive electrode layer 10 . When charging is further performed beyond the capacity of the negative electrode active material layer 22, as shown in FIG. A lithium deposit layer 23 is formed by this lithium. The lithium deposition layer 23 is mainly composed of lithium (mainly metallic lithium), although it contains trace amounts of elements other than lithium.
Such a phenomenon occurs when the negative electrode active material contains a specific substance, that is, an alloying element that forms an alloy or compound with lithium. During discharge, lithium in the negative electrode active material layer 22 and lithium deposition layer 23 is ionized and moves toward the positive electrode layer 10 side.
Therefore, in the all-solid secondary battery 1 according to this embodiment, deposited lithium can be used as a negative electrode active material. Furthermore, since the negative electrode active material layer 22 covers the lithium deposition layer 23, it functions as a protective layer for the lithium deposition layer 23 and can suppress the deposition and growth of dendrites. As a result, the short circuit and capacity reduction of the all-solid secondary battery 1 are suppressed, and the characteristics of the all-solid secondary battery 1 are thereby improved.

wherein said capacitance ratio is greater than 0.01. If the capacity ratio is 0.01 or less, the characteristics of the all-solid secondary battery 1 deteriorate. One reason for this is that the negative electrode active material layer 22 does not sufficiently function as a protective layer. For example, when the negative electrode active material layer 22 is very thin, the capacity ratio can be 0.01 or less. In this case, repeated charging and discharging may cause the negative electrode active material layer 22 to collapse and dendrites to precipitate and grow. As a result, the characteristics of the all-solid secondary battery 1 deteriorate. In Patent Document 1, it is presumed that the characteristics of the all-solid secondary battery were not sufficiently improved because the interface layer or carbon layer was too thin. Also, the capacity ratio is preferably smaller than 0.5. This is because when the capacity ratio is 0.5 or more, the amount of lithium deposited on the negative electrode is reduced, and the battery capacity may be reduced. For the same reason, it is considered more preferable for the capacity ratio to be less than 0.25. Moreover, when the capacity ratio is less than 0.25, the output characteristics of the battery can be further improved.

The thickness of the negative electrode active material layer 22 is not particularly limited as long as it satisfies the requirements of the above formula (1), but is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. If the thickness of the negative electrode active material layer 22 is less than 1 μm, the characteristics of the all-solid secondary battery 1 may not be sufficiently improved. When the thickness of the negative electrode active material layer 22 exceeds 20 μm, the resistance value of the negative electrode active material layer 22 increases, and as a result, the characteristics of the all-solid secondary battery 1 may not be sufficiently improved.
The thickness of the negative electrode active material layer 22 described above can be estimated, for example, by assembling the all-solid secondary battery and observing the average thickness of the cross section after pressure molding with a scanning electron microscope (SEM).

<2. Characteristic configuration of the solid secondary battery according to the present embodiment>
Thus, the all-solid secondary battery 1 according to this embodiment is characterized in that the binding force between the negative electrode active material layer 22 and the solid electrolyte layer 30 is 14 mN/mm or more and 100 mN/mm or less. This binding force is more preferably 14 mN/mm or more and 98 mN/mm or less, and particularly preferably 20 mN/mm or more and 61 mN/mm or less.
The binding force between the negative electrode active material layer 22 and the solid electrolyte layer 30 can be measured by measuring the peel strength using AGS-X manufactured by Shimadzu Corporation.

Various methods are conceivable as a method for making the binding force between the negative electrode active material layer 22 and the solid electrolyte layer 30 within the range described above. is, for example, Sa (arithmetic mean height) in the range of 0.05 μm or more and 0.6 μm or less, more preferably 0.1 μm or more and 0.5 μm or less, particularly preferably 0.2 μm or more and 0.4 μm or less. A method can be mentioned. By adjusting the surface roughness of the contact surface of the solid electrolyte layer 30 with the negative electrode active material layer 22 in this manner, the contact area between the solid electrolyte layer 30 and the negative electrode active material layer 22 is increased, and the bond therebetween is increased. It can improve adhesion. The surface roughness of the solid electrolyte layer 30 can be controlled by the polishing conditions for the solid electrolyte layer 30 and the strength of the acid treatment, which will be described later. It can be controlled by the processing temperature or the like.

Further, the negative electrode active material layer 22 of the all-solid secondary battery 1 according to the present embodiment preferably has a film strength of 16 MPa or more and 85 MPa or less, more preferably 18 MPa or more and 82 MPa or less.
The film strength of the negative electrode active material layer 22 can be measured by measuring shear strength using SAICAS manufactured by Daipla Wintes Co., Ltd.
The film strength of the negative electrode active material layer 22 can be controlled, for example, by adjusting a combination of the type of amorphous carbon contained as the negative electrode active material, the type of binder, and the binder content. For example, when amorphous carbon having a relatively large specific surface area is used, the number of binders bound to one amorphous carbon tends to increase. need to be higher. In addition, if a string-like binder or the like with a large molecular weight is used, even if the content of the binder is kept low, it becomes easier for the binders to adhere to each other or between the binder and the amorphous carbon, so that the film strength can be increased. tend to be able to

In the present embodiment, binders that can be used for the negative electrode active material layer 22 include, for example, styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene difluoride, polyethylene, Examples include polymethacrylic methyl (PMMA) and the like. One of these binders may be used, or two or more of them may be used in combination.

A preferred binder content for achieving ideal film strength is 0.5% by mass or more and 30% by mass or less, and 3% by mass or more and 20% by mass or less with respect to the total mass of the negative electrode active material layer 22. It is more preferable to have
It is preferable that the content of the alloying element with respect to the entire negative electrode active material layer 22 is 5% by mass or more and 25% by mass or less, and the content of the amorphous carbon is 50% by mass or more and 90% by mass or less. is 8% by mass or more and 23% by mass or less, and the content of amorphous carbon is preferably 60% by mass or more and 86% by mass or less.

<3. Method for manufacturing a solid secondary battery according to the present embodiment>
Next, a method for manufacturing the all-solid secondary battery 1 according to this embodiment will be described. The all-solid secondary battery 1 according to the present embodiment can be manufactured by manufacturing the positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30, and then laminating the above layers.

(3-1. Positive electrode layer preparation process)
First, after mixing the materials (positive electrode active material, binder, etc.) constituting the positive electrode active material layer 12, they are laminated on the positive electrode current collector 11, and the obtained laminated body is pressurized (for example, pressurized using hydrostatic pressure). Then, the positive electrode layer 10 is produced. The pressurizing step may be omitted. The positive electrode layer 10 may be produced by compressing and molding a mixture of materials constituting the positive electrode active material layer 12 into a pellet or by stretching it into a sheet. When the positive electrode layer 10 is produced by these methods, the positive electrode current collector 11 may be pressure-bonded to the produced pellet or sheet. The positive electrode active material layer 12 is a slurry prepared by adding a material that constitutes the positive electrode active material layer to a nonpolar solvent (the slurry may be a paste; the same applies to other slurries). ) on the positive electrode current collector 11 and dried.

(3-2. Negative electrode layer preparation process)
First, slurry is prepared by adding negative electrode active material layer materials (negative electrode active material, non-alloying elements, binders, etc.) constituting the negative electrode active material layer 22 to a polar solvent or a nonpolar solvent. Then, the obtained slurry is applied onto the negative electrode current collector 21 and dried. Then, the obtained laminate is pressed (for example, pressurized using hydrostatic pressure) to fabricate the negative electrode layer 20 . The pressurizing step may be omitted. Alternatively, the negative electrode layer 20 may be manufactured by a method of stacking and pressurizing the negative electrode current collector 21 after forming the negative electrode active material layer 22 separately.

(3-3. Solid electrolyte layer preparation process)
The solid electrolyte layer 30 can be produced, for example, by the following procedures or steps.

The solid electrolyte layer 30 has a blending step of blending raw materials to obtain a blended material and a firing step of firing the obtained blended material. Here, as an example, a method for producing the solid electrolyte layer 30 made of a sintered body of garnet-type oxide will be described.

In the blending step, materials containing at least Li component, La component and Zr component are blended as raw materials to obtain a blended material.
Each component contained in the blended material has a component ratio that yields a lithium ion conductive ceramic material having a garnet-type crystal structure or a crystal structure similar to a garnet-type crystal structure.

Precursor powder is obtained by adding a sintering aid such as boron oxide to this compounded material and mixing the mixture with a ball mill, a jet mill, or the like.
Precursor pellets are obtained by putting the precursor powder thus obtained into a mold or the like and subjecting it to pressure molding.

Next, in the firing step, the precursor pellets molded as described above are heated at a temperature of about 900° C. to 1250° C. for about 1 hour to 36 hours. The sintering temperature and sintering time at this time can be appropriately changed depending on the composition of the materials.

A heating method is not particularly limited, and resistance heating, microwave heating, or the like can be used. The sintering process may be carried out in two steps of preliminary sintering and main sintering, or the above-mentioned forming process may be included in the sintering process, and an electric current sintering method, a discharge plasma sintering method, or the like may be adopted. You can make it work.

In this embodiment, the surface of the solid electrolyte layer 30 thus produced is acid-treated.
This acid treatment step can be performed using, for example, phosphoric acid. The concentration of phosphoric acid used for the phosphoric acid treatment is preferably 1 mol/L or more and 10 mol/L or less, more preferably 5 mol/L or more and 6 mol/L or less. The temperature during the acid treatment is preferably 20° C. or higher and 60° C. or lower, more preferably 30° C. or higher and 55° C. or lower. The acid treatment time is preferably 5 seconds or more and 10 minutes or less, more preferably 30 seconds or more and 5 minutes or less, and particularly preferably 1 minute or more and 2 minutes or less. The acid concentration, treatment temperature, treatment time, and the like used in the acid treatment can be appropriately changed depending on the material of the solid electrolyte layer to be used.

The surface roughness may be adjusted by polishing the surface of the solid electrolyte layer 30 before the acid treatment. Polishing is preferable because the solid electrolyte layer 30 is less likely to become brittle than when the surface roughness of the solid electrolyte layer 30 is adjusted only by acid treatment.
When the solid electrolyte layer 30 is polished, it is preferable, for example, to perform final polishing with a lapping film of #300 to #10000 after polishing with abrasive paper of #280 to #5000. As the abrasive paper, it is more preferable to use #300 or more and #2000 or less, and #600 or more and #1500 or less is particularly preferable. As the lapping film for final polishing, it is more preferable to use #400 or more and #4000 or less, and particularly preferably #600 or more and #2000 or less.

(3-4. Assembly process of all-solid secondary battery)
The positive electrode layer 10, the negative electrode layer 20, and the solid electrolyte layer 30 produced by the above method are laminated so that the solid electrolyte layer 30 is sandwiched between the positive electrode layer 10 and the negative electrode layer 20, and pressurized (for example, hydrostatic pressure The all-solid-state secondary battery 1 according to the present embodiment can be produced by performing pressurization using the same.

When operating the all-solid secondary battery 1 produced by the above method, it is preferable to operate the all-solid-state battery while applying pressure.

The pressure may be 0.5 MPa or more and 10 MPa or less. Further, pressure may be applied by a method such as sandwiching the all-solid-state battery between two hard plates of stainless steel, brass, aluminum, glass or the like and tightening the two plates with screws.

<4. Charging method for solid secondary battery according to the present embodiment>
Next, a method for charging the all-solid secondary battery 1 will be described. In this embodiment, as described above, the all-solid secondary battery 1 is charged beyond the charge capacity of the negative electrode active material layer 22 . That is, the negative electrode active material layer 22 is overcharged. At the beginning of charging, lithium is occluded in the negative electrode active material layer 22 . When charging is performed beyond the charge capacity of the negative electrode active material layer 22, for example, as shown in FIG. Lithium is deposited, and this lithium forms a lithium deposition layer 23 that was not present at the time of manufacture. During discharge, lithium in the negative electrode active material layer 22 and lithium deposition layer 23 is ionized and moves toward the positive electrode layer 10 side.
The charge amount is preferably between 2 times and 100 times the charge capacity of the negative electrode active material layer 22, more preferably between 4 times and 100 times the charge capacity.
The thickness of the lithium deposition layer 23 deposited in the negative electrode layer during charging is preferably 10 μm or more, more preferably 20 μm or more, and should be within the range of 60 μm or less, which is the upper limit that can be realized as a solid secondary battery. is preferred. The thickness of the lithium deposition layer 23 can be estimated by observing the average thickness of the cross section of the all-solid secondary battery 1 after charging with a scanning electron microscope (SEM).

<5. Effect of the present embodiment>
According to the all-solid secondary battery 1 configured as described above, since the binding force between the negative electrode active material layer 22 and the solid electrolyte layer 30 is 20 mN/mm or more and 100 mN/mm or less, the negative electrode active material layer 22 Lithium deposited by overcharging can be suppressed from depositing between the negative electrode active material layer 22 and the solid electrolyte layer 30 .

Moreover, since the film strength of the negative electrode active material layer 22 is 16 MPa or more and 85 MPa or less, deposition of lithium inside the negative electrode active material layer 22 can be suppressed.

As described above, according to the all-solid secondary battery 1 according to the present embodiment, lithium can be selectively deposited only between the negative electrode active material layer 22 and the negative electrode current collector 21 . As a result, the negative electrode active material layer 22 functions as a protective layer for the lithium deposition layer 23 deposited between the negative electrode current collector 21 and the negative electrode active material layer 22, thereby further effectively suppressing the deposition and growth of lithium dendrites. can be done.

For the reasons explained above, the all-solid secondary battery 1 according to the present embodiment is prevented from short-circuiting and capacity reduction, and as a result, the characteristics of the all-solid secondary battery are improved.

<6. Other Embodiments of the Present Invention>
The solid electrolyte layer is not limited to containing only the oxide-based solid electrolyte as described above, and may contain an oxide-based solid electrolyte and a binder or the like.

In the above embodiment, the all-solid secondary battery in which the positive electrode layer, the negative electrode layer, and the solid electrolyte layer are all solid has been described. Any secondary battery can be applied. For example, it can be applied to a solid secondary battery in which part or all of the positive electrode layer is not solid, a solid secondary battery containing an electrolytic solution in addition to a solid electrolyte, and the like. be.

Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, 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 belongs can conceive of various modifications or modifications within the scope of the technical idea described in the claims. It is understood that these also naturally belong to the technical scope of the present invention.

EXAMPLES The solid secondary battery according to the present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

<Production of solid secondary battery>
(Preparation of negative electrode layer)
For each example and comparative example, a mixed particle thin film of silver and carbon or a mixed particle thin film of silicon and carbon was produced as a negative electrode active material by the following method. Silver or silicon having a particle size of about 60 nm was used, and carbon black (2 types, CB-1 and CB-2) manufactured by Asahi Carbon Co., Ltd. and acetylene black (AB) manufactured by Denka Co., Ltd. were used. Put 6 g of carbon and 2 g of silver particles (or silicon particles) in a container, add an NMP solution containing a binder, and stir while adding NMP little by little to prepare a slurry. A blade coater was used to apply this slurry onto a negative electrode current collector made of SUS with a thickness of 10 microns. and PVdF, PMMA or SBR was used as the binder. Table 1 below shows the composition of the negative electrode active material layer for each example and comparative example. The ratio described in the column of composition of the negative electrode active material layer in Table 1 represents the mass ratio of each component. CB-1, which is the carbon mentioned above, has a specific surface area of about 50 m ² /g, CB-2 has a specific surface area of about 300 m ² /g, and AB has a specific surface area of about 50 m ² /g. It is a thing.

(Preparation of solid electrolyte layer)
For the solid electrolyte layer, an oxide-based solid electrolyte (LLZO) Pellet (Li7La3Zr2-xTaxO12 (LLZ-Ta) manufactured by Toyoshima Seisakusho Co., Ltd.) was phosphoricated with 5 mol/L phosphoric acid for 1 minute, and vacuum-dried. .
In Examples 15 and 16, the polishing treatment was performed before the acid treatment. More specifically, after polishing with #1000 abrasive paper, final polishing was performed with #600 lapping film. The grit of the abrasive paper and the lapping film is not limited to those described above, and similar effects can be obtained by using grit in the range of about ±20% of the grit mentioned above.

(Preparation of positive electrode layer)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ (NCA) was used as the positive electrode active material, Li ₆ PS ₅ Cl solid electrolyte, carbon nanofiber (CNF) as the conductive agent, and PTFE binder (manufactured by Daikin) as the binder. Materials:solid electrolyte:CNF:PTFE binder=88:12:2:1 (mass) were mixed and stretched into a sheet to form a positive electrode active material layer. An aluminum foil having a thickness of 10 μm was used as the positive electrode current collector, and the positive electrode layer was produced by laminating the above-described positive electrode active material layer on this aluminum foil.

(Production of solid secondary battery)
The positive electrode layer, the solid electrolyte layer, and the negative electrode layer thus produced were stacked in order, sealed in a laminate film in vacuum, and hydrostatically treated at 490 MPa for 30 minutes to produce an all-solid secondary battery. A part of each of the positive electrode layer and the negative electrode layer protruded from the laminate film so as not to break the vacuum of the battery, and the protruded parts were used as terminals of the positive electrode layer and the negative electrode layer, respectively.

<Measurement of binding force between negative electrode active material layer and solid electrolyte layer>
It was measured by measuring the peel strength using AGS-X manufactured by Shimadzu Corporation. Specifically, a negative electrode layer was laminated on one side of the solid electrolyte layer and subjected to hydrostatic pressure treatment in the same procedure as in the manufacturing process of the secondary battery described above. The 90° peel strength was measured when the was pulled and peeled. The peel strength was measured by calculating the average value of the measured values from the start of peeling to the end of peeling. For samples in which the negative electrode active material layer could not be peeled off from the surface of the solid electrolyte layer even when the negative electrode current collector was peeled off, an adhesive tape was attached to the surface of the negative electrode active material layer after the negative electrode current collector was peeled off. The 90° peel strength was measured when the pressure-sensitive adhesive tape was peeled off.

<Measurement of film strength of negative electrode active material layer>
It was measured by measuring the shear strength (deemed shear strength) using SAICAS manufactured by Daipla Wintes Co., Ltd. Specifically, the film strength of the negative electrode active material layer was measured by cutting the negative electrode active material layer with a diamond blade and measuring the shear strength at that time. Specific experimental conditions are as follows.
Cutting edge specifications: Clearance angle 10°, edge angle 60°, rake angle 20°, edge width 1mm
Cutting blade movement conditions: Horizontal 2 μm/sec, Vertical: 0.2 μm/sec

<Evaluation of charging current density characteristics>
The charging current density characteristics of the all-solid secondary battery thus produced were evaluated under the following conditions.
The measurement was performed by placing the all-solid secondary battery in a constant temperature bath at 25°C. Charging to 4.25 V and discharging to 2.5 V were repeated, and discharging after charging was performed at a constant current of 0.3 mA/cm ² . On the other hand, the current density was increased for each cycle of charging, 0.3mA/cm ² in the first cycle and then increased by 0.1mA/cm ² . At this time, the column of CCD (Critical Current Density) in Table 1 shows the maximum current that could be charged without causing a short circuit. A CCD of 2.6 mA/cm ² means that a charging current of up to 2.6 mA/cm ² can be achieved without a short circuit. The binding strength in Table 1 indicates the binding strength between the negative electrode active material layer and the solid electrolyte layer, and the film strength indicates the film strength of the negative electrode active material layer.

From the results in Table 1, in Examples 1 to 16 in which the binding force between the negative electrode active material layer and the solid electrolyte layer is in the range of 14 mN/mm or more and 100 mN/mm or less, the CCD value is 1.0. As described above, it was found that a charging current density suitable for practical use could be achieved.
On the other hand, in Comparative Examples 1 and 2, in which the binding force was outside the range of 14 mN/mm or more and 100 mN/mm or less, it was found that a short circuit occurred at a charging current density of less than 1.0.
From this, as long as the binding force between the negative electrode active material layer and the solid electrolyte layer is in the range of 14 mN/mm or more and 100 mN/mm or less, even when an oxide-based solid electrolyte is used, lithium It is considered that the short circuit of the solid secondary battery due to the deposition of can be sufficiently suppressed.
It should be noted that the higher the binding force, the more it is possible to suppress the deposition of lithium between the solid electrolyte layer and the negative electrode active material layer, which is considered preferable. put away. Therefore, it is more preferable that the range of the binding force is in the range of 14 mN/mm or more and 61 mN/mm or less as a range in which these are easily balanced. With the current technology, there arises a problem that the solid electrolyte becomes brittle when attempting to achieve a binding force exceeding 100 mN/mm. However, in the future, if a technique is developed that can increase the binding force of the solid electrolyte while minimizing embrittlement, it is possible that the upper limit will be exceeded and solid secondary batteries can be implemented.

From the results of Examples 1, 3 and 6, it can be seen that the binding force tends to increase as the acid treatment temperature for the solid electrolyte layer increases.
From the comparison of Example 1, Example 4, and Example 5, it was found that similar binding strength can be obtained even if the type of carbon contained in the negative electrode active material layer is changed.
From the results of Examples 5, 9, and 10 under almost the same conditions except for the type of binder used, similar binding strength was obtained even when the type of binder contained in the negative electrode active material layer was changed. I found out that I can.
Moreover, from the comparison between Example 2 and Example 15, and between Example 6 and Example 16, it was found that by polishing the surface of the solid electrolyte layer before the acid treatment, embrittlement of the solid electrolyte was suppressed as much as possible, and It was found that the adhesion can be further improved.
From these results, it can be seen that the binding force between the negative electrode active material layer and the solid electrolyte layer can be controlled by changing conditions such as the acid treatment temperature and polishing conditions for the solid electrolyte layer.

The surface roughness (Sa) of the surface of the solid electrolyte layer used in Example 1, which is in contact with the negative electrode active material layer, was measured and found to be 0.21 μm. Since the Sa of the solid electrolyte layer was 0.28 μm in Example 6 and the Sa of the solid electrolyte layer was 0.35 μm in Example 16, the binding force between the negative electrode active material layer and the solid electrolyte layer was It is believed that it can be changed by adjusting the surface roughness of the solid electrolyte layer.

Furthermore, among Examples 1 to 16, in Examples 1 to 7 and 12 to 16 in which the film strength of the negative electrode active material layer is 16 MPa or more and 85 MPa or less, the CCD is 1.5 or more, which indicates higher charging. It was found that charging is possible without short-circuiting even at current densities.

From the results of Examples 4 and 5, it is considered that the film strength can be controlled by the type of amorphous carbon used in the negative electrode active material layer.
Further, from a comparison of Example 6, Example 7, and Example 11, it can be seen that the film strength of the negative electrode active material layer tends to be increased by increasing the content of the binder in the negative electrode active material layer.
Moreover, from the results of Examples 5, 9, and 10, it can be seen that the film strength of the negative electrode active material layer also changes depending on the type of binder contained in the negative electrode active material layer.
From this result, it is considered that the film strength of the negative electrode active material layer can be controlled by changing the type of amorphous carbon used in the negative electrode active material layer, the type of binder, and the binder content.

1 solid secondary battery 10 positive electrode layer 11 positive electrode current collector 12 positive electrode active material layer 20 negative electrode layer 21 negative electrode current collector 22 negative electrode active material layer 23 lithium deposition layer 30 solid electrolyte layer

Claims (7)

A solid secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer,
The solid electrolyte layer contains only an oxide-based solid electrolyte as a solid electrolyte,
A solid secondary battery, wherein the binding force between the negative electrode layer and the solid electrolyte layer is 14 mN/mm or more and 100 mN/mm or less. The negative electrode layer includes a plate-shaped or foil-shaped negative electrode current collector and a negative electrode active material layer laminated on the negative electrode current collector, and the film strength of the negative electrode active material layer is 16 MPa or more and 85 MPa. 2. The solid secondary battery according to claim 1, wherein: 3. The solid secondary battery according to claim 2, wherein said negative electrode active material layer contains a negative electrode active material that forms an alloy or compound with lithium. an acid treatment step of acid-treating the solid electrolyte layer;
Assembly in which the solid electrolyte layer that has undergone the acid treatment step is laminated between the positive electrode layer and the negative electrode layer so that the binding force between the solid electrolyte layer and the negative electrode layer is 14 mN/mm or more and 100 mN/mm or less. A method for manufacturing a solid secondary battery, comprising the steps of: 5. The method of manufacturing a solid secondary battery according to claim 4, further comprising a polishing step of polishing the surface of said solid electrolyte layer. A method for charging the solid secondary battery according to any one of claims 1 to 3,
A method of charging a solid secondary battery, characterized in that charging is performed beyond the charging capacity of the negative electrode active material layer. 7. The solid secondary battery charging method according to claim 6, wherein charging is performed so that the thickness of the lithium deposition layer deposited on the negative electrode layer is in the range of 20 [mu]m to 60 [mu]m.

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