Solid-state secondary battery and method for manufacturing a solid-state secondary battery

Abstract

To provide a solid secondary battery capable of obtaining a preferable durability with high output characteristic and high durability.SOLUTION: A solid secondary battery comprises a negative electrode layer, a positive electrode layer, and a solid electrolyte layer. In the solid electrolyte layer, a binding material is contained. The larger binding materials are contained in the side of the negative electrode layer and the positive electrode layer from a center side of a thickness direction of the solid electrolyte layer. In the solid electrolyte layer, a region where the binding material is not included may be provided to the center side of the thickness direction, and a content of the binding material is preferably sequentially changed so that the content of the binding material becomes larger as directed to the side of an electrode layer from the center side of the thickness direction of the solid electrolyte layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a solid-state secondary battery and a method for manufacturing a solid-state secondary battery. [Background technology]

[0002] Traditionally, secondary batteries such as lithium-ion batteries, which have high energy density, have been widely used. In recent years, the use of secondary batteries has been considered in various applications, such as in automobiles, from the perspectives of improving energy efficiency, mitigating adverse impacts on the global environment by increasing the proportion of renewable energy, and reducing CO2 emissions. A secondary battery has a structure in which a solid electrolyte (separator) is present between the positive electrode and the negative electrode, and the battery is filled with a liquid or solid electrolyte (electrolyte solution).

[0003] Solid-state rechargeable batteries using solid electrolytes offer higher thermal safety and can meet the demand for compactness compared to rechargeable batteries using liquid electrolytes. Solid-state rechargeable batteries are molded by mixing a binder with the electrode layer and solid electrolyte layer to form a slurry, which is then applied. For example, Patent Document 1 discloses a method for manufacturing a solid electrolyte sheet, which includes the steps of coating the surface of the skeletal part surrounding the voids of a porous substrate with an adhesive, and filling the voids with an inorganic solid electrolyte material. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6498335 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The technology described in Reference 1 has the problem that the adhesive (binding agent) coats the surface of the solid electrolyte, inhibiting lithium ion conduction and reducing the output characteristics and durability of the solid-state secondary battery. In addition, the presence of the adhesive (binding agent) inside the solid electrolyte layer causes grain boundaries to form, which can lead to cracks and thus reduce durability.

[0006] This invention has been made in view of the above problems, and aims to provide a solid-state secondary battery that has excellent output characteristics and durability characteristics, and that can achieve desirable durability. [Means for solving the problem]

[0007] (1) The present invention relates to a solid-state secondary battery having a negative electrode layer, a positive electrode layer, and a solid electrolyte layer, wherein the solid electrolyte layer contains a binder, and the binder is contained in greater quantities on the negative electrode layer side and the positive electrode layer side than on the central side in the thickness direction of the solid electrolyte layer.

[0008] According to the invention of (1), it is possible to provide a solid-state secondary battery that has excellent output characteristics and durability characteristics, and that can achieve desirable durability.

[0009] (2) The solid secondary battery according to (1), wherein the solid electrolyte layer is provided with a binder-free region in the central part in the thickness direction, in which the binder is not contained.

[0010] According to the invention of (2), the decrease in conductivity of the charge transfer medium due to the binder and the occurrence of cracks in the solid electrolyte layer can be preferably suppressed.

[0011] (3) The solid electrolyte layer comprises a first layer which is the layer on the negative electrode side or the positive electrode side of the solid electrolyte layer, and a second layer which is the layer on the central side in the thickness direction of the solid electrolyte layer, wherein the amount of the binder in the first layer is greater than the amount of the binder in the second layer, and the amount of the binder in at least one of the first layer and the second layer changes such that the amount of the binder increases towards the negative electrode side or the positive electrode side.

[0012] According to the invention of (3), it is possible to desirablely achieve both bonding between the electrode layer and the solid electrolyte layer and conductivity of the charge transfer medium.

[0013] (4) The present invention also relates to a method for manufacturing a solid secondary battery having an electrode layer and a solid electrolyte layer, comprising a coating step of applying an electrode mixture containing a binder to a solid electrolyte layer that does not contain a binder.

[0014] According to the invention of (4), a solid-state secondary battery can be easily manufactured in which a larger amount of binder is contained on the electrode layer side than on the central side in the thickness direction of the solid electrolyte layer.

[0015] (5) The method for manufacturing a solid secondary battery according to (4), wherein the coating step is performed by dip coating.

[0016] According to the invention of (5), a solid-state secondary battery can be easily manufactured in which a larger amount of binder is contained on the electrode layer side than on the central side in the thickness direction of the solid electrolyte layer. [Brief explanation of the drawing]

[0017] [Figure 1] This is a cross-sectional view showing the configuration of a solid-state secondary battery according to an embodiment of the present invention. [Figure 2] This graph shows the prevalence of binders in Figure 1. [Figure 3] This is a cross-sectional view showing the configuration of a conventional solid-state rechargeable battery. [Figure 4A]A graph showing the output characteristics of the solid secondary battery according to the examples and comparative examples of the present invention. [Figure 4B] A graph showing the durability characteristics of the solid secondary battery according to the examples and comparative examples of the present invention. [Figure 5A] A graph showing the binder occupancy on the positive electrode side of the solid secondary battery according to the examples and comparative examples of the present invention. [Figure 5B] A graph showing the binder occupancy on the negative electrode side of the solid secondary battery according to the examples and comparative examples of the present invention. [Embodiments for Carrying Out the Invention]

[0018] [Solid Secondary Battery] Hereinafter, the solid secondary battery 1 according to the embodiment of the present invention will be described. As shown in FIG. 1, the solid secondary battery 1 according to this embodiment includes a negative electrode layer 20 as an electrode layer, solid electrolyte layers 40a and 40b, and a positive electrode layer 30 as an electrode layer, which are laminated in this order. The solid secondary battery 1 is, for example, a lithium ion solid secondary battery that uses lithium ions as a charge transfer medium. Hereinafter, the solid secondary battery 1 will be described as a lithium ion solid secondary battery.

[0019] [Negative Electrode Layer] The negative electrode layer 20 is formed, for example, by forming a negative electrode composite layer on a negative electrode current collector 22. As shown in FIG. 1, the negative electrode composite layer includes a negative electrode active material 21, a binder 5, a conductive assistant 6, and a solid electrolyte 7.

[0020] The negative electrode active material 21 is not particularly limited, and a known material as a negative electrode active material of a solid secondary battery can be applied. Examples of the negative electrode active material 21 include lithium transition metal oxides such as lithium titanate (Li4Ti5O 12 ), transition metal oxides such as TiO2, Nb2O3, and WO3, metal sulfides, metal nitrides, carbon materials such as graphite, soft carbon, and hard carbon, and metal lithium, metal indium, and lithium alloys.

[0021] The negative electrode current collector 22 is not particularly limited, and any material known as a negative electrode current collector for a solid-state secondary battery can be used. Examples of the negative electrode current collector 22 include copper and stainless steel. The above-mentioned copper, stainless steel, etc., can be used, for example, in the form of foil.

[0022] The binder 5, acting as a binder, is included together with the negative electrode active material 21 in the slurry-like negative electrode mixture used when coating the negative electrode current collector 22 with the negative electrode mixture. This improves the bonding between the negative electrode mixture layer and the negative electrode current collector 22, and between the negative electrode mixture layer and the solid electrolyte layer 40a. Furthermore, the negative electrode layer 20 can be made thicker, increasing the amount of negative electrode active material 21 per unit area. On the other hand, since the binder 5 acts as a resistor in the operation of the solid secondary battery 1, increasing the amount of binder 5 added will increase the internal resistance of the battery and reduce the battery output. The content of binder 5 in the negative electrode mixture layer can be 0.1 wt% to 2.0 wt% of the total mass of the negative electrode mixture layer.

[0023] As binder 5, known binders used in solid-state secondary batteries can be used. Examples include nitrile polymers, polyester polymers, acrylic acid polymers, cellulose polymers, styrene polymers, styrene-butadiene polymers, vinyl acetate polymers, urethane polymers, fluoroethylene polymers, and the like.

[0024] As the conductive additive 6, known conductive additives used in solid-state secondary batteries can be used. Examples of conductive additives 6 include acetylene black, natural graphite, artificial graphite, carbon nanotubes (CNTs), and carbon nanofibers. The negative electrode layer 20 does not necessarily have to contain the conductive additive 6.

[0025] As the solid electrolyte 7, the same substance as the solid electrolyte 41 contained in the solid electrolyte layer described later can be used.

[0026] (Positive electrode layer) The positive electrode layer 30 is formed, for example, by forming a positive electrode composite material layer on a positive electrode current collector 32. As shown in Figure 1, the positive electrode composite material layer includes a positive electrode active material 31, a binder 5, a solid electrolyte 7, and a conductive additive.

[0027] The positive electrode active material 31 is not particularly limited, and any known material used as a positive electrode active material for a solid-state secondary battery can be applied. Examples of positive electrode active material 31 include LiCoO2, LiNiO2, LiCo x Ni y Mn z Layered cathode active material particles such as O2(x+y+z=1), LiVO2, LiCrO2, LiMn2O4, Li(Ni 0.25 Mn 0.75 Examples of positive electrode active materials include spinel-type positive electrode active materials such as 2O4, LiCoMnO4, and Li2NiMn3O8; olivine-type positive electrode active materials such as LiCoPO4, LiMnPO4, and LiFePO4; conductive polymers such as solid solution oxides (Li2MnO3-LiMO2 (M=Co, Ni, etc.)), polyaniline, and polypyrrole; sulfides such as Li2S, CuS, Li-Cu-S compounds, TiS2, FeS, MoS2, and Li-Mo-S compounds; and mixtures of sulfur and carbon. The positive electrode active material may consist of one of the above materials or a composition of two or more of the above materials.

[0028] The positive electrode current collector 32 is not particularly limited, and any material known as a positive electrode current collector for a solid-state secondary battery can be used. Examples of the positive electrode current collector 32 include aluminum and stainless steel. The above-mentioned aluminum and stainless steel can be used, for example, in the form of foil. In addition to the above, conductive carbon sheets (for example, graphite sheets or CNT sheets) may also be used.

[0029] The binder 5, solid electrolyte 7, and conductive additive contained in the positive electrode composite layer of the positive electrode layer 30 can be configured in the same way as the configuration in the negative electrode composite layer.

[0030] (solid electrolyte layer) The solid electrolyte layers 40a and 40b include a solid electrolyte 41 and a binder 5. Figure 1 shows the state of the solid electrolyte layer 40a formed on the negative electrode layer 20 and the solid electrolyte layer 40b formed on the positive electrode layer 30. A solid-state secondary battery 1 is obtained by overlapping the surface F1 of the solid electrolyte layer 40a and the surface F2 of the solid electrolyte layer 40b and joining them by pressing or the like. That is, the above surfaces F1 and F2 are surfaces located on the central side in the thickness direction of the entire solid electrolyte layer.

[0031] The solid electrolyte 41 is not particularly limited as long as it is a material capable of conducting lithium ions, and any material known as a solid electrolyte used in solid-state secondary batteries can be applied. Examples of solid electrolytes 41 include sulfide-based solid electrolytes, oxide-based solid electrolytes, nitride-based solid electrolytes, and halide-based solid electrolytes.

[0032] The solid electrolyte layers 40a and 40b contain a binder 5. The binder 5 can be of the same type as that contained in the negative electrode layer 20 and the positive electrode layer 30. In this embodiment, the binder 5 content of the solid secondary battery can be lower than in conventional designs. For example, the binder 5 content in the solid electrolyte layers 40a and 40b can be 3 wt% or less of the total mass of the solid electrolyte layers 40a and 40b.

[0033] Figure 2 schematically shows the presence rate X of the binder 5 content in the solid-state secondary battery 1. The vertical axis of Figure 2 corresponds to Figure 1 and indicates the position in the stacking thickness direction of the solid-state secondary battery 1. The horizontal axis of Figure 2 shows the binder presence rate X, with the right side of Figure 2 indicating a higher binder presence rate X.

[0034] As shown in Figure 2, the presence rate X of binder 5 is highest in the negative electrode layer 20 and the positive electrode layer 30, and remains almost constant. This ensures bonding between the negative electrode layer 20 and the solid electrolyte layer 40a, and between the positive electrode layer 30 and the solid electrolyte layer 40b. On the other hand, in the solid electrolyte layers 40a and 40b, the presence rate X of binder 5 decreases as it approaches the surfaces F1 and F2 from the negative electrode layer 20 and positive electrode layer 30 side. That is, the presence rate X of binder 5 is lowest in the center of the thickness direction of the solid electrolyte layers 40a and 40b. This reduces the occurrence of grain boundaries in the solid electrolyte layer and suppresses the occurrence of cracks in the solid electrolyte layer. In addition to the above, the total amount of binder 5 contained in the solid electrolyte layer can be reduced. Therefore, the output characteristics and cycle durability of the solid secondary battery 1 can be improved.

[0035] As shown in Figure 2, the solid electrolyte layer 40b includes a first layer R1, which is the layer on the positive electrode layer 30 side, and a second layer R2, which is the central layer in the thickness direction of the solid electrolyte layer 40b. The presence rate of binder 5 in the first layer R1 is higher than the presence rate of binder 5 in the second layer R2. Furthermore, the presence rate of binder 5 in the second layer R2 increases as it approaches the positive electrode layer 30 side. According to the manufacturing method of the solid-state secondary battery described later, it is possible to form a solid electrolyte layer having a layer such as the second layer R2 in which the presence rate of binder 5 increases as it approaches the electrode layer side. Alternatively, the first layer R1, which is the layer on the positive electrode layer 30 side, may be a layer in which the presence rate of binder 5 increases as it approaches the electrode layer side.

[0036] The binder 5 abundance X shown in Figure 2 is just an example; for example, the binder abundance X on surfaces F1 and F2 may be zero. That is, a binder-free region where binder 5 is absent may be provided on the central side in the thickness direction of the solid electrolyte layers 40a and 40b. This more preferably provides the effect of suppressing the occurrence of the cracks.

[0037] <Manufacturing method for solid-state rechargeable batteries> The method for manufacturing a solid-state secondary battery according to this embodiment includes a step of applying an electrode mixture containing a binder 5 as a binder to a solid electrolyte layer that does not contain a binder 5 as a binder. As a result, the binder 5 contained in the slurry-like electrode mixture gradually impregnates the solid electrolyte layer from the electrode layer side surface toward the center of the solid electrolyte layer. Therefore, a solid electrolyte layer can be formed in which the binder 5 is most abundant in the electrode layer side (first layer R1 or second layer R2), so that the binder 5 is least abundant in the center of the solid electrolyte layer.

[0038] The preferred method for applying the above electrode mixture to the solid electrolyte layer is by dip coating.

[0039] The method for manufacturing a solid-state secondary battery according to this embodiment includes the steps of: forming a solid electrolyte layer 40a by applying a solid electrolyte slurry containing a solid electrolyte without binder 5 to a negative electrode layer 20 which includes a negative electrode composite layer containing a negative electrode active material 21 and a binder 5, and a negative electrode current collector 22; similarly forming a solid electrolyte layer 40b by applying a solid electrolyte slurry containing a solid electrolyte without binder 5 to a positive electrode layer 30 which includes a positive electrode composite layer containing a positive electrode active material 31 and a binder 5, and a positive electrode current collector 32; and joining the surface F1 of the layered body of the negative electrode layer 20 and the solid electrolyte layer 40a and the surface F2 of the layered body of the positive electrode layer 30 and the solid electrolyte layer 40b by applying predetermined pressure and temperature. Note that the interface between surface F1 and surface F2 of a solid-state secondary battery manufactured by the above manufacturing method is not necessarily clearly distinguishable.

[0040] The above-described method for manufacturing a solid-state secondary battery is merely an example. The method for manufacturing a solid-state secondary battery may include the step of applying a negative electrode mixture slurry and a positive electrode mixture slurry to the layered solid electrolyte layers 40a and 40b, respectively. Alternatively, using a single solid electrolyte layer, the method may include the steps of applying a negative electrode mixture slurry to one side of the solid electrolyte layer, applying a positive electrode mixture slurry to the other side of the solid electrolyte layer, and joining current collectors to the formed electrode mixture layers.

[0041] (Conventional solid-state secondary batteries) Figure 3 is a cross-sectional view showing the configuration of a conventional solid-state secondary battery 1a. In the following description, components similar to those in Figure 1 are denoted by the same reference numerals as in Figure 3, and their descriptions may be omitted.

[0042] The negative electrode layer 20a, positive electrode layer 30a, and solid electrolyte layer 40 of the solid secondary battery 1a contain a binder 5 to ensure inter-layer bonding. Since each layer is bonded after the slurry has hardened, the binder 5 is uniformly distributed in each layer. In this configuration, regions R3 containing the binder 5 are formed on the surfaces of the negative electrode layer 20a and the positive electrode layer 30a. Similarly, regions R4 containing the binder 5 are formed on the surface of the solid electrolyte layer 40.

[0043] In regions R3 and R4 described above, the surfaces of solid electrolyte 7 and solid electrolyte 41 are covered with binder 5, which inhibits the conduction of lithium ions between the negative electrode layer 20a and positive electrode layer 30a and the solid electrolyte layer 40. As a result, the output characteristics and durability characteristics of the solid-state secondary battery 1a are reduced. Furthermore, the binder 5 covering the surface of solid electrolyte 41 inside the solid electrolyte layer 40 inhibits the conduction of lithium ions within the solid electrolyte layer 40. In addition, grain boundaries are formed inside the solid electrolyte layer 40, making it more susceptible to cracking.

[0044] On the other hand, in the solid secondary battery 1 according to the present embodiment, an amount of binder 5 necessary for ensuring the interlayer bonding property is contained between the electrode layer and the solid electrolyte layer, and the central side in the thickness direction of the solid electrolyte layer is configured to have a low content of binder 5. Thereby, while ensuring the interlayer bonding property, the conductivity of lithium ions can be improved, and the generation of cracks inside the solid electrolyte layer 40 can also be suppressed.

[0045] As described above, the preferred embodiments of the present invention have been explained. The present invention is not limited to the description of the above embodiments, and can be appropriately changed within the scope not departing from the gist of the present invention.

[0046] In the above embodiment, the content of the binder 5 in the solid electrolyte layer has been described as changing such that the content of the binder increases from the central side in the thickness direction of the solid electrolyte layer toward the electrode layer side. The content of the binder 5 in the solid electrolyte layer may change continuously or stepwise such that the content of the binder increases from the central side in the thickness direction of the solid electrolyte layer toward the electrode layer side.

Example

[0047] Hereinafter, the present invention will be described in more detail based on examples. The present invention is not limited by these examples.

[0048] <Example> A sulfide-based solid electrolyte was used as the solid electrolyte, graphite was used as the negative electrode active material, SUS foil was used as the negative electrode current collector, and LiCo x Ni y Mn z O2 (x + y + z = 1) was used as the positive electrode active material, Al (aluminum) foil was used as the positive electrode current collector, and an SBR (styrene-butadiene rubber) - based binder was used as the binder. The binder content in the negative electrode mixture layer was 1.0 wt% with respect to the total mass of the negative electrode mixture layer. After forming the negative electrode layer and the positive electrode layer, a solid electrolyte slurry having a binder content of 0 wt% was coated on the negative electrode layer and the positive electrode layer, respectively, to form a solid electrolyte layer, and then the solid electrolyte layers were joined to produce a solid secondary battery according to the example.

[0049] <Comparative Example> A solid-state secondary battery according to the comparative example was prepared in the same manner as in the example, except that the binder content of the solid electrolyte slurry was set to 3 wt%.

[0050] [Measurement of Binder Presence] The binder presence rate in the thickness direction of the solid-state secondary batteries in the examples and comparative examples was measured using the TOF-SIMS TOFSIMS.5 manufactured by IONTOF. The results are shown in Figures 5A and 5B. Figure 5A is a graph of the positive electrode 30, 30a side, and Figure 5B is a graph of the negative electrode 20, 20a side. In Figures 5A and 5B, the vertical axis represents the binder presence rate, with higher values ​​indicating a higher binder presence rate. In Figures 5A and 5B, the horizontal axis represents the distance in the thickness direction of the solid-state secondary battery. In Figure 5A, "30, 30a" indicates the positive electrode layer, "L1" indicates the positive electrode layer of the solid electrolyte layer, and "L2" indicates the central layer in the thickness direction of the solid electrolyte layer. In Figure 5B, "20, 20a" indicates the negative electrode layer, "L3" indicates the central layer in the thickness direction of the solid electrolyte layer, and "L4" indicates the negative electrode layer of the solid electrolyte layer.

[0051] As shown in Figures 5A and 5B, it was confirmed that in the solid-state secondary battery according to the example, there was more binder on the negative electrode side and positive electrode side than on the central side in the thickness direction of the solid electrolyte layer. In contrast, it was confirmed that in the solid-state secondary battery according to the comparative example, there was more binder on the central side in the thickness direction of the solid electrolyte layer than on the negative electrode side and positive electrode side.

[0052] [Output Characteristics Evaluation] Using the solid-state secondary batteries described in the examples and comparative examples, the resistance value (Ωcm) was measured after repeating the charge-discharge cycle 400 times. 2 The output characteristics of the solid-state secondary battery were evaluated by measuring the following. The results are shown in Figure 4A.

[0053] [Durability characteristics evaluation] The durability characteristics of the solid-state secondary batteries were evaluated by measuring the capacity retention rate (%) after repeating charge and discharge cycles 400 times using the solid-state secondary batteries described in the examples and comparative examples. The results are shown in Figure 4B.

[0054] The results shown in Figures 4A and 4B confirm that the solid-state secondary battery according to the example exhibits superior output characteristics and durability compared to the solid-state secondary battery according to the comparative example. [Explanation of Symbols]

[0055] 1 Solid state secondary battery 20 Negative electrode layer 30 Positive electrode layer 40a, 40b solid electrolyte layer 5. Binder (binding material)

Claims

1. A method for manufacturing a solid-state secondary battery, The aforementioned solid-state secondary battery has a negative electrode layer, a positive electrode layer, and a solid electrolyte layer. The solid electrolyte layer contains a binder, The binder is present in greater quantities on the negative electrode layer side and the positive electrode layer side than on the central side in the thickness direction of the solid electrolyte layer. The solid electrolyte layer is provided with a binder-free region in the central part of its thickness direction, where the binder is not contained. A step of forming the negative electrode layer containing the binder and the positive electrode layer containing the binder, A coating step to form a solid electrolyte layer by applying a solid electrolyte slurry containing a solid electrolyte but not containing the binder to the negative electrode layer and the positive electrode layer, respectively, A method for manufacturing a solid-state secondary battery, comprising a bonding step of bonding a solid electrolyte layer formed on the negative electrode layer and a solid electrolyte layer formed on the positive electrode layer.

2. The solid electrolyte layer is A first layer which is the layer on the negative electrode side or the positive electrode side of the solid electrolyte layer, It includes a second layer which is the central layer in the thickness direction of the solid electrolyte layer, The amount of the binder in the first layer is greater than the amount of the binder in the second layer. The method for manufacturing a solid-state secondary battery according to claim 1, wherein the content of the binder in at least one of the first layer and the second layer changes such that the content of the binder increases towards the negative electrode layer side or the positive electrode layer side.

3. The method for manufacturing a solid-state secondary battery according to claim 1, wherein the coating step is performed by dip coating.

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