Secondary battery

Abstract

To solve the problem in a secondary battery including a solid electrolyte between a negative electrode and a positive electrode, in which the capacity decreases due to a charging and discharging cycle, that is, cycle deterioration, and realize a secondary battery in which the deterioration in cycle characteristic of the secondary battery due to volume change of the electrode can be suppressed.SOLUTION: A secondary battery includes a current collector 1, a positive electrode 3, and a solid electrolyte 4 that are stacked in order, and includes a conductive layer 2 held between the current collector 1 and the positive electrode 3. Here, the positive electrode 3 is a conversion electrode, and the conductive layer 2 has lower conductivity than the current collector 1. When a central part and an end part of the conductive layer 2 in a direction along a surface of the current collector 1 that faces the conductive layer 2 are compared, the thickness of the central part of the conductive layer 2 is larger than the thickness of the end part of the conductive layer 2.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a secondary battery comprising a solid electrolyte and a conductive layer between an electrode and a current collector. [Background technology]

[0002] In recent years, the use of lithium-ion secondary batteries has expanded as rechargeable secondary batteries. Among them, solid-state batteries (semi-solid-state batteries) that use solid electrolytes as electrolytes are highly safe because they use less flammable electrolyte. In addition, solid-state batteries have the theoretical possibility of achieving high energy density, so many companies are conducting research into them.

[0003] Patent Document 1 (JP 2019-87420 A) describes a solid electrolyte that constitutes an all-solid-state battery, and a negative electrode in contact with the solid electrolyte, the negative electrode comprising a stainless steel mesh and a metal layer covering its surface.

[0004] Patent Document 2 (Publication No. 24143 / 1986) describes gold plating on the surface of a stainless steel current collector so that the current collector does not come into direct contact with the positive electrode active material that constitutes the battery. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-87420 A [Patent Document 2] Publication No. 61-24143 Summary of the Invention [Problem to be solved by the invention]

[0006] In solid-state batteries using a conversion electrode, a significant deterioration in cycle characteristics due to volume expansion and change of the electrode is an issue. The object of the present invention is to realize a secondary battery capable of suppressing the deterioration of cycle characteristics of the secondary battery due to volume change of the electrode.

[0007] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0008] A brief outline of a representative embodiment of the present invention will be described below.

[0009] The secondary battery according to one embodiment includes a current collector, an electrode, and a solid electrolyte, which are stacked in this order, and a conductive layer interposed between the current collector and the electrode. Here, the electrode is a conversion electrode, and the conductive layer has a lower conductivity than the current collector. When comparing a center portion and an end portion of the conductive layer in a direction along a surface of the current collector facing the conductive layer, the thickness of the center portion of the conductive layer is greater than the thickness of the end portion of the conductive layer. Effect of the Invention

[0010] A secondary battery can be realized that can suppress deterioration of the cycle characteristics of the secondary battery due to volumetric changes in the electrodes. [Brief description of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a secondary battery according to an embodiment. [Diagram 2] 2 is a table showing the compounding ratios of materials constituting an electrode (positive electrode) of a secondary battery according to an embodiment. [Diagram 3] 3A and 3B are a plan view and a cross-sectional view showing a conductive layer that covers a current collector of the secondary battery according to the embodiment. [Figure 4] 1 is a graph showing the relationship between capacity and voltage when a secondary battery of a comparative example is charged and discharged at room temperature. [Diagram 5] 4 is a graph showing the relationship between capacity and voltage when charging and discharging the secondary battery according to the embodiment at room temperature. [Figure 6] 5A to 5C are schematic diagrams illustrating the contraction mode of electrodes of a secondary battery as a comparative example. [Figure 7] 5A to 5C are schematic diagrams illustrating the contraction mode of electrodes of a secondary battery as a comparative example. [Figure 8] 3A to 3C are schematic diagrams illustrating a contraction mode of an electrode of a secondary battery according to an embodiment of the present invention. [Figure 9] 3A to 3C are schematic diagrams illustrating a contraction mode of an electrode of a secondary battery according to an embodiment of the present invention. [Figure 10] 3A to 3C are schematic diagrams illustrating a contraction mode of an electrode of a secondary battery according to an embodiment of the present invention. [Figure 11] 11A and 11B are a plan view and a cross-sectional view showing a conductive layer that covers a current collector of a secondary battery that is a modified example of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings for explaining the embodiments, the same reference numerals are used for the members having the same functions, and the repeated explanations are omitted. In addition, in the following embodiments, the explanations of the same or similar parts are not repeated in principle, except when particularly necessary. In addition, in the drawings for explaining the embodiments, hatching may be used even in plan views or perspective views, etc., in order to make the configuration easier to understand. Furthermore, in the drawings for explaining the embodiments, hatching may be omitted in cross-sectional views in order to make the configuration easier to understand.

[0013] (Embodiment) Lithium-ion secondary batteries based on conversion reactions are secondary batteries that can achieve higher energy density than lithium (Li)-ion secondary batteries based on intercalation reactions. In lithium-ion secondary batteries based on conversion reactions, the host material not only provides a site for accommodating the active material, but the host material itself can also participate in the redox reaction.

[0014] Lithium-ion secondary batteries that use a conversion reaction include all-solid-state batteries that do not use a liquid electrolyte (electrolytic solution), and solid-state batteries (hereinafter sometimes referred to as semi-solid batteries) that use an electrolyte in part and separate the electrolyte from the positive electrode.

[0015] In solid-state batteries, the electrode (positive electrode) expands and contracts every time the battery is charged and discharged, and so repeated charging and discharging can cause poor contact between the current collector and the electrode, resulting in a decrease in capacity. In the following, we will explain how to solve this problem by making the center of the conductive layer that covers the current collector of a secondary battery thicker than the ends.

[0016] <Structure of secondary battery> A cross-sectional view of a secondary battery according to the present embodiment is shown in Fig. 1. As shown in Fig. 1, secondary battery 10, which is a solid-state battery (semi-solid battery), has a structure in which sheet-like members extending in an XY plane along the X and Y directions and a portion of a liquid (electrolytic solution) are laminated. Secondary battery 10 is composed of a current collector 1, a conductive layer 2, a positive electrode 3, a solid electrolyte 4, an electrolytic solution 5a, a separator 6, an electrolytic solution 5b, a negative electrode 7, and a current collector 8, which are laminated in this order from the bottom in Fig. 1.

[0017] The current collectors (current collector foils) 1 and 8 are made of, for example, stainless steel. Each of the current collectors 1 and 8 has a protrusion (not shown) extending in the lateral direction (X direction or Y direction), and these protrusions are used as terminals for connecting the secondary battery 10 to the outside. The conductive layer 2 is, for example, an Au (gold) plating layer (hereinafter, may be referred to as a gold plating layer). The positive electrode 3 is a conversion electrode, which is one of the two electrodes that the secondary battery 10 has. The conversion electrode is an electrode in which a reversible decomposition and regeneration reaction (conversion reaction) occurs as an electrode reaction. The specific configuration of the positive electrode 3 will be described later. The solid electrolyte 4 is a substance that can easily move ions by applying an electric field from the outside while remaining in a solid state. The solid electrolyte 4 is, for example, LICGC (registered trademark) containing lithium oxide.

[0018] The separator 6 is soaked in the electrolytes 5a and 5b. The separator 6 has a role of separating the negative electrode 7 from the solid electrolyte. In FIG. 1, the solid electrolyte 4 and the negative electrode 7 are separated from the separator 6, but in reality, the solid electrolyte 4 and the negative electrode 7 may be in close contact with the separator 6. The separator 6 is made of a polymer material (e.g., polyethylene) having high lithium ion conductivity. Since LICGC (registered trademark) is reduced to lithium metal that constitutes the negative electrode 7, direct contact between the solid electrolyte 4 and the negative electrode 7 is prevented by using the separator 6 together with the electrolytes 5a and 5b. Although the electrolytes 5a and 5b are shown separately in FIG. 1, the electrolytes 5a and 5b are impregnated into the separator 6 and are not separated from each other. The electrolytes 5a and 5b have a role of smoothly moving lithium ions between the negative electrode 7 and the solid electrolyte 4. The electrolytes 5a and 5b are mainly composed of lithium salts, organic solvents, and additives. The negative electrode 7 is one of two electrodes included in the secondary battery 10. The negative electrode 7 is, for example, a metal layer containing lithium.

[0019] The laminated structure shown in FIG. 1 is sealed by a laminate film (not shown). However, the protruding portions of the current collectors 1 and 8 are exposed to the outside of the laminate film and used as terminals. The laminated structure and the laminate film that seals it constitute a cell of a solid-state battery. In the laminate film, the electrolytes 5a and 5b are only permeated into the separator 6, and the amount of the electrolyte is small, so that the electrolytes 5a and 5b do not come into contact with the positive electrode 3. In other words, the secondary battery 10 of this embodiment is a semi-solid battery having the electrolytes 5a and 5b and the solid electrolyte 4, and the electrolytes 5a and 5b are separated (separated) from the positive electrode 3.

[0020] The positive electrode 3 of the secondary battery of this embodiment contains an active material, a conductive additive, an electrolyte, and a binder. However, the binder is not an essential element. A lithium ion secondary battery is a chemical battery that extracts a chemical reaction as a capacity. A material involved in this chemical reaction is called an active material. The conductive additive has a role of ensuring the flow of electricity in the secondary battery 10, and is not directly involved in the chemical reaction.

[0021] The electrolyte is a material for smoothly causing a chemical reaction in the entire positive electrode 3. In other words, it is believed that the chemical reaction in the positive electrode 3 of a solid-state battery occurs in order from the surface that is in contact with the solid electrolyte 4, and that the chemical reaction is difficult to occur in any other part of the positive electrode 3 than that surface. Therefore, by mixing the electrolyte into the positive electrode 3, the contact points between the positive electrode 3 and the electrolyte can be increased, making it easier for a chemical reaction to occur in the entire positive electrode 3. The binder realizes a uniform thickness of the positive electrode 3. In other words, the positive electrode 3 is formed by solidifying a semi-liquid slurry through a drying process. The binder imparts viscoelasticity to the slurry and prevents the slurry from flowing when the slurry is applied to the current collector 1.

[0022] Next, examples of materials constituting the positive electrode 3 are given below. The active material is copper chloride (CuCl2). The electrolyte is a material containing lithium oxide, for example LICGC (registered trademark). Two types of conductive assistants are blended here, one is acetylene black (carbon) and the other is furnace black. This furnace black has the characteristic of being easily deformable. The binder is a mixture of PEO (polyethylene oxide) and CNF (cellulose nanofiber).

[0023] The current collector 1 includes a first main surface on the side of the conductive layer 2 and a second main surface on the opposite side of the first main surface. Here, the conductive layer (plating base material) 2, which is a gold plating layer covering the first main surface of the current collector 1, has a curvature such that the upper surface (the side of the positive electrode 3) bulges upward. The planar shape of each of the configurations shown in FIG. 1 is substantially rectangular. As shown in the upper part of FIG. 3, the planar shape of the conductive layer 2 is also rectangular. In FIG. 3, the shape of the conductive layer 2 in plan view is square, but this may be rectangular. Also, in the lower part of FIG. 3, a cross-section along the diagonal of the conductive layer 2 with a rectangular planar shape is shown. As shown in this cross-sectional view, when comparing the central part and the end part of the conductive layer 2 in the direction along the first main surface (X-Y plane) of the current collector 1 (see FIG. 1) facing the conductive layer 2, the thickness of the central part is larger than the thickness of the end part. The thickness referred to here means the length of the layer in the Z direction (thickness direction), which is the direction in which each member constituting the secondary battery 10 shown in FIG. 1 is laminated.

[0024] One of the main features of the present embodiment is that, as described above, the thickness of the central part of the conductive layer 2 in the direction along the first main surface of the current collector 1 is larger than the thickness of the end part of the conductive layer 2. The central part of the conductive layer 2 with a rectangular planar shape is a portion (point A1) including the intersection of the diagonals in the plan view of the conductive layer 2. Also, the end part of the conductive layer 2 with a rectangular planar shape is a position on the diagonal in the plan view of the conductive layer 2, which is the position at 5% from the corner when the distance from the corner to the intersection is taken as 100% (point A2).

[0025] As shown in the cross-section in the lower part of FIG. 3, the thickness 1a at the central part (point A1) of the conductive layer 2 is larger than the thickness 2a at the end part (point A2). The radius of curvature R formed by the three points of the central part (point A1) and the two end parts (point A2) of the conductive layer 2 on the diagonal is represented by 800000 mm < R.

[0026] Here, the case where the conductive layer 2 is a gold plating layer has been described, but the material of the conductive layer 2 is not limited to gold as long as it has a higher conductivity than the current collector (for example, a stainless steel current collector) 1. A metal layer made of a precious metal such as Au (gold) can be used as the conductive layer 2. Furthermore, the conductive layer 2 does not have to be a plating layer, and may be attached to the current collector 1.

[0027] <Secondary battery manufacturing procedure> Next, a procedure for manufacturing a cell of a secondary battery (solid-state battery) will be described. Here, the secondary battery during the manufacturing process may be called a laminate. First, a conductive layer 2, which is a gold plating layer, is formed by electrolytic plating on a first main surface of a current collector 1 made of stainless steel. At this time, as described with reference to FIG. 3, the conductive layer 2 has a structure in which the center is thicker than the ends.

[0028] Next, a slurry is prepared by mixing materials for forming a positive electrode. The slurry is a mixture of the above-mentioned active material (e.g., copper chloride), conductive additive (e.g., lithium oxide-containing material), conductive additive (acetylene black and furnace black), and binder (PEO / CNF). These materials are liquid, and the slurry containing the binder is semi-liquid.

[0029] Next, the slurry is applied onto the current collector 1 via the conductive layer 2. Next, a plate-shaped solid electrolyte 4 is attached onto the slurry. Next, the current collector 1 with the solid electrolyte 4 disposed thereon via the slurry is placed in a vacuum-drawn drying device, and heated by a heater to be vacuum-dried. This solidifies the slurry, forming a positive electrode 3. Next, a separator 6 into which electrolytic solutions 5a and 5b have been injected, a negative electrode 7, and a current collector 8 are stacked in this order on the upper surface of the laminate taken out of the drying device, that is, on the upper surface of the solid electrolyte 4. The secondary battery 10 made of the laminate thus formed is sealed with a laminate film. With the above steps, the secondary battery (solid battery, semi-solid battery) of this embodiment is substantially completed.

[0030] <Effects of this embodiment> In a comparative semi-solid battery (secondary battery), the first main surface of the positive electrode side current collector may be covered with a metal layer or a primer coat layer, which is a conductive layer containing carbon. In this case, the surface of the conductive layer that contacts the electrode (positive electrode) of the comparative secondary battery is flat. When such a comparative secondary battery is repeatedly charged and discharged, a deterioration phenomenon of cycle characteristics (cycle deterioration) occurs, in which the capacity that can be extracted from the secondary battery decreases each time the battery is charged and discharged, as shown in FIG. 4.

[0031] In FIG. 4 and the graph shown in FIG. 7 described later, the horizontal axis indicates the capacity of the secondary battery, and the vertical axis indicates the voltage of the secondary battery. In FIG. 4, the graphs of the first charge and discharge are shown by solid lines, the graphs of the second charge and discharge are shown by dashed lines, and the graphs of the third charge and discharge are shown by dashed lines. The graphs of the charge are upward sloping, and the graphs of the discharge are downward sloping. The charge and discharge performed when measuring the graphs shown in FIG. 4 and FIG. 7 were performed under the conditions of an environmental temperature of 50 degrees, a cutoff voltage of 3.6V or more and 2.2V or less, and a charge and discharge rate of 0.05C. Here, the secondary battery is discharged until cutoff, and then the next charge is performed.

[0032] This cycle deterioration is due to the positive electrode 3, which is the conversion electrode, undergoing volume changes (expansion and contraction) every time a charge / discharge reaction occurs. In other words, the positive electrode 3 expands during discharge and contracts during charging. This causes stress in the positive electrode 3, which in turn deforms the positive electrode 3, causing the contact between the positive electrode 3 and the current collector 1 to deteriorate as charge / discharge cycles are repeated. This causes the cycle characteristics to deteriorate.

[0033] That is, as shown in FIG. 5, a positive electrode 3 is provided between a conductive layer 2c and a solid electrolyte 4 constituting the secondary battery of the comparative example. The surface of the conductive layer 2c of the comparative example on the side of the positive electrode 3 is flat. The positive electrode 3 is an integral layer as a whole, but here, as an example, the positive electrode 3 is shown divided into three parts. In FIG. 5 and in FIG. 6 and FIG. 8 to FIG. 10 used in the following explanation, the current collector below the conductive layer and the structure above the solid electrolyte are omitted.

[0034] When a charge / discharge reaction occurs in the secondary battery of the comparative example, the positive electrode 3 expands (or contracts) as shown in FIG. 6. At this time, since the thickness of the conductive layer 2c is uniform, a current flows uniformly in any region in the direction along the XY plane (horizontal direction). In other words, in the positive electrode 3, a reaction occurs uniformly in any region in the direction along the XY plane (horizontal direction). Here, when the volume of the positive electrode 3 expands, the inner part is compressed by the expansion of both adjacent parts, but it must also expand. If a large stress is applied to the positive electrode 3 due to the resulting pushing, there is a risk of the positive electrode 3 cracking. In addition, the fact that the reaction causes expansion means that a clean reaction cannot occur unless it expands, so side reactions increase and capacity deterioration occurs. In addition, the inner part of the positive electrode 3 may be significantly deformed in the vertical direction by being pushed from both sides. As a result of these occurrences, cycle deterioration occurs due to deterioration of the contact between the positive electrode 3 and the current collector 1 side (conductive layer 2c) during repeated charge / discharge cycles.

[0035] In contrast, in the secondary electrode of the present embodiment, the center of the conductive layer 2 covering the current collector 1 has a greater thickness than the end. The cycle characteristics of such a secondary battery 10 are shown in FIG. 7. In FIG. 7, the graphs of the first charge and discharge are shown by solid lines, the graphs of the second charge and discharge are shown by dashed lines, the graphs of the third charge and discharge are shown by dashed lines, the graphs of the tenth charge and discharge are shown by dashed two-dot lines, and the graphs of the 30th charge and discharge are shown by dashed three-dot lines. The graphs of the charge are upward sloping, and the graphs of the discharge are downward sloping. This charge and discharge were performed under the conditions of an environmental temperature of 50 degrees, a cutoff voltage of 3.6V or more and 2.2V or less, and a charge and discharge rate of 0.05C.

[0036] As shown in Fig. 7, even after 30 charge / discharge cycles, the secondary battery of the present embodiment can extract a larger capacity than the comparative example shown in Fig. 4, which extracts the second charge / discharge cycle. The secondary battery of the present embodiment also has an improved capacity compared to the comparative example.

[0037] In the secondary battery of this embodiment, as shown in FIG. 8, the thickness of the conductive layer 2 on the current collector 1 (see FIG. 1) is greater at the center than at the ends. That is, the conductive layer 2 is designed to be thicker at the center. In such a case, because the conductive layer 2 is thick at the center, electricity flows more easily through the center and the positive electrode 3 directly above it. That is, in the positive electrode 3, reaction and expansion occur preferentially from the center.

[0038] As a result, expansion begins from the center of the positive electrode 3, and then expansion begins on both sides (end sides) of the positive electrode 3, making it difficult for the center and both sides to be pushed together. As a result, the generation of internal stress in the positive electrode 3 is suppressed, and the positive electrode 3 is prevented from cracking. In addition, capacity deterioration due to stress can be prevented. In addition, the positive electrode 3 in the center, which is pushed from both sides, is prevented from being deformed in the up and down direction. In other words, the contact between the positive electrode 3 and the conductive layer 2 is prevented from deteriorating. As a result, in this embodiment, the cycle characteristics of the secondary battery are improved.

[0039] The conductive layer 2 of this embodiment may have a thickness that increases toward the core. In other words, the conductive layer 2 continuously increases in thickness from the end portion toward the center. The ideal thickness of the conductive layer 2 is 0.2 μm or more and 5 μm or less, and preferably 3 μm or more and 5 μm or less. It is effective if the thickness difference at the center is 130% (0.26 μm) or more of the reference value of the thickness at the end portion (lower limit, for example, 0.2 μm).

[0040] <Modification> In the following, a case where the planar shape of the conductive layer 2 is circular will be described. In this case, it is assumed that the planar shapes of the respective components shown in FIG.

[0041] As shown on the upper side of FIG. 11, the planar shape of the conductive layer 2 is circular. Also, on the lower side of FIG. 11, a cross-section along the radial direction of the conductive layer 2 with a circular planar shape is shown. As shown in this cross-sectional view, when comparing the center portion and the end portion of the conductive layer 2 in the direction along the first main surface (X-Y plane) of the current collector 1 (see FIG. 1) facing the conductive layer 2, the thickness of the center portion is larger than the thickness of the end portion.

[0042] The center portion of the conductive layer 2 with a circular planar shape is a portion (point B1) including the center point of the circle. Also, the end portion of the conductive layer 2 with a circular planar shape is a position on the radius in the plan view of the conductive layer 2, and is a position (point B2) at 5% from the peripheral edge when the distance from the peripheral edge to the center point is taken as 100%.

[0043] As shown in the lower cross-section of FIG. 3, the thickness 1b at the center portion (point B1) of the conductive layer 2 is larger than the thickness 2b at the end portion (point B2). The radius of curvature R formed by three points, i.e., the center portion (point B1) and two end portions (point B2) on the diameter of the conductive layer 2 in the plan view, is represented by 800000 mm < R.

[0044] As described above, the invention made by the present inventors has been specifically described based on the embodiments. However, it goes without saying that the present invention is not limited to the above embodiments and can be variously modified without departing from the gist thereof.

[0045] For example, in the above embodiment, a semi-solid battery has been described as an example, but this embodiment is also applicable to the positive electrode of an all-solid battery.

Explanation of Reference Numerals

[0046] 1, 8 Current collector 2, 2c Conductive layer 3 Positive electrode 4 Solid electrolyte 5a, 5b Electrolyte solution 6 Separator 7 Negative electrode 10 Secondary battery

Claims

1. A secondary battery having a current collector, an electrode, and a solid electrolyte, which are laminated in this order, a conductive layer interposed between the current collector and the electrode; the electrode is a conversion electrode, the conductive layer has a lower electrical conductivity than the current collector; A secondary battery, wherein when comparing a center and an end of the conductive layer in a direction along a surface of the current collector facing the conductive layer, the thickness of the center of the conductive layer is greater than the thickness of the end of the conductive layer.

2. 2. The secondary battery according to claim 1, The conductive layer has a rectangular planar shape, the central portion of the conductive layer includes an intersection point between diagonal lines of the conductive layer in a plan view, A secondary battery, wherein the end of the conductive layer is located on the diagonal line and is located 5% from the corner when the distance from the corner of the conductive layer to the intersection is 100%.

3. 2. The secondary battery according to claim 1, The conductive layer has a circular planar shape, the central portion of the conductive layer includes a center point of the circular shape; A secondary battery, wherein the end of the conductive layer is a position on a radius of the conductive layer, and is located 5% from the periphery when the distance from the periphery of the conductive layer to the center point is 100%.

4. 2. The secondary battery according to claim 1, A secondary battery, wherein a radius of curvature R formed by three points, namely, the two ends of the conductive layer and the center portion therebetween, which are aligned in a straight line in a plan view, is expressed as 800,000 mm<R.

5. 2. The secondary battery according to claim 1, A secondary battery, wherein the thickness of the central portion of the conductive layer is 130% or more of the thickness of the end portion.

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