Secondary battery

Abstract

Secondary batteries provided with a solid electrolyte between a negative electrode and a positive electrode have the issue of cycle deterioration where capacity decreases due to the charge-discharge cycle. Accordingly, the present invention realizes a secondary battery capable of suppressing deterioration, in the cycle characteristics of the secondary battery, that occurs due to changes in electrode volume. For this purpose, a secondary battery is used that has a current collector, a positive electrode, and a solid electrolyte which are sequentially laminated, and that comprises a conductive layer interposed between the current collector and the positive electrode. The positive electrode is a conversion electrode. The conductive layer has a lower conductivity than the current collector. If the central portion and the end portions of the conductive layer in a direction extending along the surface of the current collector facing the conductive layer are compared, the thickness of the central portion of the conductive layer is greater than the thickness of the end portions of the conductive layer.

Description

secondary battery

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

[0002] In recent years, the use of lithium-ion secondary batteries has expanded as a rechargeable secondary battery. Among them, solid-state batteries (semi-solid batteries) that use a solid electrolyte as the electrolyte are highly safe because they use a small amount of 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 constituting an all-solid-state battery, and a negative electrode in contact with the solid electrolyte, the negative electrode including a stainless steel mesh and a metal layer covering the surface of the stainless steel mesh.

[0004] Patent Document 2 (Showa 61-24143) describes gold plating 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.

[0005] JP 2019-87420 A

[0006] In solid-state batteries using conversion electrodes, significant deterioration of cycle characteristics due to volumetric expansion and changes in the electrode is a problem. The object of the present invention is to provide a secondary battery that can suppress deterioration of the cycle characteristics of the secondary battery due to volumetric changes in the electrode.

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

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

[0009] In one embodiment, the secondary battery includes a current collector, an electrode, and a solid electrolyte 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 and an end of the conductive layer in a direction along the 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.

[0010] A secondary battery can be realized that can suppress deterioration of the cycle characteristics of the secondary battery due to volume changes of the electrodes.

[0011] FIG. 1 is a cross-sectional view showing a secondary battery according to an embodiment; FIG. 2 is a table showing the compounding ratios of materials constituting an electrode (positive electrode) of a secondary battery according to an embodiment; FIG. 3 is a plan view and a cross-sectional view showing a conductive layer covering a current collector of a secondary battery according to an embodiment; FIG. 4 is a graph showing the relationship between capacity and voltage when a secondary battery according to a comparative example is used and charged and discharged at room temperature; FIG. 5 is a schematic diagram illustrating the contraction mode of an electrode of a secondary battery according to a comparative example; FIG. 6 is a schematic diagram illustrating the contraction mode of an electrode of a secondary battery according to an embodiment; FIG. 7 is a schematic diagram illustrating the contraction mode of an electrode of a secondary battery according to an embodiment; FIG. 8 is a schematic diagram illustrating the contraction mode of an electrode of a secondary battery according to an embodiment;

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, components having the same functions are designated by the same reference numerals, and repeated explanations thereof will be omitted. Furthermore, in the following embodiments, explanations of identical or similar parts will not be repeated unless specifically required. Furthermore, in the drawings for explaining the embodiments, hatching may be used even in plan views or perspective views to make the configuration easier to understand. Furthermore, in the drawings for explaining the embodiments, hatching may be omitted in cross-sectional views to make the configuration easier to understand.

[0013] (Embodiment) A lithium ion secondary battery based on a conversion reaction is a secondary battery that can achieve a higher energy density than a lithium (Li) ion secondary battery based on an intercalation reaction. In a lithium ion secondary battery based on a conversion reaction, the host material not only provides a site for accommodating the active material, but also allows the host material itself to participate in a 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 that use an electrolytic solution in part and separate the electrolytic solution from the positive electrode (hereinafter, sometimes referred to as semi-solid batteries).

[0015] In solid-state batteries, expansion and contraction occur in the electrode (positive electrode) each time charge and discharge are performed, which causes the contact between the current collector and the electrode to deteriorate with repeated charge and discharge, 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 covering the current collector of a secondary battery thicker than the edges.

[0016] <Structure of Secondary Battery> Figure 1 shows a cross-sectional view of a secondary battery according to this embodiment. As shown in Figure 1, secondary battery 10, which is a solid-state battery (semi-solid battery), has a structure in which sheet-like members extending in the X-Y plane along the X and Y directions and a portion of a liquid (electrolyte) are laminated together. 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 Figure 1.

[0017] The current collectors (current collecting 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 laterally (in the X or Y direction), which serves as a terminal connecting the secondary battery 10 to the outside. The conductive layer 2 is, for example, an Au (gold) plating layer (hereinafter sometimes referred to as a gold plating layer). The positive electrode 3 is a conversion electrode, one of the two electrodes of the secondary battery 10. The conversion electrode is an electrode where a reversible decomposition / 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 material that can easily migrate ions in a solid state by applying an external electric field. The solid electrolyte 4 is, for example, LICGC (registered trademark) containing lithium oxide.

[0018] The separator 6 is impregnated with electrolyte solutions 5a and 5b. The separator 6 serves to separate the negative electrode 7 from the solid electrolyte. While the solid electrolyte 4 and the negative electrode 7 are shown separated from the separator 6 in FIG. 1 , in practice, the solid electrolyte 4 and the negative electrode 7 may be in intimate contact with the separator 6. A polymer material (e.g., polyethylene) with high lithium ion conductivity is used for the separator 6. Because LICGC (registered trademark) is reduced to lithium metal, which constitutes the negative electrode 7, the use of the separator 6 together with the electrolyte solutions 5a and 5b prevents direct contact between the solid electrolyte 4 and the negative electrode 7. While the electrolyte solutions 5a and 5b are shown separately in FIG. 1 , the electrolyte solutions 5a and 5b are impregnated into the separator 6 and are not separated from each other. The electrolyte solutions 5a and 5b facilitate the smooth movement of lithium ions between the negative electrode 7 and the solid electrolyte 4. The electrolyte solutions 5a and 5b are primarily composed of a lithium salt, an organic solvent, and an additive. 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 laminate structure shown in FIG. 1 is sealed at its periphery with a laminate film (not shown). However, protruding portions that are part of each of the current collectors 1 and 8 are exposed to the outside of the laminate film and used as terminals. The laminate structure and the laminate film that seals it constitute a solid-state battery cell. Within the laminate film, the electrolyte solutions 5a and 5b are only absorbed into the separator 6, and because the amount of liquid is small, the electrolyte solutions 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 that has the electrolyte solutions 5a and 5b and the solid electrolyte 4, and in which the electrolyte solutions 5a and 5b are separated (spaced apart) 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 capacity from a chemical reaction. A material involved in this chemical reaction is called an active material. The conductive additive has the role of ensuring the flow of electricity within the secondary battery 10, but is not directly involved in the chemical reaction.

[0021] The electrolyte is a material that ensures smooth chemical reactions throughout the positive electrode 3. In other words, it is believed that chemical reactions in the positive electrode 3 of a solid-state battery occur sequentially, starting from the surface that is in contact with the solid electrolyte 4, and that chemical reactions are unlikely to occur anywhere else within the positive electrode 3. Therefore, by mixing an electrolyte into the positive electrode 3, the number of contact points between the positive electrode 3 and the electrolyte can be increased, making it easier for chemical reactions to occur throughout the positive electrode 3. The binder ensures a uniform thickness for 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 it from flowing when applied to the current collector 1.

[0022] Next, examples of materials constituting the positive electrode 3 will be described. The active material is copper chloride (CuCl ２). The electrolyte is a material containing lithium oxide, such as LICGC (registered trademark). Two types of conductive additives are blended here: one is acetylene black (carbon) and the other is furnace black. This furnace black has the characteristic of being easily deformed. The binder is a mixture of PEO (polyethylene oxide) and CNF (cellulose nanofiber).

[0023] The current collector 1 has a first main surface on the conductive layer 2 side and a second main surface opposite the first main surface. The conductive layer (plating substrate) 2, which is a gold-plated layer covering the first main surface of the current collector 1, has a curvature such that the upper surface (near the positive electrode 3) bulges upward. Each of the configurations shown in FIG. 1 has a substantially rectangular planar shape. As shown in the upper part of FIG. 3, the planar shape of the conductive layer 2 is also rectangular. While the shape of the conductive layer 2 in the planar view in FIG. 3 is square, it may also be rectangular. The lower part of FIG. 3 shows a cross section of the rectangular planar conductive layer 2 taken along a diagonal line. As shown in this cross section, when comparing the center and the edge 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 is greater than the thickness of the edge. The "thickness" here refers to the length of the layer in the Z direction (thickness direction), which is the direction in which the components constituting the secondary battery 10 shown in FIG. 1 are stacked.

[0024] One of the main features of the present embodiment is that, as described above, the thickness of the central portion of the conductive layer 2 in the direction along the first main surface of the current collector 1 is greater than the thickness of the end portion of the conductive layer 2. The central portion of the conductive layer 2 having a rectangular planar shape is a point (point A1) that includes the intersection of diagonal lines in a planar view of the conductive layer 2. The end portion of the conductive layer 2 having a rectangular planar shape is a position on a diagonal line in a planar view of the conductive layer 2, and is a position (point A2) that is 5% from the corner when the distance from the corner to the intersection is 100%.

[0025] 3, the thickness 1a at the center (point A1) of the conductive layer 2 is greater than the thickness 2a at the end (point A2). The radius of curvature R formed by the three points on the diagonal line, the center (point A1) and the two end (point A2) of the conductive layer 2, is expressed as 800,000 mm < R.

[0026] Here, the case where the conductive layer 2 is a gold plated 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 (e.g., a stainless steel current collector) 1. A metal layer made of a noble metal such as Au (gold) can be used as the conductive layer 2. Furthermore, the conductive layer 2 does not have to be a plated layer, and may be attached to the current collector 1.

[0027] <Secondary Battery Fabrication Procedure> Next, the fabrication procedure for a secondary battery (solid-state battery) cell will be described. Here, the secondary battery during the manufacturing process may be referred to as 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 edges.

[0028] Next, a slurry containing the materials for forming the positive electrode is prepared. The slurry is a mixture of the 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 with a heater to dry under vacuum. 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 layered in this order on the top surface of the laminate removed from the drying device, i.e., on the top 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-state battery, semi-solid battery) of this embodiment is substantially completed.

[0030] <Effects of the Present Embodiment> In a comparative example of a semi-solid battery (secondary battery), the first main surface of the positive electrode-side current collector may be covered with a primer coating layer or a metal 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 example of a secondary battery is flat. When such a comparative example of a secondary battery is repeatedly charged and discharged, a deterioration phenomenon of cycle characteristics (cycle degradation) occurs, in which the capacity that can be extracted from the secondary battery decreases with each charge and discharge, as shown in FIG. 4 .

[0031] In the graphs shown in FIG. 4 and FIG. 7 (described later), the horizontal axis represents the capacity of the secondary battery, and the vertical axis represents the voltage of the secondary battery. In FIG. 4 , the graphs for the first charge and discharge are shown with solid lines, the graphs for the second charge and discharge are shown with dashed lines, and the graphs for the third charge and discharge are shown with dashed lines. The graphs for charge are shown with an upward slope, while the graphs for discharge are shown with a downward slope. The charge and discharge tests performed to measure the graphs shown in FIGS. 4 and 7 were performed under the following conditions: an ambient temperature of 50°C, a cutoff voltage of 3.6 V or higher and 2.2 V or lower, and a charge / discharge rate of 0.05 C. Here, the secondary battery was discharged until cutoff, and then the next charge was performed.

[0032] This cycle degradation is due to the volume change (expansion and contraction) of the positive electrode 3, which is the conversion electrode, each time a charge / discharge reaction occurs. In other words, the positive electrode 3 expands during discharge and contracts during charge. This causes stress in the positive electrode 3, which in turn deforms the positive electrode 3. As a result, the contact between the positive electrode 3 and the current collector 1 deteriorates over repeated charge / discharge cycles. This deteriorates the cycle characteristics.

[0033] That is, as shown in Fig. 5, a positive electrode 3 is provided between a conductive layer 2c and a solid electrolyte 4 that constitute 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 Figs. 6 and 8 to 10 used in the following explanation, the current collector below the conductive layer and the structure above the solid electrolyte are not shown.

[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 Figure 6. Because the conductive layer 2c has a uniform thickness, current flows uniformly in all regions along the X-Y plane (lateral direction). In other words, the reaction occurs uniformly in all regions along the X-Y plane (lateral direction) in the positive electrode 3. Here, as the volume of the positive electrode 3 expands, the inner portion is compressed by the expansion of the adjacent portions, but it must also expand. This resulting pressure exerts a large stress on the positive electrode 3, potentially causing cracks in the positive electrode 3. Furthermore, expansion due to the reaction means that a clean reaction cannot occur without expansion, resulting in an increase in side reactions and capacity degradation. Furthermore, the inner portion of the positive electrode 3 may undergo significant vertical deformation due to pressure from both sides. As a result, repeated charge-discharge cycles can cause cycle degradation due to factors such as poor contact between the positive electrode 3 and the current collector 1 (conductive layer 2c).

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

[0036] As shown in Fig. 7, the secondary battery of this embodiment can extract a larger capacity even after 30 charge / discharge cycles than the comparative example shown in Fig. 4 after the second charge / discharge cycle. Furthermore, the secondary battery of this 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 in the center than in the edges. That is, the conductive layer 2 is designed to be thicker toward the center. In this case, because the conductive layer 2 is thick in 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 pressed against each other. As a result, the generation of internal stress in the positive electrode 3 is suppressed, preventing the positive electrode 3 from cracking. Furthermore, capacity degradation due to stress can be prevented. Furthermore, the positive electrode 3 in the center, which is pressed from both sides, can be prevented from deforming in the vertical direction. In other words, deterioration of contact between the positive electrode 3 and the conductive layer 2 can be prevented. As described above, 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 the closer to the core. That is, the conductive layer 2 has a thickness that continuously increases 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. The thickness difference is effective if the thickness 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] <Modifications> The following describes a case where the planar shape of the conductive layer 2 is circular. In this case, it is assumed that the planar shapes of the respective components shown in FIG.

[0041] As shown in the upper part of Fig. 11, the planar shape of the conductive layer 2 is circular. Moreover, the lower part of Fig. 11 shows a cross section along the radial direction of the conductive layer 2 having a circular planar shape. As shown in this cross section, when comparing the center and the end 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 is greater than the thickness of the end.

[0042] The center of the conductive layer 2 having a circular planar shape is a point (point B1) that includes the center point of the circle. The end of the conductive layer 2 having a circular planar shape is a position on the radius of the conductive layer 2 in a planar view, and is a position (point B2) that is 5% from the periphery when the distance from the periphery to the center point is 100%.

[0043] 3, the thickness 1b at the center (point B1) of the conductive layer 2 is greater than the thickness 2b at the end (point B2) of the conductive layer 2. The radius of curvature R, which is formed by three points on the diameter of the conductive layer 2, the center (point B1) and the two end (point B2), is expressed as 800,000 mm < R.

[0044] The invention made by the present inventors has been specifically described above based on the embodiments, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention.

[0045] For example, although the above embodiment has been described taking a semi-solid battery as an example, the present embodiment can also be applied to the positive electrode of an all-solid battery.

[0046] REFERENCE SIGNS LIST 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 stacked in this order, comprising a conductive layer interposed between the current collector and the electrode, the electrode being a conversion electrode, the conductive layer having a lower conductivity than the current collector, and 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. A secondary battery as described in claim 1, wherein the planar shape of the conductive layer is rectangular, the central portion of the conductive layer includes an intersection of diagonals of the conductive layer in a planar view, and the end portion of the conductive layer is located on the diagonal line and is located 5% from the corner when the distance from the corner to the intersection is 100%.

3. A secondary battery as claimed in claim 1, wherein the planar shape of the conductive layer is circular, the central portion of the conductive layer includes the center point of the circle, and the end portion of the conductive layer is a position on the 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 taken as 100%.

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

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

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