Secondary battery

Abstract

To solve the problem of a solid battery including a solid electrolyte between a negative electrode and a positive electrode, in which a charging and discharging reaction at room temperature is difficult and realize charging and discharging of the secondary battery including the solid electrolyte at room temperature.SOLUTION: A secondary battery 10 includes a solid electrolyte 4, and an electrode used as a positive electrode 3 in contact with the solid electrolyte 4. Here, the positive electrode 3 includes an electrolyte and an additive, and the additive includes a halide with a metal-based counter cation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a secondary battery including a solid electrolyte. [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 2004-47416 A) and Patent Document 2 (JP 2016-131081 A) describe secondary batteries in which a positive electrode and a negative electrode are immersed in a liquid non-aqueous electrolyte. In Patent Document 1, a lithium compound (such as LiCl) is contained in either the positive electrode, the non-aqueous electrolyte, or the negative electrode. In Patent Document 2, LiCl is contained in the positive electrode as a lithium salt.

[0004] Patent Document 3 (WO 2019 / 146236) describes a solid-state battery containing lithium in the solid electrolyte. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-47416 A [Patent Document 2] JP 2016-131081 A [Patent Document 3] International Publication No. 2019 / 146236 Summary of the Invention [Problem to be solved by the invention]

[0006] In solid-state batteries, charge / discharge reactions occur under high temperature conditions, but there is a problem in that it is difficult to cause charge / discharge reactions at room temperature. An object of the present invention is to realize a secondary battery that can be charged and discharged at room temperature.

[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 solid electrolyte and an electrode that is in contact with the solid electrolyte and serves as a positive electrode. The electrode includes an electrolyte and an additive, and the additive includes a halide having a metal counter cation. Effect of the Invention

[0010] It is possible to realize a secondary battery that can be charged and discharged at room temperature. [Brief description of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a secondary battery according to an embodiment. [Diagram 2] 1 is a table showing the compounding ratios of materials constituting an electrode (positive electrode) of a secondary battery as a comparative example. [Diagram 3] 2 is a table showing the compounding ratios of materials constituting an electrode (positive electrode) of a secondary battery according to an 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] FIG. 4 is a schematic diagram showing the active material concentration gradient after charging and discharging at room temperature using a secondary battery as a comparative example. [Figure 7] FIG. 2 is a schematic diagram showing an active material concentration gradient of a secondary battery according to an embodiment. [Figure 8] FIG. 2 is a schematic diagram showing the active material concentration gradient after charging and discharging at room temperature using a secondary battery according to an 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, there is a problem that charging and discharging at room temperature is difficult. The inventors considered that the cause of this is that the solidification of the electrolyte makes it difficult to exchange chlorine ions to cause a reaction. In the following, we will explain how to solve this problem by adding lithium chloride as an additive to the positive electrode of a secondary battery.

[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 current collector 1, primer coat layer 2, positive electrode 3, solid electrolyte 4, electrolytic solution 5a, separator 6, electrolytic solution 5b, negative electrode 7, and 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 primer coat layer 2 is, for example, a coating layer containing carbon. The primer coat layer 2 may be, for example, an Au (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, which is an electrode of the secondary battery of this embodiment, contains an active material, an additive, an electrolyte, and a binder. However, the binder is not an essential element. The positive electrode 3 may further contain a conductive assistant. The secondary battery 10 is a solid-state battery equipped with a solid electrolyte. A lithium-ion secondary battery including this solid-state 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 assistant has a role of ensuring the flow of electricity in the secondary battery 10, but is not directly involved in the chemical reaction. The additive is directly involved in the chemical reaction, and has a role of causing the electrode reaction to occur from the collector (substrate) side.

[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 film 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 it is applied to the current collector 1.

[0022] Next, examples of materials constituting the positive electrode 3 are shown. The active material is copper chloride (CuCl2). The electrolyte is a material containing lithium oxide, for example LICGC (registered trademark). The additive is lithium chloride (LiCl). The binder is a mixture of PEO (polyethylene oxide) and urethane. One of the main features of this embodiment is the use of lithium chloride as the additive material. In addition to LiCl, halides having metal counter cations such as NaCl (sodium chloride) or KCl (potassium chloride) can be used as the additive material. A cation is an ion with a positive charge, and an anion is an ion with a negative charge. In an ionized substance, negative ions (for example, Cl - ion) to the other ion of the opposite sign (e.g. Li + ion) is called a counter cation.

[0023] <Secondary battery manufacturing procedure> Next, the procedure for manufacturing a secondary battery (solid-state battery) cell will be described. Here, the secondary battery in the manufacturing process may be called a laminate. First, 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 assistant (e.g., lithium oxide-containing material), additive (lithium chloride), and binder (PEO / urethane). These materials are liquid, and the slurry containing the binder is semi-liquid.

[0024] Next, the above slurry is applied onto a current collector 1 made mainly of stainless steel and the upper surface of which is covered with a primer coat layer 2. Next, a plate-shaped solid electrolyte 4 is attached onto the slurry. Next, the current collector 1 with the solid electrolyte 4 arranged thereon via the slurry is placed in a drying device that has been evacuated, and is heated by a heater and dried in a 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 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 almost completed.

[0025] <Effects of this embodiment> In the semi-solid battery (secondary battery) of the comparative example, lithium chloride is not used as an additive for the positive electrode, and for example, acetylene black (carbon black) made of carbon is used as a conductive additive for the positive electrode. In the case of such semi-solid battery or all-solid battery without electrolyte, the reaction efficiency is significantly lower than that of a secondary battery (liquid battery) that extracts electricity between the positive and negative electrodes via electrolyte, and charging and discharging at low temperatures (below room temperature, below 25°C) becomes very difficult.

[0026] In other words, even if the materials and structures used in liquid batteries are directly replaced with solid-state or semi-solid batteries, the same performance cannot be easily obtained. This is because in the case of solid-state batteries, the parts that contribute to the reaction do not contain electrolyte, and the diffusion of lithium ions does not occur as easily as in liquid batteries. Also, in the case of conversion reactions using multi-step reactions, the diffusion of three types of ions, lithium ions, chloride ions, and copper ions, and the re-encounter between the ions must be repeated in order to recharge after discharging.

[0027] The inventors have considered that one of the reasons why it is difficult to cause charge / discharge reactions at room temperature in solid-state batteries is that it is difficult to exchange chloride ions at the positive electrode. The chemical reactions that occur at the positive electrode during charge / discharge are shown below as reaction formulas (1) and (2).

[0028] CuCl2+Li + +e - ←→CuCl+LiCl(3.41V vs. Li + / Li) · · · (1) CuCl+Li + +e - ←→Cu+LiCl(2.74V vs. Li + / Li) · · · (2) That is, during discharge, the reaction (1) CuCl2 + Li + +e - → The reaction of CuCl + LiCl occurs, and then the reaction of CuCl + Li + +e - →Cu+LiCl occurs. In this way, CuCl2 becomes Li + During charging, the reverse reaction occurs. That is, during charging, the reaction is Cu+LiCl→CuCl+Li + +e - The reaction then proceeds as shown in reaction (1): CuCl+LiCl→CuCl2+Li + +e - In this way, Cu is reduced to CuCl2 by LiCl. This redox reaction allows the solid-state battery to function as a secondary battery.

[0029] In the solid-state battery of the comparative example, a surface reaction occurs in the entire positive electrode, starting from the outermost surface in contact with the solid electrolyte, and the active material (product) that has completed the reaction is concentrated near the outermost surface on the solid electrolyte side, which is thought to inhibit the movement of ions in the ion conduction path. The active material (product) that has completed the reaction here is LiCl generated in reaction formula (1) (see the right side of reaction formula (1) in Figure 1).

[0030] Therefore, the present inventors have considered using LiCl (lithium chloride) as an additive constituting the positive electrode, unlike the secondary battery of the comparative example. Here, FIG. 2 shows the compounding ratio of the materials constituting the positive electrode of the secondary battery of the comparative example, and FIG. 3 shows the compounding ratio of the materials constituting the positive electrode of the secondary battery of the present embodiment. As shown in FIG. 2, acetylene black is compounded as a conductive assistant constituting the positive electrode in the comparative example, and lithium chloride is compounded as an additive constituting the positive electrode in the present embodiment. The compounding ratio of each material (unit: weight percent) is as shown in FIG. 2 and FIG. 3.

[0031] The compounding ratio (weight ratio) of lithium chloride, which is the additive of this embodiment, in the positive electrode is preferably 2% or more and 30% or less. This is because, when the compounding ratio of lithium chloride is less than 2%, the capacity during charging and discharging of the secondary battery using lithium chloride as an additive is not sufficiently obtained, and when the compounding ratio is more than 30%, the compounding ratio of the active material and the electrolyte becomes small, and the charge and discharge reaction is inhibited. Here, as an example, the compounding ratio of lithium chloride in the positive electrode is 12.54%. As long as the compounding ratio of lithium chloride in the positive electrode is within the above numerical range, other materials (e.g., acetylene black) may be compounded as a conductive assistant.

[0032] The inventors conducted a charge-discharge test using a comparative secondary battery having a positive electrode with the blending ratio shown in FIG. 2, and as a result, the characteristics shown in the graph in FIG. 4 were obtained. In the graphs shown in FIG. 4 and FIG. 5 described later, the horizontal axis indicates the capacity of the secondary battery, and the vertical axis indicates the voltage of the secondary battery. In addition, in FIG. 4 and FIG. 5, the graphs during discharge are shown by solid lines, and the graphs during charging are shown by dashed lines. This charge-discharge is the first cycle of charge-discharge performed under conditions of room temperature, cutoff voltage of 3.6V or more and 2.2V or less, and charge-discharge rate of 0.05C.

[0033] As shown in FIG. 4, the capacity of the secondary battery of the comparative example is very small at all voltages. Although it is difficult to confirm in FIG. 4, the graph extends vertically between voltages of 2.2 and 3.6 V in the vicinity of a capacity of 0 mAh / g. Even the secondary battery of the comparative example shows a large capacity in a high temperature situation. However, a battery with a very small capacity at room temperature is difficult to use as a secondary battery. In contrast, the secondary battery of the present embodiment has a large capacity even in a room temperature environment, and can be used as a secondary battery.

[0034] The present inventors' consideration of the reason why there is a difference in performance of the secondary battery between the comparative example and the present embodiment will be described below with reference to Figures 6 to 8. Figures 6 to 8 show schematic diagrams of the positive electrode of the secondary battery, and indicate areas with high LiCl (lithium chloride) concentration with darker colors (high density dots).

[0035] As shown in Figure 6, in the positive electrode 3a of the comparative secondary battery in which acetylene black is used as a conductive assistant and lithium chloride is not added as an additive, chemical reactions occur in sequence from the surface on the side of the solid electrolyte 4 (see Figure 1), and lithium chloride, the active material (product) that has completed the reaction, is concentrated on the surface side. As a result, the ionic conductivity on the surface side is low, and it is no longer possible to rapidly exchange chloride ions.

[0036] 7, the positive electrode 3 of the present embodiment contains lithium chloride as an additive. Therefore, lithium chloride is distributed almost uniformly throughout the positive electrode 3. In other words, the difference between the lithium chloride blend ratio on the surface of the positive electrode 3 facing the solid electrolyte 4 and the lithium chloride blend ratio on the opposite surface is ±2% or less.

[0037] As a result, the positive electrode 3 is filled with Cl - In this way, Cl ions are added in excess around the active material. -By creating a situation where there is an excess of ions, the chemical reaction occurs from the surface of the current collector 1. In other words, MCl acts as a catalyst (M = Li (lithium), Na (sodium) or K (potassium) metal cation catalyst) and Cl - Ions are released, allowing rapid charging and discharging from the current collector 1 side.

[0038] As shown in Fig. 8, the concentration gradient of the active material (LiCl) after the instantaneous reaction gradually increases from the solid electrolyte 4 side toward the current collector 1 (see Fig. 1) side. In other words, after charging or discharging the secondary battery, the lithium chloride compounding ratio on the surface of the positive electrode 3 of the secondary battery on the solid electrolyte 4 side is smaller than the lithium chloride compounding ratio on the opposite surface.

[0039] This prevents the active material from concentrating on the solid electrolyte 4 side. In other words, the concentration of the active material is low on the solid electrolyte 4 side in the positive electrode 3 after the reaction, so the ion conductivity is high. Therefore, chloride ions can be exchanged quickly in the vicinity of the solid electrolyte 4. This is thought to enable charging and discharging at room temperature.

[0040] The invention made by the present inventors has been specifically described above based on an embodiment, but it goes without saying that the present invention is not limited to the above embodiment and can be modified in various ways without departing from the gist of the invention.

[0041] 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. [Explanation of symbols]

[0042] 1 Current collector 2. Primer coat layer 3, 3a positive electrode 4 Solid electrolyte 5a, 5b Electrolyte 6 Separator 7 Negative electrode 8 Current collector 10 Secondary battery

Claims

1. A solid electrolyte; an electrode used as a positive electrode in contact with the solid electrolyte; Equipped with The electrode includes an electrolyte and an additive; The additive comprises a halide having a metal counter cation.

2. 2. The secondary battery according to claim 1, A secondary battery, wherein a compounding ratio of the halide among materials constituting the electrode is 2% or more and 30% or less.

3. 2. The secondary battery according to claim 1, The secondary battery, wherein the halide is lithium chloride, sodium chloride or potassium chloride.

4. 2. The secondary battery according to claim 1, The electrode further comprises a binder.

5. 2. The secondary battery according to claim 1, The secondary battery, wherein the electrode further contains carbon as a conductive additive.

6. 2. The secondary battery according to claim 1, Contains lithium chloride, a difference between a lithium chloride compounding ratio in a first surface of the electrode facing the solid electrolyte and a lithium chloride compounding ratio in a second surface of the secondary battery opposite to the first surface is ±2% or less.

7. 2. The secondary battery according to claim 1, Contains lithium chloride, a lithium chloride blending ratio in a first surface of the electrode facing the solid electrolyte after charging or discharging the secondary battery is smaller than a lithium chloride blending ratio in a second surface of the secondary battery opposite to the first surface.

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