Secondary cell

Abstract

There is a problem to be addressed in that in a solid-state cell provided with a solid electrolyte between a negative electrode and a positive electrode, it is difficult to cause the charge / discharge reaction at room temperature. The present invention therefore achieves room-temperature charging and discharging of a secondary cell provided with a solid electrolyte. Used as a means therefor is a secondary cell comprising: a solid electrolyte; and an electrode that is in contact with the solid electrolyte and is used as a positive electrode. Herein, the positive electrode includes an electrolyte and an additive. The additive contains a halide that has a metal countercation.

Description

secondary battery

[0001] The present invention relates to a secondary battery having a solid electrolyte.

[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 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 (e.g., 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 a solid electrolyte.

[0005] JP 2004-47416 A JP 2016-131081 A International Publication No. 2019 / 146236

[0006] In solid-state batteries, charge-discharge reactions occur under high-temperature conditions, but 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.

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

[0009] A secondary battery according to one embodiment includes a solid electrolyte and an electrode in contact with the solid electrolyte and used as a positive electrode. The electrode includes an electrolyte and an additive, and the additive includes a halide having a metal counter cation.

[0010] A secondary battery that can be charged and discharged at room temperature can be realized.

[0011] FIG. 1 is a cross-sectional view showing a secondary battery according to an embodiment. FIG. 2 is a table showing the compounding ratio of materials constituting an electrode (positive electrode) of a secondary battery according to a comparative example. FIG. 3 is a table showing the compounding ratio of materials constituting an electrode (positive electrode) of a secondary battery according to an embodiment. FIG. 4 is a graph showing the relationship between capacity and voltage when charging and discharging is performed at room temperature using a secondary battery according to a comparative example. FIG. 5 is a graph showing the relationship between capacity and voltage when charging and discharging is performed at room temperature using a secondary battery according to an embodiment. FIG. 6 is a schematic diagram showing an active material concentration gradient and the like after charging and discharging is performed at room temperature using a secondary battery according to a comparative example. FIG. 7 is a schematic diagram showing an active material concentration gradient and the like after charging and discharging is performed at room temperature using 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] Solid-state batteries have a problem in that they are difficult to charge and discharge at room temperature. The inventors believe that this is because the solidification of the electrolyte makes it difficult to exchange chloride ions that 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> 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 primer coating 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 primer coating layer 2 is, for example, a coating layer containing carbon. The primer coating layer 2 may also be, for example, an Au (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 in which 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, 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 additive. 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 capacity. A material involved in this chemical reaction is called an active material. The conductive additive serves to ensure the flow of electricity within the secondary battery 10, but is not directly involved in the chemical reaction. The additive is directly involved in the chemical reaction, thereby causing the electrode reaction to occur from the current collector (substrate) side.

[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 film 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, 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 with metal counter cations such as NaCl (sodium chloride) or KCl (potassium chloride) can also 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, an anion (for example, Cl － ion) to the other ion of the opposite sign (e.g., Li ＋ ions) are called countercations.

[0023] <Secondary Battery Manufacturing Procedure> Next, the manufacturing 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 slurry containing materials for forming a positive electrode is prepared. The slurry is a mixture of the above-mentioned active material (e.g., copper chloride), conductive additive (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 slurry is applied to a current collector 1 made primarily of stainless steel, the upper surface of which is covered with a primer coating layer 2. Next, a plate-shaped solid electrolyte 4 is attached to the slurry. Next, the current collector 1 with the solid electrolyte 4 disposed thereon 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, a negative electrode 7, and a current collector 8, each filled with electrolytic solutions 5a and 5b, are stacked in this order on the top surface of the stack removed from the drying device, i.e., on the top surface of the solid electrolyte 4. The secondary battery 10 made of the stack thus formed is sealed with a laminate film. This substantially completes the secondary battery (solid-state battery, semi-solid battery) of this embodiment.

[0025] <Effects of the Present Embodiment> In the semi-solid battery (secondary battery) of the comparative example, it is conceivable to use, for example, acetylene black (carbon black) made of carbon as a conductive additive for the positive electrode instead of using lithium chloride as an additive for the positive electrode. In the case of such semi-solid or all-solid batteries that do not use an electrolyte solution, reaction efficiency is significantly reduced compared to secondary batteries (liquid-based batteries) that extract electricity between the positive and negative electrodes via an electrolyte solution, and charge / discharge at low temperatures (room temperature or below, 25°C or below) becomes extremely difficult.

[0026] In other words, simply replacing the materials and structures used in liquid-based batteries with solid-state or semi-solid-state batteries does not easily achieve the same performance. This is because solid-state batteries do not contain electrolyte in the reaction-related areas, making lithium ions less likely to diffuse than liquid-based batteries. Furthermore, in the case of a multi-step conversion reaction, the diffusion and reunion of three types of ions—lithium ions, chloride ions, and copper ions—repeatedly becomes necessary for recharging after discharge.

[0027] The present 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. Reaction formulas (1) and (2) of the chemical reactions that occur at the positive electrode during charge / discharge are shown below. CuCl ２ +Li ＋ +e － ←→CuCl+LiCl (3.41V vs. Li ＋ / Li)...(1) CuCl+Li ＋ +e － ←→Cu+LiCl (2.74V vs. Li ＋ / Li)...(2)

[0028] That is, during discharge, CuCl in reaction formula (1) ２ +Li ＋ +e － → The reaction of CuCl + LiCl occurs, and then the reaction of CuCl + Li in reaction formula (2) ＋ +e － → Cu + LiCl occurs. In this way, CuCl２ Li ＋ During charging, the reverse reaction occurs. That is, during charging, the reaction proceeds as follows: Cu + LiCl → CuCl + Li ＋ +e － The reaction then proceeds as follows: CuCl + LiCl → CuCl ２ +Li ＋ +e － In this way, Cu is converted to CuCl by LiCl. ２ 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 which the reaction proceeds sequentially from the outermost surface in contact with the solid electrolyte throughout the entire positive electrode, 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 (see the right side of reaction formula (1) in Figure 1) that is generated in reaction formula (1).

[0030] Therefore, the inventors considered using LiCl (lithium chloride) as an additive to form the positive electrode, unlike the secondary battery of the comparative example. Here, FIG. 2 shows the compounding ratio of materials constituting the positive electrode of the secondary battery of the comparative example, and FIG. 3 shows the compounding ratio of 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 additive to form the positive electrode in the comparative example, while lithium chloride is compounded as an additive to form the positive electrode in the present embodiment. The compounding ratios of each material (unit: weight percent) are as shown in FIGS. 2 and 3.

[0031] The blending 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 if the blending ratio of lithium chloride is less than 2%, the secondary battery using lithium chloride as an additive will not achieve sufficient capacity during charge and discharge, and if the blending ratio is greater than 30%, the blending ratio of the active material and the electrolyte will be small, inhibiting the charge and discharge reaction. Here, as an example, the blending ratio of lithium chloride in the positive electrode is 12.54%. As long as the blending ratio of lithium chloride in the positive electrode is within the above numerical range, other materials (e.g., acetylene black) may be blended as a conductive additive.

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

[0033] As shown in FIG. 4, the capacity of the comparative secondary battery is very small at all voltages. Although it is difficult to see in FIG. 4, the graph extends vertically between voltages of 2.2 and 3.6 V when the capacity is near 0 mAh / g. Note that even the comparative secondary battery exhibits a large capacity under high temperature conditions. 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 this embodiment can achieve a large capacity even in a room temperature environment, making it possible to use it as a secondary battery.

[0034] The present inventors' considerations as to why there is a difference in performance between the secondary battery of the comparative example and the present embodiment will be explained below with reference to Figures 6 to 8. Figures 6 to 8 show schematic diagrams of the positive electrode of the secondary battery, and areas with high LiCl (lithium chloride) concentrations are indicated by dark colors (high density dots).

[0035] As shown in Figure 6, in the positive electrode 3a of the comparative secondary battery, which uses acetylene black as a conductive additive and does not contain lithium chloride as an additive, chemical reactions occur sequentially 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, accumulates at a high concentration on this surface. As a result, the ionic conductivity on this surface is low, making it impossible to quickly 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 blending ratio on the surface of the positive electrode 3 facing the solid electrolyte 4 and the lithium chloride blending ratio on the surface opposite to that surface is ±2% or less.

[0037] As a result, the positive electrode 3 is entirely 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 in order from the surface of the current collector 1. In other words, the Cl － Ions are released, and this allows quick charging and discharging from the 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 content on the surface of the positive electrode 3 of the secondary battery facing the solid electrolyte 4 is lower than the lithium chloride content on the surface opposite to that surface.

[0039] This prevents the active material from concentrating on the solid electrolyte 4 side. That is, after the reaction, the active material concentration on the solid electrolyte 4 side in the positive electrode 3 is low, resulting in high ionic conductivity. Therefore, chloride ions can be exchanged quickly near the solid electrolyte 4. This is thought to enable charge / discharge at room temperature.

[0040] 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.

[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.

[0042] REFERENCE SIGNS LIST 1 Current collector 2 Primer coating layer 3, 3a Positive electrode 4 Solid electrolyte 5a, 5b Electrolyte solution 6 Separator 7 Negative electrode 8 Current collector 10 Secondary battery

Claims

1. A secondary battery comprising: a solid electrolyte; and an electrode used as a positive electrode in contact with the solid electrolyte, wherein the electrode contains an electrolyte and an additive, and the additive contains a halide having a metal counter cation.

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

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

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

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

6. A secondary battery as claimed in claim 1, comprising lithium chloride, wherein the difference between the lithium chloride content on a first surface of the electrode facing the solid electrolyte and the lithium chloride content on a second surface of the secondary battery opposite the first surface is within ±2%.

7. A secondary battery as claimed in claim 1, comprising lithium chloride, wherein after charging or discharging the secondary battery, the ratio of lithium chloride on a first surface of the electrode facing the solid electrolyte is smaller than the ratio of lithium chloride on a second surface of the secondary battery opposite to the first surface.

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