Solid electrolyte and solid-state battery

Abstract

Provided is a solid-state battery that has superior performance. This solid-state battery comprises a positive electrode, a solid electrolyte layer, and a negative electrode in the stated order. The solid electrolyte layer is divided into three equal portions in the thickness direction from the positive electrode toward the negative electrode, said portions constituting a first region, a second region, and a third region. The first region contains at least Li, P, S, and O, and each of the second region and the third region contains at least Li, P, and S. The average oxygen concentration of the first region is higher than the average oxygen concentration of the second region, and the average oxygen concentration of the second region is equal to or higher than the average oxygen concentration of the third region. The average oxygen concentration of the first region is higher than 0.08 and lower than 0.55. The average oxygen concentration of the third region is equal to or higher than 0 but lower than 0.3.

Description

Solid electrolytes and solid-state batteries

[0001] The present disclosure relates to a solid electrolyte and a solid-state battery including the solid electrolyte.

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight power sources that can provide high energy density. These secondary batteries include a positive electrode, a negative electrode, and an electrolyte housed inside an exterior member.

[0003] In recent years, solid-state batteries, which are secondary batteries equipped with solid electrolytes instead of liquid or gel electrolytes containing organic solvents or the like, have been developed (see, for example, Patent Documents 1 and 2).

[0004] Japanese Patent Application Laid-Open No. 2021-106075

[0005] As described in the above-mentioned prior art documents, various studies have been conducted to improve the performance of solid-state batteries, but there is still room for improvement in the performance of solid-state batteries.

[0006] Therefore, it is desirable to provide a solid-state battery with superior performance and a solid electrolyte for use therein.

[0007] A solid-state battery according to an embodiment of the present disclosure includes, in this order, a positive electrode, a solid electrolyte layer, and a negative electrode. The solid electrolyte layer has a first region, a second region, and a third region, which are divided into three equal parts in the thickness direction from the positive electrode to the negative electrode. The first region contains at least Li, P, S, and O, and the second and third regions each contain at least Li, P, and S. The average oxygen concentration of the first region is higher than the average oxygen concentration of the second region, and the average oxygen concentration of the second region is equal to or greater than the average oxygen concentration of the third region. The battery further satisfies the following conditional expressions (1) and (3): 0.08<C1<0.55 (1) 0≦C3<0.3 (3) where C1 is the average oxygen concentration of the first region, and C3 is the average oxygen concentration of the third region.

[0008] According to the solid state battery of an embodiment of the present disclosure, by having the above configuration, it is possible to achieve excellent performance, such as ensuring a high capacity while achieving a long life.

[0009] Note that the effects of the present disclosure are not necessarily limited to the effects described here, but may be any of a series of effects related to the present disclosure described below.

[0010] Fig. 1 is a schematic cross-sectional view showing an example of a configuration of a solid-state battery according to an embodiment of the present disclosure. Fig. 2 is an enlarged schematic cross-sectional view showing a portion of the solid-state battery shown in Fig. 1. Fig. 3 is an explanatory diagram for explaining the thickness of the solid electrolyte layer in the solid-state battery shown in Fig. 1.

[0011] An embodiment of the present disclosure will be described in detail below with reference to the drawings. The description will be given in the following order: 0. Overview of the Present Technology 1. One Embodiment 1.1 Configuration of a Solid-State Battery 1.2 Manufacturing Method of a Solid-State Battery 1.3 Actions and Effects of a Solid-State Battery 2. Examples The "solid-state battery" of the present disclosure refers to a battery whose components are solid. For example, the "solid-state battery" of the present disclosure is a stacked solid-state battery formed by stacking multiple layers. The multiple layers are made of, for example, a sintered body. The "solid-state battery" of the present disclosure includes not only secondary batteries that can be repeatedly charged and discharged, but also primary batteries that can only be discharged.

[0012] [0. Outline of the Present Disclosure] First, an outline of the present disclosure will be described. Various studies have been conducted to date on improving the performance of solid-state batteries. Because solid-state batteries include a solid electrolyte, they generally have superior high-temperature resistance and higher safety compared to batteries that use a liquid electrolyte.

[0013] In such solid-state batteries, for example, a sulfide solid electrolyte containing Li (lithium), P (phosphorus), and S (sulfur) (hereinafter referred to as Li-P-S solid electrolyte) is used. Li-P-S solid electrolytes have characteristics such as high ionic conductivity compared to, for example, oxide solid electrolytes, and can be molded at room temperature. However, Li-P-S solid electrolytes have also been pointed out to have problems such as a narrow potential window and low oxidation resistance.

[0014] Therefore, the use of sulfide solid electrolytes (hereinafter referred to as Li-P-S-O-based solid electrolytes) in which part of the S (sulfur) in Li-P-S-based solid electrolytes is substituted with O (oxygen) has been investigated. Li-P-S-O-based solid electrolytes have better oxidation-reduction resistance and moisture resistance than Li-P-S-based solid electrolytes, and the greater the oxygen substitution, the greater the effect. However, Li-P-S-O-based solid electrolytes have lower ionic conductivity than Li-P-S-based solid electrolytes, resulting in a problem of increased battery resistance. Furthermore, the greater the oxygen substitution, the greater the decrease in lithium ion conductivity.

[0015] The above-mentioned Patent Document 1 discloses a solid-state battery using a sulfide-based solid electrolyte having an oxygen-enriched layer at the contact surface with the oxide-based positive electrode active material, the oxygen-enriched layer having a higher oxygen element concentration than the portion outside the contact surface. In Patent Document 1, charging is performed until the positive electrode potential exceeds the upper limit of normal use, thereby forming an oxygen-enriched layer (1 to 100 nm thick) at the contact surface between the sulfide-based solid electrolyte and the positive electrode active material. However, in this technology, the oxygen-enriched layer and the other solid electrolyte layer exist as two clearly distinguishable layers, which is expected to result in high interfacial resistance between the oxygen-enriched layer and the other solid electrolyte layer. Furthermore, since the thickness of the oxygen-enriched layer is 100 nm or less, it is thought that sufficient oxidation resistance cannot be obtained. Furthermore, since the oxygen-enriched layer is formed by high-potential charging in Patent Document 1, the initial efficiency of the battery is likely to be low. This is because the oxygen source in the solid-state battery of Patent Document 1 is the positive electrode active material, and the formation of the oxygen-enriched layer deactivates a portion of the positive electrode active material. Furthermore, since a powdered positive electrode active material is used, the area of ​​the interface between the positive electrode and the solid electrolyte layer is large, and there is concern that the oxygen-enriched layer will have a large effect on the increase in interface resistance.

[0016] In view of the above circumstances, the present applicant proposes a solid-state battery that has excellent long-term reliability while suppressing a decrease in ionic conductivity, and a solid electrolyte layer used therein.

[0017] 1. One Embodiment 1.1 Configuration of Solid-State Battery A configuration of a solid-state battery 100 according to one embodiment of the present disclosure will be described with reference to FIGS. 1 and 2 . FIG. 1 is a schematic cross-sectional view illustrating the configuration of the solid-state battery 100. The solid-state battery 100 has a laminated structure in which a positive electrode 10, a solid electrolyte layer 20, and a negative electrode 30 are laminated in this order. As described below, the positive electrode 10 has a positive electrode current collector 11 and a positive electrode active material layer 12. The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material layer 32. The solid-state battery 100 may have a structure in which a plurality of units are repeatedly laminated, each unit being formed by laminating the positive electrode 10, the solid electrolyte layer 20, the negative electrode 30, and the solid electrolyte layer 20 in this order.

[0018] 2 is an enlarged schematic cross-sectional view of a portion of the positive electrode 10 and the solid electrolyte layer 20 of the solid battery 100. The positive electrode 10 is an electrode layer containing at least a positive electrode active material. The positive electrode 10 has a positive electrode current collector 11 and a positive electrode active material layer 12 provided on the positive electrode current collector 11.

[0019] The positive electrode current collector 11 is, for example, a metal foil. Examples of materials for the positive electrode current collector 11 include a metal (single metal) selected from the group consisting of Al (aluminum), Cu (copper), Mg (magnesium), Ti (titanium), Fe (iron), Co (cobalt), Ni (nickel), Zn (zinc), Ge (germanium), In (indium), Au (gold), Pt (platinum), Ag (silver), and Pd (palladium), or an alloy containing two or more metal elements selected from the above group. The positive electrode current collector 11 may also be a sintered body. This is to enable the solid-state battery 100 to be formed by co-firing or to reduce the internal resistance of the positive electrode current collector 11. When the positive electrode current collector 11 is a sintered body, the positive electrode current collector 11 may contain a conductive additive and a sintering additive. The conductive additive contained in the positive electrode current collector 11 may be the same type as the conductive additive contained in the positive electrode active material layer 12, for example. The sintering aid contained in the positive electrode current collector 11 may be the same type as the sintering aid contained in the positive electrode active material layer 12, for example.

[0020] The positive electrode current collector 11 may have, for example, a plate, foil, or mesh shape. The surface of the positive electrode current collector 11 may be smooth or may have an irregular surface.

[0021] (Positive Electrode Active Material Layer 12) The positive electrode active material layer 12 contains a positive electrode active material as a main component. The positive electrode active material contained in the positive electrode active material layer 12 is a material that participates in the occlusion and release of ions in the solid-state battery 100 and in the transfer of electrons to and from an external circuit. Ions move between the positive electrode 10 and the negative electrode 30 via the solid electrolyte (i.e., ion conduction). The occlusion and release of ions into the positive electrode active material is accompanied by the oxidation or reduction of the positive electrode active material. Electrons or holes for such an oxidation-reduction reaction are transferred to the positive electrode 10 or the negative electrode 30, thereby allowing charge and discharge to proceed. The positive electrode active material layer 12 contains, for example, lithium ions, sodium ions, protons (H ＋ ), potassium ions (K ＋ ), magnesium ions (Mg ２＋ ), aluminum ions (Al ３＋ ), silver ions (Ag ＋ ), fluoride ion (F － ) or chloride ions (Cl － In other words, the solid-state battery 100 is an all-solid-state secondary battery in which charging and discharging are performed by the ions moving between the positive electrode 10 and the negative electrode 30 via the solid electrolyte.

[0022] (Positive electrode active material) The positive electrode active material contained in the positive electrode 10 may be, for example, at least one selected from the group consisting of a lithium-containing phosphate compound having a Nasicon structure, a lithium-containing phosphate compound having an olivine structure, a lithium-containing layered oxide, and a lithium-containing oxide having a spinel structure. An example of a lithium-containing phosphate compound having a Nasicon structure is Li ３ V ２ (P.O. ４ ) ３ Examples of lithium-containing phosphate compounds having an olivine structure include Li ３ Fe ２ (P.O. ４ )３ , LiFePO ４ , LiMnPO ４ , LiFe ０．６ Mn ０．４ P.O. ４ An example of a lithium-containing layered oxide is LiCoO ２ , LiCo １／３ Ni １／３ Mn １／３ O ２ , LiCo ０．８ Ni ０．１５ Al ０．０５ O ２ An example of a lithium-containing oxide having a spinel structure is LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ etc.

[0023] In addition, the positive electrode active material capable of absorbing and releasing sodium ions may be at least one selected from the group consisting of sodium-containing phosphate compounds having a Nasicon structure, sodium-containing phosphate compounds having an olivine structure, sodium-containing layered oxides, and sodium-containing oxides having a spinel structure.

[0024] As shown in FIG. 2 , the cathode active material layer 12 is provided on the cathode current collector 11 and is a porous molded body including a cathode active material structure 13 to which a solid electrolyte SE is attached. The cathode active material structure 13 is a single continuous structure extending from the cathode current collector 11 to the solid electrolyte layer 20. The cathode active material structure 13 is formed, for example, by sintering a plurality of cathode active material particles together. Therefore, the cathode active material structure 13 includes a plurality of granular portions 13A having a granular appearance and connecting portions 13B that connect the plurality of granular portions 13A and have an outer diameter smaller than that of the granular portions 13A. In other words, the connecting portions 13B are narrow, constricted portions of the cathode active material structure 13. Note that although the cross section of FIG. 2 depicts the cathode active material structure 13 as if it were separated into a plurality of portions, in reality, the portions are connected at a position different from the cross section shown in FIG. 2 .

[0025] A solid electrolyte SE may be present around the positive electrode active material structure 13. The solid electrolyte SE will be described in detail later.

[0026] A coating 14 containing an ion-conducting material may be present at at least a portion of the interface between the positive electrode active material structure 13 and the solid electrolyte SE. The thickness of the coating 14 is, for example, 1 nm or more and 100 nm or less. The ion-conducting material here is, for example, a compound represented by LiXY. Here, X is at least one element selected from the group consisting of B (boron), Al (aluminum), Si (silicon), P (phosphorus), Ti (titanium), Ge (germanium), Zr (zirconium), Nb (niobium), In (indium), Sn (tin), Hf (hafnium), Ta (tantalum), and W (tungsten). Y is at least one element selected from the group consisting of O (oxygen), S (sulfur), N (nitrogen), F (fluorine), Cl (chlorine), Br (bromine), and I (iodine). The ion-conducting material constituting the coating 14 is, for example, LiNbO ３ is.

[0027] (Solid Electrolyte Layer 20 and Solid Electrolyte SE) The solid electrolyte layer 20 has a first region R1, a second region R2, and a third region R3, which are divided into three equal parts in the thickness direction from the positive electrode 10 toward the negative electrode 30. The thickness T1 of the first region R1, the thickness T2 of the second region R2, and the thickness T3 of the third region R3 are equal to one another. Each of the thicknesses T1 to T3 is one-third of the overall thickness T of the solid electrolyte layer 20. As shown in FIG. 3 , the thickness T is the distance between the portion 12P of the positive electrode active material layer 12 that protrudes most toward the negative electrode 30 and the portion 32P of the negative electrode active material layer 32 that protrudes most toward the positive electrode 10 in the thickness direction of the solid battery 100, i.e., the stacking direction of the positive electrode 10, the solid electrolyte layer 20, and the negative electrode 30. FIG. 3 is an explanatory diagram for explaining the thickness T of the solid electrolyte layer 20 in the solid battery 100. Each of the first region R1, the second region R2, and the third region R3 has a solid electrolyte SE, which is a Li-P-S-O-based solid electrolyte containing Li (lithium), P (phosphorus), S (sulfur), and O (oxygen). The solid electrolyte SE may be present not only in the solid electrolyte layer 20 but also in the positive electrode active material layer 12. Furthermore, a portion of the solid electrolyte SE may be in contact with the positive electrode current collector 11 near the interface between the positive electrode active material layer 12 and the positive electrode current collector 11. The solid electrolyte SE has a plurality of voids.

[0028] The solid electrolyte SE is a material capable of conducting ions such as lithium ions between the positive electrode 10 and the negative electrode 30. The solid electrolyte SE may further contain a halogen element in addition to the above-mentioned Li (lithium), P (phosphorus), S (sulfur), and O (oxygen). Examples of the halogen element include F (fluorine), Cl (chlorine), Br (bromine), and I (iodine). Specific examples of the solid electrolyte SE include Li, ２ S-P ２ S ５ -Li ２ O. Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS２ -Li ｘ P.O. ｙ (where x and y are positive numbers). ２ S-P ２ S ５ -Li ２ The description "O" is Li ２ S and P ２ S ５ and Li ２ The term "solid electrolyte SE" refers to a material obtained using a raw material composition containing O, and the same applies to other descriptions. The molar ratio of each element in the solid electrolyte SE can be controlled by adjusting the content of each element in the raw materials. The molar ratio and composition of each element in the solid electrolyte SE can be measured, for example, by ICP atomic emission spectrometry.

[0029] In the solid electrolyte layer 20, the average oxygen concentration in the first region R1 is higher than the average oxygen concentration in the second region R2, and the average oxygen concentration in the second region R2 is equal to or higher than the average oxygen concentration in the third region R3. Specifically, when the average oxygen concentration in the first region R1 is C1, the average oxygen concentration in the second region R2 is C2, and the average oxygen concentration in the third region R3 is C3, it is preferable that all of the following formulas (1) to (3) are satisfied: 0.08<C1<0.55 (1) 0≦C2<0.35 (2) 0≦C3<0.3 (3)

[0030] The average oxygen concentrations C1 to C3 were determined by performing element mapping using an energy dispersive X-ray analyzer (EDS) on cross-sectional images of the first to third regions R1 to R3 of the solid electrolyte layer 20 taken by an SEM (scanning electron microscope), and determining the oxygen atomic concentration C Ｏ , phosphorus atom concentration C Ｐ , and sulfur atom concentration C Ｓ That is, the oxygen atomic concentration C obtained from each cross-sectional image of the first to third regions R1 to R3 can be calculated by the following formula (4). Ｏ , the oxygen atom concentration C Ｏ and phosphorus atom concentration C Ｐ and sulfur atom concentration C Ｓ It is obtained by dividing by the sum of

[0031] C1 to C3 = C Ｏ / {C Ｏ+C Ｐ +C Ｓ …(4)

[0032] The solid electrolyte layer 20 may contain a sintering aid. The sintering aid that can be contained in the solid electrolyte layer 20 may be selected from, for example, the same materials as the sintering aids that can be contained in the positive electrode 10 and the negative electrode 30.

[0033] (Negative electrode 30) The negative electrode 30 is an electrode layer containing at least a negative electrode active material. The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode current collector 31 is, for example, a metal foil such as copper foil. The negative electrode current collector 31 may also be a sintered body. This is to enable the solid state battery 100 to be formed by integral firing or to reduce the internal resistance of the negative electrode current collector. When the negative electrode current collector is a sintered body, the negative electrode current collector may contain a conductive additive and a sintering additive.

[0034] (Negative Electrode Active Material) The negative electrode active material contained in the negative electrode 30, like the positive electrode active material contained in the positive electrode 10, is a material that participates in the occlusion and release of ions in the solid state battery 100 and in the transfer of electrons to and from an external circuit. Ions move between the positive electrode 10 and the negative electrode 30 via the solid electrolyte layer 20 (i.e., ion conduction). The occlusion and release of ions in the negative electrode active material is accompanied by the oxidation or reduction of the negative electrode active material. Electrons or holes for such an oxidation-reduction reaction are transferred to and from the positive electrode 10 or the negative electrode 30, thereby allowing charge and discharge to proceed. The negative electrode active material may be, for example, lithium ions, sodium ions, protons (H ＋ ), potassium ions (K ＋ ), magnesium ions (Mg ２＋ ), aluminum ions (Al ３＋ ), silver ions (Ag ＋ ), fluoride ion (F － ) or chloride ions (Cl －) can be absorbed and released. Examples of the negative electrode active material contained in the negative electrode 30 include at least one selected from the group consisting of an oxide containing at least one element selected from the group consisting of Ti, Si, Sn, Cr, Fe, Nb, and Mo, a graphite-lithium compound, a lithium alloy, a lithium-containing phosphate compound having a Nasicon structure, a lithium-containing phosphate compound having an olivine structure, and a lithium-containing oxide having a spinel structure. An example of a lithium alloy is Li-Al. An example of a lithium-containing phosphate compound having a Nasicon structure is Li. ３ V ２ (P.O. ４ ) ３ , LiTi ２ (P.O. ４ ) ３ Examples of lithium-containing phosphate compounds having an olivine structure include Li ３ Fe ２ (P.O. ４ ) ３ , LiCuPO ４ Examples of lithium-containing oxides having a spinel structure include Li ４ Ti ５ O １２ etc.

[0035] In addition, examples of negative electrode active materials capable of absorbing and releasing sodium ions include at least one selected from the group consisting of sodium-containing phosphate compounds having a Nasicon structure, sodium-containing phosphate compounds having an olivine structure, and sodium-containing oxides having a spinel structure.

[0036] <1.2 Manufacturing Method of Solid-State Battery 100> An example of a manufacturing method of the solid-state battery 100 will be described. When manufacturing the solid-state battery 100, a printing method such as a screen printing method, a green sheet method using a green sheet, or a combination of these methods can be used. One manufacturing method will be described below as an example, but the present disclosure is not limited to the manufacturing method described below. Furthermore, the chronological matters such as the order of description below are merely for the convenience of explanation, and the present disclosure is not limited to those matters.

[0037] First, the positive electrode 10 is fabricated. Specifically, a plurality of positive electrode active materials (e.g., LiCoO ２ ) particles are pressure-molded and then sintered at a predetermined temperature for a predetermined time to produce a positive electrode pellet. A sputtering device is then used to form a coating of a composite oxide containing lithium and phosphorus (e.g., Li-P-O-N composite oxide) on the surface of the positive electrode pellet. Furthermore, a positive electrode current collector 11 is layered on the back surface of the positive electrode pellet to obtain a positive electrode 10.

[0038] Next, the solid electrolyte layer 20 is formed on the positive electrode 10. Specifically, for example, in an argon gas atmosphere, a sulfide solid electrolyte (e.g., Li ３ P.S. ４ ) is dissolved in a solvent such as ethanol and then stirred to prepare a solid electrolyte solution. Next, the solid electrolyte solution is dropped onto the cathode 10, and the cathode 10 onto which the solid electrolyte solution has been dropped is heated and dried to remove the solvent. Thereafter, a heat treatment is performed for a predetermined time under a predetermined temperature environment, and further, pressure is applied using a uniaxial press to form a laminate of the cathode 10 and the solid electrolyte layer 20.

[0039] Furthermore, the negative electrode 30 is produced. Specifically, a negative electrode active material mixture is produced by kneading a plurality of negative electrode active material particles and a solid electrolyte SE at a predetermined volume ratio (for example, 5:5). Next, a negative electrode current collector 31 is prepared, and the above-described negative electrode active material mixture is applied to the surface of the negative electrode current collector 31. Thereafter, the negative electrode active material mixture applied to the negative electrode current collector 31 is pressure-molded using a press, thereby forming a negative electrode active material layer 32 on the negative electrode current collector 31. In this manner, the negative electrode 30 is obtained.

[0040] Finally, the negative electrode 30 is placed on the laminate of the positive electrode 10 and the solid electrolyte layer 20, and then compressed using a press, thereby completing the solid battery 100.

[0041] <1.3 Actions and Effects of Solid-State Batteries> In general, important properties required of solid-state batteries include volumetric energy density, high-speed charge / discharge characteristics, and durability.

[0042] In the solid-state battery 100 of this embodiment, the solid electrolyte layer 20 has a first region R1, a second region R2, and a third region R3, which are divided into three equal parts in the thickness direction from the positive electrode 10 toward the negative electrode 30. Each of the first region R1, the second region R2, and the third region R3 has a solid electrolyte SE containing Li (lithium), P (phosphorus), S (sulfur), and O (oxygen). The average oxygen concentration C1 of the first region R1 is higher than the average oxygen concentration C2 of the second region R2, and the average oxygen concentration C2 of the second region R2 is equal to or greater than the average oxygen concentration C3 of the third region R3. The solid-state battery 100 having this configuration can achieve excellent performance, such as a long life while maintaining a high capacity. Specifically, the solid electrolyte layer 20 contains Li (lithium), P (phosphorus), S (sulfur), and O (oxygen), thereby improving redox resistance and moisture resistance. In addition, the first region R1, which has a higher average oxygen concentration than the second region R2 and the third region R2, is located closest to the positive electrode 10 in the solid electrolyte layer 20, thereby suppressing a decrease in the conductivity of lithium ions. Furthermore, the first region R1 has a different average oxygen concentration from the second region R2 and the third region R3, which allows smooth movement of lithium ions in the solid electrolyte layer 20 and suppresses an increase in resistance in the solid electrolyte layer 20.

[0043] In the solid state battery 100 of this embodiment, the positive electrode active material layer 12 has a positive electrode active material structure 13 that is a single continuous structure extending from the positive electrode current collector 11 to the solid electrolyte layer 20. Therefore, a good ion conduction path is formed in the solid state battery 100, and better performance can be achieved, such as improved rate characteristics without impairing energy density.

[0044] In the solid-state battery 100 of this embodiment, at least a part of the interface between the positive electrode active material structure 13 and the solid electrolyte SE contains LiNbO ３ The presence of the coating 14 containing an ion-conductive material such as ZnO can form a better ion-conductive path, thereby achieving even better performance.

[0045] 2. Examples Examples of the present disclosure will be described.

[0046] Example 1 As described below, a solid-state battery (half cell) for evaluation was fabricated including the positive electrode (evaluation electrode) of the present disclosure shown in Figure 1, a reference electrode as its counter electrode, and a solid electrolyte layer provided between the positive electrode and the reference electrode, and the battery characteristics were evaluated. However, an indium-lithium alloy foil was used as the reference electrode.

[0047] (Preparation of Positive Electrode) First, an aluminum (Al) foil was prepared as a positive electrode current collector. Next, LiCoO ２ 20 mg of the particles were pressure-molded to a thickness of 60 nm and then sintered at 1100 °C for 60 hours to produce a positive electrode pellet. Then, using a magnetron sputtering device, a Li-P-O-N composite oxide coating was formed to a thickness of 10 nm on the surface of the positive electrode pellet in an argon gas and nitrogen gas atmosphere. Furthermore, a positive electrode was obtained by overlaying Al (aluminum) foil as a positive electrode current collector on the back surface of the positive electrode pellet.

[0048] (Fabrication of Solid-State Battery (Half Cell)) A solid electrolyte layer was formed on the cathode obtained as described above. ３ P.S. ４ (NEI Corp.) was dissolved in ethanol (Fujifilm Wako Co., Ltd.) ３ P.S. ４ The solid electrolyte solution was then applied to the positive electrode at a concentration of 210 μL / cm ２ After the solid electrolyte solution was dropped, the positive electrode onto which the solid electrolyte solution had been dropped was placed on a hot plate and dried at 90°C for 17 hours to remove the solvent. Thereafter, the temperature of the hot plate was raised to 210°C, and heat treatment was carried out for 3 hours. Thereafter, the positive electrode was pressed with a uniaxial press at 3.7 ton / cm. ２The laminate was then placed in a Macol die (outer diameter 30 mm, inner diameter 10 mm, height 20 mm), and a 200 μm thick indium foil (manufactured by Nilaco) and a 200 μm thick lithium foil (manufactured by Honjo Metals) were stacked as a negative electrode on the opposite side of the solid electrolyte layer from the positive electrode to obtain a solid-state battery (half cell).

[0049] (Evaluation of Battery Characteristics) The battery characteristics of the fabricated solid-state battery of Example 1 were evaluated, and the results shown in Table 1 were obtained. Here, CC-CV charge-discharge was performed on the solid-state battery of Example 1 at room temperature within a potential range of 4.3 V vs. Li / Li+ at the upper limit and 3.0 V vs. Li / Li+ at the lower limit. The C rate was 0.02 C, and the cutoff rate was 0.01 C. The ratio of the discharge capacity at the initial discharge to the charge capacity at the initial charge was calculated to determine the initial Coulombic efficiency (charge-discharge efficiency) [%]. Furthermore, the overvoltage at the start of discharge [mV] was calculated by subtracting the potential 5 seconds after the start of discharge from the open-circuit voltage immediately before the start of discharge. Note that 1 C is the current magnitude required to fully charge (or discharge) the theoretical capacity of the battery in 1 hour during CC-CV charge-discharge. Furthermore, the solid-state battery of Example 1 was cut in the stacking direction to expose a cross section along the stacking direction, and an image of the cross section was obtained using a scanning electron microscope (SEM). By analyzing the image of the cross section, the average oxygen concentrations C1 to C3 of the first to third regions R1 to R3 were determined. Specifically, the solid-state battery of Example 1 before charge and discharge was subjected to cross-section polishing using a cross-section polisher (IM-4000, manufactured by Hitachi) without exposure to the atmosphere, thereby obtaining a cross-section observation sample. Next, the prepared cross-section observation sample was observed using a scanning electron microscope (SEM, S4800, manufactured by Hitachi), and elemental mapping was performed using an energy dispersive X-ray spectrometer (EDS) to determine the oxygen atomic concentration C Ｏ , phosphorus atom concentration C Ｐ , and sulfur atom concentration C Ｓ were calculated, and the average oxygen concentrations C1 to C3 were calculated using the above formula (4).

[0050]

[0051] Example 2 When preparing a solid electrolyte layer, Li in the solid electrolyte solution ３ P.S. ４ The concentration of the argyrodite-type solid electrolyte Li was 5 mass %, and 6 mg of the argyrodite-type solid electrolyte Li, which corresponds to a thickness of 50 μm, was placed between the solid electrolyte layer and the negative electrode. ６ P.S. ５ The total thickness of the solid electrolyte layer was adjusted to 100 μm by inserting Cl (manufactured by NEI Corp.). Except for this, a solid-state battery was fabricated in the same manner as in Example 1, and the same evaluation was carried out. The results are also shown in Table 1.

[0052] Example 3 When preparing a solid electrolyte layer, Li in the solid electrolyte solution ３ P.S. ４ A solid-state battery was fabricated in the same manner as in Example 2, except that the concentration of was 20 mass %, and the same evaluation was carried out. The results are also shown in Table 1.

[0053] <Comparative Example 1> When a solid electrolyte layer was produced, a solid electrolyte solution was not used, and Li ３ P.S. ４ 6 mg of powder at 3.7 ton / cm ２ A solid-state battery was fabricated in the same manner as in Example 1, except that the solid electrolyte layer was formed by pressure molding at a pressure of 1000 kJ / cm 2 , and the same evaluation was carried out. The results are also shown in Table 1.

[0054] <Comparative Example 2> As in Prior Art Document 1, a positive electrode potential of 4.6 V vs. Li / Li was measured in an environment of 25°C. ＋ The battery was charged at constant current and constant voltage (CC-CV) until the battery reached a full charge, and then stored in an argon gas-filled container at 60°C for 1 hour. Except for these details, a solid-state battery was fabricated in the same manner as in Comparative Example 1, and the same evaluation was carried out. The results are also shown in Table 1.

[0055] <Comparative Example 3> When preparing the solid electrolyte layer, the solid electrolyte solution was not dropped onto the positive electrode, but was dropped onto a separately prepared Al foil under the same conditions as in Example 1, followed by drying and heat treatment. The solid electrolyte on the Al foil was then recovered with a spatula, and 6 mg of the solid electrolyte powder was placed in a Macol die (outer diameter 30 mm, inner diameter 10 mm, height 20 mm) and heated at 3.7 ton / cm ２A solid-state battery was fabricated in the same manner as in Example 1, except that the solid electrolyte layer was formed by pressure molding at a pressure of 1000 kJ / cm 2 , and the same evaluation was carried out. The results are also shown in Table 1.

[0056] <Comparative Example 4> When preparing a solid electrolyte layer, Li in the solid electrolyte solution ３ P.S. ４ A solid-state battery was fabricated in the same manner as in Example 1, except that the concentration of was 50 mass %, and the same evaluation was carried out. The results are also shown in Table 1.

[0057] <Comparative Example 5> When preparing a solid electrolyte layer, Li in the solid electrolyte solution ３ P.S. ４ A solid-state battery was fabricated in the same manner as in Example 2, except that the concentration of was 2 mass %, and the same evaluation was carried out. The results are also shown in Table 1.

[0058] <Comparative Example 6> When preparing the solid electrolyte layer, 6 mg of the powder of the solid electrolyte described in Comparative Example 3 was placed between the solid electrolyte layer and the negative electrode in a Macol die (outer diameter 30 mm, inner diameter 10 mm, height 20 mm), and a pressure of 3.7 ton / cm ２ A solid-state battery was fabricated in the same manner as in Example 2, except that the battery was inserted after being press-molded at a pressure of 1000 kJ / cm 3 , and the same evaluation was carried out. The results are also shown in Table 1.

[0059] [Discussion] As shown in Table 1, Examples 1 to 3 exhibited higher initial coulombic efficiencies than Comparative Examples 1 to 6. Furthermore, Examples 1 to 3 were able to suppress the overvoltage at the start of discharge compared to Comparative Examples 1 to 6. This is believed to be because, in Examples 1 to 3, the first region R1, which has a higher average oxygen concentration C1 compared to the average oxygen concentration C2 in the second region R2 and the average oxygen concentration C3 in the third region R3, is positioned closest to the positive electrode in the solid electrolyte layer, thereby suppressing the decrease in lithium ion conductivity. Furthermore, in Examples 1 to 3, there is no clear interface between the first region R1 and the second region R2, or between the second region R2 and the third region R3 in the solid electrolyte layer, allowing for smooth migration of lithium ions in the solid electrolyte layer. Therefore, it is believed that Examples 1 to 3 were able to suppress an increase in resistance in the solid electrolyte layer and suppress the voltage at the start of discharge. Furthermore, in Examples 1 to 3, oxygen was not introduced during the charging process as in Comparative Example 2, so side reactions are less likely to occur in the solid electrolyte layer during charging. For this reason, it is believed that a higher initial efficiency was obtained compared to Comparative Example 2. Furthermore, in Comparative Example 3, the average oxygen concentration C1 in the first region R1 was lower than the average oxygen concentration R2 in the second region R1, which is thought to have resulted in a slightly lower initial coulombic efficiency and a higher overvoltage at the start of discharge. Furthermore, in Comparative Example 4, the average oxygen concentration C1 in the first region R1 was low at 0.08, which is thought to have resulted in a slightly lower initial coulombic efficiency and a higher overvoltage at the start of discharge. Furthermore, in Comparative Example 5, the average oxygen concentration C1 in the first region R1 was high at 0.55, which is thought to have resulted in a slightly lower initial coulombic efficiency and a higher overvoltage at the start of discharge. Furthermore, in Comparative Example 6, the average oxygen concentration C3 in the first region R1 was high at 0.30, which is thought to have resulted in insufficient battery reaction.

[0060] Although the present disclosure has been described above with reference to several embodiments and examples, the configuration of the present disclosure is not limited to the above-described configuration and can be modified in various ways. For example, in the above-described embodiments and examples, a coating containing an ion-conductive material is provided on at least a portion of the interface between the cathode active material structure and the solid electrolyte, but the present disclosure is not limited to the case where such a coating is present. However, the provision of such a coating is expected to improve ion permeability and suppress side reactions.

[0061] The effects described in this specification are merely examples, and the effects of the present disclosure are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present disclosure.

[0062] Furthermore, the present disclosure may take the following forms: <1> A solid-state battery including, in this order, a positive electrode, a solid electrolyte layer, and a negative electrode, wherein the solid electrolyte layer has a first region, a second region, and a third region that are equally divided into three in a thickness direction from the positive electrode to the negative electrode, wherein the first region contains at least Li (lithium), P (phosphorus), S (sulfur), and O (oxygen), and the second region and the third region each contain at least Li (lithium), P (phosphorus), and S (sulfur), wherein the average oxygen concentration of the first region is higher than the average oxygen concentration of the second region, and the average oxygen concentration of the second region is equal to or higher than the average oxygen concentration of the third region, and wherein the following conditional expressions (1) and (3) are satisfied: 0.08<C1<0.55 (1) 0≦C3<0.3 (3) where C1 is the average oxygen concentration of the first region, and C3 is the average oxygen concentration of the third region. <2> The solid state battery according to <1> above, further satisfying the following conditional formula (2): 0≦C2<0.35 (2), where C2 is the average oxygen concentration of the second region. <3> The solid state battery according to <1> above or <2>, wherein the positive electrode includes: a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector, the positive electrode active material layer having a positive electrode active material structure that is continuous from the positive electrode current collector to the solid electrolyte layer. <4> The solid-state battery according to any one of <1> to <3>, further comprising a coating provided on a surface of the positive electrode active material structure and containing an ion-conductive material, wherein the coating contains, as the ion-conductive material, a compound containing Li (lithium), at least one of B (boron), Al (aluminum), Si (silicon), P (phosphorus), Ti (titanium), Ge (germanium), Zr (zirconium), Nb (niobium), In (indium), Sn (tin), Hf (hafnium), Ta (tantalum), and W (tungsten), and at least one of O (oxygen), S (sulfur), N (nitrogen), F (fluorine), Cl (chlorine), Br (bromine), and I (iodine). <5> The solid-state battery according to <4>, wherein the coating has a thickness of 1 nm or more and 100 nm or less.<6> A solid electrolyte layer provided between a positive electrode and a negative electrode, the solid electrolyte layer having a first region, a second region, and a third region divided into three equal parts in a thickness direction from the positive electrode toward the negative electrode, the first region containing at least Li (lithium), P (phosphorus), S (sulfur), and O (oxygen), the second region and the third region each containing at least Li (lithium), P (phosphorus), and S (sulfur), the first region having an average oxygen concentration higher than the second region, the second region having an average oxygen concentration equal to or higher than the third region, and satisfying the following conditional expressions (1) and (3): 0.08<C1<0.55 (1) 0≦C3<0.3 (3) where C1: average oxygen concentration of the first region, and C3: average oxygen concentration of the third region. <7> The solid electrolyte layer according to <6> above, further satisfying the following conditional expression (2): 0≦C2<0.35 (2) where C2 is the average oxygen concentration in the second region.

Claims

1. A solid-state battery comprising, in that order, a positive electrode, a solid electrolyte layer, and a negative electrode, the solid electrolyte layer having a first region, a second region, and a third region that are divided into three equal parts in a thickness direction from the positive electrode toward the negative electrode, the first region containing at least Li (lithium), P (phosphorus), S (sulfur), and O (oxygen), the second region and the third region each containing at least Li (lithium), P (phosphorus), and S (sulfur), the average oxygen concentration of the first region is higher than the average oxygen concentration of the second region, the average oxygen concentration of the second region is equal to or higher than the average oxygen concentration of the third region, and the following conditional expressions (1) and (3) are satisfied: 0.08<C1<0.55 ... (1) 0≦C3<0.3 ... (3) where C1 is the average oxygen concentration of the first region, and C3 is the average oxygen concentration of the third region.

2. The solid-state battery according to claim 1, further satisfying the following conditional formula (2): 0≦C2<0.35 (2), where C2 is the average oxygen concentration of the second region.

3. The solid-state battery according to claim 1 or 2, wherein the positive electrode comprises: a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector, and the positive electrode active material layer has a continuous positive electrode active material structure extending from the positive electrode current collector to the solid electrolyte layer.

4. The solid-state battery according to any one of claims 1 to 3, further comprising a coating provided on a surface of the positive electrode active material structure and containing an ion conductive material, the coating including a compound containing, as the ion conductive material, Li (lithium) and at least one of B (boron), Al (aluminum), Si (silicon), P (phosphorus), Ti (titanium), Ge (germanium), Zr (zirconium), Nb (niobium), In (indium), Sn (tin), Hf (hafnium), Ta (tantalum), and W (tungsten), and at least one of O (oxygen), S (sulfur), N (nitrogen), F (fluorine), Cl (chlorine), Br (bromine), and I (iodine).

5. The solid-state battery according to claim 4, wherein the coating has a thickness of 1 nm or more and 100 nm or less.

6. A solid electrolyte layer provided between a positive electrode and a negative electrode, the solid electrolyte layer having a first region, a second region, and a third region that are divided into three equal parts in a thickness direction from the positive electrode toward the negative electrode, the first region contains at least Li (lithium), P (phosphorus), S (sulfur), and O (oxygen), the second region and the third region each contain at least Li (lithium), P (phosphorus), and S (sulfur), the average oxygen concentration of the first region is higher than the average oxygen concentration of the second region, the average oxygen concentration of the second region is equal to or higher than the average oxygen concentration of the third region, and the solid electrolyte layer satisfies the following conditional expressions (1) and (3): 0.08<C1<0.55 ... (1) 0≦C3<0.3 ... (3) where C1 is the average oxygen concentration of the first region, and C3 is the average oxygen concentration of the third region.

7. The solid electrolyte layer according to claim 6, further satisfying the following conditional formula (2): 0≦C2<0.35 (2), where C2 is the average oxygen concentration of the second region.

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