Positive electrode active material and all-solid-state battery

Abstract

To provide a positive electrode active material or an all-solid-state battery using the same that is inexpensive, can be supplied stably, and has higher energy efficiency. [Solution] The invention comprises positive electrode active material particles and a coating film covering at least a portion of the positive electrode active material particles, wherein the coating film is mainly composed of an oxide containing sodium.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for use in an all-solid-state battery, or to an all-solid-state battery using the same. [Background technology]

[0002] Traditionally, lithium-ion rechargeable batteries using lithium have been widely used. However, lithium-ion rechargeable batteries are expensive because they use lithium, a rare earth element, and they have safety issues such as concerns about leakage and the risk of fire due to internal short circuits, as they use a liquid (electrolyte) as the substance that transports Li ions. From this perspective, the development of all-solid-state rechargeable batteries is progressing as the next generation of rechargeable batteries. All-solid-state rechargeable batteries use a solid electrolyte, which essentially eliminates the risk of leakage, and the use of a hard solid can also prevent internal short circuits. In addition, because the safety risks are inherently low, safety design is reduced, and cost benefits can be enjoyed. Furthermore, all-solid-state rechargeable batteries can use inexpensive materials such as sodium, which are less susceptible to resource depletion, instead of rare earth elements such as lithium. However, all-solid-state rechargeable batteries have the drawback of lower output compared to liquid-based batteries that use an electrolyte. In particular, oxide-based all-solid-state batteries have powder particles that are physically harder and less flexible than sulfide-based all-solid-state batteries, making it difficult for the particles to adhere closely to each other and ensuring sufficient contact area between particles. This insufficient contact between particles leads to increased interfacial resistance, which in turn tends to reduce current output, making the decrease in output a major problem. Furthermore, because oxides are hard, inflexible, and brittle materials, they are prone to cracking due to volume changes associated with repeated charging and discharging. The occurrence of these cracks disrupts the current conduction path, resulting in a significant decrease in current output and capacity, which is also a major problem.

[0003] In recent years, various research and developments have been carried out to increase the output of all-solid-state batteries. For example, in Patent Document 1, in a sulfide-based all-solid-state battery, an unintended reaction occurs due to the direct contact between the sulfide solid electrolyte and the positive electrode active material particles, and the substance generated thereby inhibits ion conduction. To solve this problem and reduce the interfacial resistance, a technique has been proposed to form a coating film coated with a phosphorus compound on the surface of the positive electrode active material particles.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1, it is disclosed that a positive electrode active material coated with a phosphoric acid compound has a lower interfacial resistance and higher output compared to a positive electrode active material not coated with a phosphoric acid compound. However, the output is smaller compared to a liquid-based battery, and there is a demand for an all-solid-state battery having a higher output or a positive electrode active material for manufacturing an all-solid-state battery having a higher output. Furthermore, since sulfur is used, there is a risk of generating hydrogen sulfide, and a more safe positive electrode active material and an all-solid-state battery using such a material are desired. In addition, since lithium, which has a risk of resource procurement, is required as a material, there are problems in terms of cost and stable supply. Therefore, an all-solid-state battery and a positive electrode active material that are larger in output, have cost advantages, can be stably supplied, and are safer are desired.

[0006] An object of the present invention is to provide a positive electrode active material or an all-solid-state battery using the same that is inexpensive, can be stably supplied, is safer, and has a higher output.

Means for Solving the Problems

[0007] The present invention is characterized by the following positive electrode active materials (1) to (8). (1) A positive electrode active material comprising positive electrode active material particles and a coating film covering at least a portion of the positive electrode active material particles, wherein the coating film is mainly composed of an oxide containing sodium. (2) The sodium-containing oxide is the positive electrode active material according to (1) above, wherein the oxide has a cationic species element that can form an intermediate oxide. (3) The sodium-containing oxide is the positive electrode active material according to (1) above, wherein the oxide has a cationic species element capable of forming a glass-forming oxide. (4) The positive electrode active material according to (3) above, characterized in that the sodium-containing oxide is phosphorus, which is a cationic species element capable of forming the glass-forming oxide. (5) The positive electrode active material described in (1) above, wherein the thickness of the coating film is 20 nm or less. (6) The positive electrode active material according to (1) above, wherein the sodium-containing oxide is coated in an amorphous state. (7) The positive electrode active material according to (1) above, wherein the positive electrode active material particles are a compound containing sodium, phosphorus, and oxygen. (8) The positive electrode active material according to (1) above, wherein the positive electrode active material particles are particles having a phosphate structure. Furthermore, the present invention is essentially based on the all-solid-state battery described in (9) below. (9) A solid-state battery having a positive electrode active material as described in any of (1) to (8) above. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a positive electrode active material or an all-solid-state battery using the same that is inexpensive, can be supplied stably, and has higher energy efficiency. [Brief explanation of the drawing]

[0009] [Figure 1] This is a diagram showing the configuration of the sintered body according to this embodiment. [Figure 2] This figure shows the charge and discharge curves of the all-solid-state battery according to Comparative Example 1. [Figure 3] This figure shows the charge and discharge curves of the all-solid-state battery according to Comparative Example 2. [Figure 4] This figure shows the charge and discharge curves of the all-solid-state battery according to Example 1. [Figure 5] This figure shows the charge and discharge curves of the all-solid-state battery according to Example 2. [Figure 6] This table shows the measurement results of the average capacity of all-solid-state batteries in Comparative Examples 1 and 2 and Examples 1 and 2. [Figure 7] This is an image of the positive electrode active material particles according to this embodiment, captured with an electron microscope. [Modes for carrying out the invention]

[0010] Embodiments of the positive electrode active material and all-solid-state battery 1 according to the present invention will be described with reference to the figures. In this embodiment, "average particle size" is defined as the particle diameter (median diameter, D50) as the integrated value in the volume-based particle size distribution obtained by laser diffraction-scattering.

[0011] Figure 1 is a diagram showing the configuration of an all-solid-state battery 1 according to this embodiment. As shown in Figure 1, the all-solid-state battery 1 according to this embodiment is constructed by stacking a positive electrode layer 10, a negative electrode layer 20, and an electrolyte layer 30. In this embodiment, an all-solid-state battery 1 consisting of three layers, a positive electrode layer 10, a negative electrode layer 20, and an electrolyte layer 30, is described as an example, but the all-solid-state battery 1 according to the present invention only needs to have at least three layers, a positive electrode layer 10, a negative electrode layer 20, and an electrolyte layer 30. The above three layers may be stacked repeatedly, or for example, the electrolyte layer 30 may be interposed between multiple positive electrode layers 10 and negative electrode layers 20, such as positive electrode layer 10, electrolyte layer 30, negative electrode layer 20, electrolyte layer 30, positive electrode layer 10, electrolyte layer 30.

[0012] The positive electrode layer 10 and the negative electrode layer 20 are constructed by adding a positive electrode active material or a negative electrode active material, respectively, to the material that makes up the electrolyte layer 30. First, the electrolyte layer 30 will be described.

[0013] The electrolyte layer 30 according to the present invention is composed of a solid electrolyte. The solid electrolyte is not particularly limited as long as it can be used in an all-solid-state battery, and more preferably in an all-solid-state sodium ion secondary battery. For example, a sodium ion-conductive oxide solid electrolyte having a NASICON structure can be used. As the sodium ion-conductive oxide solid electrolyte having a NASICON structure, for example, Na3Zr2Si2PO 12 、Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 、Na3Zr 1.6 Ti 0.4 Si2PO 12 、Na3Hf2Si2PO 12 、Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 、Na3Zr 1.7 Nb 0.24 Si2PO 12 、Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9、Na3Zr 1.88 Y 0.12 Si2PO 12 、Na 3.12 Zr 1.88 Y 0.12 Si2PO 12 、Na 3.6 Zr 0.13 Yb 1.67 Si 0.11 P 2.9 O 12 、Na5YSi4O 12 and the like. In this embodiment, Na3Zr2Si2PO 12 (hereinafter referred to as NZSP) shall be used. In addition, the average particle size of the solid electrolyte is preferably 5 μm or less. The positive electrode layer 10 and the negative electrode layer 20 also contain the same solid electrolyte as the electrolyte layer 30.

[0014] Furthermore, the electrolyte layer 30 according to this embodiment may also contain sodium borate as a sintering aid. In particular, from the viewpoint of ionic conductivity and mechanical strength, it is preferable to contain sodium tetraborate (Na2B4O7·10H2O) among the sodium borates, but sodium metaborate (NaBO2), sodium diborate (Na4B2O5), sodium pentaborate (Na2B 10 O 16 ), sodium hexaborate (Na2B6O 10 ), sodium octaborate (NaB8O 13 ), the structure may also include an amorphous material of Na2O-B2O3=x:100-x (x=1~99). Furthermore, in this embodiment, the electrolyte layer 30 may also include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, acrylic acid, etc., as a sintering aid.

[0015] The positive electrode layer 10 comprises at least a positive electrode active material and a solid electrolyte. In particular, in this embodiment, the positive electrode active material is formed by coating at least a portion of the positive electrode active material particles with a sodium-containing oxide.

[0016] Here, the positive electrode active material particles are not particularly limited as long as they are raw materials for the positive electrode active material of an all-solid-state battery, but compounds containing sodium, phosphorus, and oxygen, or crystalline structures of a sodium-containing phosphate structure are preferred. For example, as compounds containing sodium, phosphorus, and oxygen, sodium-containing oxides such as Na3V2(PO4)3, Na2FePO4F, Na3V2(PO4)2F3, Na4Ni3(PO4)2P2O7, and Na4Mn3(PO4)2P2O7 can be used, and more specifically, sodium-containing polyanionic phosphate materials can be used. Also, as crystalline structures of a sodium-containing phosphate structure, thorium iron olivine (NaFeSiO4) and maranger olivine (Na(Mg,Fe)4(Si,Al)8O) can be used. 20 ), Lasalite (Na3(Mg,Fe)2(AlSi4O 10(OH)2 is one example. In this embodiment, nickel phosphate compounds (Na4Ni3(PO4)2P2O7) particles (hereinafter also referred to as NNPP particles) are used. In this embodiment, the positive electrode active material particles are spherical or ellipsoidal in shape. Furthermore, the larger the specific surface area of ​​the positive electrode active material particles, the easier it is for ions and electrons inside the positive electrode active material particles to reach the surface, which has the advantage of improving battery performance. For this reason, in this embodiment, it is preferable that the average particle size of the positive electrode active material particles is 100 μm or less in order to increase the specific surface area.

[0017] Furthermore, the positive electrode active material according to this embodiment is formed by coating positive electrode active material particles with a substance mainly composed of a sodium-containing oxide. In other words, the positive electrode active material according to this embodiment has a coating film mainly composed of a sodium-containing oxide. In this invention, "main component" means that the coating film contains a sodium-containing oxide in a proportion of 1 wt% or more, preferably 50 wt% or more, more preferably 80 wt% or more, and even more preferably 95 wt% or more. Hereinafter, the above sodium-containing oxide will also be referred to as the "oxide that forms the coating film".

[0018] The sodium contained in the oxide that forms the coating film functions as an ion carrier. Furthermore, it is preferable that the oxide that forms the coating film contains at least one cationic species element other than sodium that can form an intermediate oxide. An intermediate oxide is a substance that does not vitrify on its own but can be incorporated into the glass in significantly large quantities. Specifically, examples include Al2O3, BeO, ZnO, CdO, PdO, TiO2, ThO2, NbO2, and Ta2O5. Therefore, examples of the above cationic species elements include Be, Zn, Cd, Pb, Ti, Th, Nb, Ta, and Al. Among these, Al is preferred from the viewpoint of cost and ease of handling in manufacturing. The inclusion of cationic species elements that can form intermediate oxides in the oxide that forms the coating film has the effect of improving the ionic conductivity of sodium, which is an ion carrier.

[0019] Furthermore, the oxide forming the above-mentioned coating film can also have a composition containing cationic species elements capable of forming glass-forming oxides. Glass-forming oxides are substances that become amorphous on their own, and examples include B2O3, SiO2, GeO2, P2O5, As2O5, Sb2O3, Bi2O3, P2O3, V2O5, Sb2O5, As2O3, Cb2O5, and ZrO2. In other words, cationic species elements capable of forming the above-mentioned glass-forming oxides include B, Si, Ge, P, As, Sb, Bi, V, Cb, and Zr. By having cationic species elements capable of forming glass-forming oxides in the oxide forming the coating film, the coating film material can be made amorphous. Since amorphous materials are relatively flexible, covering the inherently hard positive electrode active material with such a coating film imparts flexibility to the surface of the positive electrode active material. As a result, when the battery is manufactured, the positive electrode active material can adhere more closely to other positive electrode active materials and electrolytes contained in the positive electrode layer 10, reducing interfacial resistance and improving the ion conductivity of the all-solid-state battery 1.

[0020] Furthermore, the oxide forming the coating film according to this embodiment is preferably an amorphous substance, and is a solid material mixed with sodium, oxygen, aluminum, and phosphorus, which is coated at the elemental level by sputtering or the like, without undergoing the crystallization process such as heating at high temperatures. It is not necessary for it to be a compound in which elements such as sodium, oxygen, aluminum, and phosphorus are chemically bonded, as in sodium phosphate or aluminum oxide. In addition, in this embodiment, the sodium oxide forming the coating film is 1 × 10⁻¹⁶ even in an amorphous state. -10 ~1 × 10 -6It is preferable to use sodium borate or sodium phosphate because they have an ionic conductivity of approximately S / cm. Sodium phosphate is particularly preferable because it contains fewer deliquescent substances, is easy to prepare as a target material for sputtering, facilitates the handling of the coated cathode active material during manufacturing, and is less prone to undesirable reactions with the cathode active material. Among sodium phosphates, it is preferable to use one with a sodium-to-phosphorus ratio of 3:7 to 9:1, as this range is preferable for suitably forming an amorphous structure.

[0021] Since the coating film is preferably made of components that do not crystallize after firing, it is desirable that the temperature at which crystallization occurs in the coating film material is above the firing temperature. Generally, firing requires at least 300°C, so a crystallization temperature higher than 300°C is preferable, for example, it can be 400°C or higher, 500°C or higher, or 700°C or higher. Generally, the higher the firing temperature of a solid-state battery, the higher the density, which improves the battery performance and mechanical strength, but it must be below the melting point of the positive electrode active material. Since the melting point of the positive electrode active material used in this embodiment is 700°C or higher, it is even preferable that the temperature at which crystallization occurs in the coating film material is higher than 700°C, as firing can be arbitrarily performed at a temperature below 700°C. In this embodiment, since the coating film does not crystallize even after firing, the contact area between positive electrode active materials or between the positive electrode active material and the solid electrolyte can be increased, thereby increasing the ionic conductivity.

[0022] The method for forming the coating film is not particularly limited, but a dry process and gas phase method that allows for relatively easy control of the composition is preferred (a method of coating the positive electrode active material without using solutions such as water or organic solvents is preferred). Barrel sputtering is an example of such a method. Since good battery performance cannot be obtained, or the battery will not function, if electrons do not conduct when sodium ions are added to the coating film, it is desirable to keep the thickness of the coating film to a thickness that allows electron conduction. The thickness should preferably be so thin that it cannot be observed even with a transmission electron microscope (TEM), and specifically, it is preferably 0.01 nm to 20 nm or less, and more preferably 0.1 nm to 10 nm or less. The coating film only needs to cover at least a portion of the positive electrode active material, but for example, a thin coating can be applied to the positive electrode active material particles by sputtering for a long period of time, such as 4 to 150 hours, using a low-power device of 100 W.

[0023] In this embodiment, the positive electrode layer 10 may further contain a conductive additive and / or a sintering additive. Examples of conductive additives include spheroidal graphite, acetylene black, carbon fiber, VGCF, or mixtures thereof. Examples of sintering additives, similar to those for the electrolyte layer 30, include sodium tetraborate, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, acrylic acid, or mixtures thereof.

[0024] The negative electrode layer 20 includes at least a negative electrode active material and a solid electrolyte. The negative electrode active material is not particularly limited as long as it is a negative electrode active material used in all-solid-state batteries, but for example, Na2Ti3O7, NaTi2(PO4)3, NaAlTi3O8, Na 1.3 Al 0.3 Ti 1.7 (PO4)3, Na 1.5 Al 0.5 Ti 1.5 (PO4)3, Na 1.3 Al 0.3 TiZr 0.7Sodium oxides and sodium phosphate compounds such as (PO4)3 and Na3V2(PO4)3 can be used. In this embodiment, Na3V2(PO4)3 having a NASICON-type crystal structure is used. The average particle size of the negative electrode active material is preferably 100 μm or less.

[0025] Furthermore, the negative electrode layer 20 may also contain a conductive additive and / or a sintering additive. The negative electrode layer 20 may also contain the same conductive additive and / or sintering additive as the positive electrode layer 10.

[0026] As described above, the all-solid-state battery 1 according to this embodiment has a positive electrode layer 10, a negative electrode layer 20, and an electrolyte layer 30. The positive electrode active material of the positive electrode layer 10 is formed by coating at least a portion of the positive electrode active material particles with a substance mainly composed of an oxide containing sodium. Furthermore, the positive electrode active material particles are preferably composed of a compound containing sodium, phosphorus, and oxygen, such as Na4Ni3(PO4)2P2O7, or a crystalline body with a sodium-containing phosphate structure. In addition, it is preferable that the specific surface area of ​​the positive electrode active material particles be large, specifically, that the average particle size is 100 μm or less. Furthermore, it is preferable that the oxide forming the coating film contains sodium, which acts as an ion carrier, a cationic species element that can form an intermediate oxide, and a cationic species element that can form a glass-forming oxide. Furthermore, it is preferable that the coating film is amorphous and does not crystallize even after firing.

[0027] (Manufacturing method) The positive electrode active material according to this embodiment can be manufactured as follows. First, positive electrode active material particles, which will serve as the core of the positive electrode active material, are prepared. The positive electrode active material particles can be prepared by known manufacturing methods, similar to conventional positive electrode active materials that do not have a coating film. In particular, in this embodiment, the positive electrode active material is preferably a compound containing sodium, phosphorus, and oxygen, such as Na4Ni3(PO4)2P2O7, or a crystalline body with a sodium-containing phosphate structure, and among the known manufacturing methods for positive electrode active materials, it is preferable to use a liquid-phase method such as the sol-gel method or the precipitation method. Furthermore, it is preferable to prepare the positive electrode active material particles so that the average particle size is 100 μm or less.

[0028] The positive electrode active material according to this embodiment is manufactured by forming a coating film containing a sodium-containing oxide on positive electrode active material particles. In this embodiment, a coating film can be formed on the positive electrode active material particles by depositing a sodium-containing oxide onto the positive electrode active material particles using a dry process vapor phase method (e.g., PVD or CVD), and in particular, the barrel sputtering method is preferably used in this embodiment. For example, by barrel sputtering, sodium, phosphorus, oxygen, and aluminum can be deposited onto the positive electrode active material particles using trisodium phosphate (Na3PO4) and alumina (Al2O3) as targets, forming a mixed film of an oxide containing sodium, phosphorus, oxygen, and aluminum as a coating film. Alternatively, by barrel sputtering, sodium, oxygen, and aluminum can be deposited onto the positive electrode active material particles using β-alumina (Na2O·11Al2O3) as a target, forming a mixed film of an oxide containing sodium, oxygen, and aluminum as a coating film on the positive electrode active material particles.

[0029] Next, a method for manufacturing the all-solid-state battery 1 according to this embodiment will be described. A positive electrode active material and a solid electrolyte are mixed to prepare a positive electrode layer material. Similarly, a negative electrode active material and a solid electrolyte are mixed to prepare a negative electrode layer material. Note that conductive additives and sintering aids may be added when preparing the positive electrode layer material and / or the negative electrode layer material. Furthermore, an electrolyte layer material containing a solid electrolyte is prepared. In addition to the solid electrolyte, a sintering aid may be added to the electrolyte layer material.

[0030] Then, the prepared materials for each layer are placed in a carbon mold layer by layer and pressed together to stack the layers. Preferably, the electrolyte layer 30 is designed to have a thickness of 2 mm or less after firing. The three layers together in the carbon mold are then fired under pressure at a temperature of 700°C or less, or 600°C or less. In this embodiment, the fired sintered body is polished to obtain the all-solid-state battery 1, but it is also possible to obtain the all-solid-state battery without polishing the sintered body. The firing method is not particularly limited, and firing can be performed by hot pressing, SPS (discharge plasma sintering), air firing, etc. When carbon is used as a conductive additive in the electrode layer, it is desirable to have an atmosphere-controlled environment due to the reaction between oxygen in the atmosphere and carbon. Thus, since the all-solid-state battery 1 according to this embodiment can be sintered as a three-layer unit, the manufacturing process can be shortened compared to a method in which each layer is fired separately and then joined together.

[0031] (Properties of all-solid-state batteries 1) In the all-solid-state battery 1 according to this embodiment, the positive electrode active material contained in the positive electrode layer 10 is in the form of a coating film having a sodium-containing oxide on the positive electrode active material particles, thereby increasing the discharge capacity of the all-solid-state battery 1 compared to conventional batteries. Specifically, the all-solid-state battery 1 according to this embodiment can obtain a discharge capacity of 40 mAh / g or more, more preferably 45 mAh / g or more. Although the detailed mechanism of action is unknown, in the positive electrode active material according to this embodiment, the oxide according to this embodiment is amorphous, and since an amorphous oxide is used to form the coating film, it is thought that not only is the interfacial resistance between the positive electrode active material and the solid electrolyte reduced, but the contact area between the positive electrode active material and the solid electrolyte, and the contact area between the positive electrode active materials themselves can be increased during the pressurization and / or firing process, thereby obtaining higher ionic conductivity and discharge capacity than conventional batteries. Furthermore, conventional all-solid-state batteries had the problem of large variations in discharge capacity between manufactured all-solid-state batteries, but the all-solid-state battery 1 according to this embodiment can suppress variations in discharge capacity to a smaller extent compared to conventional batteries. Although the details of this mechanism are unclear, it is thought that forming a coating film with amorphous oxide mitigates the volume change of active material particles associated with the charge-discharge reaction, thereby suppressing the occurrence of cracks. Furthermore, if the oxide contains aluminum, unwanted reactions between different particles are suppressed, allowing for a stable charge-discharge reaction. In addition, in the all-solid-state battery 1 according to this embodiment, the ionic conductivity of the solid electrolyte is 10 -5 It can be set to S / cm or higher. [Examples]

[0032] In this example, an all-solid-state battery was fabricated as follows, and the charge and discharge capacity of the fabricated all-solid-state battery was measured. Specifically, first, an aqueous precursor solution for the positive electrode active material was prepared using disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), and ammonium dihydrogen phosphate (NH4H2PO4) as raw materials. Next, the aqueous precursor solution was evaporated while spraying it into hot air to obtain a dry powder. Then, the dry powder was calcined to crystallize it, and NNPP particles with an average particle size of 100 μm or less were obtained as the positive electrode active material particles of this example. Figure 7 is an image of the positive electrode active material particles fabricated in this example, magnified 5000 times with a scanning electron microscope (SEM). As shown in Figure 7, the positive electrode active material particles according to this example were spherical particles with multiple indentations or pores on their surface.

[0033] In this embodiment, four types of positive electrode active materials were prepared: Examples 1 and 2, and Comparative Examples 1 and 2. Specifically, in Example 1, a positive electrode active material was prepared by coating the prepared NNPP particles with a mixed film of sodium, oxygen, and aluminum using a barrel sputtering method, targeting β-alumina (Na2O·11Al2O3). More specifically, sodium carbonate (Na2CO3), alumina (Al2O3), and lithium carbonate (Li2CO3) were mixed, then calcined at 1000-1400°C to synthesize and pelletize, and then calcined again at 1400-1700°C. The calcined pellets were then pulverized to produce β-alumina (Na2O·11Al2O3) powder. Furthermore, the prepared β-alumina powder was sintered in a hot press at 800-1400°C under pressure conditions of 15 MPa or higher to form a plate. Alternatively, the β-alumina powder may be formed into pellets, then fired at 1400-1700°C to form a plate. The plate-shaped β-alumina was then used as a target for barrel sputtering and bonded to a dedicated backing plate of the barrel sputtering apparatus. NNPP particles, the coating base material, were then introduced into the barrel sputtering apparatus. After creating a high vacuum inside the apparatus, the barrel section of the apparatus was oscillated to agitate the NNPP particles while sputtering the coating was performed. During sputtering, argon was injected at a flow rate of 10 sccm and oxygen at a flow rate of 1 sccm inside the chamber at a pressure of 1.5 Pa. After coating, the chamber was opened to the atmosphere, the sputtered powder was collected, and stored in an argon glove box.

[0034] In Example 2, a positive electrode active material was prepared by coating the fabricated NNPP particles with a mixed film of sodium, phosphorus, oxygen, and aluminum using a barrel sputtering method, with trisodium phosphate (Na3PO4) and alumina (Al2O3) as targets. More specifically, trisodium phosphate (Na3PO4) and alumina (Al2O3) were dry-mixed in a weight ratio of 80-99:1-20 to prepare a mixed powder. Then, as in Example 1, the prepared mixed powder was sintered in a hot press at 800-1400°C and under pressure of 15 MPa or higher to form a plate. The plate-shaped trisodium phosphate (Na3PO4) and alumina (Al2O3) were used as targets for barrel sputtering and were bonded to a dedicated backing plate of the barrel sputtering apparatus. Next, as in Example 1, NNPP particles, which are the coating base material, were introduced into the barrel sputtering apparatus. After creating a high vacuum inside the barrel sputtering apparatus, sputter coating was performed while the barrel portion of the barrel sputtering apparatus was oscillated to agitate the NNPP particles inside. After coating, the chamber was opened to the atmosphere, the sputter-coated powder was collected, and stored in an argon glove box.

[0035] Furthermore, in Comparative Example 1, the fabricated positive electrode active material particles were used as the positive electrode active material without coating. In addition, in Comparative Example 2, a positive electrode active material was prepared by coating the fabricated positive electrode active material particles with a mixed film of sodium, phosphorus, and oxygen, targeting trisodium phosphate (Na3PO4). The film thickness of the coating film in the positive electrode active materials of Examples 1 and 2 and Comparative Example 2 was set to 20 nm or less.

[0036] Furthermore, in this embodiment, the material for the positive electrode layer 10 is 25-30 wt% of the positive electrode active material according to Examples 1 and 2 and Comparative Examples 1 and 2, and a solid electrolyte (Na3Zr2Si2PO 12 The following was weighed at 60-70 wt%, and then a conductive additive (carbon) and a sintering additive (Na2B4O7·10H2O) were added and mixed. In addition, as the material for the negative electrode layer 20, 20-25 wt% of the negative electrode active material (Na3V2(PO4)3) and a solid electrolyte (Na3Zr2Si2PO 12The following was weighed at 50-60 wt%, and a conductive additive (carbon) and a sintering additive (Na2B4O7·10H2O) were added and mixed. Furthermore, as the electrolyte layer 30, a solid electrolyte (Na3Zr2Si2PO 12 A sintering aid (Na2B4O7·10H2O) was added to the mixture and mixed. Then, the materials of each layer were uniformly mixed using a mortar and pestle or a ball mill. The mixing method is not particularly limited, and mixing may be done using a planetary ball mill or a mechanical milling ring. In this example, mixing was performed using an agate mortar and pestle.

[0037] In this embodiment, the positive electrode active material had an average particle size of 90 μm or less, the negative electrode active material had an average particle size of 20 μm or less, the solid electrolyte had an average particle size of approximately 0.1 to 5 μm, and the conductive additive had an average particle size of approximately 10 μm.

[0038] (2) Fill into a mold for sintering Next, the mixed powders for each layer are filled into a mold for sintering. In this example, first, the mixed powders for each layer were placed in a carbon mold (10 mm in diameter) and pressed down evenly, repeating this process three times until the three layers—the negative electrode layer 20, the electrolyte layer 30, and the positive electrode layer 10—were stacked on top of each other. The order in which the mixed powders are filled is not fixed; it is sufficient that the middle layer is the electrolyte layer 30. In this example, a φ10 mm carbon mold was used to manufacture a 10 mm diameter all-solid-state battery, but the size of the carbon mold can be appropriately changed according to the size of the battery to be manufactured. For example, carbon molds with a diameter of 50 mm or 106 mm can be used. The material of the sintering mold is not limited to carbon; materials with a melting point of 1000°C or higher, such as SUS or tungsten carbide, may also be used as the mold.

[0039] (3) Sintering Next, each layer, which had been stacked while still in the carbon mold, was placed in a sintering apparatus and heated to 600-700°C over 1-10 minutes. At this temperature, it was then fired for 30 seconds to 5 minutes under pressure of 100 MPa. In this example, sintering was performed using the SPS method.

[0040] (4) Polishing The fired sintered body was removed from the carbon mold and polished. Polishing was performed to remove excess carbon adhering to the sintered body and to smooth the surface of the sintered body. The polished sintered body was then obtained as the all-solid-state battery according to Examples 1 and 2 and Comparative Examples 1 and 2.

[0041] (5) Measurement of charge / discharge capacity The charge and discharge capacities of the polished solid-state batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were measured. In this example, four solid-state batteries were prepared for each of Examples 1 and 2 and Comparative Example 1 (two solid-state batteries were prepared for Comparative Example 2). The charge and discharge capacities of the solid-state batteries were measured while repeatedly charging and discharging them in a 60°C environment, thereby creating charge and discharge curves for the solid-state batteries of Examples 1 and 2 and Comparative Examples 1 and 2. Figure 2 shows the charge and discharge curve of the solid-state battery of Comparative Example 1. Figure 3 shows the charge and discharge curve of the solid-state battery of Comparative Example 2. Furthermore, Figure 4 shows the charge and discharge curve of the solid-state battery of Example 1, and Figure 5 shows the charge and discharge curve of the solid-state battery of Example 2. Furthermore, Figure 6 is a table showing the average measured discharge capacities of Examples 1 and 2 and Comparative Examples 1 and 2. Figures 2 to 5 show the charge and discharge capacities of each solid-state battery at a 20-hour rate.

[0042] As shown in Figure 2, in the all-solid-state battery according to Comparative Example 1, the discharge capacity when the voltage reached 1.0V was approximately 0 mAh / g (<1 mAh / h). This is thought to be because the all-solid-state battery according to Comparative Example 1 did not have a coating film on the positive electrode active material, resulting in high interfacial resistance. Also, as shown in Figure 3, in the all-solid-state battery according to Comparative Example 2, the discharge capacity when the voltage reached 1.0V was also approximately 0 mAh / g. In the all-solid-state battery according to Comparative Example 2, the positive electrode active material had a coating film, but the target used to construct the coating film was trisodium phosphate, and since the cationic species element in the oxide forming the coating film consisted only of sodium, the coating film crystallized, and the gaps between the solid electrolyte and the positive electrode active material could not be sufficiently filled, which is thought to have reduced ion conductivity. Furthermore, when fabricating the all-solid-state battery according to Comparative Example 2, deliquescence was also confirmed in the positive electrode active material under atmospheric conditions, indicating that there were also stability problems.

[0043] On the other hand, as shown in Figure 4, the all-solid-state battery according to Example 1 had an average discharge capacity of 15 mAh / g when the voltage reached 1.0V. This is thought to be because, in the all-solid-state battery according to Example 1, the positive electrode active material has a coating film consisting of a mixed film of sodium, oxygen, and aluminum, and a part of this coating film reacts with phosphorus contained in the positive electrode active material, so that a part of the coating film of the positive electrode active material is maintained in an amorphous state. As a result, the positive electrode active material deforms in part during pressurization and sintering, filling the gaps between the solid electrolyte and the positive electrode active material, and consequently reducing the interfacial resistance and improving ionic conductivity. Furthermore, as shown in Figure 5, the all-solid-state battery according to Example 2 had an average discharge capacity of 46 mAh / g when the voltage reached 1.0V. The improved discharge capacity of the all-solid-state battery according to Example 2 compared to the all-solid-state battery according to Example 1 is thought to be due to the presence of a coating film consisting of a mixed film containing phosphorus in addition to sodium, oxygen, and aluminum. This coating film maintains an amorphous state over a wider area, making it easier to fill the gaps between the solid electrolyte and the positive electrode active material, thus improving ionic conductivity. Furthermore, the all-solid-state batteries according to Examples 1 and 2 were able to suppress the deliquescence of the positive electrode active material compared to the all-solid-state battery according to Comparative Example 2.

[0044] As described above, the positive electrode active material according to this embodiment has a coating film that covers at least a portion of the positive electrode active material particles, and the coating film mainly contains an oxide containing sodium. The sodium contained in the oxide forming the coating film functions as an ion carrier and can improve the ionic conductivity in the battery. In addition, the oxide forming the coating film contains a cationic species element that can form an intermediate oxide such as aluminum. By including a cationic species element that can form an intermediate oxide, unwanted reactions between the positive electrode active material and the solid electrolyte are suppressed and an increase in interfacial resistance is prevented, and the ionic conductivity of sodium, which acts as an ion carrier, can be further enhanced. Furthermore, by including a cationic species element that can form a glass-forming oxide such as phosphorus in the oxide forming the coating film, the surface can be made amorphous and flexible, so that the contact area between the positive electrode active material and the solid electrolyte, or the contact area between the positive electrode active materials themselves, can be increased in heating and pressurizing processes, and as a result the ionic conductivity can be increased, the discharge capacity can be increased and the output can be increased, and the mechanical strength of the all-solid-state battery 1 can also be improved. Furthermore, in this embodiment, instead of using rare elements that pose resource procurement risks, readily available and inexpensive elements such as sodium, aluminum, and phosphorus can be used. Therefore, it is possible to provide an all-solid-state battery 1 and positive electrode active material that offer cost advantages and stable supply compared to conventional methods. Moreover, by composing the positive electrode active material, electrolyte, and coating film all from essential oxides, it is possible to provide a battery that is safer than sulfide all-solid-state batteries.

[0045] Furthermore, in this embodiment, the discharge capacity of the positive electrode active material can be further increased by using a compound having sodium, phosphorus, and oxygen, or particles having a phosphate structure crystal structure containing sodium, and forming the coating film with an oxide containing sodium. In particular, it is preferable to form the coating film with an oxide containing sodium, oxygen, and aluminum, or an oxide containing sodium, phosphorus, oxygen, and aluminum.

[0046] Although preferred embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the embodiments described above. Various modifications and improvements can be made to the above embodiments, and such modified or improved forms are also included in the technical scope of the present invention. [Explanation of symbols]

[0047] 1…All-solid-state battery 10…Positive electrode layer 20... Negative electrode layer 30...Electrolyte layer

Claims

1. Positive electrode active material particles, The positive electrode active material particles have a coating film that covers at least a portion of them, The coating film is a positive electrode active material characterized by having a sodium-containing oxide as its main component.

2. The positive electrode active material according to claim 1, wherein the sodium-containing oxide has a cationic species element capable of forming an intermediate oxide.

3. The positive electrode active material according to claim 1, wherein the sodium-containing oxide has a cationic species element capable of forming a glass-forming oxide.

4. The positive electrode active material according to claim 3, characterized in that, in the sodium-containing oxide, the cationic species element capable of forming the glass-forming oxide is phosphorus.

5. The positive electrode active material according to claim 1, wherein the thickness of the coating film is 20 nm or less.

6. The positive electrode active material according to claim 1, wherein the sodium-containing oxide is coated in an amorphous state.

7. The positive electrode active material according to claim 1, wherein the positive electrode active material particles are a compound containing sodium, phosphorus, and oxygen.

8. The positive electrode active material according to claim 1, wherein the positive electrode active material particles are particles having a phosphate structure crystal structure.

9. A solid-state battery having a positive electrode active material according to any one of claims 1 to 8.

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