Positive electrode active material, positive electrode, and solid-state battery

Abstract

Provided is a positive electrode active material that exhibits an enhanced performance. The positive electrode active material is formed by the aggregation of a plurality of primary particles each having a disordered rock salt structure and being represented by formula (1). The plurality of primary particles have a particle diameter of at least 3 nm and not more than 10 nm as determined using the Scherrer equation from the X-ray diffraction spectrum yielded by an X-ray diffraction method. The difference between the particle diameter of a plurality of primary particles as determined using the Scherrer equation from the X-ray diffraction spectrum yielded by an X-ray diffraction method, and the particle diameter of a plurality of primary particles as determined from the TEM image, is equal to or greater than 0 nm and not more than 2 nm.　(1) LiaMbX1-bOc (where M includes at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe); X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C); and the following are satisfied: 0 < a ≤ 2, 0.67 ≤ b < 1, and 1 < c ≤ 3.)

Description

Cathode active material, cathode, and solid-state battery

[0001] The present disclosure relates to a positive electrode active material, a positive electrode including the positive electrode active material, and a solid-state battery.

[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] Recently, positive electrode active materials with high electrochemical activity have been investigated (see, for example, Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2022-140018

[0005] Although various studies have been conducted to improve the performance of secondary batteries and positive electrode active materials, there is still room for improvement in the performance of secondary batteries and positive electrode active materials.

[0006] Therefore, there is a demand for a positive electrode active material with better performance, as well as a positive electrode and a solid-state battery including the same.

[0007] The positive electrode active material according to one embodiment of the present disclosure is formed by agglomeration of a plurality of primary particles each having a disordered rock salt structure and represented by the following formula (1): The plurality of primary particles have a particle diameter of 3 nm to 10 nm determined using the Scherrer equation from the diffracted X-ray spectrum obtained by the X-ray diffraction method. The difference between the particle diameter of the plurality of primary particles determined using the Scherrer equation from the diffracted X-ray spectrum obtained by the X-ray diffraction method and the particle diameter of the plurality of primary particles determined from the TEM image is 0 nm to 2 nm. Li ａ M ｂ X １－ｂ O ｃ ... (1) (wherein M includes at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe), and X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C), and 0<a≦2, 0.67≦b<1, and 1<c≦3 are satisfied.)

[0008] According to one embodiment of the positive electrode active material of the present disclosure, the primary particles have a particle diameter of 3 nm to 10 nm, as determined from the diffracted X-ray spectrum by X-ray diffraction using the Scherrer equation, and the difference between the particle diameters of the primary particles determined from the diffracted X-ray spectrum by X-ray diffraction using the Scherrer equation and the particle diameters of the primary particles determined from the TEM image is 0 nm to 2 nm. This reduces the diffusion distance of electrons and lithium ions. Therefore, when used in a battery, a good ion conduction path is formed in the battery, resulting in a higher capacity.

[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 cross-sectional schematic diagram illustrating an example of a configuration of a solid-state battery according to an embodiment of the present disclosure. Fig. 2 is an X-ray diffraction spectrum obtained by X-ray diffraction (XRD) using CuKα radiation for a powder of a positive electrode active material according to an embodiment of the present disclosure.

[0011] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. The description will be made in the following order: 0. Overview of the Present Disclosure 1. First Embodiment 1.1 Configuration of Positive Electrode Active Material 1.2 Method for Manufacturing Positive Electrode Active Material 1.3 Function and Effects of Positive Electrode Active Material 2. Second Embodiment 2.1 Configuration of Solid-State Battery 2.2 Method for Manufacturing Solid-State Battery 2.3 Function and Effects of Solid-State Battery 3. 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, the outline of the present disclosure will be described. Various studies have been conducted so far on improving the performance of positive electrode active materials. For example, in the above-mentioned Patent Document 1, LiMO synthesized by a solid-state reaction is ２Patent Document 1 discloses a method for producing a positive electrode active material by mechanically pulverizing M (where M is at least one selected from vanadium, chromium, manganese, iron, cobalt, and nickel) to obtain a positive electrode active material. Patent Document 1 further discloses a production method in which the material is mechanically pulverized so that the crystallite size is 120 Å or less. Patent Document 1 states that a positive electrode active material with high electrochemical activity can be easily obtained.

[0013] However, in the method disclosed in Patent Document 1, LiMO ２ Furthermore, the cycle characteristics in the charge-discharge cycle test shown in Patent Document 1 are also insufficient for practical use.

[0014] In view of the above circumstances, the present applicant proposes a battery that can provide a higher battery capacity, as well as a positive electrode active material and a positive electrode that can be used in the battery.

[0015] [1. First embodiment] <1.1 Configuration of positive electrode active material> The positive electrode active material of this embodiment has a disordered rock salt structure and is formed by aggregating a plurality of primary particles represented by the following formula (1): Li ａ M ｂ X １－ｂ O ｃ ... (1) (wherein M includes at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe), and X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C), and 0<a≦2, 0.67≦b<1, and 1<c≦3 are satisfied.)

[0016] Furthermore, the positive electrode active material of the present embodiment does not have a so-called layered rock salt structure in which lithium ions and transition metal ions are regularly arranged, but has a disordered rock salt structure in which lithium ions and transition metal ions are irregularly arranged. The disordered rock salt structure can be determined by the fact that the space group determined by X-ray diffraction (XRD) is Fm-3m and that there are multiple elements (Li, M, and X in Formula (1)) measured by composition analysis using inductively coupled plasma atomic emission spectrometry.

[0017] In the lithium metal composite oxide represented by the above formula (1), it is preferable that M contains at least nickel (Ni), because this allows a higher capacity to be achieved when used in a battery.

[0018] In the lithium metal composite oxide represented by the above formula (1), it is preferable that X contains at least boron (B), because this allows a higher capacity to be achieved when used in a battery.

[0019] The primary particles each having a rock salt structure preferably have a particle diameter (outer diameter) Rs calculated using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction and ranging from 3 nm to 10 nm. The particle diameter Rs of the primary particles referred to here refers to the crystallite size obtained by applying the Scherrer equation to the peak of the (200) plane assigned to a crystal structure having the space group Fm-3m using X-ray diffraction (XRD) with CuKα radiation. The Scherrer coefficient used is 0.9. Furthermore, the difference ΔR between the particle diameter Rs of the primary particles calculated using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction and the particle diameter Rt of the primary particles calculated from a TEM image should preferably be 0 nm to 2 nm. The particle diameter Rs of the primary particles calculated using the Scherrer equation from XRD measurement is a weighted average diameter along the direction of the diffraction vector of the crystal particles, and amorphous regions are not reflected in the size calculated by this method. On the other hand, the particle diameter Rt of primary particles obtained from a TEM image is the particle size that includes some amorphous regions. Therefore, a small difference ΔR between the particle diameter Rs obtained using the Scherrer equation and the particle diameter Rt obtained from a TEM image (specifically, 0 nm or more and 2 nm or less) means that the primary particles contain few amorphous regions. The particle diameter Rt obtained from a TEM image is calculated as follows: First, images are taken at three different locations under a microscope with a magnification of 1,000,000 times, and the image data is processed using image analysis software (ImageJ) to calculate the size. Crystal particles are observed as lattice fringes in the microscope image at a magnification of 1,000,000 times. The microscope image is Fourier-transformed to synthesize an FFT image, which emphasizes only the bright spots caused by the rock salt structure. This is then combined with an inverse Fourier-transformed image to obtain a composite image that emphasizes the crystal particles. The contrast difference in this composite image is used to detect the contours of the crystal particles, and the longitudinal and lateral lengths of the contours of the detected particles are calculated, and the average of these lengths is calculated as the particle diameter Rt.

[0020] The positive electrode active material of the present embodiment preferably exhibits a maximum peak at 2θ=45±5° in an X-ray diffraction spectrum measured by X-ray diffractometry, with the half-width of the maximum peak being 1.0° to 2.5°. Furthermore, the ratio of the second peak appearing at 2θ=40±10° to the first peak appearing at 2θ=20±10° in the X-ray diffraction spectrum measured by X-ray diffractometry is preferably 1.72 or greater.

[0021] <1.2 Manufacturing Method of Positive Electrode Active Material> A manufacturing method of the positive electrode active material of this embodiment will be described. The positive electrode active material of this embodiment can be manufactured using, for example, a mechanochemical method.

[0022] First, as a raw material, for example, LiMO ２ (wherein M is at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe)) or Li ２ MO ３ (wherein M is at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe)) powder and, for example, Li ｎ X ｍ O ｌ (wherein X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C)) powder. ｎ X ｍ O ｌ Specific examples of Li ２ SO ４、 Li ３ P.O. ４、 Li ３ BO ３ , Li ２ CO ３ Examples include:

[0023] Next, the LiMO ２ Powder of Li ｎ X ｍ O ｌThe powders are weighed out to a predetermined ratio, and then mixed using a mortar and pestle. The mixed powders are then placed in a container made of zirconia, for example, and mixed by ball milling using zirconia balls. Mixing using a mortar and pestle and mixing by ball milling may be repeated alternately multiple times. The weighing and mixing operations are preferably performed in an inert gas atmosphere, for example, argon gas.

[0024] In this manner, the powdered positive electrode active material of the present embodiment is obtained.

[0025] <1.3 Function and Effect of Cathode Active Material> The cathode active material of this embodiment is composed of an aggregate of a plurality of primary particles each having a disordered rock salt structure and represented by the above formula (1). Furthermore, according to the cathode active material of this embodiment, the plurality of primary particles have a particle diameter (outer diameter) of 3 nm to 10 nm, as determined from a diffracted X-ray spectrum obtained by X-ray diffraction using the Scherrer equation. Furthermore, in the cathode active material of this embodiment, the difference ΔR between the particle diameter Rs of the plurality of primary particles obtained from a diffracted X-ray spectrum obtained by X-ray diffraction using the Scherrer equation and the particle diameter Rt of the plurality of primary particles obtained from a TEM image is 0 nm to 2 nm. This reduces the diffusion distance of electrons and lithium ions. As a result, when used in a battery, a good ion conduction path is formed in the battery, resulting in a higher capacity.

[0026] 2. Second Embodiment 2.1 Configuration of Solid-State Battery 100 The configuration of a solid-state battery 100 according to one embodiment of the present disclosure will be described with reference to FIG. 1 . 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. 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.

[0027] (Positive Electrode 10) The positive electrode 10 is an electrode layer containing the positive electrode active material described in the first embodiment. The positive electrode 10 has a positive electrode current collector 11 and a positive electrode active material layer 12.

[0028] 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.

[0029] 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.

[0030] (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.

[0031] The composition of the positive electrode active material in the positive electrode active material layer 12 can be measured as follows: First, the solid state battery 100 is fully charged. Next, a portion of the positive electrode active material is taken out of the solid state battery 100, and composition analysis is performed using STEM-EDX and inductively coupled plasma optical emission spectroscopy, thereby allowing the composition of the positive electrode active material to be calculated.

[0032] (Solid Electrolyte Layer 20) The solid electrolyte layer 20 has a solid electrolyte. The solid electrolyte is a material capable of conducting ions such as lithium ions between the positive electrode 10 and the negative electrode 30. The solid electrolyte is, for example, a material capable of conducting lithium ions. In particular, the solid electrolyte constituting the battery structural unit in a solid-state battery constitutes the solid electrolyte layer 20 capable of conducting lithium ions between the positive electrode 10 and the negative electrode 30. Specific examples of the solid electrolyte include lithium-containing phosphate compounds having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet or garnet-like structure. Examples of lithium-containing phosphate compounds having a NASICON structure include Li ｘ M ｙ (P.O. ４ ) ３ (1≦x≦2, 1≦y≦2, M is at least one selected from the group consisting of Ti, Ge, Al, Ga and Zr). An example of a lithium-containing phosphate compound having a Nasicon structure is Li １．２ Al ０．２ Ti １．８ (P.O. ４ ) ３ Examples of oxides having a perovskite structure include La ０．５５ Li ０．３５ TiO ３ Examples of oxides having a garnet-type or garnet-like structure include Li ７ La ３Zr ２ O １２ Examples of the solid electrolyte capable of conducting sodium ions include sodium-containing phosphate compounds having a NASICON structure, oxides having a perovskite structure, and oxides having a garnet or garnet-like structure. Examples of the sodium-containing phosphate compounds having a NASICON structure include Na ｘ M ｙ (P.O. ４ ) ３ (1≦x≦2, 1≦y≦2, M is at least one selected from the group consisting of Ti, Ge, Al, Ga and Zr).

[0033] 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.

[0034] (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.

[0035] (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 absorption and release of lithium ions in the solid-state battery 100 and the transfer of electrons to and from an external circuit. Lithium ions move between the positive electrode 10 and the negative electrode 30 via the solid electrolyte layer 20 (i.e., ion conduction). The absorption and release of lithium ions into the negative electrode active material is accompanied by the oxidation or reduction of the negative electrode active material. Charging and discharging proceeds as electrons or holes for such oxidation-reduction reactions are transferred to the positive electrode 10 or the negative electrode 30. Examples of the negative electrode active material contained in the negative electrode 30 include at least one selected from the group consisting of oxides containing at least one element selected from the group consisting of Ti, Si, Sn, Cr, Fe, Nb, and Mo; graphite-lithium compounds; lithium alloys; lithium-containing phosphate compounds having a NASICON structure; lithium-containing phosphate compounds having an olivine structure; and lithium-containing oxides 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.

[0036] <2.2 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, after preparing the positive electrode current collector 11 and the powdered positive electrode active material described in the first embodiment, the positive electrode active material powder is placed on the surface of the positive electrode current collector 11. Alternatively, a binder and a conductive additive may be mixed with the positive electrode active material powder to prepare a positive electrode active material mixture, and the positive electrode active material mixture may be applied to the surface of the positive electrode current collector 11. Next, the positive electrode current collector 11 and the positive electrode active material powder (or the positive electrode active material mixture) are pressure-molded using a press, thereby forming the positive electrode active material layer 12 on the positive electrode current collector 11. In this manner, the positive electrode 10 is obtained.

[0038] Next, the negative electrode 30 is fabricated. Specifically, a negative electrode current collector 31 and a powdered negative electrode active material are prepared, and the negative electrode active material powder is placed on the surface of the negative electrode current collector 31. Alternatively, a binder and a conductive additive may be mixed with the negative electrode active material powder to prepare a negative electrode active material mixture, and then the negative electrode active material mixture may be applied to the surface of the negative electrode current collector 31. Next, the negative electrode current collector 31 and the negative electrode active material powder (or the negative electrode active material mixture) are 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.

[0039] Finally, the positive electrode 10, the solid electrolyte layer 20, and the negative electrode 30 are stacked in this order to form a laminate, and then the laminate is compressed using a press, thereby producing the solid battery 100.

[0040] 2.3 Functions and Effects of Solid-State Battery 100 The solid-state battery 100 of this embodiment uses the positive electrode active material described in the first embodiment. Therefore, the solid-state battery 100 forms a good lithium ion conduction path, and can achieve excellent performance such as high capacity.

[0041] 3. Examples Examples of the present disclosure will be described.

[0042] Example 1 (Preparation of Positive Electrode Active Material) A positive electrode active material of Example 1 was prepared according to the procedure described below. Specifically, first, LiNiO ２ powder and Li ３ BO ３ Next, the powders of LiNiO ２ powder and Li ３ BO ３ The powders were weighed out so that the molar ratio was 4:1, and then LiNiO ２ powder and Li ３ BO ３ The powders were placed in a mortar and mixed using a pestle. 4 g of the mixed powder was then placed in a zirconia container (capacity 45 ml) and mixed by ball milling using a zirconia ball (70 g) with a diameter of 5 mm. The ball milling was carried out at a rotation speed of 400 rpm for 48 hours (with a 10-minute break every 12 hours and the rotation direction reversed). The mixing using the mortar and pestle and the mixing by ball milling were alternately repeated twice. The weighing and mixing operations were carried out in an argon gas atmosphere. As a result, the positive electrode active material powder of Example 1 was obtained.

[0043] (Evaluation of Positive Electrode Active Material) The powder of the obtained positive electrode active material was analyzed by X-ray diffraction (XRD) using CuKα radiation, and the X-ray diffraction spectrum shown in FIG. 2 was obtained. From the X-ray diffraction spectrum in FIG. 2, the obtained positive electrode active material had a space group assigned to Fm-3m. Furthermore, the obtained positive electrode active material contained multiple elements (Li, M, and X in formula (1)) as measured by composition analysis using inductively coupled plasma atomic emission spectrometry. Therefore, it was confirmed that this positive electrode active material had a disordered rock salt structure in which lithium and transition metal ions were disordered. No peaks attributable to the raw materials were observed in the X-ray diffraction spectrum in FIG. 2.

[0044] In the X-ray diffraction spectrum of the positive electrode active material according to Example 1, a peak due to the (200) plane was observed near 45°. The crystallite size of the positive electrode active material, i.e., the particle diameter Rs, was calculated using the Scherrer equation and found to be 6 nm. Furthermore, when a high-resolution TEM image and an electron beam diffraction image were obtained for the positive electrode active material according to Example 1, it was confirmed that primary particles with an average particle diameter Rt of 8 nm were aggregated. That is, the difference ΔR between the particle diameter Rs calculated using the Scherrer equation from XRD measurement and the particle diameter Rt calculated from the TEM image was 2 nm. That is, the positive electrode active material had few amorphous crystallite regions. The exact position of the peak near 45° was 44.4°, and the half-width of the peak was 1.5°. Furthermore, in the X-ray diffraction spectrum, the ratio of the second peak, which is the maximum peak appearing at 2θ = 40 ± 10°, to the first peak, which is the maximum peak appearing at 2θ = 20 ± 10°, was 1.78. The composition of the positive electrode active material was analyzed by STEM-EDX and inductively coupled plasma optical emission spectroscopy. The composition ratios of the positive electrode active material are shown in Table 1. The measurement method was as follows: First, a 75 nm x 75 nm scale STEM-EDX mapping of the positive electrode active material was obtained. The EDX signal intensity of each of the obtained primary particles (100 particles) was converted into atomic composition percentages (at%), and the composition of each element was calculated. Then, 100 particles were randomly selected and histogrammed to determine the mode of distribution of element ratios of elements excluding lithium and boron. The composition ratios of boron, lithium, and nickel were calculated for the positive electrode active material by inductively coupled plasma optical emission spectroscopy. The element ratios of elements excluding boron and lithium obtained from the STEM-EDX mapping and the composition ratios of boron, lithium, and nickel obtained from inductively coupled plasma optical emission spectroscopy were used to obtain the composition ratio of the positive electrode active material. The obtained composition ratios roughly matched the mixing ratios of the raw materials. The evaluation results of the above positive electrode active materials are shown in Tables 1 and 2.

[0045] (Fabrication of Solid-State Battery (Half Cell)) Furthermore, a solid-state battery (half cell) for evaluation was fabricated, which included a positive electrode (evaluation electrode) containing the positive electrode active material, a reference electrode as the counter electrode, and a solid electrolyte layer provided between the positive electrode and the reference electrode, and the battery characteristics were then evaluated. The positive electrode was fabricated using the positive electrode active material powder of Example 1 above, VGCF (vapor grown carbon fiber, a trade name of Resonac Co., Ltd.), and an Argyrodite-type electrolyte Li ６ P.S. ５ Cl and PTFE were measured in a ratio of 57:2:40:1 (wt%), mixed in a mortar, and pressed to form a powder compact, which was then laminated with aluminum foil as a current collector. The solid electrolyte layer was made of an argyrodite-type electrolyte Li ６ P.S. ５ Furthermore, a laminated metal body in which a lithium metal foil was sandwiched between two indium metal foils was used as the reference electrode.

[0046] (Evaluation of Battery Characteristics) The battery characteristics of the solid state battery of Example 1 were evaluated, and the results shown in Table 2 were obtained. Here, the solid state battery of Example 1 was subjected to two cycles of constant current charging and discharging (4.3 V - 2 V) at 11 mA / g in an environment of 60°C, and the discharge capacity at the second cycle was measured.

[0047]

[0048]

[0049] Example 2: Li as a raw material ３ BO ３ Instead of powder of Li ３ P.O. ４ A positive electrode active material and a solid battery of Example 2 were fabricated in the same manner as in Example 1, except that the powder of Example 2 was used. The fabricated positive electrode active material and solid battery of Example 2 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0050] Example 3: Li as a raw material ３ BO ３ Instead of powder of Li ２ SO ４A positive electrode active material and a solid battery of Example 3 were fabricated in the same manner as in Example 1, except that the powder of Example 3 was used. The fabricated positive electrode active material and solid battery of Example 3 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0051] Example 4: Li as a raw material ３ BO ３ Instead of powder of Li ２ CO ３ A positive electrode active material and a solid battery of Example 4 were fabricated in the same manner as in Example 1, except that the powder of Example 4 was used. The fabricated positive electrode active material and solid battery of Example 4 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0052] Example 5: LiNiO as raw material ２ Instead of powder of LiNi ０．８ CO ０．１ Mn ０．１ O ２ Powder of Li ３ BO ３ Instead of powder of Li ２ SO ４ A positive electrode active material and a solid battery of Example 5 were fabricated in the same manner as in Example 1, except that the powder of Ni, Co, and Mn was used. The fabricated positive electrode active material and solid battery of Example 5 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2. Note that the value of b for Example 5 in Table 1 is the sum of the composition amounts of Ni, Co, and Mn.

[0053] Example 6: LiNiO as raw material ２ Instead of powder of LiFeO ２ A positive electrode active material and a solid battery of Example 6 were fabricated in the same manner as in Example 1, except that the powder of Example 6 was used. The fabricated positive electrode active material and solid battery of Example 6 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0054] Example 7: LiNiO as raw material ２ Instead of powder of LiMnO ２ Powder of Li ３ BO ３Instead of powder of Li ３ P.O. ４ A positive electrode active material and a solid battery of Example 7 were fabricated in the same manner as in Example 1, except that the powder of Example 7 was used. The fabricated positive electrode active material and solid battery of Example 7 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0055] Example 8 Manganese (II) acetate tetrahydrate, lithium acetate, and boric acid were used as raw materials. These compounds were weighed to a molar ratio of Li:Mn:B = 7:4:1 and dissolved in acetic acid as a solvent to prepare a solution. This solution was dried under reduced pressure at 200°C and then calcined at 300°C for 1 hour and then at 750°C for 5 hours to prepare a calcined body. The calcined body was then processed in a ball mill to obtain an active material powder. 1 g of the powder was transferred to a 45 ml zirconia container along with 70 g of 5 mm diameter zirconia balls and milled at 400 rpm for 3 hours (with a 10-minute break every hour, after which the direction of rotation was reversed), for a total of 6 hours. The cathode active material and solid-state battery prepared in Example 8 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0056] Example 9: LiNiO as raw material ２ Instead of powder of LiMnO ２ A positive electrode active material and a solid battery of Example 9 were fabricated in the same manner as in Example 1, except that the powder of Example 9 was used. The fabricated positive electrode active material and solid battery of Example 9 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0057] Example 10: LiNiO as raw material ２ Instead of powder of LiCoO ２ A positive electrode active material and a solid battery of Example 10 were fabricated in the same manner as in Example 1, except that the powder of Example 10 was used. The fabricated positive electrode active material and solid battery of Example 10 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0058] Example 11: LiNiO ２ powder and Li ３ BO３ The positive electrode active material and the solid battery of Example 11 were fabricated in the same manner as in Example 1, except that the powders of and were weighed and mixed in a molar ratio of 6.7:3.3. The positive electrode active material and the solid battery of Example 11 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0059] Comparative Example 1: Li as a raw material ３ BO ３ Instead of powder of Li ２ SO ４ A positive electrode active material and a solid battery of Comparative Example 1 were fabricated in the same manner as in Example 1, except that the powder of Comparative Example 1 was used. The fabricated positive electrode active material and solid battery of Comparative Example 1 were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0060] Comparative Example 2: LiNiO ２ Instead of powder of LiFeO ２ powder and LiFeO ２ powder and Li ３ BO ３ The positive electrode active material and the solid battery of Comparative Example 2 were fabricated in the same manner as in Example 1, except that the powders of and were weighed and mixed in a molar ratio of 1:1. The positive electrode active material and the solid battery of Comparative Example 2 thus fabricated were evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0061] [Discussion] As shown in Table 2, the batteries of Examples 1 to 11 achieved higher discharge capacities than the batteries of Comparative Examples 1 and 2. This is thought to be because Examples 1 to 11 each used a positive electrode active material that had a disordered rock salt structure and was formed by agglomerating a plurality of primary particles represented by the above formula (1), thereby achieving good ionic conductivity in the positive electrode active material layer.

[0062] The present disclosure has been described above with reference to several embodiments, modifications, and examples, but the configuration of the present disclosure is not limited to the configuration described above and can be modified in various ways.

[0063] Specifically, the raw materials for producing the cathode active material of the present disclosure are not limited to those described in the above embodiments and examples. Furthermore, the synthesis method for the cathode active material of the present disclosure is not limited to the mechanochemical method. For example, a solid-phase method or a liquid-phase method may be used.

[0064] In addition, although the second embodiment has been described by exemplifying a solid-state battery to which the cathode active material of the present disclosure is applied, the present disclosure is not limited thereto. For example, the cathode active material of the present disclosure can also be used in batteries having a liquid or gel electrolyte.

[0065] Furthermore, 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.

[0066] Furthermore, the present disclosure may take the following forms: <1> A positive electrode active material comprising a plurality of primary particles each having a disordered rock salt structure and represented by the following formula (1) aggregated together, the plurality of primary particles having a particle diameter of 3 nm to 10 nm determined using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction, and a difference between the particle diameter of the plurality of primary particles determined using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction and the particle diameter of the plurality of primary particles determined from a TEM image is 0 nm to 2 nm. ａ M ｂ X １－ｂ O ｃ... (1) (wherein M includes at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe), and X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C), and 0<a≦2 is satisfied, 0.67≦b<1 is satisfied, and 1<c≦3 is satisfied.) <2> The positive electrode active material according to <1> above, which exhibits a maximum peak at 2θ=45±5° in a diffracted X-ray spectrum measured by an X-ray diffraction method, and has a half-width of 1.0° or more and 2.5° or less. <3> The positive electrode active material according to <1> or <2> above, which exhibits a ratio of a second peak appearing at 2θ=40±10° to a first peak appearing at 2θ=20±10° in a diffracted X-ray spectrum measured by an X-ray diffraction method of 1.72 or more. <4> The cathode active material according to any one of <1> to <3> above, wherein M in formula (1) contains at least nickel (Ni). <5> The cathode active material according to any one of <1> to <4> above, wherein X in formula (1) contains at least boron (B). <6> A cathode comprising: a cathode current collector; and a cathode active material layer provided on the cathode current collector, wherein the cathode active material layer contains the cathode active material according to any one of <1> to <5> above. <7> A solid-state battery comprising, in this order, the cathode according to <6> above, a solid electrolyte layer, and a cathode.

Claims

1. A positive electrode active material comprising a plurality of primary particles each having a disordered rock salt structure and represented by the following formula (1) aggregated together, wherein the plurality of primary particles have a particle diameter of 3 nm to 10 nm as determined using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction, and the difference between the particle diameter of the plurality of primary particles as determined using the Scherrer equation from a diffracted X-ray spectrum obtained by X-ray diffraction and the particle diameter of the plurality of primary particles as determined from a TEM image is 0 nm to 2 nm. ａ M ｂ X １－ｂ O ｃ ... (1) (wherein M includes at least one of nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe), and X is at least one of phosphorus (P), boron (B), sulfur (S), and carbon (C), and 0<a≦2, 0.67≦b<1, and 1<c≦3 are satisfied.) 2. The positive electrode active material according to claim 1, which exhibits a maximum peak at 2θ=45±5° in a diffraction X-ray spectrum measured by X-ray diffraction method, and the half-value width of the maximum peak is 1.0° or more and 2.5° or less.

3. The positive electrode active material according to claim 1 or 2, wherein in a diffraction X-ray spectrum obtained by X-ray diffraction, the ratio of a second peak appearing at 2θ=40±10° to a first peak appearing at 2θ=20±10° is 1.72 or more.

4. The positive electrode active material according to any one of claims 1 to 3, wherein M in formula (1) contains at least nickel (Ni).

5. The positive electrode active material according to any one of claims 1 to 4, wherein X in formula (1) contains at least boron (B).

6. A positive electrode comprising: a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector, wherein the positive electrode active material layer contains the positive electrode active material according to any one of claims 1 to 5.

7. A solid-state battery comprising the positive electrode according to claim 6, a solid electrolyte layer, and a negative electrode, in that order.

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