Positive electrode active material, lithium-ion secondary battery, and method for manufacturing a positive electrode active material

Abstract

To provide a positive electrode active material with excellent cycle characteristics.SOLUTION: A positive electrode active material has a layered rock salt structure, and elements constituting the layered rock salt structure include at least one transition metal element selected from Ni, Co, and Mn, Mg, Li, and O, the amount of Mg in the entire positive electrode active material is 0.1 mass% or more and 5.0 mass% or less, and the c-axis length of the layered rock salt structure is 13.46 Å or more and 14.20 Å or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses a positive electrode active material, a lithium-ion secondary battery, and a method for manufacturing the positive electrode active material.

Background Art

[0002] Patent Documents 1 and 2 disclose a positive electrode active material containing, as constituent elements, at least one transition metal element selected from Ni, Co, and Mn, Mg, Li, and O.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventional positive electrode active materials have room for improvement in terms of cycle characteristics.

Means for Solving the Problems

[0005] As means for solving the above problems, this application discloses the following multiple aspects. <Aspect 1> A positive electrode active material having a layered rock salt structure, [[ID=五十二]]The elements constituting the layered rock salt structure include at least one transition metal element selected from Ni, Co, and Mn, Mg, Li, and O, <明 The amount of Mg in the entire positive electrode active material is 0.1% by mass or more and 5.0% by mass or less, The c-axis length of the layered rock salt structure is 13.46 Å or more and 14.20 Å or less, Positive electrode active material. <Aspect 2> A method for manufacturing a positive electrode active material, To obtain a first mixture containing at least one transition metal element from Ni, Co, and Mn, and Li, but without Mg. The first mixture is calcined to obtain a precursor having a layered rock salt structure. The aforementioned precursor and the Mg source are mixed to obtain a second mixture, and The second mixture is calcined to obtain a positive electrode active material having a layered rock salt structure and containing 0.1% by mass or more and 5.0% by mass or less of Mg. A method for producing a positive electrode active material, including the active material. <Aspect 3> A lithium-ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The positive electrode active material layer includes the positive electrode active material of embodiment 1, Lithium-ion rechargeable battery. [Effects of the Invention]

[0006] The cathode active material of this disclosure has excellent cycle characteristics. [Brief explanation of the drawing]

[0007] [Figure 1] This diagram illustrates the Li layer, transition metal layer, and c-axis length of the crystal structure contained in the positive electrode active material. [Figure 2] This shows an example of a manufacturing process for positive electrode active material particles. [Figure 3] This diagram schematically shows an example of the configuration of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0008] The following describes a positive electrode active material and a method for producing the same according to one embodiment, as well as a lithium-ion secondary battery using the positive electrode active material.

[0009] 1.Cathode active material An embodiment of the positive electrode active material of this disclosure will be described with reference to Figure 1. The positive electrode active material of this disclosure has a layered rock salt structure. Here, the elements constituting the layered rock salt structure include at least one transition metal element from Ni, Co, and Mn, Mg, Li, and O. The amount of Mg in the total positive electrode active material is 0.1% by mass or more and 5.0% by mass or less. The c-axis length of the layered rock salt structure is 13.46 Å or more and 14.20 Å or less.

[0010] 1.1 Chemical composition The positive electrode active material of this disclosure comprises, as constituent elements, at least one transition metal element selected from Mn, Ni, and Co, as well as Mg, Li, and O. In particular, when the constituent elements include at least Ni, Mg, Li, and O, or when the constituent elements include at least Mn, Mg, Li, and O, or when the constituent elements include at least Ni, at least one of Mn and Co, as well as Mg, Li, and O, or when the constituent elements include at least Mn, at least one of Ni and Co, as well as Mg, Li, and O, and even more so when the constituent elements include at least Li, Mg, Mn, Ni, Co, and O, higher performance is more easily ensured. Furthermore, the positive electrode active material of this disclosure may contain other impurity elements.

[0011] The positive electrode active material of the present disclosure contains Mg as described above. In the Li layer of the crystal structure of the positive electrode active material, Mg, which has an ionic radius close to that of Li and a larger ionic radius than the above transition metal, suppresses the cation mixing (movement to the Li layer) of the transition metal, suppresses the distortion of the crystal structure, stabilizes the crystal structure, and is considered to improve the cycle characteristics. In the positive electrode active material of the present disclosure, if the amount of Mg is too small, it is difficult to obtain the effect of stabilizing the crystal structure. Further, if the amount of Mg is too large, although the effect of stabilizing the crystal structure can be obtained, the amount of Li relatively decreases and the capacity tends to decrease. In this regard, it is important that the positive electrode active material of the present disclosure contains Mg in an amount of 0.1% by mass or more and 5.0% by mass or less. Thereby, both the stability of the crystal structure and the high capacity can be achieved. The amount of Mg in the entire positive electrode active material may be 0.5% by mass or more, and may be 4.0% by mass or less, 3.0% by mass or less, or 2.0% by mass or less.

[0012] The specific chemical composition of the positive electrode active material of the present disclosure is not particularly limited as long as the above constituent elements are included, the above amount of Mg is satisfied, and the following crystal structure is maintained. The positive electrode active material of the present disclosure is, for example, Li a Mg x Mn b Ni c Co d O 2±α (where 0.95 ≦ a ≦ 1.05, 0 ≦ b ≦ 1.00, 0 ≦ c ≦ 1.00, 0 ≦ d ≦ 1.00, 0.95 ≦ b + c + d ≦ 1.05, and x is a value such that the amount of Mg in the entire positive electrode active material is 0.1% by mass or more and 5.0% by mass or less) may have a chemical composition represented by. Here, 0 ≦ b <0.50, 0.50 ≦ c ≦ 1.00, 0 ≦ d <0.50 may be satisfied, 0.50 ≦ b ≦ 1.00, 0 ≦ c <0.50, 0 ≦ d <0.50 may be satisfied, and 0 ≦ b <0.50, 0 ≦ c <0.50, 0.50 ≦ d ≦ 1.00 may be satisfied. The chemical composition of the positive electrode active material can be specified by various elemental analyses.

[0013] 1.2 Crystal Structure The positive electrode active material of the present disclosure has a layered rock salt structure (α-NaFeO2-type layered rock salt structure) as its crystal structure. Specifically, it has an O3-type layered structure including a Li layer and a transition metal layer as shown in FIG. 1. Here, when Mg is inserted into the transition metal layer of the layered rock salt structure, since the ionic radius of Mg is larger than those of Mn, Ni, and Co, it is considered that distortion occurs in the octahedral structure of the transition metal layer, and as a result, the layered rock salt structure becomes unstable and the cycle characteristics become insufficient. On the other hand, when Mg is inserted into the Li layer, the above-mentioned distortion is less likely to occur, cation mixing is also less likely to occur, the layered rock salt structure is stabilized, and the cycle characteristics are considered to be improved. According to the findings of the present inventor, when Mg is inserted into the Li layer, the c-axis length (see FIG. 1) becomes smaller than when Mg is inserted into the transition metal layer. More specifically, when the c-axis length of the layered rock salt structure not containing Mg is used as a reference, the c-axis length of the layered rock salt structure in which Mg is inserted into the Li layer is smaller than the reference, and the c-axis length of the layered rock salt structure in which Mg is inserted into the transition metal layer is larger than the reference. Also, the larger the insertion amount of Mg into the Li layer, the smaller the c-axis length.

[0014] As described above, the positive electrode active material of the present disclosure contains Mg in an amount of 0.1% by mass or more and 5.0% by mass or less, and depending on the amount of Mg, the c-axis length of the layered rock salt structure can be within a predetermined range. Specifically, in the positive electrode active material of the present disclosure, the c-axis length of the layered rock salt structure is 13.46 Å or more and 14.20 Å or less. When the c-axis length is below the lower limit, the amount of Mg inserted into the Li layer becomes excessive, the amount of Li relatively decreases, and the capacity is likely to decrease. When the c-axis length exceeds the upper limit, the amount of Mg inserted into the Li layer is insufficient, or the amount of Mg inserted into the transition metal layer increases, and it is difficult to obtain the stabilizing effect of the crystal structure. In the positive electrode active material of the present disclosure, the c-axis length of the layered rock salt structure may be 13.50 Å or more, 13.60 Å or more, 13.70 Å or more, or 13.84 Å or more, and may also be 14.15 Å or less or 14.11 Å or less. Incidentally, the crystal structure of the positive electrode active material and its c-axis length can be specified by X-ray diffraction measurement or the like.

[0015] 1.3 Others The shape of the positive electrode active material in this disclosure is not particularly limited and may be particulate, for example. The positive electrode active material particles may be solid particles, hollow particles, or particles with voids. The positive electrode active material particles may be primary particles or secondary particles formed by the aggregation of multiple primary particles. It is preferable that each of the primary particles of the positive electrode active material has the above-described chemical composition and crystal structure. The average particle diameter (D50) of the positive electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may also be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. In this application, the average particle diameter D50 refers to the particle diameter (median diameter) at 50% of the cumulative value in the volume-based particle size distribution determined by laser diffraction-scattering.

[0016] 2. Method for manufacturing positive electrode active material As described above, the positive electrode active material of this disclosure is obtained by inserting Mg into a layered rock salt structure Li layer. According to the inventors' knowledge, whether Mg is inserted into the Li layer or the transition metal layer can be controlled, for example, by the timing of Mg addition during the synthesis of the positive electrode active material. For example, after obtaining a composite oxide (precursor) having a layered rock salt structure containing Li and a transition metal but not Mg, Mg can be inserted into the layered rock salt structure Li layer by calcining a Mg source together with the composite oxide.

[0017] Figure 2 shows an example of the flow of the method for producing the positive electrode active material of the present disclosure. As shown in Figure 2, in one embodiment, the method for producing the positive electrode active material of the present disclosure is as follows: S1: To obtain a first mixture containing at least one transition metal element from Ni, Co, and Mn, and Li, and not containing Mg. S2: The first mixture is calcined to obtain a precursor having a layered rock salt structure. S3: Mixing the precursor and the Mg source to obtain a second mixture, and S4: The method involves calcining the second mixture to obtain a positive electrode active material having a layered rock salt structure and containing 0.1% by mass or more and 5.0% by mass or less of Mg.

[0018] 2.1 Preparation of the first mixture In step S1, a first mixture containing at least one transition metal element from Ni, Co, and Mn and Li is obtained. The first mixture does not contain Mg. The first mixture may be obtained, for example, by mixing a transition metal source and a Li source. The transition metal source may be various salts such as sulfates and carbonates, or hydroxides or oxides. The transition metal source may be a mixture of multiple types of transition metal sources. The multiple types of transition metal sources may be mixed in the liquid phase or in the solid phase. For example, the transition metal source may be obtained by dissolving multiple types of transition metal sources in a solvent and then crystallizing them to obtain a precipitate. The Li source may be various salts such as sulfates and carbonates, or hydroxides or oxides. The transition metal source and the Li source may be mixed in the liquid phase or in the solid phase. For example, the transition metal source and the Li source may be mixed using a mortar and pestle or a ball mill. The mixing ratio of the transition metal source and the Li source in the first mixture can be appropriately determined according to the chemical composition of the final product, the positive electrode active material.

[0019] 2.2 Primary firing In step S2, the first mixture is calcined to obtain a precursor having a layered rock salt structure. The calcination conditions can be any conditions that yield a layered rock salt structure. The calcination atmosphere may be an oxygen-containing atmosphere such as an air atmosphere or an oxygen atmosphere. The calcination temperature may be, for example, 500°C to 1100°C. The calcination time may be, for example, 1 hour to 20 hours. Various calcination furnaces (such as muffle furnaces) may be used as the calcination means.

[0020] 2.3 Preparation of the second mixture In step S3, the precursor and the Mg source are mixed to obtain a second mixture. Before mixing the precursor and the Mg source, the precursor may be crushed. Various types of grinders (such as a jet mill) may be used for crushing. The Mg source may be various salts such as sulfates and carbonates, or it may be a hydroxide or an oxide. The precursor and the Mg source may be mixed in the liquid phase or in the solid phase. For example, the precursor and the Mg source may be mixed using a mortar and pestle or a ball mill.

[0021] 2.4 Second firing In step S4, the second mixture is calcined to obtain a positive electrode active material having a layered rock salt structure and containing 0.1% by mass or more and 5.0% by mass or less of Mg. The calcination conditions are as long as the layered rock salt structure is maintained. The calcination atmosphere in step S4 may be the same as or different from the calcination atmosphere in step S2, for example, an oxygen-containing atmosphere such as an air atmosphere or an oxygen atmosphere. The calcination temperature in step S4 may be the same as or different from the calcination temperature in step S2, for example, 800°C or more and 1100°C or less. The calcination time in step S4 may be the same as or different from the calcination time in step S2, for example, 1 hour or more and 20 hours or less. Various calcination furnaces (such as muffle furnaces) may be used as the calcination means. The positive electrode active material obtained through step S4 may be crushed by various pulverizers (such as jet mills).

[0022] 3. Lithium-ion rechargeable batteries The technology of this disclosure also has aspects as a lithium-ion secondary battery. Figure 3 schematically shows the configuration of a lithium-ion secondary battery 100 according to one embodiment. As shown in Figure 3, the lithium-ion secondary battery 100 of this disclosure has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. The positive electrode active material layer 10 contains the positive electrode active material of this disclosure described above.

[0023] 3.1 Cathode active material layer The positive electrode active material layer 10 contains a positive electrode active material and may optionally contain an electrolyte, a conductive additive, a binder, various additives, etc. The respective contents of the positive electrode active material, electrolyte, conductive additive, binder, etc. in the positive electrode active material layer 10 can be appropriately determined according to the desired battery performance. For example, if the entire positive electrode active material layer 10 (total solid content) is 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 100% by mass or less, or 90% by mass or less. The shape of the positive electrode active material layer 10 is not particularly limited, and for example, it may be a sheet-shaped positive electrode active material layer 10 having a substantially flat surface. The thickness of the positive electrode active material layer 10 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, 2 mm or less, or 1 mm or less.

[0024] The positive electrode active material layer 10 may contain only the positive electrode active material described above as the positive electrode active material. Alternatively, the positive electrode active material layer 10 may contain, in addition to the positive electrode active material described above as the positive electrode active material, a different type of positive electrode active material (other positive electrode active material). For example, with the total positive electrode active material contained in the positive electrode active material layer 10 as 100% by mass, the content of the positive electrode active material described above as the positive electrode active material may be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more. The surface of the positive electrode active material may be covered with a protective layer containing a lithium ion conductive oxide. This makes it easier to suppress reactions between the positive electrode active material and sulfides (for example, sulfide solid electrolytes described later). The lithium ion conductive oxide may be a Li-containing composite oxide containing at least one element selected from B, C, Al, Si, P, S, Ti, Zr, Nb, Mo, and W. The coverage rate (area ratio) of the protective layer may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more, 1 nm or more, 100 nm or less, or 20 nm or less.

[0025] The electrolyte contained in the positive electrode active material layer 10 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. Only one type of electrolyte may be used, or two or more types may be used in combination. For the solid electrolyte, any known solid electrolyte for lithium-ion secondary batteries may be used. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ionic conductivity and heat resistance. The inorganic solid electrolyte may be at least one selected from, for example, oxide solid electrolytes, sulfide solid electrolytes, hydride solid electrolytes, halide solid electrolytes, etc. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be, for example, particulate. The electrolyte may be an aqueous electrolyte or a non-aqueous electrolyte. The composition of the electrolyte may be the same as that known for lithium-ion secondary battery electrolytes. For example, the electrolyte may be a solution of lithium salt dissolved in a carbonate-based solvent at a predetermined concentration.

[0026] Examples of conductive additives that may be included in the positive electrode active material layer 10 include carbon materials such as acetylene black (AB), Ketjen black (KB), carbon black (CB), carbon nanotubes (CNT), vapor-processed carbon fibers (VGCF), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The shape and size of the conductive additives are not particularly limited. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0027] Examples of binders that may be included in the positive electrode active material layer 10 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, polyimide (PI) binders, carboxymethylcellulose (CMC) binders, and polyacrylic acid (PAA) binders. A single binder may be used alone, or two or more may be used in combination.

[0028] 3.2 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. If the lithium-ion secondary battery 100 is a solid-state battery (a battery containing a solid electrolyte, which may also contain a liquid electrolyte in part, or it may be an all-solid-state battery that does not contain a liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte and may further optionally contain a binder, etc. In this case, the content of the solid electrolyte and binder, etc. in the electrolyte layer 20 is not particularly limited. On the other hand, if the lithium-ion secondary battery 100 is an electrolyte battery, the electrolyte layer 20 contains an electrolyte and may further have a separator, etc., to hold the electrolyte and prevent contact between the positive electrode active material layer 10 and the negative electrode active material layer 30. The thickness of the electrolyte layer 20 is not particularly limited; for example, it may be 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less. The electrolyte and binder included in the electrolyte layer 20 may be appropriately selected from the examples of those that may be included in the positive electrode active material layer described above. The separator can be any separator commonly used in lithium-ion secondary batteries, such as those made of polyethylene (PE), polypropylene (PP), polyester, and polyamide resins. The separator may have a single-layer structure or a multi-layer structure. The separator may also be made of a nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

[0029] 3.3 Negative electrode active material layer The negative electrode active material layer 30 contains at least a negative electrode active material and may optionally also contain an electrolyte, a conductive additive, a binder, various additives, etc. The respective contents of the negative electrode active material, electrolyte, conductive additive, binder, etc. in the negative electrode active material layer 30 may be appropriately determined according to the desired battery performance. The shape of the negative electrode active material layer 30 is not particularly limited and may, for example, be a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 30 is not particularly limited and may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less. As the negative electrode active material, for example, silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium and lithium alloys, etc. The negative electrode active material may be used alone or in combination of two or more types. The electrolyte, conductive additive, and binder that may be included in the negative electrode active material layer 30 can be appropriately selected from, for example, those exemplified above as potentially included in the positive electrode active material layer 10.

[0030] 3.4 Others As shown in Figure 3, the lithium-ion secondary battery 100 may include a positive electrode current collector 40 electrically connected to the positive electrode active material layer 10 and a negative electrode current collector 50 electrically connected to the negative electrode active material layer 30. Any known configuration of the current collectors may be used. In addition to the above configuration, the lithium-ion secondary battery 100 may also include other configurations common to secondary batteries, such as tabs and terminals. Furthermore, the lithium-ion secondary battery 100 may have all of the above configurations housed inside an outer casing. Any known battery casing can be used. Multiple batteries 100 may be electrically connected and stacked as desired to form a battery pack. Examples of lithium-ion secondary battery shapes include coin-type, laminate-type, cylindrical, and prismatic types. The lithium-ion secondary battery 100 can be manufactured, for example, by a process of molding each of the above layers using a dry or wet molding method.

[0031] 4. Vehicles equipped with lithium-ion secondary batteries The lithium-ion secondary battery of this disclosure has excellent cycle characteristics. A lithium-ion secondary battery having such excellent performance can be suitably used in, for example, at least one type of vehicle selected from hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). That is, the technology of this disclosure also has an aspect as a vehicle having a lithium-ion secondary battery, wherein the lithium-ion secondary battery has a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, and the positive electrode active material layer contains the positive electrode active material of this disclosure. [Examples]

[0032] The technology of this disclosure will be described in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0033] 1. Preparation of positive electrode active material 1.1 Example 1 The positive electrode active material was prepared through steps S1 to S4 shown in Figure 2. Specifically, the process was as follows: Step S1 (Preparation of the first mixture): NiSO4, CoSO4, and MnSO4 were dissolved in deionized water as transition metal sources to obtain a raw material solution. The Ni:Co:Mn ratio was 8:1:1 (atm%). The concentration of the raw material solution was 30% by mass. Subsequently, a fixed amount of NH3 aqueous solution was added to the reaction vessel and nitrogen was purged while stirring with a stirrer. Next, NaOH was added to the reaction vessel to adjust the pH to alkaline. Subsequently, while controlling the pH of the solution in the reaction vessel to be constant, the raw material solution and NH3 were added dropwise to precipitate the transition metal hydroxide. Subsequently, the transition metal hydroxide was recovered by filtration and washed with deionized water. Subsequently, the washed material was filtered to recover the transition metal hydroxide. Subsequently, the recovered transition metal hydroxide was dried at 120°C for 16 hours. Subsequently, the dried transition metal hydroxide and Li2CO3 as a Li source were mixed in a mortar to obtain the first mixture. Step S2 (Primary calcination): The first mixture was calcined in a muffle furnace to obtain a precursor having a layered rock salt structure. The calcination atmosphere was an oxygen atmosphere, the calcination temperature was 600°C, and the calcination time was 10 hours. Step S3 (Preparation of the second mixture): The obtained precursor was crushed using a jet mill, and then the precursor and Mg(OH)2 as a Mg source were mixed in a mortar to obtain the second mixture. Step S4 (Secondary Calcination): The second mixture was calcined in a muffle furnace to obtain a positive electrode active material having a layered rock salt structure. The calcination atmosphere was an oxygen atmosphere, the calcination temperature was 900°C, and the calcination time was 10 hours. Subsequently, the positive electrode active material was crushed in a jet mill to obtain the positive electrode active material according to Example 1. The positive electrode active material according to Example 1 contained Ni, Co, and Mn in a molar ratio of Ni:Co:Mn = 8:1:1, and also contained Li, Mg, and O. The Mg content in the positive electrode active material was 1.0% by mass.

[0034] 1.2 Reference example 2, Examples 3 ~5, Comparative Example 1, 5 The positive electrode active material was obtained in the same manner as in Example 1, except that the mixing ratio of the Mg source to the precursor was changed in step S3. Reference example 2, Examples 3 The positive electrode active materials in Comparative Example 5 all contained Ni, Co, and Mn in a molar ratio of Ni:Co:Mn = 8:1:1, and also contained Li, Mg, and O. reference The Mg content in the positive electrode active material of Example 2 was 0.1% by mass, Example 3 was 0.5% by mass, Example 4 was 2.0% by mass, Example 5 was 5.0% by mass, Comparative Example 1 was 0% by mass (no Mg source added), and Comparative Example 5 was 7.0% by mass.

[0035] 1.3 Comparative Example 2 In step S1, Mg(OH)2 was added as a Mg source at the time of crystallization of the raw material solution to obtain a mixed hydroxide of transition metal hydroxide and Mg(OH)2. This mixed hydroxide was then mixed with a Li source to obtain a first mixture containing the transition metal, Li, and Mg. This first mixture was fired under the same conditions as the secondary firing in Example 1 to obtain the positive electrode active material according to Comparative Example 2. The positive electrode active material according to Comparative Example 2 contained Ni, Co, and Mn in a molar ratio of Ni:Co:Mn=8:1:1, and also contained Li, Mg, and O. The Mg content in the positive electrode active material according to Comparative Example 2 was 1.0% by mass.

[0036] 1.4 Comparative Example 3 In step S1, Mg(OH)2 was mixed with a transition metal hydroxide and a Li source to obtain a first mixture containing the transition metal, Li, and Mg. This mixture was then fired under the same conditions as the secondary firing in Example 1 to obtain the positive electrode active material according to Comparative Example 3. The positive electrode active material according to Comparative Example 3 contained Ni, Co, and Mn in a molar ratio of Ni:Co:Mn=8:1:1, and also contained Li, Mg, and O. The Mg content in the positive electrode active material according to Comparative Example 3 was 1.0% by mass.

[0037] 1.5 Comparative Example 4 Without performing steps S2 and S3, the first mixture obtained in the same manner as in Example 1 was calcined under the same conditions as the secondary calcination in Example 1, and then Mg(OH)2 was added to the calcined product to obtain the positive electrode active material according to Comparative Example 4. The positive electrode active material according to Comparative Example 4 contained Ni, Co, and Mn in a molar ratio of Ni:Co:Mn=8:1:1, and also contained Li, Mg, and O (where Mg exists as Mg(OH)2). The Mg content in the positive electrode active material according to Comparative Example 4 was 1.0% by mass.

[0038] 2. X-ray diffraction measurement X-ray diffraction measurements were performed on the positive electrode active material using an X-ray diffraction measuring device (SmartLab, manufactured by Rigaku Corporation), and the X-ray diffraction peaks were confirmed. The X-ray diffraction measurement conditions were an angle (2θ): 10 to 120°, a velocity: 10° / min, the full width at half maximum was calculated using the value automatically calculated by the accompanying software (SmartLab Studio II), and the c-axis length was calculated by fitting the diffraction pattern (Ritveld analysis) using analysis software (fullprof). When the X-ray diffraction peaks of each positive electrode active material in the examples and comparative examples were confirmed, all of them had diffraction peaks attributable to an α-NaFeO2 type layered rock salt structure, and both had this layered rock salt structure as the main phase.

[0039] 3. Creating evaluation cells A positive electrode slurry was obtained by mixing the above positive electrode active material, acetylene black as a conductive additive, and PVdF as a binder with a solvent in a mass ratio of 88:10:2. The positive electrode slurry was coated onto a current collector foil using a film applicator (manufactured by Allgood Co., Ltd.) and dried at 80°C for 5 minutes to obtain the positive electrode. On the other hand, a negative electrode slurry was obtained by mixing natural graphite as a negative electrode active material with SBR and CMC as binders with a solvent. The negative electrode slurry was coated onto a current collector foil using a film applicator (manufactured by Allgood Co., Ltd.) and dried at 80°C for 5 minutes to obtain the negative electrode. The above positive and negative electrodes were wound together with a separator into a cylindrical shape and housed in a case with an electrolyte to create a cylindrical cell. As the electrolyte, a solution of LiPF6 dissolved at a concentration of 1 M in a solvent with EC / DMC / EMC = 3 / 4 / 4 (volume %) was used.

[0040] 4. Evaluation of cycle characteristics For the fabricated evaluation cells, 100 charge-discharge cycle tests were performed at 1C, 1.5-4.1V. The discharge capacity at 1CCCV 1.5-4.1V before and after these tests was measured, and the capacity retention rate after 100 cycles was calculated based on the following formula. Capacity retention rate after 100 cycles = (Capacity after 100 cycles) / (Initial capacity)

[0041] 5. Evaluation Results Table 1 below shows the chemical composition (Mg content), crystal structure (c-axis length, Li occupancy in the Li layer), initial capacity of the evaluation cell (relative to theoretical capacity), and capacity retention rate after 100 cycles for each of the examples and comparative examples.

[0042] [Table 1]

[0043] The results shown in Table 1 indicate the following: (1) In Comparative Examples 2 and 3, the c-axis length in the layered rock salt structure of the positive electrode active material increased compared to Comparative Example 1. This means that Mg was inserted into the transition metal layer of the layered rock salt structure. In Comparative Examples 2 and 3, the insertion of Mg with a large ionic radius into the transition metal layer of the layered rock salt structure caused strain, and the Li occupancy in the Li layer also decreased due to cation mixing. As a result, it is thought that the cycle characteristics were worse than in Comparative Example 1. (2) Comparative Example 4 is a positive electrode active material in which Mg(OH)2 is added after calcination. In this case, the c-axis length of the layered rock salt structure is the same as in Comparative Example 1, and there is no change in the cycle characteristics. (3) Example 1 Reference Example 2, Example 3 In Example 5, the c-axis length in the layered rock salt structure of the positive electrode active material decreased compared to Comparative Example 1. This means that Mg was inserted into the Li layer of the layered rock salt structure. Example 1 Reference Example 2, Example 3 In steps ~5, the distortion of the crystal structure and cation mixing seen in Comparative Examples 2 and 3 were suppressed, and the crystal structure stabilization effect of Mg was obtained. As a result, it is considered that the cycle characteristics were improved compared to Comparative Example 1. (4) In Comparative Example 5, the amount of Mg in the positive electrode active material was excessive, which relatively reduced the amount of Li and thus the capacity decreased.

[0044] 6. Summary From the above examples, a positive electrode active material that (1) has a layered rock salt structure, (2) the elements constituting the layered rock salt structure include at least one transition metal element from Ni, Co, and Mn, Mg, Li, and O, (3) the amount of Mg in the total positive electrode active material is 0.1% by mass or more and 5.0% by mass or less, and (4) the c-axis length of the layered rock salt structure is 13.46 Å or more and 14.20 Å or less can be said to have excellent cycle characteristics. [Explanation of symbols]

[0045] 10 Cathode active material layer 20 Electrolyte layer 30 Negative electrode active material layer 40 Positive electrode current collector 50 Negative electrode current collector 100 Lithium-ion rechargeable batteries

Claims

1. A positive electrode active material having a layered rock salt structure, The elements constituting the aforementioned layered rock salt structure are Li, Mg, Ni, Co, Mn, and O. In the Li layer of the aforementioned layered rock salt structure, Mg is inserted. The amount of Mg in the total positive electrode active material is 0.5% by mass or more and 5.0% by mass or less. The c-axis length of the aforementioned layered rock salt structure is 13.46 Å or more and 14.08 Å or less. Cathode active material.

2. A method for producing a positive electrode active material, To obtain a first mixture containing Li, Ni, Co, and Mn, but not containing Mg, The first mixture is calcined to obtain a precursor having a layered rock salt structure. The aforementioned precursor and the Mg source are mixed to obtain a second mixture, and The second mixture is calcined to obtain a positive electrode active material having a layered rock salt structure, wherein the elements constituting the layered rock salt structure are Li, Mg, Ni, Co, Mn, and O, and which contains 0.5% by mass or more and 5.0% by mass or less of Mg. A method for producing a positive electrode active material, including the active material.

3. A lithium-ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The positive electrode active material layer includes the positive electrode active material described in claim 1. Lithium-ion rechargeable battery.

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