Positive electrode active material particles, positive electrode and lithium-ion battery

Abstract

Positive electrode active material particle (5), wherein the positive electrode active material particle (5) has a lamellar rock salt type crystal structure, the positive electrode active material particle (5) contains a magnesium-containing lithium composite oxide, and a magnesium concentration in an inner section of the positive electrode active material particle (5) is higher than a magnesium concentration in a surface section of the positive electrode active material particle (5).

Description

REFERENCE TO RELATED APPLICATIONSThis non-provisional application is based on Japanese Patent Application JP 2023 - 127 491 filed on Aug. 4, 2023 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.BACKGROUND OF THE DISCLOSUREField of the InventionThe present disclosure relates to a positive electrode active material particle, a positive electrode, and a lithium ion battery.Description of Related ArtJapanese Patent Application JP 2018-88 383 A discloses a positive electrode active material for a nonaqueous electrolyte secondary battery, wherein the positive electrode active material contains magnesium (Mg) and has a concentration gradient of the Mg concentration that decreases from the surface of the particle toward the center.SUMMARY OF THE DISCLOSUREWhen the Mg concentration at the surface of the particle is high, it inhibits, among other things, lithium (Li) diffusion and also electron conduction, whereby the resistance, particularly the initial resistance, can be increased. When Mg is present only on the surface of the particle, the degree of expansion / shrinkage of the c-axis length occurring in charging and discharging is different between the outer and inner parts of the positive electrode active material, which may lead to the formation of cracks in the inner part, possibly leading to deterioration of the capacity retention.An object of the present disclosure is to prevent an increase in initial resistance and prevent deterioration in capacity retention. [1] A positive electrode active material particle, wherein the positive electrode active material particle has a lamellar rock salt type crystal structure, the positive electrode active material particle contains a magnesium-containing lithium composite oxide, and a magnesium concentration in an inner portion of the positive electrode active material particle is higher than a magnesium concentration in a surface portion of the positive electrode active material particle.When the Mg concentration in the inner portion is higher than that in the surface portion, it is conceivable that the inhibition of Li diffusion, the inhibition of electron conduction, and / or the like may be less influenceable, and thereby an increase in initial resistance may be prevented. It is also conceivable that, by including Mg, the elongation of the c-axis of the positive electrode active material particle that may occur in charging and discharging may be inhibited, and thereby the formation of cracks in the inner portion may be inhibited, resulting in less deterioration of capacity retention.[2] The positive electrode active material particles according to [1], wherein the magnesium concentration in the inner portion is from 0.5 mol % to 8.0 mol %, and the magnesium concentration in the surface portion is 5.0 mol % or less.[3] The positive electrode active material particles according to [1] or [2], wherein a difference between the magnesium concentration in the inner portion and the magnesium concentration in the surface portion is 1.0 mol % or more.[4] A positive electrode comprising the positive electrode active material particle according to any one of [1] to [3].[5] The positive electrode according to [4], further comprising carbon nanotubes.[6] A lithium ion battery containing the positive electrode according to [4] or [5].The above and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE FIGURESFIG. 1 is a cross-sectional view of a positive electrode active material particle according to the present embodiment. FIG. 2 is a conceptual view showing an example of a lithium ion battery according to the present embodiment. FIG. 3 is a table showing a first battery configuration. FIG. 4 is a table showing a second battery configuration. FIG. 5 is a table showing a third battery configuration. FIG. 6 is a table showing the configurations of the samples in the examples and comparative examples, and the evaluation results.DESCRIPTION OF THE PREFERRED EMBODIMENTSHereinafter, an embodiment of the present disclosure (which may also be simply referred to as "the present embodiment" hereinafter) and an example of the present disclosure (which may also be simply referred to as "the present example" hereinafter) will be described. Note that neither the present embodiment nor the present example limits the technical scope of the present disclosure.A stoichiometric composition formula represents a typical example of a compound. A compound may have a non-stoichiometric composition. For example, "Al 2 O 3" is not limited to a compound in which the molar ratio (molar ratio) is "Al / O=2 / 3." "Al 2 O 3" means a compound containing Al and O in an arbitrary composition ratio unless otherwise specified. Moreover, the compound may be doped with a trace element, or a part of Al and / or O may be replaced with another element, for example."Derivative" refers to a compound derived from its original compound by at least one partial modification selected from the group consisting of introduction of a functional group, replacement of an atom, oxidation, reduction and other chemical reactions. The position of the change may be one position or a plurality of positions. "Substituent" may include, for example, at least one group selected from the group consisting of alkyl group, alkenyl group, alkynyl group, cycloalkyl group, unsaturated cycloalkyl group, aromatic group, heterocyclic group, halogen atom (F, Cl, Br, I, etc.), OH group, SH group, CN group, SCN group, OCN group, nitro group, alkoxy group, unsaturated alkoxy group, amino group, alkylamino group, dialkylamino group, aryloxy group, acyl group, alkoxycarbonyl group, acyloxy group, aryloxycarbonyl group, acylamino group, Alkoxycarbonylamino group, aryloxycarbonylamino group, sulfonylamino group, sulfamoyl group, carbamoyl group, alkylthio group, arylthio group, sulfonyl group, sulfinyl group, ureido group, phosphoramide group, sulfo group, carboxy group, hydroxamic acid group, sulfino group, hydrazino group, imino group, silyl group, and the like. These substituents may be further substituted. When there are two or more substituents, these substituents may be the same or different from each other. A plurality of substituents may be linked together to form a ring."Copolymer" includes at least one copolymer selected from the group consisting of unspecified type, random type, random type, alternating type, periodic type, block type and graft type.<<Positive Electrode Active Material Particles>>FIG. 1 is a cross-sectional view of a positive electrode active material particle according to the present embodiment. A positive electrode active material particle 5 has a lamellar rock salt type crystal structure. Positive electrode active material particles 5 contain a Mg-containing lithium composite oxide. The Mg concentration in an inner portion 1 of positive electrode active material particles 5 is higher than the Mg concentration in a surface portion 2 of positive electrode active material particles 5.Positive electrode active material particles 5 have a lamellar rock salt type crystal structure. The crystal structure of positive electrode active material particles 5 can be identified by, for example, inductively coupled plasma atomic emission spectroscopy (ICP-AES).Positive electrode active material particles 5 contain a Mg-containing lithium composite oxide. The Mg-containing lithium composite oxide is not limited in composition as long as positive electrode active material particles 5 have a lamellar rock salt type crystal structure.The Mg concentration (mol %) in the inner portion 1 of positive electrode active material particles 5 is higher than the Mg concentration (mol %) in the surface portion 2 of positive electrode active material particles 5. in other words, the composition of the inner portion 1 of the positive electrode active material particle 5 is different from that of the surface portion 2. when the Mg concentration in the inner portion 1 of the positive electrode active material particle 5 is increased, it is expected that the inhibition of Li diffusion, the inhibition of electron conduction, and / or the like may be less influenceable, and as a result, an increase in initial resistance may be prevented. It is also expected that the c-axis length expansion of the positive electrode active material particle 5 that may occur in charging and discharging can be prevented, and thereby the formation of cracks in the inner portion 1 can be prevented, resulting in less deterioration of the capacity retention. The Mg concentration in the inner portion 1 of the positive electrode active material particle 5 is not particularly limited as long as it is higher than the Mg concentration in the surface portion 2 of the positive electrode active material particle 5. Moreover, the Mg concentration of the positive electrode active material particle 5 may continuously increase from the surface portion 2 of the positive electrode active material particle 5 toward the inner portion 1.In the present embodiment, "the inner portion of the positive electrode active material particle" refers to a region at and near the center of gravity of the positive electrode active material particle, and typically refers to a region that is 100 nm or more inward from the outermost surface of the positive electrode active material particle toward the center of gravity. For example, when the positive electrode active material particle is a substantially spherical particle, a smallest circle circumscribing the particle is drawn, and the region extending up to 100 nm from the center of gravity may be regarded as the inner portion of the positive electrode active material particle. "The surface portion of the positive electrode active material particle" refers to a region extending up to 50 nm inward from the surface of the positive electrode active material particle.For example, the composition of the inner portion 1 of the Mg-containing lithium composite oxide is represented by the following formula (1). Li a Ni (1-b) Mg b O c(1)In the above formula (1), a satisfies the relationship of 1.00≤a≤1.20, b satisfies the relationship of 0.005≤b≤0.08, and c satisfies the relationship of 2.0≤c<2.1. b is preferably 0.01≤b≤0.06, and it may be 0.02≤b≤0.05.The composition of the surface portion 2 of the Mg-containing lithium composite oxide is represented by, for example, the following formula (2). Li x Ni (1-y) Mg y O z(2)In the above formula (2), x satisfies the relationship of 1.00≤x≤1.20, y satisfies the relationship of 0.00≤y≤0.05, and z satisfies the relationship of 2.0≤z<2.1.The Mg concentration in the inner portion 1 and the surface portion 2 of the positive electrode active material particle 5 according to the present embodiment can be checked by, for example, transmission electron microscopy (TEM) energy dispersive X-ray spectroscopy (EDX). More specifically, a measurement object is first prepared by embedding a positive electrode active material particle 5 in a suitable resin and subjecting it to cross-sectional polishing processing and / or the like to expose a cross section of the positive electrode active material particle 5. Then, this cross section is examined with the TEM at an appropriate magnification. In the TEM observation image thus obtained, the center of gravity and the outermost surface of positive electrode active material particles 5 are analyzed by EDX to determine the concentration (mol %) of the Mg atoms in the respective regions. It is also preferable to perform the EDX line analysis along a straight line extending from a point on the outermost surface of the positive electrode active material particle 5 toward the center of gravity. The line analysis makes it possible to accurately track the change in the concentration of Mg atoms from the surface of the positive electrode active material particle 5 toward the center of gravity.The Mg concentration in the inner portion 1 of the positive electrode active material particle 5 may be from 0.5 mol % to 8.0 mol %, preferably from 1.0 mol % to 6.0 mol %, or from 2.0 mol % to 5.0 mol %. The Mg concentration in the surface portion 2 of positive electrode active material particles 5 may be 5.0 mol % or less, preferably 0.1 mol % to 5.0 mol %, or 0.2 mol % to 4.0 mol %.The difference between the Mg concentration in the inner portion 1 of positive electrode active material particles 5 and the Mg concentration in the surface portion 2 of positive electrode active material particles 5 is preferably 1.0 mol % or more. When the difference satisfies the above range, both the inhibition of the initial resistance increase and the inhibition of the degradation of the capacity retention can be further achieved. Preferably, the difference between the Mg concentration in the inner portion 1 of the positive electrode active material particle 5 and the Mg concentration in the surface portion 2 of the positive electrode active material particle 5 may be 2.0 mol % or more, or 2.5 mol % or more, or 3.0 mol % or more. From the viewpoint of producing positive electrode active material particles 5, for example, it may be 5.0 mol % or less.The crystallite diameter of positive electrode active material particles 5 may be 450 Å to 900 Å. Here, a crystallite refers to a region (a chunk) in the crystal structure of a single positive electrode active material particle 5 that can be regarded as a single crystal, and the crystallite diameter refers to the size of the crystallite. As the crystallite diameter, a value calculated from the line profile of X-ray diffraction (XRD) by the Scherer equation may be used. As the crystallite diameter of positive electrode active material particles 5 according to the present embodiment, the crystallite diameter based on a (003) plane that appears as a main peak in the X-ray diffraction line profile can be used. The crystallite diameter of positive electrode active material particles 5 can be obtained by, for example, calculating the half width of a (003) plane observed in the range of 2θ=19,1 to 20.1 of the spectrum obtained by the XRD measurement using the Scherer equation.Typically, positive electrode active material particles 5 is a secondary particle formed of primary particles aggregated together. For example, positive electrode active material particles 5 may have an average particle size (D50) of 4.0 μm to 8.0 μm or a D50 of 5.0 μm to 7.0 μm. Here, D50 refers to a particle size in the volume-based particle size distribution at which the cumulative frequency on the small particle size side reaches 50%. D50 can be measured by laser diffraction.The shape of the positive electrode active material particles 5 is not particularly limited; for example, they may be substantially spherical, flake-shaped, columnar, and / or the like, and are preferably substantially spherical. "Substantially spherical" is a term that includes spherical, rugbyspheric, polygonal, and the like, and refers to, for example, a shape having an average aspect ratio (which is the ratio of the length of the long axis to the length of the short axis of a smallest rectangle circumscribing the particle) of 1 to 2.Positive electrode active material particle 5 may include, for example, a transition metal oxide, a polyanion compound, and / or the like.<Übergangsmetall Oxide: Space Group R-3m>The transition metal oxide may have any crystal structure. For example, the transition metal oxide may have a crystal structure belonging to a space group R-3m and / or the like. For example, a compound represented by the general formula "LiMO 2" may have a crystal structure belonging to a space group R-3m. The transition metal oxide can be represented by the following formula (A-1), for example. Li 1-a Ni x M 1-x O 2( A-1)In the above formula, the relationships -0.5 a≤≤0.5, 0 x≤≤1 are satisfied.M may contain, for example, at least one element selected from the group consisting of Co, Mn and Al.In the above formula (A-1), x may satisfy the relationship 0<x≤0.1, 0.1≤x≤0.2, 0.2≤x≤0.3, 0.3≤x≤0.4, 0.4≤x≤0.5, 0.5≤x≤0.6, 0.6≤x≤0.7, 0.7≤x≤0.8, 0.8≤x≤0.9, or 0.9≤x≤1, for example, a may satisfy the relationship -0.4≤a≤0.4, -0.3≤a≤0.3, -0.2≤a≤0.2, or -0.1≤a≤0.1.The transition metal oxide may include, for example, at least one selected from the group consisting of LiCoO 2, LiMnO 2, LiNi 0.9 Co 0.1 O 2, LiNi 0.9 Mn 0.1 O 2, and LiNiO 2.<NCM>The transition metal oxide can be represented by, for example, the following formula (A-2). A compound represented by the following formula (A-2) may also be referred to as "NCM". Li 1-a Ni x Co y Mn z O 2( A-2)In the above formula, the relationships of -0.5≤a≤0.5, 0<x<1, 0<y<1, 0<z<1, x+y+z=1 are satisfied.In the above formula (A-2), x may have the relationship 0<x≤0.1, 0.1≤x≤0.2, 0.2≤x≤0.3, 0.3≤x≤0.4, 0.4≤x≤0.5, 0.5≤x≤0.6, 0.6≤x≤0.7, 0.7≤x≤0.8, 0.8≤x≤0.9, or 0.9≤x<1, for example.In the above formula (A-2), y may satisfy the following relationship: 0<y≤0.1, 0.1≤y≤0.2, 0.2≤y≤0.3, 0.3≤y≤0.4, 0.4≤y≤0.5, 0.5≤y≤0.6, 0.6≤y≤0.7, 0.7≤y≤0.8, 0.8≤y≤0.9, or 0.9≤y<1, for example.In the above formula (A-2), z may satisfy the relationship 0<z≤0.1, 0.1≤z≤0.2, 0.2≤z≤0.3, 0.3≤z≤0.4, 0.4≤z≤0.5, 0.5≤z≤0.6, 0.6≤z≤0.7, 0.7≤z≤0.8, 0.8≤z≤0.9, or 0.9≤z<1, for example.NCM may include, for example, at least one selected from the group consisting of LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2, LiNi 0.4 Co 0.3 Mn 0.3 O 2, LiNi 0.3 Co 0.4 Mn0.3O2, LiNi0.3Co0.3Mn0.4O2, LiNi 0.5 Co 0.2 Mn 0.3 O 2, LiNi 0.5 Co 0.3 Mn 0.2 O 2, LiNi 0.5 Co 0.4 Mn0.1O2, LiNi0.5Co0.1Mn0.4O2, LiNi0.6Co0.2Mn0.2O2, LiNi 0.6 Co 0.3 Mn 0.1 O 2, LiNi 0.6 Co 0.1 Mn 0.3 O 2, LiNi 0.7 Co 0.1 Mn0.2O2, LiNi0.7Co0.2Mn0.1O2, LiNi0.8Co0.1Mn0.1O2, and LiNi 0.9 Co 0.05 Mn 0.05 O 2.<NCA>The transition metal oxide can be represented by, for example, the following formula (A-3). A compound represented by the following formula (A-3) may also be referred to as "NCA". Li 1-a Ni x Co y Al z O 2( A-3)In the above formula, the relationships of -0.5≤a≤0.5, 0<x<1, 0<y<1, 0<z<1, x+y+z=1 are satisfied.In the above formula (A-3), x may have the relationship 0<x≤0.1, 0.1≤x≤0.2, 0.2≤x≤0.3, 0.3≤x≤0.4, 0.4≤x≤0.5, 0.5≤x≤0.6, 0.6≤x≤0.7, 0.7≤x≤0.8, 0.8≤x≤0.9, or 0.9≤x<1, for example.In the above formula (A-3), y may satisfy the following relationship: 0<y≤0.1, 0.1≤y≤0.2, 0.2≤y≤0.3, 0.3≤y≤0.4, 0.4≤y≤0.5, 0.5≤y≤0.6, 0.6≤y≤0.7, 0.7≤y≤0.8, 0.8≤y≤0.9, or 0.9≤y<1, for example.In the above formula (A-3), z may satisfy the relationship 0<z 0<z≤0.1, 0.1≤z≤0.2, 0.2≤z≤0.3, 0.3≤z≤0.4, 0.4≤z≤0.5, 0.5≤z≤0.6, 0.6≤z≤0.7, 0.7≤z≤0.8, 0.8≤z≤0.9, or 0.9≤z<1, for example.NCA may include, for example, at least one selected from the group consisting of LiNi 0.7 Co 0.1 Al 0.2 O 2, LiNi 0.7 Co 0.2 Al 0.1 O 2, LiNi 0.8 Co 0.1 Al0.1O2, LiNi0.8Co0.17Al0.03O2, LiNi 0.8 Co 0.15 Al 0.05 O 2, and LiNi 0.9 Co 0.05 Al 0.05 O 2.< System>Positive electrode active material particles 5 may contain, for example, two or more NCMs and / or the like. Positive electrode active material particle 5 may contain, for example, NCM (0.6≤x) and NCM (x<0.6). "NCM (0.6≤x)" refers to a compound in which x (Ni ratio) in formula (A-2) above is 0.6 or more. NCM (0.6≤x) may also be referred to as "high nickel content material", for example. NCM (0.6≤x) includes, for example, LiNi 0.8 Co Mn O 0.10.12 and / or the like. "NCM(x<0,6)" refers to a compound in which x (Ni ratio) in the above formula (A-2) is less than 0.6. NCM (x<0.6) includes, for example, LiNi 1 / 3 Co Mn O 1 / 31 / 32 and / or the like. The mixing ratio (mass ratio) between NCM (0.6≤x) and NCM (x<0.6) may be "NCM (0.6≤x) / NCM (x<0.6)=9 / 1 to 1 / 9", or "NCM (0.6≤x) / NCM (x<0.6)=9 / 1 to 4 / 6", or "NCM (0.6≤x) / NCM (x<0.6)=9 / 1 to 3 / 7", for example.Positive electrode active material particles 5 may include, for example, NCA and NCM. The mixing ratio (mass ratio) between NCA and NCM may be, for example, "NCA / NCM=9 / 1 to 1 / 9", "NCA / NCM=9 / 1 to 4 / 6", or "NCA / NCM=9 / 1 to 3 / 7". Between NCA and NCM, the Ni ratio may be the same or different. The Ni ratio of NCA may be greater than the Ni ratio of NCM. The Ni ratio of NCA may be less than the Ni ratio of NCM.<Übergangsmetall Oxide: Space Group C2 / m>The transition metal oxide may have, for example, a crystal structure belonging to space group C2 / m and / or the like. The transition metal oxide can be represented by, for example, the following formula (A-4). Li 2 MO 3( A-4)In the above formula, M may include, for example, at least one element selected from the group consisting of Ni, Co, Mn, and Fe.Positive electrode active material particles 5 may include, for example, a mixture of LiMO 2( space group R-3m), and Li 2 MO 3( space group C2 / m). Positive electrode active material particles 5 may include, for example, a solid solution formed of LiMO 2 and Li 2 MO 3( Li 2 MO 3- LiMO 2) and / or the like.<Übergangsmetall Oxide: Space Group Fd-3m>The transition metal oxide may have, for example, a crystal structure belonging to space group Fd-3m, and / or the like. The transition metal oxide can be represented by the following formula (A-5), for example. LiMn 2-x M x O 4( A-5)In the above formula, the relationship 0≤x≤2 is satisfied.M may contain, for example, at least one element selected from the group consisting of Ni, Fe and Zn.LiM 2 O 4( space group Fd- 3 m) may contain, for example, at least one element selected from the group consisting of LiMn 2 O 4 and LiMn 1.5 Ni 0.5 O 4. The positive electrode active material particle 5 may include, for example, a mixture of LiMO 2( space group R-3m), and LiM 2 O 4( space group Fd-3m). The mixing ratio (mass ratio) between LiMO 2( space group R-3m), and LiM 2 O 4( space group Fd-3m) may be, for example, LiMO 2 / LiM 2 O 4=9 / 1 to 9 / 1", or "LiMO 2 / LiM 2 O 4=9 / 1 to 5 / 5", or "LiMO 2 / LiM2O4=9 / 1 to 7 / 3".< Compound>The polyanion compound may include, for example, a phosphoric acid salt (such as LiFePO 4), a silicic acid salt, a boric acid salt, and / or the like. The polyanion compound can be represented by, for example, the following formulas (A-6) to (A-9). LiMPO 4( A-6) Li 2-x MPO 4 F (A-7) Li 2 MSiO 4( A-8) LiMBO 3( A-9)In the above formulas (A-6) to (A-9), M may include, for example, at least one element selected from the group consisting of Fe, Mn, and Co. In the above formula (A-7), for example, the relationship 0≤x≤2 may be satisfied.Positive electrode active material particles 5 may include, for example, a mixture of LiMO 2( space group R-3m), and the polyanion compound. The mixing ratio (mass ratio) between LiMO 2( space group R-3m), and the polyanion compound may be, for example, "LiMO 2 / ( polyanion compound)=9 / 1 to 9 / 1" or "LiMO 2 / ( polyanion compound)=9 / 1 to 5 / 5" or "LiMO 2 / ( polyanion compound)=9 / 1 to 7 / 3".<>A dopant may be added to the positive electrode active material particle 5. The dopant may be diffused into the entire particle or locally distributed. The dopant may be, for example, locally distributed on the surface of the particles. The dopant may be a substituted solid atom or an penetrating solid atom. The amount of the dopant to be added (the molar proportion relative to the total amount of the positive electrode active material particle 5) may be, for example, 0.01 to 5%, 0.1 to 3%, or 0.1 to 1%. A single type of dopant or two or more types of dopants may be added. The two or more dopants may form a complex.The dopant may include, for example, at least one of the following elements: B, C, N, a halogen, Si, Na, Mg, Al, Mn, Co, Cr, Sc, Ti, V, Cu, Zn, Ga, Ge, Se, Sr, Y, Zr, Nb, Mo, In, Pb, Bi, Sb, Sn, W, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and an actinoid.For example, to NCA, a combination of "Zr, Mg, W, Sm", a combination of "Ti, Mn, Nb, Si, Mo", or a combination of "Er, Mg" may be added.For example, Ti may be added to NCM. For example, a combination of "Zr, W", a combination of "Si, W", or a combination of "Zr, W, Al, Ti, Co" may be added to NCM.<Surface Coating>The positive electrode may contain a composite particle. The composite particle contains a core particle and a cover layer. The core particle contains the positive electrode active material particle 5. The cover layer can have a thickness of 1 to 3000 nm, 5 to 2000 nm, 10 to 1000 nm, 10 to 100 nm or 10 to 50 nm, for example. The thickness of the cover layer can be measured, for example, in an SEM photograph (scanning electron microscope) of a cross section of the particle and / or the like. More specifically, the composite particle is embedded in a resin material to produce a sample. A cross section of the sample is exposed by means of an ion milling device. For example, an ion milling machine with the trade name "ArLade (registered trademark) 5000" of Hitachi High Technologies (or a similar product) may be used. The cross section of the sample is examined with an SEM apparatus. For example, an SEM device with the trade name "SU8030" of Hitachi High Technologies (or a similar product) may be used. For each of ten composite particles, the thickness of the cover layer is measured in twenty fields of view. The arithmetic mean of a total of 200 thickness measurements is used.The portion of the surface portion of the core particle covered by the covering layer is also called "coverage rate". The coverage may be, for example, 1% or more, 10% or more, 30% or more, 50% or more, or 70% or more. The coverage may be, for example, 100% or less, or 90% or less, or 80% or less.The coverage can be measured, for example, by XPS (X-ray photoelectron spectroscopy). For example, an XPS device having the trade name "PHI X-tool" from ULVAC-PHI (or a similar product) may be used. A powder sample consisting of the composite particle is loaded into the XPS apparatus. Narrow scan analysis is performed. The measurement data are processed with analysis software. For example, analysis software having the trade name "MulTiPak" from ULVAC-PHI (or a similar product) may be used. The measurement data is analyzed to recognize a plurality of element types. From the area of each peak, the ratio of the detected element is determined. The coverage rate is determined using the following equation.0: Coverage [%] I0: Ratio of the elements to be assigned to the core particle1: Ratio of Elements to Be Associated with OvercoatFor example, when the core particle contains NCM, I 0 represents the total ratio of elements "Ni, Co, Mn". For example, when the core particle contains NCA, I 0 represents the total ratio of elements "Ni, Co, Al". For example, when the overcoat layer contains P and B, I 1 represents the total ratio of elements "P, B".The cover layer may contain any component. The covering layer may contain, for example, an elemental substance, an organic substance, an inorganic acid salt, an organic acid salt, a hydroxide, an oxide, a carbide, a nitride, a sulfide, a halide, and / or the like. The covering layer may, for example, include at least one selected from the group consisting of B, Al, W, Zr, Ti, Co, F, lithium compound (such as Li 2 CO 3, LiHCO 3, LiOH, Li 2 O, for example), tungsten oxide (such as WO 3, for example), titanium oxide (such as TiO 2, for example), zirconium oxide (such as ZrO 2, for example), boron oxide, boron phosphate (such as BPO 4, for example), aluminum oxide (such as Al 2 O 3, for example), Boehmite, aluminum hydroxide, phosphoric acid salt [such as Li 3 PO 4, ( NH 4)3 PO 4, AlPO 4, for example], boric acid salt (such as Li 2 B 4 O 7, LiBO 3, for example), polyacrylic acid salt (such as Li salt, Na salt, NH 4 salt), acetic acid salt (such as Li salt), CMC (such as acid type, Na salt, Li salt, NH_NER197 salt), LiNbO 3, Li 2 TiO 3, and Li-containing halide (such as LiAlCl 4, LiTiAlF 6, LiYBr 6, LiYCl 6,, for example).< Particles, Solid Particles>"Hollow particle" refers to a secondary particle in which, in a cross-sectional image, the proportion of the area of the central cavity is 30% or more of the total cross-sectional area of the particle. In a hollow particle, the proportion of the void may be, for example, 40% or more, 50% or more, or 60% or more. "Solid particles" refers to a secondary particle in which, in a cross-sectional image of the particle, the proportion of the area of the central void is less than 30% of the total cross-sectional area of the particle. The proportion of the void in a solid particle may be, for example, 20% or less, 10% or less, or 5% or less. The positive electrode active material particles 5 may be hollow particles or solid particles. A mixture of hollow particles and solid particles may also be used. The mixing ratio (mass ratio) between hollow particles and solid particles may be, for example, "(hollow particles) / (solid particles)=1 / 9 to 9 / 1" or "(hollow particles) / (solid particles)=2 / 8 to 8 / 2" or "(hollow particles) / (solid particles)=3 / 7 to 7 / 3" or "(hollow particles) / (solid particles)=4 / 6 to 6 / 4".< Particles / Small Particles>"Electrode active material" refers to a positive electrode active material and a negative electrode active material. The electrode active material may have, for example, a monomodal particle size distribution (based on the number). The electrode active material may have a multimodal particle size distribution, for example. The electrodeactive material may have, for example, a bimodal particle size distribution. That is, the electrode active material may contain large and small particles. When the particle size distribution is bimodal, the particle size corresponding to the peak of the larger particle size is considered to be the particle size of the large particles (d L). The particle size corresponding to the peak value of the smaller particle size is considered as the particle size of the small particles (d S). The particle size ratio (d L / d S) may be, for example, 2 to 10, 2 to 5, or 2 to 4. d L may be, for example, 8 to 20 μm or 8 to 15 μm. d S may be, for example, 1 to 10 μm or 1 to 5 μm.For example, with the aid of waveform analysis software, peak separation for particle size distribution can be performed. The ratio between the peak area of the large particles (S L) and the peak area of the small particles (S S) may be, for example, "S / S LS=1 / 9 to 9 / 1" or "S / S LS=5 / 5 to 9 / 1" or "S / S LS=7 / 3 to 9 / 1.".The number-average particle size distribution is measured by a microscopic method. A plurality of cross-sectional samples are taken from the layer of active electrode material. The cross-sectional sample can comprise, for example, a cross-section perpendicular to the surface of the electrode-active material layer. The cleaning is carried out, for example, by ion milling and / or the like on the side to be observed. The cross-sectional sample is examined with the SEM. The magnification for the examination is adjusted so that 10 to 100 particles are contained in the field of view of the examination. The Feret diameters of all particles on the image are measured. "Feret Diameters" refers to the distance between two points that are furthest from each other on the outline of the secondary particle. The plurality of cross-sectional samples are examined to obtain a total of 1000 or more Feret diameters. A number-based particle size distribution is produced from the 1000 or more Feret diameters.The bimodal particle size distribution may be formed by two types of particles which are mixed together. These two types of particles have different particle size distributions. For example, the two kinds of particles may have different D50. "D50" refers to a particle size in the volume-based particle size distribution at which the cumulative frequency accumulated on the small particle size side reaches 50%. D50 can be measured by laser diffraction. The sample to be measured is a powder. For example, the large particles may have a D50of from 8 to 20 μm or from 8 to 15 μm. The small particles may have a D50of from 1 to 10 μm or from 1 to 5 μm, for example. The ratio of D50of the large particles to D50of the small particles may be, for example, 2 to 10, 2 to 5, or 2 to 4. The mixing ratio (mass ratio) between the large particles and the small particles may be, for example, "(large particles) / (small particles)=1 / 9 to 9 / 1" or "(large particles) / (small particles)=5 / 5 to 9 / 1" or "(large particles) / (small particles)=7 / 3 to 9 / 1.".The large particles and the small particles may have the same composition or may have different compositions. For example, the large particles may be NCA and the small particles may be NCM. For example, the large particles may be NCM (0.6≤x) and the small particles may be NCM (x<0.6).<<Method for Producing Positive Electrode Active Material Particles>>As a method for producing a positive electrode active material particle according to the present embodiment, for example, the method described below may be mentioned. The following description is given by way of example and does not limit the scope of the manufacturing method.The method for producing a positive electrode active material particle according to the present embodiment includes, for example, "(A) production of a precursor" and "(B) calcination".<(A) Production of Precursor>The present manufacturing method includes manufacturing a precursor of a positive electrode active material particle. The method for producing the precursor is not particularly limited. For example, a mixed hydroxide of Mg and a transition metal (such as Ni) is prepared. For example, the powder of the mixed hydroxide is pulverized in a mortar. For example, the pulverized powder of the mixed hydroxide is dissolved in an aqueous alkali solution, thereby forming a mixed solution. The alkali aqueous solution may be, for example, an NaOH aqueous solution, an ammonia aqueous solution, and the like. To the mixed solution, for example, an aqueous acidic solution and an aqueous alkali solution are simultaneously added dropwise, whereby a sediment may be formed. During the dropwise addition, the pH can be adjusted. It is conceivable that the sediment contains a composite hydroxide (a precursor). The sediment is rinsed and dried, so that a dried product is formed.Alternatively, a sulfate of Mg and a transition metal (such as Ni) is prepared. The sulfate is dissolved in water to form an aqueous acidic solution. To the aqueous acidic solution, for example, an aqueous alkali solution is added dropwise, whereby a neutralization reaction can take place. The alkali aqueous solution may contain, for example, an NaOH aqueous solution, an aqueous ammonia solution, or the like. A sediment can be formed by the neutralization reaction. During the neutralization reaction, the pH can be adjusted. It is conceivable that the sediment contains a precursor. The sediment is rinsed and dried, so that a dried product is formed.<(B) Calcination>The dried product thus obtained is mixed with a lithium compound to form a mixture. The lithium compound may include, for example, Li 2 CO 3, LiOH, and / or the like. The mixture is subjected to a heat treatment (calcination). The calcination temperature may be, for example, between 500 and 1000° C. The calcination time may be, for example, 5 to 30 hours. In this way, a positive electrode active material particle is produced.<<Lithium Ion Battery>FIG. 2 is a conceptual view showing an example of a lithium ion battery according to the present embodiment. A battery 100 includes a power generation element 50 and an electrolyte (not shown).< Packaging>The battery 100 may include an outer package (not shown). The outer package may house the power generating element 50 and the electrolyte. The outer package may have any configuration. The outer package may be a metal case or a pouch made of, for example, a metal foil laminated film. The case may have any shape. The case may be, for example, cylindrical, prismatic, flat, coin-shaped, and / or similar. The outer package may contain, for example, Al and / or the like. The outer package may house, for example, a single power generating element 50 or a plurality of power generating elements 50. The plurality of power generation elements 50 may form, for example, a series circuit or a parallel circuit. In the outer package, the plurality of power generation elements 50 may be stacked in the thickness direction of the battery 100.< Element>Power generation element 50 may also be referred to as "an electrode group", "an electrode assembly", and the like. The power generation element 50 includes a positive electrode 10 and a negative electrode 20. The separator 30 is interposed between the positive electrode 10 and the negative electrode 20. The power generation element 50 may have any configuration. For example, the power generation element 50 may be a stack type. For example, the positive electrode 10 and the negative electrode 20 may be alternately stacked with the separator 30 interposed between the positive electrode 10 and the negative electrode 20 to form the power generating element 50. The power generation element 50 may be, for example, a wound element. For example, the positive electrode 10 having a belt-like shape, the separator 30 having a belt-like shape, and the negative electrode 20 having a belt-like shape may be stacked into a stack. The resulting stack may be spirally wound to form the power generating element 50. After winding, the wound power generation element 50 may be formed into a flat shape.<Positive electrode>Positive electrode>The positive electrode 10 may be in a sheet form, for example. Positive electrode 10 includes positive electrode current collector 11 and positive electrode active material layer 12. The positive electrode current collector 11 supports the positive electrode active material layer 12, for example, the positive electrode current collector 11 may be in a sheet form. Positive electrode current collectors 11 may have a thickness of 5 to 50 μm, for example. The positive electrode current collector 11 may include, for example, a metal foil. The positive electrode current collector 11 may include, for example, at least one metal selected from the group consisting of Al, Mn, Ti, Fe, and Cr. The positive electrode current collector 11 may include, for example, an Al foil, an Al alloy foil, a Ti foil, a stainless steel foil (SUS), and / or the like.Between the positive electrode current collector 11 and the positive electrode active material layer 12, an intermediate layer (not illustrated) may be formed. The intermediate layer does not contain a positive electrode active material particle. The intermediate layer may have a thickness of 0.1 to 5 μm, for example. The intermediate layer may include, for example, a conductive material, an insulating material, a binder, and / or the like. The conductive material and the binder will be described below. The insulating material may include, for example, alumina, boehmite, aluminum hydroxide, and / or the like.Positive electrode active material layer 12 is placed on the surface of positive electrode current collector 11. The positive electrode active material layer 12 may be disposed on only one side of the positive electrode current collector 11. The positive electrode active material layer 12 may be disposed on both sides of the positive electrode current collector 11. The positive electrode active material layer 12 may have a thickness of 10 μm or more, or have a thickness of 50 μm or more, or have a thickness of 80 μm or more, or have a thickness of 100 μm or more, for example. It is conceivable that, in the case of a thick positive electrode active material layer 12, in particular, in the case of a thickness of 80 μm or more, retention after curing is improved even further. The positive electrode active material layer 12 may have a thickness of 1000 μm or less, or a thickness of 500 μm or less, or a thickness of 300 μm or less. The positive electrode active material layer 12 contains a positive electrode active material particle. The positive electrode active material layer 12 may also include a conductive material, a binder, carbon nanotubes, and the like.<<Leitfähiges Material>>The conductive material may form an electron conducting path within the positive electrode active material layer 12. The amount of the conductive material to be used may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material particle. The conductive material may include any component. The conductive material may include, for example, at least one component selected from the group consisting of graphite, acetylene black (AB), ketjen black (registered trademark), vapor phase grown carbon fibers (VGCFs), carbon nanotubes (CNTs), and graphene flakes (GFs).<<>>The binder is capable of fixing the positive electrode active material layer 12 to the positive electrode current collector 11. The amount of the binder to be used may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material particle. The binder may contain any component. The binder may include, for example, at least one member selected from the group consisting of polyvinylidene difluoride (PVdF), vinylidene difluoride-hexafluoropropylene copolymer (PVdF-HFP), tetrafluoroethylene (PTFE), CMC, PAA, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyoxyethylene alkyl ethers, and derivatives thereof.<<Carbon Nanotubes>>Carbon nanotubes (CNTs) have high electronic conductivity. That is, when the positive electrode active material layer 12 contains CNTs, the electronic conductivity is ensured, and it is expected that the initial resistance and the capacity retention thereby improve. In addition, CNTs are preferably contained in the inner portion of the positive electrode active material particle. When CNTs are contained in the inner portion of the positive electrode active material particle, the CNTs are expected to form composites that can more reliably ensure electronic conductivity.The CNTs may have an aspect ratio of 20 or more, for example. When the CNTs have an aspect ratio of 20 or more, the CNTs tend to be included in the inner portion 1 of the positive electrode active material particle 5, and thereby the resistance is expected to be reduced. The CNTs may have an aspect ratio of 100 or more.The aspect ratio is the ratio of length to diameter. The aspect ratio is determined by dividing the average length of the CNTs by the average diameter of the CNTs. The average length and the average diameter may each be the arithmetic mean of values measured for ten or more CNTs. The length and diameter of each of the CNTs are measured with a TEM or a scanning electron microscope (SEM).The CNTs may have an average diameter of, for example, 10 nm to 50 nm. The CNTs may have an average length of, for example, 1 μm to 5 μm.The proportion of the CNTs in the positive electrode active material layer 12 may be, for example, between 0.1 mass % and 5.0 mass %. When the proportion of the CNTs in the positive electrode active material layer 12 satisfies the above range, the initial durability and the retention of the capacitance are expected to be improved. Preferably, the proportion of the CNTs in the positive electrode active material layer 12 is 0.5 mass % to 4.0 mass %.<<Andere Constituents>>The positive electrode active material layer 12 may also include, for example, an inorganic filler, an organic filler, a solid electrolyte, a surface modifier, a lubricant, a flame retardant, a protective agent, a flux, a coupling agent, an adsorbent, and / or the like. The positive electrode active material layer 12 may include, for example, polyoxyethylene allyl phenyl ether phosphate, zeolite, silane coupling agent MoS 2, WO 3, and / or the like.<Negative Electrode>The negative electrode 20 may be in a sheet form, for example. Negative electrode 20 may include, for example, a negative electrode current collector 21 and a negative electrode active material layer 22. Negative electrode current collector 21 is electrically conductive. Negative electrode current collector 21 carries negative electrode active material layer 22, and negative electrode current collector 21 may be in a plate form, for example. Negative electrode current collector 21 may have a thickness of 5 to 50 μm, for example. Negative electrode current collector 21 may include, for example, a metal foil and / or the like. The negative electrode current collector 21 may include, for example, at least one metal selected from the group consisting of Cu, Ni, Fe, Zn, Pb, Ag, and Au. Negative electrode current collector 21 may include, for example, a Cu foil, a Cu alloy foil, and / or the like.Negative electrode active material layer 22 is placed on the surface of negative electrode current collector 21. Negative electrode active material layer 22 may be disposed on only one side of negative electrode current collector 21. Negative electrode active material layer 22 may be disposed on both sides of negative electrode current collector 21. The negative electrode active material layer 22 may have a thickness of, for example, 10 to 1000 μm, or 50 to 500 μm, or 100 to 300 μm. The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material layer may also include, for example, a conductive material, a binder, and the like.Negative electrode active material layer 22 may have a thickness of 10 to 1000 μm, or 50 to 500 μm, or 100 to 300 μm, for example. The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material layer may include, for example, a conductive material, a binder, and the like.<<Leitfähiges Material>>The conductive material may form an electron conduction path within the negative electrode active material layer 22. The proportion of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material. The conductive material may include any component. The conductive material may include at least one component selected from the group consisting of graphite, AB, ketjenblack, VGCFs, CNTs, and GFs, for example. The CNTs may include at least one selected from the group consisting of single-wall CNTs (SWCNTs) and multi-wall CNTs (MWCNTs).<<>>The binder is capable of fixing the negative electrode active material layer 22 to the negative electrode current collector 21. The content of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material. The binder may contain any component. The binder may include, for example, at least one component selected from the group consisting of styrene-butadiene rubber (SBR), acrylate-butadiene rubber (ABR), sodium alginate, carboxymethylcellulose (such as CMC-H, CMC-Na, CMC-Li, CMC-NH 4), polyacrylic acid (such as PAA-H, PAA-Na, PAA-Li), polyacrylonitrile (PAN), polyvinylidene difluoride (PVdF), polytetrafluoroethylene (PTFE), acrylic resin (acrylic ester copolymer), methacrylic resin (methacrylic ester copolymer), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and derivatives of these. For example, the term "CMC-Na" refers to a Na salt of CMC. The term "CMC-H" refers to, for example, an acid-type CMC. The same applies to "PAA-Na" and the like.<<Weitere Constituents>>The negative electrode active material layer 22 may further include, for example, an inorganic filler, an organic filler, a solid electrolyte, a surface modifier, a dispersant, a lubricant, a flame retardant, a protective agent, a flux, an adhesive, an adsorbent, and / or the like. The negative electrode active material layer may include, for example, a layered silicate (such as smectite, montmorillonite, bentonite, hectorite), an inorganic filler (such as solid alumina, hollow silica, boehmite), a polysiloxane compound, and / or the like.<<Negative Electrode Active Material>>The negative electrode active material may include, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, hard carbon, silicon (Si), SiO, Li silicate, Si-based alloy, tin (Sn), SnO, Sn-based alloy, and Li 4 Ti 5 O 12.<Carbon-Based Active Material>The term "graphite" refers to natural graphite and artificial graphite. The graphite may be a mixture of natural graphite and artificial graphite. The mixing ratio (mass ratio) may be, for example, "(natural graphite) / (artificial graphite)=1 / 9 to 9 / 1" or "(natural graphite) / (artificial graphite)=2 / 8 to 8 / 2" or "(natural graphite) / (artificial graphite)=3 / 7 to 7 / 3".The graphite may contain a dopant. The dopant may include, for example, at least one selected from the group consisting of B, N, P, Li, and Ca. The amount to be added may be, for example, 0.01 to 5%, 0.1 to 3%, or 0.1 to 1% in molar proportions.The surface of the graphite may be covered with amorphous carbon, for example. The surface of the graphite may be covered with another type of material, for example. This other type of material may include, for example, at least one selected from the group consisting of P, W, Al, and O. The other type of material may include, for example, at least one selected from the group consisting of Al(OH) 3, AlOOH, Al 2 O 3, WO 3, Li 2 CO 3, LiHCO 3, and Li 3 PO 4.<Legierungs-Based Active Material>SiO can be represented by the following formula (B-1), for example. SiO x( B-1)In the above formula, the relationship 0<x<2 is satisfied.In the above formula (B-1), x may satisfy, for example, 0.5≤x≤1.5 or 0.8≤x≤1.2.Li silicate may include, for example, at least one selected from the group consisting of Li 4 SiO 4, Li 2 SiO 3, Li 2 Si 2 O 5, and Li 8 SiO 6. A second negative electrode active material may include, for example, a mixture of Si and Li silicates. The mixing ratio (mass ratio) may be, for example, "Si / (Li silicate)=1 / 9 to 9 / 1" or "Si / (Li silicate)=2 / 8 to 8 / 2" or "Si / (Li silicate)=3 / 7 to 7 / 3" or "Si / (Li silicate)=4 / 6 to 6 / 4".The alloy-based active material (such as Si, SiO) may contain an additive. The additive may be, for example, a substituted solid solution atom or an penetrating solid solution atom. The additive may be an element adhering to the surface of the alloy-based active material. The adhesive agent may be, for example, an elemental substance, an oxide, a carbide, a nitride, a halide, and / or the like. The amount to be added may be, for example, between 0.01 and 5%, 0.1 and 3%, or 0.1 and 1% in molar proportions. The additive may include, for example, at least one of Li, Na, K, Rb, Be, Mg, Ca, Sr, Fe, Ba, B, Al, Ga, In, C, Ge, Sn, Pb, N, P, As, Y, Sb, and S. That is, SiO may be doped with Mg and / or Na. For example, Mg silicate and / or Na silicate may be formed. For example, SiO may be doped with boron oxide (such as B O 23), yttrium oxide (such as Y O 23) and / or the like.<Si-C Composite Material>The negative electrode active material may include a composite material of the carbon-based active material (such as graphite) and the alloy-based active material (such as Si). A composite material containing Si and carbon may also be referred to as "Si-C composite material". For example, Si microparticles may be dispersed in carbon particles. For example, Si microparticles may be dispersed in graphite particles. For example, Li silicate particles may be covered with a carbon material (such as amorphous carbon). The Si-C composite material and the graphite may be mixed together for use.< System>The negative electrode active material may contain two or more components. The negative electrode active material may include the carbon-based active material (such as graphite) and the alloy-based active material (such as Si, SiO). The mixing ratio (mass ratio) of the carbon-based active material and the alloy-based active material may be, for example, "(carbon-based active material) / (alloy-based active material)=1 / 9 to 9 / 1" or "(carbon-based active material) / (alloy-based active material)=2 / 8 to 8 / 2" or "(carbon-based active material) / (alloy-based active material)=3 / 7 to 7 / 3" or "(carbon-based active material) / (alloy-based active material)=4 / 6 to 6 / 4".<>The separator 30 is capable of separating the positive electrode 10 from the negative electrode 20. The separator 30 is electrically insulating. The partition member 30 may include, for example, at least one layer selected from the group consisting of a resin film, an inorganic particulate layer, and an organic particulate layer. The partition member 30 may include, for example, a resin film and an inorganic particle layer.<<>>The resin film is porous. The resin film may include, for example, a microporous film, a nonwoven fabric, and / or the like. The resin film contains a resin skeleton. The resin skeleton may be continuous in the form of a mesh, for example. Voids in the resin skeleton form pores. The electrolyte can permeate through the resin film. The resin film may have, for example, an average pore size of 1 μm or less. The resin film may have, for example, an average pore size of from 0.01 to 1 μm or from 0.1 to 0.5 μm. The "average pore size" can be measured by mercury porosimetry. The resin film may have, for example, a Cucumbery value of 50 to 250 s / 100 cm 3. The "Cucumbery value" can be measured by a Cucumbery test method.The resin film may include, for example, at least one resin selected from the group consisting of an olefin-based resin, a polyurethane-based resin, a polyamide-based resin, a cellulose-based resin, a polyether-based resin, an acryl-based resin, a polyester-based resin, and the like. The resin film may include, for example, at least one of polyethylene (PE), polypropylene (PP), polyamide (PA), polyamideimide (PAI), polyimide (PI), aromatic polyamide (aramid), and polyphenylene ether (PPE), and derivatives thereof. The resin film may be formed by, for example, stretching, phase separation and / or the like. The resin film may have a thickness of, for example, 5 to 50 μm or 10 to 25 μm.The resin film may have a monolayer structure. The resin film may be composed of, for example, a PE layer. The framework of the PE layer is formed of PE. The PE layer may have a shutdown function. The resin film may have, for example, a multilayer structure. The resin film may include, for example, a PP layer and a PE layer. A skeleton of the PP layer is made of PP. The resin film may have a three-layer structure, for example. The resin film may be formed by stacking a PP layer, a PE layer, and a PP layer in this order, for example. The thickness of the PE layer may be, for example, 5 to 20 μm. The thickness of the PP layer may be, for example, 3 to 10 μm.<<Anorganische Particle Layer>>The inorganic particle layer may be formed on the surface of the resin film. The inorganic particle layer may be formed on only one side of the resin film or on both sides of the resin film. The inorganic particle layer may be formed on the side facing the positive electrode 10, or may be formed on the side facing the negative electrode 20. The inorganic particle layer may be formed on the surface of the positive electrode 10 or on the surface of the negative electrode 20.The inorganic particle layer is porous. The inorganic particle layer contains inorganic particles. The inorganic particles may also be referred to as "inorganic filler". Voids between the inorganic particles form pores. The inorganic particle layer may have a thickness of 0.5 to 10 μm or, for example, 1 to 5 μm. The inorganic particles may contain, for example, a heat-resistant material. The inorganic particle layer containing a heat-resistant material is also called "HRL (Heat Resistance Layer)". The inorganic particles may include at least one of boehmite, alumina, zirconia, titania, magnesia, silica, and the like. The inorganic particles may have any shape. The inorganic particles may be, for example, spherical, rod-shaped, platelet-shaped, fibrous and / or similar. The inorganic particles may have a D50of 0.1 to 10 μm or, for example, of 0.5 to 3 μm. The inorganic particle layer may further contain a binder. The binder may include, for example, at least one selected from the group consisting of an acrylic-based resin, a polyamide-based resin, a fluorine-based resin, an aromatic polyether-based resin, and a liquid crystal polyester-based resin, and the like.<<Organic Particle Layer>>The separator 30 may include, for example, an organic particle layer. For example, the partition member 30 may include an organic particle layer instead of the resin film. For example, the separator 30 may include an organic particle layer instead of the inorganic particle layer. The partition member 30 may include both the resin film and an organic particle layer. The separator 30 may include both the inorganic particle layer and an organic particle layer. The partition member 30 may include the resin film, the inorganic particle layer, and an organic particle layer.The organic particle layer may have a thickness of from 0.1 to 50 μm, or from 0.5 to 20 μm, or from 0.5 to 10 μm, or from 1 to 5 μm, for example. The organic particle layer comprises organic particles. The organic particles can also be referred to as "organic filler". The organic particles may contain a heat-resistant material. The organic particles may include, for example, at least one selected from the group consisting of PE, PP, PTFE, PI, PAI, PA, aramid, and the like. The organic particles can be, for example, spherical, rod-shaped, platelet-shaped, fibrous and / or the like. The organic particles may have, for example, a D50of 0.1 to 10 μm or of 0.5 to 3 μm.The separator 30 may include, for example, a mixed layer. The mixed layer contains both inorganic and organic particles.<>The electrolyte dissolves Li ions therein. The electrolyte may be a liquid electrolyte or a gelled electrolyte. The liquid electrolyte may be, for example, an electrolyte solution. The electrolyte solution contains a solvent and a solute.<<Solvent>><Solver Based on Ether>The electrolyte solution may include, for example, an ether-based solvent and / or the like. The solvent may include, for example, hydrofluoroether (HFE) and / or the like. For example, HFE may include at least one selected from the group consisting of a difluoromethyl group, a 2,2-difluoroethyl group, a 2,2,2-trifluoroethyl group, a 1,1,2,2-tetrafluoroethyl group, a 2,2,3,3,3-pentafluoropropyl group, a 2,2,3,3-tetrafluoropropyl group, a 1,1,1,3,3,3-hexafluoroisopropyl group, a 1,1,2,3,3,3-hexafluoropropyl group, a 2,2,3,3,4,4,4-heptafluorobutyl group, a 2,2,3,3,4,4-hexafluorobutyl group, and a 2,2,3,3,4,5,5-octafluoropentyl group.The solvent may also contain an ether other than HFE (the ether may also be referred to as "second ether" hereinafter). The second ether may include, for example, at least one ether selected from the group consisting of tetrahydrofuran (THF), 1,4-dioxane (DOX), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), ethylglyme, triglyme, tetraglyme, and derivatives thereof. For example, the solvent may contain the second ether (such as DME) in a volume fraction of 1 to 50%, the balance consisting of HFE. For example, the solvent may contain the second ether in a volume fraction of 10 to 40%, the balance being HFE.< Based Carbonate>The electrolyte solution may contain, for example, a carbonate-based solvent (a carbonate ester-based solvent). The solvent may include, for example, a cyclic carbonate, a chain carbonate, a fluorinated carbonate, and / or the like. The solvent may, for example, include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate, 4,4-difluoroethylene carbonate, trifluoroethylene carbonate, perfluoroethylene carbonate, fluoropropylene carbonate, difluoropropylene carbonate, and derivatives thereof.The solvent may contain a cyclic carbonate (such as EC, PC, FEC) and a chain carbonate (such as EMC, DMC, DEC). The mixing ratio between the cyclic carbonate and the chain carbonate (volume ratio) may be, for example, "(cyclic carbonate) / (chain carbonate)=1 / 9 to 4 / 6", or "(cyclic carbonate) / (chain carbonate)=2 / 8 to 3 / 7", or "(cyclic carbonate) / (chain carbonate)=3 / 7 to 4 / 6".The solvent may include a cyclic carbonate (such as EC, PC) and a fluorinated cyclic carbonate (such as FEC). The mixing ratio between the cyclic carbonate and the fluorinated cyclic carbonate (volume ratio) may be "(cyclic carbonate) / (fluorinated cyclic carbonate)=9 / 1 to 90 / 10", or "(cyclic carbonate) / (fluorinated cyclic carbonate)=9 / 1 to 1 / 9", or "(cyclic carbonate) / (fluorinated cyclic carbonate)=9 / 1 to 7 / 3", or "(cyclic carbonate) / (fluorinated cyclic carbonate)=3 / 7 to 1 / 9", for example.The solvent may contain, for example, EC, FEC, EMC, DMC and DEC. The volume ratio of these components may satisfy the relationship represented by the following equation (C-1), for example. V EC+ V FEC+ V EMC+ V DMC+ V DEC=10 ( C-1)In the above equation, each of V EC, V FEC, V EMC, V DMC and V DEC represents the volume ratio of EC, FEC, EMC, DMC and DEC, respectively.The following relationships are satisfied: 1≤V EC ≤4, 0≤V FEC ≤3, V EC+ V FEC ≤4, 0≤V EMC ≤9, 0≤V DMC ≤9, 0≤V DEC ≤9, 6≤V EMC+ V DMC+ V DEC ≤9.In the above equation (C-1), for example, the relationship 1≤V EC≤2or 2≤V EC≤3may be satisfied.The relationship 1<V FEC ≤2or 2<V FEC ≤4may be satisfied, for example.For example, the relationship 3≤V EMC≤4, or 6≤V EMC≤8 may be satisfied.For example, the relationship 3≤V DMC≤4, or 6≤V DMC≤8 may be satisfied.For example, the relationship 3≤V DEC≤4, or 6≤V DEC≤8 may be satisfied.The solvent may have a composition of "EC / EMC=3 / 7", "EC / DMC=3 / 7", "EC / FEC / DEC=1 / 2 / 7", "EC / DMC / EMC=3 / 4 / 3", "EC / DMC / EMC=3 / 3 / 4", "EC / FEC / DMC / EMC=2 / 1 / 4 / 3", "EC / FEC / DMC / EMC=1 / 2 / 4 / 3", "EC / FEC / DMC / EMC=2 / 1 / 3 / 4", "EC / FEC / DMC / EMC=1 / 2 / 3 / 4" (volume ratio), and / or the like, for example.<Additive>The electrolyte solution may contain any additive. The amount to be added (the mass proportion of the total amount of the electrolyte solution) may be, for example, 0.01 to 5%, 0.05 to 3%, or 0.1 to 1%. The additive may include, for example, a solid electrolyte biphasic (SEI) formation promoter, an SEI formation inhibitor, a gas forming agent, an overcharge inhibitor, a flame retardant, an antioxidant, an electrode protective agent, a surfactant, and / or the like.The additive may, for example, include at least one selected from the group consisting of vinylene carbonate (VC), vinylethylene carbonate (VEC), 1,3-propane sultone (PS), tert-amylbenzene, 1,4-di-tert-butylbenzene, biphenyl (BP), cyclohexylbenzene (CHB), ethylene sulfite (ES), propane sultone (PS), ethylene sulfate (DTD), γ-butyrolactone, phosphazene compound, carboxylate esters [such as methyl formate (MF), methyl acetate (MA), methyl propionate (MP), diethyl malonate (DEM), for example], fluorobenzene (such as monofluorobenzene (FB), 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-Trifluorobenzene, 1,2,4-Trifluorobenzene, 1,3,5-Trifluorobenzene, 1,2,3,4-Tetrafluorobenzene, 1,2,3,5-Tetrafluorobenzene, 1,2,4,5-Tetrafluorobenzene, Pentafluorobenzene, Hexafluorobenzene, for example), Fluorotoluene (such as 2-Fluorotoluene, 3-Fluorotoluene, 4-Fluorotoluene, 2,3-Difluorotoluene, 2,4-Difluorotoluene, 2,5-Difluorotoluene, 2,6-Difluorotoluene, 3,4-Difluorotoluene, Octafluorotoluene, for example), Benzotrifluoride (such as Benzotrifluoride, 2-Fluorobenzotrifluoride, 3-Fluorobenzotrifluoride, 4-Fluorobenzotrifluoride, 2-Methylbenzotrifluoride, 3-Methylbenzotrifluoride, 4-Methylbenzotrifluoride, for example), Fluoroxylene (such as 3-Fluoro-o-xylene, 4-Fluoro-o-xylene, 2-Fluoro-m-xylene, 5-fluoro-m-xylene, for example), sulfur-containing heterocyclic compound (such as benzothiazole, 2-methylbenzothiazole, tetrathiafulvalene, for example), nitrile compound (such as adiponitrile, succinonitrile, for example), phosphate (such as trimethyl phosphate, triethyl phosphate, for example), carboxylic anhydride (such as acetic anhydride, propionic anhydride, oxalic anhydride, succinic anhydride, maleic anhydride, phthalic anhydride, benzoic anhydride, for example), alcohol (such as methanol, ethanol, n-propyl alcohol, ethylene glycol, diethylene glycol monomethyl ether, for example), and derivatives thereof.The components described above as solvents and solvents can be used as a trace component (an additive). The additive may include, for example, at least one selected from the group consisting of LiB 4, LiFSI, LiTFSI, LiBO, LiDFOB, LiDFOP, LiPO 2 F 2, FSO 3 Li, LiI, LiBr, HFE, DOX, PC, FEC, and derivatives thereof.<<Ionische Liquid>>The liquid electrolyte may comprise an ionic liquid. The liquid electrolyte may contain, for example, at least one of the following salts: a sulfonium salt, an ammonium salt, a pyridinium salt, a piperidinium salt, a pyrrolidinium salt, a morpholinium salt, a phosphonium salt, an imidazolium salt, and derivatives thereof.<< Electrolyte>>The gelled electrolyte may include a liquid electrolyte and a polymer material. The polymer material may form a polymer matrix. The polymeric material may include, for example, at least one material selected from the group consisting of PVdF, PVdF-HFP, PAN, PVdF-PAN, polyethylene oxide (PEO), polyethylene glycol (PEG), and derivatives thereof.< Configuration>FIG. 3 is a table showing a first battery configuration. FIG. 4 is a table showing a second battery configuration. FIG. 5 is a table showing a third battery configuration. When multiple materials are described in a single cell in each table, this description refers to both one of the materials and a combination of them. For example, when the materials "α, β, γ" are described in a single cell, this description is intended to mean "at least one selected from the group consisting of α, β, and γ". Certain elements may be extracted from the first through third battery configurations and optionally combined with each other.The present embodiment can be incorporated into the first to third battery configurations, for example. The positive electrode in the first battery configuration may be replaced with the positive electrode (positive electrode current collector 11, positive electrode active material layer 12) according to the present embodiment. For example, the combination of the first to third battery configurations with the present embodiment may improve battery performance.[Examples]Hereinafter, the present embodiment will be described by way of examples, but the present embodiment is not limited to these examples. Hereinafter, the method for producing a positive electrode active material particle of Nos. 2 to 4 may be referred to as "doping method", the method for producing a positive electrode active material particle of Nos. 5 to 7 may be referred to as "MHP method", and the method for producing a positive electrode active material particle of Nos. 8 to 10 may be referred to as "late addition method".<Production of Positive Electrode Active Material Particles>(No. 1)An aqueous nickel sulfate solution (1.8 mol / L) (hereinafter, "mol / L" may also be simply expressed as "M") was prepared. To the nickel sulfate aqueous solution, an NaOH aqueous solution (1.0 M) was added dropwise, thereby forming a sediment. The dropwise addition of the aqueous NaOH solution was controlled so that the pH was maintained at 11.6. The sediment was rinsed and dried to form a dried product. The dried product was mixed with LiOH to give a mixture. The mixture thus formed had a mixing ratio of Li and Ni (Li / Ni) (molar ratio) of 1.04. The mixture was calcined. The calcination temperature was 880° C. The calcination time was 6 hours. In this manner, a positive electrode active material particle of No. 1 having a lamellar rock salt structure was synthesized. It was also checked that the positive electrode active material particles of Nos. 2 to 10 described below also have a lamellar rock salt structure.(No. 2)Nickel sulfate and magnesium sulfate were dissolved in water to prepare an aqueous acidic solution (1.8 M). The mixing ratio of the solutes (molar ratio) was (Nickelsulfat):(Magnesiumsulfat)=96,8:3,2 To the aqueous acidic solution, an aqueous NaOH solution (1.0 M) was added dropwise, thereby forming a sediment. The dropwise addition of the aqueous NaOH solution was controlled so that the pH was maintained at 11.6. The sediment was rinsed and dried to form a dried product. The dried product was mixed with LiOH to give a mixture. The mixture thus formed had a mixing ratio of Li to Ni and Mg (Li / (Ni+Mg)) (molar ratio) of 1.04. The calcination temperature was 880° C. The calcination time was 6 hours. In this manner, a positive electrode active material particle of No. 2 was synthesized.(Nos. 3 to 4)Positive electrode active material particles of Nos. 3 and 4 were synthesized under the same conditions as for No. 2 except that the solute mixing ratio of the aqueous acidic solution was changed to (Nickelsulfat):(Magnesiumsulfat)=94.4:5,6 for No. 3 and the solute mixing ratio of the aqueous acidic solution was changed to (Nickelsulfat):(Magnesiumsulfat)=98,7:1,3 for No. 4.(No. 5)A mixed hydroxide of nickel hydroxide and magnesium hydroxide was prepared. The mixing ratio (molar ratio) was (Nickelhydroxid):(Magnesiumhydroxid)=96,7:3,1 The powder of the mixed hydroxide was pulverized in a mortar until the D50 reached about 3 μm. The pulverized powder of the mixed hydroxide was dissolved in an aqueous ammonia solution (mass concentration, 10%), thereby preparing a mixed solution. To the resulting mixed solution, an aqueous nickel sulfate solution (1.8 M) and an aqueous NaOH solution (1.0 M) were simultaneously added dropwise, thereby forming a sediment. The dropwise addition of the nickel sulfate aqueous solution and the NaOH aqueous solution was controlled so that the pH was maintained at 11.6. The sediment was rinsed and dried to form a dried product. The dried product was mixed with LiOH to give a mixture. The mixture thus formed had a mixing ratio of Li to Ni and Mg (Li / (Ni+Mg)) (molar ratio) of 1.04. The calcination temperature was 880° C. The calcination time was 6 hours. In this manner, a positive electrode active material particle of No. 5 was synthesized.(No. 6 to 7)Positive electrode active material particles of Nos. 6 and 7 were synthesized under the same conditions as for No. 5 except that the mixing ratio in the mixed hydroxide was for No. 6 (Nickelhydroxid):(Magnesiumhydroxid)=94,2:5,8 and the mixing ratio of the solutes in the mixed hydroxide was for No. 7 (Nickelhydroxid):(Magnesiumhydroxid)=99,1:0,9.(No. 8)The positive electrode active material particle of No. 1 was prepared. The positive electrode active material particle was mixed with magnesium oxide to obtain a mixture. The mixture thus formed had a mixing ratio of Ni and Mg (Ni:Mg) (molar ratio) of 96.9:3.1. The calcination temperature was 500°C. The calcination time was 5 hours. In this manner, a positive electrode active material particle of No. 8 was synthesized.(No. 9 to 10)Positive electrode active material particles of Nos. 9 and 10 were synthesized under the same conditions as for No. 8, except that the mixing ratio of Ni and Mg for No. 9 was 94.6:5.4 and the mixing ratio of Ni and Mg for No. 10 was 98.9:1.1.< Of Lithium Ion Battery>(Example 1)The materials described below were used to produce a positive electrode of Example 1.<<Positive Electrode>>Positive electrode active material particles: No. 2 Conductive material: AB (Denka Company Limited) Binder: PVDF (Kureha Corporation)In addition, the materials described below were used to produce a test battery for the evaluation purpose of Example 1.<<Negative Electrode>>Negative electrode active material: natural graphite (Hitachi Chemical Company, Ltd.) Binder: CMC (Nippon Paper Industries Co., Ltd.), SBR (JSR Corporation)<<>>Separator: Porous sheet (thickness, 24 μm) Electrolyte: Electrolyte solution [LiPF 6(1 M), EC+DMC+EMC](Example 2)A positive electrode and a test battery for evaluation purposes of Example 2 were produced under the same conditions as in Example 1, except that the positive electrode active material particle of Example 2 was No. 3.(Examples 3 to 5)The positive electrodes and the test batteries for evaluation purposes of Examples 3 to 5 were manufactured under the same conditions as in Example 1, except that 0.4 mass % of CNTs was added to the positive electrode of Example 1, 1.0 mass % of CNTs was added to the positive electrode of Example 1, and 4.0 mass % of CNTs was added to the positive electrode of Example 1.(Examples 6 to 8)The positive electrodes and test batteries of Examples 6 to 8 were manufactured under the same conditions as in Example 1, except that the positive electrode active material particles of Example 6 were No. 5, the positive electrode active material particles of Example 7 were No. 6, and the positive electrode active material particles of Example 8 were No. 7.(Examples 9 to 11)The positive electrodes and the test batteries of Examples 9 to 11 were produced under the same conditions as in Example 1, except that 0.4 mass % of CNTs was added to the positive electrode of Example 6, 1.0 mass % of CNTs was added to the positive electrode of Example 6, and 4.0 mass % of CNTs was added to the positive electrode of Example 6, respectively.(Comparative Example 1)A positive electrode and a test battery of Comparative Example 1 were produced under the same conditions as in Example 1, except that the positive electrode active material particle of Comparative Example 1 was No. 1.(Comparative Example 2)A positive electrode and a test battery of Comparative Example 2 were produced under the same conditions as in Example 1, except that the positive electrode active material particle of Comparative Example 2 was No. 2. 4.(Comparative Examples 3 to 5)The positive electrodes and the test batteries for evaluation purposes of Comparative Examples 3 to 5 were manufactured under the same conditions as in Example 1, except that the positive electrode active material particle of Comparative Example 3 was No. 8, the positive electrode active material particle of Comparative Example 4 was No. 9, and the positive electrode active material particle of Comparative Example 5 was No. 10.(Comparative Examples 6 to 8)The positive electrodes and the test batteries for evaluation purposes of Comparative Examples 6 to 8 were manufactured under the same conditions as in Example 1, except that 0.4 mass % of CNTs were added to the positive electrode of Comparative Example 3, 1.0 mass % of CNTs were added to the positive electrode of Comparative Example 3, and 4.0 mass % of CNTs were added to the positive electrode of Comparative Example 3.<Analysis><<>>ICP-AES was performed to check the composition of the inner portion and the surface portion of the positive electrode active material particles of Nos. 1 to 10. As the composition of the inner portion of the positive electrode active material particle, the composition at the center of gravity of the positive electrode active material particle was checked, and as the composition of the surface portion of the positive electrode active material particle, the composition of the outermost surface portion of the positive electrode active material particle was checked. The composition of each positive electrode active material particle is shown in FIG. 6.<<magnesium concentration>>TEM-EDX was performed as described above to check the Mg concentration (mol %) in the inner portion and the surface portion of the positive electrode active material particles of Nos. 1 to 10. As the Mg concentration in the inner portion of the positive electrode active material particle, the Mg concentration in the center of gravity of the positive electrode active material particle was checked, and as the Mg concentration in the surface portion of the positive electrode active material particle, the Mg concentration in the outermost surface portion of the positive electrode active material particle was checked. The Mg concentration in the inner portion and the surface portion of each positive electrode active material particle is shown in FIG. 6. In FIG. 6, the difference between the Mg concentration in the inner portion of the positive electrode active material particle and the Mg concentration in the surface portion of the positive electrode active material particle, (concentration in the inner portion) (concentration in the surface portion), is expressed as "Mg concentration difference".<>(Initial Resistance)Each test battery was charged to 3.6 V and then discharged in an environment at room temperature for 10 seconds at a current value of 10 C. The voltage at a time of 10 seconds after the start of discharge was measured, and from the result, the IV resistance (electric resistance) was calculated to determine the initial resistance. The results are shown in Fig. 6. The results are expressed as relative values with respect to the initial resistance of the test battery of Comparative Example 1 which was set at 100. When the initial resistance was 150 or less, it was judged to be good.(Capacity Retention)After the measurement of the initial resistance, 100 cycles of charging and discharging were performed in an environment at room temperature with a constant current of 1C at a voltage in the range of 3.0 to 4.2 V. After the first charging and discharging operation, charging and discharging operations were performed with an electric current of 0.1 C and a voltage in the range of 3.0 to 4.2 V, and the capacitance discharged at this time was defined as the first discharged capacitance. After 100. Charging and discharging operations were performed with an electric current of 0.1 C and a voltage in the range of 3.0 to 4.2 V, and the capacitance discharged at this time was defined as 100. The 100 discharged capacity was divided by the initial discharged capacity, thus determining the capacity retention (in percent). The results are shown in Fig. 6. A capacity retention of 75% or more was evaluated as good.<>(Initial Resistance)Referring to FIG. 6, in the examples in which the Mg concentration in the inner portion of the positive electrode active material particle was higher than the Mg concentration in the surface portion of the positive electrode active material particle, the value of the initial resistance was good.In addition, when Example 1 was compared with the positive electrode active material particle of No. 2 produced by the doping method and Example 6 was compared with the positive electrode active material particle of No. 5 produced by the MHP method and Comparative Example 3 containing the positive electrode active material particle of No. 8 produced by the late addition method, the initial resistance was most inhibited in Example 6 produced by the MHP method, and the initial resistance was highest in Comparative Example 3 produced by the late addition method.(Capacity Preservation)Referring to FIG. 6, in the examples in which the Mg concentration in the inner portion of the positive electrode active material particle was higher than the Mg concentration in the surface portion of the positive electrode active material particle, good capacity retention was maintained even after repeated charging and discharging.In addition, when Example 1 was prepared with the positive electrode active material particle of No. 2 prepared by the doping method, Example 6 was prepared with the positive electrode active material particle of No. 5 prepared by the MHP method, and Comparative Example 3 was prepared with the positive electrode active material particle of No. 8 prepared by the late addition method, the capacity retention was highest in Example 6 prepared by the MHP method, and the capacity retention was lowest in Comparative Example 3 prepared by the late addition method.When Examples 9 to 11 containing the positive electrode active material particle of No. 5 prepared by the MHP method and CNTs were compared with Example 6 containing no CNTs, improvement in capacity retention was observed as the proportion of the CNTs was increased.Moreover, in Examples 6 to 11 including the positive electrode active material particles produced by the MHP method, the Mg concentration in the inner portion of the positive electrode active material particles is greatly different from the concentration in the surface region, as compared with Examples 1 to 5 including the positive electrode active material particles produced by the doping method.Although the embodiments of the present disclosure have been described, the embodiments disclosed herein are illustrative in all respects and are not restrictive. The scope of the present disclosure is defined by the terms of the claims, and is intended to include all modifications within the meaning and scope that correspond to the terms of the claims.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2023 - 127 491

[0001] JP 2018-88 383 A

[0003]

Claims

A positive electrode active material particle (5), wherein the positive electrode active material particle (5) has a lamellar rock salt type crystal structure, the positive electrode active material particle (5) comprises a magnesium-containing lithium composite oxide, and the magnesium concentration in an inner portion of the positive electrode active material particle (5) is higher than the magnesium concentration in a surface portion of the positive electrode active material particle (5).The positive electrode active material particle (5) according to claim 1, wherein the magnesium concentration in the inner portion is 0.5 mol% to 8.0 mol%, and the magnesium concentration in the surface portion is 5.0 mol% or less.The positive electrode active material particle (5) according to claim 1 or 2, wherein a difference between the magnesium concentration in the inner portion and the magnesium concentration in the surface portion is 1.0 mol% or more.A positive electrode (10) comprising the positive electrode active material particle (5) according to any one of claims 1 to 3.The positive electrode (10) of claim 4, further comprising carbon nanotubes.A lithium ion battery (100) comprising the positive electrode (10) according to claim 4 or 5.

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