A positive electrode active material having a crystal structure that is unlikely to be broken by repeated charging and discharging is provided. A positive electrode active material with high charge and discharge capacity is provided. A positive electrode active material including lithium, cobalt, nickel, magnesium, and oxygen, in which the a-axis lattice constant of an outermost surface layer of the positive electrode active material is larger than the a-axis lattice constant of an inner portion and in which the c-axis lattice constant of the outermost surface layer is larger than the c-axis lattice constant of the inner portion. A rate of change between the a-axis lattice constant of the outermost surface layer and the a-axis lattice constant of the inner portion is preferably larger than 0 and less than or equal to 0.12, and a rate of change between the c-axis lattice constant of the outermost surface layer and the c-axis lattice constant of the inner portion is preferably larger than 0 and less than or equal to 0.18.

A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and an electrolyte solution, wherein the positive electrode includes a composite oxide including lithium as a first metal, and a second metal X other than lithium; in the composite oxide, the second metal X includes Ni, Al, and Mn; the second metal X does not contain Co or the atomic ratio Co/X of Co to the second metal X is 0.02 or less; and the electrolyte solution contains a chain carboxylic acid ester having 2 to 4 carbon atoms, and a cyclic compound having a ring structure composed of 2 oxygen atoms and 3 to 5 carbon atoms.

Provided is a lithium secondary battery comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and a first functional layer between the positive electrode and the negative electrode, wherein the first functional layer includes plate-like polyolefin particles having an average diameter of 1 μm to 8 μm, and the positive electrode includes a positive electrode active material layer including a positive electrode active material and a flame retardant, or has a stacked structure including a positive electrode active material layer and a second functional layer including a flame retardant.

Provided is a lithium secondary battery comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and a first functional layer between the positive electrode and the negative electrode, wherein the first functional layer includes plate-like polyolefin particles having an average diameter of 1 μm to 8 μm, and the positive electrode includes a positive electrode active material layer including a positive electrode active material and a flame retardant, or has a stacked structure including a positive electrode active material layer and a second functional layer including a flame retardant.

This positive electrode active substance for a non-aqueous electrolyte secondary battery contains a lithium-transition metal composite oxide which has a crystal structure belonging to the space group Fm-3m and is represented by the compositional formula Li.sub.xMn.sub.yM.sub.aO.sub.bF.sub.c (in the formula: M is at least one type of metal element excluding Mn; x+y+a=b+c=2; 1<x≤1.35; 0.4≤y≤0.9; 0≤a≤0.2; and 1.3≤b≤1.8). In addition, the lattice constant a of the lithium-transition metal composite oxide is 4.09-4.16.

This positive electrode active substance for a non-aqueous electrolyte secondary battery contains a lithium-transition metal composite oxide which has a crystal structure belonging to the space group Fm-3m and is represented by the compositional formula Li.sub.xMn.sub.yM.sub.aO.sub.bF.sub.c (in the formula: M is at least one type of metal element excluding Mn; x+y+a=b+c=2; 1<x≤1.35; 0.4≤y≤0.9; 0≤a≤0.2; and 1.3≤b≤1.8). In addition, the lattice constant a of the lithium-transition metal composite oxide is 4.09-4.16.

Provided are a lithium plating detection method, a battery management method and apparatus for safety diagnosis using the lithium plating detection method. The lithium plating detection method according to the present disclosure includes detecting, accumulating and tracking each of an open circuit voltage (OCV) after fully charging and an OCV after fully discharging of a battery every charge/discharge cycle of the battery, and determining if lithium plating took place in the battery using a result of tracking the OCV after fully charging and the OCV after fully discharging, wherein a range satisfying a condition of the reduced OCV after fully charging and the reduced OCV after fully discharging in the tracking result is determined as a range in which lithium plating starts to take place.

Provided are a lithium plating detection method, a battery management method and apparatus for safety diagnosis using the lithium plating detection method. The lithium plating detection method according to the present disclosure includes detecting, accumulating and tracking each of an open circuit voltage (OCV) after fully charging and an OCV after fully discharging of a battery every charge/discharge cycle of the battery, and determining if lithium plating took place in the battery using a result of tracking the OCV after fully charging and the OCV after fully discharging, wherein a range satisfying a condition of the reduced OCV after fully charging and the reduced OCV after fully discharging in the tracking result is determined as a range in which lithium plating starts to take place.

A positive electrode for a secondary battery including a positive electrode current collector, and a positive electrode mixture layer containing a positive electrode active material and disposed on a surface of the positive electrode current collector. The positive electrode mixture layer contains a first positive electrode active material having a compressive strength of 400 MPa or more, and a second positive electrode active material having a compressive strength of 250 MPa or less. When the positive electrode mixture layer is divided into a first region and a second region having the same thickness, the first positive electrode active material is contained more in the first region than in the second region, and the second positive electrode active material is contained more in the second region than in the first region.

A positive electrode for a secondary battery including a positive electrode current collector, and a positive electrode mixture layer containing a positive electrode active material and disposed on a surface of the positive electrode current collector. The positive electrode mixture layer contains a first positive electrode active material having a compressive strength of 400 MPa or more, and a second positive electrode active material having a compressive strength of 250 MPa or less. When the positive electrode mixture layer is divided into a first region and a second region having the same thickness, the first positive electrode active material is contained more in the first region than in the second region, and the second positive electrode active material is contained more in the second region than in the first region.