Provided is a lithium ion secondary battery including Li.sub.4Ti.sub.5O.sub.12 particles in a negative electrode active material layer and having both high heat generation suppressing performance during overcharging, and high storage stability in a high SOC region. The lithium ion secondary battery herein disclosed includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode has a positive electrode active material layer. The positive electrode active material layer includes Li.sub.3PO.sub.4 as a secondary material. The negative electrode has a negative electrode active material layer. The negative electrode active material layer includes Li.sub.4Ti.sub.5O.sub.12 as a secondary material. The Li.sub.3PO.sub.4 content in the positive electrode active material layer is 0.5 mass % or more and 5.0 mass % or less. The Li.sub.4Ti.sub.5O.sub.12 content in the negative electrode active material layer is 0.5 mass % or more and 5.0 mass % or less.

A lithium secondary battery includes a cathode including a cathode current collector and a cathode active material layer disposed on at least one surface of the cathode current collector, the cathode active material layer including a cathode active material including first cathode active material particles, each of which has a single particle shape; an anode facing the cathode; and a non-aqueous electrolyte including a non-aqueous organic solvent that contains a fluorine-based organic solvent, a lithium salt and an additive.

A lithium secondary battery includes a cathode including a cathode current collector and a cathode active material layer disposed on at least one surface of the cathode current collector, the cathode active material layer including a cathode active material including first cathode active material particles, each of which has a single particle shape; an anode facing the cathode; and a non-aqueous electrolyte including a non-aqueous organic solvent that contains a fluorine-based organic solvent, a lithium salt and an additive.

A positive electrode material powder including a lithium nickel-based oxide represented by Chemical Formula 1 (Li.sub.aNi.sub.bCo.sub.cM.sup.1.sub.dM.sup.2.sub.eO.sub.2) and having a degree of single-particle formation, represented by the following Equation (1), of 0.3 to 0.8:

[00001]$\frac{\underset{i=1}{\overset{n}{.Math.}}\frac{4\pi }{3}{R}_i^3}{n}\times \frac{1}{D_{50}}.$

In Equation (1), R.sub.i is a radius of the i.sup.th grain as measured by subjecting an electrode manufactured using the positive electrode material powder to ion milling and then analyzing the cross section of the electrode by electron backscatter diffraction (EBSD), n is the total number of grains as measured by the EBSD analysis and ranges from 350 to 450, and D.sub.50 is a volume-cumulative average particle diameter of the positive electrode material powder as measured using a laser diffraction particle size analyzer.

A positive electrode material powder including a lithium nickel-based oxide represented by Chemical Formula 1 (Li.sub.aNi.sub.bCo.sub.cM.sup.1.sub.dM.sup.2.sub.eO.sub.2) and having a degree of single-particle formation, represented by the following Equation (1), of 0.3 to 0.8:

[00001]$\frac{\underset{i=1}{\overset{n}{.Math.}}\frac{4\pi }{3}{R}_i^3}{n}\times \frac{1}{D_{50}}.$

In Equation (1), R.sub.i is a radius of the i.sup.th grain as measured by subjecting an electrode manufactured using the positive electrode material powder to ion milling and then analyzing the cross section of the electrode by electron backscatter diffraction (EBSD), n is the total number of grains as measured by the EBSD analysis and ranges from 350 to 450, and D.sub.50 is a volume-cumulative average particle diameter of the positive electrode material powder as measured using a laser diffraction particle size analyzer.

The positive electrode material for lithium ion secondary batteries includes a mixture including a positive electrode active material in which a length of a longest side of a primary particle is 1 nm or more and 1000 nm or less and a NASICON-type compound in which a length of a longest side of a primary particle is 1 nm or more and 1000 nm or less.

The positive electrode material for lithium ion secondary batteries includes a mixture including a positive electrode active material in which a length of a longest side of a primary particle is 1 nm or more and 1000 nm or less and a NASICON-type compound in which a length of a longest side of a primary particle is 1 nm or more and 1000 nm or less.

A cathode including: a cathode current collector; and a cathode active material layer disposed on the cathode current collector and including a first surface, and a second surface opposite the first surface and adjacent to the cathode current collector, wherein the cathode active material layer includes a cathode active material including a dopant, and wherein a concentration gradient of the dopant decreases in a direction from the first surface to the second surface.

A cathode including: a cathode current collector; and a cathode active material layer disposed on the cathode current collector and including a first surface, and a second surface opposite the first surface and adjacent to the cathode current collector, wherein the cathode active material layer includes a cathode active material including a dopant, and wherein a concentration gradient of the dopant decreases in a direction from the first surface to the second surface.

Positive electrode active material particles that inhibit a decrease in capacity due to charge and discharge cycles are provided. A high-capacity secondary battery, a secondary battery with excellent charge and discharge characteristics, or a highly-safe or highly-reliable secondary battery is provided. A novel material, active material particles, and a storage device are provided. The positive electrode active material particle includes a first region and a second region in contact with the outside of the first region. The first region contains lithium, oxygen, and an element M that is one or more elements selected from cobalt, manganese, and nickel. The second region contains the element M, oxygen, magnesium, and fluorine. The atomic ratio of lithium to the element M (Li/M) measured by X-ray photoelectron spectroscopy is 0.5 or more and 0.85 or less. The atomic ratio of magnesium to the element M (Mg/M) is 0.2 or more and 0.5 or less.