Lithium ion secondary battery

1. A lithium ion secondary battery, comprising:

a lithium nickel composite oxide represented by the following formula, and carbon nanotubes in a positive electrode mixture layer in a positive electrode,

wherein a ratio (a)/(b) of an average length (a) of the carbon nanotubes to an average particle size (b) of primary particles of the lithium nickel composite oxide is 0.5 or more, and an average length of the carbon nanotubes is 100 nm or more and 840 nm or less,

an average diameter of outermost cylinders of the carbon nanotubes is 40 nm or less,

an average split width of gaps between the primary particles is 50 nm or more and 700 nm or less,

an average particle size of secondary particles of the lithium nickel composite oxide is 1 μm or more and 16 μm or less, and

the carbon nanotubes are disposed within the gaps between the primary particles of the lithium nickel composite oxide;

Li y Ni (1-x) M x O 2

wherein 0≤x≤0.4, 0<y≤1.2, and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, and B.

2. The lithium ion secondary battery according to claim 1 , wherein an average particle size of the primary particles is 50 nm or more and 700 nm or less.

3. The lithium ion secondary battery according to claim 1 , wherein a density of a positive electrode mixture layer is 3.25 g/cm 3 or more.

4. The lithium ion secondary battery according to claim 1 , wherein an average split width of the primary particles before charging-discharging the lithium ion secondary battery is 50 nm or more and 700 nm or less.

5. A vehicle equipped with the lithium ion secondary battery according to claim 1 .

6. A method for manufacturing a lithium ion secondary battery, comprising the steps of:

preparing a slurry comprising a positive electrode active material comprising a lithium nickel composite oxide represented by the following formula, a conductive material comprising carbon nanotubes, a binder and a solvent, and applying the slurry to a positive electrode current collector to form a positive electrode mixture layer,

obtaining a positive electrode by pressing the positive electrode mixture layer,

fabricating an electrode element by stacking the positive electrode and a negative electrode via a separator, and

encapsulating the electrode element and an electrolyte solution into an outer package,

wherein a ratio (a)/(b) of an average length (a) of the carbon nanotubes to an average particle size (b) of primary particles of the lithium nickel composite oxide is 0.5 or more, and an average length of the carbon nanotubes is 100 nm or more and 840 nm or less,

an average diameter of outermost cylinders of the carbon nanotubes is 40 nm or less,

an average split width of gaps between the primary particles is 50 nm or more and 700 nm or less,

an average particle size of secondary particles of the lithium nickel composite oxide is 1 μm or more and 16 μm or less, and

the carbon nanotubes are disposed within the gaps between the primary particles of the lithium nickel composite oxide;

Li y Ni (1-x) M x O 2

wherein 0≤x≤0.4, 0<y≤1.2, and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, and B.

7. The lithium ion secondary battery according to claim 1 , wherein the carbon nanotubes form a conductive path in the gaps between the primary particles of the lithium nickel composite oxide.