Cathode active material used for lithium ion secondary battery and lithium ion secondary battery using same
Provided are a cathode active material used for a lithium ion secondary battery capable of sufficiently realizing both high charge/discharge capacities and excellent cycle properties, and a lithium ion secondary battery using the cathode active material. The cathode active material contains a plurality of secondary particles formed via agglomeration of a plurality of primary particles of a lithium transition metal composite oxide. Spreading resistance distributions of the secondary particles respectively observed in cross-sections at optional three positions of the cathode active material are measured so as to afford average values of spreading resistance of the secondary particles in the respective cross-sections. The average values of spreading resistance of the secondary particles are further averaged. The resultant averaged value of spreading resistance is made to enter the range of 1.0×10 6 Ω/cm or more and 1.0×10 10 Ω/cm or less.
1. A cathode active material used for a lithium ion secondary battery, comprising a plurality of secondary particles formed via agglomeration of a plurality of primary particles of a lithium transition metal composite oxide, wherein
spreading resistance distributions of the secondary particles observed in cross-sections at three selected positions of the cathode active material are measured, and average values of spreading resistance of the secondary particles observed in the cross sections are calculated;
the calculated three average values of spreading resistance are further averaged; and
when an averaged value thus obtained is defined as a value of spreading resistance of the cathode active material, the averaged value thus defined is 1.0×10 6 Ω/cm or more and 1.0×10 10 Ω/cm or less.
2. The cathode active material used for a lithium ion secondary battery according to claim 1 , wherein the secondary particles comprise solid secondary particles.
3. The cathode active material used for a lithium ion secondary battery according to claim 1 , wherein porosity measured by mercury intrusion porosimetry is 1% or more and 20% or less.
4. The cathode active material used for a lithium ion secondary battery according to claim 1 , wherein the averaged value of spreading resistance is 0.90×10 9 Q/cm or less.
5. The cathode active material used for a lithium ion secondary battery according to claim 1 , wherein porosity measured by mercury intrusion porosimetry is 3% or more and 15% or less.
6. The cathode active material used for a lithium ion secondary battery according to claim 1 , wherein the lithium transition metal composite oxide is represented by the following Formula (1):
Li 1+a Ni b Co c Mn d M 1 e O 2+α Formula (1)
where is a metal element other than Li, Ni, Mn and Co; “a”, “b”, “c”, “d”, “e” and “α” are respectively numerals satisfying the following conditions: −0.03≤a≤0.11, 0.7≤b≤1.0, 0<c<0.3, 0≤d<0.3, 0≤e<0.1, b+c+d+e=1, −0.2<α<0.2.
7. A lithium ion secondary battery, comprising a cathode active material according to claim 1 .
8. The lithium ion secondary battery according to claim 7 , comprising a cathode active material included in a cathode of a lithium ion secondary battery subjected to 100 charge/discharge cycles, wherein
a charge/discharge cycle is carried out via charging the secondary battery to an upper limit potential of 4.3 V, and subsequently discharging the secondary battery down to a lower limit potential of 2.7 V under 25° C. environment with charge/discharge rate of 1 C.