| CPC H01M 4/525 (2013.01) [C01G 45/1228 (2013.01); C01G 53/50 (2013.01); H01B 1/08 (2013.01); H01M 4/131 (2013.01); H01M 4/366 (2013.01); H01M 4/505 (2013.01); H01M 10/052 (2013.01); C01P 2002/01 (2013.01); C01P 2004/04 (2013.01); C01P 2006/40 (2013.01); H01M 2004/021 (2013.01); H01M 2004/028 (2013.01)] | 10 Claims |

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1. A positive electrode active material, comprising:
an overlithiated lithium manganese-based oxide comprising at least lithium, nickel and manganese,
in which a phase belonging to a C2/m space group and a phase belonging to an R3-m space group coexist in the lithium manganese-based oxide,
wherein, in the lithium manganese-based oxide, there are regions in which a proportion of the phase belonging to the C2/m space group and the phase belonging to the R3-m space group are different,
wherein a concentration of a metal element in the lithium manganese-based oxide satisfies Expression 2 below:
0.40≤M2′/M1′≤0.70 [Expression 2]
wherein,
M1′ is the number of moles of all metal elements (excluding lithium) in the phase belonging to the R3-m space group, and
M2′ is the number of moles of nickel based on all metal elements (excluding lithium) in the phase belonging to the R3-m space group.
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