A method for making a sulfur-graphene composite material is provided. In the method, an elemental sulfur solution and a graphene dispersion are provided. The elemental sulfur solution includes a first solvent and an elemental sulfur dissolved in the first solvent. The graphene dispersion includes a second solvent and graphene sheets dispersed in the second solvent. The elemental sulfur solution is added to the graphene dispersion, a number of elemental sulfur particles are precipitated and attracted to a surface of the graphene sheets to form the sulfur-graphene composite material. The sulfur-graphene composite material is separated from the mixture.

A positive electrode active material comprises a first active material and a second active material having an average particle size (D50) smaller than an average particle size (D50) of the first active material. The first active material is a lithium-(transition metal) composite oxide containing Ni at 75 mol % or more and Ti at 0.5 to 2.8 mol % relative to a total number of moles of metallic element except Li. The second active material is a lithium-(transition metal) composite oxide containing Ni at 75 mol % or more relative to a total number of moles of metallic element except Li. A content of Ti in the second active material is 0.1 mol % or less relative to the total number of moles of metallic element except Li. Ni disorder of the first active material is from 2.1 to 2.6%, and Ni disorder of the second active material is 2.0% or less.