LITHIUM MANGANESE COMPOSITE OXIDE, SECONDARY BATTERY, ELECTRONIC DEVICE, AND METHOD FOR FORMING LAYER

To increase the volume density or weight density of lithium ions that can be received and released in and from a positive electrode active material to achieve high capacity and high energy density of a secondary battery. A lithium manganese composite oxide represented by LiMnMOthat includes a region belonging to a space group C2/c and is covered with a carbon-containing layer is used as the positive electrode active material. The element M is an element other than lithium and manganese. The lithium manganese composite oxide has high structural stability and high capacity.