The present invention relates to a mixed oxide containing a) a mixed-substituted lithium manganese spinel in which some of the manganese lattice sites are occupied by lithium ions and b) a boron-oxygen compound.

Furthermore, the present invention relates to a process for its preparation and the use of the mixed oxide as electrode material for lithium ion batteries.

The present invention provides a positive electrode active material for nonaqueous electrolyte secondary battery including a lithium transition metal composite oxide represented by the following formula:
Li.sub.aNi.sub.xMn.sub.2-xFe.sub.yB.sub.zO.sub.4
wherein 1.00a1.30, 0.30x0.60, 0.003y0.200, and 0.003z0.200.

A mixed oxide containing a) a mixed-substituted lithium manganese spinel in which some of the manganese lattice sites are occupied by lithium ions and b) a boron-oxygen compound. Furthermore, a process for its preparation and the use of the mixed oxide as electrode material for lithium ion batteries.