This paper reports on the electrochemistry of a new series of layered manganese oxide cathodes for lithium-ion cells. The layered structure is stabilized by the partial substitution of the manganese by chromium and lithium atoms and by the partial or complete oxidation of the manganese to the +4 oxidation state. The series covers the range of compositions Li x Cr y Mn2−y O4+z for which 2.2<x<4, 0<y<2 and z≥0.