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Manganese-rich and cobalt-free compounds of LixNi0.25Mn0.75Oy(0.5 ≤ x≤ 2, 2 ≤ y≤ 2.75) were investigated as the positive electrode materials for high energy lithium-ion batteries. Compounds with x= 0.5, 1, 1.25, 1.5, and 2 were prepared by a solid-state reaction from the same carbonate precursor, Ni0.25Mn0.75CO3, with an appropriate amount of Li2CO3. The structural and physical characteristics of these phases were determined by X-ray diffraction and scanning electron microscopy. With an increase of the lithium content, the LixNi0.25Mn0.75Oyevolved from a spinel (Fd3m) structure (x= 0.5) to a mixed spinel-layered (Fd3m and C2/c) structure (x= 1 and 1.25), to a more layered (R3m and C2/c) structure (x= 1.5 and 2). A similar structural trend was found for samples prepared from NiMn2O4–Mn2O3mixed oxide, itself prepared by thermal decomposition of Ni0.25Mn0.75CO3carbonate precursor, to which appropriate amounts of Li2CO3were added. An increase of the lithium content also affected the size of the primary particles and the roughness of the secondary particles, without any substantial change of their spherical morphology and packing densities. Further results showed that the electrochemical performance and safety characteristics of the LixNi0.25Mn0.75Oymaterials were primarily governed by their structures. [ABSTRACT FROM AUTHOR]