Determination of Valence State of Transition metal ions in Li(Fe, M)PO4 (M $=$ Cr, Ni, Ti, Cd, and V) Cathode Materials.

A clear determination of the valence states of transition metal ions in cathode materials of lithium-ion batteries is important for understanding the improvement of electrochemical performance of cation doped LiFePO4 cathode materials. In this work, the x-ray absorption spectroscopy (XAS) determination of valence states of transition metal ions Cr, Ni, Ti, Fe, Cd, and V in Li(Fe, M)PO4 (with M $=$ Cr, Ni, Ti, Cd, and V) cathode materials is presented. Our XAS results clearly indicate that in these transition metal doped cathode materials, the valence values of Cr, Ni, Ti, Fe and Cd are $+$3, $+$2, $+$4, $+$2, and $+$3, respectively. The valence of V in V-doped LiFePO4, however, is between $+$3 and $+$4. The improvement of the electrochemical performance (or the increase of specific discharge capacity) due to the transition metal M for Fe doping for some transition metals M is discussed in association with the XAS results.