In Situ XAFS Analysis of Li(Mn, M)2O4 (M=Cr, Co, Ni) 5V Cathode Materials for Lithium-Ion Secondary Batteries

Chemical states and structural changes accompanying the electrochemical Li deintercalation of Li1−x(Mn, M)2O4 (M=Cr, Co, Ni) were studied by the in situ X-ray absorption fine structure (XAFS) technique. The X-ray absorption near-edge structures (XANES) of Mn and M as a function of x showed that the high voltage (∼5 V) in the cathode materials of an Li secondary battery is due to the oxidation of M3+ to M4+ (M=Cr, Co) and M2+ to M4+ (in the case of M=Ni), while the origin of the low voltage (3.9–4.3 V) can be ascribed to the oxidation of Mn3+ to Mn4+. The extended X-ray absorption fine structure (EXAFS) analysis of Li1−x(Mn,Ni)2O4 revealed that Ni2+ is oxidized to Ni4+ via the Ni3+ state with a Jahn–Teller distorted Ni3+–O octahedron.