Comparative kinetic study of olivine LixMPO4 (M = Fe, Mn)

A huge kinetic difference in olivine LixMPO4 (M = Fe, Mn) is demonstrated in a quantitative manner. Galvanostatic discharge profiles and the current relaxation to the stepwise anodic overvoltage (chronoamperometry) are comparatively measured for the LixFePO4 and LixMnPO4 under identical extrinsic conditions, which are carefully controlled and confirmed using Rietveld refinement for the X-ray diffraction profiles, direct texture observation by scanning electron microscope, Brunauer-Emmett-Teller surface area measurements, and tap density measurements. The current durability for LixMnPO4 is orders-of-magnitude inferior to that of LixFePO4, the origin of which is clearly attributed to their intrinsic crystallographic and transport property differences. Heavy polaronic holes localized on the Mn3+ sites are suggested as an important rate-limiting factor. In spite of the higher open-circuit voltage of Mn3+/ Mn2+ (4.05 V) compared to that of Fe3+/Fe2+ (3.45 V) in the olivine framework, the abnormally large polarization may eliminate pure LiMnPO4 as a practical lithium battery cathode due to much lower effective energy density than LiFePO4. (C) 2004 The Electrochemical Society.