Electrochemically Induced Phase Transformation in Nanoscale Olivines Li1-xMPO4 (M = Fe, Mn)

The phase stability and phase transformation kinetics of Li1-xMPO4 olivines are critical to their performance as lithium storage electrodes. In this work, nanoscale (< 100 nm primary particle size) Li1-xFePO4 and Li1-xMnPO4 are chosen as model systems for comparison with a coarser-rained LiFePO4 that exhibits a conventional two-phase reaction. The nanoscale materials first exhibit time and state-of-charge dependences of the electrochemical potential and structural parameters which show that stable two-phase coexistence is not reached. The evolution of structural parameters supports the existence of a coherency stress influenced crystal-crystal transformation. However, an additional response, the preferential formation of amorphous phase at nanosize scale, is identified. In Li1-xFePO4, at 34 nm average particle size, at least one amorphous phase of varying Li content coexists with the crystalline phases. In Li1-xMnPO4 of 78 nm particle size, the electrochemically formed delithiated phase is highly disordered. These phenomena are interpreted front the effect Of Surface and bulk energetics on phase stability of a nanoscale material.