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The phase stability and phase transformation kinetics of Li 1− x MPO 4 olivines are critical to their performance as lithium storage electrodes. In this work, nanoscale (<100 nm primary particle size) Li 1− x FePO 4 and Li 1− x MnPO 4 are chosen as model systems for comparison with a coarser-grained LiFePO 4 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 Li 1− x FePO 4, at 34 nm average particle size, at least one amorphous phase of varying Li content coexists with the crystalline phases. In Li 1− x MnPO 4 of 78 nm particle size, the electrochemically formed delithiated phase is highly disordered. These phenomena are interpreted from the effect of surface and bulk energetics on phase stability of a nanoscale material.
Meethong et al. (Tue,) studied this question.