Size-Dependent Staging and Phase Transition in LiFePO4/FePO4

The LiFePO4/FePO4 phase transition process is remarkable in terms of its excellent reversibility, making this redox system extremely promising for high rate lithium storage. The recent observation of ordering effects (Li0.5FePO4) during the phase transition challenges the traditional two phase models. In this work, the phenomenon of staging for LiFePO4 for different sizes (70 nm and 50 nm) by high resolution aberration corrected annular bright electron microscopy is detected, investigated, and discussed along with previous results on larger crystals. In the small crystals, staging is found throughout with a decrease of order from center to the surface. For the larger crystal, a staging phase occurs constituting the interfacial zone (width around 15 nm) between LiFePO4 and FePO4. A comparison is made to recent experiments on even larger crystals showing such an interphase of smaller extent (around 2 nm). Thus it appears that these zones narrow with increasing size. These findings are discussed in the light of phase transition thermodynamics and kinetics. In particular, the possibility is discussed that the staging interphase may constitute a low energy solution to the LiFePO4/FePO4 contact.