Direct observation of a metastable crystal phase of Li_xFePO ₄ under electrochemical phase transition

The phase transition between LiFePO₄ and FePO₄ during nonequilibrium battery operation was tracked in real time using time-resolved X-ray diffraction. In conjunction with increasing current density, a metastable crystal phase appears in addition to the thermodynamically stable LiFePO ₄ and FePO₄ phases. The metastable phase gradually diminishes under open-circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase and posit the new phase's role in decreasing the nucleation energy, accounting for the excellent rate capability of LiFePO₄. This study is the first to report the measurement of a metastable crystal phase during the electrochemical phase transition of Li_xFePO₄.