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 LiFePO4 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 LixFePO4.

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Transient states of phase transition in LiFePO 4 /FePO 4 for lithium ion battery positive electrodes are investigated by timeresolved measurements. To directly detect changes in electronic and crystal structures under battery operation, in situ time-resolved X-ray absorption and diffraction measurements are performed, respectively. The phase fraction change estimated by the iron valence change is similar to the electrochemically expected change. The transient change of lattice constant during two phase reaction is clearly observed by the time-resolved X-ray diffraction measurement. The nonequilibrium lithium extraction behavior deviates from the thermodynamic diagram of the two phase system, resulting in continuous phase transition during electrochemical reactions.

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