Quantitative Elucidation of the Non-Equilibrium Phase Transition in LiFePO₄ via the Intermediate Phase

Phase-transition route according to compositional change strongly affects the reaction kinetics within materials in energy devices such as lithium-ion batteries. The promising electrode material of LiFePO4 exhibits a high rate performance due to the crucial but controversial act of the metastable intermediate phase in addition to the end members of the two phases at room temperature. Here, we investigated the electrochemical and crystal structural behavior of the intermediate phase in LixFePO4 to be thermodynamically stable at elevated temperature. The current-induced intermediate phase was detected by electrochemical measurements as well as operando X-ray diffraction using a molten salt electrolyte at 230 °C and shows hysteretic charge/discharge characteristics. The nucleation of the intermediate phase occurs at its composition of x = 0.64–0.65 in the independent reaction direction. Both the composition of the intermediate phase and the total composition of LixFePO4 are equal on the charge and not on the discharge. This discrepancy produces the unexpected results that the sequential phase transition via the intermediate phase as a single phase proceeds on the charge, but the three phases coexist in a whole reaction on the discharge. The charge process is a kinetically favorable direction for a current-induced phase transition being responsible for the intermediate phase. This phase-transition mechanism could be deduced to the actual environment. The formation of the intermediate phase is important, as its further stabilization leads to an extension of a single-phase reaction, realizing high-rate electrode materials.