Solid Solution Domains at Phase Transition Front of Li_xNi_0.5Mn_1.5O₄

Nickel-substituted manganese spinel LiNi_0.5Mn_1.5O₄ (LNMO) is a promising 5 V class positive electrode material for lithium-ion batteries. As micron-sized LNMO particles show high rate capability in its two-phase coexistence regions, the phase transition mechanism is of great interest in understanding the electrode behavior at high rates. Here, the phase transition dynamics of Li_xNi_0.5Mn_1.5O₄ is elucidated on high rate charging-discharging using operando time-resolved X-ray diffraction (TR-XRD). The TR-XRD results indicate the existence of intermediate states, in addition to the thermodynamically stable phases, and it is shown that the origin of such intermediate states is ascribed to the solid-solution domains at the phase transition front, as supported by the analysis using transmission electron microscopy coupled with electron energy-loss spectroscopy. The phase transition pathways dependent on the reaction rate are shown, together with possible explanation for this unique transition behavior.