Phase transition kinetics of LiNi_0.5Mn_1.5O₄ analyzed by temperature-controlled operando X-ray absorption spectroscopy

LiNi_0.5Mn_1.5O₄ (LNMO) is a promising positive electrode material for lithium ion batteries because it shows a high potential of 4.7 V vs. Li/Li⁺. Its charge-discharge reaction includes two consecutive phase transitions between LiNi_0.5Mn_1.5O₄ (Li1) 虠 Li_0.5Ni_0.5Mn_1.5O₄ (Li0.5) and Li0.5 虠 Ni_0.5Mn_1.5O₄ (Li0) and the complex transition kinetics that governs the rate capability of LNMO can hardly be analyzed by simple electrochemical techniques. Herein, we apply temperature-controlled operando X-ray absorption spectroscopy to directly capture the reacting phases from -20 °C to 40 °C under potential step (chronoamperometric) conditions and evaluate the phase transition kinetics using the apparent first-order rate constants at various temperatures. The constant for the Li1 虠 Li0.5 transition (process 1) is larger than that for the Li0.5 虠 Li0 transition (process 2) at all the measured temperatures, and the corresponding activation energies are 29 and 46 kJ mol^-1 for processes 1 and 2, respectively. The results obtained are discussed to elucidate the limiting factor in this system as well as in other electrode systems.