Slow-Moving Phase Boundary in Li_{4/3+ x} Ti_5/3 O₄

Lithium titanate is one of the most promising anode materials for high-power demands but such applications desire a complete understanding of the kinetics of lithium transport. The poor diffusivity of lithium in the completely lithiated and delithiated (pseudo spinel) phases challenges to explain the high-rate performance. This study aims at clearing the kinetics of lithium transport using an innovative technique that employs optical microscopy in a constrained region of sputter-deposited thin-film samples. It enables the in situ observation of the transport of lithium through the electrode. Furthermore, with a thermostatically controlled cell, the Arrhenius-like temperature dependence is revealed. The quantitative findings demonstrate that indeed the end phases have poor diffusivity which is, however, accelerated at intermediate Li concentrations in the spinel structured Li_{4/3+ δ} Ti_5/3 O₄ phase. Surprisingly, the slow migration of the phase boundary hinders the formation of the Li-rich (rock-salt) phase in the initial stages. Such kinetic control by the phase boundary stands in obvious contrast to a prior (theoretical) study postulating almost "liquid" behavior of the interface. Only after the Li diffusion into the Li-poor (spinel) phase has faded, when approaching the solubility limit, the further growth of the rock-salt phase becomes diffusion controlled.