Substantial Oxygen Loss and Chemical Expansion in Lithium-Rich Layered Oxides at Moderate Delithiation

Abstract

Delithiation of layered oxide electrodes triggers irreversible oxygen loss, one of the primary degradation modes in lithium-ion batteries. However, the delithiation-dependent mechanisms of oxygen loss remain poorly understood. Here, we investigate the oxygen nonstoichiometry in Li- and Mn-rich Li1.18-xNi0.21Mn0.53Co0.08O2-δ electrodes as a function of Li content by utilizing cycling protocols with long open-circuit voltage steps at varying states of charge. Surprisingly, we observe significant oxygen loss even at moderate delithiation, corresponding to 2.5, 4.0 and 7.6 mL O2 g-1 after resting at 135, 200, and 265 mAh g-1 (relative to the pristine material) for 100 h. Our observations suggest an intrinsic oxygen instability consistent with predictions of high equilibrium oxygen activity at intermediate potentials. From a mechanistic viewpoint, we show that cation disorder greatly lowers the oxygen vacancy formation energy by decreasing the coordination number of transition metals to certain oxygen ions. In addition, we observe a large chemical expansion coefficient with respect to oxygen nonstoichiometry, which is about three times greater than those of classical oxygen-deficient materials such as fluorite and perovskite oxides. Our work challenges the conventional wisdom that deep delithiation is a necessary condition for oxygen loss in layered oxide electrodes and highlights the importance of calendar aging for investigating oxygen stability.