Deciphering the critical effect of cathode-electrolyte interphase by revealing its dynamic evolution

Abstract

Cathode electrolyte interphase (CEI) layer plays a crucial role in determining the electrochemical performance of lithium-ion batteries. Limited by the sensitive nature of CEI and the lack of characterization techniques, its dynamic evolution during cycling, its formation mechanism, and its specific impact on battery performance are not yet fully understood. Herein, we systematically investigate the dynamic evolution of CEI layer and its critical effect on the cycling performance of LiCoO2 cathode by diverse characterization techniques. We find that cycling voltage plays a key role in affecting CEI formation and evolution, and a critical potential (4.05 V vs. Li) is identified, which acts as the switching potential between CEI deposition and decomposition. We show that CEI starts deposition in the discharge process when the potential is below 4.05 V, and CEI decomposition occurs when the potential is higher than 4.05 V. When the battery is cycled below such a critical potential, a stable CEI layer is developed, which leads to superior cycling stability. When the battery is cycled above such a critical potential, a CEI-free cathode interface is observed, which also demonstrates good cycle stability. However, when the critical potential falls in the cycling voltage range, CEI deposition and decomposition are repeatedly switched on during cycling, leading to the dynamically unstable CEI layer. The unstable CEI layer causes continuous interfacial reaction and degradation, resulting in battery performance decay. Our work deepens the understanding of the CEI formation and evolution mechanisms, and clarifies the critical effect of CEI layer on cycling performance, which provides new insights into stabilizing the electrode–electrolyte interface for high-performance rechargeable batteries.