Reviewing Li₂O in solid electrolyte interphases: from intrinsic properties and interfacial chemistry to rational...

Lithium batteries play an indispensable role in powering everything from portable electronics to electric vehicles. The solid electrolyte interphase (SEI) is pivotal for the stable operation of lithium metal batteries (LMBs), with its composition and structure critically influencing battery performance. Traditionally, lithium fluoride (LiF) has been considered the most important passivating component in the SEI, while lithium oxide (Li₂O) was often regarded as a background component, with its role frequently overlooked. However, recent breakthroughs in advanced characterizations have challenged this conventional paradigm. In this review, we comprehensively reassess the role of Li₂O within the SEI. First, we compare the intrinsic properties of common inorganic components such as Li₂O and LiF, re-evaluating the potential of Li₂O to facilitate lithium ion transport and maintain interfacial stability within the SEI. Second, we elucidate the fundamental formation principles of Li₂O, including the decomposition of oxygen-containing electrolyte components, electrochemical reaction at the electrode surface, and induction through inorganic component regulation. Furthermore, we systematically summarize regulation strategies, aimed at directionally constructing Li₂O-rich interphases, encompassing electrolyte system optimization, pre-construction of artificial SEI layers, and electrode material modification. Advanced characterization techniques tailored for accurately probing Li₂O are also discussed. Finally, we provide perspectives on future research directions, aimming to provide new insights for the development of high-performance, long-lifespan lithium batteries.