Comparative Insights and Overlooked Factors of Interphase Chemistry in Alkali Metal‐Ion Batteries

ABSTRACT Although Na‐ and K‐ion batteries are emerging as cost‐effective and sustainable alternatives to Li‐ion batteries (LIBs) for large‐scale energy storage, their distinct physicochemical characteristics present unique challenges in achieving long‐term stability and high electrochemical performance. Among the various performance‐limiting factors, the electrode–electrolyte interphase, including the solid‐electrolyte interphase (SEI) and the cathode–electrolyte interphase (CEI), plays a crucial, yet still insufficiently understood, role in determining battery performances. From this perspective, this review offers critical insights into the interphase characteristics, with a special focus on not only clarifying the comparative characteristics of interphases across alkali‐ion battery systems, but also addressing several key issues often overlooked or misunderstood even in LIB research, including: (i) the dynamic and metastable nature of SEI and CEI, (ii) the limitations of fluorine‐rich CEI, (iii) the critical role of binders for interphase optimization, (iv) the overgeneralization of CEI functionality and formation mechanisms, and (v) the impact of interphase stability and passivation on self‐discharge. In doing so, this review emphasizes the pivotal role of interphase design in electrochemical performance, attempts to redefine the so‐called ideal interphases, and highlights the need for a clear and accurate understanding of the fundamental nature and functionalities of the interphases, beyond the conventional definitions, in alkali metal‐ion battery systems.