Potassium-ion batteries (PIBs) have aroused considerable interest as large-scale energy storage candidates owing to a naturally abundant potassium resource and low cost. Since the birth of PIBs, the solid electrolyte interphase (SEI) has been a critical concern, which plays a vital role in the coulombic efficiency, cycling stability and even safety of batteries. However, the SEI of PIBs remains poorly understood. In this review, we begin with a discussion regarding the differences in the chemical composition and structure of the K-ion SEI versus the much better-known Li-ion and Na-ion SEI. Unanswered scientific questions on the nature of the K-ion SEI are clearly identified. We review findings used to understand the formation mechanism of the SEI on graphite, and the existence of the SEI in ether-based electrolytes is openly debated. The current understanding of the SEI on different types of anodes is comprehensively summarized and discussed, with an emphasis on how the enhanced stability of the SEI fundamentally affects the efficiency and reversibility of electrochemical reactions. Finally, we detail the major challenges of anodes and discuss the emerging research directions for further development of PIBs.
