The interfacial stability of solid-state batteries is critical to achieving long-term stability. It is often facilitated by a minuscule amount of liquid/polymer/gel electrolyte, leading to the formation of the solid–liquid interphase (SOLI) in the vicinity of the electrolyte/electrolyte interface. This article highlights the importance/challenges and possibilities of altering the SOLI to modulate cell performance.

Solid-state sodium batteries (SSSBs) offer high energy density with improved safety, making them an appealing candidate for long-range mobility applications. Considering the advances in SSSBs, ionic conduction is no longer a critical barrier. However, the instability of the electrode/electrolyte interface remains a hurdle, limiting cycling stability and cathode utilization. The compatibility between the electrode/electrolyte is often established by a small amount of liquid/polymer electrolyte. The solid–liquid interphase (SOLI) instead of the solid–solid interface plays a crucial role in deciding the performance of SSSBs. SOLI is a key component of the existing SSSBs, facilitating ion transport while mitigating interfacial resistance. The intricate, but essential, characteristics of SOLI, namely the composition, distribution, and ionic properties of the interfaces, are highlighted. This review highlights the key design strategies for optimizing the SOLI, including electrolyte engineering, interphase material selection, and the use of multiphase interphases to balance cell performance. Moreover, advanced characterization techniques are discussed, along with recent breakthroughs in SOLI research. This review aims to provide insights into overcoming the challenges of SOLI to enhance the electrochemical performance and long-term stability of SSSBs. A thorough understanding of SOLI engineering will pave the way for practical, safe, and long-lasting high-performance SSSBs.