Advanced in situ technology for Li/Na metal anodes: an in-depth mechanistic understanding

Li/Na metal anodes, based on their high theoretical capacity and low electrochemical potential, provide promising alternatives for next-generation high energy batteries. However, their unstable solid-electrolyte interphase and dendrite growth remain ubiquitous issues that have led to the decline of cycle performance and even safety problems. In the past 5 years, research interest and achievements in in situ technologies have surged globally, including in situ reactions to form a specific interface layer and in situ characterization to capture transient metastable information of metal anodes continuously as a function of time. It is desirable to provide an overview with a comprehensive understanding of the reaction process, degradation mechanism and structure evolution of metallic Li/Na through advanced in situ techniques. A critical appraisal of recent advances in the in situ technology is also presented for addressing the key issues in metallic Li/Na anodes, with a special emphasis on the emerging in situ electrode design and advanced in situ electrochemistry mechanism analysis. Finally, we provide a roadmap regarding the remaining challenges and integrated improvement strategies toward next-generation reliable and stable metal anodes.