In-situ observation of electrolyte-dependent interfacial change of the graphite anode in sodium-ion batteries by atomic force microscopy

Graphite has proved to be inactive for Na⁺ storage in ester-based electrolytes when used as the anode material. Recent studies have shown the feasibility of a graphite anode for Na⁺ storage with a large capacity and a high initial Coulombic efficiency (ICE) in linear ether-based electrolytes. Understanding such solvent-dependent electrochemical behavior at the nanometer scale is essential but has remained elusive, especially the direct visualization of the graphite/electrolyte interface. We report the in-situ observation by atomic force microscopy of a working battery that allowed us to monitor and visualize the changes of the graphite/electrolyte interface in both linear ether and ester-based electrolytes. Results indicate that there is no solid electrolyte interphase (SEI) formation in the linear ether-based electrolytes and the co-intercalation is reversible and stable in the following cycles, which are responsible for the relatively high ICE, large capacity and excellent stability. In the ester-based electrolytes, SEI deposition is obvious during the sodiation process, but not in the desodiation process, leading to a serious consumption of the electrolyte, and thus a low ICE and irreversible Na⁺ storage. Our findings provide insights into the dynamics of changes in the graphite/electrolyte interface and reveal the solvent-dependent Na⁺ storage at the nanometer scale, paving the way to develop high-performance Na⁺ batteries.