Operando Discovery of LiNO₃-Induced Ultrathick Solid Electrolyte Interphase Formation on Li Metal Anodes

Solid electrolyte interphases (SEIs) play essential roles in lithium metal batteries by dictating the deposition morphology and Coulombic efficiency (CE) of lithium metal anodes. However, the understanding of the formation and evolution of SEIs remains elusive so far. Herein, we present an operando investigation of the formation and evolution dynamics of LiNO₃-induced SEI under liquid electrolytes using high-resolution electrochemical atomic force microscopy (EC-AFM). For the first time, we discovered the in situ formation of an ultrathick SEI (∼648 nm) under liquid electrolytes, which far exceeded the thicknesses observed via ex situ characterization techniques. This ultrathick SEI formed through a sequential reduction process, wherein an inorganic-rich inner layer was initially formed, followed by the deposition of a soft outer layer with a modulus of approximately 24 MPa. We also observed that the outer SEI layer was metastable and susceptible to dissolution in liquid electrolytes. Our findings provide a comprehensive investigation of the formation and evolution processes of the LiNO₃-induced SEI, deepening the understanding of the in situ structure of SEIs under liquid electrolytes.