On the Hidden Transient Interphase in Metal Anodes: Dynamic Precipitation Controls Electrochemical Interfaces in Batteries

The Solid-Electrolyte Interphase (SEI) formed on a battery electrode has been a central area of research for decades. SEI consists of a variety of byproducts generated by spontaneous and electrochemical decomposition of the electrolyte. This thin, structurally complex layer profoundly impacts the electrochemical deposition morphology and stability of the metal in battery anodes. Departing from conventional approaches, we investigate metal dissolution—the reverse reaction of deposition—in battery environments using a state-of-the-art electroanalytical system combining a rotating-disk electrode and in-operando visualization. Our key finding is the presence of a Transient Solid-Electrolyte Interphase (T-SEI) that forms during fast discharging at high dissolution rates. We attribute T-SEI formation to transient local supersaturation and resultant electrolyte salt deposition. The T-SEI fundamentally alters the dissolution kinetics at the electrochemical interface, leading to a self-limiting morphological evolution and eventually yielding a flat, clean surface. Unlike a classical SEI formed due to electrolyte decomposition, the T-SEI is fully “relaxable” upon removal of the enforced dissolution current; That is, the T-SEI completely dissolves back into the electrolyte when rested. The formation of T-SEI, surprisingly, plays a critical role in the subsequent electrodeposition. When the metal is redeposited on a fully relaxed T-SEI surface, the morphology is remarkably different from that deposited on pristine or low-rate discharged metal electrodes. Electron backscatter diffraction analysis suggests a homoepitaxial relationship with the original grains in the electrode. This is in stark contrast to the isolated, particulate nuclei seen on standard metal electrodes without T-SEI formation. Using 3D profilometry, we observe a 42% reduction in surface roughness due to T-SEI formation. Our findings provide important insights into the electrochemical kinetics at the metal-electrolyte interface, particularly in concentrated or “water-in-salt” electrolytes that are close to the salt saturation limit. The results suggest a new dimension for electrochemical engineering in next-generation batteries cycled at high rates.