Resolving the Origin of Poor Li-Metal Cycling in Nonflammable Electrolytes

Phosphate-based nonflammable electrolytes offer a low-cost, environmentally benign route to safer lithium batteries, but their use is limited by poor anode cycling stability, commonly attributed to continuous electrolyte decomposition and uncontrolled solid electrolyte interphase (SEI) growth. Using operando electrochemical quartz crystal microbalance with dissipation monitoring and operando Fourier-transformed infrared spectroscopy, we reveal a different failure mechanism. Contrary to the prevailing view, SEI formation is largely completed during the initial reduction and rapidly passivates the electrode. Failure is instead governed by SEI transport properties. In 1 m LiFSI in TEP, solvent-dominated reduction forms an organic-rich, Li + -transport-limiting interphase that prevents reversible lithium plating/stripping. Increasing the salt concentration to 2 m promotes FSI- reduction, yielding an inorganic-rich, Li + -conductive SEI that supports stable cycling. This transition arises from enhanced Li-FSI association, which suppresses solvent reduction and promotes anion-derived interphase formation.