Elucidating Passivation Layer Effects on Low-Temperature Performance of Nitrile-Based Electrolytes in Lithium-Ion Batteries

The increasing demand for lithium-ion batteries (LIBs) capable of functioning in extreme environments underscores the limitations of conventional carbonate-based electrolytes, particularly at subzero temperatures. This study presents a nitrile-based electrolyte formulated with butyronitrile (BN), which offers an exceptionally low freezing point and high dielectric constant, critical for efficient lithium-ion transport at temperatures as low as −40 °C. The reductive behavior of BN in the presence of bis(fluorosulfonyl)imide (FSI) anions is effectively controlled by incorporating optimized concentrations of 1.5% vinylene carbonate (VC) and 10% fluoroethylene carbonate (FEC). Electrochemical testing reveals the optimized electrolyte achieves 145 mAh/g at C/20 and 105 mAh/g at C/1 at +25 °C, with minimal polarization and excellent capacity retention over 250 cycles. At −40 °C, the electrolyte retains 82.6% capacity at C/20 relative to room temperature, significantly outperforming conventional systems. Coulombic efficiency (CE) remains near 100% at room temperature and above 80% at −40 °C, emphasizing the stability of the passivation layer. Arrhenius analysis indicates lower activation energy, highlighting improved ion conduction under cryogenic conditions. This work demonstrates the critical role of passivating additives in forming a robust, ion-conductive solid-electrolyte interface (SEI), establishing the optimized 1.5% VC+10% FEC formulation as a transformative electrolyte composition for LIBs in aerospace, defense, and cold-climate applications.