Ion–Solvent Interplay in Concentrated Electrolytes Enables Subzero Temperature Li-Ion Battery Operations
Soohwan Kim, Bumjoon Seo, Hari Vignesh Ramasamy, Zhongxia Shang, Haiyan Wang, Brett M. Savoie, Vilas G. Pol
Abstract
Despite the essential role of ethylene carbonate (EC) in solid electrolyte interphase (SEI) formation, the high Li+ desolvation barrier and melting point (36 °C) of EC impede lithium-ion battery operation at low temperatures and induce sluggish Li+ reaction kinetics. Here, we demonstrate an EC-free high salt concentration electrolyte (HSCE) composed of lithium bis(fluorosulfonyl)imide salt and tetrahydrofuran solvent with enhanced subzero temperature operation originating from unusually rapid low-temperature Li+ transport. Experimental and theoretical characterizations reveal the dominance of intra-aggregate ion transport in the HSCE that enables efficient low-temperature transport by increasing the exchange rate of solvating counterions relative to that of solvent molecules. This electrolyte also produces a <5 nm thick anion-derived LiF-rich SEI layer with excellent graphite electrode compatibility and electrochemical performance at subzero temperature in half-cells. Full cells based on LiNi0.6Co0.2Mn0.2O2||graphite with tailored HSCE electrolytes outperform state-of-the-art cells comprising conventional EC electrolytes during charge–discharge operation at an extreme temperature of −40 °C. These results demonstrate the opportunities for creating intrinsically robust low-temperature Li+ technology.