Interfacial Lithium Redox Kinetics Regulated by Solvent Asymmetry and Moment of Inertia in Li Metal Batteries

Abstract

Lithium metal batteries (LMBs) offer high energy density, but interfacial charge-transfer kinetics remain a bottleneck for high-rate operation. In this work, we construct a library of 12 ether solvents with systematic variations in symmetry and fluorination, confirming that molecular asymmetry is an important feature that results in enhanced Li+ redox kinetics and more stable solid electrolyte interphases on Li0. Furthermore, we developed a mechanistic understanding of the underlying process. We show that more tilted dipoles and lower moments of inertia strongly correlate with higher exchange current densities. Among the solvents in our study, the asymmetric F5MPE (1-methoxy-2-(2,2,3,3,3-pentafluoropropoxy) ethane) molecule, a previously unreported solvent molecule, enabled >170 stable cycles in high-rate Li||NMC full cells, outperforming the previously reported best single-salt-single-solvent ether solvent, F5DEE molecule. Overall, this work links molecular-level information with the behavior at electrochemical interfaces and provides an understanding of molecular design considerations for next-generation electrolytes tailored for high-power, fast-charging LMBs.