Concentration-Dependent Thermodynamics and Kinetics in Lithium-Metal Battery Electrolytes: Implications for Coulombic Efficiency

Abstract

Lithium (Li)-metal batteries (LMBs) present a promising avenue for high-energy applications. However, their practical adoption is constrained by challenges such as dendrite formation and unstable interphases. This study investigates the intricate interplay between electrolytedependent thermodynamics, kinetics, and transport properties in LMBs, focusing on the concentration effects in fluoroethylene carbonate (FEC) and 1,2-dimethoxyethane-based electrolytes containing lithium bis(fluorosulfonyl)imide. Due to FEC’s unique properties, these electrolytes facilitate significant upshifts in the Li redox potential and contribute to stable interphases and voltage profiles. Our findings reveal that the redox potential is primarily governed by the solvent’s electron-donating ability, reflecting underlying solvation dynamics, while the electrolyte permittivity influences reaction entropy trends. The results show entropy changes from increased molecular disorder at moderate concentrations to reduced entropy in highly concentrated regimes, driven by the formation of ion–solvent complexes. Kinetic analyses demonstrate a volcano-shaped dependence of exchange current density on concentration, centered at 2 M. Two prevailing perspectives propose that either kinetic–transport interplay or thermodynamic properties govern Coulombic efficiency (CE). However, separating these contributions is complex, since both higher exchange current density and upshifts in the Li redox potential enhance CE. Furthermore, CE strongly aligns with the combined effects of kinetics, thermodynamics, and transport, emphasizing the need for a holistic electrolyte design approach. Optimizing these three factors makes it possible to stabilize the interphase, promote uniform Li deposition, and elevate the overall safety and performance of next-generation LMBs.