The Effect of Salt Concentration on Electrochemical Stability in Li-ion Battery Electrolytes

Electrolyte stability dictates the performance and lifetime of Li-ion batteries, yet a molecular level understanding of how salt concentration governs both thermodynamics and electronic structure remains incomplete. Here, we present an integrated study that links solvation environments, activity coefficients, and electronic structure across a wide range of LiPF6 concentrations in ethylene carbonate:ethyl methyl carbonate (EC:EMC, 3:7 v:v) electrolytes. From molecular dynamics simulations and Raman spectroscopy, we find increasing salt concentration reduces the number of species coordinating Li+ and suppresses free PF6- , EC, and EMC populations, which we connect to a lattice fluid theory that demonstrates systematic shifts in redox potentials from the changes in activity coefficients. In parallel, density functional theory calculations reveal how salt concentration reshapes the density of states, shifts frontier orbital energies, and thereby tunes electrode interfacial reactivity. X-ray absorption spectroscopy confirms these electronic structure variations. By combining thermodynamic and electronic structure perspectives, an approach not previously applied to electrolyte design, we show how concentrated electrolytes stabilize solvents and anions through Li+ association, shift the Li/Li+ redox couple to more positive potentials, and enhance PF−6- reactivity, promoting inorganic rich cathode electrolyte interphases. Importantly, this work establishes a direct link between solvation, electrochemical stability, and electronic structure, offering design principles for next-generation electrolytes with improved stability in high voltage Li-ion batteries.