Linking solvation structure, activity, and electronic structure to electrochemical stability in lithium-ion battery electrolytes

Electrolyte stability governs the performance and lifetime of Li-ion batteries, yet a quantitative molecular-level framework linking salt concentration to electrochemical stability remain absent. Here, we develop an integrated theoretical–experimental framework that quantitatively connects solvation structure, thermodynamic activities, and electronic structure to electrochemical stability for LiPF6:EC:EMC (3:7 v:v). Molecular dynamics (MD) simulations, a lattice fluid theory, X-ray absorption spectroscopy (XAS), and density functional theory (DFT) are combined to predict electrolyte stability across concentration. We derive simple, analytic expressions for the activities of species, which are then experimentally parameterized using Raman spectroscopy and validated against MD simulations and experiment. Increasing salt concentration is found to enhance PF6− participation in the Li+ coordination shell while suppressing free solvent populations, leading to systematic shifts in the activities of species and an expanded electrochemical stability window. The lattice fluid theory directly links solvation structure to measurable shifts in redox potentials via the Nernst equation, which we confirm experimentally. DFT calculations predict concentration-dependent shifts in unoccupied electronic states, which are validated by O and F K-edge X-ray absorption spectroscopy. These shifts provide the physical origin for the activity changes predicted by lattice fluid theory, while frontier orbital analysis further rationalizes the evolution of interphase composition arising from enhanced PF6− reactivity at high concentrations. This work establishes a predictive protocol connecting solvation structure, thermodynamic activity, and electronic structure with electrolyte stability, enabling rational design of electrolytes with tailored stability windows.