DFT and ATR-FTIR investigation of cation-anion-solvent coordination in LiPF₆-based carbonate electrolytes for lithium-ion batteries

Electrolytes critically influence lithium-ion battery performance and safety, yet detailed ion-solvent interactions remain poorly understood. While Li⁺-solvent interactions have been studied extensively, the role of the PF₆⁻ counterion in mixed-solvent systems has been investigated less extensively. Here, gas-phase DFT calculations and ATR-FTIR spectroscopy were combined to systematically investigate the molecular interactions and solvation structures in LiPF₆-based electrolyte systems composed of polar aprotic carbonate solvents-ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC)-and their binary/ternary mixtures. DFT calculations were performed at the B3LYP/6–311++G(d,p) level to optimize cluster geometries and evaluate electronic, thermochemical, and vibrational properties, including incremental and relative Gibbs free energies, adiabatic reduction energetics, global reactivity indices, HOMO-LUMO gaps, and binding energies for Li⁺(S)₂PF₆⁻, Li⁺(S₁)(S₂)PF₆⁻, and Li⁺(EC)(DMC)(PC)PF₆⁻ structures, where S, S₁, S₂ ∈ {EC, DMC, PC}. Calculated infrared spectra of the Li⁺-solvent-PF₆⁻ clusters were compared with ATR-FTIR spectra of 1 M LiPF₆ in pure EC, PC, and DMC, as well as in the binary EC-DMC, EC-PC, and DMC-PC (1:1 v/v) and ternary EC-DMC-PC (1:1:1 v/v/v) mixtures. Good qualitative agreement between the calculated and experimental vibrational features supported structural assignments and provided deeper insight into cation-anion-solvent coordination. Within the present gas-phase cluster framework, the results show that explicit PF₆⁻ plays an active role in shaping mixed-solvent coordination and ion-pairing behavior, providing molecular-level guidance for the design of advanced lithium-ion battery electrolytes.