Highly Concentrated Electrolytes: Electrochemical and Physicochemical Characteristics of LiPF₆ in Propylene Carbonate Solutions

Highly concentrated electrolytes (HCEs) based on LiPF₆ in propylene carbonate (PC) have been examined as lithium-ion battery electrolytes. These HCEs have lower ionic conductivities and higher viscosities than ethylene carbonate (EC) electrolytes with 1.2 M LiPF₆, but they have higher Li⁺ ion transference numbers. Electrochemical cycling behaviour of LiNi_0.8Co_0.015Al_0.05O₂//graphite cells with 3.2 M LiPF₆ in PC resembles that of cells with EC-based electrolytes; the HCE cells have higher impedance, which can be lowered by increasing test temperature. By employing Raman and infrared spectroscopy, combined with density functional theory and ab initio molecular dynamics simulations, we reveal that the Li⁺ solvation structure and speciation are key factors that determine cell performance. Two distinct regimes are observed as a function of salt concentration—in the conventional regime, the solvation number (SN) is mostly constant, while in the HCE regime it decreases linearly. Graphite exfoliation is suppressed only at very high salt concentrations (>2.4 M), where [PC]_free/[Li⁺] < 1 and {[P{F}₆⁻]}_free>{[PC]}_free. Results from the Advanced Electrolyte Model indicate that Li⁺ desolvation improves at higher LiPF₆ concentrations, thereby mitigating PC co-intercalation into the graphite. However, Li⁺ ion transport is hindered in the HCEs, which increases impedance at both the oxide-positive and graphite-negative electrodes.