Ultrafast ion transport at a cathode–electrolyte interface and its strong dependence on salt solvation

To access the full performance potential of advanced batteries, electrodes and electrolytes must be designed to facilitate ion transport at all applicable length scales. Here, we perform electrodynamic measurements on single electrode particles of ~6 nAh capacity, decouple bulk and interfacial transport from other pathways and show that Li intercalation into LiNi0.33Mn0.33Co0.33O2 (NMC333) is primarily impeded by interfacial kinetics when using a conventional LiPF6 salt. Electrolytes containing LiTFSI salt, with or without LiPF6, exhibit about 100-fold higher exchange current density under otherwise identical conditions. This anion group effect is explained using molecular dynamics simulations to identify preferred solvation structures, density functional theory calculations of their binding energies and Raman spectroscopy confirmation of solvation structure. We show that TFSI− preferentially solvates Li+ compared to PF6−, and yet its preferred solvation structures provide a lower Li+ binding energy, suggesting a lower desolvation energy consistent with ultrafast interfacial kinetics. Understanding Li transport is important in the development of fast-rate batteries. Here the authors uncover an ultrafast charge transfer across a cathode–electrolyte interface with the aid of single-particle measurements and ascribe the solvation of electrolyte salts to be key for the interfacial kinetics.