Electron Density Engineering at the Bond Critical Points in Solvation Sheath of Sodium Ions for High-Rate Hard Carbon in Ether-Based Electrolyte

Rationally designing the electrolyte system toward improving the electrochemical performance, especially the rate capability, of sodium ion batteries (SIBs) is very important for accelerating their large-scale commercialization. Herein, it is shown that by refining the molar ratio of two ether solvents, namely dimethoxyethane (DME) and 2-methyl tetrahydrofuran (MeTHF), a binary solvent electrolyte system forms a sodium ion solvation structure that facilitates high rate charge/discharge of hard carbon (HC) electrodes. It is demonstrated that the boosted rate capability can be attributed to the enhanced sodium ion transportation and desolvation kinetics, resulting from the participation of weak-coordinating MeTHF molecule with low steric hindrance in the sodium ion solvation sheath, which weakens the interaction between sodium ion and solvent molecules/anions through electron density regulation at the bond critical points (BCPs). The thin and uniform solid electrolyte interphase film on HC electrodes formed in such an ether-based electrolyte is also beneficial for improving the rate performance and cycling stability. The results of the present study shed more light on how the electron density engineering at the BCPs in sodium ion solvation sheath affects the rate capability of HC electrodes and promote its practical application prospect in future sodium-based battery chemistries.