Unraveling the underlying mechanism of good electrochemical performance of hard carbon in PC/EC–Based electrolyte

Although hard carbon in propylene carbonate / ethylene carbonate (PC/EC)–based electrolytes possesses favorable electrochemical characteristics in rechargeable sodium–ion batteries, the underlying mechanism is still vague. Numerous hypotheses have been proposed to solve the puzzle, but none of them have satisfactorily unraveled the reason at the molecular–level. In this study, we firstly attempted to address this mystery through a profound insight into the disparity of the ion solvation/desolvation behavior in electrolyte. Combining the results of density functional theory (DFT) calculations and experiments, the work explains that compared to the sole PC–based electrolyte, Na+–EC4 molecules in the PC/EC–based electrolyte preferentially undergo reduction and contribute to the emergence of a more stable protective film on the surface of hard carbon, leading to the preferable durability and rate capability of the cell. Nevertheless, applying the ion solvation/desolvation model, it also reveals that Na+–(solvent)n molecules in the PC/EC–based electrolyte can achieve faster Na+ desolvation processes than in the PC–based electrolyte alone, contributing to the enhancement of charge transfer kinetics. This research holds great importance in uncovering the possible mechanism of the remarkable electrochemical– properties of hard carbon in PC/EC–based electrolytes, and advancing its practical utilization in future sodium–ion batteries.