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-EC 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) 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.