Rational manipulation of electrolyte to induce homogeneous SEI on hard carbon anode for sodium-ion battery

Sodium-ion batteries (SIBs) have great potential to be the next major energy storage devices due to their obvious advantages and developing advanced electrodes and electrolytes is urgently necessary to promote its future industrialization. However, hard carbon as a state-of-the-art anode of SIBs still suffers from the low initial Coulomb efficiency and unsatisfactory rate capability, which could be improved by forming desirable solid electrolyte interphases (SEI) to some extent. Indeed, the chemistry and morphology of these interfacial layers are fundamental parameters affecting the overall battery operation, and optimizing the electrolyte to dictate the quality of SEI on hard carbon is a key strategy. Hence, this review summarizes the recent research on SEI design by electrolyte manipulation from solvents, salts, and additives. It also presents some potential mechanisms of SEI formation in various electrolyte systems. Besides, the current advanced characterization techniques for electrolyte and SEI structure analyses have been comprehensively discussed. Lastly, current challenges and future perspectives of SEI formation on hard carbon anode for SIBs are provided from the viewpoints of its compositions, evolution processes, structures, and characterization techniques, which will promote SEI efficient manipulation and improve the performance of hard carbon, and further contribute to the development of SIBs.