Hierarchical doping electrolyte solvation engineering to achieve high-performance sodium-ion batteries in wide temperature

Sodium-ion batteries are considered the most promising supplement to lithium-ion batteries in electrochemical energy storage due to their cost, reserves, and safety advantages, and they have received extensive attention. However, due to sodium ion kinetics and thermodynamics limitations, sodium-ion batteries are still challenging to operate at high rates and extreme temperature conditions. Here, we innovatively proposed a "hierarchical doping" solvation strategy and successfully designed a new type of wide-temperature sodium-ion electrolyte. The strategy is to reconstruct the first and second solvation layers of sodium ions and regulate their solvation process and behavior by introducing ethyl methyl carbonate and ethoxy (pentafluoro) cyclotriphosphazene with significant differences in solvation ability into PC-based electrolytes. It gives the electrolyte higher ionic conductivity, lower desolvation energy barrier, and the ability to form a thin and stable electrode/electrolyte interface, thereby achieving good sodium ion kinetics at low temperatures and showing excellent thermodynamic stability at high temperatures. Therefore, the Na3Fe2(PO4)P2O7 (NFP)||Na button battery containing the electrolyte has excellent high-rate cycle stability at low temperature, room temperature, and high temperature, which has a capacity retention rate of >99 % after 500 cycles at 5 C. In addition, its application in Ah-grade NFP||hard carbon (HC) pouch cells can also maintain excellent electrochemical performance such as cycle, rate, and capacity in a wide temperature range. More importantly, based on sodium ions' solvation process and behavior, this work innovatively developed a multi-functional wide-temperature electrolyte, which provides a new feasible direction for achieving high-performance batteries in wide temperatures.