An Azacrown Electrolyte for High‐Performance Sodium‐Ion Batteries

ABSTRACT Sodium (Na)‐ion batteries (SIBs) are promising energy storage devices. However, they still do not exhibit satisfactory performance metrics largely because the relatively large ionic radius of Na + results in unfavorable solvation structures. This work demonstrates that using an azacrown (i.e., 1,4,7,10,13‐pentamethyl‐1,4,7,10,13‐pentaazacyclopentadecane, PMPAC) as a co‐solvent of the electrolyte can overcome the fundamental issues associated with SIBs. PMPAC features a cavity that not only matches the ionic radius of Na + but chelates the metal ions as well. As a result, PMPAC effectively encapsulates Na + and affords a disc‐like solvation complex with a relatively small solvation radius. These characteristics bestow the PMPAC‐based electrolytes with high ion conductivity and Na + transference number, which promote battery operation by enabling Na to plate in a homogeneous manner. The PMPAC also prompts anions to locate in the inner sheath of the Na + solvation structure such that inorganic‐rich solid‐electrolyte interphase layers with high structural integrity and low charge transfer resistance are formed at the anodes. SIBs that utilize PMPAC‐based electrolytes exhibit outstanding rate capacities (38 mAh g −1 at 100 C), along with ultralow capacity decay rates (0.0024% per cycle over 20 000 cycles for a half cell and 0.0062% per cycle over 10 000 cycles for a full cell).