A Sodium Bis(fluorosulfonyl)imide (NaFSI)‐based Multifunctional Electrolyte Stabilizes the Performance of NaNi1/3Fe1/3Mn1/3O₂/hard Carbon Sodium‐ion Batteries

A sodium bis(fluorosulfonyl)imide (NaFSI)‐based multifunctional electrolyte is developed by partially replacing NaPF 6 salt in the electrolyte to improve the wide temperature range working capability of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 /hard carbon (NNFM111/HC) sodium‐ion batteries (SIBs). The capacity retention of the SIBs with NaFSI‐NaPF 6 dual salt electrolyte increases from 47.2 % to 75.5 % after 250 cycles at 25 °C, and from 51.0 % to 82.3 % after 80 cycles at 45 °C, and the 1 C discharge capacity retention at the low temperature of −20 °C also increases 26.8 %. In the single salt system, NaPF 6 effectively passivate the aluminum foil and NaFSI passivate the electrode/electrolyte interface. The synergistic effect of NaPF 6 and NaFSI greatly improves the battery performance in a wide temperature range. This NaFSI‐based dual salt electrolyte also effectively overcomes the flaws when the SIBs using NaFSI or NaPF 6 independently, and makes it more suitable for SIBs, indicating promising prospects in the commercial application of NNFM111/HC SIBs.