Tuning Electrolyte Solvation Structure and CEI Film to Enable Long Lasting FSI⁻-Based Dual-Ion Battery

Dual-ion battery (DIB) is a promising energy storage system because it can provide high power. However, the stability and rate performance of the battery depend strongly on the type of salt and solvents in the electrolyte. Herein, the use of lithium bis(fluorosulfonyl)imide (LiFSI) is studied, which has better high-temperature stability, as salt in the DIB and develop a 3 m LiFSI fluoroethylene carbonate/methyl 2,2,2-trifluoroethyl carbonate (FEC/FEMC) = 3:7 electrolyte, which stabilizes graphite–lithium DIB with 94.1% capacity retention after 2000 cycles at 5C. The DIB also exhibits excellent rate performance with 100.4 mAh g⁻¹ capacity at 30C, with a utilization of 96.3% compared to capacity at 2C. The outstanding electrochemical performance is attributed to the thin cathode electrolyte interface (CEI) layer and fast FSI⁻ transport kinetics, confirmed by X-ray photoelectron spectroscopy and activation energy calculation. Superior cycle and rate performances are also obtained from a graphite–graphite full cell. Though, increasing salt concentration to 5 and 6 Μ leads to sluggish FSI⁻ de-intercalation reaction and lower capacity, which is attributed to solvent co-intercalation. The research suggests that the electrolyte plays an important role in ion transport, surface film formation, and stability of DIB. © 2023 Wiley-VCH GmbH.