Hydrofluoroether-assisted dilution of Na-ion concentrated ionic liquid electrolyte for safe, stable cycling of high-voltage Na-metal batteries

Abstract

Sodium (Na)-powered rechargeable batteries (NRBs) are promising as sustainable energy storage systems. To overcome an inherent energy density handicap of NRBs, increasing cell voltage is necessary by building a Na metal battery (NMB), which simultaneously features coveted high-voltage stability and efficient Na dendrite protection. Although ionic liquids (ILs) are eligible to provide superior oxidative stability, their practical uses are challenging due to high viscosity and sluggish ionic transport at a higher Na concentration. Here, a localized Na+ ion concentrated ionic liquid (LNCIL) electrolyte consisting of an IL and a hydrofluoroether (HFE) as cosolvents is developed. The addition of a non-solvating HFE lowers the viscosity and improves separator wettability, thereby facilitating Na+ ion transport. Furthermore, HFE dilution promotes the involvement of dual anions (FSI/TFSI) in the development of a protective solid-electrolyte interphase, leading to Na dendrite suppression. A Na||Na3V2(PO4)3 cell incorporating the LNCIL electrolyte demonstrates excellent cyclability (~96.6% capacity retention over 500 cycles). Moreover, with oxidative stability up to 4.9 V (vs. Na/Na+) and non-flammability, the LNCIL electrolyte ensures the safe operation of high-voltage NMBs.