Fluorobenzene-Diluted Localized Highly Concentrated Electrolyte for Enhanced Electrochemical Performance of Li–O₂ Battery

Highly concentrated electrolytes (HCEs), owing to their high thermal and chemical stability, wider electrochemical stability windows (ESWs), and enhanced stability with Li metal anode, have been under the spotlight as a potential electrolyte candidate for developing Li–O2 batteries. Nonetheless, their high viscosity, poor wettability, and high cost pose great challenges in achieving the desired results. In this study, we designed an HCE diluted with a low-polarity hydrocarbon cosolvent, fluorobenzene, and investigated its electrochemical performance in a Li–O2 battery. Raman spectroscopy analysis and self-diffusivity coefficients of electrolyte components determined by nuclear magnetic resonance (NMR) technique have confirmed that the fluorobenzene-based localized highly concentrated electrolyte (FB-LHCE) conserved the unique solvation structure of contact ion pairs and cation–anion aggregates formed in HCE. Incorporating fluorobenzene into HCE improved the self-diffusivity of electrolyte components by over a magnitude, lowered the viscosity by over 40 times, and increased the ionic conductivity 5-fold. Furthermore, FB-LHCE drastically improved the electrode wettability by yielding 3 and 5 orders of magnitude higher double-layer capacitances than those of low-concentration (LCE) and HCE, respectively. Additionally, when compared with LCE as the baseline, HCE and FB-LHCE have demonstrated wider anodic ESWs (>4.6 V). Li||Li symmetric cell tests revealed significantly improved electrochemical stability of HCE and FB-LHCE with Li metal anode compared to LCE. HCE and FB-LHCE have yielded stable cycling for double the amount of time (1300 h) compared with LCE (650 h). FB-LHCE delivered the highest specific discharge capacity (6.0 Ah g–1) followed by LCE (2.25 Ah g–1) and HCE (1.07 Ah g–1) in Li–O2 cells. Full-cell cycling stability tests have shown enhanced cycling stability with FB-LHCE (11 cycles) compared with those with LCE (2 cycles) and HCE (2 cycles).