Thermally stable electrolyte for lithium‑oxygen battery and its metal-free version

Polyethylene glycol dimethyl ether with molecular weight of 250 (PEGDME 250) is considered a solvent of choice to achieve electrolytes with low volatility and high thermal stability for application in lithium oxygen (LiO₂) cells. The study focuses on the concomitant effect of the conducting salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and the sacrificial film-forming salt lithium nitrate (LiNO₃) both on chemical and electrochemical features. The electrolytes reveal conductivity ranging from ∼10 ^-3 S cm^-1 at 85 °C to ∼10 ^-4 S cm^-1 at -7 °C, and Li⁺ transference number (t₊) approaching 0.6. The concomitant incorporation of LiTFSI and LiNO₃ significantly modifies the thermal characteristics of the electrolyte due to specific salt-solvent interactions. Furthermore, LiNO₃ actually enhances the solid electrolyte interphase (SEI), stabilizes the Li, and improves the conductivity below 61 °C without significantly altering the t₊ value. Despite the LiO₂ cell can operate suitably even without LiNO₃ addition where a capacity limit of 0.5 mAh cm^-2 (500 mAh g^-1) is exploited, the one added by LiNO₃ can reversibly deliver a higher capacity (4500 mAh g^-1, 5.4 mAh cm^-2) than the bare solution (2000 mAh g^-1, 2.2 mAh cm^-2) if the voltage cutoff is used. On the other hand, the solution added by LiNO₃ enhances the performance of the metal-free (Li-ion) version of the cell using O₂ and Li_xSn@C alloying anode, with capacity ranging from 50 to 450 mAh g^-1 delivered at currents from 0.1 to 3 mA cm^-2. These findings may favor the achievement of high-energy storage systems with low cost and environmental sustainability.