Investigation of Ultra-Low Temperature Li/CF x Batteries in the Liquefied Gas Electrolyte

Lithium-fluorinated carbon (Li/CF x ) is one of the most promising chemistries to serve as a reliable high-energy density primary energy storage system in applications where rechargeability is not required. Though Li/CF x demonstrates high energy density (>2100 Wh kg –1 ) under ambient conditions, achieving such a high energy density when exposed to subzero temperatures remains challenging, particularly under high current density. Liquefied gas electrolytes (LGE) are regarded as one of the most promising electrolyte systems, offering excellent ionic conductivities at wide temperature ranges from -80 to + 60 o C due to low viscosity and low freezing point of gas molecules. Moreover, the LGE can significantly improve the capacity retention of Li-metal batteries operated at reduced temperatures. Here, we report a liquefied gas electrolyte with an anion-pair solvation structure based on dimethyl ether with a low melting point (−141 o C) and low viscosity (0.12 mPa×S, 20 o C), leading to high ionic conductivity (> 3.5 mS cm –1 ) between −70 and 60 o C. Besides that, through systematic X-ray photoelectron spectroscopy (XPS) integrated with transmission electron microscopy (TEM) characterizations, we evaluate the CF x interphase from the low-temperature discharge process. We conclude that the fast transport and anion-pairing solvation structure of the electrolyte bring about reduced charge transfer resistance at low temperatures, which resulted in significantly enhanced performance of Li/CF x cells (1690 Wh kg –1 , −60 o C;1172 Wh kg –1 , −70 o C based on active materials). Utilizing 50 mg cm -2 loading CF x electrodes, the Li/CF x displayed 1530 Wh kg –1 at −60 o C. This work provides insights into the electrolyte design that may overcome the operational limits of batteries in extreme environments.