Liquefied Gas Electrolytes for Wide-Temperature Lithium Metal Batteries

Abstract

The momentum in developing next-generation high energy batteries calls for an electrolyte that is compatible with both lithium (Li) metal anode and high-voltage cathodes that is also capable of providing high power in a wide temperature range. Here, we present a fluoromethane-based liquefied gas electrolyte with acetonitrile cosolvent and higher, yet practical, salt concentration. The unique solvation structure observed in molecular dynamics simulations and confirmed experimentally shows not only improved ionic conductivity of 9.0 mS·cm-1 at +20°C but high Li transference number (tLi+=0.72). Excellent conductivity (> 4 mS·cm-1) was observed from -78 to +75°C, demonstrating operation above fluoromethane’s critical point for the first time. The liquefied gas electrolyte also enables excellent Li metal stability with a high average coulombic efficiency of 99.4% over 200 cycles at the aggressive condition of 3 mA·cm-2 and 3 mAh·cm-2. Also, dense Li deposition with an ideal Li-substrate contact is seen in the liquefied gas electrolyte, even at -60°C. Attributed to superior electrolyte properties and the stable interfaces on both cathode and anode, the performances of both Li metal anode and Li/NMC full cell (up to 4.5 V) are well maintained in a wide-temperature range from -60 to +55°C. This study provides a pathway for wide-temperature electrolyte design to enable high energy density Li-metal battery operation between -60 to +55°C.