A High-Capacity Lithium–Gas Battery Based on Sulfur Fluoride Conversion

Abstract

Identification of novel redox reactions that combine the prospects of high potential and capacity can contribute new opportunities in the development of advanced batteries with significantly higher energy density than today’s state-of-the-art, while advancing current understanding of nonaqueous electrochemical transformations and reaction mechanisms. The immense research efforts directed in recent years toward metal–gas, and in particular lithium–oxygen (Li–O₂) batteries, have highlighted the role that gas-to-solid conversion reactions can play in future energy technologies; however, efforts have mainly focused on tailoring the anode (alkali metal) in the metal–gas couple to achieve improved reversibility. Here, in a different approach, we introduce and characterize a new gas cathode reaction that capitalizes on the full change in the oxidation state (from +6 to −2) available in redox-active sulfur, based on the cathodic reduction of highly fluorinated sulfur hexafluoride (SF₆) in a Li metal battery. In a glyme-based electrolyte (0.3 M LiClO₄ in tetra ethylene glycol dimethyl ether), we establish, using quantitative gas and 19F NMR analysis, that discharge predominantly involves an 8-electron reduction of SF₆, yielding stoichiometric LiF, as well as Li2S and modest amounts of higher-order Li polysulfides. This multiphase conversion reaction yields capacities of ∼3600 mA h gC⁻¹ at moderate rates (30 mA gC⁻¹) and potentials up to 2.2 V versus Li/Li⁺. In a nonglyme electrolyte, 0.3 M LiClO₄ in dimethyl sulfoxide, SF₆ reduction also proceeds readily, yielding higher capacities of ∼7800 mA h gC⁻¹ at 30 mA gC⁻¹. Although not at present rechargeable, the demonstration of, and insights gained, from the primary Li–SF₆ system provides a promising first step for design of novel sulfur conversion chemistries with energy densities that exceed those of today’s Li primary batteries, while demonstrating a new design space for nonaqueous gas-to-solid electrochemical reactions.