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 towards metal-gas, and in particular lithiumoxygen (Li-O 2 ) 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 oxidation state (from +6 to -2) available in redox-active sulfur, based on the cathodic reduction of highly fluorinated sulfur hexafluoride (SF 6 ) in a Li metal battery. In glyme-based electrolyte (0.3 M LiClO 4 in TEGDME), we establish, using quantitative gas and 19 F NMR analysis, that discharge predominantly involves an 8-electron reduction of SF 6 , yielding stoichiometric LiF, as well as Li 2 S and modest amounts of higher-order Li polysulfides. This multi-phase conversion reaction yields capacities of ~3600 mAh g C -1 at moderate rates (30 mA g C -1 ) and potentials up to 2.2 V vs. Li/Li + .In a non-glyme electrolyte, 0.3 M LiClO 4 in DMSO, SF 6 reduction also proceeds readily, yielding higher capacities of ~7800 mAh g C -1 at 30 mA g C -1 . Although not at present rechargeable, the demonstration of, and insights gained, from the primary Li-SF 6 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.

Nonaqueous metal-gas batteries based on halogenated reactants exhibit strong potential for future high-energy electrochemical systems. The lithium-sulfur hexafluoride (Li-SF 6) primary battery, which utilizes a safe, noncombustible, energy-dense gas as cathode, demonstrates attractive eight-electron transfer reduction during discharge and high attainable capacities (> 3000 mAh/g carbon) at voltages above 2.2 V Li. However, improved rate capability is needed for practical applications. Here, we report two viable strategies to achieve this by targeting the solubility of the passivating discharge product, lithium fluoride (LiF). Operating at moderately elevated temperatures, e.g. 50 °C, in DMSO dramatically improves LiF solubility and promotes sparser and larger LiF nuclei on gas diffusion layer (GDL) electrodes, leading to capacity improvements of ~10x at 120 µA cm-2. More aggressive chemical modification of the electrolyte by including a tris(pentafluorophenyl)borane (TPFPB) anion receptor further promotes LiF solubilization; capacity increased even at room temperature by a factor of 25 at 120 μA cm-2 , with attainable capacities up to 3 mAh cm-2. This work shows that bulk fluoride-forming conversion reactions can be strongly This article is protected by copyright. All rights reserved. 2 manipulated by tuning the electrolyte environment to be solvating towards F-, and that significantly improved rates can be achieved, leading a step closer to application. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) Significant improvement of rate capability of Li-SF 6 cells is achieved by controlling the formation of LiF. Two viable strategies, moderately elevating temperature to 50 °C or using an anion receptor (TPFPB) as additive in electrolyte, can increase attainable capacity by a factor of 10 or 25, respectively, at high current density (120 μA cm-2).

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Nonaqueous metal-gas batteries have emerged as a growing family of primary and rechargeable batteries with high capacities and energy densities. We herein report a high-capacity primary Li-gas battery that uses a perfluorinated gas, nitrogen trifluoride (NF), as the cathode reactant. Gravimetric capacities of ∼1100 and 4000 mAh/g are achieved at 25 and 55 °C, respectively (at 20 mA/g), with discharge voltages up to 2.6 V vs Li/Li. NF reduction occurs by a 3e/NF process, yielding polycrystalline lithium fluoride (LiF) on a carbon cathode. The detailed electrochemical NF conversion mechanism is proposed and supported by solid- and liquid-phase characterization and theoretical computation, revealing the origin of observed discharge overpotentials and elucidating the significant contribution of N-F bond cleavage. These findings indicate the value of exploring fluorinated gas cathodes for primary batteries; moreover, they open new avenues for future targeted electrocatalyst design and cathode materials synthesis applications benefiting from conformal coatings of LiF.

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