Perfluoroalkylated Catholyte Harnessing Multiple Fluoride Bond Breaking for Lithium Primary Batteries

Abstract

Lithium primary batteries, with theoretical energy densities 3x higher than the rechargeable systems, are critical for applications where recharging is impossible or impractical, such as medical implants, unmanned vehicles, and remote monitoring. The current energy leader among commercial primaries is the Li−carbon monofluoride (CFx) battery, which has the highest theoretical energy density (2180 Wh/kgCFx) and up to 800 Wh/kg achieved in packaged cells. The solid CFx (0 < x ≤ 1.3) particles enable excellent safety and shelf stability, but also result in limited rate capability due to low electronic conductivity. Unfortunately, despite the increasing demand for high-energy Li primaries, the field is maturing with few fundamental innovations in cell chemistries in the past 40 years. To design battery chemistries that can surpass the energy of Li−CFx cell, herein, we examined the possibility to conduct multi-electron carbon reduction in high F content perfluoroalkyl groups (RF = −CnF2n+1, with F/C ratios of x>2) using liquid phase reactants. First, using liquid perfluoroalkyl iodides (CFI) as a model system, we investigated the governing factors for C−F bond redox, including supporting solvent viscosity, CFI chain length and concentration, discharge rate, and temperature. Up to 8 e− transfer per CFI (>1.3 e− per C, or 8/13 available F) is achievable with low reactant concentrations (0.1 M) and rates (20 μA/cm2). CFI catholyte sees challenges at high reactant concentrations and/or rates, as premature cell termination caused by deactivation of intermediates was observed. Therefore, to address this issue, we examined multiple handles in molecular structure to further tune the C−F bond activity. By replacing the I-ligand in CFI with an aromatic structure, the obtained fluoro-aromatics demonstrate close-to-full defluorination of RF group, yielding up to 15 e− (out of 17 available F) transfer per molecule, and attractive gravimetric energies of up to 1785 Wh/kgfluoro-aromatic, at high potentials (up to 2.6 V vs. Li/Li+) and concentrations (up to 1 M). Additionally, the voltage compatibility of the fluoro-aromatics with CFx enabled the hybridization of the catholyte and solid cathode in one cell. The hybrid cells provide opportunity to maximize the active material loading at cell-level, showing strong potential to further improve the energy densities over existing battery systems.