Coin cell preparation. A complete description of the materials and experimental procedures used to prepare electrodes, PVDF-HFP gels, and the associated coin cells is provided in the SI. Briefly, cathodes were obtained by dropcasting a slurry containing 70% FeF3, 20% Super cells except the voltage range was 0.6 to -2.4 V (vs. LFP). Potentials for the FeF3 half-cell experiments were converted to the Li/Li + potential scale using the formula ELi/Li + = ELFP + 3.43 V, in accordance with the literature. Coulombic efficiency (CE) was determined at 1.0 mA cm -2 , over 50 cycles, for both half-cell and full-cell experiments.