Development of Fluoride-Ion Primary Batteries
Authors: Robinson Loleth E , Wang Jonah , Asare Harrison , Andrews Jessica , Tripathi Balram , Katiyar Ram S , Melot Brent C , Jones Simon , West William C , Robinson Loleth J 
Published: December 2024

The primary lithium-carbon monofluoride (Li-CFx) cell offers the highest specific energy of any commercial lithium metal battery chemistry known. However, the practical discharge voltage even at low current densities (ca. 2.5-2.7 V) is significantly lower relative to the theoretical value (4.57 V), causing a significant penalty in specific energy. To reduce the overpotential at the CFx electrode, we altered the electrochemical reaction mechanism to defluorinating the CFx electrode upon discharge, without concomitant lithiation. Here, using a room temperature fluoride-ion (F-ion) conducting electrolyte, we demonstrate the electrochemical defluorination of CFx cathodes paired with either lead (Pb) or tin (Sn) metal anodes for the first time.
The primary F-ion Pb-CFx and Sn-CFx cells yielded capacities of 700 mAh g-1 and 400 mAh g-1, without optimization. Importantly, the discharge voltage of the Pb-CFx cell suggested that polarization loss of the cell was reduced by 1 V compared to a Li-CFx cell, indicating that a much higher operating voltage could be achieved with this design. XRD measurements show that the metal fluorides PbF2 or SnF2 form on the Pb or Sn anodes, respectively. Solid-state 19F and 119Sn{19F} NMR measurements of a F-ion Sn-CFx discharged Sn electrode and electrode components revealed the presence of both SnF2 and SnF4, where the latter is the main discharge product. Solid-state 2D 19F{19F} dipolar-correlation NMR experiments of the discharged Sn electrode revealed the amorphous nature of the electrochemically formed SnF4, confirming the reason it was not observed by XRD. The presence of nano-ordered regions with some amorphous in nature is also validated by Raman scattering and surface morphology changes of the electrodes upon discharge. Metal fluorides were discovered on the discharged CFx cathode of both the F-ion Pb-CFx and Sn-CFx cells, as evidenced by XRD, XRF, and solid-state NMR. ICP and XRF, quantified at ca. 5.4%, and ca. 3% contribution to the discharge capacity due to Pband Sn cation transport and reaction. Thus, a two-fold reduction in metal fluoride dissolution, transport, and reaction at the CFx electrode was achieved when using Sn, compared to the Pb.
Solid-state 19F NMR measurements of a pristine and discharged CFx electrode from a Sn-CFx cell establish unambiguously that C-F bonds are broken upon discharge. The 19F molar percentage of CF bonds consumed (39%) was compared to the cell capacity extracted related to the theoretic capacity of CFx (36.4%), confirming the electrochemical defluorination of CFx as well as a minor contribution due to Sn2+ cation transport. Overall, we have demonstrated a new electrochemical discharge mechanism relative to conventional Li–CFx cells, which reduces polarization loss and raises the possibility of improvement to a higher practical discharge voltage by using alternative metal anodes. Future work will address the lithium metal counter electrode reactivity to enable a F-ion based Li-CFx cell.