Edge-Propagation Discharge Mechanism in CFx Batteries -- a First Principles and Experimental Study

TL;DR

This study combines first-principles calculations and experiments to reveal a new edge-propagation discharge mechanism in CFx batteries, explaining voltage limitations and suggesting pathways for rechargeability enhancement.

Contribution

It introduces a novel atomic-scale discharge mechanism based on edge propagation, supported by DFT calculations and experimental validation, advancing understanding of CFx battery behavior.

Findings

Discharge voltage range of 2.5 to 2.9 V depending on solvent involvement

Discharge mechanism involves lithium insertion at CF/C boundary

Predicted rechargeability pathway under high potentials

Abstract

Graphite fluoride (CFx) cathodes coupled with lithium anodes yield one of the highest theoretical energy densities (>860 Wh/g) among primary batteries. In practice, the observed discharge voltage (~2.5 V) is significantly lower than thermodynamic limits (>4.5 V), the discharge rate is low, and so far Li/CFx has only been used in primary batteries. Understanding the discharge mechanism at atomic length scales will improve practical CFx energy density, rate capability, and rechargeability. So far, purely experimental techniques have not identified the correct discharge mechanism or explained the discharge voltage. We apply Density Functional Theory calculations to demonstrate that a CFx-edge propagation discharge mechanism based on lithium insertion at the CF/C boundary in partially discharged CFx exhibits a voltage range of 2.5 to 2.9 V -- depending on whether solvent molecules are involved. The voltages and solvent dependence agrees with our discharge and galvanostatic intermittent titration technique measurements. The predicted discharge kinetics are consistent with CFx operations. Finally, we predict Li/CFx rechargeability under the application of high potentials, along a charging pathway different from that of discharge. Our work represents a general, quasi-kinetic framework to understand the discharge of conversion cathodes, circumventing the widely used phase diagram approach which most likely does not apply to Li/CFx because equilibrium conditions are not attained in this system.