Edge-Propagation Discharge Mechanism in CF_x Batteries—A First-Principles and Experimental Study

Abstract

Graphite fluoride (CF x ) cathodes coupled with lithium anodes yield one of the highest theoretical specific capacities (>860 mAh/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/CF x has only been used in primary batteries. Understanding the discharge mechanism at atomic length scales will improve practical CF x 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 CF x -edge propagation discharge mechanism based on lithium insertion at the CF/C boundary in partially discharged CF x exhibits a voltage range of 2.5 to 2.9 Vdepending on whether solvent molecules are involved. The voltages and solvent dependence agree with our discharge and galvanostatic intermittent titration technique measurements. The predicted discharge kinetics are consistent with CF x operations. Finally, we predict some Li/CF x 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/CF x because equilibrium conditions are not attained in this system.

Revisiting Discharge Mechanism of CFx as a High Energy Density Cathode Material for Lithium Primary Battery

“…proposing the edge‐propagation mechanism in Li‐CF x system. [ 8 ] CF x peaks started to disappear after 40% DoD, while the presence of a graphitic peak at around 9° remained through discharged to 1.5 V state. This result confirmed the presence of both the graphitic and amorphous characteristics of CF x materials and is in good agreement with previous computational work by Goddard et al.…”

“…[ 6 ] Ideally, CF x has a layered structure in which each carbon atom is bonded to three other carbon atoms and one fluorine atom, minimizing the total energy of the structure. [ 7,8 ]…”

“…Most recently, Leung et al. [ 8 ] suggested that such solvent‐coordinated Li + complex undergoes an edge‐propagation mechanism rather than a bulk‐phase reaction pathway. Using density functional theory (DFT) calculations, they demonstrated this discharge mechanism based on lithium insertion at the zigzag edge boundary of the CF x structure and showed an operating voltage range of 2.5–2.9 V that varied based on electrolyte solvent.…”

Boosting the Energy Density of Li||CFx Primary Batteries Using a 1,3-Dimethyl-2-imidazolidinone-Based Electrolyte

“…The assembled Li||CF x primary battery has been widely applied in medical, military, and spatial fields. However, the discharge voltage plateau of the practical Li||CF x battery is much lower than the theoretical value, due to the low electronic conductivity and high overpotential of CF x electrodes, as well as robust C–F covalent bonds, which inherently restricts the practical energy density. , …”

Stable Cycling of Lithium Batteries Utilizing Iron Disulfide Nanoparticles

“…Conversion chemistry-based cathodes are an attractive choice to pair with lithium anodes due to their high specific capacity, energy density, and use of less expensive materials compared to commercial intercalation-based cathodes. − Of Li-compatible conversion chemistries, iron disulfide is a promising candidate with a theoretical specific capacity of 893 mA h/g. Already known to serve well as a commercial primary system, an expansive effort toward cell chemical and electrochemical reversibility is found in the literature.…”