  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.
