Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 1 / 11 / ii

Using $365$ days per year, the required time is $2\times 365\times 24=17520\mathrm{h}$. Hencelithium transfers one electron per atom, soThis is the stoichiometric minimum for one cell, or one cell-equivalent parallel assembly, with complete utilization and no self-discharge. Practical loading must be larger. If a pack uses several cells in series, each series cell must carry the full delivered charge and the total lithium mass increases accordingly; no required pack voltage or series count is supplied.

The reaction consumes one mole of CF for each mole of electrons. Using $M_{\mathrm{CF}}=12.01+19.00=31.01\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$,This is active CF alone for the same single-cell-equivalent assumption, not the full positive-electrode or pack mass. Conductive additive, binder, electrolyte, battery separator, collectors and casing add mass, and incomplete utilization increases the required CF.

For the $x=1$ reaction there is one mole of transferred electrons per mole of reaction. Treating the supplied fresh-cell open-circuit voltage as the standard equilibrium cell potential givesA fresh-cell voltage alone would not normally establish standard conditions or equilibrium. The result above is the formal standard-free-energy estimate under the interpretation required to use the given datum; a metastable open-circuit voltage should not be mistaken for an independently measured standard thermodynamic potential.

This is a primary cell and should not be treated as rechargeable. Discharge converts the fluorinated-carbon electrode into carbon and lithium fluoride. It does not simply insert lithium into a host that can be reversibly deintercalated. Reversing electric current does not cleanly reconstruct the original distributed C–F bonding and electrode microstructure.

Insulating LiF, loss of the original reaction interfaces and unwanted electrolyte reactions obstruct reversibility. Replating lithium can also produce an irregular metallic deposit and internal shorting. A thermodynamically possible formal reverse reaction is therefore not evidence of a practical rechargeable cell.