 Solution

The ideal estimate divides fully utilized electrochemical energy by active reactant mass alone. A real cell also contains electrolyte, separator, binder, conductive additive, current collectors, tabs, casing and safety components. Electrode balancing requires excess capacity on one side, and useful discharge does not generally span complete cobalt-oxide delithiation and a constant $4.2\mathrm{V}$. Ohmic, kinetic and concentration losses lower the loaded voltage. These effects can readily bring the cell-level value well below the $663\mathrm{Wh}\mathrm{k}{\mathrm{g}}^{-1}$ active-material proxy.

Improvements include higher-capacity host materials, stable higher-average-voltage positive electrodes, thinner inactive layers, lighter current collectors and packaging, and better utilization at practical electrode thickness. Each has a trade-off: very thick coatings impede ionic transport, more energetic electrodes may be less stable, and reducing the negative-electrode safety margin can promote lithium plating. Improving active-material capacity helps only if the added support, electrolyte and safety requirements do not consume the gain.