The principle of E=mc^2 is of mass-energy equivalence. Any form of stored energy has mass, including the binding energy between atoms in a molecule, and lithium ions convert those chemical bonds to electric currents when a load is applied.

The mass associated with those chemical bonds is miniscule compared to the protons, electrons, and neutrons, but it is not zero.

All (confined?) energy is equivalent to mass, even when that mass isn’t in the form of elections and quarks.