Theoretical limit of energy density per mass in a Lithium battery

The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941 or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kg. This is a bit more than the heat of combustion of gasoline, but does not consider the other materials that go into a lithium battery and that make lithium batteries many times heavier per unit of energy.

https://en.wikipedia.org/wiki/Lithium-ion_battery

Molecular mass of lithium - 6.941 u. One mole of Lithium has a mass of about 6.941 gramms.

Faraday's constant - 96485.33 C/mol.

96485.33 C/mol    /    6.941 g/mol  = 13901 C/g

C = J / V,       3 V  *   13901 C/g  =  41.7 kJ/g