This module explains the thermodynamic origins of the open-circuit voltage in a Li-ion battery. The voltage is one of the most critical components in evaluating battery performance as it is directly related to the amount of energy stored in a battery and is heavily dependent on the materials used.

We first compared a battery to a closed fuel cell, and showed that the voltage changes with the fraction of lithium. We discussed the thermodynamic origins of the voltage in a single-phase electrode, as well as in a two-phase electrode where a phase transformation occurs. In the latter case, during the phase transformation the chemical potentials in the coexisting phases remain equal, and the voltage is constant. The Lever rule describes how the overall lithium fraction is distributed between the two phases during the transformation.

In practice, the actual battery voltage also takes into account overpotentials and resistive losses—causing the charging voltage to be higher than the discharge voltage.