Here we show the description of the full incorporation thermodynamics in
nanocrystalline LixFePO4 where the full range from x=0 to 1 is experimentally
accessible. The following points are most important: the treatment in terms of
the neutral lattice-gas model is incorrect rather ions and electrons need to be
considered separately. One needs to invoke point defect chemistry which gives a
satisfactory agreement over the full storage range if we treat the problem from
the two-sides: from the FePO4 side where Li+ is incorporated interstitially and
from the LiFePO4 side where Li+ is filling the vacancies. Furthermore, we
clarify how non-idealities such as ion-electron and electron-electron
interactions can be introduced to obtain a mechanistic understanding