Cation mixing has been demonstrated experimentally in the Li-ion battery cathode material Li 2FeSiO 4. This feature is investigated here using DFT calculations. It is shown that full reversal of Li/Fe site occupations is energetically favoured on delithiation for all three electrochemically active Li 2FeSiO 4 polymorphs. The common layered topology in the arrangement of SiO 4 and FeO 4 tetrahedra in all three polymorphs transforms into a 3D-framwork. Calculations show here that such a change in structure leads to a lowering of electrochemical insertion potential from ~ 3.1 to ~ 2.8 V, in good agreement with experimental data. Calculations also predict the correct anisotropy in the cell expansion on delithiation on Li/Fe site reversals. Partial mixing of Li and Fe site occupations is energetically less favourable, which supports a two-phase transformation mechanism.