Local structure evolution of Li 2 Fe 0.5 Mn 0.5 SiO 4 during the delithiation/lithiation processes is investigated by the first-principles calculations in an attempt to understand the capacity fading phenomenon often observed in cathode materials containing manganese. A cycle of delithiating/lithiating Li x Fe 0.5 Mn 0.5 SiO 4 compound for x between 2.0 and 0.625 is completely reversible with the local structure consisting of corner-sharing tetrahedrons centered by Fe, Mn, and Si ions. The accompanied redox couples are Fe 2+ /Fe 3+ , Mn 2+ /Mn 3+ , Mn 3+ /Mn 4+ for x from 2.0 to 1.5, 1.0, 0.5, respectively. When Li x Fe 0.5 Mn 0.5 SiO 4 is over-delithiated to x⩽0.5, the delithiation/lithiation processes are no longer reversible. The local structure in this stage is collapsed into edge-sharing 5-coordinated polyhedrons centered by Fe and Mn ions, leading to a substantial volume shrinkage and structural distortion. The reason of this irreversibility is attributed to the existence of energy barrier since the energy associated with the local structures of 5-coordinated polyhedrons stays in the deeper valley of the formation energy surface. A portion of energy injected into the compound to extract more than 1.5 Li ions is consumed to form additional bonds between O and Fe/Mn ions, which is not recoverable. Further extracting Li 0.5 Fe 0.5 Mn 0.5 SiO 4 to x=0.0 is accompanied by O 2− /O γ− (2⩾γ⩾1).