Structural and electrochemical aspects of Mn substitution into Li2FeSiO4 from DFT calculations

DFT calculations are presented which probe the effect of low-concentration Mn substitution of the Fe-sites in Li2FeSiO4: the promising new and potentially cheap cathode material for upscaled Li-ion battery applications. The LixFe0.875Mn0.125SiO4 system investigated could be achieved by replacing 12.5% of the Fe-sites in 2 × 2 × 1 and 2 × 2 × 2 supercells by Mn ions. The evolution of Bader charges and partial densities of states (DOS) have been followed under a stepwise delithiation process. A clear structural distortion is seen to occur at the Mn-site on delithiation, suggesting possible structural instability. Oxidation of Mn beyond 3+ is calculated to occur at potentials in excess of 4.7 V, implying that oxidation of well separated (>10Å), low-concentration Mn ions to Mn4+ is energetically unfavourable in the LixFe0.875Mn0.125SiO4 structure. This, together with previous DFT results for higher levels of Mn substitution into Li2FeSiO4, indicates that capacity increase in Li2Fe1 − yMnySiO4 through a >1 electron redox reaction may not be so readily attainable in practice, either for high or low Mn concentrations.