Comparative computational investigation of N and F substituted polyoxoanionic compounds. The case of Li2FeSiO4 electrode material

First principles calculations are used to anticipate the electrochemistry of polyoxoanionic materials consisting of XO4−yAy (A=F, N) groups. As an illustrative case, this work focuses on the effect of either N or F for O substitution upon the electrochemical properties of Li2FeSiO4. Within the Pmn21–Li2FeSiO4 structure, virtual models of Li2Fe22.5+SiO3.5N0.5 and Li1.5Fe2+SiO3.5F0.5 have been analyzed. We predict that the lithium deinsertion voltage associated to the Fe3+/Fe4+ redox couple is decreased by both substituents. The high theoretical specific capacity of Li2FeSiO4 (330mAh/g) could be retained in N-substituted silicates thanks to the oxidation of N3− anions, whilst Li1.5Fe2+SiO3.5F0.5 has a lower specific capacity inherent to the F substitution. Substitution of N/F for O will respectively improve/worsen the electrode characteristics of Li2FeSiO4.