Benefits of N for O substitution in polyoxoanionic electrode materials: a first principles investigation of the electrochemical properties of Li2FeSiO4−yNy (y = 0, 0.5, 1)

Abstract

First principles calculations have been used to investigate the effect of N for O substitution on the electrochemical properties of Li₂FeSiO₄. Within the Li₂FeSiO₄ structure, hypothetical models of the N-substituted Li₂FeSiO₃N and Li₂FeSiO_3.5N_0.5 have been analyzed. The computational results indicate that the lithium deinsertion voltage associated to the Fe^3+/Fe⁴⁺ redox couple can be decreased by N substitution (4.86 V in Li₂FeSiO₄, 4.7 V in Li₂FeSiO_3.5N_0.5 and 4.1 V in Li₂FeSiO₃N). The high theoretical specific capacity of Li₂FeSiO₄ (330 mA h g⁻¹) could be retained in N-substituted silicates thanks to the oxidation of N³⁻ anions. The redox activity of N ions is observed in a voltage range of ca. 3.5–4.2 V. In the light of the potential benefits of N substitution for O experimental work is encouraged, in particular to investigate the reversibility and overpotential of the N redox reaction.