Abstract
First principles calculations are used to anticipate the electrochemistry of polyoxoanionic materials consisting of XO 4 − y A y (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 Li 2FeSiO 4. Within the Pmn2 1 –Li 2FeSiO 4 structure, virtual models of Li 2Fe 2 2.5+SiO 3.5N 0.5 and Li 1.5Fe 2+SiO 3.5F 0.5 have been analyzed. We predict that the lithium deinsertion voltage associated to the Fe 3+/Fe 4+ redox couple is decreased by both substituents. The high theoretical specific capacity of Li 2FeSiO 4 (330 mAh/g) could be retained in N-substituted silicates thanks to the oxidation of N 3− anions, whilst Li 1.5Fe 2+SiO 3.5F 0.5 has a lower specific capacity inherent to the F substitution. Substitution of N/F for O will respectively improve/worsen the electrode characteristics of Li 2FeSiO 4.
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