Comparative computational investigation of N and F substituted polyoxoanionic compounds: The case of Li_2FeSiO_4 electrode material

摘要

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_(3.5)N_(0.5) and Li _(1.5)Fe~(2+)SiO_(3.5)F_(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_(1.5)Fe~(2+)SiO _(3.5)F_(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.