On the delithiation mechanism of Li₂FeSiO_4−yS_y compounds: A first-principles investigation

The first-principles calculations are performed for both Li₂FeSiO₄ and Li₂FeSiO₃S compounds to elucidate their delithiation mechanism for a full extraction of two Li ions. The structural, electronic and electrochemical characteristics of both compounds are studied in detail, and the feasibility in removal of two Li ions from the hypothetical Li₂FeSiO₃S is explored. The calculated results reveal that the delithiation mechanism of Li₂FeSiO₄ is significantly modified by S substitution because of its larger radius. The bond lengths between Li–O and Li–S in the hypothetical Li₂FeSiO₃S compound become longer, and the gap between the pseudo-layers [Fe₂Si₂O₆S₂]_∞ becomes wider, leading to lower delithiation voltages and minor structural deformations in Li₂FeSiO₃S. The extraction of the first Li ion involves Fe²⁺/Fe³⁺ for both Li₂FeSiO₄ and Li₂FeSiO₃S, with a lower delithiation voltage 2.90 V and a slight expansion in cell volume up to 4.0% for Li₂FeSiO₃S. The extraction of the second Li ion is associated with combined redox couples of Fe³⁺/Fe^(4−α)+ and O²⁻/O^γ− (0 < α < 1 and 1 < γ < 2) for Li₂FeSiO₄ or a redox couple of S²⁻/S⁻ for Li₂FeSiO₃S. The difference in redox reactions corresponded to the extraction of the second Li ion for both compounds is the major reason leading to a drop of delithiation voltage from 4.85 V for Li₂FeSiO₄ to 4.19 V for Li₂FeSiO₃S, which is well in the stable range of voltages for conventional and commercial electrolytes, and a reduction in volume variation from a large volume expansion up to 11.5% for Li₂FeSiO₄ to a slight reduction of volume up to 2.3% for Li₂FeSiO₃S