On the delithiation mechanism of Li2FeSiO4-ySy 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](infinity) 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-alpha)+ and O2-/O gamma- (0 < alpha < 1 and 1 < gamma < 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. (C) 2013 Elsevier Ltd. All rights reserved.