A DFT study of polyanion substitution into the Li-ion battery cathode material Li2FeSiO4

Density Functional Theory (DFT) has here been used to study the substitution of SiO 4 4 - for VO 4 3 - polyanions in the orthosilicate Li-ion battery cathode material Li2FeSiO4, in order to enhance electron transfer between the TM-ions and thereby achieve a capacity increase from the potential redox activity of the orthovanadate polyanion. Comparison of results for five different model structures for LiFeXO4, X = Si, P and V, reveals that VO 4 3 - substitution destabilizes the tetrahedral structures towards olivine- or spinel-type structures. Our modelling of lithiation of the hypothetical 100% substituted system LiFeVO4 to Li2FeVO4 predicts the reduction of V5+ in the VO 4 3 - anion to V4+ at a potential of 2.1 V. While complete delithiation of LiFeVO4 to FeVO4 is accompanied by Fe2+/Fe3+ oxidation at ∼3.1 V. These lithiation and delithiation processes trigger changes in the unit-cell volume: −6% and +10%, respectively. Notably, only minor structural distortions were observed in both VO 4 3 - and the more exotic VO 4 4 - tetrahedra. Thermodynamically feasible VO 4 3 - substitution levels are also shown to be <30%. This is exemplified for a 12.5% VO4-substituted system which exhibits ∼50% smaller band-gap and increased capacity at an average deintercalation potential of ∼3.2 V compared to the un-substituted system.