A DFT study of VO_4~(3-) polyanion substitution into the Li-ion battery cathode material Li_2FeSiO_4

摘要

Density Functional Theory (DFT) has here been used to study the substitution of SiO44- for VO43- polyanions in the orthosilicate Li-ion battery cathode material Li_2FeSiO_4, 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 LiFeXO_4, X = Si, P and V, reveals that VO43- substitution destabilizes the tetrahedral structures towards olivine- or spinel-type structures. Our modelling of lithiation of the hypothetical 100% substituted system LiFeVO_4 to Li _2FeVO_4 predicts the reduction of V~(5+) in the VO43- anion to V~(4+) at a potential of 2.1 V. While complete delithiation of LiFeVO_4 to FeVO_4 is accompanied by Fe ~(2+)/Fe~(3+) 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 VO43- and the more exotic VO44- tetrahedra. Thermodynamically feasible VO43- substitution levels are also shown to be <30%. This is exemplified for a 12.5% VO_4-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.