First principles study of polyanionic cathode materials LiFeSO4F and LiFeSO4OH using density functional theory

In this research, first principles techniques have been leveraged to thoroughly understand on the fundamental knowledge of Li-ion batteries. Computational materials design demonstrated that the modern modelling techniques play a valuable role that can help to achieve deeper fundamental insight into novel materials for rechargeable lithium ion batteries by computing key relevant properties. The effect of DFT + U method was investigated on the properties of cathode materials such as structural properties, electronic properties and voltage of the cathode. It is found that the electronic properties and voltage calculation are improved upon the addition of U value to the iron atom. However, the addition of U value on the structural properties calculation is not necessary as it has overestimated the data. To understand the difference of voltage between LiFeSO4F and LiFePO4, the Mulliken population analysis calculation was conducted. The result shows that the increase of voltage of LiFeSO4F compared to LiFePO4 which is due to the inductive effect. However, the difference of voltage between tavorite and layered LiFeSO4OH could not be explained using this inductive effect. The best explanation to this phenomenon, the difference between the polyhedral connectivity of the tavorite and layered LiFeSO4OH structure is taken into account. It is found that the layered LiFeSO4OH produces the overestimated result on lattice parameter using the conventional exchange correlation functional. To improve the result, the van der Waal dispersion correction was applied to the GGA-PBE and GGA-PBEsol exchange correlation functional. Upon the addition, the structural properties and the calculated voltage of the layered LiFeSO4OH have been improved near to experimental values. The density of states of LiFePO4, LiFeSO4F and LiFeSO4OH cathode materials were calculated to investigate their rate capability. It is found that those cathode materials possess low rate capability as the lithiated and delithiated states behave as n-type and p-type semiconductor respectively. Furthermore, the effect of Vanadium substitution on the layered LiFeSO4OH was also investigated. Based on the formation energy calculation, vanadium substitution in LiFeSO4OH tends to reside at the Fe site because of it more energetically stable compared to S site. The high volume of LiFe0.75V0.25SO4OH facilitates lithium ion to move easily and hence enhancing the rate number of lithium ion to channel in and out from the cathode. Thus, this contributes in increasing the ionic conductivity of such cathode material. The reduced band gap upon the vanadium substitution could improve the electronic conductivity of the cathode material. The calculated bond order values obtained upon delithiation process showing that the changes of S-O bond in LiFe0.75V0.25SO4OH are more uniform resulting the volume shrinking after the removal of lithium ion is lower compared to the pristine compound. Thus, it could improve the cycle life of the battery and could make this new LiFe0.75V0.25SO4OH as a promising cathode material candidate in lithium ion batteries.