First principles study on novel cathode material NaFeSO4OH for sodium ion battery

This research sought to investigate via calculations from first principles based on the density functional theory (DFT), various properties of a novel cathode material (positive electrode) for secondary/rechargeable sodium ion battery (SIB), namely sodium iron(II) hydroxysulphate, NaFeSO4OH. Today lithium ion batteries (LIBs) dominate portable electronics and are currently making inroads into electric vehicle and grid energy storage sectors. Its main attraction over the other energy storage technologies is its high energy density. Nevertheless, lithium suffers from natural abundance problem, hence the idea to replace it with sodium, which is far more abundant and shares similar structure as lithium, making SIBs a promising prospect as next-generation technology. The key component of the battery is the cathode, and there have been several candidates proposed, yet none has reported on NaFeSO4OH. In this research, NaFeSO4OH was virtually built from the crystallographic data of layered lithium iron(II) hydroxysulphate, LiFeSO4OH through in situ substitution of lithium with sodium. Before that, the experimentally measured lattice parameters and redox voltage of layered LiFeSO4OH were reproduced, as the computational parameters used to achieve those are said to describe the layered FeSO4OH host structure well, thus giving confidence when applying them to NaFeSO4OH. Some traits were expected beforehand and indeed confirmed from the calculations made. In particular, NaFeSO4OH should have lower redox voltage than layered LiFeSO4OH and this was confirmed (3.23 V versus 3.60 V), which is still acceptable for a Na-based cathode. Besides that, its positive attributes include robust host structure due to strong SO and Fe-O bonds (maximum bond order, BOmax values 0.27 and 0.64 respectively), thereby promising thermal and cycling stabilities. It also possesses a high theoretical capacity (140 mAh/g) while being made up of abundant constituent elements. Its disadvantages include large unit cell volume change (13.10%) and strong Na-O bond (BOmax 0.24) which may affect electrochemical activity, and large electronic band gap (3.46 eV) which implies low electronic conductivity. Considering the overall results with some insight from experiments on other related cathodes, it was concluded that NaFeSO4OH is still a viable cathode candidate for SIB, keeping in mind that its performance could yet be enhanced for which various techniques were suggested.