Improving the Electronic Conductivity of Alluaudite Na2.5Fe1.75(SO4)3 Cathodes for Na-Ion Batteries

Abstract

The increasing dependency on batteries in modern technology has led to an ever-increasing demand that cannot be met by Li-ion batteries (LIBs) alone. The push towards using renewable energy and electric vehicles, coupled with the limited reserves of lithium resources is encouraging research into alternative options for electrochemical storage devices. Na-ion batteries (SIBs) are a promising alternatives due to the similarity of their chemistry to LIBs. This allows for utilisation of analogous materials and methods already in place for LIBs. Furthermore, SIBs are more economically viable and environmentally sustainable due to the abundance of the elements that can be utilised as electrode active materials. For example, SIB cathodes, such as Na2.5Fe1.75(SO4)3, can be synthesised from precursor materials that are widely available by-products of industry. The polyanionic framework of Na2.5Fe1.75(SO4)3 has three 3-dimensional vacancies within it, in which Na-ions de/intercalate.[1] However, the performance of Na-ion batteries is hindered by the low electronic conductivity of these polyanionic compounds, affecting the flow of electrons through the material and the reversible movements of the Na ions.[2] To address this issue, previous reports have shown that using a conductive additive such as carbon black and graphene oxides can increase the electronic conductivity of Na ion cathode materials and improving battery performance.[3]In this work, we investigate a non-stoichiometric alluaudite phase Na2.5Fe1.75(SO4)3 material (NFS), produced by a ball-milling-assisted solid-state synthesis. A study of the effects of variables on the ball milling process using the was carried out. A multi-step process involving a repetition of the ball milling and annealing was used to decrease particle size and eliminate impurities. The results were compared regarding particle size, impurities present, conductivity and battery performance. The obtained materials were characterised using X-ray diffraction, scanning electron microscopy and electrochemical impedance spectroscopy. The materials were tested as cathodes in sodium half-cells with 1 mol/L NaClO4 in ethylene carbonate (EC)/ propylene carbonate (PC) electrolyte. The additional ball milling and annealing steps saw a significant decrease in impurities present across all variations in methodology. All batteries show three redox peaks in their differential capacity plots, demonstrating that all three vacancies within the polyanionic framework of Na2.5Fe1.75(SO4)3 are active during charge and discharge. The best performing electrode achieved a capacity of 80 mAh/g using only C45 as a conductive additive. These are promising results that are expected to improve with further studies researching the addition of RGOs and MXenes into the ball milling process. These composite electrodes will be prepared by combining rGOs/MXenes with the NFS precursors during the synthesis process of Na2.5Fe1.75(SO4)3 to enhance battery performance further.