Synthesis, Structural and Electrochemical Properties of Sodium Transition Metal Fluorosulfate Cathodes for Na-Ion Batteries

Abstract

The development of low-cost and sustainable rechargeable batteries is attractive for storing energy from renewable sources. At present, the expansion of Na-ion batteries (NIBs) serves as an appealing solution from the perspective of both raw material abundance as well as the cost in comparison with the existing Li-ion batteries.1 However, the energy densities of NIBs are lower compared to their Li-ion counterparts due to thermodynamic reasons. To address this issue, researchers have turned their attention towards the development of high energy density cathodes, such as Na3V2(PO4)2F3 and Na3V2(PO4)3-type materials,2 in which the operation of multi-redox couples render high storage capacities. Moving forward, the replacement of PO4 3- by SO4 2- provides higher intercalation voltages (due to inductive effect), thereby further improving the overall energy density of cathode. 3 The inclusion of fluoride into the sulphate based polyanionic frameworks will help to increase the operating voltage through inductive effects.4 This will enhance cationic mobility by reducing electrostatic interactions along conduction channels, thereby, lead to increase in cell capacity.Herein, we report synthesis, structural and electrochemical properties of two classes of sodium transition metal fluorosulfates, namely Na3MF2(SO4)2 and jarosites NaM3(SO4)2(OH/F)6 (M= V and Mn etc.,). The Na2VF3(SO4)2 framework consists of chains of trans-VO2F4 octahedra linked to each other via vertex sharing of F atoms and bridged by SO4 tetrahedron as adjacent pairs. 5 Jarosite is a natural mineral to be found in acidic and sulfate-rich environments with a general formula of AM3(SO4)2(OH)6, where A = K+, Na+, and NH4 + and M = Fe3+, Cr3+, V3+, Ga3+, Al3+, and In3+. Jarosite crystals stabilize in a trigonal system with space group R3m. The structure is composed of 2D uneven layers formed by linkage of transition metal octahedra [MO2(OH)4] and sulfate tetrahedra (SO4), that are stacked along the c direction. 6 We unveil the details of growth mechanism of these materials in hydro- and solvo-thermal conditions using X-ray diffraction and microscopy techniques. Further, the mechanism of electrochemical (de)sodiation reactions in these materials will be presented using galvanostatic cycling and in-operando XRD measurements.