Understanding Growth Kinetics and Electrochemical Reaction Mechanism of Sodium Transition Metal Fluorosulfates As High Voltage Cathodes
Authors: Ahuja Vinita , Baskar Senthilkumar , Senguttuvan Premkumar 
Published: July 2021

The intermittent nature of renewable sources demands the utilization of resilient storage systems, which can smoothen their energy production and distribution to consumer utilities. Among the storge systems, rechargeable batteries offer easy maintenance, low cost, high round trip efficiency and durability. At present, sodium-ion batteries (SIBs) have garnered widespread interest for grid storage application due to inexpensive and earth abundant raw materials. Their practical realization is nested in successful exploration of suitable electrode and electrolyte materials. So far, various Na-ion cathodes have been discovered including layered oxides, polyanionic compounds and Prussian blue frameworks, whose energy densities are slightly inferiro to their Li-ion counterparts. Polyanionic frameworks consists of tetrahedral anions (XO4)n– (X= S, P) and MeOx (Me= 3d transition metal) polyhedral. In sulfate based polyanionic systems, the presence of SO4
2– anions will ensure rapid and better pathway for alkali conduction along with structural stability to metal redox. The inclusion of fluoride anion into the sulphate frameworks increases their operating voltages through inductive effect.2 It can also enhance cationic mobility by reducing electrostatic interactions along conduction channels, thereby, lead to increase in cell capacity.3 Recently, metal fluorosulfates such as NaMSO4F, Na3MF2(SO4)2 (M=3d transition metals) have been reported as promising high voltage cathode materials for sodium-ion batteries.
In this work, we have explored a new family of Na2MF3SO4 (M= V, Cr, and Mn) fluorosulfates as potential Na-ion cathode materials. They were synthesized through hydrothermal route at ~200 °C. These compounds are isostructural with the reported Na2FeF3(SO4)2 as depicted in X-ray diffraction refinement (Figure 1a).4 The Na2VF3(SO4)2 crystallizes as light green needle like particles (Figure 1b and inset of Figure 1a), as single phase. The 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 (Figure 1c). Furthermore, its growth mechanism is studied with microscopy techniques to comprehend its formation kinetics and evolution of morphology with respect to time. Upon first desodiation, the Na2VF3SO4 structure showed a plateau at ~4.5 V Na+ (de)insertion where it exceeds the capacity beyond ~100 mAh g–1. Its subsequent sodiation profile showed two-step flat voltage with voltage at 4.0 and 3.2 V. Detailed reaction kinetics, electrochemical performances and sodium (de)intercalation mechanism will be presented.

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Figure 1