CNF/S Cathodes for RT Na-S Batteries in Carbonate Electrolyte: Understanding the Role of Sulfur Phase and Solid Electrolyte Interphase

Abstract

Over the last few years, there is growing interest towards safe and stable room temperature sodium-sulfur batteries owing to their high theoretical capacity and energy density of ~1672 mAh/g and 1230 Wh/kg respectively based on the final discharge product of Na2S. However, these Na-S batteries exhibit poor cycling properties mainly due to severe shuttle effect. Sulfur cathodes with various nanostructured carbon have been introduced to achieve long-term cycling. However, that these advances are partially due to the use of a non-carbonate solvent-based ethereal electrolytes, and the additive NaNO3 salt, in which the solubility of polysulfides is reduced or the polysulfide shuttle is passivated. Given the high flammability of ether-based electrolytes, such systems are not practically viable. Moreover, the conversion of sulfur to Na2S/Na2S2 in these reports is unclear. In this work, we have developed free-standing carbon nanofiber/S cathodes for achieving long-term cycling in RT Na-S batteries. The as-prepared CNF/S (~1.08 mg/cm2, 43.8 wt% S) cathodes were used directly as cathodes in 2032 type coin sized RT Na-S cells. The electrochemical tests were performed in EC/PC/FEC based electrolyte against Na/Na+ anode. The cyclic voltammetry and charge discharge results showed a single potential for conversion of sulfur to Na2S. The CNF/S cathodes delivered capacity of ~1229 mAh/g during the first cycle with ~51.1% coulombic efficiency. The capacity of RT Na-S cells was stabilized at ~350 mAh/g after Ist cycle and sustained up to 300 cycles with only ~.03% decay rate per cycle. The postmortem XRD results confirmed the conversion of sulfur to Na2S and vice versa. Further, we performed in situ electrochemical impedance spectroscopy, postmortem XPS and galvanostatic intermittent titration technique (GITT) to understand the underlaying conversion mechanism. The preliminary results showed that the observed single plateau and long term cycling of RT Na-S cells were the convoluted effect of deposited sulfur and stable SEI layer on the cathode side. #RP and AS have contributed equally