The bond evolution mechanism of covalent sulfurized carbon during electrochemical sodium storage process

The excellent energy storage performance of covalent sulfur-carbon material has gradually attracted great interest. However, in the electrochemical sodium storage process, the bond evolution mechanism remains an elusive topic. Herein, we develop a one-step annealing strategy to achieve a high covalent sulfur-carbon bridged hybrid (HCSC) utilizing phenylphosphinic acid as the carbon-source/catalyst and sodium sulfate as the sulfur-precursor/salt template, in which the sulfur mainly exists in the forms of C–S–C and C–S–S–C. Notably, most of the bridge bonds are electrochemically cleaved when the cycling voltage is lower than 0.6 V versus Na/Na+, leading to the appearance of two visible redox peaks in the following cyclic voltammogram (CV) tests. The in-situ and ex-situ characterizations demonstrate that S2− is formed in the reduction process and the carbon skeleton is concomitantly and irreversibly isomerized. Thus, the cleaved sulfur and isomerized carbon could jointly contribute to the sodium storage in 0.01–3.0 V. In a Na-S battery system, the activated HCSC in cut off voltage window of 0.6–2.8 V achieves a high reversible capacity (770 mA h g−1 at 300 mA g−1). This insight reveals the charge storage mechanism of sulfur-carbon bridged hybrid and provides an improved enlightenment on the interfacial chemistry of electrode materials.