Melt-diffusion and ion-exchange methods: self-triggered O-rich groups on the surface of acetylene black using S-EDA solution to boost its sulfur content for lithium–sulfur batteries

Abstract

Only a few studies have described the use of H⁺-attacking S-EDA in nucleophilic substitution reactions to bind frameworks and sulfur in cathode materials, which is also known as the ion-exchange method. The pros and cons of this method are still unclear in relation to lithium–sulfur battery applications. Here, the influences of two synthetic routes, a melt-diffusion method and H⁺ reacting with S-EDA via nucleophilic substitution, on the morphologies and electrochemical properties of cathode materials are discussed in detail based on in situ XRD and other advanced technologies. Accordingly, high S-loading is achieved when H⁺ reacts with S-EDA via ion exchange on the surface of acetylene black, and capacities of 693.8, 644.5, and 638.9 mA h g⁻¹ are obtained over the first three cycles when the C/S composite is used as a cathode in coin cells without the conductive additive Super P. In situ XRD data confirm that poor electrochemical properties can mainly be attributed to the conversion rate of S species being too rapid to thoroughly utilize the S molecules that are immobilized, which means that more fixed sulfur can form during the charge/discharge process when using the ion-exchange method to make the C/S composite. In addition, a long-chain polysulfide shuttling effect is directly noticed via AFM in tapping-KPFM mode in the C/S composite that was synthesized via the melt-diffusion method, even though polar S–O bonds exist in the composite. The increase in the cathodic surface potential from 102.8 to 141.1 mV and the increase in the morphological height from 547.7 to 829.7 nm during the discharge/charge process can be attributed to the process of S loss.