Probing the Extent of Polysulfide Confinement Using a CoNi2S4 Additive Inside a Sulfur Cathode of a Na/Li-Sulfur Rechargeable Battery.

The extent of confinement of soluble metal polysulfides inside a sulfur cathode strongly determines the performance of metal-sulfur rechargeable batteries. This challenge has been largely tackled by loading sulfur inside various conducting porous scaffolds. However, this approach has not proven to be fully effective because of poor chemical interaction between the scaffold and polysulfides. Here, we demonstrate an excellent strategy of using a sulfide additive in the sulfur cathode, viz., cobalt nickel sulfide (CoNi2S4), to efficiently trap the soluble polysulfides inside the sulfur cathode. In situ Raman and ex situ UV-vis spectroscopies clearly reveal higher retention of polysulfides inside CoNi2S4/S compared to bare sulfur and carbon-sulfur mixture cathodes. Against sodium, the CoNi2S4/S assembly showed remarkable cyclability both as a function of current density (at room temperature) and temperature (at constant current density). The versatility of CoNi2S4 is further proven by the exemplary cyclability at various current densities at room temperature against lithium.