Incorporation of layered tin (IV) phosphate in graphene framework for high performance lithium-sulfur batteries

Abstract

To anchor the polysulfide and enhance the conversion kinetics of polysulfide to disulfide / sulfide is critical for improving the performance of lithium-sulfur battery. For this purpose, the graphene-supported tin (IV) phosphate (Sn(HPO4)2·H2O, SnP) composites (SnP-G) are employed as the novel sulfur hosts in this work. When compared to the graphene-sulfur and carbon–sulfur composites, the SnP-G-sulfur composites exhibit much better cycling performance at 1.0C over 800cycles. Meanwhile, the pouch cell fabricated with the SnP-G-sulfur cathodes also exhibits excellent performance with an initial capacity of 1266.6 mAh g−1 (S) and capacity retention of 76.9% after 100cycles at 0.1C. The adsorption tests, density functional theory (DFT) calculations in combination with physical characterizations and electrochemical measurements provide insights into the mechanism of capture-accelerated conversion mechanism of polysulfide at the surface of SnP. DFT calculations indicate that the Li-O bond formed between Li atom (from Li2Sn, n = 1, 2, 4, 6, 8) and O atom (from PO3-OH in SnP) is the main reason for the strong interactions between Li2Sn and SnP. As a result, SnP can effectively restrain the shuttle effect and improving the cycling performance of Li-S cell. In addition, by employing the climbing-image nudged elastic band (ci-NEB) methods, the energy barrier for lithium sulfide decomposition (charging reaction) on SnP is proved to decrease significantly compared to that on graphene. It can be concluded that SnP is an effective sulfur hosts acting as dual-functional accelerators for the conversion reactions of polysulfide to sulfide (discharging reaction) as well as polysulfide to sulfur (charging reaction).