Boosting the high rate and durability of lithium–sulfur batteries using a bidirectional catalyst of a polyoxometalate-cyclodextrin supramolecular compound

Abstract

In this work, a supramolecular compound {NaCo(H₂O)₆H₆(PW₁₂O₄₀)₃(C₄₈H₈₀O₄₀)₃(H₂O)₆₀} (PW₁₂-CD) prepared using Keggin-type H₃PW₁₂O₄₀ (PW₁₂) and cyclodextrin (CD) as precursors is utilized as a modified material of the separator to overcome the drawbacks of lithium–sulfur batteries. The introduction of CD can limit the solubility of PW₁₂ in the electrolyte. Moreover, PW₁₂ and CD synergetically capture polysulfides, in which the Li cations of LiPSs are respectively fixed by the oxygen atoms from the {WO₆} octahedra of the PW₁₂ cluster and the hydroxyl group from CD. The catalytic efficacy of the modified material is closely related to the chemical adsorptive capacity of polysulfides. In situ Raman and ex situ XPS tests indicate that PW₁₂ can bidirectionally catalyze the conversion of lithium polysulfides, and it undergoes a reversible transformation between the reduced state and oxidized state during the charging/discharging process. Therefore, the cells with PW₁₂-CD modified separators show favorable reversible capacities of 732.5 mA h g⁻¹ and 408.6 mA h g⁻¹ after 1000 cycles under 2 C and 10 C, with low average fading rates of 0.023% and 0.042%, respectively. Even under a high sulfur loading of 10.3 mg cm⁻² and a lean electrolyte of 3.7 μL mg⁻¹, an areal capacity of 6.0 mA h cm⁻² can be obtained after 120 cycles at 0.2 C.