Ultrathin double-shell nanotubes of narrow band gap titanium oxide@carbon as efficient polysulfide inhibitors towards advanced lithium–sulfur batteries

Abstract

The low conductivity, shuttling phenomenon, and sluggish transformation of intermediate lithium polysulfides (LiPSs) constitute primary obstacles for the practical application of lithium–sulfur (Li–S) batteries. Ultrathin double-shell nanotubes of titanium oxide with oxygen-vacancy@nitrogen-doped carbon (denoted as OV-TiO_2−x@NC) as the sulfur host were designed. As demonstrated by experimental and computational results, an ultrathin TiO_2−x shell featuring a small aspect ratio can not only provide rapid charge transfer but also shorten the diffusion pathway of lithium ions. Furthermore, the special double-shell nanostructure with the large surface area can accommodate high number of sulfur species, provide effective channels for the electrolyte penetration, supply adequate buffer space for alleviating the volume expansion of sulfur upon repeated cycling, and provide both physical and chemical adsorption to efficiently capture LiPSs. The OV-TiO_2−x inner shell with a small aspect ratio is compactly packed and tightly adhered to the carbon layer, which can effectively anchor LiPSs with strong chemical affinity and prevent their diffusion from the inner to the outside layer. More importantly, the introduction of oxygen-vacancies can enhance the conductivity of electrode, restrict the loss of LiPSs into the electrolyte, strengthen electrocatalytic activity, and accelerate LiPSs conversion. Endowed with these properties, the OV-TiO_2−x@NC/S electrode enables excellent electrochemical performance, including long cycle stability of 531 mA h g⁻¹ for 3000 cycles at 5C with decay rate of only 0.0123%, excellent rate capability of 675 mA h g⁻¹ at 8C and high areal capacity of 8.01 mA h cm⁻² with sulfur mass loading of 9.5 mg cm⁻² under low electrolyte/sulfur (E/S = 5). This work provides perspective on altering the electronic states of OV–metal oxides toward high energy density Li–S batteries and promotes the progress of catalysis or energy storage systems.