MOF-Derived\nNitrogen-Rich Hollow Nanocages as a Sulfur\nCarrier for High-Voltage Aluminum Sulfur Batteries

Aluminum–sulfur batteries\n(ASBs) are emerging as promising\nenergy storage systems due to their safety, low cost, and high theoretical\ncapacity. However, it remains a challenge to overcome voltage hysteresis\nand short cycle life in the sulfur/Al<sub>2</sub>S<sub>3</sub> conversion\nreaction, which hinders the development of ASBs. Here, we studied\na high-voltage ASB system based on sulfur oxidation in an AlCl<sub>3</sub>/urea electrolyte. Nitrogen-doped hollow nanocages (HNCs)\nsynthesized from MOF precursors were rationally designed as sulfur/carbon\ncomposite electrodes (S@HNC), and the impact of the nitrogen species\non the electrochemical performance of sulfur electrodes was systematically\ninvestigated. The S@HNC-900 achieved efficient conversion at 1.9 V,\ndelivering a stable capacity of 197.3 mA h g<sup>–1</sup> and\na Coulombic efficiency of 93.28% after 100 cycles. Furthermore, the\nS@HNC-900 electrode exhibited exceptional rate capacity and 800th\nlong-term cycling stability, retaining a capacity of 87.1 mA h g<sup>–1</sup> at 500 mA g<sup>–1</sup>. Ex situ XPS and\nXRD characterizations elucidated the redox mechanism, revealing a\nfour-electron transfer process (S/AlSCl<sub>7</sub>) at the S@HNC-900\nelectrode. Density functional theory calculations demonstrated that\npyridinic nitrogen-enriched HNC-900 significantly enhanced the sulfur\nconversion reaction and facilitated the adsorption of sulfur intermediates\n(SCl<sub>3</sub><sup>+</sup>) on the carbon interface. This work provides\ncritical insights into the high-voltage sulfur redox mechanism and\nestablishes a foundation for the rational design of carbon-based electrocatalysts\nfor the enhancement of ASB performance.