MOF-Derived Nitrogen-Rich Hollow Nanocages as a Sulfur Carrier for High-Voltage Aluminum Sulfur Batteries

Aluminum-sulfur batteries (ASBs) are emerging as promising energy storage systems due to their safety, low cost, and high theoretical capacity. However, it remains a challenge to overcome voltage hysteresis and short cycle life in the sulfur/Al₂S₃ conversion reaction, which hinders the development of ASBs. Here, we studied a high-voltage ASB system based on sulfur oxidation in an AlCl₃/urea electrolyte. Nitrogen-doped hollow nanocages (HNCs) synthesized from MOF precursors were rationally designed as sulfur/carbon composite electrodes (S@HNC), and the impact of the nitrogen species on the electrochemical performance of sulfur electrodes was systematically investigated. The S@HNC-900 achieved efficient conversion at 1.9 V, delivering a stable capacity of 197.3 mA h g^-1 and a Coulombic efficiency of 93.28% after 100 cycles. Furthermore, the S@HNC-900 electrode exhibited exceptional rate capacity and 800th long-term cycling stability, retaining a capacity of 87.1 mA h g^-1 at 500 mA g^-1. Ex situ XPS and XRD characterizations elucidated the redox mechanism, revealing a four-electron transfer process (S/AlSCl₇) at the S@HNC-900 electrode. Density functional theory calculations demonstrated that pyridinic nitrogen-enriched HNC-900 significantly enhanced the sulfur conversion reaction and facilitated the adsorption of sulfur intermediates (SCl₃⁺) on the carbon interface. This work provides critical insights into the high-voltage sulfur redox mechanism and establishes a foundation for the rational design of carbon-based electrocatalysts for the enhancement of ASB performance.