MBene with Redox‐Active Terminal Groups for an Energy‐Dense Cascade Aqueous Battery

Abstract Two‐dimensional (2D) transition metal borides (MBenes), new members of the 2D materials family, hold great promise for use in the electrocatalytic and energy storage fields because of their high specific area, high chemical activity, and fast charge carrier mobility. Although various types of MBenes are reported, layered MBenes featuring redox‐active terminal groups for high energy output are not yet produced. A facile and energy‐efficient method for synthesizing MBenes equipped with redox‐active terminal groups for cascade Zn||I₂ batteries is presented. Layered MBenes have ordered metal vacancies and ─Br terminal groups, enabling the sequential reactions of I⁻/I⁰ and Br⁻/Br⁰. The I₂‐hosting MBene‐Br cathode results in a specific energy as high as 485.8 Wh kg⁻¹ at 899.7 W kg⁻¹ and a specific power as high as 6007.7 W kg⁻¹ at 180.2 Wh kg⁻¹, far exceeding the best records for Zn||I₂ batteries. The results of this study demonstrate that the challenges of MBene synthesis can be overcome and reveal an efficient path for producing high‐performance redox‐active electrode materials for energy‐dense cascade aqueous batteries.