Sodium‐Selenium Batteries with Outstanding Rate Capability by Introducing Cubic Mn2O3 Electrocatalyst**

Abstract With their high volumetric capacity and electronic conductivity, sodium‐selenium (Na−Se) batteries have attracted attention for advanced battery systems. However, the irreversible deposition of sodium selenide (Na 2 Se) results in rapid capacity degradation and poor Coulombic efficiency. To address these issues, cubic α‐Mn 2 O 3 is introduced herein as an electrocatalyst to effectively catalyze Na 2 Se conversion and improve the utilization of active materials. The results show that the addition of 10 wt% Mn 2 O 3 in the selenium/Ketjen black (Se/KB) composite enhances the conversion from Na 2 Se to Se by lowering activation energy barrier and leads to fast sodium‐ion kinetics and low internal resistance. Consequently, the Mn 2 O 3 ‐based composite delivers a high specific capacity of 635 mAh ⋅ g −1 at 675 mA ⋅ g −1 after 250 cycles as well as excellent cycling stability for 800 cycles with a high specific capacity of 317 mAh ⋅ g −1 even at the high current density of 3375 mA ⋅ g −1 . Due to the cubic Mn 2 O 3 electrocatalyst, the performance of the composites is superior to existing state‐of‐the‐art Na−Se batteries reported in the literature.