Sandwich‐Shell Structured CoMn₂O₄/C Hollow Nanospheres for Performance‐Enhanced Sodium‐Ion Hybrid Supercapacitor

Abstract Sodium hybrid supercapacitors (Na‐HSCs) are regarded as one promising electrochemical energy storage device, because of the low price of sodium, prolonged life cycle, and high‐energy/power density. Nonetheless, the imparity between the fast capacitive reactions at cathode and the sluggish Faradaic reactions at the anode leads to an imbalance in the electrochemical reaction kinetics, limiting the development of Na‐HSCs. Therefore, it is urgent to develop suitable anode materials for performance‐enhanced Na‐HSCs. Herein, sandwich‐shell‐structured CoMn₂O₄/C hollow spheres are synthesized by a facile hydrothermal reaction and subsequent calcination, where mesoporous carbon hollow spheres (CHSs) serve as nonsacrificial hard templates. CHSs with numerous mesoporous channels are beneficial for the penetration of reactant ions. Therefore, CoMn₂O₄ nanosheets are successfully deposited on the inner and outer surfaces of CHSs, generating sandwich‐shell‐structured CoMn₂O₄/C hollow spheres. Benefiting from the unique design, CoMn₂O₄/C HSs exhibit excellent sodium storage performance, including a high‐specific capacity of 290 mAh g^–1 at 0.1 A g^–1 and prolonged cycling durability. A Na‐HSC assembled by CoMn₂O₄/C HSs anode and activated carbon cathode exhibits a high‐energy density (265 Wh kg^–1) and a wide‐operating voltage range (0.01–4.0 V).