Internal−External Carbon Nanotube Network-Reinforced Ultrathin Birnessite-Type MnO2 Electrodes with Improved Mass Loading and Areal Energy Density

Abstract

High-mass-loading electrodes are pivotal to the economic feasibility and practical application of energy storage devices. In this work, strip-shaped bath flower-like birnessite nanostructures (SBFB) grown on carbon nanotubes (CNTs) with ultrathin birnessite films (∼3 nm) were synthesized via a low-temperature and alkalescent approach, which effectively reduced the growth rate of birnessite, thereby minimizing the electrochemical dead mass content. By constructing internal−external carbon nanotube (CNT) networks, SBFB@CNTs/CNT composites were fabricated to facilitate the electron transport, thereby achieving a high proportion of surface capacitive contribution. Hence, an optimal mass loading of up to 27.3 mg cm−2 was achieved with the areal capacitance as high as 6.5 F cm−2, attributed to the supported SBFB@CNT structures that enable efficient ion diffusion. On this basis, an asymmetric supercapacitor, assembled by SBFB@CNT/CNT composites (positive electrode) and hierarchical porous carbon composites (HPC/CNTs, negative electrode), delivered a high areal energy density of 1.37 mWh cm−2 and a notable volumetric energy density of 10.51 mWh cm−3. Moreover, after 10,000 cycles, approximately 90% of the original capacitance was maintained, exhibiting promising cycling stability that approaches the preliminary durability expectations for commercial energy storage applications.