Achieving Ultrahigh Capacity with Self-Assembled Ni(OH) ₂ Nanosheet-Decorated Hierarchical Flower-like MnCo ₂ O _4.5 Nanoneedles as Advanced Electrodes of Battery-Supercapacitor Hybrid Devices

Self-assembled Ni(OH) ₂ nanosheet-decorated hierarchical flower-like MnCo ₂ O _4.5 nanoneedles were synthesized via a cost-effective and facile hydrothermal strategy, aiming to realize a high-capacity advanced electrode of a battery-supercapacitor hybrid (BSH) device. It is demonstrated that the as-synthesized hierarchical flower-like MnCo ₂ O _4.5 @Ni(OH) ₂ -nanosheet electrode exhibits a high specific capacity of 318 mAh g ^-1 at a current density of 3 A g ^-1 and still maintains a capacity of 263.5 mAh g ^-1 at a higher current density of 20 A g ^-1 , with an extremely long cycle lifespan of 87.7% capacity retention after 5000 cycles. Moreover, using the unique core-shell structure as the cathode and hollow Fe ₂ O ₃ nanoparticles/reduced graphene oxide as the anode, the BSH device delivers a high energy density of 56.53 Wh kg ^-1 when the power density reaches 1.9 kW kg ^-1 , and there is an extraordinarily good cycling stability with the capacity retention rate of 90.4% after 3000 cycles. It is believed that the superior properties originate from desirable core-shell structures alleviating the impact of volume changes as well as the existence of two-dimensional Ni(OH) ₂ nanosheets with more active sites, thereby improving the cycle stability and achieving ultrahigh capacity. These results will provide more access to the rational material design of diverse nanostructures toward high-performance energy storage devices.