Ultra-rapid microwave-assisted synthesis of layered ultrathin birnessite K0.17MnO2 nanosheets for efficient energy storage

Inorganic graphene analogues (IGAs) are currently in the spotlight of nanotechnology with the aim of achieving superior energy storage performance. However, cumbersome, expensive and time-consuming synthetic routes have definitely hindered further study of these species, thus the development of facile, low-cost and ultra-rapid synthetic approaches to these species is urgently needed and has met with limited success so far. Herein, we put forward an ultra-rapid, low-cost and facile microwave-assisted strategy to achieve this goal without exfoliation for the first time. This protocol relies on microwave dielectric heating, a reducing reagent and reactive medium, which will efficiently lower the Gibbs activation energy and polarize the conducting electrons. As an example, (001)-oriented ultrathin birnessite K0.17MnO2 nanosheets with a thickness of only 2 nm were successfully synthesized within 5 min, much faster than presently known routes. Notably, this novel route can also be extended to the synthesis of ultrathin Na-type birnessite nanosheets, revealing the universality of this synthetic strategy by extending to those layered compounds. The layered ultrathin birnessite K0.17MnO2 nanosheet-based electrode exhibits remarkably improved electrochemical characteristics compared with its bulk counterpart, showing a high specific capacitance of 206 F g(-1) at 1 A g(-1) and an excellent cycling performance at a large current density of 5 A g(-1) (>93% retention over 1000 cycles). Even in lithium ion battery cathodes, the reversible capacity of 167.4 mA h g(-1) is still retained with a negligible capacity loss per cycle (0.25%), which is superior to most reported birnessite nanostructures, suggesting a remarkably promising candidate for energy storage. Such intriguing behavior is mainly attributed to the intrinsic crystal structure and the synergistic effect of IGAs, such as huge surface area, facile guest ion diffusion and electron transport. This work opens the door for the ultra-rapid and facile preparation of ultrathin nanosheets, which will significantly expand the studies of IGAs and optimize energy storage through rational materials design and synthesis.