Genuine Unilamellar Metal Oxide Nanosheets Confined in a Superlattice-like Structure for Superior Energy Storage.

Two-dimensional (2D) metal oxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Especially, genuine unilamellar nanosheets with an atomic-scale thickness are expected to exhibit the ultimate energy storage capability but have not yet achieved their potential. Here, we demonstrate the utilization of genuine unilamellar MnO2 nanosheets for high-performance Li and Na storage using an alternately stacked MnO2/graphene superlattice-like structure. Different from previous reports, all unilamellar MnO2 nanosheets are separated and stabilized between the graphene monolayers, resulting in highly reversible 2D-confined conversion processes. As a consequence, large specific capacities of 1325 and 795 mA h g-1 at 0.1 A g-1, high-rate capacities of 370 and 245 mA h g-1 at 12.8 A g-1, and excellent cycling stabilities after 5000 cycles with ∼0.004% and 0.0078% capacity decay per cycle were obtained for Li and Na storage, respectively, presenting the best reported performance to date.