Two-Dimensional Unilamellar Cation-Deficient Metal Oxide Nanosheet Superlattices for High-Rate Sodium Ion Energy Storage

Cation-deficient two-dimensional (2D) materials, especially atomically thin nanosheets, are highly promising electrode materials for electrochemical energy storage that undergo metal ion insertion reactions, yet they have rarely been achieved thus far. Here, we report a Ti-deficient 2D unilamellar lepidocrocite-type titanium oxide (Ti_0.87O₂) nanosheet superlattice for sodium storage. The superlattice composed of alternately restacked defective Ti_0.87O₂ and nitrogen-doped graphene monolayers exhibits an outstanding capacity of ∼490 mA h g^-1 at 0.1 A g^-1, an ultralong cycle life of more than 10000 cycles with ∼0.00058% capacity decay per cycle, and especially superior low-temperature performance (100 mA h g^-1 at 12.8 A g^-1 and -5 °C), presenting the best reported performance to date. A reversible Na⁺ ion intercalation mechanism without phase and structural change is verified by first-principles calculations and kinetics analysis. These results herald a promising strategy to utilize defective 2D materials for advanced energy storage applications.