Nb 2 CT x MXene Derived Polymorphic Nb 2 O 5 .

Previously, heat treatment was the only feasible route for tuning the crystal phases of niobium pentoxide (Nb 2 O 5 ). With the use of Nb 2 CT x MXene precursors, the first case of phase tuning of Nb 2 O 5 in the low-temperature hydrothermal synthesis using sulfuric acid regulating agents is presented. By varying the amount of the agent, four pure-phase Nb 2 O 5 crystals and mixed phases in-between are obtained. The required amount is found to be related to the H-covered surface energy calculated based on density functional theory. Overall, MXene-derived B-phase Nb 2 O 5 is of particular interest due to its exceptionally high capacities as lithium-ion battery anodes, which are three times higher than the routine synthesized one. Oxygen vacancies induced by crystallographic shear would be responsible for the extraordinary performance. The proposed phase tuning strategy encourages the prudent synthesis of difficult-to-obtain crystal phases.