Single-Crystal-to-Single-Crystal Transformation in a Thermally Stable All-Inorganic Polyoxoniobate Framework Boosts...

Niobium oxides are considered as promising anode materials for lithium-ion batteries (LIBs) due to their excellent rate-performance. However, the practical application is hindered by their limited specific capacity. In this work, we report the first example of an all-inorganic two-dimensional (2D) niobate framework as anode material for LIBs. The title compound is based on antimony-linked bivanadyl-capped α-Keggin polyoxoniobates as secondary building units. The compound undergoes a unique single-crystal-to-single-crystal (SCSC) transformation triggered by formic acid which results in the migration of a {VO} unit into the framework interlayer. This results in a 34% increase of the specific capacity, reaching 519 mAh g^-1 at 0.1 A g^-1, thereby surpassing most Nb-based LIB anode materials. Experimental and theoretical calculations reveal that the SCSC transformation exposes more Li-binding sites in the framework, and reduces the interlayer Li-ion diffusion barrier, leading to a capacity increase. This work presents the first example of a SCSC transformation leading to enhanced LIB performance and offers atomic-level insights into the design of advanced LIB anode materials.