Rb-Doped VO2 Enhanced Structural Stability for Aqueous Calcium-Ion Batteries

Vanadium oxide exhibits significant potential for large-scale energy storage for aqueous calcium-ion batteries (ACIBs) due to its high theoretical specific capacity and multiple valence states. However, vanadium dissolution is often caused by lattice distortion and structural collapse during Ca2+ insertion/extraction. Herein, a Rb-doped VO2 (RbVO) material was synthesized via a facile hydrothermal method, which achieved a high cycling stability in ACIBs. Experimental and theoretical calculations indicated that Rb selectively occupied the interstitial sites of the VO2 lattice, significantly expanded the lattice, and induced a high concentration of oxygen vacancies. The unique defect-associated structure suppresses vanadium dissolution by strengthening the V-O covalent network and thus enhances structural stability. Importantly, Rb doping improved Ca2+ diffusion kinetics by reducing the diffusion energy barrier from 2.74 to 1.95 eV. As expected, the RbVO cathode delivered a reversible specific capacity of 186.28 mA h g-1 at 0.1 A g-1 and achieved a capacity retention of 99.69% after 800 cycles at 2.0 A g-1. The experimental results combined with spectroscopy characterization elucidated that the RbVO cathode experienced reversible changes in lattice parameters and the stabilization of vanadium valence states during the Ca2+ insertion/extraction process. This study offers a comprehensive understanding of the fabrication of high-stability vanadium-based cathode materials for ACIBs.