Low-temperature synthesis, crystal structure, and ionic mobility in KTiOPO4-structured RbVOPO4

In this paper, we report on a new polymorph of rubidium vanadium oxophosphate (alpha-RbVOPO4) adopting a KTiOPO4-type structure, which was stabilized through an ion-exchange reaction between a hydrothermally prepared NH4VOPO4 precursor and Rb2CO3<middle dot>1.5H(2)O. The crystal structure of alpha-RbVOPO4 was refined based on X-ray powder diffraction data and verified by infrared spectroscopy, selected area electron diffraction, and high-resolution scanning transmission electron microscopy. The material was examined as a host for reversible (de)insertion of Rb+ ions using bond valence force field and density functional theory calculations disclosing a one-dimensional diffusion network with an activation barrier of similar to 0.2 eV. The electrochemical activity of RbVOPO4 was studied experimentally in two-electrode cells with Li, Na, and K metal counter electrodes. Electronic structure calculations confirmed vanadium redox during Rb+ (de)insertion. The alpha-RbVOPO4 phase was compared with the known RbVOPO4 polymorphic modification, beta-RbVOPO4, demonstrating no ionic mobility. The crystal chemistry of all known compounds in the Rb-V-P-O system is reviewed. This work supplements the existing toolkit on synthesizing the KTiOPO4-structured electrode materials for metal-ion batteries and discusses their crystal structure features.