Mass Transfer of Divalent Ions in an Oxide Host: Comparison of Mg2+ and Zn2+ Diffusion in Hexagonal KxW3O9 Bronze

Pure and isovalent cation-substituted potassium hexagonal tungsten bronze (KHTB) have been synthesized and used as a model oxide host to study the electrochemically driven bulk diffusion of divalent Mg2+ and Zn2+ ions. For the cation substitution, tungsten has been replaced by 10% Mo or 5% Cr as a strategy to mitigate the slow diffusivities of the divalent ions. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) studies reveal the superior kinetics of Zn2+ insertion compared to that of Mg2+, majorly due to the sluggish desolvation/electro-adsorption of Mg species at the electrode-electrolyte interface in an organohaloaluminate-based electrolyte. For both divalent cations, the insertion/extraction kinetics exhibits a profound improvement with the isovalent cation-substituted KHTB phases due to the synergistic effect of particle size reduction and enhanced ionic diffusivities. Hence, maximum insertion amounts of 0.18 and 0.20 mol, respectively, for Mg2+ and Zn2+ per (W1-xMx)(3)O-9 formula unit (M = Mo or Cr) have been achieved within the electrolyte stability window, whereas the unsubstituted KHTB is essentially inactive. Further EIS studies in symmetric cells indicated the inherently faster kinetics of Zn2+ diffusion compared to that of Mg2+ in identical hosts. Calculations based on density functional theory (DFT) were implemented to probe the local atomic environment of the diffusing ions. The DFT study suggests that the Zn-ion transition state is stabilized by similar to 0.4 eV compared to that of Mg2+ through a host-to-guest charge transfer, a consequence of the tendency of Zn2+ ions to form covalent bonds compared to Mg2+ with the oxide anion.