14-electron reduced Mo IV6-ε-Keggin polyoxometalates: highly stable and reversible electron/Li+ sponge materials

Abstract

First, 14-electron reduced Mo^IV₆-ε-Keggin polyoxometalates (POMs), namely14e-[Sb₂Mo^IV₆Mo^V₂Mo^VI₄O₃₂(OH)₂py₆]²⁻ (1), 14e-[(GeO)₂Mo^IV₆Mo^V₂Mo^VI₄O₃₄(OH)₂py₆]⁴⁻ (2), and 14e-[Sb₂ZnMo^IV₆Mo^V₂Mo^VI₄O₃₄py₇] (3) (py = pyridine), were prepared. X-ray structural analyses of 1–3 revealed two triangularly metal–metal Δ-bonded 6e-[Mo^IV₃O₄] incomplete cubane-type units and one 2e-[Mo^V₂O₄], which account for the hollow ε-Keggin structures free of central heteroatoms. Long-range π-stacking interactions and super electron-rich Keggin structures of 14e-1 and 14e-2 lead to their 260 times higher electrical conductivity than conventional fully oxidized Mo^VI-Keggin POMs. The hollow Mo^IV₆-ε-Keggin POM 1 exhibited structural integrity retained during 24-electron charging/discharging processes when being simultaneously monitored by ex situ XPS and IR. A dynamic study of Li-ion migration in 1 revealed its dominant capacitor-like Li⁺-storage mechanism and effective/fast absorption/desorption of Li⁺. Therefore, it exhibited a high discharge specific capacity (303 mA h g⁻¹, 50 mA g⁻¹), superior rate and cycling performance, especially at an extremely high current density (121.6 mA h g⁻¹, 100 cycles, 8.0 A g⁻¹), which remarkably enhances the performance of conventional fully oxidized Mo^VI-Keggin cathode materials, and provides a new option for using super electron-rich, hollow POMs to improve the electrochemical performance of POM electrode materials.