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

First, 14-electron reduced Mo-6(IV)-epsilon-Keggin polyoxometalates (POMs), namely14e-[(Sb2Mo6Mo2Mo4O32)-Mo-IV-Mo-V-O-VI(OH)(2)py(6)](2-) (1), 14e-[(GeO)(2)(Mo6Mo2Mo4O34)-Mo-IV-Mo-V-O-VI(OH)(2)py(6)](4-) (2), and 14e-[Sb(2)ZnMo(6)(IV)Mo(2)(V)Mo(4)(VI)O(34)py(7)] (3) (py = pyridine), were prepared. X-ray structural analyses of 1-3 revealed two triangularly metal-metal Delta-bonded 6e-[(Mo3O4)-O-IV] incomplete cubane-type units and one 2e-[(Mo2O4)-O-V], which account for the hollow epsilon-Keggin structures free of central heteroatoms. Long-range pi-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-6(IV)-epsilon-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.