Reversible lithium charge–discharge property of bi-capped Keggin-type polyoxovanadates

Abstract

As a new molecular cluster cathode material for lithium batteries, K5.72H3.28[PV14O42] (KPV), a hetero polyoxovanadate with a bi-capped Keggin structure, was synthesized. Its crystal structure was refined by X-ray Rietveld analysis, and its electrochemical properties were examined. The symmetry space group of KPV was assigned to FM3-M. The [PV14O42]9− bi-capped Keggin units are connected sharing K+ ions. KPV contains many cation site vacancies and has tunnels facing various directions. The polycrystalline KPV powder becomes amorphous when dried at 80 °C, while the molecular cluster structure of KPV, identified by Raman scattering method, is maintained.KPV showed higher discharge capacity with higher discharge voltage than K3[PMo12O40] (KPM) with Keggin-type structure. The most improved property of KPV compared with KPM was cycle stability. KPV kept 95% of its initial discharge capacity after 50 cycles of discharge–charge, whereas KPM kept only 35%. The presence of bi-capped Keggin unit is confirmed by ex situ EXAFS measurement, even though a rapid amorphization of KPV was observed at the beginning of the discharge–charge process in ex situ X-ray diffraction (XRD) measurements. The results of ex situ EXAFS measurement and the reversible charge transfer resistance of KPV estimated from the AC-impedance measurement during the discharge–charge process also indicate that the cycle stability of KPV is attributable to the stability of the KPV cluster ion unit.