Decavanadate-Type Polyoxometalate Anions Encapsulated in a MIL-100 Framework with Enhanced Mixed Ion–Electron Conduction and Potential Application as Cathode Materials for a Lithium Ion Battery

Polyoxometalates (POMs) with reversible multielectron redox are called “electron sponges”, showing promising application as electrode materials for LIBs. In this study, Na2.0(NH4)3.05[V9.05Mo0.95O28]·10H2O (NaMoV), a decavanadate-type POM, was successfully encapsulated into the pores of an Fe-based MOF (metal organic framework), MIL-100 (Fe), and a series of NaMoV@MIL-100-x (x = 1.3, 2, 3, 4, which is the feed mass ratio of NaMoV to MIL-100) were prepared and characterized using IR spectroscopy, PXRD, and N2 adsorption–desorption techniques. The maximum amount of NaMoV inserted in the pores of MIL-100 was achieved as x = 3, and this NaMoV@MIL-100-3 composite shows the intrinsic mixed ion–electron conduction and high stability to organic electrolyte in LIBs. NaMoV@MIL-100-3 was evaluated as the cathode material for LIBs, and it demonstrated high discharge capacity (210.7 mA h g–1 at 50 mA g–1) and excellent cycling stability (capacity retention of 84% after 50 cycles at 50 mA g–1). To the best of our knowledge, this is the first time a POM@MOF has demonstrated intrinsic mixed ion–electron conduction and been demonstrated as a cathode material for LIBs. This study demonstrates that, by rational design, POM@MOF materials not only become new types of potential mixed ion–electron conductors but also may serve as cathode materials with high capacity and good cycling stability for LIBs.