Ternary Cross-Vanadium Tetra-Capped POMOFs@PPy/RGO Nanocomposites with Hybrid Battery-Supercapacitor Behavior for Enhancing Lithium Battery Storage

Abstract

To improve the electrochemical performances and to understand the mechanism of lithium storage of polyoxometalate-based metal–organic frameworks (POMOFs), a generic strategy to construct nanocomposites based on POMOFs and reduced graphene oxide (RGO) assisted by polypyrrole (PPy) as an adhesive, and a conductive additive is demonstrated for the first time in this work. Here, two fascinating vanadium-capped POMs-encapsulating MOFs, [Cu12(Trz)8Cl][PMo12O42V2] (Trz = 1,2,4 triazole) (POMOF-1, and -2), were explored in detail. Although POMOF-1 and -2 have different cell parameters and morphologies caused by the different stacking directions, both are the same compound. Using PPy as an adhesive, POMOF-2 was successfully anchored on RGO, resulting in a new type of POMOF@PPy/RGO (PPG-n, n = 1, 2, 3) nanocomposites. The resulting PPG-3 nanocomposite exhibits excellent reversible capacity (985 mA h g–1 at 50 mA g–1 after 100 cycles) and outstanding rate capabilities, which is much better than those of most nanocomposite electrodes based on POMs. Meanwhile, the lithium storage mechanism of PPG-3 has been discussed by analyzing X-ray photoelectron spectroscopy and powder X-ray diffraction patterns. Moreover, the PPG-3 nanocomposite exhibits the hybrid supercapacitor-battery behavior confirmed by the analysis results of cyclic voltammetry and electrochemical impedance spectroscopy. This might open a new avenue for designing other multifunctional POMOF-based nanocomposites as high-performance electrode materials for lithium storage.