A novel molybdenum nitride composite based on polyoxometalates as anode for Li-ion batteries

Abstract

With the increasing energy crisis and environmental pollution, the development of energy storage devices has attracted widespread attention. Lithium-ion batteries (LIBs), as new energy storage devices, have made great progress in the field of portable electronic products and electric vehicles. However, traditional batteries electrode materials are difficult to meet peoples increasing demand for energy density and power density. Therefore, it is essential to find higher performance electrode materials for LIBs. Polyoxometalates (POMs) have already demonstrated great promise for LIBs owing to their electron storage, chemical structure and reversible redox. However, the application of POMs in LIBs is hindered for the poor electronic conductivity and its dissolution in electrolyte. Consequently, taking full advantage of POMs and solving the immobilization of POMs is of great significance for the research of POMs in the application of LIBs. Phosphomolybdate (H3PMo12O40· n H2O, PMo12) is a typical Keggin-type POMs, containing abundant molybdenum source, and has excellent reversible redox. To improve the conductivity of the materials, graphene oxide (GO) with good chemical stability and electrical conductivity is an ideal substrate. However, both GO and PMo12 are negatively charged, the electrostatic between them makes it difficult to conjugate preferably. Considering the polymerization conditions of conductive polymer, pyrrole (Py) can be chosen to bridge PMo12 and GO, then realizing the anchor of PMo12 on GO. Inspired by these, in the present work, by taking full advantage of good solubility, unique acidity and reversible redox of PMo12, the polyoxometalate/conductive polymer/graphene oxide (PMo12/PPy/RGO, PCG) precursor was prepared via a simple one-pot method under milder condition, which realizes the in-suit polymerization of Py and uniform loading of PMo12 on GO. Then a novel molybdenum nitride composite (NPC@Mo2N/NPRGO) was obtained by direct ammoniation of PMo12/PPy/RGO precursors. As anode materials for LIBs, NPC@Mo2N/NPRGO exhibits a good cycling reversibility and achieves a high capacity, which can deliver a discharge capacity of 1446 mAh/g at 100 mA/g at the first cycle. And a reversible capacity of 771 mAh/g is maintained after 200 cycles. In addition, at the current density of 100, 200, 500, 1000, 2000 mA/g, the reversible capacities are about 797, 725, 630, 545, 460 mAh/g, respectively. With the decrease of current density back to 100 mA/g, the capacity can almost resume to the original values, demonstrating the good reversibility and excellent rate capability of the composite. The excellent performance of NPC@Mo2N/NPRGO toward LIBs could be attributed to the synergistic effects of each component in the composite. The good solubility of PMo12 ensures the uniform distribution of molybdenum nitride on the surface of graphene substrate, so that the material does not aggregate and pulverize during the process of charging and discharging, which improves the electrochemical performance and stability of the composite. This work prepared a novel two-dimensional nanocomposite by simple method, which is not only cheap in raw materials, simple in preparation, but also has high yield. It is convenient both for materials characterization and performance research. The design could provide ideas for the application of POMs in LIBs and the preparation of molybdenum nitride-based composites for other energy storage technology.