Vanadium-Based Polyoxometalate As Anode Materials for Lithium–Ion Capacitors

Abstract

Because of the increasing demand for portable devices, renewable energy systems, and electric vehicles, energy storage system has been developed intensively over the past decade. Researchers have intensively studied low-cost, high energy density, high power density, and high-safety energy storage devices. Lithium-ion batteries (LIBs) have advantages of high energy density and no memory effect. However, the power density is relatively low. In contrast to LIBs, supercapacitors (SCs) exhibit high power density and long cycling life, but low energy density. Therefore, lithium ion capacitors (LICs) which combine the superiorities of LIBs and SCs has been announced recently in order to achieve both high energy density and power density. Polyoxometalates (POMs) are a class of anionic polynuclear transition metal oxides, and they possess multiple redox reactions which can store many charges per molecule. Hence, POMs have been employed as electrode materials for energy storage devices, such as LIBs, sodium-ion batteries (NIBs), and SCs, and most of them exhibit high capacity. In this work, a vanadium-based POM (V-POM) was utilized as anode material for LICs. V-POM exhibited high specific capacities as well as good cycling stability. A full cell comprised with V-POM anode and porous carbon cathode were assembled, and it showed high energy density and high power density. These results demonstrated that V-POM anode material is suitable for LIC applications. Furthermore, we also utilized several in operando techniques to investigate the charge storage mechanisms of this V-POM anode including X-ray Absorption Spectroscopy and X-ray diffraction.