(A) study on the electrochemical energy storage properties of hybrid devices with high capacity and cycle stability using Redox active nanomaterials

Abstract

Human beings have progressively developed technology since the industrial revolution, and the demand for energy has soared every year. On the other hand, since most of the energy produced is lost in the process of transmission, it requires the development of an energy storage system along with the effective use of electric energy. In addition, energy storage devices have become an essential part of everyday life from small electronic devices such as mobile phones and notebooks to electric vehicles, and it is necessary to develop high-performance energy storage devices to increase the utilization of large-sized electronic devices. Lithium ion battery and supercapacitor are typical energy storage devices currently used in real life. Lithium ion battery is energy storage device with high energy density (100 to 250 Wh $kg^{-1}$), but it still suffer from low power density and long charging times over few hours, as well as the risk of battery explosion and toxic electrolyte. Supercapacitor is eco-friendly due to the use of aqueous electrolytes with an ion conductivity ($10^{-2}$ to $10^{-3}$ S $cm^{-2}$) and have high power density ( > 10,000 W $kg^{-1}$), it has low energy density ( < 100 Wh $kg^{-1}$) with low operation voltage and low capacitance and is used as an auxiliary role for other energy storage devices. Although the energy storage devices are variously used according to their characteristics, the range is limited. In recent decades, interest in the development of hybrid energy storage devices has started to surge. The hybrid energy storage device is a next-generation energy storage device aiming at high energy and power densities and excellent charge-discharge cycle stability by complementing the disadvantages of each device by utilizing the electrode materials of both lithium ion battery and supercapacitor. Since hybrid energy storage devices use electrode materials with different storage mechanisms, the kinetic balance between the anode and cathode plays an important role and directly affects the performance of the device. Therefore, for well-matched anode and cathode, electrode materials have been developed by various methods, such as morphology control, particle nanosizing, structural change and material hybridization.

Nanocomposites based on carbon as an electrode material suitable for hybrid devices and combined with materials having oxidation-reduction activity are attracting attention. Carbon materials have electrical properties due to carbon-carbon bonds of sp2 orbital, and they are electrical double layer type materials storing energy using nano-pores. Also, they show physical and electrochemical stability. However, carbon materials consist only of carbon bonds, which are highly influenced by structural morphology and porosity, and consequently have limitations in energy storage capacity. On the other hand, metals, metal oxides and conductive polymers have a higher capacity than carbon materials because they store energy through redox reaction. However, as redox reaction repeatedly occurs, it is difficult to independently utilize the material due to deformation of the material and deterioration of electrical characteristics, particularly, low electrical conductivity. Therefore, carbon materials and redox active materials show synergetic effect both in capacity and electrochemical stability when used as nanocomposite through hybridization.

In this study, high capacity electrode materials were developed using conductivity enhanced carbon based nanocomposite, and high-performance hybrid energy storage devices are implemented through anode and cathode well-matched practical devices. For aqueous media, a high-capacity aqueous anode was realized on a polymer-based redox active composite material, polyaniline and reduced graphene oxide. Moreover, with cathode of sub-nanosized metal oxide and reduced graphene oxide composite having theoretical capacity, an asymmetric hybrid capacitor with a high energy density and ultra-long robust cycling stability was implemented. For organic media, a metal organic framework (MOF), a highly porous nanomaterial, has been fabricated and mesoporous structure is derived. Additionally, reduced graphene oxide wrapping has been implemented to enhance the electrical conductivity. This nanocomposite shows more than twice the theoretical capacity of Li ion storage and shows excellent properties as an anode of Li ion hybrid capacitor. Therefore, this work represents a breakthrough in realization a new class of next-generation energy storage system that can overcome the limitations of current commercialized energy storage devices.