Fundamental study on polyimide-based electrode materials for sodium-ion energy storage systems

Abstract

Sodium-ion energy storage systems such as sodium-ion batteries (NIBs) and sodium-ion capacitors (NICs) hold great promise in sustainable electrochemical energy storage due to the abundance of sodium resources. Most of the current electrode materials for the sodium-ion energy storage are inorganic compounds, which can be toxic, resource-limited, and of high cost. Alternative organic electrode materials that are environmentally friendly and cost-effective from readily available resources, have received increasing interest. Among them, polyimides are promising redox-active electrode materials due to the high density of electroactive functional groups, high mechanical strength, and excellent thermal stability. However, the utilization efficiency of the polyimides as electrode materials is low because of the poor electronic conductivity of polyimides. As a result, the charge storage capacity and rate capability of polyimide electrode materials are not satisfying. Therefore, this PhD project aims to improve the electrochemical performance of polyimide-based sodium-ion energy storage systems by improving the utilization efficiency of the polyimide electrode materials.Prior to improving the electrochemical performance of the polyimide materials, the sodium-ion storage mechanism of the polyimides needs to be understood. A mechanistic study of sodiation and desodiation during discharge and charge using attenuated total reflectance Fourier transform infrared spectroscopy confirmed that the pyromellitic dianhydride-based polyimides [C16H6O4N2]n experienced a two-step reversible enolization reaction with two sodium ions during the redox electrochemical reaction. Polyimide nanosheets with a microflower-like morphology were used as the anode materials for the sodium-ion batteries. The electrode exhibited a stable discharge capacity of 125 mAh g-1 at a current density of 25 mA g-1 at the 100th cycle. At a current density of 2000 mA g-1, the electrode delivered a discharge capacity of 43 mAh g-1. The capacity contribution of this polyimide electrode material mainly occurred below 1.5 V, indicating that it is suitable as an organic anode material for sodium-ion batteries.Having understood the charge storage mechanism of pyromellitic dianhydride-based polyimides [C16H6O4N2]n, it is meaningful to investigate and improve the performance of their application in sodium-ion energy storage system. A hybrid sodium-ion capacitor was fabricated with the polyimide as anode and the polyimide-derived carbon as cathode. The pyromellitic dianhydride-based polyimide was hydrothermally synthesized and further thermally treated to prepare porous carbon materials. The porous structure of the polyimide-derived carbon could be controlled by varying the reactant concentrations during the hydrothermal reaction and KOH activation to achieve a highly accessible specific surface area of 1302 m2 g-1. The hybrid sodium-ion capacitor could be operated at a voltage of 4.2 V, delivering an energy density of 66 Wh kg-1 at a power density of 196 W kg-1, and an energy density of 13.3 Wh kg-1 at a power density of 1200 W kg-1.Due to the poor electronic conductivity of the polyimides, the addition of a large amount of conductive additives in electrode preparation leads to low capacity of the polyimide electrode and low energy density of the entire device. Thus, free-standing polyimide-graphene composite electrode materials were synthesized and directly used as electrodes without any additives. By manipulating the interfacial chemistry and interactions between the polyimide and graphene, composite electrode materials with different polyimide particle sizes and morphologies were prepared and characterized. The sodium-ion storage capacity was significantly improved. A hybrid sodium-ion capacitor fabricated with freestanding polyimide-graphene composite as anode and reduced graphene oxide as cathode delivered energy densities of 55.5 Wh kg-1 and 21.5 Wh kg-1 at power densities of 395 W kg-1 and 3400 W kg-1, respectively. In addition, the composite electrode was found to be flexible as demonstrated in a sodium-ion capacitor, which displayed outstanding mechanical stability upon bending.This PhD thesis presents new experimental methodologies for improving the electrochemical properties of polyimide-based electrode materials for sodium-ion energy storage systems, discusses mechanistic insights into sodium-ion storage in polymer materials, and provides research perspectives and future development trends in the area of polymer electrode materials for sodium-ion-based electrochemical energy storage systems.