Nickel cobaltite-based anode materials for sodium-ion capacitors

Abstract

While lithium-ion energy storage has found wide applications, the use of lithium ions as charge carrier has a number of issues, such as safety concerns and resource scarcity. In comparison with lithium, sodium is naturally abundant and cheaper. Therefore, recent years have seen a great deal of research interest in using sodium ions as charge carrier to develop sodium-ion energy storage technologies, such as sodium-ion batteries (NIBs) and sodium-ion capacitors (NICs). NICs have emerged as a promising technology for large-scale energy storage applications because this energy storage system combines the advantages of batteries and electrochemical capacitors (ECs). A NIC cell is configured with a battery electrode as the anode, an EC electrode as the cathode and an electrolyte containing sodium ions.However, finding a high-performance anode material has been one of the great challenges in developing this sustainable electrochemical energy storage technology. Transition-metal oxides (TMOs), such as TiO2, Nb2O5, NiCo2O4, and Fe3O4, have been demonstrated to be promising anode materials for sodium-ion storage. Nickel cobaltite (NiCo2O4) is of particular importance because of its low cost, abundance in nature, and high theoretical specific capacity (890 mAh g-1). However, this material suffers from critical problems, including sluggish sodium ion diffusion kinetics, low electrical conductivity, and large volume changes during charge/discharge, resulting in poor rate capability and cycling stability in NICs.This PhD thesis project aims to improve the electrochemical properties of NiCo2O4 (NCO) with regard to a number of physical and electrochemical aspects, including morphology, structure, electrical conductivity, ion diffusivity, and cycling stability. The main innovations and key findings in this thesis work include:• Spherical NCO particles have been synthesized by using a solvothermal method. It was found that subsequent thermal treatment temperature played an important role in determining the crystalline structure and particle size of the NCO. The NCO sample thermally treated at 350 oC showed an optimal and promise performance in NICs. Hollow NCO spheres with a chestnut shell morphology have also been synthesized using the solvothermal method.• Mechanism study revealed that the NCO phase was converted to metallic nickel, cobalt and sodium oxide phases upon pre-sodiation. The pre-sodiation of NCO was found to significantly improve its energy density. A NIC assembled with the pre-sodiated NCO as the anode and an activated carbon (AC) as the cathode exhibited an energy density of 60 Wh kg-1 at a power density of 10,000 W kg-1, much better performance than that reported in the literature. • The physical and electrochemical properties of the NCO particles were improved by using nitrogen-doped reduced graphene oxide (N-rGO). A porous N-rGO framework was used to encapsulate NCO particles to form a NCO@N-rGO composite. In addition to enhance electronic conductivity and alleviated the volume changes of NCO during charge/discharge, the N-rGO network also contributed to charge storage via a capacitive mechanism. A NIC assembled with NCO@N-rGO as the anode and an AC as the cathode delivered an energy density of 48.8 Wh kg-1 at a power density of 9750 W kg-1 with a stable cycle life.• A three-dimensional (3D) nitrogen-doped holey graphene (N-HG) was prepared and used to stabilize NCO particles, forming a NCO@N-HG composite. This composite exhibited good rate capabilities with a capacity of 403 mAh g-1 at a current density of 1 A g-1, which was higher than the 358 mAh g-1 of NCO@N-rGO in a sodium half-cell. A NIC assembled with NCO@N-HG as the anode and an AC as the cathode also delivered a higher energy density of 52 Wh kg-1 at a power density of 10,000 W kg-1. The physical and electrochemical properties of magnetite (Fe3O4) nanoparticles were also improved by using the 3D N-HG. The thin graphene sheets in the composites facilitate the electron transport and buffer the volume changes during charge/discharge, while the interconnected 3D macroporous network with a pore size in several micrometers range, combined with the nanopores in the N-HG provide pathways for rapid ion transport. The good electrochemical performance of the composites indicates that using N-HG to support TMOs particles is an effective and general approach towards developing high-performance anode materials for sodium-ion storage.In summary, this PhD thesis work has significantly improved the physical and electrochemical properties of nickel cobaltite with regard to electrical conductivity, structural stability, and sodium ion diffusivity in the bulk electrode. The good electrochemical performance indicates that NCO-based materials hold a great promise as anode for high-performance NICs with competing energy and power densities.