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 sodiumion 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 TiO 2 , Nb 2 O 5 , NiCo 2 O 4, and Fe 3 O 4 , have been demonstrated to be promising anode materials for sodium-ion storage. Nickel cobaltite (NiCo 2 O 4 ) 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 NiCo 2 O 4 (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 o C showed an optimal and promise performance in NICs. Hollow NCO spheres with a chestnut shell morphology have also been synthesized using the solvothermal method. VII