Development of electrode materials for sodium-ion batteries

Abstract

The extensive implementation of renewable energy resources such as the wind and solar energy requires the development of cost-effective, efficient and large-scale electric energy storage system. Electrochemical secondary batteries especially lithium-ion batteries (LIBs) are considered as one family of the most promising candidates to store electricity in wide scale. However, due to the high cost of LIBs and the concern of the availability of lithium resources, room-temperature sodium-ion batteries (NIBs) have recently been considered as potential electricity storage devices in grid energy storage when considering the abundance, geographic distribution, low cost and environmental friendly properties of sodium sources. Though a large number of electrochemically active materials for NIBs have been reported, the development of high performance electrode materials including cathode and anode parts is still the greatest technological challenge that hurdles the practical application of NIBs. This doctoral program aims to develop electrode materials with advanced electrochemical properties in terms of long cyclability and high rate capabilities for NIBs system. Recently, a variety of cathode materials including polyanion compounds and layered transition metal oxides have been investigated for their sodium insertion capabilities. However, the electrochemical performances are still insufficient for practical implementation. In the first part of this PhD program, P2-Na2/3Ni1/3Mn2/3O2, a promising sodium layered cathode material, was explored electrochemically to achieve a balance between the capacities and cycling performances and also examined structurally to have insight into the working mechanism. In situ X-ray Diffraction (XRD) was employed to understand the structural changes upon charge/discharge processes stable capacity sustainability in the explored voltage range. Afterwards, in the following chapter, the incorporation of different amounts of lithium in the pristine materials of P2-Na2/3Ni1/3Mn2/3O2 was studied to achieve improved electrochemical performances. Over 88.8% of the initial capacity of 157 mA h g-1 can be retained after 50 cycles in Na2/3Li0.2Ni1/3Mn2/3O2+y. The absence of suitable anode electrodes is also another major obstacle for the realization of NIBs applications. Graphite, the most utilized anode for commercial LIBs, was proved to have limited sodium intercalation capability and cannot be considered as a “default” anode materials for SIBs.7 Therefore it is highly desirable to explore proper Na-ion intercalation anode materials with low-cost, good safety and long-term cyclability that play similar role of ii graphite in LIBs for the perspective of large–scale production. In the third part of this thesis, nitrogen-doped carbon materials synthesized from polydopamine demonstrated higher reversible capacities and better rate capabilities than non-nitrogen-doped carbon derived from resorcinol-formaldehyde (RF) resin. A cycle life as long as 2500 with a reasonably reversible capacity of 136.4 mA h g-1 can be achieved. Anatase (TiO2) has been widely investigated as anode materials with high safety and excellent electrochemical performances for LIBs. In the fourth part of this program, carbon coated TiO2 nanosheets have been developed as anode materials in NIBs. Long term stability and promising rate capabilities have been achieved. The thin thickness of the TiO2 nanosheets and the enhanced electronic conductivity as well as protective effects from the carbon coating enables fast and stable Na-ion transfer, ensuring the excellent electrochemical properties. Based on the superior electrochemical performances achieved and the low-cost as well as environmental friendly properties, the TiO2@C nanocomposites are considered as promising candidates of anode materials in NIBs system.