Rational synthesis and structural engineering of two-dimensional inorganic nanosheets for electrochemical energy storage

Lithium-ion batteries have dominated the portable electronics industry and solid-state electrochemical R&D for the past two decades due to the relatively high energy density. However, the limited lithium resources and their non-uniform distribution are becoming big concerns. Therefore, exploring high-performance, safe rechargeable batteries based on abundant resources is urgent. Sodium-ion batteries are of great interest as a potentially low-cost and safe alternative to the prevailing lithium-ion battery technology owing to the high abundance of sodium in earth crust, even distribution in nature and its favorable redox potential (only ~0.3 V above that of lithium). Figures of merit for future SIBs call for a breakthrough in energy (>200 Wh kg⁻¹) and power density (>2000 W kg⁻¹) as well as the cycle life (>4000 cycles) by designing new electrode structures, materials engineering and identifying new chemistries, to satisfy the requirements of many potential applications ranging from ubiquitous portable electronics to grid energy storage. Two-dimensional (2D) inorganic nanosheets offer exciting opportunities for fundamental studies and many technological applications due to their unique and fascinating physical and chemical properties. Preparation of 2D materials via exfoliation/delamination from intrinsically layered materials has been greatly limited by the categories of materials with such suitably layered host crystals. It is critically challenging to obtain 2D nanocrystals from the materials of non-layered structures. This dissertation focuses on the rational synthesis and structural engineering of 2D inorganic nanosheets for high-performance electrochemical energy storage. Several 2D energy materials, such as MnO₂, LiFePO₄, VOPO₄ nanosheets, for electrochemical energy storage are presented. The synthesis and characterizations of these 2D energy materials are discussed in details. Their electrochemical characteristics for H⁺, Li⁺ and Na⁺ storage and the corresponding energy storage mechanisms are also investigated for each case. 2D energy materials have proven effective in constructing kinetically favorable ion channels, but the irreversible restacking of the individual 2D nanosheets during materials processing or device fabrication may lead to the decrease of active sites for ion storage and the sluggish ion transport. To address this issue, two possible strategies are developed in this dissertation to improve the ion transport in 2D nanomaterials. One possible strategy is to increase the interlayer spacing to facilitate ion transport by creating a lower energy barrier for ion transport through the interlayer space. A general interlayer-engineering strategy to improve the sodium-ion transport in VOPO₄ nanosheets via organic molecules intercalation is presented in Chapter 5. Another strategy is to make porous/holey materials to facilitate the ion transport. Chapter 6 summarizes the porosity engineering of 2D transition metal oxide nanosheets for improved rate capability and cycling stability for both lithium and sodium-ion storage. Rational synthesis and structural engineering of 2D inorganic nanosheets has allowed us to make progress on (i) understanding the materials chemistry of 2D energy materials for electrochemical energy storage, (ii) developing promising strategies to address the key problems in 2D nanomaterials for energy-related applications, and (iii) fabricating high-performance lithium- and sodium-ion batteries for the next generation