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The work in this thesis focuses on the metallic anode in two of the divalent battery systems: aqueous zinc batteries and calcium-ion batteries. By performing systematic studies, we aim to provide a deep understanding of their anode degradation mechanisms: In Chapter 3, I looked at the ageing-induced degradation in ZnSO4-based aqueous zinc batteries. It was found that ageing can lead to significant anode capacity loss, in some cases over 70% efficiency loss after only 24 hours. We assessed such loss with respect to various factors: ageing time, anode loading, cycling rates etc. and investigated the root causes of such ageing-induced capacity loss using in-situ Computed X-Ray Tomography. A paper based on this chapter, 'Decoupling and Quantifying Aging-Induced Zn Anode Degradation Processes in Aqueous Zinc Batteries' has been submitted to 'Joule' and is now under review. In Chapter 4, I explored a possible strategy to mitigate the dendrite problem in the ZnSO4-based aqueous zinc batteries. By using a single-crystal (002) Zn as an ideal model anode, I promoted planar Zn deposition and defect suppression, which largely resolved the dendrite problem and allowed excellent cell performance at high-rate and long-cycle conditions. A paper based on this chapter, 'Achieving Ultra‐High Rate Planar and Dendrite‐Free Zinc Electroplating for Aqueous Zinc Battery Anodes' has been published in 'Advanced Materials'. In Chapter 5, I systematically studied and visualized the plating and stripping process in the Ca(BH4)2-based calcium-ion battery electrolyte using in-situ TEM. I revealed that despite calcium deposition being mostly globular, calcium dendrite formation can still occur as a result of high current density. In addition, the inhomogeneous CaH2 layer formation can also lead to the formation of localized 'hot spots', which can induce unexpected dendrite formation at lower current densities. Two papers based on this chapter, 'Liquid cell transmission electron microscopy and its applications' and 'Current-density-dependent electroplating in Ca electrolytes: From globules to dendrites', has been published in 'Royal Society open science' and 'ACS Energy Letters'.

More energy dense lithium ion batteries are essential for the production of long-range electric vehicles and all-electric aircraft. These next generation batteries will require high capacity cath- ode materials that move beyond the lithium intercalation chemistry of current electrodes. On paper, transition metal fluorides are among the most promising cathode materials, combining high electrode potentials (2.6 V to 3.5 V vs. Li+/Li) with three to six times the theoretical capacity of conventional cathode materials. However, their commercial application has been hindered by their inability to cycle in a truly energy dense format (i.e. at high active material concentra- tion) and ultimately the inadequate understanding of their electrochemical capabilities and limitations. Both of these issues have been largely resolved as a result of this DPhil project. First I design a materials system that is practically ideal for reversible high-capacity cycling as well as detailed mechanistic study. This is achieved through the development of a new colloidal synthesis method for single-crystalline, monodisperse transition metal fluoride (FeF2, CoF2) nanorods. Size and shape control are demonstrated, and a chemical reaction pathway and nu- cleation and growth mechanism are proposed. Next I present an ionic liquid electrolyte (1 molal lithium bis(fluorosulfonyl)imide in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide) that forms a stable solid electrolyte interphase, prevents the fusing of particles, and consequently precludes the major failure mechanisms associated with transition metal fluoride cathodes. The use of this electrolyte with the iron(II) fluoride (FeF2) nanorod system results in near theoretical capacity (570 mA h g−1) on discharge and charge, with extraordinarily stable cycling (>90% ca- pacity retention after 100 cycles). This stability is maintained for over 200 cycles at relatively high cycling rates (C/2) and temperatures (50 ◦C). By combining the unique vantage point afforded by this novel materials system with the morphology preserving qualities of our new electrolyte, I am further able to characterize the discharge/charge mechanism with an unprecedented level of detail. High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that that provide a new and comprehensive understanding of the discharge/charge (conversion) reaction mechanism. This new mechanistic understanding defines the structural and chemical origins of reversible cycling and cell failure in transition metal fluorides and unearths new concepts such as an inherently high discharge rate capability in FeF2.