Advanced Materials for High-Energy Rechargeable Batteries: Borate Polyanion Cathodes and Silicon Anodes
This thesis explores advanced electrode materials designed to enhance the energy density of next-generation rechargeable batteries, with a dual focus on cathode materials for rechargeable magnesium-ion batteries (RMBs) and anode materials for lithium-ion batteries (LIBs). The work is divided into two major parts: the development and evaluation of borate-based cathodes for RMBs, and the investigation of mechanical degradation in silicon anodes for LIBs. The first part presents a comprehensive study on borate polyanions as potential cathode materials for RMBs. Three structural families were systematically explored: orthoborates (M₃(BO₃)₂), pyroborates (M₂B₂O₅), and ludwigites (M₃BO₅). In Chapter 3, a detailed overview of the solid-state synthesis approaches is presented, including ball-milling, thermal treatment strategies, and the challenges encountered in achieving phase-pure samples. Structural characterisation using X-ray diffraction and neutron powder diffraction was employed to investigate the metal ion disorder in the synthesised compounds and confirm phase purity. Chapter 4 focuses on an in-depth electrochemical investigation of a pyroborate compound (MgFeB₂O₅), initially synthesised with a small weight percentage of Fe metal impurity. Despite encouraging first-charge capacities of up to 200 mAh g⁻¹ between 3.0–4.2 V (vs Li/Li⁺) at 55°C, advanced post-cycling analyses, including X-ray absorption near-edge spectroscopy, Mössbauer spectroscopy, and energy-dispersive X-ray analysis, revealed that the observed capacity stemmed from a faradaic side reaction involving hydrofluoric acid in the electrolyte and the Fe impurity, rather than true Mg²⁺ intercalation. In Chapter 5, the electrochemical properties of all five synthesised borates are evaluated, with a particular focus on the phase-pure orthoborate Mg₂Mn(BO₃)₂. All materials displayed similar voltage profiles and capacity behaviour, prompting a post-mortem study of Mg₂Mn(BO₃)₂ to understand the underlying mechanisms. Detailed analysis revealed the absence of any significant demagnesiation or transition metal redox activity. Instead, capacity originated from the activation of an amorphous surface layer formed during synthesis and enhanced by mechanical treatment. These findings emphasise the critical role of rigorous characterisation in distinguishing genuine magnesium intercalation processes from capacity contributions arising due to secondary effects such as surface-mediated reactions, electrolyte decomposition products, or impurity-driven redox activity, which may otherwise misleadingly appear as bulk intercalation phenomena in electrochemical measurements. The second part of the thesis, detailed in Chapter 6, transitions to lithium-ion battery technology, specifically addressing the challenge of volume changes in micron-sized crystalline silicon (μm Si) anodes. Charge photometry was, for the first time, applied to enable real-time tracking of volumetric changes in individual silicon particles during electrochemical cycling under realistic operating conditions, providing spatially resolved insights into the mechanical response of the Si active material at the single-particle level. A customised image processing pipeline was developed, enabling quantitative analysis and the extraction of state-of-charge dependent particle volume metrics, revealing substantial particle-to-particle variability. These measurements were investigated as a function of capacity usage, C-rate, voltage cut-offs, and ageing behaviour, and were validated by scanning electron microscopy. This allows for the first time to study Si volume changes at a particle level in realistic cell configurations, using commercially supplied materials. The analysis method developed provides new insights into the fracture and degradation mechanisms of silicon anodes and demonstrates the potential of charge photometry for real-time battery diagnostics of materials with significant volume changes. Taken together, the two parts of this thesis highlight the challenges in studying and understanding the complex, multiscale processes governing the performance of emerging battery materials. Only through the integration of complementary, multi-modal characterisation techniques can the true mechanisms underlying electrochemical activity and degradation be understood. Such a fundamental understanding is essential to guide the rational design of more reliable and efficient next-generation energy storage systems.
Li-ion, Mg-ion, Cathode Materials, Volume Expansion, RMBs, Rechargeable Mg-ion Batteries, Si, High Energy Density Batteries
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