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Date
2025-09-30
Abstract
This thesis is submitted through publication. Herein it explores the synthesis, application, and mechanistic investigation of alloying and conversion anode materials for sodium-ion batteries (SIBs). Specifically, it examines SnSb nanolayers with a focus on improving electrochemical performance through tailored electrolyte selection, mechanistic insights and performance enhancement of BiOCl micron-sized flowers, and growth of Cu3P wires with exploration of sodiation and failure mechanisms. The core chapters are formatted as research articles, each with an introductory section.
As global demand for green energy increases, so does the need for sustainable energy storage. SIBs provide a low-cost alternative to lithium-ion batteries (LIBs), utilizing more sustainable and ethically sourced materials. However, one of the major challenges facing SIBs is their lower energy density compared to LIBs due to the larger and heavier Na⁺ ion. To close this gap and develop more competitive SIBs, advanced electrode materials with higher capacity, rate capability, and low cost are essential. Currently, hard carbon (HC) is the leading anode material for SIBs due to its stable capacity over long-term cycling. However, their performance can vary significantly with synthesis method and precursor. Additionally, its low gravimetric and volumetric capacity limit its application beyond stationary storage. Developing alternative anodes with higher capacities will enhance SIBs grid-storage capability while also making them more viable for emerging applications such as electric vehicles (EVs). Alloying and conversion-based anodes can offer significantly higher capacities than carbonaceous electrodes, but their large volume expansion during cycling leads to cracking and pulverization, ultimately shortening battery life. Addressing this challenge is essential for enabling alloying and conversion-type anodes to achieve long-term stability comparable to HC, and positioning SIBs as a viable competitor to LIBs in energy density, cost-effectiveness, and longevity.
Chapter 3 discusses SnSb as a long-cycle-life anode for SIBs, enabled by high=concentration electrolytes (HCEs). Na-alloying metals such as Sn and Sb have attracted attention as potential SIB anodes due to their high theoretical capacities. However, their performance is often limited by pulverization resulting from significant volume changes during cycling. Nanostructuring the material can mitigate this issue, as nano-sized dimensions help accommodate volume fluctuations. In SnSb electrodes, a mutual buffering between Sn and Sb effectively alleviates the volume expansion. This study reports the enhanced cycle life of SnSb nanolayers as a Na-ion anode, achieving a capacity of 378 mAh g⁻¹ after 1500 cycles, significantly improved by using a high concentration electrolyte (HCE) compared to a standard concentration electrolyte (SCE). Dendritic Cu was also used to increase active mass loading while maintaining capacity retention.
Chapter 4 investigates the mechanistic behaviour and performance enhancement of BiOCl as a SIB anode. BiOCl is a low-cost, non-toxic conversion anode with promising long-term cycling stability. It has a respectable theoretical capacity of 307 mAh g-1, and has previously demonstrated capability to maintain close to theoretical capacity for 1000s of cycles. Rate capability is also impressive, maintaining 98.4% capacity when increasing the rate from 50 to 2000 mA g-1. However, before achieving these benefits, BiOCl exhibits substantial capacity loss in early cycles, followed by a slow recovery over hundreds of cycles. To establish BiOCl as a viable anode, understanding and improving this phenomenon is necessary. Here-in is presented BiOCl synthesised as a micron-sized flower morphology. This chapter details an in-depth analysis of BiOCl’s sodiation mechanism using SEM, ex-situ and in-operando XRD, XPS, and TEM. An activation protocol was developed to minimize early-cycle capacity loss and accelerate capacity recovery. Using this protocol the minimum capacity was increased from 165 to 218 mAh g-1 , and capacity achieved after 100 cycles is increased from 167 to 257 mAh g-1. Post-mortem SEM confirmed that this activation method facilitates the formation of a nanoporous structure earlier in the cycle life compared to conventional cycling protocols.
Chapter 5 explores Cu₃P as a conversion anode for SIBs, providing insights into its sodiation and failure mechanisms. Metal phosphides are promising SIB anodes, offering the Na+storage capability of phosphorus while potentially addressing its limitations. Among them, Cu₃P is particularly attractive due to its competitive theoretical capacity of 360 mAh g⁻¹ and relatively high electrical conductivity. While a sodiation mechanism for Cu₃P has been proposed, further investigation is needed for validation. This study introduces a novel synthesis method for Cu₃P nanowires (NWs) via phosphorisation of Cu nanoparticles (CuNPs). Examining cyclic voltammetry (CV) and differential capacity plots (DCPs) during cycling revealed a more complex sodiation profile with multiple reduction steps. Ex-situ XRD analysis unveiled the formation of intermediate phases during (de)sodiation. Complementary XPS studies provided additional insight phase transformation upon desodiation. This mechanism was further investigated using GITT to analyse sodium diffusivity variations as phase transitions occurs. Additionally, EIS was used to track interfacial evolution over various cycles. Post-mortem characterization of aged Cu3P wires identified phase and morphology evolution after extended cycling. Additionally, the failure mechanism for this material were revealed, providing insights for the development of future strategies to enhance cycle life.
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University of Limerick
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Sustainable Development Goals
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Thesis
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http://creativecommons.org/licenses/by-nc-sa/4.0/