Lu, Yi; (2025) Investigation of anode materials for potassium-ion batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Lithium-ion batteries (LIBs) perform high power and energy densities. However, they face significant challenges due to resource limitations and cost issues amid growing energy storage demands. Therefore, developing ‘beyond Li-ion’ battery technologies that can reduce costs or increase battery energy density would not only address the scarcity of lithium resources but also to alleviate market pressure on LIBs. Potassium-ion batteries (KIBs) are a considered potential candidate for ‘beyond Li-ion’ batteries, because of the abundant resources, low cost and high conductivity. Additionally, potassium, as an alkali metal, shares characteristics similar to lithium. However, research into KIBs presents several challenges due to potassium's larger ionic radius, electrolyte stability issues, and limited electrode material options. The motivation of this project is to investigate various optimization approaches for different types of anode materials previously used in LIBs, aiming to improve anode performance in KIBs. The first experimental project compares the performance of iron selenide (FexSey@C) in two different electrolytes: 2.5 M potassium bis(fluorosulfonyl)imide in ethylene carbonate/diethyl carbonate (KFSI–EC/DEC) and 2.5 M potassium bis(fluorosulfonyl)imide in triethyl phosphate (KFSI–TEP). The comparison reveals that the KFSI–TEP system is more suitable for this material, offering superior and more stable electrochemical performance (specific capacity of 397.15 mAh g-1 at 0.1 A g-1 with a capacity retention of 107% after 150 cycles). Various electrochemical analyses, structural characterization, and X-ray photoelectron spectroscopy (XPS) surface and Ar-ion sputtering techniques show that the solid electrolyte interphase (SEI) formed in the TEP electrolyte on the anode surface undergoes reversible reactions and is more stable during charge–discharge cycles. This provides an alternative optimization strategy for using metal chalcogenides as anodes in KIBs. The second project investigates an alternative approach to enhance the performance of metal chalcogenide anodes by adjusting different ratios of sulfur (S) and selenium (Se) to form dual-anion FeSexS1-x compounds, focusing specifically FeSe2/3S1/3 as the primary sample to study. Ex situ Raman spectroscopy is employed to study the reaction mechanism of the material in another highconcentration electrolyte, 5 M KFSI in diethylene glycol dimethyl ether (DEGDME). The third project compares a series of soft carbon samples with different degrees of graphitization, produced by annealing perylenetetracarboxylic dianhydride (PTCDA) at different temperatures, and assesses pure graphite as a KIB anode in TEP electrolyte. Electrochemical and structural characterization techniques are utilized to elucidate the performance differences and potassium-ion storage mechanisms of these materials. Finally, the thesis discusses the outlook and limitations of these projects.

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