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Title: Carbon materials for advanced potassium-based batteries : mechanism exploration and performance optimization
Authors: Tan, Hong
Degree: Ph.D.
Issue Date: 2021
Abstract: Potassium-based batteries, bestowed by the abundance of potassium resources and similar working mechanisms with lithium-based counterparts, have emerged as promising alternatives to the prevailing Li-ion batteries (LIBs) toward large scale energy storage. Among various electrode candidates, carbon materials are considered the most cost-effective and eco-friendly ones due to their facile manufacturing processes and natural recyclability. In this thesis, carbon materials with various microstructure are synthesized and investigated as anodes/cathodes in various potassium-based battery prototypes. The corresponding K ion storage mechanisms associated with the electrochemical performances are revealed using a series of operando and ex-situ techniques. Systematic studies are first conducted to explore the potential active sites for K ion storage on the carbon anode side. Pitch-derived soft carbon is utilized as a model material due to its high carbon purity so that the interference of heteroatoms could be minimized. Stepwise carbonization is performed to gradually tune the degree of order, allowing the establishment of the correlation between the charge storage mechanism and microstructure by in situ Raman spectroscopy. To further boost the capacity and cyclability of pitch-derived carbon anodes, a strategy combining microstructure design and electrode/electrolyte interphase regulation is implemented. Considerable amounts of mesopores are produced via a MgO-template method to provide extra active sites for K ion storage. The optimized mesoporous carbon anode delivers a remarkable capacity of 460 mAh g-1 and an outstanding rate capability up to 4.0 A g-1. In-situ Raman spectra reveal that extra capacity comes from the effective storage of K ions in the as-incorporated mesopores. The construction of robust solid electrolyte interphase in ethylene glycol diethyl ether derived electrolyte further improves the long-term stability, leading to an exceptional capacity retention of 80% after 2000 cycles under a current density of 1.0 A g-1. A comparison is made between the as-prepared carbon materials and a typical insertion-type anode KVPO4F in terms of the rates and capacities. Turn to the cathode side, activated carbon with a high surface area of over 2800 m2 g-1 is fabricated from alkali lignin through a traditional KOH activation method assisted by self-activation. The as-prepared activated carbon is then adopted as a cathode within a KICs configuration. A wide voltage window of 1.0-4.8 V (vs. K+/K) is applied where synergistic storage of anions and cations is achieved. It shows that a deep discharge down to 1.0 V is necessary for full desorption of anions, which also triggers the adsorption of cations (K+), resulting in an increased capacity. However, a compromise must be made on the energy efficiency due to the intensified charge hysteresis upon deep discharging. Then, a dual anion (PF6- and FSI-) intercalation strategy is adopted to enable a graphite cathode with both high capacity and decent stability. It reveals that the presence of PF6- helps the formation of an effective cathode electrolyte interface to allow high anionic stability up to 5.5 V, while FSI- intercalation brings about superior rate capability and long-term cyclic stability. Concurrent intercalation of FSI- and PF6- is tracked by in-situ Raman spectroscopy and ex-situ XRD. It reveals the formation of stage I graphite intercalation compounds (GICs) upon charging, leading to a reversible capacity of over 100 mAh g-1 with an average potential of 4.65 V (vs. K+/K). Furthermore, the graphite-potassium cell delivers an exceptional capacity of 94 mAh g-1 at 0.3 A g-1 and shows a capacity retention of 96% after 250 cycles.
Subjects: Potassium-ion batteries
Electric batteries -- Materials
Carbon compounds -- Electric properties
Hong Kong Polytechnic University -- Dissertations
Pages: xviii, 168 pages : color illustrations
Appears in Collections:Thesis

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