Chen, Jintao; (2023) Structural engineering of anode materials for potassium ion batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis begins with an introduction of the current stage of PIB anode material development, focusing on carbon materials, which are noted and reported as one of the most reliable PIB anode systems. Compared with LIBs, the challenges and barriers that hindered the development of PIBs were discussed. Optimisation of carbon-based materials by structural engineering can significantly change the electrochemical performance of the material for energy storage applications. An in-depth literature review based on the second chapter can be a valuable contribution to and drive forward the project. In order to produce highly porous materials with engineered nanostructure and morphology, nano-casting as a well-developed method has been improved to synthesis MoS2 with mesopores. The material was applied as an anode for PIBs by comparing with the pristine MoS2. The templated MesoMoS2 can achieve 289 mAh g-1 for the first cycle and 96 mAh g-1 at 100th cycle at current density of 200 mA g-1. To further reveal the mechanisms of the electrochemical process, an investigation on internal resistance, crystal structure, and morphology was conducted. By learning the low retention rate from the work on Meso MoS2, carbon nanotubes were selected as a candidate material and was structurally improved by chemical synthesis. CNTs, as a type of graphitic materials, is a material with excellent mechanical properties (e.g. ductility, elasticity) and more importantly, good electrical conductivity. However, CNTs cannot provide adequate active reaction sites for K+ ion storage, and hence resulted in low capacity. So, CNTs were further combined with amorphous carbon to introduce more active sites to increase the capacity. In this work, glucose, a biomass material, was employed as the amorphous carbon precursor, which is also low-cost and environmental-friendly in terms of synthesis and recycling. The GCNTs can deliver a capacity of 81 mAh g-1 at 500th cycle at current density of 200 mA g-1, which increased the retention rate by 60% over MWCNTs. Phosphorus(P) doping of carbon can not only change the nanostructure, but also enhance the electrochemical performance. By phosphorus doping, the rate capability of the anode has been largely enhanced, which allows it to be utilised as a novel anode material for commercialisation. However, in order to further improve the rate capability and the cyclability of PIBs, the intensive structural change of GCNTs during cycling of batteries is urged to be investigated and solved. Herein, I induced anionic defects with P-atom doping within the GCNTs nanostructure to boost the kinetics of PIBs anodes as a strategy of structural engineering, which allowed a mechanically strong and chemically stable structure to facilitate the volume change of materials during cycling by this novel carbon structure. And the further P-atom doping as a method of modification was studied to obtain the better rate capability of the anode. The reasons of this were also discussed and summarised: the change of the pore size distribution, electronic structures, and hence charge storage mechanism. The optimized anode in this chapter delivered a high capacity of 175 mAh g-1 at a current density of 200 mA g-1. Additionally, the improvement of rate capability (60% capacity retention with the current density increase of 50 times) was also enhanced. In this work, a rational design method was provided for the guidance of the future research on commercialisation of carbon-based anodes for PIBs. Last but not least, with a summary and overview of the whole project, some thoughts and outlooks for the future development of PIBs were concluded.

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