Understanding and Preventing the Degradation of Silicon as a Lithium Ion Battery Anode
Creators: name: Kneusels, Nis-Julian Hendrik
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: subject: Lithium Ion Batteries
subject: Silicon
subject: Phosphazenes
subject: Anode
subject: Inorganic Chemistry
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences

description: Silicon anodes are of great interest for lithium ion batteries (LIBs) due to their promising high gravimetric capacity compared to conventional carbon anodes. However, the immense volume expansion and continuous solid-electrolyte interphase (SEI) formation during operation hinder stable long-term cycling. This thesis aims to identify factors that drive the degradation of the silicon anode and propose strategies to overcome it using electrolyte additives and poly(phosphazenes) as artificial SEIs. Chapter 1 outlines the need for LIBs and their functionality and introduces silicon as an anode material and the chemistry of (poly)phosphazenes. Chapter 2 describes the relevant techniques such as scanning electron microscopy (SEM), galvanostatic cycling, etc. and their application for this work. Chapter 3 addresses the degradation of the silicon anode using thin-films of the material, as the application of nanosized silicon anodes yields reasonable electrochemical performance. Previous studies have analysed a wide range of thin-films using varying techniques, however, no clear view of the degradation processes involved can be concluded. Thin-film electrodes are created by magnetron sputtering and thoroughly tested. Electrochemical cycling demonstrate the impact of the anodes thickness and substrate structure and varying cycling conditions suggest ideal parameters to improve the performance. The electrodes undergo distinct morphological changes during operation which are identified by electron microscopy SEM and compared to data from electrochemical cycling. In Chapter 4, the impact of the electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the thin-film electrode is investigated, suggesting the formation of a mechanically robust SEI. Electrochemical experiments and electron microscopy give an insight into both structural and chemical differences that arise from the addition of the individual electrolyte additives. Chapter 5 introduces poly(phosphazene) coatings to stabilise the silicon thin-film anode and increase its performance. The design of an artificial SEI based on mechanically and chemically flexible poly(phosphazenes) is demonstrated and greatly increases the cycle life of the thin-film electrode. These inorganic-organic hybrid polymers are synthesised from simple starting materials, allowing the functionalisation with various organic side-groups to tune their properties. The polymer synthesis and deposition of thin polymer films are described in detail and the film morphologies observed before and after electrochemical cycling are presented. Stabilisation of the anode with a polyether-substituted poly(phosphazene) is further improved by introducing trifluoroethoxide side-groups. The coatings provide mechanical and chemical stability depending on its functionalisation. Chapter 6 shows proof-of-principle studies of phosphazene surface-grafting on to silicon summarised as ongoing work. Results indicate that very thin poly(phosphazene) layers can be engineered. All results are summarised in Chapter 7, drawing conclusions on the impact of this work in the field and its implications on future work.