Engineering Electrode-Electrolyte-Interface for Developing Advanced Lithium-Ion Batteries
The electrode-electrolyte interface (EEI) is recognized as one of the most crucial components inside lithium-ion batteries (LIBs) because of diverse phenomena that occur in this region: charge transfer reactions, electrolyte decomposition, and electrode (cathode, anode) degradation. Engineering the EEI to enable its superior properties and stability is the key to developing advanced LIBs. Despite the tremendous effort that has been put into this field, polymers have not attracted much attention. For example, no research papers had reported vapor-based techniques to engineer the EEI with polymers when I started my PhD research in 2016. The significance of investigating polymer-engineered EEI is further highlighted by the organic components in the naturally formed side products in the region.This dissertation focuses on developing vapor-based techniques to engineer the EEI in LIBs with organic polymers and understanding the effect of these polymers on the properties of the EEI. Chemical vapor deposition (CVD) polymerization techniques are introduced, for the first time, to engineer the EEI of battery electrodes. Various polymers have been synthesized and investigated for their effect on engineering the EEI, including poly(divinylbenzene) (PDVB), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(1H,1H,2H,2H‐perfluorodecyl acrylate-co-divinylbenzene) (P(PFDA-co-DVB)), and polythiophenes (PTs). These polymers are selected because of their good mechanical stability and high melting points, crucial to stabilize the EEI in LIBs.
LiMn2O4 is selected to study the feasibility of applying the CVD polymerization technique to engineer the EEI. By controlling the deposition parameters, we successfully engineer the LiMn2O4 cathode with nanometer-thick PDVB and PEDOT coatings. The electrochemical results show that the PEDOT coating improves both rate capability and cycling stability of LiMn2O4, while the PDVB has no significant effect. LiCoO2 is then selected to investigate the mechanisms of polymer coatings on the properties of the EEI. Advanced experimental tools and density functional theory (DFT) calculations are applied. Small binding energy with Li+ and the presence of sufficient binding sites for Li+ promote the kinetics of the PEDOT-coated LiCoO2. Chemical bonds between PEDOT and LiCoO2 allow superior cycling stability of the cathode. Additionally, the PEDOT coating promotes current homogeneity in the LiCoO2 electrode and alleviates the mechanical degradation of the electrode. These insights provide us practical design rules for polymer selection that will help shorten the research time and reduce the trial-and-error effort involved in tailoring EEIs for developing advanced batteries.
Engineering EEI with inorganic materials is also studied for a comparison with the polymer coatings. The LiCoO2 cathode is engineered with TiO2 and ZrO2 coatings via a microwave-assisted sol-gel synthesis. The results suggest that the TiO2 coating improves the rate capability of the LiCoO2 electrode, while the ZrO2 has no significant effect. Both TiO2 and ZrO2 coatings improve the 4.5 V high voltage cycling stability of LiCoO2. However, such improvement is not as good as that from the organic PEDOT coating, suggesting the superior protection ability of organic polymers.
This thesis introduces and develops novel vapor-based techniques to engineer the EEI for advanced LIBs. Because of the mild synthesis conditions and ability to form conformal coatings with precisely controlled thickness and chemical composition, CVD polymers can further improve the performance of battery anodes, solid electrolytes, and complicated interfaces inside critical renewable energy systems like solar cells and fuel cells.
History
Date
2021-03-03Degree Type
- Dissertation
Department
- Mechanical Engineering
Degree Name
- Doctor of Philosophy (PhD)