Characterization of Cathode Materials for Alkali Ion Batteries by Solid-State Nuclear Magnetic Resonance Methods

Abstract

This thesis concerns the use of advanced solid-state NMR methods to investigate local structural features and ion dynamics in a series of paramagnetic cathode materials for lithium and sodium ion batteries. A variety of polyanionic phosphate and fluorophosphate derivatives were explored to identify characteristics that ultimately improve battery performance. Solid-state NMR is an excellent method to probe such materials, as it offers the unique ability to track the charge-carrying alkali ion (Li or Na) over the course of the electrochemical process, adding insight not obtainable by bulk characterization techniques. Selective inversion exchange experiments were used to elucidate ion diffusion pathways in low-mobility Li ion conductors Li2MnP2O7 and Li2SnO3. Contrasting experimental results highlight significant differences observed when the method is applied to paramagnetic versus diamagnetic systems, with the former being much more complicated to study with traditional exchange spectroscopy methods. Selective inversion was similarly applied to a new lithium iron vanadate framework, LiFeV2O7, where the changing ion dynamics as a function of electrochemical state of charge were quantified, allowing for the development of a model to explain the corresponding phase changes in the material. This represents the first example of an ex situ Li-Li exchange study for a cathode material, particularly where the conductivity changes are linked directly to a change of ion exchange rates. Additionally, 23Na NMR spectroscopy was additionally used to investigate Na2FePO4F as a potential Na ion battery cathode, where ex situ NMR measurements successfully determined the local Na ion distribution in the electrode as a function of electrochemical cycling. In combination with density functional theory (DFT) calculations, the NMR results lead to the construction of a biphasic desodiation model for Na2FePO4F cathodes. Finally, possible defect formation in sodium iron fluorophosphate was investigated with a variety of methods including 23Na NMR, DFT calculations, powder X-ray diffraction and Mössbauer spectroscopy.