Novel Magnetic Resonance Techniques as Applied to Metallic Phases in Lithium-Ion Battery Electrodes

Abstract

Since their introduction in the late 1990s, lithium-ion batteries (LIBs) have played an increasingly important role in society, powering portable electronics, electric vehicles and grids. As such, they are critical in paving the way to net-zero carbon emissions. The prototypical anode and cathode in intercalation LIBs are graphite and lithium cobalt oxide (LCO), with the materials chemistry and battery communities treating them as “model compounds”. Despite this, the electronic properties of these electrodes remain unclear. Graphite is a semimetal which becomes metallic on intercalating lithium (when charging the battery), whilst LCO transitions from an insulating to a metallic phase on delithiation. Understanding the electronic structures of these systems—and how these structures impact the redox, degradation and failure mechanisms—is critical to the development of the next generation of LIB electrode materials. In this thesis, the nature of these semimetal/insulator to metal phase transitions and the electronic properties of the metallic phases were probed by magnetic resonance methods: Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarisation (DNP), as well as magnetic property measurements, all at variable temperature (VT). In addition to providing insight about these materials’ electronic structures, the body of work presented here describes novel methods to study these complex materials. Firstly, graphite is examined using variable frequency EPR spectroscopy, for the first time directly revealing that conduction electrons occupy bands on the graphite sheets whilst undergoing semi-localised hyperfine interactions with Li ions. By measuring the EPR spectra as functions of microwave frequency, temperature and state of charge of the graphite anodes, skin effects and metalliticty can be successfully probed. Leveraging this knowledge, the nature and degradation of the surface layers of graphite formed upon cycling, the solid electrolyte interphase (SEI), were probed using DNP and NMR. To date, the SEI has been examined using several techniques, with many conflicting reports on its properties, composition and efficacy in preventing electrolyte degradation. A novel approach to examine the SEI was developed by introducing selectivity towards the graphite-SEI and the Li dendrite-SEI interphases by exploiting the enhancement of the NMR signal provided by the graphite/Li conduction electrons on microwave irradiation (i.e., Overhauser DNP). For the first time, LiOH was identified as the main degradation product at the interphase and that Li plating on graphite dramatically changes the SEI composition. Finally, metallic phase transformations in LCO were studied. LCO is a very different metal to graphite, as it contains both localised and delocalised electrons, as per Mott-Hubbard theory. To explore the metal-insulator transitions in LCO, ex situ 7Li, 59Co and 17O VT NMR were used to probe the local electronic structure of LCO on delithiation, with operando NMR providing information about the dynamics of these local structural changes. The local structure obtained was combined with bulk electronic and chemical structural information provided by magnetometry and operando X-ray diffraction, resulting in an electronic picture where conduction holes are delocalised over Li and O ions and more localised over the Co ions.