Towards operando measurements of Li+ diffusion in batteries using muon spectroscopy

Ionic movement through solids is one of the unseen operations which maintains the functionality of our technological lives; the most widespread example is Li+ ion transfer, which underpins contemporary energy storage. High ionic conductivity is essential to enable efficient batteries, and such a macroscopic quality is derived from ionic diffusion
at the atomic level. The kinetic limitations on diffusion have a crucial influence over a range of battery performance characteristics, such as fast-charging ability and capacity fade, and diffusion rates have been found to vary significantly during battery operation. As such, a fundamental understanding of the underlying mechanisms behind ionic diffusion, and the structural properties which act to regulate them, is vital for the design and application of next-generation energy materials.

A number of experimental techniques have recently been developed to study charge transfer in battery materials during the charge/discharge process: so-called "operando" measurements. This thesis describes a collection of experiments which develop a particle accelerator-based technique, muon spin relaxation spectroscopy (μSR), towards a
consistent operando methodology for the advanced characterisation of energy materials during real battery operating conditions. μSR operates by using the time evolution of implanted muon spins to probe magnetic fields within materials, which can reveal information on the dynamics of Li+ ions. To fully interpret the information learned from such experiments, the application of complementary techniques is essential; electrochemical impedance spectroscopy and X-ray absorption spectroscopy were thus employed to aid understanding over a range of length scales.

In this thesis, Chapter 1 introduces the underlying scientific motivation, before Chapter 2 reviews prior μSR studies of energy materials, and the experimental methods are outlined in Chapter 3. Chapter 4 demonstrates the possibility of in situ μSR, whereby a significantly hampered ionic diffusion rate is identified in an all-solid-state battery at low
voltage, impeding cell cyclability. A garnet solid electrolyte is investigated in Chapter 5, uncovering a mechanism for interphase propagation through the grain surface regions. A custom-built cell for operando μSR experiments is given in Chapter 6, which provides reliable electrochemical performance and collection of high quality data. Chapter 7 describes the first operando μSR experiment, obtaining detailed information on the properties of commercially relevant cathode NMC811. Contrasting diffusivity behaviour in the particle surface and bulk was observed, with both regions significantly affected at a high state of charge. Finally, Chapter 8 provides the conclusions and future work; ultimately, a working methodology for operando measurements using μSR lays the foundations towards a variety of future opportunities in advanced material characterisation.