Secondary electron hyperspectral imaging of electrode components in lithium-ion batteries

This thesis presents secondary electron hyperspectral imaging (SEHI) as a technique to gain novel insights into the surfaces of lithium-ion battery (LIB) electrodes and their components through spatially localised surface chemical characterisation down to the nanoscale. The transition to renewable energy has placed huge demand on materials used in LIBs. There is a significant cost associated with materials degradation in LIB electrodes causing unpredictable capacity fade. Many of these degradation mechanisms are effective (and observed) at electrode material surfaces. SEHI is a proven analysis workflow in several fields including polymers, biomaterials, solar cells and carbon thin films, and has application in the focussed ion beam scanning electron microscope (FIBSEM), which is a workhorse of materials characterisation. This work is the first SEHI characterisation of LIB materials. The methods for SEHI developed here were used to image surface chemistry in LIB electrodes and identify (in-)homogeneity which can cause increased surface degradation and unpredictable capacity fade at the LIB cell level. Starting with an assessment of the suitability of SEHI for the task of characterising thin surface layers comprised of carbon, lithium and transition metal oxides, the approach was taken to build knowledge of the available spectral information with a set of reference material systems, before application to the most complex surface chemistries found in charge-discharge cycled LIB electrodes. In doing so, new sample preparation, data processing and analysis approaches were developed. These included peak fitting SE spectra of graphitic and amorphous carbons to identify spectral ranges for sp2, sp3 and amorphous carbon. Sensitivity to Li metal and compounds of Li followed. Finally, this knowledge was applied to SEHI characterisation of NMC811 cathode material pre- and post-charge-discharge cycling. The established workflows for surface chemical imaging demonstrated application to characterising graphitic carbons and show promise for further positive electrode solid-electrolyte interphase studies.