The Non-Destructive Analysis of Lithium-Ion Battery Degradation

This thesis investigates the electrochemical response and degradation of lithium-ion cells under combinations of calendar, cycle and compression conditions. Since the operating conditions of lithium-ion batteries are varied and dynamic throughout their life, the ageing of cells needs to be examined under representative individual scenarios. The following scenarios were examined separately using several non-destructive analysis methods, which respectively provide new methodologies, ageing theories and perspectives on the ageing of lithium-ion cells. Calendar ageing promoted a spoon-shaped dependence of state-of-charge (SoC) on capacity fade, which was linked to shuttle-based self-discharge. After calendar ageing, cells were cycled under aggressive or conservative conditions. A potential ageing path dependence was identified, where the outcome of cycling was influenced by previous calendar history. Reversible capacity losses were found to interfere with measurements of the degradation rate of cells during a cycling study, where the reversible loss of lithium into anode overhang regions led to greater capacity fade. Storage periods at low SoC recovered, quantified and identified the origins of the reversible capacity, and an alternative visualisation tool was proposed to identify time-related artefacts that led to such losses. To aid the design of a test-rig capable of applying uniform pressure, finite element analysis and experimental validation measurements were performed. Next, static measurements in the absence of cycling were performed to disentangle the effects of pressure on lithium-ion pouch cells. Pressure was found to have an instant effect on capacity and pulse resistance. The largest increases in resistance were observed at very low pressure levels, which were found to be almost entirely reversible upon decompression. These scenarios were then brought together to inform novel conclusions about the degradation and ageing behaviour of lithium-ion cells under dynamic conditions. Information gained from these studies provide the foundations for further examination of the effects of dynamic operational conditions