Dynamic Battery Usage and its Effect on Degradation

Doctoral thesis, 2025

The urgent need for decarbonisation has placed significant pressure on the transportation sector to transition away from fossil fuels, with lithium-ion batteries (LIBs) emerging as the leading alternative for clean transportation. However, despite rapid advancements in LIBs, the mechanisms of battery degradation – particularly under the dynamic usage conditions typical of vehicle applications – remain insufficiently understood.

This thesis contributes to the understanding of how usage conditions influence battery degradation, with a particular focus on operating conditions relevant to vehicle applications. Key factors investigated include variations in the state of charge window, dynamic cycling during charge and discharge, and temperature effects. The compiled studies present findings from battery ageing experiments on energy-optimised LIBs, addressing three main aspects of degradation:

·         State of charge dependence: Doping with SiOx in the graphite negative electrode is shown to dominate battery degradation.

·         Charge and discharge dynamics: Ageing studies reveal that dynamic cycling significantly improves durability, particularly when charging the battery.

·         Cell-to-cell variance: Statistical analysis indicates that a minimum of four replicates per test condition is required to ensure robust ageing assessments.

Beyond these main findings, this thesis also presents electrochemical analysis methods for determining the degradation of individual materials in mixed-material electrodes, statistical methods for analysing high-variance degradation data, and novel approaches for parameterisable dynamic life cycle testing of LIBs. Collectively, these contributions provide data and analytical tools for understanding battery degradation in dynamic usage conditions and its underlying causes.