Dimitrijevic, Alexander; (2024) The influence of electrochemical history on lifetime of commercial lithium-ion batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The urgent need to address global warming has propelled the development and optimisation of lithium-ion (Li-ion) batteries, particularly those with high nickel content, due to their critical role in the electrification of transportation and renewable energy integration. However, despite their high energy density, these cells are susceptible to accelerated degradation, and continuous efforts to characterise and mitigate these degradation mechanisms are crucial for enhancing their performance under diverse applications, including high-demand sectors such as the automotive industry or the aerospace sector. Accordingly, this thesis presents a comprehensive study of the longevity and performance of commercial cylindrical Li-ion, addressing various key elements influencing battery life. For instance, the predictive capability of high-precision coulometry (HPC) was explored and it was shown that high coulombic efficiency (CE) correlates with improved long-term capacity retention, as confirmed by the cycling data provided by an industrial partner. This early-life assessment became a reliable metric across different cell models, consisting of various chemistries and other design differences, reinforcing CE’s utility in identifying cells with greater longevity potential before significant capacity loss is observed. Furthermore, commercial cells were subjected to several variable current protocols at various temperatures, and it was shown, via various characterisation methods, that the protocols that prolonged time spent at high voltage and high SoC had the most detrimental effect on battery life, with dramatic power fade, solid electrolyte interphase (SEI) growth, and SiOx additives degradation at the anode. The time spent at high SoC emerged again as a prevalent metric during a calendar ageing study where the notable "spoon-shape" capacity fade was observed, marked by increased fade at 70-80% SoC with a partial recovery at 100% SoC. The effect has been shown to be temporary, as cells held at 100% SoC subsequently degraded faster upon cycling. The storage mechanisms involved were identified to be anode and SEI-dominated, as cathode characterisation revealed the material was relatively well preserved.

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