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Electrifying passenger transport is a key strategy in combating global warming, with Li-ion batteries (LIBs) being the current go-to technology. Despite LIB’s satisfactory performance and carbon-neutral operation, lifetime and safety are still public concerns. A thorough understanding of battery ageing is crucial for improving LIBs and advancing the overall sustainability of LIB technology. This thesis bridges a gap between academic and industrial research by combining commercial battery investigation with a multiscale approach using a combination of in-house and synchrotron characterization methods used with the implementation of method development to study commercial batteries.

The multiple degradation mechanisms were identified at various scales in the aged commercial cells. Specifically, the results show that the studied cells exhibit significant and distinct ageing heterogeneity in prismatic and cylindrical cell formats, where the area with the highest degradation is found on the side of the positive tab, where the current and temperature gradients are expected to be the strongest. After decoupling the performance on the electrode level, the Ni-rich layered oxide positive electrodes show a significant increase in Li⁺ diffusion resistance in the aged materials as a function of the State of Charge (SoC) range and temperature. Furthermore, heterogeneity is an issue relevant also on a secondary particle scale, where identified SoC gradients ranging from the centre to the surface of the particle might induce kinetic limitations and cause an increase in Li+ diffusion resistance. On a single particle level, the formation of a large number of voids within the grains was found. Such degradation can additionally contribute to the resistance increase in the material by changing tortuosity for Li-ions. Finally, at the atomic level, Ni was found to be the dominant charge compensator, which can decrease up to 25% of the redox activity after ageing. Compared to Ni, Co was found to be less redox-active, but more involved in charge compensation through changes in hybridization with the oxygen atom. The oxygen, in turn, was revealed to participate in anionic redox reactions at low SoC by both hybridization to TM and also through the formation of molecular oxygen at lower potentials than previously reported. The observed decrease in oxygen anion redox activity follows with material losing performance.

The results presented in the thesis demonstrate the importance of the multiscale approach in order to form a more complete understanding of the degradation processes which have effects within different scales.

Rechargeable alkali batteries (RABs) are a key technology for alleviating global energy demands, operating via reversible electrochemical reactions at the positive electrode (cathode) and negative electrode (anode). Current state-of-the-art lithium-ion batteries (LIBs) achieve a high coulombic efficiency (CE) of more than 99.98%, meaning only 0.02% of reactions are undesired, known as side reactions. This high CE is partly due to the presence of a passivating layer at the anode known as the solid electrolyte interphase (SEI) that protects the electrolyte from being reductively decomposed. However, there are still unwanted side reactions occurring at the cathode that cause long term capacity fade. This thesis presents the development and application of advanced methodologies for studying degradation processes at the cathode across various battery types. The operando gas analysis technique known as Online Electrochemical Mass Spectrometry (OEMS) was used throughout the thesis. Three variants of OEMS are explored: purging OEMS (POEMS), closed leak OEMS (CLEMS) and intermittently closed OEMS (ICEMS) of which the latter was developed in this work. A means of interfacing large-format prismatic and cylindrical cells with ICEMS & CLEMS was additionally developed, enabling the investigation of gas evolution in commercially relevant cells. POEMS was primarily used to study laboratory scale model systems. Four cathode materials (CAMs) are studied: two state-of-the-art (lithium nickel cobalt aluminium oxide, NCA, and lithium nickel manganese cobalt oxide, NMC) and two next-generation (sodium (II) hexacyanoferrate (Prussian White, PW) and lithium nickel manganese oxide, LNMO). Each CAM exhibited distinct degradation behaviours, with NMC, NCA and PW suffering from structural degradation and LNMO from electrolyte oxidation. It was found that the lattice oxygen release from NMC and NCA (which reacts with the electrolyte forming CO₂) affects commercial cells. An analogous process was observed in PW, where reconstruction at the surface resulted in CN ligand release at high potentials, which react with the electrolyte forming both CO₂ and (CN)₂. A comprehensive reaction pathway for electrolyte decomposition on LNMO positive electrodes was proposed, highlighting how oxidation products (namely protons) autocatalyze decomposition of multiple components in the electrode. Additionally, strategies to mitigate these processes were explored; including forced surface reconstruction on PW, Ta-doping in NMC, and use of cathode electrolyte interphase (CEI) forming additives on LNMO. Furthermore, a novel method for screening for effective CEI forming additives was developed using POEMS. These findings highlight the versatility of OEMS as a powerful tool for understanding and mitigating degradation in RABs.