Interfacial Degradation Processes in Electrochemical Energy Storage and Conversion

Zhao, Dongni and Mertens, Stijn (2025) Interfacial Degradation Processes in Electrochemical Energy Storage and Conversion. PhD thesis, Lancaster University.

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Abstract

The comprehensive usage of clean energy is the biggest challenge of society in the 21st century. Advanced energy storage and conversion systems play a critical role in this vision, and electrochemical energy storage and conversion are the most promising candidates. However, their premature ageing limits their wider application. One of the biggest issues is the active material dissolution under electrochemical conditions, but the limited availability of characterization methods hinders an in-depth understanding of the process as well as mitigation strategies. In this thesis, we develop real-time/in-situ/operando electrochemical techniques to study the degradation processes and mechanisms happening at the electrode-electrolyte interfaces within the Li-ion battery and water electrolyzer. We first establish online inductively coupled plasma-optical emission spectrometry (online ICP-OES) for real-time electrolyte analysis, enabling the direct detection of transition metal dissolution from Li-ion cathodes, taking LiMn2O4 (LMO) as an example. Then, we apply the rotating ring-disc electrode (RRDE) technique for real-time speciation of the dissolving transition metal ions, which is the basis of a comprehensive understanding of the dissolution mechanism, and supplements findings from online ICP-OES, which could provide extraordinary detection limits for multiple elements but is blind to the oxidation states of the dissolving species. We focus on Mn, and combine voltammetry on stationary and rotating electrodes with electrochemical impedance spectroscopy and Raman spectroscopy. With the understanding of Mn behaviour on the ring and disc electrodes, respectively, we then investigate LMO dissolution and speciation on the ring electrode in real-time. One of the widely reported Mn dissolution mitigation strategies is to replace one-fourth of the Mn in LMO with Ni, which then forms LiNi0.5Mn1.5O4 (LNMO). We compared the chemical composition, morphology, crystal structure, and electrochemical properties including cycling capacity, stability, and rate capability of 2 commercial and 3 lab-synthesized LNMO samples. Their dynamic phase transformation during (dis)charging was observed by operando Raman spectroscopy, aimed at studying the dynamic cathode-electrolyte interphase (CEI) changes under different electrochemical conditions and their impact on cycling and rate performances. Water electrolysis is considered to be the most promising way to generate green hydrogen as the next-generation energy carrier. Such a process includes the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. The complexity and sluggish kinetics of OER make it often the limit of the whole process and thus usually requires advanced electrocatalysts. Ideal OER electrocatalysts should have both excellent activity and stability. While the former can be directly observed electrochemically, the latter is usually hard to characterize due to the lack of available techniques. We here employed online ICP-OES for direct detection of active material dissolution of several promising candidates of OER electrocatalysis, which provide direct evidence of the stability of the electrocatalysts. To summarize, we established several real-time/in-situ/operando electrochemical techniques for studying the electrode-electrolyte interface in electrochemical energy storage and conversion systems. These results will help to better understand and mitigate the premature ageing of those systems.