Electron microscopy study of degradation mechanisms in Ni-rich transition metal oxide cathodes for Li-ion batteries
Li-ion batteries have a pivotal role in the transition towards electric transportation. This drastic societal change depends on high energy, sustainable, and long lifetime batteries. Among the most promising cathode active materials that enable such batteries is the class of Ni-rich layered transition metal oxides such as LiNi0.8Mn0.1Co0.1O2 (NMC811). However, they exhibit complex degradation mechanisms that impair their longevity and impede their commercial application. The degradation often has its roots in nanoscale processes that require careful characterisation with high spatial resolution techniques. The aim of this thesis is to advance the understanding of these degradation mechanisms using electron microscopy techniques. The relative importance of various aspects of NMC811 degradation is probed using tailored electrochemical protocols. Electron energy loss spectroscopy in a scanning transmission electron microscope, as well as focused-ion beam scanning electron microscopy cross sectional imaging allow for investigating the role of intergranular cracks and reduced surface layers in NMC811. The reduced surface layer evolution is found to be correlated with the impedance rise of NMC811, which suggests the importance of the surface layers for the performance of NMC811 based cells. Moreover, the underlying mechanisms of intragranular crack formation are investigated using electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and high-resolution imaging in a scanning transmission electron microscope. Briefly, it is found that local stresses can lead to opening of intragranular cracks in the charged state already in the first few cycles. Initially, the cracks are mostly reversible. However, over longer cycling they can become detrimental to the battery performance due to plane gliding and fragmentation of particles. Lastly, a step towards more reliable operando electrochemical transmission electron microscopy techniques is developed. Using aerosol-jet printing, microbatteries consisting of commercially available battery active material powders are fabricated. This way, high resolution, placement precision and the ability to deposit arbitrary shapes as well as combine materials in a facile way is achieved. Overall, this thesis provides important insights into the degradation mechanisms and their relative importance for Ni-rich layered transition metal oxide cathodes and lays foundation towards operando electrochemical electron microscopy studies of industrially relevant batteries.
electron microscopy, electrochemistry, batteries, materials science
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