Electric vehicles (EVs) have gained widespread attention in recent years as the dominant strategy for curbing CO2 emissions through transport electrification. Lithium-ion batteries (LIBs) are currently the most suitable and almost exclusively employed energy storage device for powering EVs. Nickel-rich layered oxides of the general formula LiNixCoyMnzO2 (NCM) are widely used as the cathode in LIBs due to their high capacity and low cost. However, the increasing nickel content reduces the stability for the higher energy density that is particularly required for meeting the mileage demand. Hence, understanding the degradation mechanisms of Ni-rich NCM is of great importance for optimizing the performance of battery systems. This thesis aims to develop methodologies for reproducible and reliable testing of battery systems using transmission electron microscopy (TEM) in the liquid phase in order to probe, at the nanoscale, the evolution of the cathode materials as they respond to the electrochemical charge/discharge cycling. In detail, this thesis first establishes lithium-ion battery electrochemical processes in a microcell, such as electroplating and charge/discharge analysis. The setup is optimized through the electrode configuration and preparation, while the cycling conditions are adapted for achieving similar electrochemical performance as compared to the conventional bulk cell. With the optimized cell, primary NCM particles are probed using in situ electrochemical cells coupled with a scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS). The electrochemically induced evolution of NCM that is analyzed using liquid cell TEM uncovers the various degradation mechanisms for the same material under different cycling conditions and for specimens with different Ni content that are cycled under the same conditions. The results offer insights for controlling the electrochemical parameters in the liquid cell for battery studies. Ultimately, the structural and chemical information on the degradation of NCM cathodes can guide our way for designing more advanced NCM cathodes with desirable capacity and enhanced stability simultaneously.