Lithium-Rich NMC (Li₂MnO₃-LMO₂) is an attractive cathode material for lithium ion batteries due to its high reversible capacity above 200 mAh/g. The Li₂MnO₃ component contributes to the high capacity through an activation process in the first delithiation cycle. During activation, oxygen and lithium are removed from the structure, causing compositional stresses that lead to mechanical degradation. In this study, LR-NMC thin-films are prepared by sputtering on sapphire substrates and characterized using XRD, Raman spectroscopy, and SEM. A Multibeam Optical Stress Sensor is used to quantitatively measure stress in the cathode film during electrochemical cycling. A unique stress signature is observed during the first charge when activation occurs. Initially, a tensile stress is observed, consistent with volume contraction from lithium removal. The stress reverses and becomes compressive with continued charging beyond 4V vs Li/Li+, indicating volume expansion; this phenomenon is present in the first cycle only. The origin of this irreversible stress during the first charge is not clear; cracking and oxygen loss are explored as possible causes. Subsequent cycling is reversible, with volume expansion during lithiation and volume contraction during delithiation. Raman spectroscopy provides evidence of the layered-to-spinel phase transition after cycling past the activation plateau, as well as recovery of the original spectra upon reannealing in an oxygen environment. Atomistic modelling in conjunction with experimental results aids in understanding the effects of stress on kinetics and phase transformations, guiding the development of improved Li₂MnO₃-LMO₂ cathodes.