Li3MnO4, a Mn5+-containing phase, is a promising cathode material for Li-ion batteries due to its very high theoretical capacity of 698Ah/kg. However, the possibility to exploit only part of the theoretical capacity and its rapid capacity fading prevents the use in Li-ion batteries. An ex-situ post mortem analysis combining XRD, SEM, XPS and Raman measurements revealed that lithium extraction or insertion gives rise to amorphization of the structure with conversion to a more stable amorphous MnO2 phase, already after the first cycle. Formation of solid electrolyte interface on the surface of the particles during discharge and its dissolution during charge was also demonstrated. In conclusion, a model for the degradation mechanism of Li3MnO4 in Li-ion batteries was proposed.