Commercial battery cells with Li4Ti5O12 negative electrode and composite metal oxide positive electrode have been analyzed with respect to aging mechanisms. Electrochemical impedance spectroscopy (EIS), differential capacity analysis (dQ/dV), differential voltage analysis (dV/dQ) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) were used to identify different ageing mechanisms such as lithium inventory loss, loss of active electrode material and surface film growth. The active material of the positive electrode was also examined by X-ray diffraction (XRD). Aging mechanisms were studied for both calendar-aged and cycle-aged cells. Data from half cells prepared from post mortem harvested electrode material, using lithium foil as negative electrode and pouch material as encapsulation, were used as reference to full cell data. Electrochemical analysis of full and half cells combined with material analysis showed to be a powerful method to identify aging mechanisms in this type of commercial cells. The calendar-aged cell showed insignificant aging while the cycle-aged cell showed noticeable loss of positive electrode active material and loss of cyclable lithium, but only minor loss of negative electrode active material. The results imply that Li4Ti5O12 negative electrode material is a good alternative to other materials if high energy density is not the primary goal.