The potential distribution through plastic Li ion cells during electrochemical testing was monitored by means of three-or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn2O4/C Li-ion cells encountered at 55 degrees C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li4Ti5O12 into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (i) the state of charge of each electrode within a Li-ion cell, (ii) their polarization, and (iii) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55 degrees C performance is mainly due to the instability of the LiMn2O4 phase toward Mn dissolution in LiPF6-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn2O4/C Li-ion cells.