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 LiMn,O,/C Li-ion cells encountered at 55°C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li,Ti,O,, 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°C performance is mainly due to the instability of the LiMn,O, phase toward Mn dissolution in LiPF,-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn,O,/C Li-ion cells.

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