SELF-DISCHARGE OF LIMN2O4 C LI-ION CELLS IN THEIR DISCHARGED STATE - UNDERSTANDING BY MEANS OF 3-ELECTRODE MEASUREMENTS/

The potential distribution through plastic Li ion cells during electro chemical testing was monitored by means of three-or four-electrode mea surements 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 compoun d 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 pol arization, and (iii) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical a nalyses, 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 e lectrolytes. A mechanism, based on Mn dissolution, is proposed to acco unt for the poor storage performance of LiMn2O4/C Li-ion cells.