Self-discharge of LiMn2O4/C Li-ion cells in their discharged state: Understanding by means of three-electrode measurements

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°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°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