Abstract
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 
/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 
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 
phase toward Mn dissolution in 
‐type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of 
/C Li‐ion cells.