Aging of the LiNi[sub 1∕2]Mn[sub 1∕2]O[sub 2] Positive Electrode Interface in Electrolyte

The evolution of lithium-containing species on the surface of grains of layered material during the aging process in (ethylene carbonate/dimethyl carbonate, ) electrolyte has been followed using magic angle spinning NMR spectroscopy. Materials displaying different surface areas have been investigated in order to study the influence of the surface/volume ratio. The evolution of the NMR signal shows that the reaction of the active material with the electrolyte is extremely fast during the first moments of exposure and tends to slow down for longer exposure times. Coupled NMR, electrochemical impedance spectroscopy, and transmission electron microscopy experiments showed that the surface of the material grains is not covered by a homogeneous layer, indicating that the reaction with electrolyte cannot be considered as a real passivation reaction. The aging process performed on a sample initially stored in ambient atmosphere clearly demonstrates the dissolution of a pristine surface layer and the growth of an interphase made primarily of fluorinated compounds.