Lithium secondary batteries working at very high temperature: Capacity fade and understanding of aging mechanisms

Li(Ni,Mn,Co)O-2/carbon lithium-ion batteries designed to Work at high temperature exhibit good performances for cycling at 85 degrees C but a strong impedance increase for cycling or storage at 120 degrees C. The effects of high temperature on the aging process of positive electrode's binder, electrodes/electrolyte interfaces and positive active material were investigated by bulk and surface analysis techniques: X-ray diffraction, Li-7 and F-19 Nuclear Magnetic Resonance, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. The main phenomenon observed at 120 degrees C is a migration of PVdF binder from the outer part of the positive electrode which thus shows a bad Li reintercalation. Binder migration results in the formation of a PVdF layer at the extreme surface of the positive electrode, and in its diffusion and deposition on the surface of the negative electrode. Several changes in the composition of the Solid Electrolyte Interphase (SEI) were also evidenced between a cycle at 60 degrees C and cycling at 85 degrees C and 120 degrees C. Disappearance of carbonate species disappearance and increase of inorganic species, which both together may also affect battery's performance. Though, since the battery can undergo 25 cycles at 120 C, Li(Ni,Mn,Co)02 seems to be an appropriate material for high temperature cycling. (C) 2013 Elsevier B.V. All rights reserved.