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

Li(Ni,Mn,Co)O2/carbon lithium-ion batteries designed to work at high temperature exhibit good performances for cycling at 85 °C but a strong impedance increase for cycling or storage at 120 °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, 7Li and 19F Nuclear Magnetic Resonance, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. The main phenomenon observed at 120 °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 °C and cycling at 85 °C and 120 °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)O2 seems to be an appropriate material for high temperature cycling. ► Li(Ni,Mn,Co)O2/carbon lithium-ion batteries were cycled at +85 °C and +120 °C for 15 days. ► Good stability of chosen electrochemistry at +85 °C. ► XPS, XRD, SEM and NMR were used to identify main aging mechanisms. ► Positive electrode binder PVDF was identified as the main aging root cause. ► Evolution of the SEI composition was quantified between +85 °C and +120 °C.