Fatigue Process in Li-Ion Cells: An In Situ Combined Neutron Diffraction and Electrochemical Study

In situ high-resolution neutron powder diffraction along with electrochemical analysis was applied to study fatigue processes in commercial LiCoO 2 (18650-type, 2600 mAh, 3.0–4.2 V) cells. The Rietveld refinement technique was successfully applied for structural characterization of the cell materials as function of fatigue introduced. The capacity loss determined from the electrochemical investigation has been found proportional to the number of cycles, which the cell underwent, and was 21.9% and 12.8% for the cell cycled for 1000 times at 25°C and 50°C, respectively. This capacity fade has a close correlation to the structural changes in cathode and anode materials with the cell fatigue. A change of the lithium occupation and lattice parameters of the Li x CoO 2 were observed with proceeding battery fatigue. Transformation of the graphitic anode into LiC 6 and LiC 12 was identified after the batteries were fully charged. Their weight ratio was found to be dependent on the number of executed cycles. It was concluded that loss of the active lithium in both cathode and anode as well as microstructure changes on the anode side are important factors for the battery degradation. A favorable influence of the elevated cycling temperature on the battery performance was observed.