Nonlinear aging of lithium-ion cells linked to pressure differences: Degradation mechanisms and remedies

Lithium-ion batteries have great potential in electromobility and stationary applications. Reusing aged batteries from cars in 2nd-life-applications could lower cost and ecological impact. For such usage scenarios, information on the expected remaining lifetime at takeover is critical. Current aging studies focus mostly on the aging behavior above 80 % residual capacity. Nonetheless, some studies report on an abrupt acceleration of the aging rate upon prolonged testing [1-4]. These results may challenge second life scenarios due to the aggravated predictability of the aging trends. A comprehensive aging experiment was performed on commercial NMC//graphite based 18650 cells to identify conditions leading to such a default. Pronounced nonlinear aging behavior could be reproduced in the laboratory for cells cycled with a high depth of discharge. Post-mortem investigations reveal localized aging mechanisms as the reason for nonlinear aging trends. In this publication, nonlinear aging behavior is linked to inhomogeneous aging phenomena in graphite based cells. Patterns of lithium plating, the migration of cathodic oxidation products and pronounced lithium loss are observed in affected cells. Localized lithium depletion appears to be the origin of an abrupt decline of the whole cell's performance. Klett et al. found similar plating patterns in LiFePO4//graphite based cells and stated that differences in temperature, pressure or electrode quality may be the cause for this behavior [4]. By using thermography, micro computed tomography, scanning electron microscopy and a variety of electrochemical measuring techniques, the aging mechanisms responsible for pronounced aging could clearly be linked to pressure differences, while temperature gradients could be ruled out. The impact of pressure is confirmed by applying controlled, localized stress and electrochemical control experiments. Based upon a comprehensive aging experiment and extensive post-mortem investigations, underlying causes for pronounced nonlinear aging near automotive end-of-life are clarified. A clear cause-and-effect chain from pressure differences to plating is established. The aging mechanisms leading to localized degradation in a lithium-ion cell are described qualitatively. It is confirmed that avoiding extreme anode potentials effectively reduces the risk of pronounced aging in carbon based lithium-ion cells. The necessary limits depend on anode composition and cell design. Based upon the presented findings, different cell designs can be evaluated by a combination of cyclic aging and visual post-mortem inspection.