Detailed investigation of degradation modes and mechanisms of a cylindrical high-energy Li-ion cell cycled at different temperatures

Kurzfassung

The temperature-dependent degradation of a cylindrical high-energy NCA/graphite-SiOx cell is investigated by a combination of in situ and post-mortem techniques. Cells are cycled at 10 °C, 25 °C and 35 °C, with periodic check-up tests to identify the degradation modes during aging. Differential voltage analysis (DVA) identifies the primary degradation modes over all temperatures as the loss of the silicon active material as well as the loss of lithium inventory. Electrochemical impedance spectroscopy, evaluated by the distribution of relaxation times and equivalent circuit modeling, shows that the resistance increase is temperature dependent with a sudden increase after the sudden failure of the cells for all resistances except the cathode impedance, which is primarily cycle dependent. Notably, reversible Li plating is observed by differential open circuit voltage analysis in cells aged at 10 °C, despite moderate charging currents. The evaluation of the Coulombic efficiency shows the onset of irreversible Li plating coincides with the sudden failure of the cells. An extensive post-mortem analysis is conducted including SEM/EDS, XRD and electrochemical measurements in a three-electrode set-up. Post-mortem results reveal a more inhomogeneous degradation at lower temperatures compared to higher temperatures. In addition, it exposes the extent of the loss of the cathode active material at higher temperatures, which is masked in the DVA of the full cell. These results highlight the significant impact of temperature on battery performance and degradation mechanisms and show the benefit of post-mortem analysis in addition to non-invasive characterization techniques.