Non-invasive yet separate investigation of anode/cathode degradation of lithium-ion batteries (nickel-cobalt-manganese vs. graphite) due to accelerated aging

The non-invasive investigation of lithium-ion batteries is of great importance, e.g. for improvement of electrode materials or monitoring the state of health (SOH) in stationary or mobile applications. Electrochemical impedance spectroscopy (EIS) is a powerful tool for this task. Once the predominant processes in the impedance spectra are assigned to their corresponding electrode (i.e. anode or cathode), a tracking of both electrode's SOH becomes possible. In a previous work, this assignment has been performed. Two predominant processes were found: the impedance of the solid electroly interphase (SEI) and the NMC's charge transfer. In this work, this information is used for a non-invasive yet separate investigation of anode and cathode degradation throughout aging. Cells have been aged for about 700 days (calendric aging) or about 3000 full cycle equivalents (cyclic aging) at three different operating points each. A combination of impedance spectra analysis, differential voltage analysis (DVA) and post mortem analysis (PMA) determines the main aging mechanisms. Calendric aging: SEI-growth, CEI-growth, cathode-dissolution. Cyclic aging: anode's and cathode's particle-cracking, cathode-dissolution and possibly CEI-growth.