Observation of preferential sputtering of Si/graphite anodes from Li-ion cells by GD-OES and its validation by neutron depth profiling

Although the loss of Li inventory (LLI) is a common aging mechanism in Li-ion batteries, there are only few methods capable of comprehensive depth profiling within the bulk of the electrode to locate the residual Li. Two post-mortem analytical methods, which can be used to obtain quantified Li depth profiles to depths greater than 10 μm from the electrode surface are neutron depth profiling (NDP) and glow discharge optical emission spectroscopy (GD-OES). In this work, the validation of GD-OES using NDP by examining the Si/graphite anodes of cylindrical 21700 cells is presented. One anode was in a pristine/fresh state, two anodes were aged to the state of health (SOH) 90% and 76% at 45 °C respectively, and suffered from heavy solid electrolyte interphase (SEI) growth, and the fourth anode was aged to SOH 60% at 0 °C and exhibited Li plating. It is demonstrated that Li plating leads to a faster sputter rate of the Li-rich anode surface. An adapted method is introduced to incorporate the changing sputter rate into the depth profile calculation. For the aged anodes, the progressive aging of the anodes leads to a higher amount of irreversibly bound Li on the anode surface.