Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly study Li metal in standard batteries due to the extremely weak scattering from Li atoms. In this work, it is demonstrated the stripping and plating of Li metal can be effectively quantified during battery cycling in appropriately designed synchrotron XRD experiments that utilize an anode-free battery configuration in which a Li-containing cathode material of LiNi0.6Mn0.2Co0.2O2 (NMC622) is paired with a bare anode current collector consisting of either Cu metal (Cu/NMC) or Mo metal (Mo/NMC). In this configuration, it is possible to probe local variations in the deposition and stripping of Li metal with sufficient spatial sensitivity to map the inhomogeneity in pouch cells and to follow Li deposition and stripping with sufficient time resolution to track state of charge dependent variations in the rate of Li usage at a single point. For the Cu/NMC and Mo/NMC batteries, it was observed that the initial plating of Li occurred in a very homogeneous manner but severe macroscopic inhomogeneity arose on a mm-scale during the subsequent stripping of Li, contrasting with the conventional wisdom that the greatest challenges in Li metal batteries are associated with Li deposition.

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While there have been many synchrotron studies of the intercalation and de-intercalation of Li ions into battery cathode materials, there have been few corresponding studies of the behavior of the Li metal anodes that are commonly used in half-cell batteries due to the poor quality of the diffraction signal from Li metal. In this work, we demonstrate the suitability of synchrotron methods for quantifying both the amount and the heterogeneity of Li metal deposited and stripped during electrochemical cycling. This work provides the opportunity to better understand the formation of and the impact of “dead” Li during cycling, and contributes to the overall Battery500 Consortium goal of developing batteries with Li metal anodes that can provide both a high energy density and a long lifetime.