The suppression of dendritic lithium growth is a pivotal factor in advancing high-energy-density lithium metal batteries with solid-state or polymer electrolytes. Conventional models assume that lithium dendrites exclusively initiate at the electrode surface. However, detecting early-stage microstructural phase transformations deep within polymer electrolytes during battery operation remains an analytical challenge. In this study, the authors employ operando scanning nanofocus wide-angle X-ray scattering (nWAXS) on a symmetric lithium cell with a gel-type polymer electrolyte (PVDF-HFP/PSTFSILI) to monitor spatial and temporal structural dynamics with nanometer resolution. The researchers visualized the real-time movement and recrystallization kinetics of lithium carbonate as well as stationary lithium hydroxide crystallites. Crucially, the direct observation of metallic lithium crystallites forming inside the bulk polymer electrolyte reveals that dendrite growth can originate within the electrolyte itself, driven by residual electronic conductivity rather than sole electrode interface phenomena. These insights refine the understanding of degradation mechanisms in solid polymer electrolytes, providing critical guidance for engineering safer, next-generation lithium metal battery systems.

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