2, Florida State University, Tallahassee, Florida, United States
All-solid-state rechargeable batteries embody the promise for high energy density, low cost, and improved stability. The success of all-solid-state rechargeable batteries is largely impeded by high resistance at electrode-electrolyte interfaces. Among all the causes for high interfacial resistance, Li deficiency has been proposed as one of the major culprit. Yet the experimental evidence is elusive due to the challenges associated with probing Li distribution. In this contribution, three-dimensional 7Li Magnetic Resonance Imaging (MRI) is employed to examine the homogeneity of Li distribution in a superionic conductor Li10GeP2S12 within symmetric Li/Li10GeP2S12/Li battery cells. 7Li MRI and derived Li histograms reveal depletion of Li from the electrode-electrolyte interface and increased heterogeneity of Li distribution in the bulk upon electrochemical cycling. The degree of Li loss at electrode-electrolyte interfaces, instead of solid-electrolyte-interphase formation, is determined as the dominant cause for high interfacial resistance. Significant Li loss at interfaces is mitigated via a facile modification with a Polyethylene oxide (PEO)/LiTFSI thin film. The Li/PEO-coated Li10GeP2S12/Li shows excellent long-term cycling stability with minimized Li loss. This study demonstrates a powerful tool for non-invasively monitoring Li distribution at the interfaces and in the bulk of all-solid-state batteries and a convenient strategy for improving interfacial stability.