Cryo-ultramicrotomy enables TEM characterization of global lithium/polymer interfaces

Abstract

Lithium electrochemistry dictates the safety and cycling durability of lithium metal batteries. Although there have been numerous spectroscopy studies of lithium in liquid electrolyte batteries, electron microscopy studies of lithium in solid state batteries are scarce due to the technical difficulty of sample fabrication. In this work, we introduce an innovative specimen preparation methodology dubbed cryo-ultramicrotomy to fabricate large-scale cryo-TEM specimens that permit direct visualization of global anode/electrolyte interfaces and lithium within the polymer electrolyte from the nanometer to millimeter scale. By combining electron energy loss spectroscopy and multivariate least squares fitting, we map the lithium/PEO interface and the deposited lithium clusters, revealing a novel tri-layer SEI|Li|SEI structure on the lithium anode surface, indicating that electrons can tunnel through the SEI layer. The deposited lithium features either isolated or interconnected clusters that are different from the lithium dendrites in liquid electrolytes. The lithium clusters comprise Li⁰ and LiH cores surrounded by inorganic components such as Li₂O and LiOH but with little LiF, which are further enwrapped by the organic components. Based on this understanding, tetrabutylammonium fluoride was introduced into the electrolyte to tailor the SEI and PEO structures to significantly improve the electrochemical performance of lithium metal batteries.