Understanding the Influence of Li7La3Zr2O12 Nanofibers on Critical Current Density and Coulombic Efficiency in Composite Polymer Electrolytes

Composite polymer electrolytes (CPEs)are attractive materialsfor solid-state lithium metal batteries, owing to their high ionicconductivity from ceramic ionic conductors and flexibility from polymercomponents. As with all lithium metal batteries, however, CPEs facethe challenge of dendrite formation and propagation. Not only doesthis lower the critical current density (CCD) before cell shorting,but the uncontrolled growth of lithium deposits may limit Coulombicefficiency (CE) by creating dead lithium. Here, we present a fundamentalstudy on how the ceramic components of CPEs influence these characteristics.CPE membranes based on poly-(ethylene oxide) and lithium bis-(trifluoromethanesulfonyl)-imide(PEO-LiTFSI) with Li7La3Zr2O12 (LLZO) nanofibers were fabricated with industrially relevantroll-to-roll manufacturing techniques. Galvanostatic cycling withlithium symmetric cells shows that the CCD can be tripled by including50 wt % LLZO, but half-cell cycling reveals that this comes at thecost of CE. Varying the LLZO loading shows that even a small amountof LLZO drastically lowers the CE, from 88% at 0 wt % LLZO to 77%at just 2 wt % LLZO. Mesoscale modeling reveals that the increasein CCD cannot be explained by an increase in the macroscopic or microscopicstiffness of the electrolyte; only the microstructure of the LLZOnanofibers in the PEO-LiTFSI matrix slows dendrite growth by presentingphysical barriers that the dendrites must push or grow around. Thistortuous lithium growth mechanism around the LLZO is corroboratedwith mass spectrometry imaging. This work highlights important elementsto consider in the design of CPEs for high-efficiency lithium metalbatteries.