(Invited) Advancing Understanding of Composite Polymer Electrolytes with LLZO Nanofibers

Abstract

Ceramic Li6.55Al0.2La3Zr2O12 (LLZO) is one of the most attractive electrolyte materials for solid-state batteries that combined with metallic Li anodes holds the promise for safer and more energetically dense battery, but its performance is limited by the increases in electrolyte-electrode interfacial resistances upon cycling. The combination of Li+-conducting ceramics and polymers offers a new pathway to create better electrolytes with both high ionic conductivity and good (electro)mechanical interfacial properties. We optimized two-step annealing processing conditions for the fabrication of low temperature cubic-LLZO nanofibers and Li2CO3 removal from the surface of the nanofibers. Using industrially relevant roll-to-roll electrospinning and slot-die coating, we fabricated thin composite membranes with reproducible thickness down to 50 microns. We investigated PEO based composite polymer electrolyte (CPE) with a high, 50 wt% loading of Al-doped LLZO nanofibers in comparison with nanofiber-free PEO electrolyte. XPS measurements show that LLZO is not present at the composite electrolyte surface, and solid electrolyte interphase (SEI) formation is dictated solely by PEO and LiTFSI when reacting with Li metal. Electrochemical SEI formation, studied by cyclic voltammetry, shows SEI formation is identical with and without up to 75 wt% LLZO in the electrolyte. Galvanostatic cycling with lithium symmetric cells shows that the critical current density (CCD) can be tripled by including 50 wt% LLZO, but half cell cycling reveals this comes at the cost of CE. Varying the LLZO loading shows that even a small amount of LLZO drastically lowers the CE, from 88% at 0 wt% LLZO to 77% at just 2 wt% LLZO. Mesoscale modelling reveals that the increase in CCD cannot be explained by an increase in the macroscopic or microscopic stiffness of the electrolyte; only the microstructure of the LLZO nanofibers in the PEO-LiTFSI matrix slows dendrite growth by presenting physical barriers that the dendrites must push or grow around. This tortuous lithium growth mechanism around the LLZO is corroborated with mass spectrometry imaging. Electrochemical impedance spectroscopy (EIS) analysis showed comparable bulk ionic and interfacial resistances with and without nanofibers, indicating that PEO dominates Li+ transport and interfacial chemistry. The total Li-ion conductivity of the composite is still governed by the polymer matrix due to high interfacial resistance between the garnet particles and the PEO/LiTFSI matrix. This work highlights important elements to consider in the design of CPEs for high-efficiency lithium metal batteries.