Improved interfacial li-ion transport in composite polymer electrolytes via surface modification of LLZO

Composite polymer electrolytes (CPEs) that incorporate ceramic fillers in a polymer matrix offer mechanical strength and flexibility as solid electrolytes for lithium metal batteries. However, fast Li⁺ transport between polymer and Li⁺-conductive filler phases is not a simple achievement due to high barriers for Li⁺ exchange across the interphase. This study demonstrates how modification of Li₇La₃Zr₂O₁₂ (LLZO) nanofiller surfaces with silane chemistries influences Li⁺ transport at local and global electrolyte scales. Anhydrous reactions covalently link amine-functionalized silanes (APTES) to LLZO nanoparticles, which protects LLZO in air. APTES functionalization lowers the PEO-LLZO interphase resistance to half that of unmodified LLZO and increases effective Li⁺ transference number, while insulating Al₂O₃ completely blocks ion exchange and lowers transference number and conductivity in PEO-LiTFSI-LLZO composites. Modeling an inner resistive interphase between LLZO and PEO surrounded by an outer conductive interphase explains non-linear conductivity trends. Solid-state ⁷Li & ⁶Li NMR shows Li⁺ only exchanges between PEO-LiTFSI and some LLZO interphase, with no appreciable Li⁺ transport through bulk LLZO. Surface functionalization is a promising path toward lowering the polymer-ceramic interphase resistance. This work demonstrates that local changes in Li⁺ transport affect macroscopic performance, highlighting the intricate relationships between all interfaces in inherently heterogeneous CPEs.