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

Abstract

Composite polymer electrolytes 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 [(3-aminopropyl)triethoxysilane (APTES)] to LLZO nanoparticles, which protects LLZO in air. APTES functionalization lowers the poly (ethylene oxide) (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-lithium bis(trifluoromethanesulfonyl)imide (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 nuclear magnetic resonance 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 composite polymer electrolytes.

Keywords

Composite polymer electrolyte / LLZO / interface / interphase / silane / lithium vacancies / ion transport

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