Effect of Nanoparticle Surface Modification on Lithium-Ion Transport in Composite Polymer Electrolytes

Abstract

Solid-state lithium-metal batteries require electrolytes that combine high ionic conductivity with efficient interfacial transport. In this work, we systematically examine how nanoparticle fillers with distinct chemistries and surface functionalizations govern interfacial Li+ transport in composite polymer electrolytes (CPEs). Inert Al2O3 fillers improve CPE performance through mechanical integrity and interfacial contact with lithium; however, surface modification with γ‑methacryloxypropyltrimethoxysilane diminishes these benefits, resulting in reduced electrochemical performance relative to neat Al2O3. In contrast, modifying lithium containing Li6.25Al0.25La3Zr2O12 (LLZO) nanoparticles with the same silane agent substantially impacted cell performance: the interfacial resistance decreased, and the lithium-ion transference number increased, yielding a threefold rise in critical current density to the systems containing modified Al2O3. LLZO functionalized with (3‑aminopropyl)triethoxysilane in an aprotic solvent exhibited poorer electrochemical performance. Solid-state NMR showed reduced lithium content in both silane-modified LLZO samples, with more pronounced depletion under protic modification. Complementary T₁ measurements revealed a larger fraction of fast-relaxing lithium near the nanoparticle surface, consistent with accelerated local Li-ion dynamics. We attribute the performance enhancement with LLZO nanofillers to silane-induced lithium vacancies at or near LLZO surfaces that facilitate interfacial Li+ motion. These results demonstrate that Li-containing fillers are necessary to achieve interfacial transport improvements even when they do not contribute to bulk Li transport, and that performance is governed by changes in local lithium concentration, interfacial chemistry, and ion dynamics. This work establishes a structure–property relationship between silane functionality and ion transport, providing a practical framework for the future interface design toward enabling continuous bulk transport through Li-containing nanofillers in high-performance solid-state electrolytes.