Low-Surface-Energy Nanofillers for Fast and Selective Li+ Transport toward Stable Solid-State Lithium Metal Batteries

Abstract

Composite solid-state electrolytes (CSEs) are key to enabling safe and high-energy-density solid-state lithium metal batteries, yet achieving fast and selective lithium-ion (Li⁺) transport remains a major challenge. Conventional nanofiller strategies mainly suppress polymer crystallinity, while the role of nanofiller surface energy in Li⁺ transport is often overlooked. Here, we develop low-surface-energy nanofillers to promote fast and selective Li⁺ transport and improve interfacial stability in poly(ethylene oxide)-based CSEs. By tuning the surface energy of SiO₂ nanofillers through controlled hydrolytic condensation of vinyltrichlorosilane, low-surface-energy interfaces are introduced that facilitate Li⁺ solvation and coordination. As a result, the electrolyte delivers a total ionic conductivity of 0.62 mS cm^–1, a Li⁺ transference number of 0.81, and an electrochemical stability window up to 4.3 V vs. Li/Li⁺. Mechanistic studies show that weakened Li⁺-TFSI^– interactions, optimized Li⁺-EO coordination, and enhanced polymer segmental motion together enable fast and selective Li⁺ transport. As a result, Li symmetric cells exhibit stable cycling for over 2500 h, while Li/LFP cells retain 97.0% of their capacity after 500 cycles at 1.0 C. Flexible pouch cells can also operate under zero external pressure with stable performance under mechanical deformation. These results suggest that the apparent low-surface-energy interfacial state of nanofillers, arising from coupled changes in surface chemistry, hierarchical morphology, and filler dispersion, is closely associated with improved Li⁺ transport and enhanced battery stability, providing a practical route toward stable solid-state lithium metal batteries.