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 1.0C. 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.