In Situ Grown Hydrochar@ZIF-8 Hybrid Fillers for High-Performance PEO-Based Solid Polymer Electrolytes

All-solid-state lithium metal batteries (ASSLMBs) have attracted considerable attention owing to their high energy density and enhanced safety. However, poly(ethylene oxide) (PEO)-based solid polymer electrolytes suffer from limited lithium-ion transport due to their high crystallinity. Although metal–organic frameworks (MOFs) have been employed as functional fillers, particle aggregation often leads to poor dispersion and reduced effectiveness.

In this study, a Hydrochar@ZIF-8 hybrid filler was synthesized by directly growing ZIF-8 on glucose-derived hydrochar and incorporated into a PEO/LiTFSI-based solid polymer electrolyte. The in situ growth strategy effectively suppressed MOF aggregation and improved filler dispersion. The specific surface area increased from 13.27 to 86.06 m² g⁻¹ after ZIF-8 growth, providing porous interfaces that facilitated lithium salt dissociation and ion transport. Consequently, the glass transition temperature (Tg) decreased from −36.48 to −43.91 °C, the melting temperature (Tm) decreased from 55.8 to 48.17 °C, and the crystallinity was reduced from 37.8% to 32.6%.

The electrolyte containing 3 wt% Hydrochar@ZIF-8 exhibited the highest ionic conductivity of 1.13 × 10⁻³ S cm⁻¹ at 60 °C and a lithium-ion transference number of 0.68. In addition, the optimized electrolyte maintained excellent mechanical flexibility with an elongation at break of approximately 2300%. Li/LFP all-solid-state batteries employing the optimized electrolyte retained over 80% of their initial capacity after more than 800 cycles at 1 C, demonstrating outstanding cycling stability.

These results demonstrate that biomass-derived Hydrochar@ZIF-8 simultaneously improves ionic transport, mechanical flexibility, and cycling stability by suppressing MOF aggregation and reducing PEO crystallinity, providing an effective strategy for high-performance solid polymer electrolytes.