Upgrading Garnet–Polymer Composite Electrolytes for Solid-State Lithium Batteries: The Role of the Hydrogen Bonds and PTFE Fibers

Garnet–polymer composite electrolytes, such as Li6.4La3Zr1.4Ta0.6O12/poly(ethylene oxide) (LLZTO/PEO), integrate the benefits of organic polymer electrolytes and inorganic ceramic fillers. However, their performance is often limited by the formation of a Li2CO3 passivation layer on LLZTO surfaces, which impedes Li+ migration, and by inadequate mechanical stability, especially above PEO’s melting point (60 °C), increasing susceptibility to Li dendrite penetration. This study proposes an interfacial reconstruction strategy, pretreating LLZTO with poly(phosphoric acid) (PPA) to form PPA-LLZTO, replacing the detrimental Li2CO3 layer with a Li-PPA-OH ionic conductor shell. This modification enhances LLZTO dispersion in PEO and improves interfacial stability via hydrogen bonding between PPA-LLZTO’s −OH groups and PEO’s −O–CH2– groups. Additionally, PTFE fibers are incorporated to bolster the mechanical strength and thermal stability. The resulting PPA-LLZTO-filled, PTFE fiber-supported PEO composite electrolyte (PLPP, 25 μm thick) demonstrates high flexibility (3000 bends), excellent ionic conductivity (∼1 × 10–4 S cm–1 at 30 °C), and robust Li dendrite resistance (>500 h). This novel PLPP composite electrolyte is demonstrated to be suitable for solid-state Li-metal batteries paired with LiFePO4, delivering a high capacity of ∼150 mAh g–1 at 0.2 C and maintaining ∼90% of the initial capacity after 200 cycles. This work offers a promising approach for designing high-performance garnet–polymer composite electrolytes through the interfacial reconstruction of garnet fillers.