Composite Polymer Electrolytes with Li₇La₃Zr₂O₁₂ Garnet-Type Nanowires as Ceramic Fillers: Mechanism of Conductivity Enhancement and Role of Doping and Morphology

Composite polymer solid electrolytes (CPEs) containing ceramic fillers embedded inside a polymer-salt matrix show great improvements in Li⁺ ionic conductivity compared to the polymer electrolyte alone. Lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) with a garnet-type crystal structure is a promising solid Li⁺ conductor. We show that by incorporating only 5 wt % of the ceramic filler comprising undoped, cubic-phase LLZO nanowires prepared by electrospinning, the room temperature ionic conductivity of a polyacrylonitrile-LiClO₄-based composite is increased 3 orders of magnitude to 1.31 × 10^-4 S/cm. Al-doped and Ta-doped LLZO nanowires are also synthesized and utilized as fillers, but the conductivity enhancement is similar as for the undoped LLZO nanowires. Solid-state nuclear magnetic resonance (NMR) studies show that LLZO NWs partially modify the PAN polymer matrix and create preferential pathways for Li⁺ conduction through the modified polymer regions. CPEs with LLZO nanoparticles and Al₂O₃ nanowire fillers are also studied to elucidate the role of filler type (active vs passive), LLZO composition (undoped vs doped), and morphology (nanowire vs nanoparticle) on the CPE conductivity. It is demonstrated that both intrinsic Li⁺ conductivity and nanowire morphology are needed for optimal performance when using 5 wt % of the ceramic filler in the CPE.