Uniform Garnet Nanoparticle Dispersion in Composite Polymer Electrolytes

Abstract: Solid-state lithium batteries (SSLBs) have the potential to further boost the energy density of Li-ion batteries and improve their safety by facilitating the use of Li-metal anodes and limiting flammability, respectively. Solid electrolytes, as key SSLB materials, significantly impact battery performance, among which composite polymer/garnet electrolytes are promising materials for manufacturing SSLBs on a large scale, owing to polymer electrolyte processing ease in combination with the thermal stabilities and high ionic conductivities of garnet electrolytes, both of which are beneficial. Uniformly dispersing garnet particles in the polymer matrix is important for ensuring a highly ionically conductive composite polymer electrolyte. However, high nanoparticle surface energies and incompatible organic–inorganic interfaces lead to garnet particle agglomeration in the polymer matrix and a poorly ionically conductive composite electrolyte. With the aim of promoting Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particle dispersion in both solvents and polymer matrices, in this study, we introduced the 3-glycidyloxypropyl trimethoxy silane (GPTMS) coupling agent onto the LLZTO surface. A 5-nm-thick GPTMS shell was constructed on each LLZTO nanoparticle by covalently bonding GPTMS molecules on the surface of the nanoparticle. The lipophilic epoxy group in GPTMS enables the uniform dispersion of GPTMS-modified LLZTO nanoparticles (LLZTO@GPTMS) in organic solvents, such as acetonitrile, N-methylpyrrolidone, and N,N-dimethylformamide. Particle-size-distribution experiments reveal that LLZTO-nanoparticle dispersity is positively correlated with solvent polarity. Well-dispersed LLZTO suspensions led to superior polyethylene-oxide-based (PEO-based) composite polymer electrolyte ionic conductivities of 2.31 × 10-4 S·cm-1 at 30 °C. Both symmetric lithium batteries and SSLBs that use LiFePO4 (LFP) cathodes, lithium-metal anodes, and the optimal PEO: LLZTO@GPTMS electrolyte exhibited prolonged cycling lives. Moreover, the polyethylene separator was homogeneously coated with LLZTO nanoparticles following GPTMS modification. LFP|Li batteries with LLZTO@GPTMS-coated PE separators exhibited better cycling stabilities than those of batteries with unmodified LLZTO/PE. This study demonstrated that GPTMS effectively improves LLZTO-nanoparticle dispersibility in both organic solvents and polymer matrices, which is also instructive for other organic–inorganic composite systems.