Ionic Conductivity and Mechanical Reinforcement of Well-Dispersed Polymer Nanocomposite Electrolytes

Nanoparticles are commonly added to polymer electrolytesto enhanceboth their mechanical and ion transport properties. Previous workreports significant increases in the ionic conductivity and Li-iontransference in nanocomposite electrolytes with inert, ceramic fillers.The mechanistic understanding of this property enhancement, however,assumes nanoparticle dispersion states-namely, well-dispersedor percolating aggregates-that are seldom quantified usingsmall-angle scattering. In this work, we carefully control the inter-silicananoparticle structure (where each NP has a diameter D = 14 nm) in a model polymer electrolyte system (PEO:LiTFSI). Wefind that hydrophobically modified silica NPs are stabilized againstaggregation in an organic solvent by inter-NP electrostatic repulsion.Favorable NP surface chemistry and a strongly negative zeta potentialpromote compatibility with PEO and the resulting electrolyte. Uponprolonged thermal annealing, the nanocomposite electrolytes displaystructure factors with characteristic interparticle spacings determinedby particle volume fraction. Thermal annealing and particle structuringyield significant increases in the storage modulus, G', at 90 degrees C for the PEO/NP mixtures. We measurethe dielectric spectra and blocking-electrode (kappa(b)) conductivities from -100 to 100 degrees C, and the Li+ current fraction (rho(+)(Li) ) in symmetricLi-metal cells at 90 degrees C. We find that nanoparticles monotonicallydecrease the bulk ionic conductivity of PEO:LiTFSI at a rate fasterthan Maxwell's prediction for transport in composite media,while rho(+)(Li) does not significantly changeas a function of particle loading. Thus, when nanoparticle dispersionis controlled in polymer electrolytes, Li+ conductivitymonotonically, i.e., (kappa(b)rho(+)(Li) ), decreases but favorable mechanical properties are realized.These results imply that percolating aggregates of ceramic surfaces,as opposed to physically separated particles, probably are requiredto achieve increases in bulk, ionic conductivity.

Automated summary

Nanoparticles are commonly added to polymer electrolytes to enhance their mechanical and ion transport properties. This work carefully controls the dispersion state of silica nanoparticles in a model polymer electrolyte system and finds that controlling the dispersion leads to improved mechanical properties while reducing the ionic conductivity. These results suggest that achieving increases in bulk, ionic conductivity requires percolating aggregates of ceramic surfaces.