Comparison of active and passive fillers for the enhancement of the ionic conductivity of polymer-based electrolytes for all-solid-state batteries

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All-solid-state batteries (ASSBs) based on solid polymer electrolytes are among the most promising candidates for next-generation batteries because of their improved safety and energy storage performance. However, the low ionic conductivity of polymer electrolytes at ambient temperature is their main limitation. Hybrid solid electrolytes (HSE) constituted of an organic polymer matrix and an inorganic filler material can mitigate limitations without compromising the advantages of the respective materials by improving the interfacial contact with electrodes and suppressing the lithium dendrite growth.
In this work, the influence of fillers on the ionic conductivity of PEO-based solid electrolytes was investigated. Two types of filler were compared, the first is ZrO2 as a passive Li+ non-conductive material and the second is Li7La3Zr2O12 (LLZO) as an active Li+ conductive filler. In order to reduce the high resistance of the ceramic-polymer interface, the fillers were functionalized by adding Si-R before being used in the electrolyte synthesis. All the electrolytes were prepared by tape-casting and the 15 vol.% of fillers were added for the hybrid solid electrolytes (HSE).
Electrochemical impedance spectroscopy (EIS) was used to determine the ionic conductivity of four different electrolytes; PEO, PEO-GMPTS (filler-free), PEO-ZrO2-GMPTS (ZrO2), and PEO-LLZO-GMPTS (LLZO). The HSE with LLZO achieved low ionic conductivity of 10-6 S cm-1 at 20 °C and 10-4 – 10-3 S cm-1 at 80 °C which is significantly lower than those exhibited by the filler-free solid electrolytes. HSE with ZrO2 showed ionic conductivity of 6.66 x 10-4 S cm-1 at 20 °C and 5.43 x 10-3 S cm-1 at 80 °C, which is similar or slightly higher than the filler-free electrolytes. The distribution of fillers in composite electrolyte was observed by scanning electron microscope. It was revealed that HSE with LLZO had the poor homogeneity of the surfaces and the poor dispersion of the fillers in the polymer. While HSE with ZrO2 presented homogeneous structure due to the better dispersion of the particles, resulting in improved ionic conductivity.
This study suggests that silane ligands can serve as well as fillers for enhancing the ionic conductivity, and the effect of prevention of crystallization has a stronger influence on the conductivity than the ion conduction of the filler.

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