Publication Date

Fall 2023

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical and Materials Engineering

Advisor

Sang-Joon Lee; Dahyun Oh; David Wagner

Abstract

Developing composite polymer electrolytes is crucial for enhancing the safety, energy density, and performance of lithium-ion batteries. While it is widely accepted that introducing filler particles in PEO-based electrolytes can reduce crystallinity and enhance ionic conductivity, this conventional understanding applies primarily to electrolytes with high lithium salt concentrations. The hypothesis of this investigation is that the crystallinity change induced by LLZTO ceramic particles in PEO-LiTFSI electrolytes is contingent on the EO:Li molar ratio. This hypothesis is interrogated by measuring changes in crystallinity, spherulite morphology, and stiffness of composite polymer electrolytes with two different EO:Li molar ratios using differential scanning calorimetry, optical microscopy, and stress-strain measurements. The findings revealed that adding 5 μm LLZTO at 10 wt% increases crystallinity from 49% to 58% at low salt concentrations (43:1 EO:Li), with very little change (32% to 33%) at moderate salt concentrations (18:1 EO:Li). Image analysis showed that the addition of LLZTO results in an increase in both the area fraction and average size of spherulites. Moreover, the presence of LLZTO and correspondingly higher crystallinity contributed to higher modulus. These experiments reveal how the interplay between the EO:Li molar ratio and LLZTO influences the mechanical response of composite polymer electrolytes, potentially enhancing the structural robustness and cycling stability of lithium-ion batteries.

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