Influence of thermal history and humidity on the ionic conductivity of nanoparticle-filled solid polymer electrolytes

The crystallinity and conductivity of nanoparticle-filled solid polymer electrolytes (SPEs) are investigated as a function of thermal history and water content. Our objective is to evaluate how performance is affected by the conditions under which the SPEs are handled and tested. The samples consist of polyethylene oxide (PEO), LiClO₄, and Al₂O₃ nanoparticles. At low humidity, SPEs at ether oxygen to lithium ratios of 8:1 do not crystallize immediately; instead, 3 days are required for crystallization to occur, and this does not depend strongly on the presence of nanoparticles. The conductivity is improved by the addition of nanoparticles at low humidity, but only at an ether oxygen to lithium ratio of 10:1, which corresponds to the eutectic concentration. At high humidity, the recrystallization time is delayed for 3 weeks, and the conductivity increases in both filled and unfilled SPEs beyond that of the low humidity samples. Although we observe that water amplifies the influence of nanoparticles on conductivity, we also find that nanoparticles inhibit water uptake-but only in the presence of lithium. Because Li⁺ strongly absorbs water, this result suggests that nanoparticles may interact directly with Li⁺ ions to prevent water uptake. In filled samples at the eutectic concentration (10:1), more water is absorbed compared to the nanoparticle-filled 8:1 samples, even though less lithium is present. This suggests that nanoparticles may segregate to lithium-poor regions in the 10:1 samples, and this scenario is supported by the morphology that would be expected at the eutectic concentration.