Salt Distribution, Phase Structure, and Conductivity of Poly(ethylene oxide)-<i>block</i>-Poly(<i>n</i>-butyl acrylate) Block Copolymer Electrolytes with Double Conductive Phases

Abstract

In this work, a poly(ethylene oxide)-block-poly(n-butyl acrylate) (PEO-b-PnBA) block copolymer (BCP) was doped with lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) to fabricate BCP electrolytes containing double conductive nanophases. A Fourier transform infrared method was established to quantitatively characterize lithium salt distribution in two phases. It was confirmed that relatively more salt is located in the PEO phase due to its stronger complexation ability toward Li+ ions, but the difference in the salt content between the two phases becomes smaller at higher doping ratios (rs). Small-angle X-ray scattering shows that PEO-b-PnBA/LiTFSI hybrids form microphase-separated but disordered structures. Both hybrids with r = 1/6 and 1/3 have a broad and diffused interphase, although the latter has a slightly larger interphase thickness (Δ). In contrast, the grain size (L) of the hybrid with r = 1/6 is 2–4 times that of the hybrid with r = 1/3. At r = 1/6, both the interphase thickness and grain size are almost invariant with temperature, but at r = 1/3, they show a stronger temperature dependence. With knowing the real salt concentrations in two phases, the conductivity normalization coefficient (σn) of the BCP electrolytes was obtained by comparing with the conductivity of the corresponding blend electrolytes. It was found that σn changes little with temperature for the hybrid with r = 1/6, but it increases with a temperature rise for the hybrid with r = 1/3. Moreover, the hybrid with r = 1/3 exhibits a much larger σn. The linear relationship between σn and 1/L indicates that grain size is the major factor affecting the conductivity of PEO-b-PnBA/LiTFSI hybrids. The microphase-separated structure also endows the BCP electrolytes with better mechanical properties, as compared with the homopolymers.

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