Abstract
A triblock copolymer of benzyl methacrylate and oligo(ethylene glycol) methyl ether methacrylate was polymerized to form the general structure PBnMA-POEGMA-PBnMA, using atom transfer radical polymerization (ATRP). The block copolymer (BCP) was blended with lithium bis(trifluoro methylsulfonate) (LiTFSI) to form solid polymer electrolytes (SPEs). AC impedance spectroscopy was used to study the ionic conductivity of the SPE series in the temperature interval 30 °C to 90 °C. Small-angle X-ray scattering (SAXS) was used to study the morphology of the electrolytes in the temperature interval 30 °C to 150 °C. By using benzyl methacrylate as a mechanical block it was possible to tune the microphase separation by the addition of LiTFSI, as proven by SAXS. By doing so the ionic conductivity increased to values higher than ones measured on a methyl methacrylate triblock copolymer-based electrolyte in the mixed state, which was investigated in an earlier paper by our group. A Li|SPE|LiFePO4 half-cell was constructed and cycled at 60 °C. The cell produced a discharge capacity of about 100 mAh g−1 of LiFePO4 at C/10, and the half-cell cycled for more than 140 cycles.
Highlights
The electrolytes used today constitute a major threat to Li-battery safety [1,2,3,4]
The block copolymer was synthesized with atom transfer radical polymerization (ATRP)
The Tg increased with the amount of LiTFSI, as expected, since the addition of LiTFSI slows down the local chain movements, resulting in a hardening of the electrolytes compared to the salt-free block copolymer (BCP)
Summary
The electrolytes used today constitute a major threat to Li-battery safety [1,2,3,4]. The popular use of low-Tg polyethers, such as poly(ethylene oxide) (PEO), rely on the dissolution of lithium salt though the interaction of ether groups found in the main chain, and increasing the temperature increases the segmental motion and the ionic conductivity. From an application point of view this is not desirable, since the hypothesis is that by separating hydrophobic and hydrophilic constituents on the same polymer chain, a local nano-scale ordering of these segments with different polarity can be realized, promoting both ionic conductivity and mechanical stability. This can, in turn, result in better power performance of the batteries. The best performing electrolyte was evaluated with SAXS and TEM to study the morphology, and a battery device was constructed to evaluate the electrochemical performance
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