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NMR study of lithium mobility in polymer electrolytes

Abstract : In battery materials, the mobility of lithium cations is the key to the limitations in battery power and charging rates. NMR spectroscopy can give access to self-diffusion coefficients of spin bearing species using pulsed field gradients which measure atomic displacement over 1 − 2μm length scales. The relaxation of nuclear spins at high magnetic fields, on the other hand, is governed by fluctuations of NMR interactions resonant with the Larmor frequency, at the nanosecond timescale, and these are usually related to atomic motions over 1 Å - 1 nm. In this thesis, we recorded self-diffusion coefficients and ⁷Li relaxation rates for two polymer electrolytes: LiTFSI in polyethylene oxide (PEO) and in a block-copolymer PS-PEO(LiTFSI)-PS. We first investigated the effect of magic-angle spinning (MAS) on diffusion and relaxation, showing that MAS can help retrieve coefficient diffusion when relaxation is fast and diffusion is slow, and second, that lithium motion is not perturbed by the partial alignment of PEO under MAS induced pressure. The relaxation rates of 7Liwere measured at three high magnetic fields (4.7, 9.4 and 17.6 Tesla) allowing us to perform a simple relaxometry study of Li+motion at the nanosecond timescale. In order to reproduce the transverse and longitudinal relaxation behaviors, it proved necessary to introduce a simple model with two correlation times. It showed for the first time that the lithium dynamics in PS-PEO(LiTFSI)-PSis slowed down by the presence of PS domains compared to the pure PEO with similar chain lengths. The results are analyzed and compared to other studies based on molecular dynamics or physical models of diffusion in polymers. A second series of gel polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-HFP), PEGM(Poly ethyleneglycol methyl ether methacrylate)/PEGDM (Poly ethyleneglycol dimethacrylate), and LiTFSI in ionic liquids were also studied. Adding oxygenated polymers to increase the retention of ionic liquids slowed the diffusion down and explained why the battery performance was degraded at higher charging rates.
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Tan-Vu Huynh. NMR study of lithium mobility in polymer electrolytes. Other. Université d'Orléans, 2015. English. ⟨NNT : 2015ORLE2054⟩. ⟨tel-01362053⟩

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