Nanoscale Relaxation in "Water-in-Salt" and "Water-in-Bisalt" Electrolytes
Nanoscale Relaxation in "Water-in-Salt" and "Water-in-Bisalt" Electrolytes
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DOI：
10.1021/acs.jpclett.0c01765
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发表时间：
2020-09-03
影响因子：
5.7
通讯作者：
Saboungi, Marie-Louise
中科院分区：
文献类型：
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作者：
Gonzalez, Miguel A.;Borodin, Oleg;Saboungi, Marie-Louise
“盐中水”(WIS)和“双盐水”(WIBS)电解液最近被开发用于锂离子电池，它们结合了水电解液的安全性和环境友好性，以及通过固体-电解液界面实现的更大的操作窗口。报道了13.9和21m(摩尔)双(三氟甲烷)磺酰亚胺LiTFSI溶液和WIBS电解液(21M LiTFSI+7M三氟化锂(LIOTF))在H2O/D2O中的准弹性中子散射(QENS)测量结果。数据经傅立叶变换得到实验的中间散射函数(ISF)，并与分子动力学(MD)模拟得到的相应量进行比较。QENS和MD ISF都可以用一个拉伸指数函数很好地拟合，得到水分子的表观平动扩散系数。对于13.9m和21m溶液，QENS值与MD模拟结果吻合得很好，但MD模拟预测WIBS电解液中水的松弛比QENS慢。非相干和相干散射的比较表明，与离子骨架的结构弛豫相比，水的动力学速度要快得多，这与纳域图像一致，其中锂通过较慢弛豫的离子基质周围的曲折水域发生扩散，导致高度非高斯的水运动。
"Water-in-salt" (WIS) and "water-in-bisalt" (WIBS) electrolytes have recently been developed for Li-ion batteries, combining the safety and environmental friendliness of aqueous electrolytes with a larger operating window made possible by a solid-electrolyte interphase. We report quasielastic neutron scattering (QENS) measurements on solutions of a WIS electrolyte at two concentrations, 13.9 and 21 m (molal) lithium bis(trifluoromethane)sulfonimide LiTFSI in H2O/D2O and a WIBS electrolyte at (21 m LiTFSI + 7 m lithium triflate (LiOTf)) in H2O/D2O. The data were Fourier transformed to obtain experimental intermediate scattering functions (ISFs) and compared with corresponding quantities obtained from molecular dynamics (MD) simulations. Both QENS and MD ISFs could be fitted well by a single stretched exponential function to obtain apparent translational diffusion coefficients for the water molecules. The QENS values agree well with the MD simulations for the 13.9 and 21 m solutions, but MD simulations predict a slower relaxation of water compared to QENS for the WIBS electrolyte. Comparison of the incoherent and coherent scattering reveals much faster water dynamics compared with structural relaxation of the ionic framework, consistent with the nanodomain picture where the lithium diffusion occurs through the tortuous water domain around the slower relaxing ionic matrix, leading to highly non-Gaussian water motion.