How does the solvent composition influence the transport properties of electrolyte solutions? LiPF6 and LiFSA in EC and DMC binary solvent
How does the solvent composition influence the transport properties of electrolyte solutions? LiPF6 and LiFSA in EC and DMC binary solvent
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溶剂成分如何影响电解质溶液的传输性能？
DOI：
10.1039/d1cp00967b
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发表时间：
2020
影响因子：
3.3
通讯作者：
Tetsu KIYOBAYASHI
中科院分区：
文献类型：
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作者：
Satoshi UCHIDA;Tetsu KIYOBAYASHI
在这项研究中，我们通过在288 ≤ T/K ≤ 328的温度范围内改变EC含量从0到60体积%，即EC的摩尔分数为0 ≤ xEC ≤ 0.7，实验测量了溶解在EC和DMC二元混合溶剂中的1 mol kg−1 LiPF 6或LiFSA的电解质溶液的粘度η和离子电导率σ。通过脉冲梯度自旋回波NMR测定Li+、PF 6 −、FSA−、EC和DMC每种物质的扩散系数D。使用溶剂和阴离子的拉曼光谱检查Li+周围的分子的状态;定量分析表明，在低xEC下，EC在溶剂化壳层中是DMC的约两倍优选，而EC优选性随着xEC的增加而降低。当从D和η计算输运实体的流体动力学半径rSt时，经典的Stokes-Einstein关系仍然定量地成立，因为（i）不含溶质的rSt，EC和rSt，DMC与溶液中的rSt，EC和rSt，DMC没有显著差异：（ii）rSt，Li与由拉曼光谱测定的溶剂化数估计的尺寸大致一致，这意味着rSt，Li反映了溶剂化壳的尺寸;和（iii）rSt，阴离子接近静态尺寸，表明阴离子几乎没有溶剂化。xEC的增加导致所有物种的rSt降低，其中阴离子受影响最大，这与以下观点一致：高度Li+-溶剂化的EC具有比DMC更好的介电屏蔽效果，从Li+中释放阴离子，从而增强阴离子转移，这对离子电导率有积极贡献，直到在高xEC下粘度占优势。
In this study, we experimentally measured the viscosity, η, and ionic conductivity, σ, of the electrolyte solutions of 1 mol kg−1 of LiPF6 or LiFSA dissolved in the binary mixture solvent of EC and DMC in a temperature range of 288 ≤ T/K ≤ 328 by varying the EC content from 0 to 60 vol%, which translates into the molar fraction of EC of 0 ≤ xEC ≤ 0.7. The diffusion coefficient, D, of each species, Li+, PF6−, FSA−, EC and DMC, was determined by pulse gradient spin-echo NMR. The state of molecules around Li+ was examined using the Raman spectra of the solvents and anions; the quantitative analysis suggests that EC is about twice as much preferred as DMC in the solvation shell at low xEC, while the EC-preference decreases with an increase in xEC. The classical Stokes–Einstein relation still quantitatively holds when evaluating the hydrodynamic radius, rSt, of transporting entities from D and η, in that (i) rSt,EC and rSt,DMC without the solute do not significantly differ from those in the solution; (ii) rSt,Li roughly coincides with the size estimated from the solvation number determined by Raman spectroscopy, which implies that rSt,Li reflects the solvation shell size; and (iii) rSt,anion is close to the static size, suggesting that anions are little solvated. The increase in xEC results in a decrease in rSt for all species, among which anions are most influenced, which is consistent with the view that the highly Li+-solvating EC, with its better dielectric shielding effect than DMC, liberates the anions from Li+, whereby enhancing the anion transfer that positively contributes to the ionic conductivity until the viscosity prevails at high xEC.