Karatrantos Argyrios V, Middendorf Maleen, Nosov Daniil R, Cai Qiong, Westermann Stephan, Hoffmann Katja, Nürnberg Pinchas, Shaplov Alexander S, Schönhoff Monika
Luxembourg Institute of Science and Technology (LIST), 5 avenue des Hauts-Fourneaux, L-4362 Esch-sur-Alzette, Luxembourg.
School of Chemistry and Chemical Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7EX, United Kingdom.
J Chem Phys. 2024 Aug 7;161(5). doi: 10.1063/5.0216222.
The diffusion of cations in organic solvent solutions is important for the performance of metal-ion batteries. In this article, pulsed field gradient nuclear magnetic resonance experiments and fully atomistic molecular dynamic simulations were employed to study the temperature-dependent diffusive behavior of various liquid electrolytes representing 1M propylene carbonate solutions of metal salts with bis(trifluoromethylsulfonyl)imide (TFSI-) or hexafluorophosphate (PF6-) anions commonly used in lithium-ion batteries and beyond. The experimental studies revealed the temperature dependence of the diffusion coefficients for the propylene carbonate (PC) solvent and for the anions following an Arrhenius type of behavior. It was observed that the PC molecules are the faster species. For the monovalent cations (Li+, Na+, K+), the PC solvent diffusion was enhanced as the cation size increased, while for the divalent cations (Mg2+, Ca2+, Sr2+, Ba2+), the opposite trend was observed, i.e., the diffusion coefficients decreased as the cation size increased. The anion diffusion in LiTFSI and NaTFSI solutions was found to be similar, while in electrolytes with divalent cations, a decrease in anion diffusion with increasing cation size was observed. It was shown that non-polarizable charge-scaled force fields could correspond perfectly to the experimental values of the anion and PC solvent diffusion coefficients in salt solutions of both monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations at a range of operational temperatures. Finally, after calculating the radial distribution functions between cations, anions, and solvent molecules, the increase in the PC diffusion coefficient established with the increase in cation size for monovalent cations was clearly explained by the large hydration shell of small Li+ cations, due to their strong interaction with the PC solvent. In solutions with larger monovalent cations, such as Na+, and with a smaller solvation shell of PC, the PC diffusion is faster due to more liberated solvent molecules. In the salt solutions with divalent cations, both the anion and the PC diffusion coefficients decreased as the cation size increased due to an enhanced cation-anion coordination, which was accompanied by an increase in the amount of PC in the cation solvation shell due to the presence of anions.
阳离子在有机溶剂溶液中的扩散对金属离子电池的性能至关重要。在本文中，采用脉冲场梯度核磁共振实验和全原子分子动力学模拟，研究了各种液体电解质的温度依赖性扩散行为，这些电解质代表了锂离子电池及其他电池中常用的含双（三氟甲基磺酰）亚胺（TFSI-）或六氟磷酸根（PF6-）阴离子的1M碳酸丙烯酯金属盐溶液。实验研究揭示了碳酸丙烯酯（PC）溶剂和阴离子的扩散系数随温度的变化遵循阿伦尼乌斯行为类型。观察到PC分子是扩散较快的物种。对于单价阳离子（Li+、Na+、K+），随着阳离子尺寸增大，PC溶剂扩散增强，而对于二价阳离子（Mg2+、Ca2+、Sr2+、Ba2+），观察到相反的趋势，即扩散系数随阳离子尺寸增大而减小。发现LiTFSI和NaTFSI溶液中的阴离子扩散相似，而在含二价阳离子的电解质中，观察到随着阳离子尺寸增大阴离子扩散减小。结果表明，在一系列工作温度下，不可极化电荷标度力场能够与单价（Li+、Na+、K+）和二价（Mg2+、Ca2+、Sr2+、Ba2+）阳离子盐溶液中阴离子和PC溶剂扩散系数的实验值完美对应。最后，在计算了阳离子、阴离子和溶剂分子之间的径向分布函数后，对于单价阳离子，随着阳离子尺寸增大PC扩散系数增大的现象可通过小Li+阳离子的大水化壳层来清楚解释，这是由于它们与PC溶剂的强相互作用。在含有较大单价阳离子（如Na+）且PC溶剂化壳层较小的溶液中，由于更多溶剂分子被释放，PC扩散更快。在含二价阳离子的盐溶液中，由于阳离子 - 阴离子配位增强，阴离子和PC扩散系数均随阳离子尺寸增大而减小，这伴随着由于阴离子的存在阳离子溶剂化壳层中PC含量的增加。
