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arXiv 1502.07708cond-mat.mtrl-sci
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第一性原理和经典反应分子动力学研究锂离子在有机电解质中的溶剂化和扩散

Lithium Ion Solvation and Diffusion in Bulk Organic Electrolytes from First Principles and Classical Reactive Molecular Dynamics

Mitchell T. Ong, Osvalds Verners, Erik W. Draeger, Adri C. T. van Duin, Vincenzo Lordi, John E. Pask

摘要:

锂离子电池的性能受到电解质的离子电导率的强烈影响,这取决于锂离子在电池中迁移的速度,并与它们的溶剂化结构有关。溶剂的选择可以极大地影响Li离子的溶剂化和扩散率。本文采用第一性原理分子动力学研究了锂离子在碳酸乙烯酯(EC)、碳酸甲乙酯(EMC)以及EC/EMC混合溶剂中的溶剂化和扩散行为。我们发现,锂离子被溶剂的羰基或醚氧原子溶剂化,有时由PF $_6^-$阴离子。Li $^+$优选四面体配位的第一溶剂化壳层,而不管涉及哪种物质,具体优选的溶剂化结构取决于有机溶剂。此外,我们计算了锂在每种电解质中的扩散系数,发现线性碳酸酯EMC中的扩散系数略大于环状碳酸酯EC。扩散系数的大小与Li $^+$溶剂化的强度相关。PF $_6^-$阴离子的相应分析表明,与弱结合,定义不清的第一溶剂化壳层相关的扩散性更大。这些结果可用于帮助设计新的电解质,以改善锂离子电池的性能。

英文摘要:

Lithium-ion battery performance is strongly influenced by the ionic conductivity of the electrolyte, which depends on the speed at which Li ions migrate across the cell and relates to their solvation structure. The choice of solvent can greatly impact both solvation and diffusivity of Li ions. We use first principles molecular dynamics to examine the solvation and diffusion of Li ions in the bulk organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and a mixture of EC/EMC. We find that Li ions are solvated by either carbonyl or ether oxygen atoms of the solvents and sometimes by the PF$_6^-$ anion. Li$^+$ prefers a tetrahedrally-coordinated first solvation shell regardless of which species are involved, with the specific preferred solvation structure dependent on the organic solvent. In addition, we calculate Li diffusion coefficients in each electrolyte, finding slightly larger diffusivities in the linear carbonate EMC compared to the cyclic carbonate EC. The magnitude of the diffusion coefficient correlates with the strength of Li$^+$ solvation. Corresponding analysis for the PF$_6^-$ anion shows greater diffusivity associated with a weakly-bound, poorly defined first solvation shell. These results may be used to aid in the design of new electrolytes to improve Li-ion battery performance.

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