Borodin Oleg, Olguin Marco, Ganesh P, Kent Paul R C, Allen Joshua L, Henderson Wesley A
Electrochemistry Branch, RDRL-SED-C, US Army Research Laboratory, 2800 Powder Mill Rd., Adelphi, MD, 20783-1138, USA.
Phys Chem Chem Phys. 2016 Jan 7;18(1):164-75. doi: 10.1039/c5cp05121e. Epub 2015 Nov 25.
The composition of the lithium cation (Li(+)) solvation shell in mixed linear and cyclic carbonate-based electrolytes has been re-examined using Born-Oppenheimer molecular dynamics (BOMD) as a function of salt concentration and cluster calculations with ethylene carbonate:dimethyl carbonate (EC:DMC)-LiPF6 as a model system. A coordination preference for EC over DMC to a Li(+) was found at low salt concentrations, while a slightly higher preference for DMC over EC was found at high salt concentrations. Analysis of the relative binding energies of the (EC)n(DMC)m-Li(+) and (EC)n(DMC)m-LiPF6 solvates in the gas-phase and for an implicit solvent (as a function of the solvent dielectric constant) indicated that the DMC-containing Li(+) solvates were stabilized relative to (EC4)-Li(+) and (EC)3-LiPF6 by immersing them in the implicit solvent. Such stabilization was more pronounced in the implicit solvents with a high dielectric constant. Results from previous Raman and IR experiments were reanalyzed and reconciled by correcting them for changes of the Raman activities, IR intensities and band shifts for the solvents which occur upon Li(+) coordination. After these correction factors were applied to the results of BOMD simulations, the composition of the Li(+) solvation shell from the BOMD simulations was found to agree well with the solvation numbers extracted from Raman experiments. Finally, the mechanism of the Li(+) diffusion in the dilute (EC:DMC)LiPF6 mixed solvent electrolyte was studied using the BOMD simulations.
采用玻恩-奥本海默分子动力学（BOMD）方法，以碳酸亚乙酯：碳酸二甲酯（EC:DMC）-LiPF6为模型体系，重新研究了混合线性和环状碳酸酯基电解质中锂阳离子（Li(+)）溶剂化壳层的组成与盐浓度的关系，并进行了团簇计算。结果发现，在低盐浓度下，Li(+)对EC的配位偏好高于DMC；而在高盐浓度下，Li(+)对DMC的偏好略高于EC。对气相中以及在隐式溶剂中（作为溶剂介电常数的函数）的(EC)n(DMC)m-Li(+)和(EC)n(DMC)m-LiPF6溶剂化物的相对结合能分析表明，将含DMC的Li(+)溶剂化物浸入隐式溶剂中时，相对于(EC4)-Li(+)和(EC)3-LiPF6，它们更稳定。这种稳定作用在高介电常数的隐式溶剂中更为明显。通过校正Li(+)配位时溶剂的拉曼活性、红外强度和谱带位移的变化，对先前拉曼和红外实验的结果进行了重新分析和核对。将这些校正因子应用于BOMD模拟结果后，发现BOMD模拟得到的Li(+)溶剂化壳层组成与从拉曼实验中提取的溶剂化数吻合良好。最后，利用BOMD模拟研究了Li(+)在稀(EC:DMC)LiPF6混合溶剂电解质中的扩散机制。
