超级电容器双电层结构的多尺度模拟研究

Electric Double-layer Capacitors (EDLC) has excellent cycling and charging performance, and has broad application in energy storage devices. The underlying mechanism for EDLC's high capacitance, as well as its charging mechanism, has not been fully understood. Molecular simulations combined with theory of statistical mechanics, can give a better description of these mechanisms at microscopic level. In this project, we will use the capacitor formed by 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and carbon electrode as our model system, and perform molecular dynamics (MD) simulations to study the structure of electrical double layer (EDL). Our MD simulations will focus on the following aspects of EDLC: 1) By comparing the liquid-solid interface structure of planar carbon electrode and nanoporous carbon electrode, we will try to illustrate the mechanism that accounts for the anomalously high capacitance in nanoporous electrode.2) We will study the influence of acetonitrile concentration on the capacitance and ion transport properties. 3) We will develop coarse-grained force field for [EMIM][TFSI] and use coarse-grained MD simulations to provide more simple and intuitive insights into the collective effects of electrostatic and van der Waals interactions on solvation structure change during charging. 4) We will apply the Local Molecular Field Theory (LMFT) to the supercapacitor system by mapping the original system to a mimic system with only short-ranged interactions, and this will accelerate the speed of our simulations. This proposed project will help us to get a better understanding the the microscopic mechanism of EDCL, optimize the performance of EDCL, as well as exploring more efficient multiscale method to study EDCLs.

双电层电容器具有优异的循环性能和快速充放电特性，在储能装置中有着广泛的应用。双电层电容器的结构和充放电微观机理至今尚未被完全揭示，而分子模拟结合统计力学理论，能够在微观层次上给出更清晰的物理图像。本项研究拟以1-乙基-3-甲基咪唑双三氟甲磺酰亚胺盐([EMIM][TFSI])离子液体和碳电极构成的电容器作为研究体系，从以下方面展开多尺度分子动力学模拟的研究：1）比较离子液体与平板碳电极和多孔碳电极的界面结构异同，以解释多孔碳电极中高电容的产生机制；2）分析乙腈溶剂浓度对电容和离子输运性质的影响；3）发展离子液体的粗粒化力场，并使用粗粒化分子动力学模拟简明深入地分析静电力与范德华力协同作用对充放电过程中溶剂化结构的变化；4）将定域分子场理论应用于本体系，将体系映射到一个具有短程相互作用的等效体系，提高模拟速度。本项目旨在更深入理解双电层电容器的微观机理，并探索更高效的多尺度模拟的方法。

双电层电容器是具有广泛应用前景的储能装置，具有循环次数高、快速充放电等优异性能。基于统计力学的分子动力学模拟，能够在微观尺度上有效地揭示双电层电容器中固液界面的微观结构和动力学，对各种影响电容器性能的参数进行建模。本项目在执行期内，以受限极板-溶液体系为模型，对双电层电容器相关固液界面体系进行了细致的分子动力学模拟，具体研究内容如下：1. 使用分子动力学模拟系统研究了电极表面极性调控对超级电容器中溶剂结构与动力学的影响，揭示了不同极性下溶剂在极板表面的微观结构、吸附热力学与输运动力学特性，并进一步对有机小分子在该界面环境中的溶剂化自由能及动力学进行了系统表征；2. 使用分子动力学模拟对不同浓度下乙腈溶剂中1-乙基-3-甲基咪唑双三氟甲磺酰亚胺盐([EMIM][TFSI])离子液体在极板表面结构、动力学以及充放电性能进行了研究，对乙腈溶剂对离子液体的润滑的微观机理进行了阐释；3.采用表面团簇修饰的方法，模拟了[EMIM][TFSI]离子液体在多孔碳电极表面的结构与动力学，获得了对离子液体结构随孔深变化的微观描述；4. 探索了采用定域分子场理论(LMFT)对离子液体进行模拟的可能性，并构建了相关的程序代码 5.对金属电极表面离子熔体的吸附及输运动力学进行了系统的计算模拟，获得了结构与输运性质随受限程度变化的规律。项目在执行期内基本完成了预期的研究目标，并将模拟研究从单纯的固液界面结构逐步拓展到了固液界面上的溶剂化及输运，项目取得的研究成果将有助于增进对固液界面吸附和传递过程的微观机制的理解。