离子液体在新型碳材料表面和孔内的吸附扩散及其充放电机理的分子模拟研究

Electrostatic double layer capacitances (EDLCs) (one kind of supercapacitors) are promising devices in electronic power storage, with high power density, fast charging-discharging process and long cyclic stability. However, they typically have one order of magnitude lower energy densities than those of batteries. Ionic liquids (ILs) are low temperature molten salts with high electrochemical stable potential window (EPSW). They are excellent electrolyte in application of supercapacitors benefit by very low vapor pressure, good chemical and thermal stability and large applicable temperature ranges. Carbon-based porous materials are recently expanded their highly abundant morphologies in virtue of well-developed methods in preparation. It is an effective way to improve energy density by their combinations. Recent studies show the performance of supercapacitor is improved dramatically when the pore size is in affinity to the size of ions, indicating much more complex charging mechanisms besides the traditional electrostatic double layers. In this work, the coarse grained model is developed for ILs based on previous developed all-atom force field. Molecular simulations are performed to study the adsorption and diffusion of cations/anions in the porous carbon electrode. The matching effect between the ion and pore is explored by using various shapes and structure of the pore. In addition, we investigate the condition in which different charging-discharging mechanisms appear, by gradually changing the interactions between the ions and the electrode. Finally three dimensional porous carbon structures are designed to predict the performance of supercapacitor quantitatively by all-atom simulations. By this work, the charging-discharging mechanisms are distinguished at atomic level, which is very important to the bottom-up design of high performance supercapacitors.

超级电容输出功率大，充放电迅速、循环使用寿命长，是具有广阔应用价值和市场前景的电能存储设备，但能量密度相对较小。离子液体电化学窗口高，几乎没有蒸汽压、具有良好的化学稳定性和宽广的操作温度范围，而多孔碳材料具有极其丰富的形态，一定程度上能够可控制备，因此二者结合是提升其能量密度的有效手段。最近研究表明当孔径减小到离子尺寸附近时，电容性能显著提升，充放电也呈现出比传统双电层更复杂的机理。本项目基于离子液体的分子力场建立粗粒化模型，通过分子模拟研究其在碳材料电极中的吸附和扩散，在微观尺度上探索表面和孔结构与阴阳离子的尺寸和形状之间可能存在的匹配效应。有针对性地改变离子、溶剂和电极间相互作用力，探讨出现不同充放电机理的条件。在此基础上设计三维孔结构的新型电极材料，并通过全原子模型定量模拟其充放电性能。本研究将厘清多孔电极材料中充放电过程的微观机理，为自下而上设计高性能超级电容奠定必要的理论基础。

本项目针对离子液体为电解液的超级电容，采用分子动力学模拟研究离子液体在电极表面的吸附和扩散，为自下而上设计高性能超级电容奠定必要的理论基础。主要成果有：1）构建了简化的超级电容微观模型，通过模拟研究了阴阳离子结构对电容性能的影响及其机理，发现双电层内的离子密度分布随电压增加而逐渐增强，而且超过某一阈值后阳离子在负极出现双峰，进一步研究确认是因其在高电压下出现了取向转变，这一转变导致微分电容出现第二个峰值；而当咪唑环上碳链变长时，此峰值会消失。2) 发现在离子液体中加入乙腈在双电层会与阴阳离子形成竞争吸附，导致总体电容下降，微分电容曲线的峰值也发生改变，但有效改善了离子液体的扩散性能。3）开发了一个团簇分析的工具包可以对溶液中各种团簇展开详细分析，模拟了亲水和憎水复配的混合离子液体水溶液的结构，发现憎水离子液体中水很容易形成团簇，而亲水离子液体对水有稳定作用，防止水发生氧化还原反应，因此仅需加入少量亲水离子液体即可显著改善其整体亲水性，达到将水锁在体相的效果，使其能承受较高电压。4) 为了减小人为设定的力场参数对模拟结果的影响，采用周期性密度泛函理论计算离子液体在Au(111)表面的吸附自由能，揭示了阴阳离子在金属表面吸附和脱附的相对选择性。