双层堆叠对石墨烯材料量子电容影响的理论研究

A DFT study of the effect of stacking on the quantum capacitance of bilayer graphene materials

石墨烯材料由于比表面积大、导电性能好,被作为正极材料与多孔炭材料一起用于锂离子电容器。石墨烯材料在制备和使用过程中易发生片层堆叠积聚,难以保证单层存在。堆叠会影响材料的电子结构进而影响量子电容。为了考察层间相互作用对石墨烯电子结构和量子电容性能的影响规律,基于密度泛函理论计算,本文系统研究了堆叠对于多种缺陷结构石墨烯材料的量子电容、表面电量等性能的影响。计算发现,由于层间相互作用以及基底层提供了部分电荷,单层石墨烯堆叠后量子电容性能增加,并且相较于完整和表面带有点缺陷的石墨烯,掺N双层石墨烯的量子电容提升幅度较大。同时在层间相互作用影响下,堆叠后拥有相近结构的石墨烯之间的量子电容性能差距减小。对于顶层不含悬挂键和孤对电子的石墨烯片层,发生堆叠后量子电容曲线随电压变化的波动趋势降低。

Graphene is acknowledged as one of the ideal active electrode materials for electric double-layer capacitors because of its extremely high specific surface area and outstanding electronic conductivity.By introducing defects or heteroatoms into the graphene sheet,the electronic structure around the defects can be altered,which could lead to an increased quantum capacitance(C_(Q))and therefore te capacitive performance.One of the unavoidable problems for manufacturing and using graphene materials is that the stacking of the layers affects their electronic structure,and eventually their capacitance.DFT calculations were used to investigate the effect of layer stacking in bilayer graphene materials on C_(Q) and the surface charge density.A two layer,AB-stacked graphene model,in which the top layer is defective and the bottom one is perfect was assumed for the calculations.The defective graphenes investigated are those containing Stone-Thrower-Wales defects,single vacancies(SV),three with double vacancies(5-8-5,555-777 and 5555-6-7777),pyrrole-N graphene and the pyridine-N graphene.Results indicate that both the values and waveform of C_(Q) of the materials are changed by stacking.The C_(Q) values of most of these graphenes are significantly increased after stacking.The C_(Q) waveforms of the SV and N-doped graphene are relatively insensitive to stacking.The basal layer contributes a considerable amount of charge,which is most obvious for the pyrrolic-N double-layer graphene and 5-8-5 double-vacancy graphene.The surface charge density provided by the defective top layer is increased by interlayer interaction,especially for the N-doped graphene.The uniform distribution of charge on the bottom layer partially alleviates fluctuations in the C_(Q) waveform.These findings provide theoretical guidance for the micro-structural design of graphene materials to optimize their performance as electrode active materials.