Theoretical Study on the Quantum Capacitance Origin of Graphene Cathodes in Lithium Ion Capacitors

Fangyuan Su,Li Huo,Lijing Xie,Chengmeng Chen,Qingqiang Kong

doi:10.3390/catal8100444

Abstract

Quantum capacitance (QC) is a very important character of the graphene cathode in lithium ion capacitors (LIC), which is a novel kind of electrochemical energy conversion and storage device. However, the QC electronic origin of the graphene cathode, which will affect the electrochemical reaction at the electrode/electrolyte interface, is still unclear. In this article, the QC of various kinds of graphene cathode is investigated systematically by DFT calculation. It was found that the value and origin of QC strongly depend on the defects and alien atoms of graphene. Graphene with pentagon defects possesses a higher QC than pristine graphene due to the contribution from the electronic states localized at the carbon pentagon. The introduction of graphitic B can contribute to QC, while graphitic N and P does not work in the voltage range of the LIC cathode. Single vacant defect graphene and pyrrolic N-doped graphene demonstrate very high QC due to the presence of states associated with the σ orbital of unbonded carbon atoms. However, pyridinic graphene shows an even higher QC because of the states from the N atom. For the residual O in graphene, its QC mainly originated from the pz states of carbon atoms and the effect of O, especially the O in bridged oxygen functional group (–COC–), is very limited. These results provide new insight into further study of the catalytic behavior and the design of a high performance graphene cathode for LIC.

Highlights

Lithium ion capacitor (LIC) is a kind of hybrid electrochemical energy conversion and storage device typically consisting of a capacitive cathode and an intercalated anode
The quantum capacitance (QC) and the excessive surface charge density (ESCD) of pristine and SW, D2 defected are shown in Figure 2a,b, respectively
The result will be more based on fixed-band approximation, in which the band structure and the density of states (DOS) of graphene are accuratetoif be thenot band structure variation is extracting taken into process account [19,30,31,32,33,34,35,43]

Summary

Introduction

Lithium ion capacitor (LIC) is a kind of hybrid electrochemical energy conversion and storage device typically consisting of a capacitive cathode and an intercalated anode. The capacitive cathode can bring excellent rate performance, while the intercalated anode enhances its capacity. LICs possess high power and high energy density simultaneously and can fill the gap between the lithium ion battery and a supercapacitor. The capacitive cathode of the LIC should possesses both a high specific surface area and good electronic conductivity simultaneously. Graphene has been used as an effective novel cathode for its large specific area and high electronic conductivity [9,10,11,12,13]

Methods

Discussion

Conclusion