The ever increased development of portable devices and electrical vehicles demands the electrochemical energy storage devices with high energy and power densities. Graphite has been the most widely used anode materials for Li-ion batteries. However, because of its low theoretical capacity (372 mAh/g), there has been significant efforts in developing electrode materials with much higher energy densities. Graphene, a one-atom-thick carbon materials showed numerous advantages including superior electrical conductivity, thermal conductivity and high surface area; and is expected to be a good candidate as a high-power and high-energy electrode material for Li-ion batteries. Although, previous studies used graphene as anode materials, it showed significantly enhanced capacity >1000 mAh/g; the energy storage mechanism in graphene is still unclear. This is due to the high density of defects in chemically synthesized graphene. Few studies have been using chemical vapor deposition (CVD)-grown defect-free single layer graphene (SLG) on copper foil as anodes in Li-ion batteries.¹ Despite this, the exact mechanism of Li ions storage in the SLG is still a subject of controversy. Theoretical study has shown that the absorption of Li-ions on defect-free SLG is energetically unfavorable.² This is in controversy with the experiments which claims that CVD-grown SLG is promising electrode materials for Li anodes. In this study, we investigated the Li-ion storage in CVD-grown SLG graphene in comparison with Cu substrate, our results confirmed that the capacity obtained for SLG on Cu is mainly originated from the contribution of Cu substrate. However, by using bilayer or triple layer graphene which is prepared by a layer-by-layer transfer process, we found that the Li ion storage capacity will be significantly enhanced when additional layer graphene was placed on the top of initial SLG. The capacitance increased from 0.05 F/cm² for Cu substrate to 0.08 F/cm² for bilayer graphene on Cu and further increased to 0.129 F/cm²for triple layer graphene on Cu substrate. The areal capacity of the bilayer graphene is ~ 2 times of the capacity available for Cu substrate after 50 cycles. Our study indicates that the interfacial space between graphene layers plays a critical role for high performance Li-ion storage.

References:

1. D. Wei, S. Haque, P. Andrew, J. Kivioja, T. Ryhanen, A. Pesquera, A. Centeno, B. Alonso, A. Chuvilin and A. Zurutuza, Journal of Materials Chemistry A, 2013, 1, 3177-3181.

2. E. Lee and K. A. Persson, Nano Letters, 2012, 12, 4624-4628.