Novel Hybrid Carbon Nanofiber/Highly Branched Graphene Nanosheet for Anode Materials in Lithium-Ion Batteries Haejune Kim, † Xingkang Huang, † Xiaoru Guo, Zhenhai Wen, Shumao Cui, and Junhong Chen* Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, United States * S Supporting Information ABSTRACT: The novel hybrid carbon nanofiber (CNF)/highly branched graphene nanosheet (HBGN) is synthesized via a simple two- step CVD method and its application as the anode material in a lithium- ion battery (LIB) is demonstrated. The CNFs offer a good electrical conductivity and a robust supporting structure, while the HBGNs provide increased Li storage sites including nanoporous cavities, large surface area, and edges of exposed graphene platelets. The hybrid material showed a reversible capacity of 300 mAh g -1 with excellent cycling stability. Our study provides a new avenue for design and synthesis of carbon-carbon hybrid materials for versatile applications. KEYWORDS: carbon nanofiber, highly branched graphene nanosheet, carbon-carbon hybrid, Li-ion battery, plasma-enhanced chemical vapor deposition, graphene ■ INTRODUCTION Nanostructured carbon exists in various types of allotropes: 0- dimensional fullerenes (C60), 1-dimensional carbon nanotubes (CNTs) and carbon nanofibers (CNFs), and 2-dimensional graphenes (or GNS: graphene nanosheet). Each carbon allotrope that is characterized by its unique shape, dimension- ality, and properties can be used as a building block to synthesize new hybrid materials by combining two or more allotropes. 1 Novel hybrids with a new structure and new morphology can be realized by two representative methods: (1) mixing surface-treated carbon materials 2-4 and (2) employing catalytic seeds for 1D carbon growth on 2D graphene. 5-7 Among these carbon allotropes, the recent discovery of the wonder material graphene has totally changed our view of the nanoscopic world, owing to its special structure and proper- ties. 8-10 Graphene is a one-atom-thick 2D sheet of sp 2 -bonded carbon atoms having exceptional mechanical, electrical, and thermal transport properties. 11-13 In particular, graphene exhibits remarkably high electron mobility even at ambient temperatures. 14,15 Graphene can be produced by exfoliating graphite, 16,17 epitaxial growth from a SiC single crystal surface, 18,19 solvothermal reaction, 20 and chemical vapor deposition (CVD). 12,21,22 There have been various approaches to take advantage of the unique properties of graphene in optoelectronics, 23,24 sensing, 25,26 and energy storage. 27-30 Recently, Chae et al. designed a new hybrid of CNT/graphene integrated with a wrinkled Al 2 O 3 layer, demonstrating great potential for stretchable and transparent electronics. 31 Kim et al. developed a superelastic and fatigue-resistant 3D-CNT network by coating it with a few layers of graphene, which improves the Young’s modulus by a factor of 6. 32 Previously, our group reported a CNT hybrid material covalently bonded with graphene leaves 33 and highly branched graphene nano- sheets (HBGNs) directly grown on a planar graphene sheet. 34 HBGNs are a few layered graphene nanosheets with open boundaries, which have similar structural characteristics to carbon nanowalls (CNWs). However, different from CNWs, HBGNs are composed of a highly dense, small graphene domain with less than 5 nm in lateral dimension, and the “standing” graphene sheets are randomly oriented. In this study, we investigate novel hybrid CNF/HBGN for anode materials for LIB application. HBGNs can be grown on any electrically conductive substrate without adding any catalyst. We used direct growth of CNFs on type 304 stainless steel for hybridizing with HBGNs. In contrast to the loosely bound hybrids prepared by simply mixing two materials or catalytic growth on a graphene surface, the hybrid CNF/ HBGN will provide a continuous conduction pathway, which is expected to lead to a high charge carrier mobility. CNFs will offer good electrical conductivity and a robust support structure, while HBGNs offer increased Li storage sites. The controlled synthesis method for hybrid CNF/HBGN will offer insights into the bottom-up design of carbon-carbon bond formation. The CNF/HBGN synthesis is accomplished through a two- step CVD process. The experimental details can be found elsewhere. 33,35 In brief, the two-step CVD process consists of a CVD method for CNF growth and a plasma-enhanced CVD (PECVD) method for HBGN growth, both of which were catalyst-free atmospheric pressure growth. We modified a CNT Received: May 27, 2014 Accepted: October 13, 2014 Published: October 13, 2014 Research Article www.acsami.org © 2014 American Chemical Society 18590 dx.doi.org/10.1021/am503328w | ACS Appl. Mater. Interfaces 2014, 6, 18590-18596