VO 2 (B)/Graphene Forest for High-Rate Li-Ion Battery
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VO2(B)/Graphene Forest for High-Rate Li-Ion Battery Guofeng Ren and Zhaoyang Fan Department of Electrical and Computer Engineering and Nano Tech Center, Texas Tech University, Lubbock, Texas 79409, USA ABSTRACT 2D nanomaterials, when assembled into an ordered macrostructure, will present many great opportunities, including for Li-ion batteries (LIBs). We report densely-packed vertically-aligned VO2(B) nanobelts (NBs)-based forest structure synthesized on edge-oriented graphene (EOG) network. Using a EOG/Ni foam as a 3D scaffold, aligned VO2(B) NBs can be further synthesized into a folded 3D forest structure to construct a freestanding electrode for LIBs. Electrochemical studies found that such a freestanding VO2(B)/EOG electrode, which combines the unique merits of 2D VO2(B) NBs and 2D graphene sheets, has excellent charge-discharge rate performance. A discharge capacity of 178 mAh g-1 at a rate of 59 C and 100 mAh g-1 at 300 C was measured. A good charge-discharge cycling stability under a high current density was also demonstrated. The results indicate VO2(B)/EOG forest based freestanding electrode is very promising for developing high-rate LIBs. INTRODUCTION A controlled assembly of 2D nanomaterials in a 3D macrostructure will provide many opportunities enabling a variety of applications. An example is the employment of 2D layered nanosheets (NSs) in a nanostructured electrode for high-performance electrochemical energy storage. Nanostructured electrodes have been and continue to be the research interest pursued to address the interrelated issues of energy density, power density, charge-discharge rate, and cycling stability of electrochemical energy storage devices, including supercapacitors and LIBs [1-4]. Leveraging nanoscale dimension to increase surface area and reduce ion diffusion length will facilitate the lithiation/delithiation process for high charge-discharge rate capability and high power density. In addition, the mechanical flexibility of low-dimensional materials will buffer the crystal lattice expansion/shrinkage to prevent electrode material pulverization. This will lead to a long cycling lifetime and performance stability. Different nanomaterials have been pursued with demonstrated advantages in LIBs. However, nanomaterials also have their own issues. Selfaggregation, loose mechanical and electrical contact, and dislodging are examples of critical issues that must be addressed. Nanoparticles or quasi-2D layered materials are easily aggregated to form close-packed clusters, and thus lose the characteristics of nanomaterials. If not allowed to aggregate, loosely-collected nanomaterials may exhibit large inter-particle contact resistance as well as may be easily dislodged from the electrode. Binders and conductive fillers are used to alleviate these problems, but cannot completely eliminate them. A low volumetric energy density is frequently encountered in a nanostructured electrode due to the porous volume. Therefore, close packing of nanomaterials but without their aggregation is essential. An int
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