Letter pubs.acs.org/NanoLett
Persistent Electrochemical Performance in Epitaxial VO2(B) Shinbuhm Lee,† Xiao-Guang Sun,‡ Andrew A. Lubimtsev,§,∥ Xiang Gao,† Panchapakesan Ganesh,§ Thomas Z. Ward,† Gyula Eres,† Matthew F. Chisholm,† Sheng Dai,‡ and Ho Nyung Lee*,† †
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡
S Supporting Information *
ABSTRACT: Discovering high-performance energy storage materials is indispensable for renewable energy, electric vehicle performance, and mobile computing. Owing to the open atomic framework and good room temperature conductivity, bronze-phase vanadium dioxide [VO2(B)] has been regarded as a highly promising electrode material for Li ion batteries. However, previous attempts were unsuccessful to show the desired cycling performance and capacity without chemical modification. Here, we show with epitaxial VO2(B) films that one can accomplish the theoretical limit for capacity with persistent charging−discharging cyclability owing to the high structural stability and unique open pathways for Li ion conduction. Atomic-scale characterization by scanning transmission electron microscopy and density functional theory calculations also reveal that the unique open pathways in VO2(B) provide the most stable sites for Li adsorption and diffusion. Thus, this work ultimately demonstrates that VO2(B) is a highly promising energy storage material and has no intrinsic hindrance in achieving superior cyclability with a very high power and capacity in a Li-ion conductor. KEYWORDS: VO2(B), epitaxy, energy storage, electrode, Li ion battery
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VO2(B) has been long regarded as a promising electrode material for LIBs after being first proposed in 1994 by Li et al.,11 owing to low cost, nontoxicity, and abundant sources. In addition, the theoretical capacity and energy density of VO2(B) (323 mAh/g at a redox voltage of 2.6 V, 0.84 Wh/g) are higher than those of state-of-the-art LIB electrodes, such as LiCoO2 (274 mAh/g at 4 V, 1.1 Wh/g), LiFePO4 (165 mAh/g at 3.5 V, 0.58 Wh/g), Nb2O5 (200 mAh/g at 1.2−2.4 V, 0.24−0.48 Wh/ g), and V2O5 (294 mAh/g at 2.7 V, 0.79 Wh/g).2,12−14 In particular, VO2(B) stands out because of its unique open framework with channels formed from edge-sharing VO6 octahedra.15,16 Thus, VO2(B) has been expected to exhibit both high capacity and rapid Li ion diffusion. However, there has been no successful experimental confirmation of such intriguing properties despite many attempts with various VO2(B) nanostructures, including quantum-dots coated nanostructures, nanobelts, nanoscrolls, and mesocrystals.15−22 So far, the low capacity and rapid irreversible capacity loss have been observed in VO2(B) electrodes, hampering their further
here is a high demand to develop better performing energy storage materials to fulfill the requirements for emerging mobile and energy technologies. In order for electric vehicles and grid storage devices to become viable alternatives to current technologies, further development of energy storage materials that offer high capacity and long-term cycling endurance is essential.1−7 In particular, the development of next-generation secondary batteries must continue to maximize miniaturization without losing electrochemical performance.1−3 To address such challenges, efforts to maximize surface area have led to the development of nanostructured electrodes as the primary direction of research. However, these nanomaterials come with severe limitations, including nonuniformity in particle size, impurity phases, and binding agent free surfaces, not to mention the possible environmental risks associated with nano hazards.3,8 As there are increased concerns on the safety of conventional Li ion batteries (LIBs) with an anodic Li metal, solid-state LIBs are attracting increasing attention to improve the stability.9,10 Thus, development of electrode materials in thin film forms with enhanced electrical and electrochemical performances is highly desirable as most electrode materials are poor electrical conductors, adding another complexity of developing conducting binders to overcome the electronic conduction problem. © XXXX American Chemical Society
Received: November 19, 2016 Revised: February 8, 2017
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DOI: 10.1021/acs.nanolett.6b04831 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Electrochemical performance of VO2(B) electrodes. (a) Galvanostatic discharge (Li intercalation)−charge (Li deintercalation) profile of an epitaxial VO2(B) film in a voltage range of 1.5−3.2 V. At various current densities, a plateau related to the Li reduction into VO2(B) appears near 2.6 eV. (b) Capacity as a function of the discharging rate. The capacity of our epitaxial VO2(B) films (solid symbols) is close to the theoretical value (323 mAh/g). The capacity values of pure V2O5,14 nanobelt VO2(B),18 nanoscroll VO2(B),19 mesocrystal VO2(B),20 carbon,21 or graphene22 quantum-dots coated VO2(B) are included for comparison. (c) Cyclic capacity test up to 200 cycles reveals a Coulombic efficiency close to 100%.
binders, offering increased design flexibility to develop binderfree LIBs. Coin cells with a metallic Li foil counter electrode were assembled to investigate the electrochemical performance of epitaxial VO2(B) films. Figure 1a shows a galvanostatic discharge (Li intercalation)−charge (Li deintercalation) profile for an epitaxial VO2(B) film at various current densities in the voltage range of 1.5−3.2 V. The plateaus near 2.6 V indicate a redox process of Li with VO2(B), i.e., xLi + VO2(B) + xe− → LixVO2(B).11,16 An extra plateau was also observed at around 3 V in the deintercalation process under low current densities (
