3D WO3 nanowires/graphene nanocomposite with improved reversible capacity and cyclic stability for lithium ion batteries
Introduction
As an important n-type semiconductor, WO3 was used in a wide variety of applications such as chemical sensors, photocatalysis, energy conversion systems [1], [2], [3], [4]. It also has been examined as an anode material for lithium ion batteries due to its large theoretical capacity (∼700 mAh g−1), low cost, enhanced safety because of high melting temperature and mechanical stability [5]. More importantly, a very high volumetric capacity can be expected considering its high theoretical density of 7.61 g cm−3 [5]. Nevertheless, WO3 as an anode material suffers from large structural and volume variation during the charge/discharge processes, and the induced structure change breaks the stability of electrode material, leading to mechanical disintegration and the loss of electrical connection between the active material and current collector, severely decreasing the cycling ability of electrodes. Also, the capacities faded rapidly to lower than 75% of the initial values after only several tens of cycles even at low current rates [6].
Graphene, a two-dimensional sheet of carbon, has attracted tremendous attention for energy storage due to its superior electrical conductivity, high specific surface area and mechanical flexibility. The use of graphene in anode materials has a lot of advantages, such as increased electrode/electrolyte contact area, good accommodation of the volume variation and short Li+ and electron transport lengths. Many transition metal oxides have been composited with graphene as anode materials for lithium ion batteries (LIBs)[7], [8], [9], [10], [11], [12]. Noticeable improvements in specific capacity and rate capability denote a promising avenue to hybridize transition metal oxide/graphene nanocomposite.
In this paper, we report a hydrothermal route for fabricating hierarchical WO3 nanowires/graphene nanocomposite. The WO3 nanowires form simultaneously with the reduction of graphene oxide to graphene, resulting in a hierarchical clustering architecture with porous nanostructures. The combination of WO3 nanowires with graphene leads to enhanced electrochemical performances by synergizing high energy density WO3 with highly conductive graphene network for effective charge transfer and energy storage. The first research work investigating the WO3 nanowires/graphene nanocomposite led to promising electrochemical performance used as anode for LIBs applications.
Section snippets
Experimental
Graphene oxide (GO) was prepared from exfoliate graphite using a modified Hummers method with details described in our previous literature [13]. GO powders were dispersed in deionized water to create a homogeneous dispersion through ultrasonication for half an hour. WO3 nanowires/graphene nanocomposite was synthesized through an in situ hydrothermal process [14]. In a typical process, 1 g Na2WO4·2H2O and 0.2 g NaCl were dissolved in 40 mL GO solution (0.2 mg/mL) and kept stirring for 6 h. 2 M
Results and discussion
Fig. 1(a) shows the XRD patterns of the WO3 nanowires/graphene nanocomposite and pure WO3 nanowires. All the peaks can be well indexed to hexagonal structure of WO3 (JCPDS 75-2187) with the space group P6/mmm. For the nanocomposite, no obvious diffraction peaks of graphene can be observed due to the strong signal from WO3, which agrees well with a previous study [14]. From the Raman spectra shown in Fig. 1(b), sharp peaks located at about 245, 670 and 807 cm−1 are attributed to the O–W–O
Conclusions
A 3D porous WO3 nanowires/graphene nanocomposite was synthesized via a facile hydrothermal method. It exhibits a high reversible capacity, excellent cycling performance, and remarkable rate capability when used as anode electrode materials for lithium ion batteries. This investigation highlights a strategy of incorporating highly conductive graphene into a 3D nanostructure to mitigate capacity degradation at high current densities of electrodes for lithium ion batteries.
Acknowledgments
This work was supported by the NSF career award under the Award number of DMR 1151028. The author Mingpeng Yu thanks for the financial support from the China Scholarship Council(CSC File No. 2010646040).
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