Two-dimensional nanocomposites based on tungsten oxide nanoplates and graphene nanosheets for high-performance lithium ion batteries
Graphical abstract
Introduction
Lithium ion batteries (LIBs) have been considered as the most important energy storage devices for use in portable equipment such as cellular phones and lap-top computers as well as in high-capacity devices such as hybrid electrical vehicles. However, the rapidly advanced applications demand high capacity, fast charging/discharging rate, and improved cycle life [1], [2], [3], [4], [5]. For the advanced electrochemical characteristics in LIBs, both the composition and structure of negative and positive active materials for better LIBs characteristics have been investigated over the past few decades. In particular, electrode materials of LIBs require a high electrical conductivity specific surface area, low cost, and long-term electrochemical stability [6], [7].
Typically, carbon-based materials have been used as a promising anode, even showing lower theoretical capacity in LIBs [8], [9], [10], [11], [12]. Furthermore, transition metal oxides have been investigated to replace graphite and have exhibited high reversible capacities, resulting from their unique electrochemical properties. However, most transition metal oxide suffers from poor rate performance leading to drastic capacity drop due to low electrical conductivity and large volume change during charge-discharge cycling [13], [14], [15], [16], [17]. Accordingly, many efforts have been made to overcome the critical problems using composite structures with carbon-based or other electrical conductive materials. Among the composites consisting of transition metal oxides and carbon-based materials, much interest has been given to two-dimensional structure composites containing graphene nanosheet (GNS), leading to higher specific capacity and better high-rate stability [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Graphene nanocomposites show good potential in LIBs due to their low energy barrier for Li-ion, large surface area, and excellent electrical conductivity. Tungsten oxide (WO3) may be one of the powerful candidates in the transition metal oxides because of its environmental friendliness, and high theoretical capacity of 693 mAh g−1 [28], [29], [30], [31], [32], [33], [34], [35], [36]. The charge-discharge mechanism of WO3 is based on conversion reaction process, which requires the formation of a metal and lithium oxide, as follows:WO3 (crystalline) + 6 Li+ + 6 e− → W + 3 Li2OW + 3 Li2O → WO3 (crystalline) + 6 Li+ + 6 e−
Herein, we prepared the two-dimensional nanocomposites (WO3/GNS) containing WO3 nanoplates and GNS with varying amount of GNS synthesized using a hydrothermal method and heating process. The structure and morphology of the nanocomposites were characterized using XRD, SEM, and TEM analysis. The capacity, capacity retention, and high rate cycling performance of the nanocomposites as an anode for lithium ion batteries were evaluated using coin cells.
Section snippets
Experimental Section
WO3/GNS composite electrodes were synthesized using a hydrothermal method and heating process. To synthesize WO3/GNS, the pre-oxidized GNS (ENanoTec 4 ∼ 5 layer) powders with intended amounts of 5, 10, and 20 wt% were dispersed in 70 mL of 5 M hydrochloric acid solution (Samchun, 35–37%) with continuous sonication for 90 min at room temperature. With vigorous stirring, 1 g ammonium tungstate ((NH4)10H2(W2O7)6, Aldrich, 99.99%) was dissolved in the mixture solution. After sonication, the mixtures of
Results and discussion
The XRD patterns of the as-prepared WO3/GNS nanocomposites (denoted as WO3/GNS-0, WO3/GNS-5, WO3/GNS-10, and WO3/GNS-20, respectively) with varying amounts of graphene (0, 5, 10, and 20 wt%, respectively) are shown in Fig. 1(a). The unit cell parameters of WO3 for the as-prepared samples were determined to be a = 7.297 Å, b = 7.539 Å, and c = 7.688 Å in agreement with those of the bulk WO3 material with a space group of P21/n (JCPDS No. 43-1035). In contrast, the main diffraction peak (denoted as A) at
Conclusions
In summary, we prepared two-dimensional nanocomposites as an anode in LIBs using a hydrothermal method and heating process. In the nanocomposites based on WO3 nanoplates and graphene nanosheets, the WO3 nanoplates could be heterogeneously nucleated on the graphene nanosheets, preventing the restacking of GNS and thus maintaining the electrochemically active sites for an improved performance in LIBs. The nanocomposites exhibited high reversible capacity, good capacity retention, and excellent
Acknowledgment
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2013R1A1A2012541).
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