Introduction
With the ever-increasing electronics industry, it is of critical importance to improve the energy storage capability of current batteries by using various strategies. Because of their high energy density, good safety, long cycle life, and less pollution, LIBs have achieved great success in the past decades and have been widely integrated into portable electronics, hybrid electric vehicles, smart grids, and so on attributed to the highly efficient energy storage devices [1]. The conventional cathode, LiCoO2, and LiFePO4 with a practical capacity of 140 and 170 mAh/g and a poor rate capability, are harder to satisfy the actual requirements for the next generation cathode.
Furthermore, with the rapid development of cathode materials with high capacities, transition-metal oxide and fluoride like V2O5 [2,3] and FeF3 [4,5] have received wide attention. These intercalation/deintercalation type cathode electrode materials often display a higher capacity than traditional cathode materials. Recently, research on transition metal sulfides has increased due to their high conductivity, good chemical durability and low cost, and they have been applied in various field such as light harvesting [6,7], catalysis [8,9] and energy storage [10–13]. As a transition-metal sulfide, VS4 possesses a unique loose stacked framework structure because the chains are connected through weak van der Waals force [14,15]. The large open channels in the loose stacked framework structure provide plenty of sites for Li+ diffusion and lithiation [16–18].
Although monoclinic VS4 has been reported as an anode material for LIBs [14,19,20], it is rarely studied as a cathode material for LIBs owing to its complex property in the high electrochemical potential range and the difficulty of preparing pure VS4 phase. Therefore, the investigation of VS4 as a cathode material for LIBs is desirable and urgent. The energy storage principle of VS4 cathode can be expressed as
VS4 has attracted great interest for its rich resources, low cost, and high theoretical capacity (449 mAh/g with 3e− transfer and 1196 mAh/g with 5e− transfer) which is much higher than the current commercial cathodes, such as LiCoO2 (~140 mAh/g) and LiFePO4 (~170 mAh/g). Despite these merits, the slow Li+ diffusion and low electrical conductivity of bulk VS4 have restricted its application in LIBs [19,21]. Therefore, designing and constructing VS4-based materials with a higher conductivity for LIBs are urgent for developing the next generation LIBs.
Fabricating nanocomposite with carbon materials has been demonstrated as an effective strategy to enhance the conductivity and rate capability of electrode materials, and numerous kinds of carbon materials have been applied [5,22–24]. Compared with carbon nanotubes (CNTs), graphene possesses outstanding mechanical strength and electrical conductivity due to its unique two-dimensional monolayer structure [25–27]. The high surface area provides graphene with a low fabricating cost and a better interfacial contact compared to CNTs. Moreover, 3D graphene aerogels (GAs) with unique characteristics including a high surface area, a tunable porosity, and large pore volumes, have shown potential applications in the fields of energy storage [10,26,28], catalysis [29], electronic devices and so on. Thus, GAs could be ideal materials to hybridize with VS4.
In this work, a tactful hydrothermal method is applied to in situ synthesize VS4@GAs hybrid nanostructures for the purpose of improving the rate capability. It is first reported as a cathode material for LIBs. Benefiting from the electron transfer highways and abundant pores for Li+ diffusion in graphene, the VS4@GAs hybrid exhibits an enhanced cycle performance and rate capability. A high discharge capacity of 239 mAh/g can be remained after 15 cycles at a current density of 40 mA/g. Even at a high rate of 2000 mA/g, a discharge capacity of 191 mAh/g still can be obtained.
Results and discussion
As illustrated in the synthesis strategy toward the VS4@GAs electrode (Fig. 1), according to the spread growth mechanism induced by electrostatic interaction, VS4 nanoparticles were homogeneously dispersed on graphene. During the hydrothermal process, C2H5NS (TAA) not only serves as a sulfur source to form VS4, but also acts as a reducing agent to reduce GO to rGO partially. The photos of VS4@GAs are exhibited in Electronic Supplementary Material (Fig. S1), which reveals a typical graphene aerogels. To confirm the crystal structure and composition of the prepared VS4@GAs, the X-ray diffraction pattern (XRD) measurement was performed (Fig. S2 in Electronic Supplementary Material). The XRD pattern of the prepared VS4@GAs has a typical body-centered monoclinic VS4 phase (JCPDS No. 87-0603). There are not any peaks of other phases in the XRD pattern, indicating the high phase purity of VS4@GAs.
The microstructure and morphology of the VS4@GAs were investigated by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). As shown in Fig. 2(a), the graphene sheets are obviously crumpled. As shown in Fig. S3 in Electronic Supplementary Material, the graphene nanosheets are highly interconnected, which could effectively impede the aggregation of VS4 nanoparticles. Numerous nanosheets form porous 3D architectures which is similar to those previously reported for graphene aerogels [30,31]. Further investigations reveal that the nanorods are randomly coated on graphene nanosheets (Fig. 2(b) and Fig. S3(d) in Electronic Supplementary Material). Besides, abundant micropores can also be identified in those TEM images, which can be attributed to the water loss during the process of freeze-drying. As shown in Fig. 2(a), the building block contain wrinkles nanosheets having lateral sizes from 5 mm to tens of micrometers, and both sides of graphene are coated with nanorods which are about 5 nm in diameter. Figure 2(c) shows that a lattice periodicity of 0.56 nm is clearly observed by high-resolution TEM image, which can be attributed to the (110) planes of VS4. Figure 2(d) displays the fast-Fourier transform (FFT), which further confirms the VS4 phase of monoclinic.
Energy dispersive X-ray was used to demonstrate the composition of VS4@GAs, and the elemental mapping analysis was used to show the distribution of elements (Fig. 3). The elemental atomic ratio of V and S is 1:4, and the V and S atoms are homogeneously distributed on the nanosheets. Figure S4 in Electronic Supplementary Material depicts the thermogravimetric curve of the VS4@GAs with two steps of weight loss below 600°C. The first weight loss below 250°C is associated with the vanadium sulfide transfer to vanadium oxide, while the second weight loss can be attributed to the decomposition of the graphene in the temperature range of 300°C–500°C. The amount of graphene content in the composite is around 9.46 wt%.
The chemical composition of the VS4@GAs was probed by using X-ray photoelectron spectroscopy (Fig. 4). As shown in Fig. 4(a), the characteristic peaks of C 1s, V 2p, and S 2p bands indicate the existence of C, S, and V. The C 1s spectra can be deconvoluted into three peaks of C-C, C-OH, and HO-C=O bonds respectively, indicating the formation of functional groups in graphene (Fig. 4(b)). The spectra of S 2p are constituted with peaks of 2p1/2 and 2p3/2 (Fig. 4(c)). In the spectra of V 2p (Fig. 4(d)), the peaks located at 513.5 eV and 521.2 eV can be ascribed to the bonds of V-S. On the other hand, the peaks located at 516.7 eV and 523.7 eV can be attributed to V4+, while the small peak located at 514.7 eV arises from V3+. This result indicates that the valence of V is mainly+4, further confirming the formation of VS4@GAs.
The electrochemical performance of VS4@GAs composite as cathode material was tested. The cyclic voltammograms (CV) of VS4@GAs tested at a scan rate of 0.2 mV/s in 1.5–3.0 V are shown in Fig. 5(a). During the initial cathodic scan, only one reduction peak located at 1.94 V appeared which could be ascribed to the lithium insertion VS4 and the formation of LixVS4 phase, corresponding to the equation: xLi+ + VS4 + xe−→ LixVS4 (x≤3). Moreover, in the following anodic scan, two peaks of 2.29 V and 2.44 V were observed, which could be attributed to the multistep lithium extraction from the LixVS4. Furthermore, all redox peaks show no obvious migration in subsequent cycles, indicating a good reaction reversibility upon lithiation and delithiation processes. Figure 5(b) displays the initial five galvanostatic charge-discharge profiles of the VS4@GAs at a current density of 0.1 C (1 C= 400 mAh/g). The obviously discharge potential plateaus around 2.0 V can be assigned to the lithium ion intercalation into the VS4, which is consistent with the CV results.
The rate capability of VS4@GAs composite is illustrated at different current densities ranging from 0.1 to 5 C. (The specific capacity is calculated by the total mass of VS4@GAs composite). As shown in Fig. 5(c), the VS4@GAs electrode delivers a high specific capacity of 487, 443, 379, 326, and 244 mAh/g at a current density of 0.1, 0.2, 0.5, 1 and 2 C respectively. Moreover, even at a high rate of 5 C, the VS4@GAs could still exhibit a discharge specific of 191 mAh/g. In addition, it is notable that all the discharge-charge curves at various current densities have similar discharge and charge plateaus, demonstrating the good rate capability of VS4@GAs composite. Figure 5(d) displays the cycle performance of the VS4@GAs electrode at 0.1C. The VS4@GAs composite has a high discharge specific of 511 mAh/g and a charge specific capacity of 438 mAh/g, corresponding to an initial coulumbic efficiency (ICE) of 86%. The capacity loss can be attributed to the partially reversible lithium ion intercalation/extraction in VS4. The VS4@GAs could maintain a reversible capacity of 239 mAh/g after 15 cycles at a current density of 0.1 C. The obvious capacity decay of VS4@GAs during the discharging/charging process further reveals that delithiation from LixVS4 is much more difficult. Honestly, there is plenty of room to achieve high performance VS4-baed cathode materials for LIBs by using more advanced nanostructure and composite methods.
Conclusions
In summary, VS4@GAs was reported as a new cathode material for LIBs. The VS4@GAs showed the mechanism of intercalations/deintercalations for lithium storage. Benefiting from the electron transfer highways in graphene and the abundant pores for Li+ diffusion, the VS4@GAs hybrid exhibits an enhanced cycle performance and rate capability. A high discharge capacity of 239 mAh/g can be remained after 15 cycles at a current density of 40 mA/g. Even at the high rate of 2000 mA/g, a discharge capacity of 191 mAh/g still can be obtained. The excellent electrochemical performance will provide new opportunities for the next-generation high-performance LIBs.