Fabrication of layered structure VS₄ anchor in 3D graphene aerogels as a new cathode material for lithium ion batteries

Abstract

VS₄ has gained more and more attention for its high theoretical capacity (449 mAh/g with 3e⁻ transfer) in lithium ion batteries (LIBs). Herein, a layered structure VS₄ anchored in graphene aerogels is prepared and first reported as cathode material for LIBs. VS₄@GAs composite exhibits an exceptional high initial reversible capacity (511 mAh/g), an excellent high-rate capability (191 mAh/g at the 5 C), and an excellent cyclic stability (239 mAh/g after 15 cycles).

Keywords

VS₄ / graphene aerogels / cathode / lithium storage

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

{{\displaystyle \mathrm{V}\mathrm{S}}}_{\mathrm{4}}+x{\mathrm{L}\mathrm{i}}^{+}+{x\mathrm{e}}^{-}\rightleftharpoons {{\displaystyle \mathrm{L}\mathrm{i}}}_x{{\displaystyle \mathrm{V}\mathrm{S}}}_{\mathrm{4}}

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 2p_1/2 and 2p_3/2 (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 Li_xVS₄ phase, corresponding to the equation: xLi⁺ + VS₄ + xe⁻→ Li_xVS₄ (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 Li_xVS₄. 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 Li_xVS₄ 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.

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