Onion-like crystalline WS₂ nanoparticles anchored on graphene sheets as high-performance anode materials for lithium-ion batteries

Highlights

• C@WS₂@Gs is synthesized via ball-milling and subsequent sulfidation process.

• Graphene prevent aggregation and develop onion-like crystalline of WS₂ nanoparticles.

• Superior long-term cyclability is achieved at a high current density of 1000 mA g⁻¹.

• C@WS₂@Gs anode exhibits notable electrochemical performance due to unique structure.

Abstract

In this study, onion-like crystalline WS₂ nanoparticles uniformly anchored on graphene sheets (WS₂@Gs) were prepared via ball milling using WO₃ nanoparticles and graphene and subsequent sulfidation. They were then employed as high-performance anode materials for Li-ion batteries (LIBs). The ball-milling process facilitated uniform anchoring of WO₃ nanoparticles with a diameter of ∼15 nm on graphene nanosheets without aggregation, and the subsequent sulfidation caused phase transition of the WO₃ nanoparticles to WS₂ nanoparticles with an onion-like crystal lattice structure. As anode materials for LIB, the uniquely structured WS₂@Gs nanocomposites exhibited excellent Li-ions storage performance, with a high reversible capacity of 587.1 mA h g⁻¹ at a current density of 200 mA g⁻¹. To enhance the cyclic stability of WS₂@Gs, the C-coating method was employed by simply adding glucose during ball milling. Even at a high current density of 1000 mA g⁻¹, the C-coated WS₂@Gs (C@WS₂@Gs) electrode exhibited a remarkably high reversible capacity of 371.9 mA h g⁻¹ and appreciable cycling stability, with a high capacity retention of 62% without any drastic capacity fading after 500 cycles.

Introduction

Recently, transition-metal dichalcogenides (TMDCs) with the formula MX₂ (M = Mo, W, V, Ti; X = S, Se, Te) have received considerable attention as electrode materials for Li-ion batteries (LIBs) owing to their high theoretical specific capacities and rapid ion diffusivity, which originate from their two-dimensional (2D) layered structure [1], [2], [3], [4], [5], [6], [7], [8]. The interlayer structure of TMDCs is a stacked structure of three atomic layers (X-M-X) with strong covalent bonds between the transition metal and the chalcogenide anions [1], [2], [5]. Because the surface of each WS₂ layer is terminated by lone-pair electrons derived from the combination of the transition metal (M) and chalcogenide atoms (X), the sandwich layers are repeatedly stacked via weak van der Waals forces [1]. Thus, when TMDCs are used as electrode materials for LIBs, the well-defined internal empty space and functional wall between the sandwich layers arranged in a specific morphology enable easy insertion and extraction of the Li ions [9], [10], [11].

Among reported TMDCs, WS₂ is a promising LIB electrode material because of its high theoretical capacity of 432 mA h g⁻¹ (that of graphite is 372 mA h g⁻¹) according to the conversion WS₂ + 4Li⁺ + 4e⁻ ↔ W + 2Li₂S and high ion diffusivity with a large interlayer spacing of 0.62 nm (that of graphite is 0.34 nm) [2], [3], [5], [6], [8]. However, the application of WS₂ as a LIB electrode material is severely limited by its low electrical conductivity and the large volume expansion it undergoes during the stated conversion with Li ions, which result in poor electrochemical performance [3], [8].

The strategy usually employed for enhancing the electrical conductivity of WS₂ involves combining WS₂ with highly conductive materials [3], [5]. The most commonly used conductive additives are C-based materials such as graphene, reduced graphene oxide, single/multi-walled C nanotubes, and pyrolytic C, which enhance the electrochemical performance without side reactions involving Li ions [5], [7], [8], [12]. For example, Zhang et al. synthesized a composite of WS₂ nanosheets and Super P carbon black (WS₂/C) as an anode material for Na-ion batteries (NIBs) and LIBs, and the resulting anode exhibited a significantly enhanced electrochemical performance compared with that of a bare WS₂ anode [12], which could be attributed to the high conductivity of Super P carbon black in cycling.

In addition to this proposed route to improving electrical conductivity, poor stability caused by the intrinsic large volume expansion of WS₂ during the conversion must be resolved for its practical application [3], [5], [12], [13]. An effective method for mitigating the excessive volume expansion of WS₂ involves multidimensional nanostructuring with C-based nanomaterials [14], [15]. Among the various types of nanostructures that can be used, 2D nanocomposites consisting of WS₂ nanosheets and graphene have been widely investigated. WS₂ nanosheets are structurally similar to the corresponding graphene structure, which results in structural stability [3], [5], [7], [15]. The relaxed 2D structure of WS₂ nanosheets with graphene can not only alleviate cracks and/or pulverization of electrodes caused by the large volume expansion of WS₂ but also provide a 2D electron-transfer pathway through the graphene sheet, thereby enhancing the rate performance of the anode material [16], [17], [18]. Meng et al. synthesized heterojunction composites consisting of liquid-phase exfoliated graphene (LEGr) and WS₂ nanosheets (LEGr@WS₂) and demonstrated their improved electrochemical performance arising from the 2D hierarchical structure. The LEGr layer acted not only as a 2D framework that buffered the large volume expansion but also as a current collector facilitating fast electron transfer in the anode [7]. Although the large volume expansion of WS₂ materials is mitigated by multidimensional nanostructuring, issues such as the low columbic efficiency, caused by the large surface area resulting from nanostructuring, need to be addressed [12], [15].

In previous studies, the intrinsic electrical properties of WS₂ could be improved by controlling the crystal structure. For example, Liu et al. reported that the semiconductor properties of WS₂ armchair nanoribbons can be changed to metallic propertied by applying external strains in the crystal structure [19]. Among the various possible crystal structures, faceted onion-like crystalline WS₂ nanoparticles exhibit low charge-transfer resistance owing to the exposure of a high density of edge sites [20]. However, so far, onion-like crystalline WS₂ nanoparticles have hardly been studied as anode materials for LIBs because of their low electrochemical activity caused by their saturated edge structure and low ion diffusivity [21], [22]. Recently, Hong et al. prepared inorganic fullerene-like (IF) Re-doped MoS₂ nanoparticles as intercalation materials for NIBs by doping IF-MoS₂ via heat treatment under H₂ gas flow. The modified IF-MoS₂ exhibited improved electrochemical performance owing to doping being an effective method for generating defects in a case involving ion transport [23]. Thus, the crystallographic approach should be considered as an effective way to enhance the electrochemical performance of TMDC-based anode materials along with the fabrication of composites and nanostructures.

In the present study, onion-like crystalline WS₂ nanoparticles with a diameter of <15 nm uniformly anchored on graphene sheets (WS₂@Gs) were successfully prepared, and their Li-ion storage properties were investigated toward application in anode materials for LIBs. The onion-like crystalline WS₂ nanoparticles were developed via sulfidation of WO₃ nanoparticles anchored on graphene sheets prepared using ball milling. For bare WS₂ nanoparticles prepared via sulfidation of WO₃ nanoparticles without graphene sheets, a typical polycrystalline structure was observed, owing to the sintering and/or growth of particles during the sulfidation process. The uniquely structured WS₂@Gs electrode exhibited better Li-ion storage performance—with a high reversible capacity of 587.1 and 415.5 mA h g⁻¹ at low and high current densities of 200 and 1000 mA g⁻¹, respectively—than the WS₂ electrode. To enhance the cycling stability, the C-coated WS₂@Gs (C@WS₂@Gs) electrode was prepared by simply adding glucose during the ball-milling process. Consequently, even at a high current density of 1000 mA g⁻¹, the C@WS₂@Gs electrode exhibited a high reversible capacity of 371.9 mA h g⁻¹ and remarkable cycling stability, with a high capacity retention of 62% after 500 cycles. The excellent electrochemical performance of the C@WS₂@Gs electrode can be attributed to its high electrical conductivity, structural stability, and large specific surface area (SSA).