MgO-template-assisted synthesis of worm-like carbon@MoS₂ composite for lithium ion battery anodes

Abstract

In this study, a facile MgO template-assisted route has been proposed for the preparation of carbon@MoS₂ composites via a one-step hydrothermal method followed by a thermal annealing process. By using sucrose as a carbon source and commercial MgO powder as a structure-directing agent, a three-dimensional sphere-like carbon@MoS₂ nanoarchitecture was assembled by one-dimensional worm-like nanorods which were composed of few-layered MoS₂ nanosheets and amorphous carbon matrix. The influence of different amount of MgO template on the morphology, structure, and lithium storage performance of the carbon@MoS₂ composites is systematically investigated. The carbon@MoS₂-2 composite synthesized with a Mo precursor: MgO molar ratio of 1:1 exhibits much higher reversible capacity (905.3 mAh g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹) and better high-rate capability (503.4 mAh g⁻¹ at 5000 mA g⁻¹) than those of bare MoS₂ and template-free carbon@MoS₂-0 composite. Impressively, superior cycling stability is achieved for the carbon@MoS₂-2 composite with a capacity retention of 93.4% even after 400 charge/discharge cycles at 5000 mA g⁻¹. The excellent electrochemical performance for carbon@MoS₂-2 composite can be ascribed to its unique hierarchical nanoarchitecture and synergetic effect between MoS₂ nanosheets and amorphous carbon.

Introduction

In recent years, lithium ion batteries (LIBs) have attracted extensive attention and become the main power sources for electric devices and grid energy storage systems [1], [2], [3], [4]. However, traditional anode electrode materials such as commercial graphite cannot fulfill the increasing demand for high energy density and power density, due to its low theoretical specific capacity (372 mAh g⁻¹) and poor rate-capability [5], [6]. Therefore, alternative anode materials with large specific capacities, high rate performance, and long-cycle properties are desirable for high-performance LIBs.

Molybdenum disulfide (MoS₂), a typical layered transition metal sulfide, is comprised of covalently bonded three stacked atom layers (S-Mo-S) held together by weak van der Waals interaction [7], [8]. Owing to its unique chemical peculiarities and high capacity (∼670 mAh g⁻¹), MoS₂-based nanostructured materials have been demonstrated to be the ideal anode materials for LIBs [9], [10]. However, most MoS₂-based electrodes still suffer from the low electronic/ionic conductivity, rapid capacity fading as well as poor cycling stability [9], [11], [12]. To address these issues, one feasible strategy is to construct the MoS₂/carbon composites through introducing conductive carbonaceous materials (such as graphene [13], carbon nanotubes [14], and amorphous carbon [15]). These conducting agent in carbon/MoS₂ composites can serve as a good conductive network and a buffering layer to accommodate substantial volume change of MoS₂ during repeated lithiation/delithiation process, thereby leading to improved rate and cycling performances [13], [14], [15]. Another strategy is to design various morphologies of nanostructured MoS₂ (such as nanosheets [16], nanotubes [17], nanoplates [18], nanoboxes [19], nanocables [20], and microspheres [21]). Owing to their large surface area, short diffusion path, and abundant active sites for lithium storage, nanostructured MoS₂ electrodes delivered greatly enhanced electrochemical properties of LIBs with higher reversible capacities and prolonged lifespan [22].

To fabricate nanostructured MoS₂-based hybrids, various metal oxides with unique morphologies (such as Fe₂O₃ nanocubes [19], TiO₂ nanotubes [23], TiO₂ nanospheres [24], and SiO₂ nanospheres [25]) have been utilized as the hard templates. These templates usually act as the supporting matrix and the core of the hybrid nanoarchitectures to grow MoS₂, which plays a key role in the formation of the MoS₂-based hybrids with rational nanoscale structure [1], [19]. However, the drawbacks of the hard template method are the complicated, time-consuming steps to synthesize the solid scaffolds by complex techniques. Compared with the above mentioned templates, commercial magnesium oxide powder exhibits the following advantages: (1) natural abundance and low cost; (2) the preparation of MoS₂ and magnesium hydroxide nanoplate template (converted from commercial MgO powder) can be achieved in one step via one-pot hydrothermal technique in the absence of any surfactants [15], [26], which reduces the preparation steps and saves time; (3) the template can be easily removed by HCl solution at room temperature. In addition, MoS₂ nanocomposites obtained via a MgO template method have not been reported on its application as high-performance LIBs anodes.

In the present work, we present a simple and facile process to synthesize carbon@MoS₂ composite via one-pot hydrothermal method with the aid of commercial MgO powder followed by annealing in H₂/N₂ atmosphere at 750 °C. With the synergistic effects from sucrose as a binder and the MgO template as a structure-directing agent, a three-dimensional (3D) sphere-like carbon@MoS₂ composite is constructed by loosely stacked, worm-like one-dimensional (1D) nanorods, which are homogeneously coated with amorphous carbon. When evaluated as anode materials for LIBs, the carbon@MoS₂ composite shows a high reversible capacity, high-rate capability and excellent cycling performance.