Facile synthesis of SnS₂@g-C₃N₄ composites as high performance anodes for lithium ion batteries

Highlights

• Facile one-step synthesis of the SnS₂@g-C₃N₄ composites.

• Significant enhancement in the electrochemical performance of the SnS₂@g-C₃N₄ composites.

• Large contribution of pseudocapacitive effect.

• Clarifying the lithium storage mechanism of the SnS₂@g-C₃N₄ composites.

Abstract

Today, the demand of high performance and low-cost anodes for lithium ion batteries becomes more serious to meet the requirement of electric vehicles and portable devices. In this study, a facile synthesis technique using a solid state reaction has been successfully applied for SnS₂@g-C₃N₄ composites. The obtained composites are constructed by SnS₂ nanosheets well-dispersed in porous matrix of g-C₃N₄. The enhancement in the specific capacity, rated performance, and cycling behavior of the SnS₂@g-C₃N₄ composites is assigned to the buffering effect of the large-surface-area g-C₃N₄ matrix as well as the predominant contribution of pseudocapacitive effect. The improved mobility of charge carriers is raised from not only the high exfoliation of the SnS₂ nanosheets, the high porosity of the composites but also the formation of heterointerfaces between semiconductors of SnS₂ and g-C₃N₄. Accordingly, an internal electric field is generated inside the synthesized composites, which accelerates the migration of electrons and lithium ions.

Introduction

The energy crisis is currently considered one of the major challenges in sustainable human development. Two fundamental energy strategies that have been of great interest in recent years include (i) the transfer of electricity production from fossil fuels to sustainable power sources, and (i) the replacement of traditional transportation using gasolines to those using electrical propulsion such as electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1],. Among the sustainable energy technologies, lithium ion batteries (LIBs) are identified as the most promising energy storage devices for a wide range of practical applications owing to their high-rate performance, no memory effect and prolonged cycle life [2,3]. For currently commercial LIBs, graphite is commonly employed as main anode active material, due to its various notable advantages, such as high availability, low price, and good mechanical stability. Structurally, graphite is composed of plenty of graphene layers bonded to each other by weak van der Waals interactions to generate a typical layered architecture [4], providing suitable channels for lithium ions to intercalate and transport. Nevertheless, current graphite anodes with a low theoretical capacity of 372 mAh g⁻¹, a poor lithium diffusion rate (10⁻⁹–10⁻⁷ cm² s⁻¹), and limited cycling lifetimes have insufficiently fulfilled the demands of different widespread applications from consumer electronics, EVs to grid‐scale energy storage [5,6]. In this context, considerable effort has been devoted to the development of alternative anodes with superior properties for the next generation of high-energy and high-power LIBs.

To date, numerous anode materials based on group IVA elements (Sn, Ge, and Si) [7], metal oxides [8,9], and sulfides [10], have been actively investigated, not only for LIBs, but also for other rechargeable batteries, such as, sodium-ion batteries, and potassium-ion batteries. Among these materials, transition metal dichalcogenides (TMDs), particularly tin disulfide (SnS₂) with 2D layered structures analogous to that of graphite, have recently received special attention as innovative anode material for practical applications. This results from its unique physicochemical characteristics [11],. Compared to the commercial graphite anode, the SnS₂-based anode, shows superior properties regarding capacity, power, and durability. In term of the structure, crystalline SnS₂ possesses a large interlayered spacing of 0.590 nm, much larger than the ionic radius of Li⁺ (0.076 nm) [12],. This enables favorable lithium-ion insertion. In addition, SnS₂ also offers a theoretical capacity of 1136 mAh g⁻¹, much higher than that of graphite (372 mAh g⁻¹). This suggests the great potential of this material as alternative candidate for graphite in highly efficient LIBs [13],.

Unfortunately, in addition to the inherent advantages as mentioned above, the pure SnS₂ anodes generally suffer from some drawbacks such as insufficient electronic conductivity and the adverse volumetric expansion throughout the lithiation/delithiation process. This leads to the poor rate performance and cycling stability for the anodes. To address these issues, two critical strategies have been proposed. The former is to fabricate nanostructured SnS₂-based materials with various morphologies, such as 0D nanoparticles [14], 1D nanowires [15], 2D nanosheets [16], and 3D-hiarechical nanoassemblies [17],. This approach shows a variety of admirable properties such as a large surface area, low lithium-ion diffusion length, and high electrical and ionic conductivity, thereby enabling increased SnS₂ reaction kinetics [11],. The latter is to synthesize SnS₂-based composite materials, in which SnS₂ is combined with another electrochemically less active or inactive supporting material. This unique construction not only provides an efficient accommodation space that cushions the internal stress induced by the volumetric variation during repeated lithiation/delithiation but also improves the conductivity of the composites [12],. As demonstrated in previous reports, the combination of nanostructured SnS₂ with carbonaceous materials, such as graphene nanosheets [11], carbon nanofibers [18], and mesoporous carbon [19], improves the electrochemical properties of anode materials, viz. increasing the specific capacity, rate capability, and cycling durability. Accordingly, SnS₂-based composites are frequently regarded as new and advanced anode materials.

Recently, a polymeric semiconductor, namely graphitic carbon nitride (g-C₃N₄) with the 2D porous graphene-like structure, has been extensively investigated as a prospective mechanical support for scaffolding anode active materials for LIBs. This results from its conjugated configuration, abundance of C and N elements, and good physicochemical stability [13],. It has been reported that combining g-C₃N₄ with other suitable materials, such as SnO₂ [20], MoS₂ [21], WS₂ [22], could significantly improve the electrochemical performance of the resultant composites because of their synergistic function and unique structural superiority. Moreover, not only can porous g-C₃N₄ architecture provide favorable networks for electrolyte diffusion to much more active sites, but it can also offer a strain-relief buffering matrix during cycling, thus significantly promoting the electrochemical performance [23],.

To date, composites of SnS₂ and various supporting matrices such as graphene [[24], [25]],[26], reduced graphene oxide [27], carbon nanotubes [28], have been investigated as high-performance anodes for LIBs via several synthesis methods including hydrothermal,[26], wet-chemistry reaction [29], microwave assisted reaction [13],. In general, these composites are high cost due to the requirement of high-end technology in the synthesis process of related matrix materials [30], as well as complications during sample preparations. Although fabrication of the SnS₂/g-C₃N₄ composites as anode materials for LIBs was reported previously [13,23], the fabrication processes are complex and associated to multi-steps, leading to unsatisfactory wide-scale deployment. Therefore, the development of a scalable and facile synthesis method for SnS₂ based-composites, especially for the composites of SnS₂ and g-C₃N₄, is needed.

In this study, we introduce a facile one-pot synthesis method for the preparation of the SnS₂@g-C₃N₄ composites. In this route, the different composites are synthesized directly from the precursors of tin (IV) acetate and thiourea via solid-state reaction. The resulting samples exhibit a well-dispersion of SnS₂ nanosheets on the entire porous framework of g-C₃N₄. This g-C₃N₄ supporting matrix not only provides a sufficiently large contact area to accommodate the volume variation but also facilitates the formation of an intimate heterojunction between SnS₂ and g-C₃N₄ phases, which accelerates the transport kinetics of electrons and lithium ions. Accordingly, the electrochemical performance of the composites is remarkably improved. In addition, the outstanding Li-storage performance of the SnS₂@g-C₃N₄ nanohybrids is mainly accounted for the pseudocapacitive contribution. The synthesized SnS₂@g-C₃N₄ composites demonstrate their great potential in large-scale applications for the next-generation anode materials.