Boosting reaction kinetics and reversibility in Mott-Schottky VS₂/MoS₂ heterojunctions for enhanced lithium storage

Abstract

Heterostructures have lately been recognized as a viable implement to achieve high-energy Li-ion batteries (LIBs) because the as-formed built-in electric field can greatly accelerate the charge transfer kinetics. Herein, we have constructed the Mott-Schottky heterostructured VS₂/MoS₂ hybrids with tailorable 1T/2H phase based on their matchable formation energy, which are made of metallic and few-layered VS₂ vertically grown on MoS₂ surface. The density functional theory (DFT) calculations unveil that such heterojunctions drive the rearrangement of energy band with a facilitated reaction kinetics and enhance the Li adsorption energy more than twice compared to the MoS₂ surface. Furthermore, the VS₂ catalytically expedites the Li–S bond fracture and meantime the enriched Mo⁶⁺ enables the sulfur anchoring toward the oriented reaction with Li⁺ to form Li₂S, synergistically enhancing the reversibility of electrochemical redox. Consequently, the as-obtained VS₂/MoS₂ hybrids deliver a very large specific capacity of 1273 mAh g⁻¹ at 0.1 A g⁻¹ with 61% retention even at 5 A g⁻¹. It can also stabilize 100 cycles at 0.5 A g⁻¹ and 500 cycles at 1 A g⁻¹. The findings provide in-depth insights into engineering heterojunctions towards the enhancement of reaction kinetics and reversibility for LIBs.

Introduction

Developing higher energy density lithium-ion batteries (LIBs) has been identified as the key determinant to achieve the prolonged driving range for electric vehicles [1], [2], [3], [4]. According to the ambitious targets of multiple national governments, the energy density of LIBs is expected to reach 400 Wh kg⁻¹ in 2025 [5]. Nevertheless, the current capacity of intercalation-type graphite anode is close to the theoretical value (372 mAh g⁻¹), without room for improvement [6], [7], [8]. Two-dimensional (2D) metal sulfides have been extensively investigated as a kind of promising candidate to date because of their high theoretical capacity and relatively higher lithium insertion potential [9], [10], [11]. In particular, the typical MoS₂ usually exhibits nearly twice the theoretical capacity (about 1200 mAh g⁻¹) in virtue of its unique 2D interlayer and rich edges [12], [13], [14]. However, the thermodynamically stable 2H-MoS₂ suffers from low conductivity with band gap of 1.3–1.9 eV while the metallic 1T-MoS₂ is very unstable [15], besides the nanosheets restacking and polysulfides dissolution, which results in rapid capacity attenuation and unsatisfied rate performance.

Building heterogeneous nanostructures enables the Fermi energy rearrangement of the adjacent semiconductors for achieving tailorable energy bands, which will trigger some unusual physicochemical properties [16], [17], [18]. For example, Huang and co-workers [19] have designed and synthesized SnS₂/CoS₂ p-n heterojunction, in which the induced built-in electric field greatly enhances charge transfer capability at the interface with accelerated electrochemical reaction kinetics. However, the intrinsic semiconducting feature is still unsolved and the ion diffusion barrier in bulk is also not obviously reduced. Fortunately, it has been found that the Mott-Schottky heterojunctions of metal and semiconductor can generate an oriented and stronger built-in electric field, and meantime significantly boost the transfer kinetics of electrons and ions. On the other hand, it is reported that the V⁴⁺ in metallic VS₂ possesses a smaller radius and a higher polarity compared with Mo⁴⁺ in MoS₂, which is more conductive to anchoring the polysulfides in Li-sulfur batteries with favorable reversibility [20], [21], [22], [23], [24]. Nevertheless, only micro-/submicro-sized VS₂ aggregations have been reported in the literature mainly due to their thermodynamic and kinetic instability at small scale [25], [26], [27]. Even with the sophisticated techniques and the abundant noxious organic solvents, it is still difficult to achieve few-layered VS₂ nanostructures [28], [29]. Therefore, it is very meaningful to construct Mott-Schottky VS₂/MoS₂ heterojunctions microscopically to significantly improve the electrochemical performance.

Based on their matchable formation energy [30], [31], we have successfully realized the metallic VS₂ nanosheets vertically grown on the MoS₂ surface with abundant Mott-Schottky heterojunctions. Through revealing the evolution process of heterostructure, it is found that the vanadate promotes the morphology evolution of the products from irregular bulks to ultrathin hierarchical nanostructures with tailorable 1T/2H ratio. The density functional theory (DFT) calculations indicate the VS₂/MoS₂ hybrids enable the rearrangement of energy band towards the fermi level, hence realizing the self-improvement of electrical conductivity. The build-in electrical field greatly enhances the Li adsorption energy at the heterointerface (–3.60 eV), much stronger than the MoS₂ surface (–1.57 eV). We also found the Li–S bond fracture catalyzed by VS₂ and the sulfur anchored by high Mo⁶⁺/Mo⁴⁺ ratio can synergistically improve the redox reversibility according to the charge/discharge mechanism. Therefore, a high specific capacity of 1273 mAh g⁻¹ can be obtained at 0.1 A g⁻¹ with 61% capacity retention even at 5 A g⁻¹. It can also stabilize 100 cycles at 0.2 A g⁻¹ and 500 cycles at 1 A g⁻¹.