Metallic phase W_0.9Mo_0.1S₂ for high-performance anode of sodium ion batteries through suppressing the dissolution of polysulfides

Abstract

WS₂ with layered graphite-like structure as anode for sodium ion batteries has high specific capacity. However, the poor cycling performance and rate capability of WS₂ caused by the low electronic conductivity and structure changes during cycles inhibit its practical application. Herein, metallic phase (1T) W_xMo_1−xS₂ (x = 1, 0.9, 0.8 and 0.6) with high electronic conductivity and expanded interlayer spacing of 0.95 nm was directly prepared via a simple hydrothermal method. Specially, 1T W_0.9Mo_0.1S₂ as anode for sodium ion batteries displays high capacities of 411 mAh g⁻¹ at 0.1 A g⁻¹ after 180 cycles and 262 mAh g⁻¹ at 1 A g⁻¹ after 280 cycles and excellent rate capability (245 mAh g⁻¹ at 5 A g⁻¹). The full cell based on Na₃V₂(PO₄)₂O₂F/C cathode and 1T W_0.9Mo_0.1S₂ anode also exhibits high capacity and good cycling performance. The irreversible electrochemical reaction of 1T W_0.9Mo_0.1S₂ with Na ions during first few cycles results in the main products of W-Mo alloy and S. The strong adsorption of W-Mo alloy with polysulfides can effectively suppress the dissolution and shuttle effect of polysulfides, which ensures the excellent cycling performance of 1T W_0.9Mo_0.1S₂.

Introduction

Recently, sodium ion batteries (SIBs) receive increasing attention due to their abundant natural resources for sodium and related salt, which in turn gives rise to the low cost and high potential for mass production [1], [2], [3], [4], [5]. However, SIBs still face great challenges in practical application in terms of the insufficient cycling stability and low rate capability [6], [7], [8], [9]. Layered transition-metal dichalcogenides (TMDs), such as MoS₂ [10], [11], [12] and WS₂ [13], [14], [15], have been reported as promising anode candidates for SIBs owing to their relatively high theoretical specific capacity. Especially, in the case of WS₂, the large interlayer spacing of 0.62 nm and weak van der Waals interaction enable easier Na⁺ intercalation and smaller volume expansion during the sodiation process in contrast to the alloying-dealloying anode materials [16], [17], [18]. However, the low electronic/ionic conductivity, inevitable reaggregation and structural changes lead to the fast capacity fading and poor rate capability, which limit the practical application of WS₂ as anode for SIBs. One effective way is enlarging the interlayer spacing of WS₂ to decrease ions diffusion resistance and restrain re-stacking [19], [20], [21]. Another effective method is preparing metallic phase WS₂ in order to improve electronic conductivity [22], [23].

In addition, the charge-storage mechanism of WS₂ during Na ions insertion/extraction is also unclear and controversial [24], [25], [26]. Owing to the similar structure of WS₂ with MoS₂, the charge-storage mechanism of MoS₂ can be referenced by WS₂ anode. However, two different discharge and charge mechanisms about MoS₂ during the Li⁺/Na⁺ insertion/extraction process have been proposed. Some researches prove that the electrochemical reaction of MoS₂ with Li⁺/Na⁺ is reversible. Li₂S/Na₂S and Mo can be reversibly converted into MoS₂ when Li⁺/Na⁺ extract [27], [28], [29], [30]. Some studies observe the different phenomena and draw an opposite conclusion. LiS₂/NaS₂ are oxidized to S during Li⁺/Na⁺ extraction process, and the conversion reaction is irreversible [31], [32], [33]. The charge-storage mechanism of WS₂ in SIBs has not been clarified. Fundamental understanding the charge-storage mechanism of WS₂ can accelerate the practical application of WS₂-based electrodes. Therefore, it is very important and urgent to real the charge-storage mechanism of WS₂ as anode in SIBs.

Herein, the conversion mechanism of 1T W_xMo_1−xS₂ as anode for SIBs has been revealed for the first time. The enhanced electrochemical performance of 1T W_xMo_1−xS₂ as anode for SIBs has also been invetigated in detail. Firstly, 1T W_xMo_1−xS₂ (x = 1, 0.9, 0.8 and 0.6) nanoplates with expanded layer spacing of 0.95 nm have been prepared through a simple hydrothermal process. Expanded-interlayer 1T W_xMo_1−xS₂ nanoplates provide fast electrons transport and ions diffusion paths. Specially, 1T W_0.9Mo_0.1S₂ exhibits superior long-term cycling performance and rate capability as anode for SIBs. Besides, the discharge and charge mechanism of 1T W_0.9Mo_0.1S₂ in SIBs has been investigated by ex-situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). These results indicate that 1T W_0.9Mo_0.1S₂ during the initial discharge and charge process irreversibly converts into W-Mo alloy and S. The enlarged interlayer spacing of 1T W_0.9Mo_0.1S₂ can decrease Na ions diffusion resistance during the initial cycle. During the following cycles, the electrochemical reaction of S with Na ions (S + 2Na⁺ + 2e⁻↔Na₂S) is main reaction. The W-Mo alloy acts as the electronically conducting phase on the interfaces and strongly absorb polysulfides, which can effectively suppress the dissolution and shuttle effect of polysulfides and avoid capacity loss caused by dissolution of polysulfides. Density functional theory (DFT) calculation further confirms the stronger adsorbability of W-Mo alloy with polysulfides (Na₂S_x, x = 1, 2, 3, 4, 5 and 6) than metal W. This result can provide a reference to prepare high-performance metal sulfides anode in SIBs.