Understanding the Limited Electrochemical Zn-Ion Insertion into 2H-MoS₂ and 2H-WS₂: A Case Study of 2H-NbS₂

Abstract

Electrochemical insertion of Zn ions into 2H-NbS2 has been investigated, for the first time, to delineate whether the limited insertion of Zn ions into 2H phases of MoS2 and WS2 is related to the geometrical features of the 2H phase. Electrochemical and ex situ structural and elemental characterization analyses reveal that significant and reversible Zn-ion insertion into 2H-NbS2 can be achieved by reducing the particle size to facilitate Zn-ion diffusion, unlike that in 2H-MoS2 or 2H-WS2. By qualitatively comparing the electronic structures of the 2H phases of NbS2, MoS2, and WS2, it is determined that Zn-ion insertion can occur with metallic phases such as 2H-NbS2 as electrochemical reduction by Zn is thermodynamically favored, while in the semiconducting phases such as 2H-MoS2 and 2H-WS2, Zn-ion insertion would not occur as electrochemical reduction by Zn is thermodynamically unfavored.

Hexagonal Carbon Nanoplates Decorated with Layer-Engineered MoS2: High-Performance Cathode Materials for Zinc-Ion Batteries

“…The electrochemical performance of HCN@MoS 2 materials was compared with those of the recent MoS 2 -based cathode materials for aqueous ZIBs in the literature (Tables S1 and S2). − It has been reported that conventional bulk MoS 2 materials show very poor electrochemical performance as cathode materials for ZIBs, matching with our electrochemical results of C-MoS 2 in this work (Figures f,l,r,x and a). , The MoS 2 or MoS 2 /carbon composites with the 5–15 layered MoS 2 showed the capacities of 139–198 mAh/g (@0.1 A/g) in the literature, − matching with the electrochemical performance of HCN@MoS 2 -3 bearing the 6–7 layered MoS 2 , displaying 167 mAh/g (@0.1 A/g) (Figure d,j,p,v). Recently, the partially oxidized MoS 2 with a 10 layered MoS 2 showed structural defects and the enhanced insertion/desertion of zinc ions with a capacity of 261 mAh/g (@0.1 A/g). , It is also noteworthy that according to the density functional theory (DFT)-based calculation, carbon–MoS 2 interfaces with carbon vacancies have been suggested to be efficient Li storage sites .…”

“…The capacitive contributions of HCN@MoS 2 -3, HCN@MoS 2 -4, and C-MoS 2 were 54.0, 75.7, and 76.2%, respectively (Figure p–r,v–x). As the scan rate increased, the capacitive contribution gradually increased, matching the trend in the literature (Figure s–x). − While the diffusion-controlled redox process was dominant in HCN@MoS 2 -1 and HCN@MoS 2 -2, the capacitive process was dominant in HCN, HCN@MoS 2 -3, HCN@MoS 2 -4, and C-MoS 2 . These observations indicate that the number of MoS 2 layer is critical in the electrochemical process and the 2–3 layered MoS 2 materials in HCN@MoS 2 -1 and HCN@MoS 2 -2 are favorable for the Faradaic ion intercalation process.…”

“…It is noteworthy that the 5–15 layered MoS 2 materials in the literature showed the D Zn values in the range of 10 –9 –10 –12 cm 2 /s (Table S2). − We suggest that the facilitated Zn 2+ diffusion of HCN@MoS 2 -2 is attributable to the 2–3 layered structures of MoS 2 with reduced lattice energy, carbon matrix-induced water compatibility, and enhanced conductivity.…”

“…More detailed electrochemical processes of HCN, HCN@MoS 2 , and C-MoS 2 were investigated by the analysis of scan rate-dependent cyclic voltammograms based on the equation, i p = aν b ( i p : peak currents, ν : scan rate, a and b : variable parameters) (Figure S8). In general, b = 0.5 implies the Faradaic process and b = 1 means the capacitive process. − While b values of HCN were close to 1, those of HCN@MoS 2 -1 and HCN@MoS 2 -2 were analyzed to be 0.58 and 0.54–0.69, respectively, indicating that while the main electrochemical behavior of HCN is the capacitive process, those of HCN@MoS 2 -1–2 are the diffusion-controlled redox process (Figure g–i and Table S2). As the amount of MoS 2 in HCN@MoS 2 -3, HCN@MoS 2 -4, and C-MoS 2 further increased, the b values gradually increased from 0.70 to 0.83 (Figure j–l).…”

“…Two-dimensional (2D) inorganic nanomaterials with a layered structural motif have been investigated as cathode materials for aqueous ZIBs. − For example, recently, MoS 2 with a theoretical capacity of 670 mAh/g has been intensively studied for aqueous ZIBs. − However, reversible insertion/desertion of multivalent zinc ions to/from bulky MoS 2 is nearly impossible. , Main strategies of recent literature studies for enhancing electrochemical performance are the defect utilization of MoS 2 − and the formation of carbon composites − (Tables S1 and S2). For example, the partial oxidation of MoS 2 induced structural defects, resulting in facilitated Zn insertion/desertion and enhanced electrochemical performance for aqueous ZIBs with the discharge capacities of 228–261 mAh/g (@0.1 A/g). , Recently, the graphene or carbon nanotube composites of MoS 2 have been engineered to improve conductivities and applied as cathode materials for ZIBs, showing the discharge capacities up to 180–284 mAh/g (@0.1 A/g). ,, The MoS 2 -based cathode materials for ZIBs in the recent literature studies have the 5–15 layered MoS 2 (Tables S1 and S2).…”