Defect Induced Performance Enhancement of Monolayer MoS₂ for Li- and Na-Ion Batteries

Abstract

Dexterity in the application of defect engineering implicates modification in the physical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) to enhance their effectiveness toward nanoelectronics applications. Subsequently, the existence of various types of defects in monolayer MoS2 has been employed to inculcate and implement their significance in enhancing the Li/Na-ion storage capability of MoS2 monolayers as anodes of lithium ion batteries (LIBs) and sodium ion batteries (SIBs). DFT calculations have guided us to traverse the effect of various point and antisite defects on Li/Na adsorption energy and the diffusion barrier of monolayer MoS2. Before looking into Li/Na adsorption properties of defective MoS2, the structural stability of various defects is explored with relevance to their formation energy. This also germinates the quest for the most stable defective structures that could be a reliable anode material for LIBs and SIBs. Enhanced adsorption is found for both Li/Na ions in the case of defective MoS2 over that corresponding to pristine MoS2. To study the level of interaction between Li/Na and defective MoS2, electronic structure analysis has been performed. To evaluate the possible migration pathways and rate of migration of Li/Na over defective MoS2, we calculated the diffusion barrier energy through the CI-NEB method. Our study demonstrates that the formation of vacancy improves the diffusion performance of both lithium and sodium at the defective region which are prerequisites for LIBs and SIBs. Additionally, we demonstrated that the formation of vacancy could improve the specific capacity of monolayer MoS2 due to a decrease of the molecular mass of defective MoS2 in comparison to pristine MoS2. However, the OCV is not affected much due to enhanced adsorption. Hence, designing MoS2 nanostructures with defects is a useful strategy to achieve an effective anode material for obtaining high capacity LIBs and SIBs by precisely tailoring its properties for desired applications, such as enhancing the adsorption energy, modulating the reaction pathway, and raising the specific capacity.

A Review on the Current Progress and Challenges of 2D Layered Transition Metal Dichalcogenides as Li/Na‐ion Battery Anodes

“…In some cases, higher stability is more desirable over high capacity which can be achieved by limiting the potential window. The details will be explained during the discussion of different reports where researchers have explored a variety of approaches to enrich the electrochemical performance of MoS 2 , such as pristine MoS 2 with different physical properties, [74d,e,77] composite with different carbon structure, [74a,b,78] graphene, [79] carbon nanotubes, [74c,80] other nanostructures, [81] etc .…”

“…Amorphous nature and large interlayer spacing facilitate the faster Li‐ion diffusion which enhanced the electrochemical performance of the material as LIB anode with an exceptionally high rate (reversible capacities at 1 C and 50 C are 917 mA h g −1 and 700 mA h g −1 , respectively). Not only the interlayer spacing, several other factors also play pivotal roles in the enhancement of the electrochemical performance of MoS 2 , such as the influence of defects, the effect of pseudocapacity, morphology with high surface reaction sites, etc [74d,e,77b] . In one of their reports, Barik et al .…”

Engineering 2D Materials: A Viable Pathway for Improved Electrochemical Energy Storage

“…[65] Although other geometric factors were roughly considered to be responsible for the high power performance (capacitance of 166 F g −1 at 5.7 A g −1 , energy density of ≈70 Wh kg −1 in organic electrolyte), the contribution from the edges at the wrinkled/crumpled regions should be more dominant based on the understanding from the recent density-functional theory (DFT) calculations. [30,[57][58][59] The contribution could include the increased binding energy of the electrolyte ions around the crumpled and rough surface with strains, originated from the newly formed in-gap electronic states at the Fermi level as described above (the "d" and "A" in Equation 1, and charge transfer number are regulated to improve the capacitance). [66] A similar porous and wrinkled graphene film (Figure 1f) with pentagon-heptagon fine structures ( Figure 1g) was also tested as the electrode material for a microsupercapacitor.…”

“…Other DFT investigations also verify the improved adsorption binding with both Li + and Na + along with enhanced capacitance ( Figure 2g,h) upon the formation of various S vacancies, while the ion diffusion barriers are lowered to offer better rate capabilities. [59,95,96] This result is expectable because the newly formed in-gap energy state (form new dangling bonds), localizing around the vacancies, downshifts the conduction band (CB) to the Fermi level and improves the binding force to the Li + /Na + . Similar promotions were also found from other 2D materials such as silicene, and blue and black P. [97][98][99] Nevertheless, high-performance supercapacitors using such ultrathin 2D materials have been rarely demonstrated, due to probably the limited fabrication techniques and other engineering issues.…”

“…Apart from the SASA, the improved capacity at a corrugation position can be understood based on the improved binding energy with electrolyte ions, which shortens the distance (the “ d ” in Equation (1)) between the electrode surface and the ions, and increases the charge transfer number from the ions. [ 58,59 ] The binding energy with electrolyte ions can be enhanced by the newly formed dangling bonds associated with the defects (or the improved DOS at the Fermi level) in 2D materials. Besides, the improved DOS could improve the quantum capacitance (the C q in Equation (3)).…”