Structure Adjustment And Study On Magnesium Storage Properties Of Two-dimensional Layered Materials Based On MoS₂

Research and development of large-scale electrochemical energy storage devices with good safety performances and high environmentally friendly have become a research hotspot in recent years.Magnesium is one of the anode materials for secondary batteries with high volumetric capacity(3383 mAh cm^-3),and is able to be electrochemically deposited/solvated without the formation of dendrite.The above-mentioned advantages of Mg anodes make magnesium ion batteries（MIBs）considered as one of the high-performance electrochemical energy storage devices.However,development and research of MIBs are hindered by the limited selections of suitable cathode materials.The divalent nature and small ionic radium lead to the high polarity of Mg²⁺,which strongly interacts with host lattices.The strong elecstatic interaction results in the sluggish diffusion kinetics of Mg²⁺in most of host materials.MoS₂ is one of the two-dimensional（2D）van der Waals’materials,and is capable to realize the reversible intercalation/deintercalation of Mg²⁺due to the open ionic diffusion path with abundant Mg-intercalating sites containing in the host frameworks.Moreover,the layered structure of MoS₂ is comprising of MoS₂ monolayers,and the interlayered van der Waals’interaction（vd W interaction）hold the structure on the c axis.The low strength of vd W interaction makes it realizable to incorporate foreign molecules or ions into MoS₂interlayers.The theoretical specific storage capacity of MoS₂ is 223.2 mAh g^-1 when used as MIB cathode.However,the strong electrostatic causes a high Mg-transport activation energy of 1.12 e V,resulting in the nonideal Mg-storage performance of MoS₂.This paper focuses on the structure adjustment of MoS₂,and the foreign species are incorporated into MoS₂ interlayers to construct specific 2D ionic transport paths with facilitated Mg-diffusion kinetics and enhanced durability,improving the Mg-storage performances of cathode materials.Polyvinylpyrrolidone（PVP）is incorporated into MoS₂ interlayers by a hydrothermal method,and the introduced PVP enlarges the spacing between MoS₂ monolayers from6.2 to 9.7（?）.The size of ionic diffusion paths in MoS₂ interlayers is adjusted by the enlarged interlayer spacing,facilitating the Mg²⁺diffusion kinetics.Moreover,PVP plays the role of pillars in MoS₂ monolayers.Chemical bonds between PVP and MoS₂monolayers improve avoids the exfoliation of MoS₂ monolayers during the repeated magnesiation/demagnesiation.The fast intercalation/deintercalation of Mg²⁺enhances the Mg-storage capacity of PVP-MoS₂,and the strong structural stability allows it to maintain most of Mg-storage capacity during the cycling process.At the current density of 20 m A g^-1,PVP-MoS₂ shows a high capacity of 140.3 mAh g^-1,and remains 129.1mAh g^-1 after 100 cycles.The incorporated PVP occupies the space in MoS₂ interlayers,breaking the consistency of Mg transport paths.In order to reduce the space occupancy in MoS₂interlayers,and circumvent the adverse effect on the diffusion of Mg²⁺,H₂O with small size and dipolar nature is incorporated into MoS₂ interlayers to play the role of pillars.The introduction of H₂O is realized by an electrochemical assistant method,and density functional theory（DFT）calculations are employed to investigated the introduce mechanism of interlayer H₂O.XPS analysis results demonstrate the formation of hydrous MoS₂（H-MoS₂）.Charge shielding effect of interlayer H₂O delivers a threefold increase of Mg-diffusion rate in H-MoS₂ lattices,ensuring the high rate and cycling performances of H-MoS₂.At 20 m A g^-1,H-MoS₂ shows an enhanced Mg-storage capacity of 190.3 mAh g^-1,and 173.5 mAh g^-1 remains after 300 cycles.When the rate current increases to 500m A g^-1,the corresponding capacity is 78.0 mAh g^-1,and 69.8 mAh g^-1 remains after 500cycles.In order to establish 2D Mg-diffusion paths without interlayered pillars,MoS₂monolayers and graphene（GR）monolayers are alternatively stacked with each other to construct MoS₂/graphene van der Waals heterostructure（vd WH）by an electrostatic self-assembly-assistant chemical synthesize method.DFT calculations are employed to simulate the transporting process of Mg²⁺on MoS₂/GR heterointerfaces with a reduced energy barrier of 0.4 e V.The lower energy barrier of MoS₂/GR vd WH allows the diffusion coefficient 11 orders of magnitude larger than that of pristine MoS₂.The facilitated Mg diffusion kinetics in MoS₂/GR vd WH ensures excellent rate and cycling performances of cathode materials.At 20 m A g^-1,MoS₂/GR vd WH exhibits a high Mg-sorage capacity of210.3 mAh g^-1,and 189.0 mAh g^-1 remains after 300 cycles.When the current rate increases to 500 m A g^-1,the corresponding capacity of MoS₂/GR vd WH is 90.8 mAh g^-1,and 75.4 mAh g^-1 remains after 500 cycles.