Preparation Of Molybdenum Disulfide Based Anode Materials For Their Lithium And Sodium Storage Properties

Lithium/sodium ion batteries（LIBs/SIBs）are one of the most potential electrochemical energy storage technologies,and its electrochemical performance mainly depends on electrode materials.Graphite as a commercial anode material for LIBs,its specific capacity has been optimized to close to the theoretical value,and the space for further improvement is very limited.Moreover,graphite is not as good at storing sodium ions with larger radii.Facing the increasing market demand,it is urgent to explore novel anode materials with excellent performance.Among all kinds of anode materials,MoS₂,as a classic transition metal sulfide,has attracted wide attention due to its unique two-dimensional layered structure,high theoretical specific capacity,large interlayer distance,and abundant mineral resources.However,due to its low intrinsic conductivity and large volume expansion during charge and discharge,MoS₂tends to exhibit poor rate performance and cycle stability.Moreover,MoS₂materials tend to exhibit different electrochemical properties in different kinds of electrolyte and the reasons for this are still unclear.These problems seriously hinder its practical application in LIBs and SIBs.The aim of this paper is to improve the cyclic stability and rate performance of MoS₂materials and to explore its action mechanism through nanostructure design,component optimization and compounding with high conductivity carbon materials.The Mo/PDA precursor with uniform distribution of molybdate and polydopamine（PDA）was obtained by the complexation between molybdate and the functional group of campholol in dopamine hydrochloride（DA）and the self-polymerization of DA under weak alkaline environment.The Mo source and PDA in Mo/PDA precursor were converted into MoS₂and nitrogen-doped carbon（NC）,respectively,by high temperature sulfidation reaction to obtain the flower spherical MoS₂/NC.The prepared MoS₂has a structure characteristic of few layers,and the number of layers is mainly concentrated in 3-5 layers,which not only improves the utilization rate of MoS₂but also effectively avoids the huge volume change in the process of charging and discharging.NC can not only improve the conductivity of MoS₂but also enhance the structural stability.Furthermore,the porous flower spherical structure can provide abundant active edges,which is conducive to the storage and rapid transport of lithium ions.Electrochemical test results show that the as-prepared flower spherical MoS₂/NC has good rate performance and cycle stability.At the current density of 5 and 10 A g^-1,the specific capacities can reach 335 and 208.7 m Ah g^-1,respectively.After 1000 cycles at 2 A g^-1,the specific capacity is 391 m Ah g^-1,and the corresponding capacity retention rate is 85.7%.Uniform interwoven structural MoS₂/NC were prepared by one-step solvothermal method combined with high temperature calcination using Mo O₂（acac）₂,thioacetamide and polyvinylpyrrolidone as raw materials.The combination of MoS₂and NC with high conductivity and high structural stability to form uniform interwoven structure can effectively improve the conductivity and structural stability of MoS₂,so as to improve its rate performance and cyclic stability.The sodium storage performance of MoS₂/NC in both ether-based and ester-based electrolytes is investigated.It is found that the specific capacity of MoS₂/NC in the ester-based electrolyte declined slowly,while in the ether-based electrolyte,it rose first and then stabilized.This phenomenon can be attributed to the gradual transition of phase composition of MoS₂during charge and discharge reaction in ether-based electrolyte,resulting in amorphous MoS₃and metallic Mo.At the same time,the transformation reaction results in the reconstruction of the morphology and structure of the electrode materials,and the particle size is reduced from micron to nanometer.The uniform interwoven structural MoS₂/NC shows good cyclic stability in the ether-based electrolyte,with the specific capacity remaining at 286 m Ah g^-1after 5000 cycles at 5 A g^-1,and the capacity decay rate of each cycle is only 0.0034%.To further improve the specific capacity of MoS₂/NC,the MoS₂@NC with core-shell structure was successfully prepared by solvothermal-PDA coating-pyrolysis method.The spherical MoS₂core provides high specific capacity,and the NC shell improves structural stability and electrical conductivity.The core-shell MoS₂@NC used as an anode material for SIBs shows high specific capacity,fine rate performance and long cycle lifespan.After 300activation cycles at 1 A g^-1,the specific capacity becomes stable(570 m Ah g^-1).At the super rate of 30 A g^-1,the specific capacity is still up to 467 m Ah g^-1.A highly reversible specific capacity of 428 m Ah g^-1can be obtained after 10,000 cycles at a high current density of 20 A g^-1.During the 500th to 10,000th cycles,the capacity retention rate reaches 86.5%,and the average capacity decay per cycle is only 0.00142%.The MoS₂@NC‖Na₃V₂（PO₄）₃full cell assembled from as-prepared MoS₂@NC and commercial Na₃V₂（PO₄）₃exhibits fine cycle stability with a specific capacity of 61 m Ah g^-1and a capacity retention rate of 91.4%after250 cycles at a current density of 0.4 A g^-1.In addition,DFT theoretical calculation and COMSOL simulation are used to explain the significant role of NC coating.To further improve the conductivity and structural stability of MoS₂/NC,Mo O₄^2-was anchored on Ti₃C₂T_xconductive substrate using polydopamine as a linking agent,and the flower-like Ti₃C₂T_x@Mo/PDA precursor was obtained,which is assembled by the primary building unit with layer upon layer stacking structure.After one step sulfidation,the flower-like Ti₃C₂T_x@MoS₂/NC were obtained.Two-dimensional Ti₃C₂T_xnanosheet plays the role of skeleton support and improving electrical conductivity,and the formation of ultra-small MoS₂nanoparticle gives the electrode material larger electrode/electrolyte contact area and more sodium storage sites,which improves the reaction rate of the electrode material.Ti₃C₂T_x@MoS₂/NC combines the high capacity of MoS₂with the high conductivity of Ti₃C₂T_x,demonstrating fine sodium storage properties.After 2000 cycles at a current density of 2 A g^-1,the specific capacity can reach 351 m Ah g^-1.During the 20th to 2000 cycles,the capacity retention rate is 81.4%,and the average capacity decay rate of each cycle is only0.0103%.Moreover,flower-like Ti₃C₂T_x@MoS₂/NC can still obtain a reversible specific capacity of 383 m Ah g^-1at a large current density of 30 A g^-1.