Preparation And Characterization Of MoS₂ And MoO₃ Based Composite And Their Electrochemical Properties

In recent years,the fast development of electric car and portable electronic products has greatly stimulated the demand for high-performance lithium ion batteries?LIBs?.Lots of efforts have been put into designing and looking for electrode materials with excellent electrochemical performance for LIBs.Transition metal sulfides?TMS?have attracted widely attention as electrode materials based on the lithium ion storing mechanism of conversion or alloying reaction.Compared with traditional carbonaceous anode materials,TMS show greater advantages to realize higher specific capacity.Among them,MoS₂,as a two dimensional and layered TMS,has particularly gained much interest in that it could accommodate lithium ions at a lower potential and exhibit high lithium ion capacity through conversion reaction.However,the cyclic stability and rate capability of the MoS₂ electrodes are still unsatisfactory due to the intrinsically poor electrical conductivity and the structural instability of MoS₂.As the same kind of transition metal compound,MoO₃ has many similarities with MoS₂,for example,high theoretical capacity and poor cycle stability.In order to combat these drawbacks,researchers found that constructing exceptional hybrid structure of MoS₂ and MoO₃ with conductive materials was an effective way to promote the conductivity and stability of the electrode materials.Therefore,in this paper,MoS₂ and MoO₃-based electrodes with different morphologies and structure were constructed to promote their electrochemical properties.The morphology and structure of the products are characterized by a series of measurements,such as XRD,XPS,Raman and SEM.Meanwhile,the LIBs were assembled using the samples as active materials,and then their electrochemical properties were measured.The mainly results are as follows:?1?The MoS₂-rGO composite was prepared by one-step hydrothermal method using Na2 MoO4·2H2O as the Mo source and GO as the matrix.In the obtained MoS₂-rGO composite,the MoS₂ showed flower sphere-like structure,with the diameter of about 400 nm,and they were evenly distributed on the graphene skeleton,while graphene nanosheets intertwined together to form 3D conductive network.The as-prepared MoS₂-rGO composite exhibited good electrochemical performances due to the special hybrid structure and the introduction of conductive graphene.When charging-discharging at a current of 100 mA/g,the specific capacity could be retained at about 900 mAh/g after 100 cycles.?2?The MoO₃@MoS₂ composite was synthesized by hydrothermal method using MoO₃ nanorods as the precursors and thiourea as the sulfur source.During the reaction,an ion exchange process was happened between MoO₃ and S₂-.A series of MoO₃@MoS₂ composites were prepared by adjusting the amount of thiourea.When the amount of NH2CSNH2 was 3 mM,the as-prepared MoO₃@MoS₂-II showed 1D hierarchical core-shell structure.In this structure,the shell was composed of numerous 2D ultrathin MoS₂ nanosheets and the core was the 1D MoO₃ nanorods.The MoS₂ nanosheets grew perpendicularly on the surface of MoO₃ nanorods and whether the 1D structure could be retained or not depending on the solvent during the reaction.As the electrochemical measurements shown,MoO₃@MoS₂-II exhibited excellent electrochemical performances.The average discharging capacities of the MoO₃@MoS₂-II at the different current densities of 100,300,500 and 1000 mA/g are 929,642,510,384 mAh/g,respectively.More importantly,MoO₃@MoS₂-II can retaine a capacity of 868mAh/g when current density reversed to the initial 100 mA/g.This indicated that MoO₃@MoS₂-II showed better rate performance.MoO₃@MoS₂-II showed a discharge capacity of 781 mAh/g after 100 cycles when charging-discharging at a current of 100 mA/g,indicating a better cycling performance.?3?MoO₃@GO composite was obtained after the MoO₃ nanorods were wrapped by graphene oxide under magnetic stirring.The electrochemical measurements showed that the lithium storage capacity of MoO₃@GO was obviously enhanced than that of MoO₃ nanorods.The first discharge capacity of MoO₃@GO reached about 1350 mAh/g,and the discharge capacities retained 720 mAh/g after 60 cycles.From the EIS measurements,it can be seen that the conductivity was also promoted by the introduction of graphene oxide.Subsequently,the sulfuration of MoO₃@GO was investigated.The test results so far indicated that the products of sulfuration?S-MoO₃@GO?could be the sulfur introduced MoO₃@GO composite.The oxygen vacancies may be introduced after the introduction of sulfur,leading to a larger interlayer spacing of MoO₃ crystal lattice.As a result,the faster charge storage kinetics was promoted and the MoO₃ structure intended to be more stable during the intercalation/extraction process of Li+.From the charging-discharging results,S-MoO₃@GO showed very stable capacity during the cycles except the first cycle,and exhibited excellent rate performance.