Study On Synthesis Of MoSe₂-based Sodium Ion Battery Anode Materials And Their Electrochemical Performance

MoSe2 is a two-dimensional transition metal chalcogenide with a small band gap(1.1 eV)and large interlayer distance(0.68 nm).It is a promising anode material for sodium ion batteries due to the conversion reaction,which can generate a high theoretical capacity during the sodium storage process.However,the current research results show that MoSe2 still has some deficiencies as electrode material.On the one hand,MoSe2 has a two-dimensional layered structure,which makes it have high surface energy and weak interlayer van der Waals force.Serious volume expansion will occur in the process of charging and discharging when large number of sodium ions are intercalated and deintercalated.This leads to the destruction of layered structure while bringing high capacity.In addition,the conversion reaction will introduce soluble intermediate products such as polyselenide during the sodium storage process,which will also react with the electrolyte,resulting in partial irreversible capacity.Therefore,the cycle stability of the MoSe2 electrode is still poor.On the other hand,MoSe2 is a semiconductor material with low intrinsic conductivity and have a poor rate performance.Therefore,in the field of energy storage the application of MoSe2 will need to be further improved.In this paper,the effects of different process conditions on the structure and electrochemical properties were investigated by using solvothermal method to combine MoSe2 with carbon fiber cloth(CFC).(1)MoSe2/CFC self-supporting composites were prepared by a solvothermal method.Effects of the solvothermal time,the ratio of molybdenum source to selenium source and source of molybdenum concentration on MoSe2/CFC self-supporting composite structure and electrochemical performance were investigated in the paper.When the solvothermal time is 12 h,the ratio of molybdenum source to selenium source is 1:2.0 and the concentration of molybdenum source is 15 mmol L-1,MoSe2 grows uniformly on the carbon fiber cloth,and the sample has the best electrochemical performance.In the first cycle MoSe2/CFC shows a specific capacity of 808.4 mAh g-1,the first coulomb efficiency is 73.2%.After 300 cycles,it also can maintain a stable specific capacity of 306.1 mAh g-1.(2)Besides,a three-dimensional porous structure of MoSe2/CFC self-supporting composite was synthesized.The structural characterization and electrochemical performance analysis were performed.The results show that the three-dimensional porous structure of MoSe2/CFC self-supporting composite electrode has excellent cycle stability and rate performance.At current density of 0.2,0.3,0.5,1.0,2.0,3.0,5.0 and 10.0 A g-1,the specific capacity is 775.9,657.5,591.8,472.9,374.3,248.4,147.2 and 97.2 mAh g-1,respectively.After 550 cycles,specific capacity remains at 343.2 mAh g-1.In addition,different MoSe2 morphologies were prepared by controlling the heat treatment temperature.The influence of different MoSe2 morphologies on carbon fiber cloth to the electrochemical properties was studied.It's found that the tilted-orientation MoSe2/CFC has two-dimensional nanosheet structure and the strong interface contact between MoSe2 and CFC,which prevents the self-aggregation of active materials and maintains good contact between MoSe2 nanosheet and CFC.The tilted-orientation MoSe2/CFC electrode material exhibits the best cycle stability and rate performance.It has a capacity of 253 mAh g-1 after 200 cycles at a current density of 1 A g-1 and a capacity of 140.5 mAh g-1 at a density of 5.0 A g-1.(3)Moreover,in order to reasearch the effect of the coating carbon method on the structure and performance of sample,a coating-carbon MoSe2/CFC structure self-supporting composite was prepared by a solvothermal method,which using selenium powder as the selenium source,sodium molybdate as the molybdenum source and glucose as the carbon source.The coating-carbon MoSe2/CFC has good cyclic stability.At 1 A g-1 current density,after 700 cycles it still can maintain a high specific capacity of 355 mAh g-1.The retention rate is 73.4%compared with the discharge capacity of the second cycle.Compared to the supporting-carbon MoSe2/CFC structure,coating-carbon MoSe2/CFC can effectively avoid MoSe2 self-agglomeration and improve electrical conductivity.Covered coating-carbon can prevent the dissolution of the intermediate product of polyselenide,alleviate volume expansion and further enhance conductivity.