Research On Electrochemical Lithium Storage Performances Of Magnesium Hydride And Molybdenum Carbide Thin Films As Conversion Anode Materials

Various conversion materials?Metal oxides,sulfides,nitrides,hydrides,carbides,etc.?are attractive candidates for LIB anodes,owing to much higher specific capacities than commercial graphite anode.However,these conversion reaction anodes generally suffer large irreversible capacity loss,poor rate capability and cycling performances.Design nanostructure and composite structure design is effective strategy to improve their electrochemical performances.In addition,the preparation of film electrode is an effective method to investigate the electrochemical performances and reaction mechanism of novel anode materials because of convenient composition and microstructure control.In this thesis,magnesium hydride and molybdenum carbide thin films were prepared by magnetron sputtering,the structural and electrochemical properties of films were characterized by XRD,SEM,XPS,CV and galvanostatic discharge/charge tests.The main conclusions are drawn as follows:?1?The Mg-C film was deposited by sputtering the Mg-C composite target,the resultant Mg-C film consists of Mg nanoparticles?¹60 nm?with the amorphous carbon coating.With the further hydrogenation treatment,MgH₂-C film was obtained.The MgH₂-C film delivers the initial discharge capacity of 126.5 ?Ah cm-2?¹429 mAh g-1?,of 70.8 ?Ah cm-2?⁸00 mAh g-1?is related to the conversion reaction,with the inefficiency of 52.5%.The GITT measurement determined the equilibrium potential of the conversion reaction of MgH₂ to be 0.545 V vs.Li+/Li,and the voltage hysteresis between discharge and charge is only 10 mV vs.Li+/Li.The MgH₂-C film electrode displays lower polarization effect in comparison to pure Mg film,which is attributed to the enhanced conductivity by the carbon addition.Instead of structural damage of film electrode during cycling,the passivated surface of LiH and Mg should be responsible for the poor cycleability of the cell.?2?The MoC_0.72 film was prepared by magnetron sputtering from the Mo₂ C target,and consists of ?-MoC1-x phase?fcc?and amorphous carbon.The MoC_0.72 film electrode shows good cycling performance and rate capability,which is based on conversion reaction mechanism(MoC_0.72+x Li++xe-? Mo+LixC).With the sputtering power of 100 W and the sputtering time of 3 h,the obtained MoC_0.72 film electrode still delivers a discharge capacity of 20.6 ?A cm-2?²11.6 mAh g-1?after 500 cycles,with the capacity retention of 49.1%.The excellent cyclic performance is attributed to,on the one hand,the small molybdenum carbide particles that would shorten the diffusion path for Li-ions and facilitate the electrochemical reaction kinetics,on the other hand,to the presence of carbon,that effectively alleviate the volume change and help to sustain the electrode structure.?3?The two-layered films MgH₂-C/MoC_0.72 and MgH₂/SnO₂ were prepared by introducing MoC_0.72 and SnO₂ protective layers on the surface of MgH₂ film.Only the SnO₂ protective layer could effectively improve the cycling performance of MgH₂ film.Comparing with the pure MgH₂ film electrode,the MgH₂/SnO₂-4 min film electrode has a discharge capacity of 7.8 ?Ah cm-2 after 20 cycles,much higher than the pure MgH₂ film.It is possibly related to the SnO₂ layer with electrochemical activity could prevent the direct contact between MgH₂ and the electrolyte,which would improve the surface passivation of LiH and Mg and be beneficial to the reversible formation of MgH₂.This result indicates that the surface modicaition could greatly improve the cyclic performance of hydride anode materials.