The Preparation And Electrochemical Performance Of Mg-Based Conversion Reaction Anodes For LIBs

Magnesium-based materials have the advantages of abundance,low-cost and environmental friendliness,they have great potentials in the fields of hydrogen storage,thermal storage,solid-state electrolyte for magnesium batteries and anode materials for lithium-ion battery?LIBs?.Compared with traditional graphite anode,Mg-based anode materials such as MgS,MgH₂ show the higher theoretical specific capacities through conversion reaction and thus have been widely investigated as promising anode materials.However,the low electronic conductivity,large volume variation of conversion reaction and the environmental sensitivity for Mg-based anode materials cause the problems of poor reversibility,poor rate performance,and poor cycling performance.In this thesis,MgS,MgH_2,and Mg₂FeH₆ and their composite materials were investigated as LIBs anode,and their electrochemical performances were enhanced by designing nanostructure and composite structure,as well as surface modification.The structure and electrochemical performance of these composites were investigated by XRD,SEM,TEM,XPS,CV,EIS and galvanostatic discharge/charge tests.The main conclusions are drawn as follows:?1?The reversible lithium-storage mechanism and performance of MgS composite were investigated.MgS was prepared by plasma-assisted ball-milling from the starting Mg and S powders.In the discharge process,MgS could react with Li⁺and reversibly form the Mg and Li₂S?MgS+2Li⁺+2e^-?Li₂S+Mg?.Besides,the effect of the plasma-assistant milling,electrode preparing method and Mn addition on the electrochemical performances were also studied.It is found that the plasma can promote the reaction between Mg and S to form MgS completely within milling time of 10h.Compared with the cold-pressing method for electrode preparation,the coating method ensured better contact between active material and the current collector,and thus resulting in higher capacity.The composite with the addition of Mn exhibited better electrochemical cycling performance at a current density of 500 m A g^-1,the discharge capacity after 100 cycles increased from 149.0 mAh g^-1 to 262.1 mAh g^-1 as the content of Mn increased from 0%to 25%.?2?The lithium-storage performances of MgH₂ composite were investigated.MgH₂ were composited with expanded graphite?EG?by plasma-assisted ball-milling.The as-prepared MgH₂-EG composite showed rather high initial discharge capacity of 1780.2 mAh g^-1,but the capacity retention was only 19.5%at 100^th cycle.By adding TiO₂ into the composite,the volume expansion can be accommodated and the the capacity of MgH₂-TiO₂-EG was increased up to 305.4 mAh g^-1 with the highest capacity retention of 52.8%.However,the obvious capacity fading still exist for the milled MgH₂ composite.?3?Using the precursor Mg₂FeH₆ to prepare the composite MgH₂-Fe-G and electrode coating of TiO₂,and the lithium-storage performance of MgH₂ could be greatly improved.Firstly,the single-phased Mg₂FeH₆ was synthesized by a combination of planetary ball-milling and heat treatment under hydrogen pressure by using Mg and Fe as raw materials.In the initial discharge process,the Mg₂Fe H₆ decomposed into nanoscale Mg and Fe,and only MgH₂ could be reversibly formed.The as-prepared Mg₂FeH₆ showed poor reversibility,and the initial charge capacity is only 280.2 mAh g^-1 with initial Columbic efficiency?ICE?of 23.3%.Then,the Mg₂FeH₆ was composited with graphite?G?by ball milling.It is assumed that the graphite could effectively buffer the volume change of MgH₂ during charging and discharging process and improve the reversibility and cycling performance of Mg H₂.The initial capacity of Mg₂FeH₆-G was increased to 433.7 mAh g^-1 with higher ICE of 37.6%.Besides,the amorphous TiO₂ film was coated on the surface of the Mg₂FeH₆-G electrode by magnetron sputtering.The resultant Mg₂FeH₆-G/TiO₂ electrode exhibited a stable charge capacity of 412.6 mAh g^-1 over100 cycles with much higher capacity retention of 84.7%.It was found that the TiO₂ film played an important role in maintaining the integral structure of the electrode and preventing the direct contact of Mg/MgH₂ with the electrolyte.Further,the nanostructure of active materials could be well preserved during cycling,which enables the reversible formation of MgH₂ and high capacity retention.This work provides a new way to improve the electrochemical performance of metal hydride anodes.