Modification Of Magnesium-Ion Cathode Material MgMn₂O₄

Compared with lithium-ion batteries,magnesium-ion batteries can provide more electrons with a larger theoretical specific capacity?2205 mAh/g?.In addition,magnesium is abundant compared to lithium,which can effectively reduce the battery cost.So,the research of cathode materials for magnesium-ion batteries has important scientific significance and practical value.MgMn₂O₄ as magnesium-ion battery cathode has attracted more attention.Based on our previous studies,this thesis attempts to improve the capacity and cycle performance of the MgMn₂O₄ by substitution,non-stoichiometry and composite,respectively.The effects of phase compositions and microstructure on the electrochemistry properties of the prepared samples have been systematically studied by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,thermogravimetry/differential thermal?TG/DTA?,raman spectroscopy,specific surface area?BET?and electrochemical performance.Fe-doped MgMn₂O₄ samples have been prepared by sol-gel method and subsequently calcination technique.The effect of calcination temperature?550-750??and holding time?5-12 h?on the phase compositions and microstructure have been investigated.The results show that Fe-doped MgMn₂O₄ samples have a cubic crystal spinel structure with good crystallinity.Unfortunately,Fe-doped samples show poor cycle performance.The samples prepared at 750?for 5 h have relatively good electrochemical performance.The first discharge specific capacity is only 40.6 mAh/g,but decreases to 11 mAh/g after 50cycles.The effect of non-stoichiometry,calcination temperature?550-750??and holding time?5-12 h?on the phase compositions,microstructure and properties of Mg_xMn₂O₄?norminal composition x=0.3,0.5,0.7?have been studied.The results show that the Mg_0.3Mn₂O₄ sample prepared by calcination at 750?for 5 h shows good crystallinity and good electrochemical performance with a lamellar and spherical structure.The BET analysis shows that the pore diameter is mesoporous and the specific surface area is 22.8m²/g,and the electrochemical tests show that the discharge specific capacity is 103.7mAh/g after 50 cycles.MgMn₂O₄/CNT composite samples have been prepared by ultrasonic liquid phase dispersion and subsequently solid state sintering.The effect of calcination temperature?300-500??,holding time?1-4 h?and carbon nanotubes content?1-5wt%?on the phase compositons and microstructure have been investigated.The results show that the MgMn₂O₄/5wt%CNT composite sample prepared at 400?for 2 h shows good crystallinity and good electrochemical performance.The morphology analysis shows that carbon nanotubes intersperse the MgMn₂O₄ grains.The first specific discharge capacity is113.3 mAh/g,and the specific discharge capacity after 30 cycles is 86.1 mAh/g.