Study On The Preparation And Electrochemical Performance Regulation Of MnO And MnF₂ With Micro-nano Structure

Recently,MnO and MnF2,as the representative of manganese based metal oxides and fluorides anode materials,have attracted wide attention due to their good energy storage properties.MnO and MnF2 have the advantages of high theoretical specific capacity(755.6 and 577 mAh g-1),relatively low electromotive force value and abundant resources.However,the conductivity of such materials is low,and the large volume change in the electrochemical process makes the electrode materials pulverize and fall off,thus resulting in poor cycle stability and rate performance.In order to solve these problems,we adjusted the morphology,micro-nano structure and electrical conductivity of the materials by optimizing the reactant concentration,magnetic field induction,carbon composite and element doping,and studied the electrochemical performance of the material.The main results are as follows:(1)The MnCO3 precursors were synthesized by hydrothermal method and magnetic field assisted hydrothermal method respectively,and then porous structure microcube and mesoporous microchain structure MnO materials were prepared by thermal decomposition.By adjusting the urea content and changing the magnetic field strength,the morphology and micro-nano structure of the material are optimized,so as to obtain good electrochemical performance.The porous microcubic structure MnO has a specific discharge capacity of 914.6 mAh g-1 after 200 cycles at 0.5 C.However,after 250 cycles of mesoporous microchain structure MnO,the specific discharge capacity of 768.7 mAh g-1 was maintained at 2 C,even after 2000 cycles at 8 C,the specific discharge capacity of 198.3 mAh g-1 was still maintained.The superior electrochemical energy storage characteristics of porous microcubic and mesoporous microchains structure MnO are attributed to their high specific surface area and relatively stable micro-nano structure,which is conducive to the rapid transmission of lithium ions and electrons,while adapting to the change of volume in the lithium ion electrochemical process,thus effectively improving the electrochemical performance.(2)In-situ coated glucose precursors were synthesized by hydrothermal method,and then the precursors were calcined to prepare MnO@C microcube.The effect of glucose content on the electrochemical performance of MnO@C was studied.The results showed that 0.2 g glucose coated MnO had the best electrochemical energy storage characteristics.After 200 cycles at 1 C,its discharge specific capacity reached 1006.5 mAh g-1,and the capacity retention rate was 105.3%.Even at 8 C,it can still provide a specific discharge capacity of 284.9 mAh g-1.Its superior electrochemical energy storage characteristics may be attributed to porous structure,good crystallinity,and appropriate amount of carbon in situ coating.(3)The Co-doped Mn site is realized by hydrothermal method,which can effectively control the intrinsic conductivity of MnO and optimize its electrochemical energy storage characteristics.Mn0.9Co0.1O showed the best electrochemical performance.After 250 cycles at 1 C current density,the discharge specific capacity reached 738.9 mAh g-1,and even at 8 C the reversible specific capacity could still maintained 287.0 mAh g-1 even at 8 C ratio.The obvious improvement of the electrochemical performance of the material is mainly due to the fact that the doping of Co element at Mn position can effectively enhance the conductivity of the material,alleviate the polarization phenomenon in the material and improve the diffusion rate of Li+.(4)MnF2 were fabricated by thermal decomposition of the NH4MnF3 precursor synthesized by solvent-thermal method.The morphology of the final product was controlled by changing the reactant concentration gradient,so as to improve the electrochemical performance of the material.The core-shell structure MnF2 microspheres showed the best electrochemical performance.After 250 cycles at 0.5 C,the discharge specific capacity reached 721.5 mAh g-1.Besides,the reversible specific capacity at 4 C still had 211.8 mAh g-1.The outstand electrochemical performance of MnF2 may be attributed to its high specific surface area and stable core-shell structure,which helps the rapid transport of lithium ions and electrons,while adapting to the dramatic volume changes during the electrochemical cycle.(5)Mesopore MnF2@CNT cube was prepared by solvothermal method.Hhighly conductive CNT were dispersed on the inner and surface of MnF2 cube to improve their conductivity.Compared with pure MnF2,the electrochemical properties of MnF2@CNT composite were significantly improved.After 100 cycles at 0.5 C,the discharge specific capacity reached 292.9 mAh g-1.In addition,it also has a reversible discharge capacity of 151.7 mAh g-1 at 8 C.The addition of CNT and the combination of the hierarchical structure improve the rate performance of MnF2@CNT,and make its cycle performance relatively stable.