Design And Construction Of Micro-nano Structure And Lithium-ion Storage Performance Of Manganese Oxides

Lithium-ion batteries?LIBs?play a key role in the application of portable electronic devices,grid energy storage systems and electric vehicles serving as important energy storage devices.It is urgently for developing the state of art LIBs with higher energy/power density,higher capacity and longer life.In recent years,great efforts have been made for searching the ideal anode materials with high capacity and long cycle life due to the significant role of anode materials in LIBs.Among them,in term of safety,natural abundance,environmentally friendly,as well as a high theoretical specific capacity and a relatively low charge potential?¹.2 V?compare to other oxides,contributing to a highly desirable of manganese oxides for LIBs.However,manganese oxides suffer several inherent disadvantages,which need to make significant efforts to improve its electrochemical property.The main reasons of manganese oxides with the rapid decreased of capacity and poor rate capability are the low inherent electronic conductivity,relatively larger volume variation and the existence of self-agglomeration and pulverization.Therefore,in this paper,building a rational micro/nano-structure of manganese oxides composites by reasonable nanotechnology,in order to improve the electronic conductivity and maintain a stable structure come from volume change during lithiation/delithiation process.Herein,MnO₂ nanorods were obtained through hydrothermal method served as precursor,and the one-dimensional Na0.55Mn2O4·1.5H2O@C yolk-shell nanorods?SMOH@C?and three-dimensional nitrogen-doped graphene modified with MnO composites?3DNG/MnO?were constructed by rational design.And then,we also investigated the relationship of property and different structure,as well as the mechanism of lithium storage.In addition,in order to discuss the effect of nanoscale and construction strategy for micro-nano structure on optimized electrochemical performance,the constructed process and samples were characterized by various testing techniques.The main results are as follows:?1?Hydrothermal method was introduced to fabricate MnO₂ nanorods,which were used as precursor.One-dimensional SMOH@C yolk-shell nanorods with considerable performance were prepared through the combination of liquid phase synthesis and solid-phase reaction.In addition,in the combination of nitrogen-doped strategy and high temperature reduction reaction to synthesis three-dimensional nitrogen-doped graphene modified MnO composites?3DNG/MnO?accompany with MnO@rGO peapod-like structure.Meanwhile,the phase,chemical state,morphology as well as structure of the as-prepared samples were characterized by advanced technology systematically,such as XRD,Raman,XPS,SEM,TEM and TG/DSC etc.?2?Furthermore,SMOH@C and 3DNG/MnO composites were tested and investigated for electrochemical property as anode materials for LIBs.SMOH@C composites with excellent property were obtained through multi-step synthesis strategy,which showed a high reversible capacity of⁷50 mAh g^-1 at 0.1 A g^-1.In comparison with other samples,SMOH@C electrode showed the best rate capability under different current density.It also demonstrated a stable reversible capacity(⁴48mAh g^-1)even at a high current density of 4.0 A g^-1 after 3000 cycles.In addition,3DNG/MnO composites were fabricated by rational design and graphene modified,which showed distinguished performance in the comparison of bare MnO and MnO/rGO.3DNG/MnO showed a reversible capacity of⁹80 mAh g^-1 at 0.1 A g^-1,a relative high of capacity retention rate with stable capacity of⁴25 mAh g^-1 was observed after 3000 cycles at 4.0 A g^-1.?3?The mechanism of charge-discharge process,as well as the relationship between property and structure was investigated.In order to analysis the change of chemical state of Mn and the mechanism of lithiation/delithiation,XPS was adopted to investigate the chemical state of SMOH@C and 3DNG/MnO composites before/after cycling,demonstrated that the oxidized of Mn²⁺to high state will enhance reversible capacity.We also used SEM/TEM to explore the integrity of morphology and structure after many cycles at high current density,and confirm the positive effect of rational design of structure for electrochemical performance.Besides,we also investigated the process of electrochemical reaction and the contribution of capacity through analyzed CV curves,and lithium storage mechanism of 3DNG/MnO.The main possible reasons of as-prepared products with relative high capacity and cycle stability come from nitrogen-doped,the synergistic effect of active material and stable structure.