Morphology Control And Electrochemical Properties Of Spinel Zinc Manganate For Lithium Ion Batteries

Along with the economic and social development,the attendant environmental pollution and global warming have become growing problems,and the global energy consumption structure is gradually moving towards to the development and utilization of clean energy.The lithium ion secondary battery is considered to be the most promising energy storage and conversion device due to its environmentally friendly,high quality/volume energy density,low self-discharge effect and long cycle life.Moreover,with the constant improvement of the requirements for high performance electronic products,the demand for high energy density lithium ion battery is more and more severe.However,the traditional commercial graphite anode has already been unable to meet the people's demand due to the low theoretical specific capacity(372 mAg h^-1).Therefore,it is very important to develop new type anode materials with high specific capacity.For the past few years,the binary transition metal oxides are increasingly employed as anode materials because their better cycle and rate properties than the unitary transition metal oxides.And the Zn Mn₂O₄ material has become the potential high-performance secondary battery cathode material benefiting from its environmentally friendly,abundant reserves,low price and many other advantages.However,the problems such as low conductivity and large volume change during charge-discharge process lead to poor cycle and rate performance that restrict its practical application.In this doctoral dissertation,we focous on the study of ZnMn₂O₄,and adopt a series of methods to improve the electrochemical property of ZnMn₂O₄ material,such as nanostructuring strategy,morphology control and conductive material recombination.The points are as follows:Firstly,we prepare spinel ZnMn₂O₄ nanoparticles via a simple combustion method.By adjusting the burn agent and heat treatment temperature,we obtain the material with most excellent performance under 800?.The material is well dispersed and possesses the smallest particle size.After been cycled for 90 times under the current density of 100 mA g^-1,the ZnMn₂O₄ material can still presents a high capacity of 716 mAh g^-1.This is mainly because that the nanoparticle materials can effectively shorten the transmission distance of lithium ions and electrons,increase the contact area of material and electrolyte as well as reducing the charge transfer resistance,thus effectively improve the electrochemical performance of materials.Secondly,we adopt a co-precipitation method and successfully fabricated ZnMn₂O₄ anode for lithium-ion batteries.And as changing the heat treatment temperatures,we obtain the pure phase ZnMn₂O₄ material with different morphology.Electrochemical research shows that the core-shell microspherical material exerts the best performance.First,the nano-sized ZnMn₂O₄ particles can not only shorten the lithium ion diffusion path,but also increase the specific surface area of the material,thus reducing the charge transfer resistance of the material.In addition,the porous structure is beneficial to the infiltration of electrolyte and can facilitate the transfer of lithium ions in the electrolyte.Moreover,the microsphere structure can improve the structural stability of the material,thus effectively reducing the capacity fading caused by volume expansion during cycle.Therefore,the electrochemical property of the material has been significantly enhanced.Thirdly,we carried out a moderate microemulsion method and by means of adjusting the reactive solvent and additives,ZnMn₂O₄ materials with distinct morphological features?hexahedron,microsphere and core-shell microshphere?have been obtained.The electrochemical performances of the three samples are obviously affected by their different mophorlogies.The porous core-shell microsphere material not only exhibit high specific capacity and excellent rate performance,also emerge a phenomenon of gradually improving capacity upon cycling.Through electrochemical charge-discharge result,CV,dQ/dV,Raman and impedance results,we can find that the higher capacity mainly comes from the oxide of Mn²⁺during cycle as well as the resulted interfacial lithium storage.This is due to the unique structural of the porous core-shell microsphere morphology,which provides excellent kinetics properties for the reaction that leads to the appearance of capacitive lithium storage effect and increasing as cycling.Then,we prepare hollow ZnMn₂O₄ microspheres through a hydrothermal process and then perform graphene wrapping on them.As seen from the SEM and TEM results,the RGO sheets have covered on the surface of material.The hollow structure can effectively improve the structural stability,and graphene modification can increase the electronic conductivity of the material.Therefore,compared with ZnMn₂O₄,the electrochemical performance of ZnMn₂O₄/RGO composite was improved significantly.The initial discharge capacity of this material is as high as1082 mAh g^-1.And been cycled for 50 times at the current densities of 100 A g^-1,the material still doesn't show obvious capacity fading.Finally,we introduce a simple one-step hydrothermal method and successfully prepare the composite of CMK-3 and ZnMn₂O₄.The ZnMn₂O₄ material appears as a nano-layer structure,and is uniformly growing on the skeleton of CMK-3,forming a unique two-dimensional link three-dimensional structure.In this composite material,the high conductive three-dimensional CMK-3 skeleton can work synergy with the ultrathin two-dimensional ZnMn₂O₄ nanosheets,which is not only construct a conductive path that provide help for electronic transmission,also effectively alleviate the structure collapse during the charge-discharge process.Therefore,ZnMn₂O₄@CMK-3 composite material exhibits excellent electrochemical performance:after been cycled for 70 times at the current density of 100 mA g^-1,it can still maintain a high specific capacity of nearly 1000 mAh g^-1.Even at a high rate of 2000 mA g^-1,it can also reach a high capacity of 693 mAh g^-1.In summary,we adopt many methods to adjust the morphology,structure and electrical conductivity of ZnMn₂O₄ material,and improve its electrochemical performance successfully.And to some extent promote the practical application of transition metal oxide as lithium anode material.