Studies On The Hydrolthermal Synthesis And Electrochemical Property Of MnO₂ Nanomaterials

MnO₂ nanomaterials have shown versatile applications in sewage treatment,catalysts,primary batteries,supercapacitors and lithium-ion batteries due to their high theoretical specific surface area,high theoretical capacity,considerable electrocatalytic activity,high abundance,low price,and environmental friendliness.As the performance and application of nanomaterials were affected by the phase,the dimensions,the physical structure and the size of the microstructure,many scientists focus on development of nanomaterials with different dimension,size and morphologies,and then do massive research projects in exploring their physicochemical properties.Therefore,this dissertation is aimed at preparation of MnO₂ nano-materials with different morphologies through feasible and facile hydrothermal methods,and exploring impact regulations of microstructures on their electrochemical properties.In addition,MnO₂/graphene oxide composites showing excellent electrochemical properties were prepared through optimal experiments.The main findings are as follows:First,choosing KMnO₄ as manganese source,MnO₂ nanomaterials were prepared through hydrothermal reaction.The influence of reaction temperature and reaction time,the content of manganese ions and hydrogen chloride on the crystal structure and morphology of MnO₂ nanomaterials were also studied.Different morphologies,i.e.nanosheet?-MnO₂,nanoflower?-MnO₂ and nanorod?-MnO₂ were obtained.It can be found that the crystal transformation of?-MnO₂,?-MnO₂ and?-MnO₂ based on Ostwald ripening process and we can regulate the morphology of the manganese dioxide particles by changing the experimental conditions.Second,MnO₂ with different morphologies were assembled into Li-ion batteries,using Galvanostatic charge-discharge measurements,cyclic voltammetry,AC impedance measurements to study its electrochemical performance.Electrochemical properties?i.e.cycle specific capacity and rate properties?were sorted by order of?-MnO₂ nanorods>?-MnO₂ nanosheet>?-MnO₂ nanoflowers,after 100 cycles at 500mA·g^-1,?-MnO₂,?-Mn O₂ and?-MnO₂ showed specific capacity of 143.8 mAh·g^-1,64.4 mAh·g^-1,53.5 mAh·g^-1.Due to better crystallization properties,?-MnO₂nanorods exhibited better rate capability as it withstand drastic volume change during the electrochemical reaction.In addition,?-MnO₂ nanorods possess 2×2 scale one-dimensional tunnel structure,which is in favor of fully inserting and extracting of lithium ions,so it showed larger capacity.What's more,AC impedance measurements showed that solid?-MnO₂ nanorod having smaller electric double layer resistance and transfer resistance,so it exhibited larger specific capacity.Third,Limitted by the low electrical conductivity and poor structural stability,the test specific capacity of Mn O₂ was much less than the theorectical value.Complexing MnO₂ with carbon-based materials could improve the electrochemical properties.In the experiment,freeze-drying method had been used to produce?-MnO₂/reduced graphene oxide carbon-based materials.Electrochemical testing revealed that the specific capacity of?-MnO₂/reduced graphene oxide carbon-based materials could reach to 505.2 mAh·g^-1 after 100 circles at 500 mAh·g^-1,which was far better than?-MnO₂.The result of AC impedance had shown that the electrical transport properties of the electrode reaction had been improved greatly after the introduction of reduced graphene oxide,so that the electrochemical performance such as specific capacity and cycling stability could strengthen.