Controlled Synthesis And Electrochemical Properties Of Porous Manganese Oxide Nanomaterials

Lithium ion batteries have dominated the portable electronic markets and have potential applications in electric vehicles and stationary energy storage system due to their advantages of high capacity, high operation voltage, long cycling life, fast charge/discharge ability and no memory effect. Electrode materials are the key limitation in the progress of lithium ion batteries. The challenge is to construct novel electrode materials of lithium ion batteries to meet the requirements of high energy density, long cycling life and low cost. The lithium storage mechanism of transition metal oxides is based on conversion reaction. During electrochemical reaction process, the degradation of electrode materials take negative influence on the life of electrode materials due to the large volume change and occurance of side reactions. Therefore, electrode materials are optimized by constructing novel nanostructure to obtain anode materials with long cycling life and high capacity.Herein, manganese oxides are investigated by constructing meosporous structure and carbon-coating structure. By investigating and comparing the electrochemical performance, internal principles between structures and electrochemical properties are further studied. The main conclusions are as follows.Mn2O3microspheres with controlled pore size were successfully synthesized by morphology-conserved transformation. As-prepared samples annealed at500,700, and900℃are mesoporous Mn2O3microspheres, porous Mn2O3microspheres and non-porous Mn2O3microspheres, respectively. As anodes for lithium ion batteries, mesoporous Mn2O3microspheres annealed at500℃show the highest discharge capacity, the minor capacity fading per cycle and ultralong cycling life. At current density of200mA/g, it can deliver reversible capacity of524mAh/g after200cycles. Meanwhile, it can realize1000cycles stable charge/discharge process with125mAh/g reversible capacity at current density of1000mA/g. The remarkable electrochemical performance can be attributed to the relatively high surface area and abundant surface active sites of mesoporous structure, which can improve the contact area between electrolyte and active materials, shorten Li ion diffusion distance and make more active materials to take part into the reaction, leading to improve capacity of electrode materials. Meanwhile, the existence of pore structure can accomodate the volume change caused by Li ion insertion/extraction, leading to keep the integrated structure of electrode materials and enhance the cycling life.Carbon coating stategy was utilized to further improve the electrochemical performance of manganese oxides at high current density. Mn3O4@C nanomateials synthesized by the reaction between Mn2O3and glucose have the highest reversible capacity. Although Mn3O4electrode materials are short of an electrochemical reaction compared with Mn2O3, its reversible capacity still improves15%after1000th cycle. The reversible capacity can reach142mAh/g. It is attributed to carbon coating, which can effectively increase the electrode conductivity, improve the surface chemisty of the active material, and protect the electrode from direct contact with electrolyte, leading to enhanced cycle life of batteries.