Hierarchical Carbon Nanocages For Li-O₂ Batteries

Driven by the growing concern about serious energy and environmental issues,increasing attention has been paid to the development of high-performance energy storage and conversion devices.Li-O2 batteries have attracted much attention due to their high energy density.However,it suffers from many challenges for practical applications,such as high discharge/charge overpotential,low rate capability and poor cycle stability which mainly depend on its cathode material.Generally,a suitable O2 electrode material should possess good electronic conductivity,large specific surface area,suitable porous structure,good ORR and OER activity and stability.Very recently,we reported a novel 3D hierarchical carbon nanocages(hCNC)with good conductivity,large specific surface area and coexistence of micro-,meso-and macropores,which demonstrated excellent electrochemical performances for lithium-ion batteries,lithium-sulfur batteries and supercapacitors.In this thesis,by applying hCNC as cathode material for Li-O2 batteries,some progresses have been made.1.The 3D hCNCs with high electrical conductivity,hierarchical porous structure and large specific surface area were prepared using benzene as carbon source and hierarchical basic magnesium carbonate as precursor.With the synthetic temperature rising,there is an increasing trend of electrical conductivity and a decreasing trend of specific surface area and defects.2.The hCNCs delivers excellent electrochemical performance including a high discharge capacity of 9963 mAh g-1 @0.1 A g-1,the excellent rate capability,low overpotentials(<0.74V),and comparable cycle stability(50 cycles).By increasing the synthetic temperature,better cycle stability has been obtained.hCNC900 shows>95%capacity retention even after 80 cycles.The excellent electrochemical performances which far exceed those of rpCNC and XC-72 mainly derive from the unique mesoscopic structures.The 3D hierarchical structure and good graphitization degree facilitates electron transfer.Large specific surface area can provide more ORR and OER activities and is beneficial for loading and dispersing Li2O2.Multi-scaled porous structure is good for and mass transfers and the accommodation of formed Li2O2.3.By a microwave-assisted glycol method,we have immobilized the manganese dioxide nanoparticles onto the surface of hCNC.The MnO2/hCNC composite delivered a larger discharge capacity of 10742@0.1 A/g higher than 9963 mAh g-1 of hCNC.With the current density rising from 0.1 to 1.0 A/g,the MnO2/hCNC based electrode maintains lower charge overpotential of 0.46 to 0.79 V,equivalent to hCNC.Moreover,even at the high rate of 2.0 A/g,the tested cell can achieve good cycle stability of 100 cycles with less capacity decay.The excellent electrochemical performance may come from the good catalytic activity of MnO2 and its protective effect to carbon,as well as its special meso structure.All these progresses have provided the basis for developing high performance Li-air batteries.