Preparation And Electrochemical Properties Of Metal Oxide/carbon Composite For Lithium-ion Batteries

Owing to the high energy density,long lifespan and memoryless effect,Li-ion batteries?LIBs?as the most promising energy-storage device are playing essential roles in military instruments,aerospace,medical treatment facility,telecommunication and other renewable energy storages.The low theoretical capacity of commonly used graphite anode is only 372mAh g^-1,which can not satisfy the increasing requirement for LIBs with higher capacity.Therefore,many alternative anode materials such as metal oxide,metal sulfide,and nonmetal with larger specific capacity are explored to substitute graphite.However,the severe volume variation during Li⁺injection and ejection processes results in electrode disintegration and rapid capacity fading and severely restricts its appliciaiton.Due to the high electrical conductivity,aboundant active sites and good stability,various nanocarbon materials can be good alternatives to the commertial graphite in LIBs.Besides,carbon coating on highly dispersed metal oxide nanoparticles is also an advisable way for energy storage and conversion.The detailed contents and results in this thesis are summarized as follows:?1?A convenient,facile,and efficient method has been proposed for in-situ synthesis of ZnO/porous carbon?ZnO/C?nanocomposites.Zn²⁺is highly dispersed and anchored by polyvinyl alcohol?PVA?,which is converted into three dimensional conductive network under subsequent heat-treatment.The precursor of Zn O,the weight ratio of ZnO to carbon matrix,high-temperature treatment,structure and electrochemical performance are systematicly investigated.Results show that 57.5 wt%ZnO/C nanocompsite treated under 500°C can deliver a reversible specific capacity of 637 mAh g^-1 after 200 cycles at 100 mA g^-1.?2?Petroleum asphalt based carbon shell?PACS?has been successfully prepared from cheap and aboundant petroleum asphalt via ZnO template,which was finally etched by HNO₃.The chemical composition,morphology,pore structure have been investigated.The effect of heat treatment on polycondensation and transformation of aromatic constituents is also studied to obtain the appropriate synthesis condition.The prepared PACS has hierarchical porous structure?i.e.macropores,mesopores and micropores?,in which macropores and big mesopores are formed due to the removal of ZnO templates,and small mesopores and micropores are formed during the heat-treatment.The microstructure can promote the transportation of ions,and the amorphous structure also enhances the lithium storage capacity.As expected,obtained PACS has high specific capacity and good rate capacity.After 1000cycles,PACS still shows a reversible specific capacity of 334 mAh g^-1 at a current density of1 A g^-1.?3?ZnMn₂O₄/petroleum asphalt based porous carbon framwork?ZnMn₂O₄/PCF?nanocomposite has been prepared through a facile self-assembly.The effect of surface oxidation technique on the elemental composition and surface properties has been systematically investigated.The pore structure,microstructure and morphology of PCF before and after loading ZnMn₂O₄ have also been studied.Besides,the microstructure and electrochemical properties of as-made composites with different ZnMn₂O₄ contents are measured systematically.The results illustrate that there is a synergistic effect between ZnMn₂O₄ and hierarchical PCF,which promots the lithium storage performance of ZnMn₂O₄/PCF,and shows a specific capacity of 420 mAh g^-1 at 1 A g^-1 after 550 cycles.