Preparation And Research On Sodium Storage Performance Of MOFs Derived Porous Carbon/Selenium Composites

The fossil energy crisis and harmful gases generated from the burning of fossil fuels have caused air pollution issues,which has greatly promoted people's demand for renewable clean energy.Lithium-ion batteries(LIBs),one of the most promising rechargeable energy-storage devices,have been extensively applied to portable electronic products such as smart phones,notebook computers,and smart watches.However,the limits of the natural abundance and uneven global distribution of lithium-source prevent the application of LIBs in large-scale energy storage application.For large-scale applications,it is emergency to develop a low-cost and sustainable energy storage system.Recently,sodium ion batteries(SIBs)are expected to replace LIBs because of the low cost and abundant resources of sodium,dominating in the large-scale energy storage devices in the future.Among the NIBs,Na-Se batteries based on selenium cathode have attracted more and more attention due to their high theoretical volumetric capacity(3253 mAh/cm3),higher electronic conductivity(1×10-3S/m)and stable performance in carbonate-based electrolyte.All these properties demonstrate that Na-Se batteries are promising high-energy density sodium ion batteries.However,selenium suffers from huge volume changes and shuttle effect of polyselenides during cycling,thus results in rapid capacity loss.To mitigate these issues,researchers have prepared various C/Se composites to improve its cycling performance and rate capability.Metal organic frameworks(MOFs)have attracted much more attention due to their diversity of structure and composition,tunable porosity,high specific surface area and porosity,thus exhibiting various types of applications in hydrogen storage,gas adsorption and separation,catalysis,electrochemical energy storage devices,etc.One can expect that Se embedded in MOF-derived porous carbon to form C/Se composites can effectively alleviate volume changes and shuttle effect of polyselenides,thus improving its cycling performance and rate capability.Chapter one,we firstly introduced sodium-selenium batteries briefly,such as properties of selenium,working principle,issues,research progress of selenium-based cathode.Then we introduced MOFs,such as structure and composition,forming principle and application in energy storage devices.Finally we proposed research strategies and contents based on the summarization of the research background.Chapter two,we summarized experimental reagents,experimental instruments material characterization method,preparation process of batteries,electrochemical characterization method of electrode materials related to this paper.Chapter three,we prepared selenium-carbon composites as cathode materials for sodium-selenium batteries to improve the utilization of selenium by embedding selenium in ZIF-8 derived N-doped microporous carbon polyhedrons(denoted as Se@N-MCPs).Then we conducted electrochemical tests of Se@N-MCPs electrode.The Se@N-MCPs cathode delivers excellent discharge capacity of 612 mAh/g after 100 cycles at a current density of 0.1 A/g and superior rate capability of 496 mA h/g at 5 A/g for Na-Se batteries.It also shows superior cyclic life of ?460 mAh/g at current density of 1A g-1 after 500 cycles with only 0.049%capacity decline per cycle.Chapter four,we summarized the innovations and inadequacies of the paper,then put forward the outlook of research plan in the future.