Structural Manipulation Of MOF-Derived Carbon Materials And Their Application In Sodium/Potassium Secondary Batteries

Sodium/potassium secondary batteries are considered to be one of the most promising candidates for the next-generation large-scale energy storage devices due to the abundance of sodium/potassium resources and low price.Electrode materials are one of the key factors that determine battery performance,and the study of carbon materials is of great significance for the application of positive and negative electrodes of sodium/potassium secondary batteries.Firstly,carbon materials themselves can store sodium/potassium ions,which are one of the typical embedded materials.Actually,carbon materials are considered as one of the most promising anode materials for commercial application due to their low price,good electrical conductivity,and stable physicochemical properties.Secondly,using functionalized carbon materials as the carriers of sulfur(selenium)can not only improve the conductivity of the electrode material,but also can inhibit the dissolution and shuttling of the reaction intermediates and alleviate the volume expansion of the material during the charging and discharging processes,thus improving the electrochemical properties of sulfur(selenium)cathode.At present,carbon materials have the problems of lacking storage sites and slow diffusion kinetics when used as potassium ion anode materials,resulting in poor rate performance and cycle stability.When used as sulfur(selenium)carriers,the difficulty lies in how to ingeniously design the structure and composition of the carbon material so that they possess the best captivity effect towards sulfur(selenium).Carbon materials derived from metal-organic frameworks(MOFs)possess the advantages of developed pores and controllable structural components.In this dissertation,starting from MOF compounds,a series of MOF-derived carbon materials have been developed by composite hybridization,pore structure design,morphology control,and heteroelement doping.The formation mechanism and regulation methods have been explored,and the controllable preparation of materials has been realized.They are used as negative electrode of potassium ion battery and hosts of sulfur and selenium-based positive electrode,which greatly improve the electrochemical performance of sodium/potassium secondary battery.Furthermore,the energy storage mechanism of the materials is also revealed in this dissertation.The main research contents are as follows:(1)Construction of amorphous carbon and graphitized carbon composite nanosheets for potassium ion batteries:Core-shell structure of 2D Zn-MOF-coated CoMOF nanosheets was designed using epitaxial growth method,which was futher carbonized into core-shell nanoplate of amorphous carbon core(Zn-MOF derivatives)and graphitic carbon shell(Co-MOF derivatives).The core-shell structure leads to the synergistic physical and chemical properties of the amorphous carbon(AC)and graphitic carbon(GC),exhibiting a variety of advantages:abundant N content in carbon structure,high conductivity,fully exposing active sites to reactive electrolytes and shorten diffusion length for both ions/electrons.When used as anode for KIBs,the AC@GC electrode exhibits a high reversible capacity(310 mAh g-1 at 0.1 A g-1 after 200 cycles),high rate capabilityes(170 and 120 mAh g-1 at 2 and 5 A g-1)and unprecedented cycle life(192 mAh g-1 at 1A g-1 after 5200 cycles).The in-situ Raman study confirms high reversibility of the carbon layer structure,leading to the superior electrochemical performance.(2)Manipulating selenium molecular configuration through micro-mesoporous carbon for high performance potassium-ion storage:ZIF-8 particles with uniform size of 40 nm and polyvinylpyrrolidone(PVP)were electrospun into PVP/ZIF-8 nanofibers,which was carbonized to obtain N/O dual-doped porous carbon fibers(MMCFs).In the hierarchical porous structure,homogeneous mesoporous of 20 nm is acted as carbon skeleton and abuntdant ZIF-8-drived micropores are distributed on the walls of the mesopores.The unique pore structure of the MMCFs enables effective regulation of selenium chain length.The micropores play a role in confining small Se molecule(Se23),which could inhibit the formation of polyselenides and work as physical barrier to stabilize the cycle performance.While the mesopores can confine long-chain Se(Se47),promising sufficient Se loading and contributing to higher discharge voltage of the whole Se@MMCFs composite.The N/O co-doping and the 3D interpenetrating nanostructure improve electrical conductivity and keep the structure integrity after cycling.The obtained Se2-3/Se4-7@MMCFs electrode exhibits an unprecedented cycle life(395 mA h g-1 at 1 A g-1 after 2000 cycles)and high specific energy density(400 Wh kg-1,nearly twice the specific energy density of the Se2-3@MMCFs).(3)An efficient strategy toward multi-chambered carbon nanoboxes with multiple spatial confinement for advanced sodium-sulfur batteries:Selective etching combined with stepwise carbonization strategy was proposed to convert ZIF-8 nanocubes into N/O co-doped multi-compartment carbon nanoboxes(MCCBs),which can work as efficient sulfur hosts to improve the performance of room-temperature sodium-sulfur batteries.The formation mechanism of the MCCBs was investigated using a series of characterization methods.First,a {110} truncated cube ZIF-8 was synthesized.Using the synergistic function of tannic acid internal etching and surface protection,as well as the crystal plane-directed etching mechanism,a hexapod-like structure was formed inside the cube by adjusting the etching time.In the subsequent carbonization process,the outer tannic acid layer and the hexapod-like outer wall are firstly carbonized into amorphous carbon,imprisoning ZIF-8 to shrink outwards during pyrolysis to form a multi-chamber structure.The MCCBs consist of porous carbon shells on the outside and connected carbon grids with hollow structure inside,bringing about a multidimensional chambered-carbon nanoboxes structure.As a sulfur host,the multichambered structure has better spatial encapsulation and integrated conductivity via the inner interconnected carbon grids,which combines the characteristics of short charge transfer path,and superb physicochemical adsorption along with mechanical strength.As expected,the S@MCCBs cathode realizes outstanding cycle stability(0.045%capacity decay per cycle over 800 cycles at 5 A g-1)and excellent rate performance(328 mA h g-1 at 10 A g-1).Furthermore,in-situ transmission electron microscopy observation confirms the excellent structural stability of the S@MCCBs during the(de)sodiation process.In addition,in view of the unstable solid electrolyte interface and Na dendrites growth of metal anodes in room-temperature sodium-sulfur batteries.,we improved the cycling of anodes by in-situ construction of an alloy/solid electrolyte hybrid interface layer on the Na metal surface,which is the appendix part of this thesis:In situ construction of alloy/solid electrolyte hybrid interphase to improve the sodium metal anode:We designed alloys&solid electrolytes hybrid protective layers for both Na metal using a facile and effective BiOCl pretreatment method.As demonstrated by theory and experiments,the alloys/solid electrolytes hybrid interphase integrates the synergistic advantages of high ionic conductivity,electronic insulation,and interfacial stability,leading to uniform dendrite-free Na deposition beneath the hybrid interphase and highly reversible metal anode.The protected Na anode demonstrates improved electrochemical performances in symmetric cells(900 h at 1.0 mA cm-2)as well as full batteries(83 mA h g-1 after 1500 cycles at 15 C).