| With the increasing demand for mobile energy storage devices,the exploitation of lithium is increasing year by year.The limited lithium resources limit the sustainable application,forcing people to seek alternative high energy density,low price and sustainable energy storage system.Abundant sodium and potassium resources can meet the needs of sustainability.Therefore,the development of sodium and potassium secondary batteries can solve the dilemma of resource shortage,which is an energy storage system with sustainable development.The new energy storage system of alkaline metal(Na/K)-chalcogen(S/Se)batteries possess a high specific capacity,which can meet people’s demand for high energy density and low price.Due to the soluble polysulfide(polyselenide)produced in the stepwise transformation reaction,the tremendous volume changes during the conversion process,and the low reactivity of the electrode material,the S and Se cathodes exhibit a low capacity and fast capacity decay.For Na/K anodes,due to their high reactivity with the electrolyte,the generation of Na/K metal dendrites and the loose stacking of Na/K during stripping/deposition can lead to short-circuit safety hazards,increased polarization,and continued electrolyte consumption.Therefore,to achieve high energy density and long cycle life of alkali metal(Na/K)-chalcogen(S/Se)secondary battery,both S/Se cathodes and Na/K anodes are required to be improved.In our dissertation,starting from the derivatives of Metal-Organic-Framework(MOF)materials,by regulating the morphology,nanopore structure and chemical composition of MOF-derived carbon materials,the volume expansion of S and Se cathodes is suppressed,and the catalytic conversion and adsorption process of polysulfides/polyselenides are promoted.In addition,the surface modification of the metal anode achieves a metal dendrite-free deposition process.In the third and fourth chapters of this dissertation,the mechanisms of S and Se cathode are revealed and their electrochemical properties are improved.In the fifth chapter,the Na metal anode is further studied and the improvement method of long cycle life of Na metal anode is explored.The details are as follows:In Chapter 1,the working principles of Na/K secondary batteries,cathode materials and Na anode materials are summarized.The status of the development of alkali metal(Na/K)-chalcogen(S/Se)secondary battery and the application of MOF materials in batteries are also presented.In Chapter 2,the instruments and reagents used in this dissertation are introduced.The characterization method of the experiment and the test process of the batteries are described in detail.In Chapter 3,porous carbon anchored with Mo2C electrocatalyst promotes the polysulfide redox kinetics for Na-S batteries.ZIF-8 and ZIF-67 composites were used as precursors to obtain hollow porous carbon material(HPC),which enormous specific surface was easy to load Mo ions,and Mo2C/HPC was synthesized by further heat treatment.The highly dispersed Mo2C as the conductive polar mediator has strong chemical adsorption to polysulfide and can further catalyze the conversion process of sodium polysulfide to Na2S.In addition,the hierarchical porous hollow carbon plays an important role in physically confining of S.Benefiting from these synergistic effects,the S@HPC/Mo2C displays the stable cycle property(503 mAh g-1 after 800 cycles at 5 A g-1)and superior rate property(483 mAh g-1 at 10.0 A g-1).In Chapter 4,we prepare the N/O co-doping porous carbon nanocages modified with CNT as Se host for K-Se batteries.The modified carbon(NO-nanocage/CNT)derived from a ZIF-8@ZIF-67 coupled structure was prepared as the host to confine Se.The hollow structure with abundant micropores and mesopores shortened the diffusion distance of K+and accommodated volume variation during repeated(de)potassiation.The N/O dual-doping engineering improved the chemical affinity for polyselenide and enhanced the utilization of Se.The in-situ growth of CNT boosted the electrical conductivity of the electrode and promoted redox kinetics.Strong chemical affinity between the NO-nanocage/CNT and K2Se was demonstrated via DFT calculations.The obtained Se@NO-nanocage/CNT cathode possesses a high initial capacity of 623 mAh g-1.It also displays stable cycle property(274 mAh g-1 after 3500 cycles at 1.0 A g-1)and superior rate property(304 mAh g-1 at 5 A g-1).In Chapter 5,we prepare the heterogeneous interfacial layers derived from the in situ reaction of CoF2 nanoparticles with sodium metal and applied in Na metal anodes.With ZIF-67 as the precursor,highly dispersed CoF2 nanoparticles were obtained after heat treatment and fluorination.The NaF/Co heterogeneous interface layer was prepared by coating CoF2 nanoparticles on the surface of a bare Na foil at the temperature of 60℃,where a replacement reaction happened and then a NaF/Co hybrid layer was generated on the sodium surface(denoted as NaF/Co/Na).The produced NaF/Co layer offers a high Young’s modulus,strong sodiophilicity and a high Na+conductivity,which is capable of inhibiting the dendrite formation and promoting the uniform deposition of sodium metal.Inorganic substances of NaF,Co,Na2CO3,and Na2O are the essential components of the artificial interface layer,which can stabilize the solid-liquid interface,reduce the occurrence of side reactions,and protect the Na anode.As a result,the NaF/Co/Na symmetric cells exhibit a long cycling lifespan of 1000 h in carbonate electrolytes(1 mA cm-2,1 mAh cm-2).And the NVP‖NaF/Co/Na coin cells also display a long cycle life(65.5 mAh g-1 after 1000 cycles at 15 C)and superior rate property(52 mAh g-1 at 60 C).In Chapter 6,the innovation points,deficiencies and future research directions of this dissertation are discussed.In addition,the pore structure of MOF-derived carbon materials is studied in Appendix.By regulating the structure of the pores,the rapid transport of potassium ions is realized. |
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