Synthesis And Electrochemical Properties Of MOF-derived Energy Storage Materials For Batteries

With the development of large-scale applications such as electrified transportation and smart grids,it is urgent to build stable,efficient,and inexpensive electrochemical energy storage systems.Therefore,it is crucial to seek novel secondary battery systems with low cost,high safety and long cycle life.Among them,sodium-ion batteries（SIBs）and potassium-ion batteries（PIBs）are expected to be potential commercialized secondary batteries for large-scale energy storage applications due to their abundant elemental resources and low cost.In addition,lithium-sulfur（Li–S）batteries have emerged as a competitive next-generation high-energy-specific secondary battery due to their high theoretical capacity and high energy density,as well as their low cost and abundant resources.Therefore,it is crucial to develop electrode materials with high specific capacity,long cycle life and low cost.Metal-organic frameworks（MOF）are a class of organic-inorganic hybrid materials with periodic network structures,and their customizable structural advantages provide new ideas for the design of electrode materials in secondary batteries.The main research work in this thesis mainly uses MOF as the precursor,by synthesizing a variety of different MOF precursors and controlling their pyrolysis and conversion processes,to design and synthesize energy storage and conversion materials with different structural and functional components,which are applied in the high specific energy sodium/potassium ion secondary battery and Li–S battery system,the material synthesis and energy storage mechanism have been deeply studied to solve the key problems in the secondary battery energy storage system.The research contents of this thesis are mainly divided into three parts.In the first part,the cubic phase ternary metal phosphoselenide Co PSe is synthesized for the first time,which is used as the anode material for the electrochemical sodium/potassium-ions storage.In the second part,the new Co PSe material is used as the separator modification component in Li–S battery system,we study the inhibition of polysulfide shuttle effect based on the Co PSe material.In the third part,aiming at the limitations of crystalline materials,the MOF-derived amorphous material of Co–P cluster served as a dual-atom site catalyst is further designed and synthesized,we explore the role of catalytic acceleration and inhibition of polysulfide shuttle effect in all stages for the Co–P cluster in Li–S batteries.The specific research contents are as follows:（1）Herein by employing a layered Co-MOF as both a precursor and template,we design and synthesize a new ternary cobalt phosphoselenide（Co PSe）that is in the form of nanoparticle embedded in the layered MOF-derived N-doped carbon matrix（Co PSe/NC）via in situ synchronous phosphorization/selenization reactions.The energy storage mechanism of Co PSe as the anode material for sodium/potassium ion batteries is deeply studied through experiments combined with theoretical calculations.The results show that,（i）Co PSe has an enhanced theoretical Na/K storage capacity and lower mechanical stress during discharge compared to Co Se_2.Moreover,the Co PSe shows enhanced reaction kinetics and lower redox potential with higher energy density in a full battery than that of Co Se₂.（ii）Co PSe possesses a higher intrinsic electrical conductivity and,especially,suppressed shuttle effect compared to Co PS,due to the replacement of S atoms by Se atoms.（iii）The unique 3D layered carbon matrix of the Co PSe/NC composite can not only accelerate the transport kinetics of ions and electrons,but also inhibit the aggregation of Co PSe nanoparticles,which effectively enhance the electrode stability and alleviate the volume changes during the Na/K insertion/extraction process.Therefore,the Co PSe/NC as the anode material for sodium/potassium ion batteries exhibits superior cycling stability and rate performance.Notably,the Na-ion full cell based on the Co PSe/NC anode can achieve an energy density of 274 Wh kg^-1,surpassing that of the Co Se₂/NC-based full cell and most state-of-the-art Na-ion full cells based on P-,Se-,or S-containing binary/ternary anodes.This work provides the basic principle for the design of new electrode materials and their applications in electrochemical energy storage.（2）Herein,we report the fabrication of MOF-derived ternary cobalt phosphoselenide Co PSe and MXene heterostructure（Co PSe@NC-TNS）via pyrolysis and synchronous phosphorization/selenization reactions,where Co PSe@NC nanoparticles are in situ grown on 2D MXene nanosheets（TNS）.Compared with TNS and Co Se₂@NC-TNS,the Ti and Co sites and dianionic P and Se sites in the Co PSe@NC-TNS heterostructure can effectively immobilize and catalyze the conversion of LIPS intermediates,enabling fast immobilization-diffusion of LIPS during conversion reactions.As the separator modification layer of Li–S batteries,the Co PSe@NC-TNS heterostructure can realize the rapid diffusion of Li-ions and improve the interfacial charge transfer kinetics.At the same time,LIPS can be effectively blocked and the LIPS conversion kinetics can be significantly enhanced,thereby effectively inhibiting the shuttle effect of LIPS and avoiding the corrosion of metal Li anode.When the Co PSe@NC-TNS-modified separator was applied to Li–S batteries,it exhibited significantly improved rate capacity and cycling stability,which can achieve an initial capacity of 1284 m Ah g^-1at a current density of 0.2 C.Even at a current density of 5 C,it can achieve a high rate capacity of 703 m Ah g^-1.The long-cycle test results show that a reversible capacity of 785 m Ah g^-1 can be achieved after1000 cycles at a current density of 2 C,with a capacity fading rate of only 0.016%per cycle.This work provides ideas for the design of novel heterostructure electrocatalysts.（3）Based on spherical ZIF-67 as precursor,by controlling the carbonization temperature of MOF and using in situ phosphatizing method,we design the synthesis of novel cobalt-phosphorus clusters confined in N-doped carbon framework（Co–P cluster/NC）as a dual-atom-site electrocatalyst for Li–S batteries.The electronic structure and coordination configuration of the Co-P cluster in the Co–P cluster/NC are identified by X-ray absorption fine structure（XAFS）combined with XPS and other characterization methods.The results show that,in contrast to Co P₃/NC and Co/NC,the Co-P cluster in the Co–P cluster/NC with abundant unsaturated bonds has moderate adsorbability and high catalytic activity for polysulfides（LIPS）,which can serve as a dual-atom site electrocatalyst to facilitate the kinetics of LIPS conversion.The theoretical calculation results show that the Co–P cluster can significantly reduce the LIPS conversion energy barrier and promote Li₂S decomposition.Therefore,Co–P clusters/NC as cathode electrocatalyst for Li–S batteries exhibits excellent rate and cycling performance.Under the high sulfur loading(6.2 mg cm^-2)and lean electrolyte(E/S=5μL mg^-1),the S@Co–P cluster/NC cathode can achieve a high areal capacity of6.5 m Ah cm^–2.Combined with the experimental and theoretical analysis,the Co–P dual-atom site synergistic catalytic mechanism is proposed.This study brings new insight to structural design and catalytic synergy mechanism at the atomic level of electrocatalysts in Li–S batteries.