Design,Synthesis And Modification Of New-type Metal-organic Frameworks (MOFs) As Cathodes Or Anodes Materials For Lithium Ion Batteries

Since commercializing in 1990s,lithium-ion batteries（LIBs）,with their high energy density,high power density,long cycle life and nonexistent memory effect,have gradually become the main energy storage devices for mobile phones,laptops and other portable electronic devices.Although the performance of lithium-ion batteries has exceeded other types of batteries such as nickel-chromium and lead-acid,emerging vehicles such as electric cars have put forward higher requirements for lithium-ion batteries in energy density,safety,production cost and environmental impact.Therefore,new-type LIBs with higher performance are highly required.Lithium-ion batteries are composed of the anode,cathode,electrolyte and porous membrane,in which the performance is mainly determined by anode and cathode materials.In commercial LIBs,graphite is used as anode,while lithium cobalt oxide,NMC ternary material or lithium iron phosphate are used as cathode.Development of new anode and cathode materials to replace these traditional materials is one of the most important research directions.Metal-organic frameworks（MOFs）,as a new type of crystalline materials,are composed of metal ions or clusters and polydentate organic ligands in the aid of coordination bonds.With the adjustability of metal centers and organic linkers,their pores and surface areas could be designed for the extensive use in catalysis,sensing,drug delivering,proton conduction,gas separation and adsorption.In recent years,people have gradually realized that MOFs have the pore structure for ion storage and can alleviate the consequent volume expansion.Moreover,if endowed with redox-active metals and ligands,MOFs can be used for lithium-ion batteries.With the rapid progress of related work,more and more researchers have participated in this topic,and the reported achievements have attracted wide attention.However,the specific capacity and cyclic stability of MOF materials still cannot meet the commercial needs.Therefore,it is still urgent to develop and construct high-performance MOF anode and cathode materials through appropriate methods.The composition and structure determine the performance.Therefore,to solve the problems in application of MOFs for LIBs,we should firstly get a clear understanding of the structure-activity relationship,and then build up relatively unified theories.After that it will be easily to choose appropriate metals,ligands,and structures to meet the performance demands.Taking the opportunity of developing high-performance anode and cathode materials,we systematically demonstrate the structure-activity relationship of MOF materials in lithium battery.More importantly,this work also puts forward solutions to some problems in MOF anode/cathode materials.The main work is summarized as follows:1.High-voltage N-redox MOF for high-rate anion-intercalation batteriesThe research of MOF anode materials is in full swing,but there are few reports about MOF cathodes.Moreover,the existing MOF cathodes are mostly composed of quinones or metals with low voltage of about 1.5³.5 V,which can not meet the commercial requirement for power density.In this chapter,by exploring the redox property of organic ligands,we propose a p-type MOF-cathode strategy with nitrogen as the active center.With appropriate ligands,we successfully construct a porous ZnDAnT MOF with pcu topology.After thermal activation,with abundant nitrogen active sites and porous cage structure,the MOF can rapidly absorb and desorb the hexafluorophosphate anion in the electrolyte.In the voltage of 2.5⁴.0 V,the discharge capacity can reach to 60 mAh g^-1 at a current density of 100 mA g^-1,equalling to its theoretical capacity.At 1000 mA g^-1,the discharge capacity can still reach half of the theoretical capacity.This work provides a feasible strategy for developing high-voltage MOF cathode materials.2.The effect of MOF topology structure on its performanceThe performance is determined by the composition and structure.However,there is no reports about the effect of topology structure on its performance in LIBs.In this chapter,we select MOFs based on azide and viologen ligands and Mn/Co metals.On the basis of similar composition,two types of MOF materials with chain structure and pillared-layer structure were synthesized and tested.The results show that although all MOF materials lose their long-range order after the first cycle,MOFs with pillared-layer structure show better reversibility than MOFs based on chain structure.We propose that the performance may be due to the retained short-range ordered pillared-layer structure,which can ensure shorter diffusion path and faster Li-ion conduction,thus maintaining more active sites.This study provides a reference for the construction of high-performance MOF electrode materials in terms of topological structure design.3.High-performance MOF with novel benzene carboxylic acid ligandWith the development of several years,especially after the terephthalic acid ligand being introduced to MOF anodes,reversible capacities of MOFs anodes have increased to 1000 mAh g^-1.Increase of the capacities to the level of silicon and phosphorus is important for the practical application of MOFs.We anticipate that more active oxygen sites in polycarboxylic acid ligands may contribute to higher circulating specific capacity.Therefore,we synthesize a Ni-BHC MOF with homobenzene hexacarboxylic acid ligand.Its theoretical capacity is as high as 1570mAh g^-1,and its discharge capacity is more than 1650 mAh g^-1 after 5 cycles and is1261 mAh g^-1 after 50 cycles.Through sXAS tests,we find that the rich lithium-ion insertion sites in polycarboxylic acid ligands,coupled with the layered structure of MOF that can quickly conduct lithium ions,are the key to extremely high performance.4.Stable MOF with nitrogen-oxygen dual-coordinated ligandThe specific capacities of MOF anodes have reached more than 1000 mAh g^-1,but their cycling stability has not been well improved,and related work is also rare.Inspired by stable metal-nitrogen coordination in ZIFs,we try a nitrogen-oxygen dual-coordinated strategy.A Co-pydc MOF is synthesized with pyridine-carboxylic ligand and used as anode for LIBs.The electrochemical tests show that the MOF has higher cycling stability than the Co-BDC which is synthesized with terephthalic acid ligand under similar conditions.A high specific capacity of 589 mAh g^-1 is still maintained after 400 cycles under a current density of 1 A g^-1.This work provides a good solution to improve the cycling stability of MOF.5.The internal mechanism of bimetallic synergistic effectBimetallic MOF has been reported in catalysis,gas storage and other aspects.Our group has also found that bimetallic MOF has synergistic effects in LIBs,but the relevant mechanism is still unclear.With this concept,we improve the cycle performance of ZIF materials by introducing two kinds of bivalent metals,namely cobalt and zinc,into the ZIF structure.Then we study the mechanism of bimetallic synergistic effect using sXAS,EPR,and some other technique.It is found that,compared with the corresponding mono-metal ZIFs,the excellent performance of bimetallic ZIF can be mainly attributed to the synergistic activation of bimetal coordination sites,which can enable the originally inert nitrogen sites to participate in the lithium-ion intercalation.This study provides a strategy for further development of high performance MOF anode and cathode materials in metal center selection.