| Porous coordination crystals contain tunable micro/nano-cavities,thus show great promise in storage and separation of guest molecules or ions.Porous coordination crystals and their derivate can reversibly accommodate sodium or potassium ions storage through electrochemical redox reaction,thus are recognized as promising electrode materials for sodium-ion batteries or potassium ion batteries.According to solid-state chemistry,ion storage of the porous coordination crystals is mainly correlated to their or crystal structure.However,experimental investigations suggested that the kinetics or performance of the ion storages varied by changing the crystal sizes and mesostructures of the porous coordination crystals.Therefore,it is necessary to investigate the sodium or potassium ion storage of the meso-structured porous coordination crystals through precise control the meso-structure.This thesis focused on porous coordination crystals and their derivate,tried to clarify the correlations between their meso-structures and mechanism for sodium or potassium ion storage,developed high-performance electrode materials for sodium-ion batteries and potassium ion batteries.1.The effect of size on sodium ions storage.According to the crystallization kinetics,sodium citrate was utilized to reduce the crystallization rate of Prussian Blue analogues?PBAs?by chelation with metal ions.The size of the Ni3[Fe?CN?6]2 crystals was controlled from 50 nm to 150 nm by varying the usage of sodium citrate?200 mg—400 mg?.The morphology,size and crystal structure were investigated by Transmission Electron Microscope?TEM?,Scanning Electronic Microscopy?SEM?and X-Ray Diffractomer?XRD?.The morphology of all samples were cube with Fm3?m structure.When size of the PBAs crystals was less than 110 nm,the surface energy became un-ignorable,therefore helped the PBAs to maintain their crystals structure upon insertion/extraction of sodium ions.When the size of the PBAs crystals was larger than 110 nm,the effect of the surface energy could be ignored,therefore a temporary plastic phase-change of the PBAs crystals could happen during intercalation of the sodium ions,leading to an increased storage capacity for the sodium ions from 2 mmol/g?50 nm,Ni3[Fe?CN?6]2?to 2.8 mmol/g?150 nm,Ni3[Fe?CN?6]2?.2.The effect of nanoframe on sodium ions storage.On the basis of the disassociation kinetics of the PBAs crystals,the etching rate of the coordination crystals by acids varied at different positions.Therefore,the facets of the PBAs crystals could be etched by protons,leading to the nanoframes.Na2Ni[Fe?CN?6]nanoframe was successfully fabricated by controlled preferential etching,utilizing hydrochloric acid as a dissociation agent.Nanoframe structure increased the area of the solid-liquid interface between the Na2Ni[Fe?CN?6]crystals and the electrolyte,and shortened the transportation of the sodium ions inside the Na2Ni[Fe?CN?6]crystals.As a result,the diffusion coefficient of the sodium ions in the electrodes made by the Na2Ni[Fe?CN?6]crystals.When the current density was 1A/g,the capacity retention of Na2Ni[Fe?CN?6]nanoframe could reach 60%.While,only 17%of initial capacity could be obtained for Na2Ni[Fe?CN?6]solid.Besides,Na2Ni[Fe?CN?6]nanoframe could deliver a capacity of 50 mAh/g after 500 cycles at a current density of 100 mh/g.In contrast,the capacity of Na2Ni[Fe?CN?6]solid was only 29 mAh/g.3.The effect of nanobubble on sodium/potassium ions storage.By manipulation of the disassociation kinetics of the crystals,cavities inside zeolitic imidazolate frameworks?ZIF-8?crystals were fabricated through etching by a polyphenol,tannic acid,leading to ZIF-8 nanobubbels.Carbonaceous polyhedral nanobubbles were synthesized by thermal pyrolysis of the ZIF-8 nanobubbles.The carbonaceousnanobubbles exhibited a pseudocapacitance behavior for sodium/potassium ion storage,while the carbon nanoparticles showed a battery type.The super thin?10 nm?shells of the carbon nanobubbles enlarged the area of the solid-liquid interface between the electrodes and electrolyte,minimized diffusion of the guest ions inside the electrode materials,finally resulted in a pseudo-capacitor type ion-storage.At a current density of 10 A/g,carbon nanobubbles could deliver a specific capacity of 100 mAh/g.Besides,the capacity retention was about 100%after1000 cycles.On the contrary,the capacity of carbon particles was only 5 mAh/g.4.The effect of network on potassium ions storage.With control of the crystallization kinetics,the sizes of the ZIFs crystals were adjusted from 50 nm to 2000 nm by using trimethylamine for de-protonation.Graphitic carbon networks were obtained by pyrolysis of the nanosized Co-incorporated ZIFs crystals.Both of the graphitic carbon networks and carbon particles fabricated by thermal pyrolysis of the Co-incorporated ZIFs crystals stored potassium ions in a pseudo-capacitor way.The crucial factor which can influence insertion/extraction of the ions was changed to the electron conductivity.The carbon networks have better electron conductivity than the carbon particles,thus showed excellent cycling stability and rate performance.As anode materials of potassium ion batteries,carbon networks exhibited superior performance.At a current density of 50mA/g,carbon networks delivered a capacity of 200 mAh/g after 500 cycles.On the contrary,the capacity of carbon particles was only 30 mAh/g.When the current density was 2000 mA/g,carbon networks could exhibit a specific capacity as high as100 mAh/g.In contrast,carbon particles will loose the capacity of potassium ions storage. |
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