Design,Synthesis And Electrochemical Properties Of Metal-Organic Framework And Its Derived Transition-Metal Oxide As Anode Materials For Lithium Ion Batteries

Lithium ion batteries?LIBs?have now become the dominant power source in today's portable electronics market due to their high energy density,long life span,and low-environmental burden.However,they are not ready for large-scale applications in areas like electric vehicles,which present higher demand for energy.Thus,people are eager for the invention of long-life battery,featuring better energy and power characteristics.As the key component of LIBs,electrode materials have a signifiant effect on the final performance of battery.With regard to anodes,the specific capacity of commercial graphite is limited to 372 mAh g^–1.Therefore,searching for novel materials accompnanied by charge storage mechanism study,as well as novel nanotechnology development is of both significant theoretical and practical significance for designing and developing high-performance anodes.In recent years,metal-organic frameworks?MOFs?,also known as coordination polymers,have been successfully implemented in LIBs as a neotype electrode material for electrochemical energy conversion and storage,and have triggered widespread interest.As a burgeoning issue,the exploitation of high-performance MOF anodes moves slowly.The reason is that the relationship between MOF structure and functions,the redox chemistry in MOF electrodes,and lots of bewildering phenomena have not been clearly disclosed.On the other hand,apart from pristine MOFs energy storage materials,MOFs-derived transition metal oxides?TMOs?are also considered to be one of the promising anodes towads practical use for LIBs.Unfortunately,at the moment come two problems.First,the composition and structure of the MOFs-templated TMOs are mostly simple,and hence their performance remain to be improved.Second,there are no more choice for the preparation of TMOs with MOF templates,except the thermal decomposition approach based on MOF precursors,which,however,approves to be to be inefficient,and involves the very high energy consumption and causes the environmental risk.Besides,the deuterogenic nanostructures based on this method are somewhat difficult to be precisely regulated.Rational design and controllable synthesis of TMO materials with desired textural properties and high through-put for practical application in structure-dependent lithium storage still remain formidable challenges.To address these bottlenecks,this thesis mainly focuses on the design,synthesis,and electrochemical behavior of high-performance MOF compounds and their derived TMO anode materials.Specifically,we investigate systematically the effect of amorphization and disordering,particle size,and oxygenic groups of MOF materials on battery performance,as well as the abnormal capacity rise during cycling.On the other hand,we explore the preparation of mixed metal oxides with tunable composition and size,by introducing the conventional MOF-templated synthesis via thermolysis.On this basis,we have developed a novel and efficient wet-chemical route to prepare advanced TMO energy storage materials from MOF template,in view of the defects of the conventional pyrolysis method.The main work is summarized as follows:1.Co₃?BTC?₂·12H₂O MOF crystals were prepared based on a reported procedure and used as anode for LIBs for the first time.By the comparative study on cycling performance of the fresh sample and collapsed disordered Co-based MOF structure upon evacuation after thermal treatment,the effect of amorphization and disordering of MOF materials on electrochemical performance was discussed.It was found that,compared to the crystalline ordered counterpart,disordered MOF electrode showed much improved lithium storage properties,which also suggests that,contrary to popular belief,the structure integrity of MOFs might not be indispensable for their high capacity retention.2.Commercial Basolite F300-like Fe-BTC MOF,that feature a broad range of industrial applications,were directly synthesized in quantity using the protonated carboxylated linker by choosing a suitable Fe source,avoiding the use of a basic solution as in previous work,and used as anode for LIBs for the first time.Detailed characterization showed that the synthetic Fe-BTC resembled the commercial counterpart and the one prepared under basic condition a lot in many physicochemical characteristics.In addition,the Fe-BTC was scaled down to nanometer scale,resulting nanoscale MOF?nMOFs?,which is beneficial to its potential applications.By the comparative study on lithium storage properties of bulk Basolite F300 and Fe-BTC MOF with similar amorphous structure,the effect of particle size of MOF on the electrochemical performance was discussed.It turnes out that the nanoscale Fe-BTC shows a more attractive electrochemical performance with a large reversible capacity approaching 1021 mAh g^-1 after the 100th cycle at a current density of 100 mA g^-1and capacities approaching 436 and 408 mAh g^-1 after the 400th cycle at a higher current density of 500 and 1000 mA g^-1,respectively.The results of the Fe-BTC suggest the potential advantages of nMOFs for high-rate LIBs applications.3.A green ligand,i.e.,citric acid that contains many oxygenic groups,based MOF material,[Cu₂?cit??H₂O?₂]_n MOF,was designed and synthesized via a modified solvothermal method according to the lithium adsorption by MOF ligand discovered previously,and used as anode for LIBs for the first time.Electrochemical measurement showed that such electrode exhibited drastically boosted specific capacity(above 950 mAh g^-1 at 0.1 A g^-1)and cycling stability(500 cycles for 2 A g^-1),which was the best among all reported Cu-based MOF anodes.In addition,we notice an abnormal phenomenon associated with capacity increase,that has also been reported in some other MOF anodes,which,however,remained unsolved.Consequently,the underlying mechanism for such a behavior was studied through ex-situ X-ray absorption fine structure spectroscopy and electron paramagnetic resonance techniques.It was found that the Cu²⁺center of MOF was also electrochemically active during cycling,and it was almost completely converted to metal Cu after the first discharge,while the copper in Cu⁰ oxidation states produced in the first discharge was oxidized back to Cu_x O in a step-by-step manner with prolonged cycles,resulting in the gradual increase of specific capacity during cycling.4.A simple and scaleable strategy was successfully developed to synthesize uniform Co_xMn_3-xO₄ nanoparticles with an adjustable composition and grain size,by adopting conventional pyrolysis approach with bimetal-organic frameworks as templates.A series of mixed-metal organic framework precursors with an accurate mole ratio of Mn/Co were firstly constructed solvothermally by solely tuning the molar ratio of two metal salts.After post calcination,phase pure Co_xMn_3-xO₄ was obtained.The grain sizes of the Co_xMn_3-xO₄ products were readily fine-tuned via altering the decomposition temperature.Specifically,the electrochemical characteristics of the MnCo₂O₄ nanomaterials for LIBs were examined as a case study.Our experimental results showed that an optimal size ensures maximum capacity maintenance.5.An eco-friendly,mild,easy,and scalable strategy was exploited with success to elaborately construct 3-D layer-by-layer manganese oxide?MnO_x?hierarchical mesoporous microcuboids from a Mn-MOF-74-based template,employing a one-step solution-phase protocol at room temperature.Through the regulated exchange of MOF linker with OH^-in alkali aqueous solution and in situ oxidation of manganese hydroxide intermediate,the Mn-MOF-74 precursor were easily transformed into Mn₃O₄ and?-MnO₂ products composed of basic nanoparticles and nanosheets units,respectively,with well-preserved morphology.By various characterization analysis,their structures were unambiguously revealed.Particularly,their Li storage properties were well assessed,and it was found that these special 3D microcuboids could realize a maintained predominant Li storage capacities especially at high current discharge because of the rationally designed structural features(Mn₃O₄ microcuboids:890.7 mA h g^-1 after 400 cycles at 0.2 A g^-1,and 767.4 mAh g^-1,560.1 mAh g^-1 and 437.1 mAh g^-1 after 400 cycles at 0.5 A g^-1,1 A g^-1 and 2 A g^-1,respectively;?-MnO₂microcuboids:991.5 mA h g^-1 at 0.2 A g^-1 after 400 cycles,and 660.8 mA h g^-1,504.4mA h g^-1 and 362.1 mA h g^-1 after 400 cycles at 0.5 A g^-1,1 A g^-1 and 2A g^-1,respectively).As far as we know,this is the most stable high-rate performance,as well as largest reversible capacity ever reported for pure MnO_x anodes,and even exceeds majority of their composites.This mild,green and economical strategy updates the conventional MOF-based approach for highly tailorable functional materials and offers new opportunities for high-performance metal oxide electrodes.6.Hierarchical porous CuO octahedron?HPCO?electrode was successfully constructed based on the abovementioned solid-solid transformation route by treating a copper MOF template,i.e.,Cu-BTC,with an alkaline solution at room temperature.Through the exchange of MOF ligand with OH^-in alkaline aqueous solution and in situ reconstructive transformation of Cu?OH?₂ intermediate,the Cu-BTC template/precursor were easily transformed into HPCOs assembed by numerous ultrathin nanosheets with average lateral sizes of ca.250 nm,which substantially retained the morphology and size of Cu-BTC.When serving as a lithium storage material,the as-constructed HPCO electrode displayed unparalleled properties due to its preponderant structural characteristics,with an ultralarge capacity of 1201 mAh g^-1and excellent rate capability with predominant cycling stability(1062,615,and 423mAh g^-1 at 0.5,2,and 5 A g^-1,after 200,400,and 400 repeated cycles,respectively).Noticeably,we also found that a remarkable surface redox pseudocapacitive behavior led to the high capacity and rate capability of the HPCO electrode.This inspiring result may further promote the progress of LIBs by a smart strategy of metal oxide-based electrode architecture design.