Synthesis Of Al-MIL-96 Derivatives And Their Application In Li-S Batteries

With the large-scale popularity of green power generation technologies such as nuclear,wind,and solar energy in China.The current progress of nuclear fusion in the field of electric energy is a foreseeable future.Therefore,electric mobility devices have great potential for future sustainable development compared to fuel mobility devices.And the battery,one of the most important components,is a key factor in determining the popularity of electric mobility devices.Currently the most used lithium-ion battery(LIB)is facing a shortage of cathode materials.For example,the prices of cobalt,nickel,and manganese have been increasing in recent years,the limited theoretical specific capacity and safety of LIB cathode materials are also constraints to further improve their performance.Lithium-sulfur battery(LSB)is attracting attention due to the advantages of high theoretical specific capacity,abundant sulfur reserves as cathode material.However,due to the poor electrical conductivity of sulfur and the intermediate product produced at the anode during the discharge process,lithium polysulfide(LPS,chemical formula:Li2Sn,4≤n≤8)dissolves into the electrolyte and diffuses through the separator to the negative electrode.The LPS can react directly with the lithium metal at the negative electrode,resulting in irreversible loss of active material,this is the shuttle effect,which makes the rapid drop of cyclic performance.Researchers have found that selecting suitable carriers to load the sulfur and reduce the direct contact with organic electrolyte can effectively solve the above problems.Among many reported carrier materials,metal-organic framework(MOF)is the most attractive one because of its porous structure,rich Lewis acid sites,easy to synthesize in large quantities,and various morphologies.However,the low electrical conductivity of most MOF materials,the poor utilization of active sites due to the separation of metal active sites by organic ligands,and the lack of catalytic effect on LPS have greatly limited their practical applications.In this thesis,different functionalized modification strategies are designed to address the problems of Al-MIL-96 as a carrier material for sulfur cathode,such as synthesizing Al-MIL96 crystals with different degrees of crystalline growth through the selection of different polar solvents,introducing a second metal to increase the pore structure diversity of Al-MIL-96,increasing the Lewis acid active site,obtaining high conductivity with high temperature selfreduction and having the composite materials of metal nanocrystals and heterogeneous structure of the catalytic effect,and the core-shell structure obtained by homogeneous coating of conductive polymers modified Al-MIL-96,which effectively increased the sulfur fixation effect,adsorption and catalytic performance of the carrier material and prepared sulfur cathode with high specific capacity and cyclic stability.The morphology,composition,and electrochemical properties of the materials were analyzed by various characterization methods,and the growth mechanism and charging/discharging mechanism of the aluminum-based MOF composites were discussed.The thesis contains five main parts as follows:1.Through the selection of different polar solvents,Al-MIL-96 crystals with different morphologies,named:hexagonal sheet crystal(HPC),hexagonal biconical crystal(HBC),and hexagonal prismatic cone crystal(HPBC),were obtained and used as sulfur carriers to study the effect of morphology on the suppression of shuttle effect.Later,HBC crystals were selected as the target,and a series of micro/nanoscale crystals with sizes ranging from 3.33 to 0.78μm were obtained by changing the volume ratio of co-solvent,so as to investigate the effects of different sizes of MOF as sulfur carriers on LSB performance.Density functional theory(DFT)calculations illustrate that different crystalline faces of Al-MIL-96 have different binding energies for Li2S4,implying different shuttle effect suppression effects,and macroscopic Li2S4 adsorption test verify the results of this calculation.Galvanostatic charge discharge(GCD)tests also demonstrate that the HBC crystals with(101)the largest proportion of active crystalline surface area have the highest specific capacity and cycle stability,and the sulfur loading gradually increases as the particle size decreases,which not only obtains higher active material utilization efficiency,but also obtains higher cycle stability.The reversible specific capacity of 448.0 mAh g-1 was obtained after 200 charge/discharge cycles at 0.5 C.2.Compared with other MOFs,Al-MIL-96 contains three different types of pores,and thus has a "flexible" structure to allow the introduction of other metal ions.In addition,AlMIL-96 contains a large number of hydroxyl groups,which can effectively bind metal ions to participate in coordination.Based on the above favorable factors,in this chapter,four metal ions(Mn2+,Co2+,Ni2+,Zn2+)were introduced into HBC crystals,and the effect on the morphology was studied in the ratio of three different metal ions to finally determine the appropriate metal ions and ratio to obtain a homogeneous bimetallic MIL-96.DFT calculations and Li2S4 macroscopic adsorption test show that the Ni2+-doped bimetallic MOF has the highest binding energy to Li2S4 and S8 molecules,indicating that it has the strongest adsorption and sulfur fixation effect on LPS when used as a sulfur carrier.The results of cyclic voltammetry and GCD tests demonstrate that the Ni2+-doped bimetallic MOF has a high current response and a stable cyclic performance.3.Recent studies on the charging and discharging mechanism of LSB have found that,in addition to the effect of shuttle effect,the intermediate product of LPS is further converted to short-chain lithium sulfides as a slow kinetic process.It is shown that metal nanocrystals can be used as effective catalysts to accelerate the conversion of LPS and improve the reaction kinetics.In this chapter,the composite of amorphous Al2O3/C and metal nanocrystals as sulfur carriers was prepared by the limiting domain self-reduction method under nitrogen atmosphere using the bimetallic MIL-96 obtained above as the precursor and guided by the oxygen potential diagram.The characterization results revealed that Ni2+ introduced bimetallic MIL96 successfully obtained Ni nanocrystals(NiNC),which were uniformly dispersed inside the composites,giving them effective chemisorption and catalytic activity towards LPS.This synthetic route can confine NiNC inside the nanomaterials,thus exhibiting more effective catalytic activity and stability.The rigidity of Al2O3 can support the integrity of the nanocomposite skeleton for sulfur storage and exhibit chemical inertness with LPS,thus acting as a "rigid unit" in the composite;while NiNC can catalyze the conversion of long-chain LPS,acting as a "flexible unit" in the composite.NiNC can catalyze the conversion of long-chain LPS and act as a "flexible unit" in the composite.This "rigid-flexible" carrier material is characterized by both stable framework and catalytic activity during the long cycling process,thus achieving excellent LSB performance(initial specific capacity of 1233.8 mAh g-1 at 0.5 C).SEM images of the electrodes after long cycling showed the stability of the microstructure,and in situ UV-vis tests demonstrated the catalytic activity of NiNC.In order to fully demonstrate the catalytic activity of NiNC,the contents of this chapter were used to remove the NiNC inside the nanocomposite by acid washing in a counterfactual method,and the resulting comparison samples were loaded with sulfur and assembled cells by the same process,and the performance of the resulting LSB decreased dramatically.4.The Ni/Ni5P2 heterostructure was prepared by converting part of NiNC in the amorphous Al2O3/C composite into nickel phosphide by in situ phosphorylation,which enabled the composite to combine the advantages of different components and realized the simultaneous promotion of adsorption and catalysis of the carrier material in LSB with only one material.Among them,Ni5P2 inhibits the shuttle effect by chemisorption of LPS;NiNC acts as a catalyst for LPS conversion to accelerate the formation of short-chain lithium polysulfide.During the synthesis process,samples with homogeneous phosphorylation degree were obtained by controlling the mass ratio of precursor to phosphorylate,and the obtained heterostructured materials obtained significant effects in suppressing shuttle effect and catalyzing LPS conversion.The inherent amorphous Al2O3 and carbon skeleton of the composites serve to support the stability of the carrier material and improve the electrical conductivity,respectively.Macroscopic adsorption experiments proved its high adsorption effect,and electrochemical test results showed that the heterostructured composites obtained a specific capacity of 1596.0 mAh g-1 at a current density of 0.5 C and retained a reversible specific capacity of 619.7 mAh g-1 after 100 cycles.5.The chemical stability of Al-MIL-96 in water facilitates the compounding of conducting polymers in aqueous reaction media,while the higher thermal stability allows the structural stability to be maintained when loaded with sulfur in the hot-melt process.In this chapter,the HBC-type Al-MIL-96 in chapter 2 was chosen as the sulfur carrier,and after sulfur was loaded by hot-melt method,a layer of conductive polymer,polypyrrole(PPy),was successfully coated on its surface by aqueous-phase oxidative polymerization.Interestingly,the PPy coating process produces a core-shell structure,whose inner core can act as a loading surface,providing more active sites and therefore significantly improving the space utilization efficiency;the inner cavity structure can act as a buffer when the electrode reaction causes a huge volume change.Therefore,this core-shell structure materials have high cycling stability when used as LSB cathode materials.Macroscopic adsorption experiments demonstrate that the core-shell structure can effectively adsorb LPS and show good cycling performance,which can be stabilized at 308.7 mAh g-1 after 200 cycles at a current density of 0.5 C.