Study On The Application Of Metal Organic Frameworks (MOFs) In Energy Storage

With the increasingly severe energy problems and the utilization of renewable energy,energy storage is becoming more and more important.Therefore,more and more energy storage devices are being researched and applied.To meet the requirements of high-efficient and high-speed in energy storage,the energy density of energy storage devices has attracted more and more attentions.Among the commercial energy storage devices,lithium iron phosphate(LiFePO₄,LFP)batteries are widely applied because of their advantages such as low cost,environmental protection,non-toxicity,and stable discharge voltage.Its energy density can be further enhanced by changing graphite anode to lithium anode,and improving the high-C rate performance of LiFePO₄ cathode.Meanwhile,the research and development of new-type energy storage devices with high energy density are also important.As a kind of new-type energy storage devices,zinc-air battery is a typical high energy density device because the cathode material of oxygen does not take up the volume of the battery and zinc anode has high energy density.However,LiFePO₄ battery and zinc-air battery face some problems due to the unideal transports of electrons and ions.For example:(1)unbalanced transports of electrons and lithium ions on the surface of lithium anode lead to the growth of lithium dendrite,resulting in reducing battery cycle life;(2)low-speed transports of electrons and lithium ions in LiFePO₄ cathodes deliver the poor high-C rate performance;(3)low-speed transports of electrons and OH*ions in cathode catalysts result in low cathode catalytic kinetics.Aim to these problems,this thesis studies on the application of metal organic frameworks(MOFs)in the conductivity enhancements of LiFePO₄ batteries and zinc-air batteries.The main contents and conclusions of this research are as follows:1.To solve the problem of lithium dendrite growth caused by unbalanced transports of electrons and lithium ions on the surface of lithium anode,porous carbons(PCs)were obtained by annealing zeolitic imidazolate framework-8(ZIF-8)precursor,as the surface modified agency,suppressing lithium dendrite growth.ZIF-8 nanocrystal materials with multi-ordered porous structures were synthesized by precipitation method.The graphite-type carbon materials with multi-stage porous structures and ordered mesoporous properties were optimized by annealing ZIF-8 under N₂atmosphere.As the surface modifier,the optimized porous carbon materials were coated on the surface of lithium anode,they can effectively balance the distribution of electrons and lithium ions during the charge and discharge cycle to avoid the orientation deposition of lithium;on the other hand,the mesoporous structure and particle accumulation voids in the porous carbon material can provide rich nucleation sites for the deposition of lithium by increasing the specific surface area,and control the deposition of lithium ions along the surface of the carbon material,thereby suppressing the growth of lithium dendrites.Combined with LiFePO₄ cathode in lithium ion batteries,the capacity retention rate of modified lithium anode was increased from 42.3%to 90.1%after 200 cycles at 0.5 C between 2.5 to 4.2 V.2.For the problems of low-speed transports of electrons and lithium ions in LiFePO₄ cathodes delivering the poor high-C rate performance,the commercial LiFePO₄ cathode materials were modified by utilizing the order mesoporous carbon materials containing graphite and element zinc derived from ZIF-8,through the physical(adding)and chemical(coating)strategies.(1)Physical strategy:When the mesoporous carbon composites(MC-GZQDs)containing hetero-core(metal Zn)and shell(graphite)quantum dots were used as additives,the mesoporous structure,high electron conductivity,and quantum tunneling effected act synergistically on LiFePO₄cathode materials,the electron and lithium ion transmission speeds of commercial LiFePO₄ cathode materials were effectively improved(the lithium ion transmission coefficient is increased to 12 times).Between 2.5 and 4.2 V,the specific discharge capacity of the LFP@MC-GZQDs cathode material was increased from 148.5 m Ah g^-1 to 154.6 m Ah g^-1 at 0.5 C compared with LFP cathode material;the capacity retention rate can still reach 99.9%after 60 cycles at 10.0 C.(2)Chemical strategy:The non-uniform nucleation and carbonization of the ZIF-8 crystal materials were used to coat the commercial LiFePO₄ cathode materials to strengthen the bonding between the mesoporous carbon material and LiFePO₄,and further increase its electron and lithium ion transmission speeds.The coating layer thickness of the optimical LFP/C_ZIF-8 sample was about 10 nm,and the electron and ion transmission speeds of the LiFePO₄ cathode material were further increased(the lithium ion transmission coefficient was increased to 17 times).Between 2.5 to 4.2 V,the specific discharge capacity of the LFP/C_ZIF-8composite cathode material was increased from 122.0 m Ah g^-1 to 159.3 m Ah g^-1 at 0.1C;the discharge energy density was increased from<50 m Wh g^-1 to 141.7 m Wh g^-1after 200 cycle at 5.0 C.3.Aiming at the problem of low cathode catalytic kinetics caused by low-speed transports of electrons and OH*ions in zinc-air battery cathode,zeolitic imidazolate framework-67(ZIF-67)was selected as template precursor to design the composite bifunctional electrocatalyst with cobalt compound and carbon materials.The sulfur,phosphorus,nitrogen tri-doped carbon(SPNC)with cobalt phosphide nanoparticles(Co PS)composite electrocatalyst(Co PS@SPNC)was prepared by the carbonization,sulfurization and phosphating of ZIF-67.The overpotential of the catalytic reaction was effectively reduced by the doped carbon materials with sulfur,phosphorus,and nitrogen elements.The ordered mesoporous structure of catalyst and Co PS nanoparticles provided rich catalytic sites for catalytic reactions,accelerating the transmissions of electron and OH*ions.Therefore,the oxygen reduction(ORR,Tafel slope was 18.2m V dec^-1)and oxygen evolution(OER,Tafel slope was 22.2 m V dec^-1)of the electrocatalyst were effectively improved.The first principles calculations(DFT)show that the Co PS@SPNC-600 sample has the lowest overpotential;the rate determination step has the lowest reaction free energy;the interaction between Co PS@SPNC-600electrocatalyst and OH*becomes weaking after phosphorus doping,then reducing the energy barrier of the OH*desorption process;Co PS combined with SPNC shows more positive charges,indicating its higher catalytic activity.A zinc-air battery containing the Co PS@SPNC-600 electrocatalyst showed an overpotential of 0.20 V and a discharge voltage of 1.36 V.In addition,the flexible zinc-air battery assembled with Co PS@SPNC-600 electrocatalyst also exhibited excellent electrochemical performance:the cycle life exceeded 80 h at a current density of 2 m A cm^-2,the energy efficiency was always maintained above 80%,and the stable electrochemical performance was maintained at different angles.