Devices Construction And Storage Mechanism For Sodium(Potassium)-Ion Capacitors

Currently,with the explosive development of smart grids,near space aerocraft systems,and electric vehicles,the highly efficient energy storage devices will play a more and more important role.Unfortunately,the current energy storage devices,such as lithium-ion batteries(LIBs),supercapacitors(SCs),are unsatisfactory due to their inherent defects.LIBs provide high energy density,but the power density is low.On the contrary,SCs suffer from the low energy density,although the power density and lifespan are very good.Besides,the Li-based energy storage may eventually render Li resources unaffordable since Li reserves are limited and its distribution is geopolitically restricted.Accordingly,pursuing sustainable energy storage devices with high energy density and high power density is becoming increasingly important.Na and K have the similar physical/chemical characteriatics with Li,while Na and K resources distribute everywhere and the cost is low.Therefore,Na and K-based energy storage can be perceived as one of the next generation energy storage technologies.Aiming to develop highly efficient energy storage devices with high energy-power densities,long lifespan,and low cost,this dissertation builds Na(K)-based hybrid capacitors including rational design of electrode materials,charge storage mechanism,and full cells optimization,etc.(1)Preparation of conductive metal-organic framework(MOF)and its application in Na-ion capacitors:The conductive MOFs possess high intrinsic conductivity,ordered porous structure,which provide the rapid transmission path of electrons and ions,and enhance the rate performance of Na-ion storage.Here we synthesise a 2D Ni-based conductive MOF(denoted as Ni-MOF)with a Ni(II)center and an active linker,hexaaminobenzene(HAB)under stirring at room temperature.As an anode material of Na-ion storage,Ni-MOF provides the maximum capacity of 517.2 m Ah g^-1.During the discharge process,the reduction reaction mainly occurs on the linker of HAB instead of the center of Ni(II)above 0.5 V(vs.Na⁺/Na).The reversible capacity is about 297.3 m Ah g^-1 with good rate performance and cycling stability during the potential window of 0.5-3.0 V.Furthermore,we construct a Na-ion capacitor based on Ni-MOF negative electrode and AC/PEDOT@Na₃V₂O₂(PO₄)₂F(mass ratio of AC and PEDOT@Na₃V₂O₂(PO₄)₂F is 1:1)positive electrode with high energy-power densities and long cycling performance.The AC/PEDOT@Na₃V₂O₂(PO₄)₂F//Ni-MOF configuration achieves a high energy density of 70.6 Wh kg^-1 and high power density of 4000 W kg^-1 on the basis of the active mass of both electrodes,and a prominent cycling stability over 5000 cycles(capacitance retention 73.2%)under 1 A g^-1.(2)Preparation of graphene-based electrode materials and the construction of quasi-solid-state Na-ion capacitors:A 3D self-surpported grephene foam(donated as GFs)positive electrode is prepared by using Ni foam as a template.We also prepare self-surpported Na₂Ti₃O₇nanoribbon array/grephene foam(NTO/GFs)negative electrode,without any binders,conducting additives and metal current collectors.The Na-ion conducting gel polymer electrolyte is prepared by soaking P(VDF-HFP)membrane in an organic electrolyte.Benefiting from the unique 3D self-supported positive electrode and negative electrode,the GFs//NTO/GFs quasi-solid-state configuration achieves a high energy density of 70.6 Wh kg^-1 and high power density of 4000 W kg^-1on the basis of the active mass of both electrodes,and a prominent cycling stability over 5000 cycles(capacitance retention 73.2%).(3)Preparation of flexible Na₂Ti₃O₇ electrode and the construction of flexible Na-ion capacitors:In order to meet the demonds of wearable devices,we construct a new class of flexible power called flexible Na-ion capacitor.Firstly,the flexible Na₂Ti₃O₇/carbon textile(NTO/CT)negative electrode is fabricated by directly growing of NTO nanosheet arrays on CT with robust through a simple hydrothermal process and the following thermal treatment.The 3D highly electronic conductive carbon fibers provide an expressway for electron transmission.The nano-structured NTO nanosheet arrays shorten the Na-ions and electrons diffusion paths.The flexible NIC is assembled by using NTO/CT negative electrode and reductive graphene oxide(r GO)positive electrode,and then seal with poly(dimethyl siloxane)(PDMS).The flexible r GO//NTO/CT configuration achieves a high energy density of 55 Wh kg^-1 and high power density of 3000 W kg^-1.Taking the fully packaged flexible NIC into consideration,the maximum practical volumetric energy density and power density reach up to 1.3 m Wh cm^-3 and 70 m W cm^-3,respectively.In addition,the flexible NIC demonstrates a stable electrochemical performance with almost 100%capacitance retention under harsh mechanical deformation.(4)Preparation and K-ion storage of K₂Ti₆O₁₃ nanocages:Because of the richness of potassium resources,K-ion batteries(KIBs)have attracted more and more attention.However,the previous reported KIBs are still unacceptable for practical applications due to their poor cycling performance.To maximize the K-based systems'cycling life,we use a capacitive positive electrode(N-doped nanoporous graphenic carbon,donated as NGC),K₂Ti₆O₁₃(KTO)nanocages negative electrode,and then assemble a K-ion capacitor.KTO nanocages construct by cross-linked nanorods with outstanding capacity retention of 84%after 1000 cycles.In addition,the NGC//KTO KIC delivers a high energy density of 58.2 Wh kg^-1 based on the active mass in both electrodes,high power density of 7200 W kg^-1(based on the total active mass of both electrodes),and outstanding cycling stability over 5000cycles(capacity retention of 75.5%).(5)Preparation of V₂O₅?n H₂O nanoribbon and its pseudocapacitive behavior:Compared with Li-ion and Na-ion,hydrated K-ion possesses the smallest hydration radius,lowest de-hydration energy and highest ion mobility in aqueous electrolytes.Therefore,K-ion storage usually has the best electrochemical performance.Although NH₄-ion,with similar hydration radius,ion mobility and de-hydration energy of K-ion,we find that NH₄-ion storage has a better electrochemical performance than K-ion storage in V₂O₅?n H₂O nanoribbons.The further studies find that the H-bonding between NH₄-ion and V₂O₅ will strengthen the pseudocapactive behavior.The presence of highly directional H-bonding and charge sharing in the case of NH₄-ion and V₂O₅·0.5H₂O is demonstrate by experiment and first principle calculation.The evidence for this H-bonding is seen experimentally in the transformed vibrational behavior in FTIR as well as the ¹H chemical shifts in solid-state NMR.Furthermore,first principles electronic structure calculations show charge transfer from VO layers to the inserted NH₄-ion.This phenomenon is analogous to the chemisorption of NH₄-ion between the V₂O₅ bilayers.This new discovery will strengthen the understanding of intercalation pseudocapacitance.We tentatively name this category of chemisorption-involved intercalation pseudocapacitance as CI pseudocapacitance.