Investigations Of High-performance Electrode Materials For Dual-ion Batteries And The Energy Storage Mechanism

With the rapid development of portable electronic devices and electric vehicles,energy storage systems with excellent performance have attracted much attention from all over the world.Batteries have high energy densities but their energy storage mechanism is controlled by diffusion resulting in low power densities;on the other hand,supercapacitors own excellent rate capabilities but suffer from low energy densities because they store charges on the surface of active materials.The integration of high energy density,high power density and high stability compatible in one system is always the goal in energy storage and material science.To address this issue,one should pursue innovations from the system.Dual-ion batteries（DIBs）,using anions and cations in the electrolyte for intercalation/de-intercalation in both negative and positive electrode,is a novel attempt in system innovation,which can pave a new path to realize the compatibility of high energy density,high power density and high stability.Herein,based on the current situation,this thesis has focused on constructing novel DIB configurations,synthesis and characterization of electrode materials for intercalation/de-intercalation of various ions and probing the intercalation mechanism and the interaction of electrolyte ions and active materials.The main contents of this thesis are concluded as follows:1.We have fabricated a novel DIB that exhibits the compatibility of high energy density and high power density.In this DIB,graphite,graphene oxide and[EMIm]⁺PF₆^-ionic liquid are used as the positive,negative electrodes and the electrolyte,respectively.PF₆^-ions intercalate into graphite which proceeds in a high potential（5 V）that can boost the energy density;[EMIm]⁺ions can be stored or released through fast surface adsorption/desorption,which can provide the power density.PF₆^-ion intercalation/de-intercalation results in a periodic variation of interlayer spacing in graphite.Combined with ex situ XRD and electrochemistry characterization,the stage mechanism of PF₆^-ion intercalation/de-intercalation in graphite has been proved.On the basis of quick absorption/desorption of electrolyte ions in graphene electrode,the present dual-ion battery yields a high power density of 1333 W/kg while delivering energy density of 70 Wh/kg.2.We have synthesized Co₃O₄/carbon fiber paper（CFP）composite throughelectrodeposition and high temperature annealing methods.The electrodeposition introduces the chemical bonding between Co₃O₄ and CFP which bypasses the use of binders and conductive additives.The vertical orientation of Co₃O₄ in CFP promises the high utilization ratio of the active material and this structure can also buffer the volume changes during the charge/discharge cycles.After 40 cycles,the energy retention of Co₃O₄/CFP reaches 90%of its initial capacitance.On the other hand,CFP can also be used as the active material for ion intercalation because of its unique structure.The three-dimensional structure offers both conductivity and structure integrity,while the highly graphitic fiber paper endows the high intercalation potential towards high energy density.High stability is resulted from such integrative electrode structure and the mixing crystal and amorphous structure in CFP can buffer the volume expansion during the intercalation process.After 1000 cycles,the energy retention of CFP can reach 90%of its initial capacitance.Then a DIB has been fabricated by using Co₃O₄/CFP composite,pure CFP and Li PF₆ as the negative,positive electrodes and the electrolyte,respectively.This DIB shows stable and remarkable performance,the cell voltage is 2.5 V and the energy density can reach 72 mAh/g.3.Lithium titanium oxide(Li₄Ti₅O₁₂,LTO)is one appealing active material because it can offer a higher operating voltage of¹.55 V versus lithium,which not only prevent the lithium dendrite,but also avoid the formation and growth of the anode SEI that slows down Li insertion and induces Li losses in graphite anode.However,the safety advantage of LTO reduces the cell voltage,which together with its low capacity（175 mAh/g）might cut down the energy density.But in DIBs the anion intercalation potential can reach as high as 5 V,which can effectively neutralize the negative effect of LTO’s 1.55 V operating potential.Herein,by employing commercial graphite,LTO and LiPF₆ as positive,negative electrode and the electrolyte,we successfully fabricated a delicate-designed DIB,which can deliver a maximum energy density of 109 Wh/kg at a power density of 35 W/kg,and the maximum power density of 2933 W/kg can be obtained with a remaining energy density of 11 Wh/kg.This DIB is compared to a Li Fe PO₄（LFP）/LTO cell,which shows inferior performance at all current densities.We also conducted a series of density functional theory（DFT）calculation to investigate the mechanism of PF₆^-ion intercalation in graphite.The slow Li ion diffusion in LFP is replaced by the fast ion intercalation mechanism in graphite,and ion transport path in electrolyte is shortened to enhance the rate capability of the DIB cell.And this DIB shows an excellent stability,after 50 cycles the capacitance holds 98%retention of its initial capacitance.This novel DIB configuration not only neutralize the negative effect of LTO’s high operating potential,but also draw the advantages of LTO to enhance the overall performance of the DIB.4.Understanding the interaction between the active material and the electrolyte is the premise to improve the overall performance of DIBs.Tuning the interlayer spacing of the active material for ion intercalation is one feasible method to investigate the relationship between the interlayer spacing,ion size and the subsequent performance.Since the interlayer spacing of two-dimensional transition metal carbides（MXenes）is tunable,we have systematically investigated the interaction between the structure and intercalation ion.To synthesize Ti₃C₂T_x MXene,we selectively etched Al from the Ti₃AlC₂ compound.Ti₃C₂T_x MXene is thoroughly characterized by using XRD,SEM,HR-TEM and XPS measurements.Ti₃C₂T_x shows the similar intercalation behavior of Nb₂O₅ as the negative electrode in DIB,and the capacitance of Li⁺ion intercalation can reach 200 mAh/g under the current density of 100 mA/g.As the positive electrode,cations from the electrolyte are stored in the surface of Ti₃C₂T_x,which shows the characteristics of supercapacitors.And the capacitance can reach 25 F/g under the current density of 500 mA/g.During the charge/discharge cycle,the Ti₃C₂T_x structure shows periodic variation detected by ex situ XRD.Based on the cyclic voltammetry curves,we have calculated the dynamics of the intercalation in Ti₃C₂T_x indicating the fast charging rate.