Controlled Synthesis Of Sodium Titanate And Sodium Titanate-based Composites For Na Ion Capacitors

Sodium-ion energy storage systems（SIESS）have received considerable attention due to the wide availability of sodium resources and similar intercalation electrochemistry compared with lithium-ion energy storage systems,which are regarded as promising candidate for electrochemical energy storage devices for large-scale applications in the future.Among the electrode materials for SIESS,layered sodium titanate materials（with a large interspacing of≈0.7 nm）have low potentials for Na⁺intercalation/de-intercalation.Additionally,sodium titanate nanoarrays can be easily synthesized by simple hydrothermal growth.Therefore,sodium titanates are considered as one of the potential negative materials for SIESS.However,the major challenge is that its low initial coulombic efficiency（ICE）and sluggish Na⁺reaction kinetics seriously restrict the application of sodium titanate electrode.In this thesis,taking Na₂Ti₂O₅ anode as an example,we detailly report the synergistic effect between ether electrolyte and binder-free array to simultaneously achieve superior ICE and ultrafast Na⁺insertion/extraction kinetics for Na ion capacitors.Furthermore,with the introduction of heterogeneous species into the layered structure of sodium titanate,significant Na⁺intercalation becomes possible even in aqueous electrolytes for aqueous Na ion capacitors.The main contents are as follows:（1）Sodium titanate array electrode and powder electrode are synthesized by simple hydrothermal method.Based on electrochemical performance of electrode in six different electrolytes,it is found that nanoarray with ether electrolyte displays superior ICE of 91%,outstanding cyclic stability,and excellent rate performance with66%capacity retention at high current density of 120 C.The excellent electrochemical performance of the array electrodes in ether electrolytes is explained based on the results from transmission electron microscopy,impedance measurement and dynamic analysis.The key to the superior performance lies in the synergistic promotion between electrolyte with thinner and more stable solid electrolyte interphase and 3D electrode architecture with short ion/electron transport path.Moreover,the Na ion capacitor using sodium titanate as anode and the activated carbon as cathode also delivers the high ICE and superior cyclic stability（>10000cycles）.Our findings open opportunities for taking advantage of nanoscale effect to the most extent,while achieving high-CEs for electrodes.（2）It is further revealed that the cycling and rate performance of sodium titanate electrode in aqueous electrolytes can be improved by introducing iron-based active species into the layered structure.Iron-sodium titanate is synthesized by in situ introducing iron active substances during hydrothermal process,and the electrochemical performance of iron-sodium titanate electrode is also investigated in detail in sodium sulfate electrolyte.Interestingly,with the increase of sweeping rate,the positions of redox peaks almost remain unchanged.To certain extent,the electrode can suppress the hydrogen evolution and work efficiently within a wide electrochemical window of-1-0.2 V vs.saturated calomel electrode.The long-term cyclic performance（8661 cycles）of iron-sodium titanate is also presented with capacity retention of 86%at 0.5 mA cm^-2.In addition,in different neutral/alkaline electrolytes of lithium,sodium and potassium ions,the electrode can always exhibit stable electrochemical performance.It is expected that aqueous Na ion capacitors can be designed using commercial activated carbon as cathode and iron-sodium titanate as the anode in the future.