Structure And Electrochemical Performance Tailoring Of SrCoO₃-Based Perovskite

In the current research on supercapacitors,it’s critical to improve energy density while ensuring high power density and high cycle life.As a new type of intercalated electrode material,perovskite oxide（ABO₃）has received widespread attention due to its unique structural compatibility and physicochemical properties,which is expected to obtain excellent electrochemical performance.Among them,SrCoO₃ is a promising electrode material for supercapacitors owing to the excellent mixed ion and electronic conductivity（MIEC）,good oxygen permeability,and electrocatalytic activity.In this thesis,structure and electrochemical performance of SrCoO₃ perovskites were systematically tailored by different B-site cation substitution.And hybrid supercapacitors were designed to explore their potential application in energy storage.The main conclusions are as follows:（1）A series of Ti-substituted perovskites Sr Co_1-xTi_xO_3-δ（x=0,0.05,0.10,0.15,0.20）were synthesized as anion-intercalated supercapacitor electrode materials.The parent SrCoO_3-δcontained two phase:a hexagonal structure with space group R^<sub>3:H（87.09 wt%）and a simple cubic structure with space group Pm^<sub>3m（12.91 wt%）.After Ti substituted,a stable cubic single-phase structure with a space group of Pm^<sub>3m was obtained,suggesting that Ti substitution stabilized the phase structure of SrCoO_3-δ.At the same time,the electrochemical performance was optimized notably after Ti substitution.Wherein,Sr Co_0.9Ti_0.1O_3-δexhibited the best electrochemical performance and Sr Co_0.9Ti_0.1O_3-δ@CC electrode supported by carbon cloth obtained a specific capacity（Q_s）of 349.1 C g^-1 at 1 A g^-1.The excellent electrochemical performance of Sr Co_0.9Ti_0.1O_3-δwas attributed to its stable cubic structure,high conductivity,and abundant oxygen vacancies.In addition,a hybrid supercapacitor（HSC）Sr Co_0.9Ti_0.1O_3-δ@CC//AC@CC combined with an activated carbon electrode could provide an energy density of 36.2 Wh kg^-1@830.6 W kg^-1 at 1 A g^-1 in an operating voltage of 1.6 V.（2）To further explore the impact of high valence ion substitution and temperature characteristics on SrCoO_3-δelectrodes,a series of Sr Co_1-xTa_xO_3-δ（x=0.05,0.10,0.15,0.20）perovskites were designed and synthesized,and the temperature dependent electrochemical properties was systematically studied.The Ta-substituted SrCoO_3-δperovskite transformed to tetragonal structure with space group of P4/nnc.The introduced of Ta⁵⁺not only stabilized the crystal structure,but also significantly improved the electrochemical activity of SrCoO_3-δ.And the Sr Co_0.95Ta_0.05O_3-δsample had the highest specific capacity of all components.The Q_s of Sr Co_0.95Ta_0.05O_3-δ@CC electrode increased from 226.35 C g^-1 to 545.10 C g^-1 with a temperature change from25 to 85℃at 2 m A cm^-2,and the capacity increased by 140.8%.The specific capacity（Q_A）of the Sr Co_0.95Ta_0.05O_3-δ@CC//AC@CC device could reach 102.7 C g^-1at 1 A g^-1,and the energy density was 22.82 Wh kg^-1@775.09 W kg^-1 at 25℃.When the temperature rose to 85℃,the Q_A increased to 127.19 C g^-1 with an amplification of23.8%due to the enhanced conductivity of electrolyte.In addition,the assembled hybrid supercapacitor Sr Co_0.95Ta_0.05O_3-δ@CC//AC@CC obtained a stable long-term cycle life with a capacity retention of 90.7%after 5000 charge-discharge cycles.（3）In order to establish the tailoring rules of structure and electrochemical performance of SrCoO₃-based perovskite,Sr Co_0.875M_0.125O_3-δ（M=Nb,Ti,Fe,Ta）perovskites with different B site cation substitution were prepared by solid state reaction method.Sr Co_0.875Ti_0.125O_3-δwas identified to a simple cubic structure,while the Nb-,Ta-,and Fe-substituted SrCoO_3-δperovskites were all defined to a tetragonal structure.At a current density of 1 A g^-1,the specific capacity of Sr Co_0.875M_0.125O_3-δ（M=Nb,Ti,Fe,Ta）perovskites were 984.83,853.33,503.53,452.00 C g^-1,respectively.When the current density came to higher,the performance of Sr Co_0.875Fe_0.125O_3-δcame to the worst.Wherein,the[CoO₆]octahedron in Sr Co_0.875Fe_0.125O_3-δwas severely twisted,and two Co atoms in the cell lost four coordination oxygen,thus seriously preventing the oxygen diffusion in Sr Co_0.875Fe_0.125O_3-δand leading to the lowest specific capacity.In addition,compared with the high valence state of Ta⁵⁺and Nb⁵⁺,Ti⁴⁺substituted SrCoO₃ perovskite contained more oxygen vacancies available for O^2-intercalated theoretically,which would perform a better charge storage characteristic.However,the valence variation of Nb ions could compensate partial capacity,making it exhibit superior electrochemical behavior to that of non-variable valence Ta ions.Besides,the Sr Co_0.875Nb_0.125O_3-δsample had shorter Co2-O2,Co4-O5 bond length,and smaller twist of Co3-O6-Co4 bond angle,therefore,the Sr Co_0.875Nb_0.125O_3-δperovskite exhibited the optimal electrochemical performance.