Electrochemical Energy-storage Mechanism And Performance Optimization Of Lanthanium-based High-entropy Perovskite Oxides

Preparation of high-performance electrode materials is the key to improve the performance of battery-supercapacitor hybrid（BSH）energy-storage devices for grid energy storage and power supplies,and it is of great significance in promoting the process of"carbon neutrality".High-entropy perovskite oxides possess adjustable conductivity and ionic adsorption,expected to be used as electrode material in BSH energy-storage devices.However,so far,high-entropy perovskite oxides show low specific capacity and low cyclic stability in aqueous alkaline electrolyte,which makes it difficult to meet the electrode requirements of alkaline aqueous BSH energy storage device.Herein,the specific capacity and cyclic stability of the high-entropy perovskite oxide electrode are optimized through electronic structure design,crystal structure design and composite structure design,and the failure mechanism of the capacity decay during the cycle test is explained by electrochemical behavior analysis.Firstly,the double-exchange interaction of high-entropy perovskite oxide is regulated based on the law of extranuclear electron gain and loss to optimize electronic structure.As-prepared La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ high-entropy perovskite oxide with good conductivity shows a high specific capacity.Secondly,for La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ high-entropy perovskite oxide with poor cyclic stability in aqueous alkaline electrolyte,its electrochemical behavior is studied to clarify the failure mechanism.Based on the failure mechanism,the material design strategies for La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ high-entropy perovskite oxide are formulated to optimize cyclic stability.Finally,based on the proposed material design strategies,the specific capacity and cyclic stability of La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃-based high-entropy perovskite oxide electrode are further optimized by the designs of crystal structure and composite structure.The research content is mainly divided into the following three aspects.1.La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ electrode achieving high specific capacity through the regulation of double-exchange interactionsBased on the law of electronic gain and loss,Bi²⁺and Fe²⁺tends to lose an electron to achieve relatively stabler electronic orbitals,and Mn³⁺and Cu²⁺tends to gain an electron.If Bi²⁺and Fe²⁺act as electron donors with Mn³⁺and Cu²⁺as electron acceptors in an ion pair,the ion pair will form a strong electron-electron coupling.Herein,the proportion of five elements in high-entropy perovskite oxides is regulated,making La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ with the highest total concentration of Bi²⁺and Fe²⁺and a lot of Mn³⁺and Cu²⁺.The A-site Bi²⁺is expected to promote electronic exchange in Cu²⁺-O^2--Mn³⁺,Cu²⁺-O^2--Cu²⁺,and Mn³⁺-O^2--Mn³⁺.In addition,an electron in the t_2g orbital of B-site Fe²⁺can be transferred to the hole in the e_g orbital of Mn³⁺and Cu²⁺through the bridged oxygen atom,accelerating electronic transfer of Mn³⁺-O^2--Fe²⁺and Fe²⁺-O^2--Cu²⁺.Due to the random distribution of elements in La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ makes it with abundant doubl-exchange ion pairs,conducive to improving conductivity and increasing the electrochemical reaction rate.Thus,La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ possesses high specific capacity of 480.95 C g^-1 at current density of 0.5 A g^-1 in 6 mol L^-1 KOH.Therefore,La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃as an example of high-entropy perovskite oxides coulde be used as a potential electrode materials for aqueous alkaline BSH energy-storage devices.Meanwhile,the components design idea based on the law of electronic gain and loss can optimize the specific capacity of high-entropy perovskite oxide through the accelerated double-exchange interactions,which is expected to further guide the preparation of high-entropy perovskite oxides with high specific capacity.2.The energy-storage failure mechanism of La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ in aqueous alkaline electrolyteAs-prepared La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ high-entropy perovskite oxides with high specific capacity suffers from poor cycle stability,whose capacity retention rate is only10.67%after 5000 cycles at 10 A g^-1.Herein,the mechanism of capacity decay during long-term cycle is studied to guide the formulation of stability optimization strategies.In aqueous alkaline electrolyte,the serious lattice distortion of high-entropy perovskite oxides results in oxygen ions only intercalating into（sub）surface oxygen vacancies of La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃.Therefore,oxygen-ion deintercalation accelerates hydrogen ions intercalate into bulk during charging,but the（sub）surface oxygen vacancies quickly filled by oxygen ions hinder hydrogen-ions deintercalation during discharging.Therefore,residual hydrogen ions in La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ generate impurity phase,volume expansion and electrode pulverization.At the same time,La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ occurs the evolution of the surface oxygen species in the energy-storage process.Surface metal-oxygen octahedrons carry on distortion because of the ligand environment change,leading to the reduced combination between surface metal-oxygen octahedrons and matrix,thus cation leaching.Active cation leaching results in the aggregation of surface inactive La（OH）₃,which hinders the electrolyte ions in contact with the active matrix.Thus,the specific capacity of La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ is attenuated during cycles.Based on the above failure mechanism,optimization strategies for high-entropy perovskite oxides are proposed to improve their cyclic stability by crystal structure design,to belance kinetic process between oxygen-and hydrogen-ion diffusions and restrain cation leaching.3.Interface stability optimization of La_0.7-ySr_yBi_0.3Mn_0.4-xFe_0.3Cu_0.3Cr_xO₃-based electrode in alkaline aqueous electrolyteBased on the failure mechanism of La_0.7Bi_0.3Mn_0.4Fe_0.3Cu_0.3O₃ in aqueous alkaline electrolyte,the content of Cr and Sr in La_0.7-ySr_yBi_0.3Mn_0.4-xFe_0.3Cu_0.3Cr_xO₃ is regulated to achieve slow oxygen-ion intercalation into the bulk of La0.56Sr0.14Bi0.3Mn0.3Fe0.3Cu0.3Cr0.1O3（Cr,Sr-HEPO）for providing plenty of time for hydrogen-ion deintercalation,improving the reversibility of redox reaction.Meanwhile,the content of metal-oxygen octahedrons prone to Jahn-Teller distortion on the surface is reduced due to the charge compensation effect between Cr⁶⁺and Sr²⁺.Thus,Cr,Sr-HEPO shows optimized cyclic stability with a capacity retention rate of 31.97%after5000 cycles at 10 A g^-1.However,Cr,Sr-HEPO still have spontaneous surface reconstruction during the cycle,accompanied by a small amount of cation leaching.In order to further inhibit cation leaching and improve the stability of Cr,Sr-HEPO-based electrodes,the composite structure of reduced graphene oxide（r GO）coating Cr,Sr-HEPO on carbon paper was further constructed.Due to the structural characteristics of r GO,the oxygen ion exchange between Cr,Sr-HEPO and the electrolyte is inhibited,and the reversibility of hydrogen-ion（de）intercalation is improved.The cladding composite structure effectively prevent the active impurity phase on the surface of Cr,Sr-HEPO from falling off,and the leaching cation is absorbed by r GO.Thus,the capacity retention rate of Cr,Sr-HEPO-based composite electrode is as high as 69.39%after 5000 cycles at 10 A g^-1.Meanwhile,the Cr,Sr-HEPO-based composite electrode achieve a high specific capacity of 531.93 C g^-1 at 0.5 A g^-1,benefit by the good conductivity of r GO.These results further verify that the proposed electrode optimization strategy based on failure mechanism is expected to improve the interfacial stability of high-entropy perovskite oxides during energy storage in alkaline aqueous electrolyte.