Preparation Of Cathode Materials For Sodium-Ion Batteries And Its Compatibility With Electrolyte

Due to the low cost and even distribution of sodium resources,sodium-ion batteries have become one of the hotspots in the research of new energy storage batteries.Although sodium-ion batteries have similar working principles to lithium-ion batteries,better low-temperature performance,and higher safety,they still need to overcome some key obstacles before commercialization.The first challenge is to develop cathode materials with high specific capacity and good cycling stability.As an important class of cathode materials for sodium-ion batteries,iron-based Prussian blue analogues（Fe HCF）have attracted widespread attention due to their open framework structure,environmental friendliness,low cost,simple preparation process,and high theoretical specific capacity.However,in this type of material,there are usually many vacancies and crystal water,which cannot be completely removed,resulting in poor cycling stability and rate performance when used as a cathode material for sodium-ion batteries,which seriously limits their application prospects.This thesis focuses on the poor cycling stability and rate performance of iron-based Prussian blue analogues as cathode materials for sodium-ion batteries.Based on the academic research progress at home and abroad,three different principles,including the addition of sodium salt-assisted synthesis,carbon coating,and optimization of the electrolyte,were used to improve their morphology and electrochemical performance.The reasons why the cycling stability and rate performance of the samples were improved after the addition of sodium salt-assisted synthesis and carbon coating were analyzed through morphology and structure characterization and kinetic testing.Finally,the cycling stability and rate performance of Fe HCF were further improved by using Na PF₆-based carbonate electrolytes with high ionic conductivity and sodium-ion transfer number.The specific research contents are as follows.（1）By adding sodium sulfate to assist in the synthesis,Fe HCF with low vacancy defects was produced,which effectively improved its cycling stability and rate performance.Through kinetic tests on the products and characterization tests of the post-cycled electrodes by ex-situ SEM,HRTEM,and XRD,the reason for the improvement in cycling stability and rate performance was further analyzed.Finally,the effects of different sodium-supplying compounds and different amounts of additives on the sodium storage performance of Fe HCF were also discussed.（2）In the room-temperature co-precipitation method,Fe HCF was grown in situ on the substrate of oxidized graphene.By comparing the differences in morphology and electrochemical properties of different carbon materials,it was found that the graphene coating could reduce the redox activity of low-spin iron,thereby alleviating the lattice distortion caused by low-spin iron in the charge-discharge process,and achieving a significant improvement in cycling stability.Finally,the effects of different amounts of oxidized graphene on the morphology and electrochemical properties of the coated samples were further discussed.（3）A series of studies were conducted based on the compatibility between Fe HCF and Na PF₆-based carbonate electrolytes.By comparing the charge-discharge voltage curves under different solvent formulations,the reason for the abnormal charging of Fe HCF in Na PF₆-based electrolyte was analyzed,and then it was proposed to add 10%DMC to EC/PC（1:1）mixed solvent to suppress the decomposition of Na PF₆ during charging.Finally,the reasons for the improvement of the electrochemical performance of Fe HCF in the 1M Na PF₆+EC/PC/DMC（0.45:0.45:0.1）+5%FEC electrolyte were analyzed by testing the physical properties of the electrolyte and post-cycling electrode characterization.There are 48 figures,5 tables,and 134 references in this thesis.