Design And Preparation Of Prussian Blue And Its Analogues As Cathode Materials And Their Applications In Sodium Storage

Prussian blue（PB）and its analogues（PBAs）have made a great progress since2012 as cathode materials for sodium-ion batteries.PB and PBAs have an open three-dimensional framework structure,two redox sites,large internal space in the framework and the structure itself provide theoretical feasibility for high capacity,fast Na⁺transport and stable crystal structure.In addition,their chemical composition and structure can be adjusted by transition metal ions doping technology to meet their practical application requirements.However,the cycle stability of this kind of materials still needs to be improved when applied to sodium-ion batteries,and there is a certain gap between the actual charge-discharge specific capacity and the theoretical value,which is due to the common problems of high vacancy and crystal water content in the synthesis process.Based on the above basis,this thesis aims to study the relationship between the crystal structure of PB and PBAs and their sodium storage properties,and to improve the structure and crystallinity,reducing vacancies of the materials by doping inert elements,surface coating and reducing the nucleation rate,in order to achieve the purpose of improving the discharge specific capacity and cycle stability of Prussian blue and its analogues in sodium-ion batteries.The main research contents and results are as follows:（1）Prussian blue and its analogues are promising cathode materials for sodium-ion batteries due to the open three-dimensional framework that can accommodate fast Na⁺transport.However,in the conventional rapid synthesis process,this kind of material will lead to defects in the crystal structure of the material,which may easily cause the problems of rapid capacity decay and poor cycle stability in the battery.Therefore,the usability of an inert metal ion Ni²⁺is used to stabilize its crystal structure,and a low-defect,nanoscale,"zero-strain"structure nickel-based Prussian blue analogue（Ni Fe-PBA）is synthesized using a sodium citrate chelator-assisted method.Ni Fe-PBA exhibits capacity retention of 85.8%after 1120 cycles at a current density of 0.1 A g^-1 in liquid sodium-ion batteries,and 86.7%even in quasi-solid-state batteries.The crystal structure phase transition of ions during de-intercalation is analyzed by ex-situ X-ray diffractometer.The experimental results show that the excellent cycling stability and rate capability of Ni Fe-PBA in sodium-ion batteries are related to the factors of inert metal ions,low defects and low crystal water content.（2）Ni Fe-PBA exhibits excellent cycling stability in sodium-ion batteries,but such single-electron-transfer materials exhibit low energy density,making them less competitive in sodium-ion batteries.Therefore,cobalt-based Prussian blue analogue（Co Fe-PBA）with a high specific capacity double electron transfer type is selected as the method to increase the specific capacity.Moreover,in order to cope with the problems of rapid capacity decay,poor cycle stability caused by high crystal water content and large crystal structure changes during charge and discharge,polydopamine（PDA）cobalt-based Prussian blue analogue with hollow structure（Co Fe-PBA@PDA）is prepared by self-template sacrificial method and surface coating method.Under the action of the hollow structure of the material itself and the thin layer of polydopamine,Co Fe-PBA@PDA exhibits initial discharge specific capacity of 123.1 m Ah g^-1 at 0.1 A g^-1,and still has 88 m Ah g^-1 after 500 cycles.In addition,in-situ X-ray diffractometer results verified the reversibility of Na⁺in the deintercalation process,indicating that the stable structure of Co Fe-PBA@PDA.The above experimental results show that this hollow structure and surface coating method are beneficial to provide more active sites,and can also buffer the volume strain caused by ions during the charging and discharging process,thereby effectively improving the cycling stability and rate performance of cobalt-based Prussian blue analogues.（3）In the context of the commercialization of Prussian blue and its analogues,it is extremely important to prepare low-cost,mass-produced Prussian blue analogues.Prussian blue（Fe Fe-PB）not only has high specific capacity,but also can enhance its cycling stability with suitable chelating agents.Therefore,Fe Fe-PB is selected as the research object.As a strong chelating agent,ethylenediaminetetraacetic acid disodium（Na₂EDTA）has a strong chelating effect with many metal ions,and can be widely used in the preparation of various types of PB and PBAs.Therefore,Na₂EDTA is selected to delay the nucleation rate,and ascorbic acid is added as a reducing agent in the preparation process to inhibit the oxidation of Fe²⁺in[Fe（CN）₆]^4-,thereby preparing EDTA-Fe Fe-PB with low vacancy defect and water content.EDTA-Fe Fe-PB exhibits superior electrochemical performance,with a specific discharge capacity of 101.1 m Ah g^-1 at 0.2 A g^-1,and the capacity retention rate is still 90.9%after 200 cycles.And the electrochemical properties of the EDTA-Mn Fe-PBA prepared by the same method are tested to verify the suitability of the chelating agent,which also shows excellent electrochemical properties.The experimental results show that the low defect and crystal water content of Prussian blue and its analogues can effectively improve the discharge specific capacity and cycle stability of SIBs,which also shows the importance of the materials structure stability to the excellent electrochemical performance of the battery.In summary,we design and synthesize different kinds of Prussian blue analogues as cathode materials in sodium-ion batteries and studied their electrochemical performance.The“zero-strain”structure is beneficial to the long-cycle stability of sodium-ion batteries.The hollow structure coated with polydopamine is beneficial to ion transport.Moreover,the facile preparation method can obtain materials with high specific capacity and excellent performance.The design idea of this work is different from the traditional preparation method,which provides a reference for the development of Prussian blue and its analogues with high stability and excellent rate performance.