Preparation And Electrochemical Properties Of Prussian Blue Analogues For Sodium Ion Batteries

Sodium ion batteries have the advantages of low cost and abundant sodium resources,which are expected to become a substitute for lithium ion batteries in the field of large-scale energy storage.Among them,Prussian blue analogues are commonly used as cathode materials for sodium ion batteries due to its open framework structure,simple synthesis and high theoretical capacity.Prussian blue analogues synthesized by coprecipitation methods in aqueous solvents inevitably introduce interstitial water and defects into the lattice structure,resulting the materials with poor electrochemical performance.The preparation of high-performance Prussian blue materials requires the control of crystal structure defects,crystal water,and structural phase transitions.This paper focuses on the high theoretical capacity iron based Prussian blue（Fe HCF）material and the high voltage manganese based Prussian blue（Mn HCF）material,aiming to improving the crystal structure and electrochemical performance of the material by regulating crystallinity,doping,and coating,respectively.The main research content is as follows:（1）Changing the type and amount of salt pre-mixed with sodium citrate to regulate the particle growth of Fe HCF materials.When sodium citrate was added to Fe SO₄solution,the pre-complexation of Fe²⁺and sodium citrate can slow down the slow down the precipitation speed,improve crystallinity,and obtain micron sized particles with low water content.The initial discharge specific capacity of Fe HCF-3 samples can reach 92 m Ah g^-1at a current density of 100 m A g^-1.At the same time,in order to reduce waste water and improve yield,based on the pre-complexation of Fe²⁺and sodium citrate,the crystal structure is further regulated in the highly concentrated Na₄Fe（CN）₆and Fe SO₄solution.By comparing the morphology,structure and electrochemical performance of the materials,the particle size distribution of Fe HCF-H-2 samples is 2.8-3.7μm.It has an initial discharge specific capacity of 116m Ah g^-1,and a capacity retention rate of 83%after 700 cycles of charging and discharging.The pre-complexation of Fe²⁺and sodium citrate achieved particle size regulation of Fe HCF materials,improved crystallinity,and thus obtained long-life Fe HCF materials.（2）Introducing rare earth metal La for bulk doping to prepare a series of La doped Fe HCF materials.The results indicate that La doping can activate the electrochemical activity of low spin iron(Fe ^LS)and prolong the high voltage discharge plateau.When the La doping amount is 3 mol%,the initial discharge specific capacity of the material can reach 123 m Ah g^-1,and the rate performance is improved.Even at a high current density of 30 C,there is still 94 m Ah g^-1of capacity retained.By doping La bulk phase,the crystal structure of the material are changed,and the electrochemical activity of Fe ^LSare activated,resulting in excellent electrochemical performance of Fe HCF material.（3）Constructing a core-shell structure on the surface of Mn HCF material to suppress the volume change of Mn HCF material during sodium ion（de）intercalation,thereby improving the cyclic stability of the material.Exploring the effects of different metal ion salt solutions on the morphology,structure,and electrochemical performance of Mn HCF materials,the results display that nickel rich and scandium rich shell layers can stabilize the Mn HCF framework structure.By further regulating the ion exchange temperature and time,a cyclically stable nickel rich shell Ex-Mn HCF-Ni material was obtained.The obtained material has an initial capacity of 122 m Ah g^-1and can achieve a capacity retention rate of 62%after 200 cycles.The XRD and XPS results indicate that ion exchange can introduce Ni into the sample structure.Compared with the particle morphology of the electrode before and after cycling,the nickel rich shell Ex-Mn HCF-Ni material still maintains a complete particle structure after long cycling.