Study On The Structure Control And Electrochemical Performance Of Fe-based Prussian Blue For Cathode Material Of Sodium Ion Battery

Owing to the open frame structure,appropriate ion pathway,abundant raw materials,and simple synthesis processes,Prussian blue is considered a valuable material with application potential in sodium-ion battery.Nevertheless,Prussian blue has shortcomings such as low electrical conductivity,poor structural stability and high sensitivity to the lattice defects and interstitial water,which lead to a large deviation of its electrochemical performance from the theoretical predictions.In this thesis,the Fe-based Prussian blue material was taken as the research object.Structure control strategies including surface coating,increasing sodium ion content,anion modification and morphology control were utilized to improve the electrical conductivity and structural stability,decrease the contents of lattice defects and interstitial water so that the electrochemical performance of Prussian blue can be enhanced.The main research contents and results of this thesis are as follows:（1）To solve the problem that the carbon-coated Prussian blue cannot be prepared by conventional carbonization methods,Prussian blue pyrolysis intermediates are used as raw materials to synthesize carbon-coated Prussian blue in situ.The experimental results show that commercial Prussian blue was pyrolyzed at 520～600℃to form a carbon-coated intermediate,which can be converted into iron-based Prussian blue by reaction with Na₄Fe（CN）₆.Carbon coating can effectively improve Prussian blue’s electrical conductivity and structural stability,thereby improving its cycling performance.The optimized carbon-coated Prussian blue sample had a discharge specific capacity of 123.5 m Ah/g at a current density of 1 C,and the capacity retention can reach 80.0%after 300 cycles.（2）To overcome the shortcomings of the dual-iron source co-precipitation method for preparing Prussian blue including time-consuming and low sodium content in the structure,a dual-iron source ascorbic acid additive ball milling method was developed.In addition,aniline was used as the electrolyte additive to realize the in-situ coating of polyaniline conductive polymer on the surface of Prussian blue during the charging and discharging process.The experimental results show that the use of ascorbic acid as the ball milling additive can effectively inhibit the oxidation of Fe²⁺and improve the sodium ion content in the Prussian blue structure.The initial discharge specific capacity at 1 C current density can be increased to 149.9 m Ah/g.The polyaniline coating improved the electronic conductivity and structural stability of Prussian blue,and the capacity retention of the optimized sample can reach 62.7%after electrochemical cycling at 1 C current density for 500 cycles.（3）Aiming at the problem that the high content of interstitial water in the structure of Prussian blue leads to poor cycle performance,different ferrous salts were used as raw materials to synthesize anion-modified Prussian blue in one step by ball milling.The experimental results showed that the interstitial water content in the carboxyl-modified Prussian blue synthesized by one-step ball milling was reduced to 12%.The Prussian blue synthesized by using ferric acetate as raw material has a discharge specific capacity of 113.7m Ah/g at 1 C current density and the capacity retention can reach 54.5%after 1000 cycles at this current density.（4）In view of the problem of low discharge specific capacity of Prussian blue due to many crystal defects and high interstitial water content in its structure,cobalt-based Prussian blue was used as raw material to synthesize hollow iron-based Prussian blue through anion exchange.This method was developed to control the morphology of Prussian blue and reduce its internal defects and interstitial water.The experimental results showed the interstitial water content in hollow Prussian blue was reduced to 9%and had lower content defects.The synthesized hollow Prussian blue had a discharge specific capacity of 133.6m Ah/g at a current density of 1 C,and the capacity retention can reach 71.6%after 100cycles of electrochemical cycling at this current density.