Electrochemical Enhancement Mechanism Of Prussian Blue Cathode In Sodium/Potassium Ion Batteries

With the increasing number of electronic products,the demand for power batteries is increasing day by day.Sodium ion batteries have gradually become a substitute for commercial lithium batteries.Among the many cathode materials for sodium ion batteries,Prussian blue（PB）,due to its open framework and good stability,does not produce harmful by-products that pollute the environment during synthesis and use.It is considered to be a promising cathode material for Na-ion storage.However,PB only presents poor electrical conductivity,which restricts its electrochemical performance.At present,most conductive agents mainly is commercial conductive carbon black,while reduced graphene oxide（RGO）has better conductivity than conductive carbon black.Therefore,in this experiment,we optimized the electrochemical performance of PB by using RGO as a conductive agent.In order to prove the reliability of the experiment,we also further analyzed the mechanism of conductivity improvement by the distribution of current image and relaxation time.According to our experimental analysis,it is found that RGO can significantly improve the conductivity of the electrode,and the conductivity of the electrode is more uniform.Although PB has made some breakthroughs in sodium battery,due to the high standard redox potential of Na⁺/Na,the voltage platform and energy density are lower,and because of the low standard redox potential of K⁺/K,we also studied PB in potassium ion batteries.Due to the rapid growth rate of Prussian blue in potassium,the synthesized grains are irregular and incomplete,and the electrochemical properties are poor.Therefore,by coprecipitation method,potassium ethylenediamine tetraacetate was used as complexing agent to reduce the crystallization growth rate of PB.Based on the progress of work,the following conclusions have been reached:1.Na₂Fe^[Fe（CN₆）^] was prepared by self-decomposition of single iron source and RGO was synthesized by Hammer method,and RGO is used as the conductive additive of the PB.Based on the excellent and uniform conductivity and mechanical properties of the RGO,the PB-RGO electrode can buffer the volume change caused by the charge and discharge process and maintain the stability of the structure.At the same time,the electrochemical properties such as good cycling performance,rate performance and long cycling performance can be obtained.The discharge capacity can remain at 80 m A/g after 200 cycles at 50 m Ah/g.2.K₂Fe^[Fe（CN）₆ was synthesized by coprecipitation and potassium ethylenediamine tetraacetate（EDTA-2K）.Relative to potassium citrate,EDTA complexing with Fe²⁺,is able to form a stronger competitive relationship with Fe（CN）₆ and thus reduce the growth rate of PB crystals to obtain a more complete PB structure.EDTA can provide K⁺,and avoid introducing more water molecules when additional K⁺is needed.In other words,it can reduce the content of crystalline water.EDTA ligands are negatively charged,which can be absorbed by the surface of crystal to inhibit the growth rate of the nucleus and prevent crystal agglomeration.Therefore,it can form monodisperse grains with better shape,and reduce the generation of defects.Therefore,EDTA-2K can improve Coulomb’s efficiency,Specific capacity and energy density.