Study On Manganese Prussian Blue Cathode Materials Of Sodium And Potassium Ion Batteries

In recent years,with the wide application of lithium ion batteries in portable electronic devices,high-power electric vehicles,large-scale energy storage power stations and smart grids,the demand for lithium ion batteries is increasing day by day.However,limited lithium resources make the price of raw materials keep rising,which also limits the sustainable development of lithium ion batteries.Sodium and lithium belong to the same main group and have similar chemical properties.Therefore,it is feasible to build sodium ion batteries that work in a similar way to lithium ion batteries.Sodium ion batteries are expected to replace lithium batteries in the future and be used in large-scale energy storage.It is an important research direction to develop high performance positive electrode materials for sodium ion batteries to explore materials with open structure and suitable for higher size cation detaching skeleton structure.Prussian blue compounds are a class of materials with open skeleton structure,which can provide sufficient clearance space for large-scale ion disinsertion,and their theoretical capacity is as high as 170 mAh g^-1.However,the Prussian blue materials synthesized by the traditional coprecipitation method often have such problems as low sodium content,vacancy defect,more crystal water and uneven particle distribution,which result in unsatisfactory electrochemical performance.Therefore,it is necessary to design a reasonable scheme to study and optimize the synthesis process to prepare Prussian blue material with good cycling stability.In view of the above,this paper carries out the following work:?1?Prussian blue material Na₂MnFe?CN?₆?NaMHCF?was prepared and synthesized by coprecipitating method.A series of samples were obtained by controlling and adjusting the temperature,the concentration of reactants,whether complexing agent was added or not.The effects of different synthesis conditions on the preparation of Prussian blue materials were systematically studied.The particle size of the sample synthesized at room temperature was small?50¹00 nm?,and the particle size of the sample prepared after increasing the synthesis temperature?85°C?was significantly larger?200⁵00 nm?.In terms of electrochemical properties,NaMHCF materials synthesized at high temperature showed higher specific capacity than those synthesized at room temperature.The particles prepared with low concentration of sodium ions in the reaction solution?without adding sodium chloride?would have severe agglomeration,while the particles prepared with increased concentration of sodium ions had relatively uniform particle size distribution,and no obvious agglomeration was observed.In addition,increasing temperature and reactant concentration can increase the sodium content of the material,reduce the vacancy defect and crystal water content of the material.On the basis of high temperature and high concentration of sodium ions,the samples prepared by adding complexing agent showed the characteristics of regular surface morphology and good crystallinity.In addition,the addition of complexing agent can further increase the sodium content and reduce the vacancy defect,almost reaching the theoretical ratio of Na₂MnFe?CN?₆,showing the optimal cycling stability.?2?On the basis of the preparation of Prussian blue by coprecipitation,high quality K₂MnFe?CN?₆?KMHCF?was prepared by coprecipitation method under the condition of high temperature and citrate.The addition of citrate solution with high concentration of complexing agent can effectively control the nucleation process of precipitation and the growth rate of grains,so as to prepare high quality KMHCF with uniform precipitation.The high quality KMHCF is characterized by low crystal water content,few defects and uniform microstructure distribution.The mixed sodium ion battery with high quality KMHCF as the positive electrode showed excellent cycling and multiplier performance.After 1000 cycles under 1 C,the capacity retention rate was up to 80%,and the high capacity of 95 mAh g^-1 was still present at the high multiplier of 20 C.Different from traditional single-cationic batteries,the hybrid sodium ion battery has obvious voltage platform change in the early cycle process,and presents three different reversible voltage platforms in the subsequent cycle.Through the analysis of non-in-situ composition and structural characterization,it can be concluded that during the electrochemical cycle,both sodium and potassium ions participate in the reversible disinsertion in the structural framework of Prussian blue compounds and form a co-existing phase of sodium and potassium ions,which will continue to exist for several hundred cycles as the intermediate transition phase of positive poles.With the increasing of the embedded sodium ions and the decreasing of potassium ions,NaMHCF cubic phase was finally formed.