Synthesis And Electrochemical Performances Of Manganese-Based Prussian Blue Cathode Materials For Sodium-Ion Battery

Sodium-ion batteries had abundant sodium resources and had many similarities with lithium-ion batteries in various energy batteries,and they had been extensively studied in recent years.It was important that develop excellent electrode materials to promote the application of sodium ion batteries.Among all kinds of cathode materials,Na₂MnFe（CN）₆showed great advantages by virtue of a unique three-dimensional open frame structure.However,it was inevitable that crystal water and vacancies occupy the channels and sites in the crystal structure during the synthesis process of this type of material,and causing structural defects,which affected its performance in sodium ion batteries.Studies had found that the integrity of the material can be improved by changing the synthesis conditions or modifying the material.So as to improve the performance of the material.In this paper,Na₂MnFe（CN）₆in the cathode material of Prussian blue analogues was used as the research object.Firstly,Na₂MnFe（CN）₆samples were prepared by solution co-precipitation method in 25℃and 80℃water-baths temperature,and the influence of different temperatures on the Na₂MnFe（CN）₆was analyzed through characterization test and electrochemical data.The results showed that although the sample prepared at 80℃water-bath temperature can increased the amount of sodium ion migration,resulting in a higher initial specific capacity,and reached 152.8m Ah·g^-1at a current density of 0.1C.However,its structure was not stable,and it was easily damaged during multiple charging and discharging processes.Then,took 80℃water bath temperature as the prerequisite,Na₂MnFe（CN）₆sample with high crystallinity was prepared by adding Trisodium Citrate to precursor solution.The results showed that the sample not only maintained a high specific discharge capacity above 150m Ah·g^-1,but also exhibited better rate performance and cycle stability.The capacity retention rate was 62.5%at 0.1C for 100 cycles,and at 0.5C,the capacity retention rate was 71.9%after 100 cycles.In order to optimize the performance of the Na₂MnFe（CN）₆positive electrode material in the sodium ion battery.In this paper,a sample with a monoclinic structure with smaller particles and higher crystallinity was prepared by doping with 20%iron ions.Compared with the material of non-doped iron ion,the specific capacity had been reduced,but the stability of the material had been greatly improved.The specific discharge capacity decreased from 114.4m Ah·g^-1to 82.1m Ah·g^-1,and the capacity retention rate was 72.1%at 0.1C for 100 cycles.In order to further explore the influence of iron ion doping on Na₂MnFe（CN）₆cathode material,The experiments were also designed with 10%and 5%iron ion doping.After comparison,it was found that with the increase of iron ion doping,the crystal particles gradually became smaller and the distribution became more and more uniform.And the crystal structure gradually changed from monoclinic phase to cubic phase.Although the sample with more iron ion doping had lower initial specific capacity,but it showed better rate performance and cycle performance.So a small amount of specific capacity of Na₂MnFe（CN）₆cathode material by doping iron ions was sacrificed to improve its cycle stability.