Precipitation Preparation And Electrochemical Performance Of Manganese-based Prussian Blue Cathode For Sodium Ion Batteries

Prussian blue analogues(PBA)show promising applications as cathodes for sodium ion batteries,owing to the unique open framework structure,the abundant raw materials,the simple synthesis method and the possibility for large-scale preparation.However,PBA are usually synthesized in a low temperature aqueous solution with a low sodium content,and its lattice contains many vacancies and interstitial water defects,resulting in low electrochemical capacity and poor cycle performance.In addition,most PBA exhibit a relatively low conductivity,leading to poor rate performance.Previous works have shown that their electrochemical performance can be improved by doping with other transitional metals,coating with conductive carbon,removal of interstitial water,and construction of special morphology structures.In this paper,manganese hexacyanoferrate(MnHCF)was synthesized byhigh-temperature coprecipitation method in aqueous solution.The effects of different synthesis temperatures on the structure,morphology and electrochemical performance of the products were investigated.The results show that crystallinity,particle size and charge/discharge capacity of the products increase with the increase of the synthesis temperature.The sample prepared at 90 ? show a monoclinic structure with a space group of 21/n.It indicates that high temperature is beneficial to increase the crystallinity and sodium content and reduce lattice defects of the precipitated product.MnHCF-90 °C exhibits high capacity and excellent cycle stability at low current.At 15 mA g-1the first charge and discharge capacity reach 142 and 139 mAh g-1 with coulombic efficiency of over 99%.At 30 mA g-1,after 300 cycles,the sample discharge capacity was attenuated from the first 134 mAh g-1 to 111 mAh g-1 with 82%retention.High-sodium content monoclinic phase NaxMnFe(CN)6 was prepared by adding sodium citrate to the precursor solution.It consists of small irregular particles in submicron scale and large cuboidal particles in micron scale.The addition of sodium citrate inhibits the release rate of manganese ions,and slows down the rate of precipitation formation.The product has a higher degree of crystallinity,a higher sodium content and lower lattice defects.So it exhibits a high capacity of 142 mAh g-1 at 0.1 C,and also exhibits good rate performance(115.6 mAh g-1 at 1 C and 86.6 mAh g-1 at 5 C)and excellent long-term cycling stability(73.4%retention after 780 cycles under a 0.5 C rate,72.7%retention after 2100 cycles under a 1 C rate).It has also been found that graphene can significantly improve the cycling performance and rate performance of MnHCF.The graphene-coated MnHCF(MnHCF/G)was prepared by vibration milling.In the composite,manganese hexacyanoferrate was finely divided into small particles and uniformly distributed on the graphene sheet.A good conductive network structure is formed between MnHCF and graphene,which improves the electrical conductivity of the material.MnHCF/G exhibits superior electrochemical performance compared to the bare MnHCF.At 0.1 C,the composite delivers a high initial discharge capacity of 142.7 mAh g-1 It also exhibits excellent rate performance,with a capacity of 115.8 mAh g-1 at 10 C and 89.7 mAh g-at 20 C.In addition,the composite MnHCF/G also exhibits excellent cycling stability.After 4000 cycles at 5 C,the discharge capacity of 90.7 mAh g-remained,and the capacity retention rate is 76.3%.Even at 10 C,MnHCF/G show a stable cycle,leaving a discharge capacity of 100.2 mAh g-1 after 2000 cycles.