Preparation Of K_xMnFe?CN?₆ Material And Their Electrochemical Mechanism

K_xMnFe?CN?₆ is one of the most promising cathode materials for potassium ion batteries.It has large reversible capacity and high working voltage.The stable frame structure allows kinetically fast insertion and extraction of potassium ions.Moreover,low lattice strain is often beneficial to the ratio and cyclic performance.Although the material has a lot of advantages theoretically,prepare the K_xMnFe?CN?₆ in large scale with fewer structural defects and impurities to get better cycle stability is still a big challenge for commercialization.Recently,adulterate,constructing different structure and morphology and controlling the particle size have been confirmed to be effective to improving cycle stability.In our work,adding other active substances,ion exchange reaction and chelation reaction were used to change the environment of crystallization and the crystallization rate.K_xMnFe?CN?₆ with different particle size and morphology were used as cathode materials for potassium ion batteries.The main research are as following:?1?Different ratios of Na_xMnFe?CN?₆,K_xMnFe?CN?₆ mixture powder can be obtained by controlling the molar ratio of sodium to potassium in the precursor during the ion exchange reaction.Testing the molar ratio and electrochemical performance of the material to evaluate the properties as a cathode.The results show that the voltage of charge and discharge is depend on the electrolyte and the counter electrode.Moreover,the mixture is more suitable for sodium batteries.The content of Na_xMnFe?CN?₆ has effects on the structure of K_xMnFe?CN?₆.When M?Na?:M?K?=3:1 or 1:1,the size of the framework is too small for the insertion and extraction of potassium ions,which lead to low coulombic efficiency.?2?Four kinds of K_xMnFe?CN?₆ with different structures and properties were prepared by direct coprecipitation,solution ion exchange,electrochemical ion exchange and slowly ion exchange.The results show that the slowly ion exchange can prepare an open flexible frame and single crystal K_xMnFe?CN?₆ with good cycle stability.Its high crystallinity and single crystal structure lead to less grain boundaries and improve the cycling life.The working voltages up to 3.95 V and 3.81 V,and the initial discharge capacity is 112 mAhg^-1,capacity retention is 62%for 350 cycles.Optimizing the synthesis conditions and packaging process will help to further improve the cycling stability.?3?K_xMnFe?CN?₆ with different morphologies were prepared by adding different amount of potassium citrate as a chelating agent.They were used as cathodes for potassium ion battery.The results show that with the increase of potassium citrate content,the surface gradually becomes rough,the structure tends to be loose,and the particle size gets an increase.The morphology change is:30 nm irregular cube,70 nm regular hollow cube,250 nm solid particle between cube and sphere?250-500 nm hollow sphere.The capacity retention of K_xMnFe?CN?₆ that co-precipitation with 30 g potassium citrate is 161%for 200 cycles,which is still superior to the other three materials without considering the capacity increase in the first ten cycles.This can benefit from its large particle size and solid structure lead to smaller specific surface area,which can reduce the side reaction with the electrolyte.