Study On The Synthesis And Electrochemical Performance Of Prussian Blue Analogues As Cathode Material For Sodium Ion Battery

With the large-scale development and high proportion of new energy connected to the grid,the innovation and breakthrough of energy storage technology has become an important leading technology to drive the subversive adjustment of global energy pattern.As the most widely used and promising energy storage technology,electrochemical energy storage has attracted much attention.At present,the best electrochemical energy storage technology is lithium-ion battery,but the limited lithium resources are restricting its bright development prospects.Sodium ion battery(SIB)has become a research hotspot in the field of energy storage battery because of its abundant sodium resources,high energy conversion efficiency,long cycle life and good safety performance.Cathode material is an important part and the biggest cost source of battery,so it is very important to study and develop the cathode materials with high cost performance.Prussian blue analogues(shorted as PB)with 3D open framework have been widely studied due to their many excellent advantages as cathode materials for SIBs.However,the structure defect and crystal water containing in the materials can destroy its electrochemical performance seriously.Among the PB materials,NiHCF has superior cycling performance but inferior capacity performance.In order to solve these problems,the author has carried out the following modification research:First,a series of PB composites were synthesized through typical coprecipitation method assisted by sodium citrate.Synthetic technique of CoHCF was optimized and the results show that the electrochemical performance for CoHCF can be enhanced by supplementing sodium,raising reaction temperature and regulating the dosage of sodium citrate.Based on these,NiHCF,FeHCF and MnHCF samples were synthesized by the same way.Among them,CoHCF,FeHCF and MnHCF have high discharge specific capacity but inferior cycling performance,while NiHCF owns great cycling performance though it has low capacity.Then,NiHCF with great cycling performance was modified by K⁺doping.Density functional theory(DFT)calculation confirmed that K⁺is feasible to enter in the lattice of NiHCF and K doping can improve the mobility of Na⁺ions and electrons in NiHCF.Then,NiHCF sample doped by K⁺was synthesized successfully by coprecipitation and marked as K-doped NiHCF.XRD results show that K doping can provide more sodium storage sites and larger de-intercalation pathway.ICP-OES results confirm that introduced K⁺ions can't get extracted from the NiHCF lattice during charge and discharge process and K⁺only play a pillar role in stabling the structure of K-doped NiHCF.As cathode material for SIBs,the K-doped NiHCF sample exhibits significantly improved electrochemical performance.Particularly,K-doped NiHCF delivers an extremely high initial capacity of 87.1 mAh g^-1 at 10 mAg^-1 and presents a very low capacity fading ratio of 0.016%per cycle at 800 mAg^-1 over 1000 cycles.At last,binary(NiCo-PB and Ni Fe-PB)and ternary(NiCoFe-PB)composites were successfully prepared by introducing active elements Fe and Co into NiHCF.The results show that the ternary NiCoFe-PB composite prepared by double doping with Co and Fe has good crystallinity,high sodium content,few structural defects and low crystallization water content,so it shows excellent electrochemical performance.At 20 mAg^-1,this ternary material can deliver a capacity as high as 120.4 mAh g^-1 and owns extremely small capacity decreasing rate of 0.0044%per cycle at 2 Ag^-1 even after 10,000 cycles.The performance improvement of NiCoFe-PB can be attributed to the optimized structure of NiHCF,the increased active sites and the improved diffusion kinetics of Na⁺ions caused by double doping of Fe and Co.