Study On Coating Modification Of Prussian White Cathode Material For Sodium Ion Battery

For the past few years,under the influence of increasingly serious resource scarcity and environmental contamination,large scale energy storage has been developed unprecedentedly,especially the renewable green energy.As the most potential energy storage technology in the past decades,secondary batteries integrate unstable interstitial-energy sources such as solar,wind and tidal energy into smart grids,thus realizing the stable operation of large scale energy storage systems.Since its commercialization,lithium-ion batteries have been broadly used in"4C"products such as computers,communications,networks and consumer electronics.However,the scarce of lithium resources and its high price limit its practical applications.In contrast,sodium is widely distributed,abundant and cheap,making sodium-ion batteries a promising alternative to lithium-ion batteries.The main technical bottleneck of sodium ion battery is the cost of cathode material and electrochemical performance.Among the cathode materials of sodium ion battery,Prussian white（Mn HCF）with low cost and high energy density has gradually become a research focus.But the practical application of Mn HCF is hampered by the large attenuation of Mn HCF capacity due to the large amount of manganese dissolution caused by the direct contact with the electrolyte,and the structural distortion caused by the Jahn-Teller effect of the oxidized Mn³⁺during cycling,which can seriously affect the electrochemical performance of the material.Therefore,by reason of the foregoing of Mn HCF problems,on the basis of taking into account the material cycle performance and energy density,this paper developed the following modification strategies:（1）Coating modification of Nickel-based Prussian Blue（Ni HCF）to Prussian White.The electrochemical properties of Mn HCF were improved by the secondary coprecipitation method.The results show that the coating amount is optimal at 5%.The electrochemical performance of the whole cell shows a more competitive application prospect.With virtually no loss of capacity,Mn HCF@5%Ni HCF achieves a capacity retention rate of up to 63.5%after 1000 weeks in 5C,showing a more competitive application prospect for full cell electrochemical performance.For the rhomboidal Mn HCF,the coating of Ni HCF can inhibit the dissolution of manganese on the surface of the material,and alleviate lattice collapse caused by Mn ion dissolution.On this basis,the feasibility of other transition metal based Prussian blue as cladding layer was explored,which widened the idea for surface modification of Mn HCF.（2）Coating modification of sodium vanadium oxyfluorophosphate（NVOPF）to Prussian White.NVOPF coating was successfully constructed on the cathode surface of Na-rich monoclinic Mn HCF by in situ coating technology,which inhibited the adverse side reactions and manganese dissolution on the surface of Mn HCF and maintained the integrity of the overall structure.In contrast,the Mn HCF@5%NVOPF cathode with the best electrochemical performance provides 85%capacity retention at5C and performs better electrochemical performance in the whole cell,providing a highly reversible discharge specific capacity of 121.6 m A h g^-1at 0.2C.In situ XRD and XPS demonstrate that both Mn HCF as the main body and NVOPF as the coating layer have highly reversible structural evolution during the charge-discharge cycle.