Studies On Synthesis And Performance Of Cathode Materials For Na-Ion Batteries

In the past 10 years,sodium-ion batteries as an alternative to lithium-ion batteries have gradually become a focus of attention for large-scale energy storage.Nowadays,the core of the development of sodium-ion batteries lies in electrode materials,and cathode materials are the key to restricting the energy density of sodium-ion batteries.Among the various cathode materials for sodium-ion batteries,Prussian blue analogs show great potential for applications due to their open three-dimensional framework,high theoretical capacity,and low cost.Among them,iron-based Prussian blue and manganese-based Prussian blue have captured widespread attention due to dual redox couples.However,the synthesis process of iron-based Prussian blue is accompanied by the oxidation of Fe,resulting in the loss of sodium.As a result,the energy density decreases when the material matches the full cell,and such materials usually have more lattice water and defects,which greatly affects the cycle performance of the material.In addition,manganese-based Prussian blue has lower capacity retention due to the Jahn-Teller effect of manganese and the dissolution of surface manganese.Therefore,this thesis proposes an effective modification strategy for the synthesis and cycle performance of Prussian blue materials.（1）Using a simple ball milling method,FeHCF materials with low Feoxidation amount were synthesized through controlling the condition of synthetic atmosphere.Due to the contact of the material with air during the washing process,the oxidation of Fewas aggravated,resulting in further loss of sodium.Here,NCO is designed as a sodium ion additive for full cell,and its decomposition during charging is used to release additional sodium ions to compensate for the Na deficiency in the full cell.In the oil-based full cell,FeHCF-1-NCO//NTP provided an initial discharge specific capacity of～130 m A h g^-1 at 1C and possessed 80%capacity retention after more than 1000 cycles.（2）Monoclinic phase MnHCF materials with different morphologies were synthesized by co-precipitation method and adjusting synthesis temperature and chelating agent concentration.On this basis,the necessary conditions for the synthesis of the rhombic phase structure were explored.The rhombic phase structure exhibited higher discharge capacity and average voltage.The reason of poor cycling for the MnHCF electrode was due to the continuous dissolution of Mnduring the charging and discharging process verified by TXRF,resulting in lattice collapse and increased polarization.To solve the cycling problem of MnHCF,by employing inert elements,the results show that MnHCF-10%Ni exhibits good stability,and the full cell also exhibits good performance.（3）For Mndissolution on the surface of MnHCF material leading to poor cycle performance,the CoB material is coated on the surface of MnHCF,which inhibits the dissolution of Mnon the surface.Firstly,CoB with good electronic conductivity was selected as the coating layer,and the coating was carried out by one-step method.The results show that the cyclic properties of the material are improved with the increase of coating amount.Preferably,the overall performance is best when the coating amount is 5%.On this basis,the inert CoB was replaced with electrochemically active NVPF.The NVOPF20%-MnHCF sample successfully exhibited three pairs of Fe²⁺/Fe³⁺,Mn²⁺/Mn³⁺and V⁴⁺/V⁵⁺redox couple,providing a new sight for MnHCF coating.