Preparation And Modification Of P2-Na_0.67Ni_0.15Fe_0.2Mn_0.65O₂ Cathode Materials For Sodium-ion Batteries And Study On Its Electrochemical Performance

P2 phase layered manganese-based cathode materials have the advantages of wide raw material sources,low price,environmental friendliness,high specific capacity,high working voltage,etc for sodium ion batteries(SIBs).However,some problems need to be solved before large-scale application:the large Na+radius results in slow dynamics during charge and discharge,and poor rate performance;The irreversible phase transition during Na+de-intercalation and the Jahn-Teller effect caused by Mn3+will destroy the cathode structure,resulting in poor cycle performance.In response to the above problems,the P2-phase layered manganese-based cathode material P2-Na0.67Ni0.15Fe0.2Mn0.65O2(NFM)was analyzed from the aspects of material preparation technology,compatibility with electrolyte and bulk phase doping modification.The crystal structure,interface chemistry and electrochemical properties were discussed.The main research contents and results are as follows:(1)NFM was prepared by three methods:high-temperature solid phase method,sol-gel method and co-precipitation method.The analysis and test results show that the material prepared by the co-precipitation method has the best electrochemical performance.The electrochemical performance of NFM used NaClO4 and NaPF6 electrolyte systems in commonly was analyzed.The results show that the compatibility of NaPF6 based electrolyte with NFM materials is more excellent.This is because the cathode/electrolyte interface film(CEI)formed in NaPF6 electrolyte is dense and uniform,which effectively inhibits the dissolution of the transition metal(TM)ions.It reduces the transmission energy barrier of Na+;while the CEI film formed by the NaClO4 electrolyte has the larger impedance,and the strong oxidizing property of perchlorate will lead to the partial CEI film falling on the surface of the cathode,resulting in the re-contaction between the electrolyte and cathode,which accelerats TM ions dissolution and destroys the battery performance.(2)Modification of NFM with ex-situ F and in-situ Mg double doping.The crystal structure,morphology and sodium storage performance of the F-doped modified Na0.67Ni0.15Fe0.2Mn0.65FxO2-x(NFMF)series of metal oxides were prepared and studied,and it was found that NFMF-005 has the most excellent electrochemistry performance.This is because the strong electronegativity of F reduces the binding energy between TM-O after a proper amount of F is doped,which increases the confusion of TM elements and the distance between the transition metal layers.This not only suppresses the Jahn-Teller effect and the structural distortion of the material,but also promotes the deintercalation of Na+.In-situ Mg doping was performed on the basis of NFMF-005 to obtain Na0.67-yMgyNi0.15Fe0.2Mn0.65F0.05O0.15(NMy-NFMF005).Since Mg occupies the Na2 site and forms a Mg-Mg dimer with the TM element in the alkali metal layer,the phase transition is suppressed and the cycle stability of the cathode material is improved.The electrochemical performance test results show that the first discharge specific capacity of NMy-NFMF005 is close to 240 mAh g-1,and the capacity retention rate after 50 cycles is 87.7%.The SIBs compatible electrolyte system selected in the thesis is conducive to giving full play to the electrochemical performance of NFM cathode materials;the doping modification method adopted is conducive to improving both the rate performance and cycle performance of NFM cathode materials.