Study On Doping Modification And Sodium Storage Performance Of Fe-Mn Based Layered Cathode Materials

In the current energy storage research system,low-cost metal ion batteries have became the hot spot due to their non-negligible price advantage and abundant reserve,sodium-ion batteries is one of them.However,for sodium ion batteries,the exploration of positive electrode materials with high specific capacity and excellent cycle stability is the only way for industrial development.In this paper,the P2 type layered oxide Na_0.6Fe_0.2Mn_0.8O₂ with high specific capacity is selected,and the elements doping modification is studied.The electrical properties,chemical reaction mechanism and phase change mechanism of lithium doped materials are analyzed by a series of characterization methods.For the Na_0.6Fe_0.2Mn_0.8O₂ material,the high-pressure phase transition above 4V is one of the reasons that affect its long-cycle stability.And the structural distortion caused by sodium ion shuttle is also one of the causes.The electrochemical properties of the material can be modified by incorporation of electrochemically active atoms and atoms having a relatively large radius to suppress high-pressure phase transformation and structural distortion.In the element doping study of this paper,Ti,K and Li elements are tried.The incorporation of Li element can maintain the layered P2 phase structure of the material,which effectively improves the working voltage and cycle stability of the material.At a current density of 0.2 C,the Na_0.6Li_0.2Fe_0.2Mn_0.8O₂ material exhibites a specific capacity of up to 165 mAh?g^-1,and still has a capacity retention rate of83%at 100 cycles.The performance of this material is still superior to other materials in the rate performance test.In the 1 C large rate performance test,the capacity retention rate of Na_0.6Li_0.2Fe_0.2Mn_0.8O₂ material is up to 70%after 250cycles.In the fine-tuning experiment of elements in Na_0.6Li_0.2Fe_0.2Mn_0.8O₂,it is confirmed that the ratio between the elements in this material is the best,and too much sodium and iron would cause a certain decrease in the properties of the material.Excessive sodium can lead to a decrease in the long-cycle stability of the material,and too much iron can cause the material to transition to a low specific capacity O3-type structure.Through structural refinement,XPS,ex-situ XRD and other characterization methods,it is found that the incorporation of lithium will support the structure of the material during charge and discharge,and it is beneficial to improve the ionic conductivity of sodium ions.It can also inhibit the OP4/"Z"harmful phase transition under high pressure and the Jan-Taylor effect of Mn³⁺.So lithium doping can effectively improve the cycle stability of the material.In summary,a series of element doping experiments are carried out by a simple and convenient experimental method.It is finally determined that lithium doping has a good modification effect on Fe-Mn-based transition metal oxide materials,and excellent comprehensive performance is obtained.The Na_0.6Li_0.2Fe_0.2Mn_0.8O₂cathode material is studied at the same time,and its modification mechanism is provided.It provides an effective and feasible solution for the research and industrialization of cathode materials for sodium ion batteries.