Synthesis And Sodium Storage Properties Of Layered Manganese Cobalt-Based Cathode Materials

Sodium-ion batteries are the most promising large-scale energy storage systems for the national grid.P2-layered manganese oxide is one of the most advantageous anode materials for sodium ion batteries due to its high theoretical capacity,high energy density,environmental friendliness and easy preparation.But at the same time,these materials face a number of problems:Jahn-Teller distortion,irreversible phase transition,Na⁺/vacancy order and electrode dissolution caused by Mn³⁺.In order to improve the above problems,Mn-Co-based oxide Na_0.75Co_0.20Mn_0.80O₂ cathode material with high sodium content was synthesized by high temperature solid phase method,and Nb⁵⁺-doping and Ga₂O₃modification were performed on the pristine,respectively,and the effects of the doping and the modification on the microstructures and morphology and sodium-storage properties of the pristine were studied by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and field emission scanning electron microscopy（FESEM）,as well as comprehensive electrochemical performance tests such as galvanostatic charge-discharge,cyclic voltammetry and electrochemical impedance spectroscopy,respectively.The parent material was doped with Nb⁵⁺by high temperature solid phase method.The P2-type structure and layered hexagonal morphology of the parent material were not changed by Nb⁵⁺-doping,and the distribution of the doped elements was uniform.The XRD Rietveld results show that the introduced niobium ion replaces the manganese ion and enters the Mn site,and the stability of the crystal structure is enhanced due to the larger Nb–O bond energy.At the same time,Nb⁵⁺-doping increases the spacing of sodium layers,and widens the insertion/extraction channel of Na⁺,inhibiting the ordered structure of Na⁺/vacancy,and significantly improving the comprehensive electrochemical performance of the parent material.Ex-situ XRD shows that the doped material has a highly reversible P2-P3-P2 phase transition during charging and discharging process.Under 0.1 C,the doped material Na_0.75Co_0.20Mn_0.78Nb_0.02O₂presents the maximum discharge specific capacity of161.7 m Ahg^-1 which is 35.4 m Ah g^-1 higher than that of parent material,the capacity retention after 100 cycles is as high as 91.0%,which is far higher than 71.7%of the partent maternal.The parent material was modified with Ga₂O₃ by high temperature solid phase method.XRD-Rietveld refinements show that Ga³⁺enters the parent lattice in the process of Ga₂O₃modification,and the strong Ga–O bond shortens the TMO₂ layer,increases the sodium layer spacing,reduces the sodium ion diffusion barrier,which is conducive to sodium ion diffusion.The pseudocapacitance effect of Ga₂O₃ modified layer improves the reactivity of the material.The synergistic effect of Ga₂O₃ surface modification and Ga³⁺doping increases the initial discharge capacity of the electrode,and improves the cyclic stability and kinetic properties of the material.At 0.1 C,the modified material Na_0.75Co_0.20Mn_0.80O₂-1 wt%Ga₂O₃ exhibits the specific discharge capacity of 143.0 m Ah g^-1,which is significantly higher than 126.3 m Ah g^-1 of the parent material.At 5 C,the capacity retention rate of the modified Na_0.75Co_0.20Mn_0.80O₂ with 1 wt%Ga₂O₃ is 84.1%after 1000 cycles,while that of the parent electrode is only 78.6%.