Reserach On Doping Modification Of P2-Na_0.66Ni_0.33Mn_0.67O₂ Layer Cathode Material For Sodium Ion Battery

Lithium-ion batteries,as the most successful and advanced rechargeable batteries at present,are widely used in portable electronic products and electric vehicles due to their advantages of long cycle life and high volumetric energy density.However,due to the uneven distribution of lithium resources and price considerations,it is difficult to meet the growing market demand.Sodium,which is in the same main group with similar physical and chemical properties,is expected to become next generation of commercial secondary batteries for large-scale energy storage applications due to its abundant content and low cost.Electrochemical properties of cathode materials,such as voltage and capacity,play a leading role in battery systems.Na-deficient P2-type Na_0.67Ni_0.33Mn_0.67O₂（NNMO）layered oxide is promising candidate for sodium-ion batteries（SIBs）,due to their superior theoretical specific capacity(173 m Ah g^-1),high operating voltage（3.6V）and two-dimensional Na⁺transport channel.However,lattice distortion of Mn O₆ octahedra induced by the Jahn-Teller effect of Mn³⁺,as well as rearrangement of ordered structure and occurrence of irreversible phase transitions,make layered manganese-based materials have insufficient Na⁺diffusion kinetics and inferior structure stability,which in turn result in poor cycling performance and rate capability,limit their progress toward commercialization.this paper adopts the modification strategy of ion doping to suppress its severe structural degradation or improve the redox activity of transition metal ions（Ni/Mn2p）,and uses multiple characterization to explore the reasons for its improved electrochemical performance.Two aspects of work carried out are as follows:1.Aiming at the problems of Na⁺/vacancy ordering and unfavorable phase transition under high voltage in Na layers,Ti⁴⁺with similar ionic radius and different redox potential as transition metal Mn⁴⁺was considered to replace transition metal（2a）site to break charge ordering in the transition metal layer and the Na⁺/vacancy ordering in the alkali metal layer.And Ni a suitable amount of Zn element substitution,through the construction of Na-O-Zn configuration,promote the reversible redox of anion,reducing the interlaminar slip is caused by the strong electrostatic repulsion of oxygen in 4.2 V,thus inhibiting the appearance of O2phase.Moreover,the Zn/Ti O₆octahedron can retain a small amount of Na⁺in Na layers due to the charge compensation mechanism,and it serve as pillar.And at the same time the preparation of Zn/Ti doped（NNZMTO）at single element doping and ion doping of synergy effect on the properties of materials.X-ray diffraction and inductively coupled plasma emission spectroscopy showed that the target product was successfully prepared.The samples were tested in the voltage range of 2.5-4.3 V for a long cycle and rate test.It was found that the dual-element doped samples had the best electrochemical performance,and could still provide specific discharge capacity of 91 m A h g^-1 at the rate of 7 C.The charge-discharge curve of NNMTO shows that the ordered rearrangement platform with sodium vacancy becomes smooth,but the phase transition problem still exists.The introduction of Zn ion can obviously inhibit the phase transition at4.2 V high voltage.Zn/Ti co-doped samples have better structural stability and diffusion kinetics in the high voltage operating range.Further studies indicate that in order to design high energy density sodium-ion batteries,inhibition of phase transition formation and rearrangement of Na⁺/vacancy ordered structures must be considered simultaneously.2.Cationic doping mainly focuses on crystal modulation and stabilization of bulk phase structure and promotion of anionic redox,while anionic doping can adjust the electrochemical activity of Mn³⁺/Mn⁴⁺and Ni²⁺/Ni⁴⁺cationic redox ions due to the principle of charge neutrality.Therefore,the more electronegative F^-is used to replace the O^2-site appropriately to regulate Ni2p and stimulate the electrochemical activity of Mn³⁺ions,and the presence of Mn³⁺ions will break the transition metal order between Ni²⁺/Mn⁴⁺ions and the Na⁺/vacancy ordered structure in the alkali metal layer.The concomitant structural modulation and stronger TM-F bond further stabilize the transition metal oxide layer and improve the diffusion coefficient of sodium ion.The same preparation process was synthesized Na_0.67Ni_0.33Mn_0.67O_2-xF_x（x=0,0.05,0.07,0.1）subsequent mark of 0.05F-NNMO,0.07F-NNMO,0.1F-NNMO.Scanning（transmission）electron microscopy and X-ray photoelectron spectroscopy（XPS）were used to characterize the microstructure of the samples and determine the elemental valence states respectively.Through the comparison of electrochemical properties,it is found that the doping molar amount of 0.07 is the best performance.Electrochemical impedance（EIS）,variable sweep rate CV and in-situ XRD tests are used to study the sodium storage kinetics and structural evolution of the materials.