Preparation And Modification Of Manganese-nickel-based Layered Oxide Cathode Materials For Sodium-ion Batteries

Due to the unique advantages of safety and cost of sodium-ion batteries?SIBs?,lithium-ion batteries?LIBs?can be replaced by SIBs with the ever-increasing demand of large-scale energy storage systems.However,short cycle life and poor rate performance restrict SIBs future commercial application.Cathode materials are considered to be the most critical component of the SIBs,which have an unparalleled effect on the electrochemical performances of SIBs.And among many cathode materials,manganese-nickel based layered oxides have received extensive attention due to high operating voltage and high capacity.However,the instability in the air and unsatisfactory cycling performance limit their 4application.Focus on the problems of manganese-nickel based oxide as a cathode material for SIBs.In this paper,the electrochemical properties of manganese-nickel-based layered oxides are improved by the strategies of element substitutions and layer coating.The innovative strategies are mainly as follows:?1?Firstly,a high electrochemical performance Co-substituted O3-type NaMn_2/3Co_1/6Ni_1/6O₂ is synthesized by hydrothermal method and high-temperature annealing.The as-prepared sample shows the uniform spherical morphology,and when cycled at a current density of 0.1 C,the material exhibits an initial specific capacity of153.6 mAh g^-1 with high capacity retention of 81.7%after 100 cycles,and the coulombic efficiency of 98%.Moreover,the sample also exhibits excellent rate performance and the reversible capacity is 91.6 mAh g^-11 at 1 C.The excellent electrochemical performances of the material is attributed to the substitution of Co to reduce the metal cation mixing of the transition metal layer,increase the diffusion rate of Na⁺,and improving the cycle performance of the material due to the effective inhibition of the phase transition of the material O3-O1.?2?The novel P3-Na_0.65Mn_0.75Ni_0.25O₂ cathode material doped with two non-metallic elements,fluorine and boron,is synthesized via hydrothermal method and subsequent low temperature annealing process.P3-Na_0.65Mn_0.75Ni_0.25O₂ exhibits excellent electrochemical performance due to the special crystal phase.The electrode material exhibits an initial specific capacity of 153.6 mAh g^-1 at a current density of 0.1C.And there is still 75.1%capacity retention after 100 cycles at a current density of0.2 C.As a comparison,the non-metallic element doped P3-Na_0.65Mn_0.75Ni_0.25O₂electrode materials show a better electrochemical performance.Among them,fluorine doped Na_0.65Mn_0.75Ni_0.25F_0.1O_1.9 can distinctly suppress P3-O1 phase transition and deliver a higher reversible capacity of 163.7 mAh g^-1 at 0.1 C,while boron doped Na_0.65Mn_0.75Ni_0.25B_0.1O₂ transforms from P3 phase to a more stable P2 phase and boron doping can effectively suppress the redox of oxygen anion.Na_0.65Mn_0.75Ni_0.25B_0.1O₂exhibits remarkable cycle life and rate performance.?3?In order to obtain a manganese-nickel-based layered oxide with excellent cycle stability and air stability.AlPO₄ and Mg₃?PO₄?₂ are employed as protective coatings to modify P3-type Na_0.65Mn_0.75Ni_0.25O₂.The metal phosphate layers cover the surfaces of the pristine particles via a co-precipitation route.The results of electrochemical performance tests show that the two modified samples exhibit an obvious improvement of cycling performance with the capacity retentions about 93%at a current density of 0.2 C within 100 cycles.In addition,after exposure to the air,the coating samples still retain a high electrochemical activity.