Study On The Preparation And Electrochemical Properties Of Sodium-manganese-based Layered Oxides

P2-type sodium-manganese-based layered oxides stand out among many cathode materials for sodium ion batteries due to their high theoretical specific capacity,rich reserves,and low cost.However,with the deepening of research,it is found that there are shortcomings and problems that cannot be ignored in the sodium-manganese-based layered oxides.For example,the Jahn-Teller effect caused by Mn³⁺,the dissolution of Mn²⁺and the irreversible phase transition caused by the repeated stripping/embedding of Na⁺.These defects reduce the stability of the structure,resulting in serious capacity attenuation.In order to solve the above problems and improve the electrochemical performance of sodium-manganese-based cathode materials,the Na-Co-Mn system was selected as the parent material and synthesized Nb⁵⁺-doped,Al³⁺-doped and Nb⁵⁺/Al³⁺co-doped cathode materials by high-temperature solid-state method.A series of structural characterizations of the materials before and after doping were carried out,and a set of electrochemical performance tests were conducted by assembling a half cell.The effects of Nb⁵⁺-doping,Al³⁺-doping and Nb⁵⁺/Al³⁺co-doping on the structure,morphology and electrochemical performance of the parent material were explored.The introduction of Nb⁵⁺and Al³⁺into the parent structure does not change the original microstructure and morphology.At the same time,Nb⁵⁺and Al³⁺partially replace the manganese ion,forming a stronger Nb–O bond and Al–O bond in the transition metal layer.The ex-situ XRD results show a good structural reversibility of the Nb⁵⁺/Al³⁺co-doped sample during charging and discharging.In addition,the CV curve shows that the three doping methods significantly improve the electrochemical activity of the electrode,resulting in better magnification performance.At the current density of 0.1 C,in the voltage range of 2.0–4.0 V,the three doped electrodes can release higher specific capacity than the original electrode after 100 cycles,and the capacity retention rates are 88.6%,81.2%and90.4%,respectively.It is worth noting that the capacity retention rate of the co-doped electrode after 1000 cycles at a current density of 5 C is 73.9%,which is 25.3%higher than the original electrode.Single Nb⁵⁺-doping,single Al³⁺-doping and Nb⁵⁺/Al³⁺co-doping all extend the sodium layer spacing and reduce the diffusion barrier of Na⁺,so that the doped sample has good electrochemical kinetic performance.The XPS results indicate that Nb⁵⁺/Al³⁺co-doping reduces the content of Mn³⁺,thus alleviating the Jahn-Teller effect caused by Mn³⁺.Meanwhile,the proportion of surface oxygen decreases from 64.85%to 52.57%,indicating that Nb⁵⁺/Al³⁺co-doping effectively inhibits the formation of surface impurity Na₂CO₃/Na OH,and improves the air stability of the material.