Solid-state Synthesis And Modification Of Na_0.67Ni_0.33Mn_0.67O₂ Cathode Material For Sodium-ion Battery

The binary layered transition metal oxide Na0.67Ni0.33Mn0.67O2 cathode material has attracted much attention due to its abundant raw materials,high discharge capacity,open two-dimensional Na+diffusion channels,and environmental friendliness.It has become one of the most promising candidates for sodium-ion battery systems.Aiming at the problem of poor cycling performance of Na0.67Ni0.33Mn0.67O2 cathode material due to P2-02 phase transition during charge and discharge process,the solid-state synthesis process of Na0.67Ni0.33Mn0.67O2 was optimized,and the modification research was carried out by multi-site ion doping and phosphate surface coating.In this work,the effects of technic parameters such as sodium source content,calcination temperature,and calcination time on the structure and performance of Na0.67Ni0.33Mn0.67O2 cathode materials synthesized by solid-state method were studied in detail.The optimum technic parameters of sodium source excess of 3.5%,calcination temperature of 900℃ and calcination time of 10 h were obtained.On this basis,electrochemically active Cu2+and electrochemically inactive Mg2+were doped in the Ni site,and Li+was doped in the Na site of the Na0.67Ni0.33Mn0.67O2 cathode material,respectively.It was found that Cu2+doping effectively inhibited the P2-O2 phase transition at high voltage,and improved the structural stability and cyclic stability of the cathode material.Among them,the capacity retention of Na.67Cu0.15Ni0.18Mn0.67O2 cathode material was 78%at a rate of 0.1 C after 200 cycles.Mg2+doping stabilized the P2 phase structure of the cathode material during charge and discharge process.Among them,the specific discharge capacity of the Na0.67Mg0.15Ni0.18Mn0.67O2 cathode material reached 67 mAh g-1 at a high rate of 20 C,and the capacity retention was up to 92.5%after 120 cycles.It was found that the introduction of Li+in the Na site improved the structural stability of the cathode material under high voltage.Among them,the capacity retention of the Li0.2Na0.8Ni0.33Mn0.67O2 cathode material was 61.5%at a rate of 0.1 C after 200 cycles.Ti4+ was doped in the Mn site of the Na0.67Ni0.33Mn0.67O2 cathode material,and the sample with the best electrochemical performance was coated with MnPO4.It was found that Ti4+doping can smooth the charge-discharge curves and effectively suppressed the P2-O2 phase transition of the cathode material.Among them,the capacity retention of Na0.67Ti0.30Ni0.33Mn0.37O2 cathode material reached 66.4%at a rate of 0.1 C after 200 cycles.It was found that the MnPO4 coating layer can suppress the side reaction between the active material and the electrolyte.Among them,the Na0.67Ti0.30Ni0.33Mn0.37O2 cathode material with a 1 wt%coating content further increased the capacity retention to 87.2%at a rate of 0.1 C after 200 cycles.