Research On Na_xNi_yNn_zO₂Cathode Material For Sodium Ion Batteries

With the increasing concerns about energy problem, the development of renewableenergy, such as solar energy, wind energy has become a hot research area. However thesekinds of energy are discontinuous and unstable, which require suitable energy storagestations. Due to the advantages of low price, high abundance and safety, sodium ionbattery has a great potential in energy storage area. Development of sodium storagematerials has become an important research project.In this work, NaxNiyMnzO2layered metallic oxides have been synthesized via spraydrying method and a two step solid state process. XRD, SEM, ICP were used tocharacterize the physical properties of the prepared products. Furthermore,electrochemical performance of the sample materials were investigated.For preparing NaNi1/2Mn3/2O4, when the metallic ion ratio of source material wasNa:Ni:Mn=2:1:3, the product presented a ratio of Na:Ni:Mn=1.88:1.05:3.00, becauseannealling at high temperate led to sodium volatilization. XRD pattern of the preparedsample showed sharp peaks of P2-type Na2/3[Ni1/3Mn2/3]O2(PDF54-0894), small peaks ofNiMn2O4could also be detected. These results indicate that the main compound in theprepared product was P2-Na2/3[Ni1/3Mn2/3]O2. SEM image showed a well definedhexagonal shape of most particles. Electrochemical performances of the sample weremeasured in three different voltage ranges. When the cycling voltage range was between2.0-4.5V, the crystal structure of P2-Na2/3[Ni1/3Mn2/3]O2was irreversibly damaged due toover extraction of Na+in4.0V-4.5V, which leads to the rapid decrease of specificcapacity. Cycle in lower voltage range (1.6-3.8) also leads to capacity lose due to overinsertion of sodium ion in the crystal structure.The effects of annealing condition on the products were investigated. Whenannealing temperature was700°C, the P2-Na2/3[Ni1/3Mn2/3]O2crystal structure was notdetected; annealing at800°C could obtain the product we expected, but impurity phasecould also be detected; when temperate improved to900°C or above, some impurity peaks disappeared. Electrochemical performance showed that the material annealed at800°C showed better capacity and stability. We also compared the samples prepared byannealing at800°C for different hours. Electrochemical performance showed that thesample obtained by annealing at800°C exbited the largest specific capacity and best cyclestability. So, the best condition for preparing NaNi1/2Mn3/2O4is annealing at800°C for8hours.Based on the above investigation results, we changed the ratio of metal ions, andprepared a material named Na2/3Ni1/3Mn2/3O2. ICP measurement showed that the finalmetal ion ratio is Na:Ni:Mn=1.90:1.05:2.00. Sharp XRD peaks were detected and wellconsistent with standard PDF54-0894, also impurity phases such as NaNi2Mn4wereobserved. SEM image of the sample powder showed well definded layered hexagonalshape, and the thickness of layer is about0.3um. Charge&discharge curve showed threeobvious charge plateaus at3.31V,3.70V and4.25V. P2type structure and O2typestructure change happened when cycle in higher voltage range (4.0-4.5V), and led tostructure broken; cycling in low voltage also lead to a gentle decrease in capacity. TheP2-Na2/3[Ni1/3Mn2/3]O2material show stable cycle performance and a relatively highcapacity in2.0-4.0V voltage range, and is a potential cathode material for sodium ionbattery.