New Observation Structural Change from O3 to P2 Type by Changing Sodium Contents for Na-Ion Batteries

Stable Na⁺-ion-storage cathodes with adequate reversible capacities are increasingly needed for the application of Na-ion batteries in large-scale, low-cost electric storage systems. Ion-storage oxides have been classified into P2- and O3-type phases based on their sodium content. There have been few studies on the structural stability and electrochemical properties of such oxides. Here, we report the synthesis of a sodium-ion battery (SIB) cathode material Na[Ni_0.8Co_0.1Mn_0.1]O₂ (x = 0.67, 1) using a hydroxide co-precipitation method. The effects of different sodium contents on the structural and electrochemical properties of this cathode material were studied. The results indicated better electrochemical performance of the P2-type materials compared to the O3-type in terms of high discharge capacity and good cycling performance. The P2-Na_0.67[Ni_0.8Co_0.1Mn_0.1]O₂ cathode exhibited a good charge storage capacity of 108.74 mAhg^-1, with a capacity retention of over 67% after 50 voltage cycles (between 2.0 and 4.5 V) at 0.1 C. The developed material may potentially be used as a cathode in sodium-ion batteries.