An O3-type NaNi_0.5Mn_0.5O₂ cathode for sodium-ion batteries with improved rate performance and cycling stability

Layered O3-type NaNi_0.5Mn_0.5O₂ (space group: R3m) was synthesized by a sol-gel method, and subjected to electrochemical characterization for sodium-ion batteries (SIBs). A Na//NaNi_0.5Mn_0.5O₂ cell can deliver a reversible capacity of 141 mA h g⁻¹ in the voltage range of 2.0-4.0 V and show a good capacity retention of 90% after 100 cycles. A highly reversible structural evolution of O3_hex.-O3′_mon.-P3_hex.-P3′_mon.-P3′′_hex. upon Na⁺ extraction and intercalation is demonstrated to be the key factor to its excellent cycling capability. Furthermore, the apparent diffusion coefficient of Na ions in the layered O3-type NaNi_0.5Mn_0.5O₂ composite electrode can be effectively increased by using good conductive CNTs, which is demonstrated by cyclic voltammograms and the galvanostatic intermittent titration technique. Thus even at a high rate of 2C, the O3-NaNi_0.5Mn_0.5O₂ cathode still exhibits a high reversible capacity of 80 mA h g⁻¹. The impressive sodium storage properties of O3-NaNi_0.5Mn_0.5O₂ make it a promising candidate as a positive electrode material for rechargeable SIBs.