A robust layered Na_0.67Mn_0.67Ni_0.33O₂ cathode with enhanced reversibility for sodium-ion batteries

The development of cathode materials with high structural stability and excellent electrochemical reversibility is critical for advancing sodium-ion battery (SIB) technology. In this work, layered Na_0.67Mn_0.67Ni_0.33O₂ was synthesized via two distinct solid-state routes: conventional dry milling and acetone-assisted wet milling. The wet-milled approach resulted in a phase-pure layered oxide, whereas the dry-milled product exhibited minor NiO impurities. Despite this, the dry-milled sample demonstrated superior electrochemical reversibility and capacity retention compared to its phase-pure counterpart. These findings highlight the complex interplay between phase purity and functional performance, offering new insights into the optimization of sodium-ion battery cathode materials. X-ray diffraction (XRD) and transmission electron microscopy (TEM) verified the development of a well-organized layered structure, whereas scanning electron microscopy (SEM) showed evenly distributed submicron particles. Inductively coupled plasma (ICP) analysis confirmed the intended stoichiometry, while X-ray photoelectron spectroscopy (XPS) offered information on the oxidation states of Mn and Ni, strengthening the material's structural integrity. Electrochemical investigations revealed an impressive initial discharge capacity of 190 mA h g^-1 across an extensive voltage range of 1.5-4.7 V, along with remarkable reversibility and prolonged cycling stability. The refined synthesis technique not only reduced sodium volatilization but also encouraged even elemental distribution, leading to diminished polarization and improved redox activity. The cohesive structural arrangement, uniform composition, and advantageous electrochemical characteristics solidly confirm Na_0.67Mn_0.67Ni_0.33O₂ as a strong layered oxide cathode. This study highlights the promise of optimized solid-state synthesis as an economical and scalable approach for creating next-generation SIB cathodes with enhanced stability and reversibility.