Novel Cathode Materials for Na-Ion Batteries Composed of Spoke-Like Nanorods of Na[Ni_0.61Co_0.12Mn_0.27]O₂ Assembled in Spherical Secondary Particles

The development of high-energy and high-power density sodium-ion batteries is a great challenge for modern electrochemistry. The main hurdle to wide acceptance of sodium-ion batteries lies in identifying and developing suitable new electrode materials. This study presents a composition-graded cathode with average composition Na[Ni_0.61Co_0.12Mn_0.27]O₂, which exhibits excellent performance and stability. In addition to the concentration gradients of the transition metal ions, the cathode is composed of spoke-like nanorods assembled into a spherical superstructure. Individual nanorod particles also possess strong crystallographic texture with respect to the center of the spherical particle. Such morphology allows the spoke-like nanorods to assemble into a compact structure that minimizes its porosity and maximizes its mechanical strength while facilitating Na⁺-ion transport into the particle interior. Microcompression tests have explicitly verified the mechanical robustness of the composition-graded cathode and single particle electrochemical measurements have demonstrated the electrochemical stability during Na⁺-ion insertion and extraction at high rates. These structural and morphological features contribute to the delivery of high discharge capacities of 160 mAh (g oxide)⁻¹ at 15 mA g⁻¹ (0.1 C rate) and 130 mAh g⁻¹ at 1500 mA g⁻¹ (10 C rate). The work is a pronounced step forward in the development of new Na ion insertion cathodes with a concentration gradient.