NIBs have attracted significant interest during the last decade due to their cost-effectiveness compared to LIBs since sodium is the 6th most abundant element in the earth’s crust to a depth of 16 km.[1-3] Additionally, NIBs undergo reactions that are similar to those of LIBs in terms of ion intercalation and solid-state diffusion, phase transitions, surface film formation, and interfacial charge transfer processes, thereby facilitating the R&D process. 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 nanorods 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.

References

[1] B. Scrosati, J. Hassoun, Y. K. Sun, Energy Environ. Sci. 2011, 4, 3287.

[2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 2011, 4, 3243.

[3] J. Emsley, Nature’s Building Blocks, Oxford University Press, Oxford, UK 2011.