Hexagonal and monoclinic NaNi0.8Co0.15Al0.05O2 (Na-NCA) for sodium ion batteries

Sodium-ion batteries have emerged as a potential alternate battery chemistry to lithium ion batteries. In this paper, NaNi_0.8Co_0.15Al_0.05O₂ (Na-NCA) materials with hexagonal O3 or monoclinic O′3 structure were synthesized by either quenching or slow cooling after heating the precursors. Oxygen vacancies present during the cooling process were found to be the reason for structural differences. Quenched O3-type material has a lower concentration of Jahn-Teller active Ni³⁺, leading to the formation of high symmetry space group $R\overline{3}\text{m}$. Both materials show better electrochemical performance than NaNiO₂. In situ and Ex situ XRD were used to study the structure transitions of both materials during cycling. The structure change during cycling was found to be more reversible for quenched O3-type materials. It is anticipated that this study offers a new strategy for the development of sodium ion battery cathode materials.