Mastering the synthesis of high Na-content, moisture-stable layered oxide cathode for Na-ion batteries

Sodium layered oxides Na_xMO₂ (x ≤ 1 and M = transition metal) are of great interest for sodium-ion batteries due to their high energy density and cost-effectiveness. However, these materials, whether they are stoichiometric (Na/M ≈ 1 as in O3 NaMO₂) or not (Na/M ≈ 0.7 as in P3/P2 Na_xMO₂), have certain disadvantages, namely sensitivity to humidity or inadequate capacity, respectively. Herein, we propose an intermediate composition Na_0.85Ni_0.38Zn_0.04Mn_0.48Ti_0.1O₂ that we succeed to stabilize in either O3 or a nanoscale mixture of O3–P3 or O3–P2 phases as proven by X-ray diffraction and transmission electron microscopy, through complex synthesis approaches including quenching, slow cooling and annealing in different atmospheres (Ar, air, O₂ etc). We rationalize the stabilization of different phases and microstructure as a function of synthesis conditions and show how it influences the electrochemical performance. Through this study we identified a single phase O3 Na_0.85Ni_0.38Zn_0.04Mn_0.48Ti_0.1O₂ synthesized at 1000 °C in air, which exhibits a high capacity of ∼170 mAh/g and good moisture stability. Furthermore, thanks to the synthesis-structure- electrochemical performance relationship identified here, we believe that this study will provide a reliable basis for optimizing the synthesis for best performing sodium layered oxides for commercialization.