Abstract
Sodium layered oxides NaxMO2 (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 NaxMO2), have certain disadvantages, namely sensitivity to humidity or inadequate capacity, respectively. Herein, we propose an intermediate composition Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 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 Na0.85Ni0.38Zn0.04Mn0.48Ti0.1O2 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.