Enabling the synthesis of O3-type sodium anion-redox cathodes via atmosphere modulation

The pursuit of advanced battery chemistries with enhanced energy density necessitates the exploration of new materials, a process intricately tied to synthesis science. Despite the promise of O3-type sodium oxygen anionic redox cathodes as high-capacity materials, their development has been severely hindered by a lack of understanding regarding synthetic mechanisms. Here, we elucidate the pivotal role of atmospheric conditions, particularly oxygen content, in the synthesis of such materials by synchronizing multiple operando characterization techniques to monitor changes in both solid and gaseous components. Utilizing the O3-Na[Li_1/3Mn_2/3]O₂ system as a model, we demonstrate that a low oxygen environment is essential and the reaction is highly complex as evidenced by multiple oxygen uptake and release processes, resulting in numerous intermediates. This behavior contrasts sharply with Na-Mn-O and Li-Mn-O ternary systems which show less significant oxygen dynamics, underscoring the unique reaction mechanism within the Na-Li-Mn-O system. We further adopt a dynamic controlled atmosphere approach to modulate oxygen concentration and demonstrate successful synthesis of Ti-substituted NaLi_1/3Mn_2/3-xTi_xO₂ materials, all exhibiting capacities surpassing 190 mAh g^-1. Our findings highlight the importance of the atmospheric conditions for the synthesis of oxide cathode materials and these fundamental insights unlock avenues for developing novel high-energy-density sodium-ion battery chemistries.