Structural Insights into Phase Formation of Sodium Layered Cathodes Materials with Prominent Electrochemical Performances.

The electrochemical performances of layered cathode materials for sodium-ion batteries (SIBs) are intimately dependent on their structural characteristics. However, realizing accurate regulation of phase structure by phase engineering is challenging, primarily due to constrained synthesis methods and the existing gaps in understanding of specialized phase structures. In this study, a series of P'2-Na0.67Fe0.05Ti0.1Mn0.85O2 cathode material with prominent electrochemical performances were successfully synthesized, based on an in-depth understanding of structural insights into P'2 phase. By analyzing the structural evolution and Mn-valence changes during synthesis process, we found that oxygen vacancies play a significant role in determining the P'2-P2 phase transition. Moreover, these structural insights not only identified the oxygen release and uptake behaviors in phase formation but also expanded synthesis strategy with enhanced operational feasibility. Benefits from expanded Mn redox range and stable oxygen vacancies during electrochemical cycling, the obtained P'2-Na0.67Fe0.05Ti0.1Mn0.85O2 demonstrated a capacity increase of over ~40 mAh g-1 at 0.1 C, maintaining ~93 mAh g-1 even after 1000 cycles at 10 C, with an impressive retention rate of 87.5%. This research significantly advances the comprehension of both synthesis mechanism and electrochemical properties optimization mechanisms of P'2 phase materials, offering a pragmatic strategy for elevating the performance of SIB materials.