High-throughput design of Na-Fe-Mn-O cathodes for Na-ion batteries

Na-ion batteries are considered to be environmentally favourable alternatives to Li-ion batteries, particularly in the extremely large-scale application of grid storage, given the abundance of Na. However, to date, the battery performance has not been competitive, and promising Na-Fe-Mn-O materials have been plagued by poor stability in air and unsatisfactory long-term cycling. Herein, high-throughput methods are used to systematically screen the entire pseudo-ternary system. The resulting phase diagram shows significant solid solution regions for 3 important layered-oxide phases: P2, P3 and O3. While only a few compositions were previously studied, we identified over 50 single-phase materials of potential interest for battery materials. The results help explain contradictions in the literature and resolve decisively the locations of each of these phases. The phase mappings were also interpreted within the context of the recently developed cationic potential approach, and we extend the approach to P3 materials for the first time and find that cationic potential cannot be used to distinguish all P2 and P3 materials. High-throughput XPS is used on aged samples to determine stability in air and demonstrates that the stability with both carbon dioxide and moisture is very sensitive to composition and structure (stability changes by an order of magnitude within the Na-Fe-Mn-O system). Importantly, the under-studied P3 and Mn-rich P2 regions show the best stability in air. High-throughput electrochemistry shows that the Mn-rich P2 material also shows the highest first discharge capacity of 206 mA h g(-1) and excellent specific energy of 504 W h kg(-1). The findings herein therefore provide many insights of importance for the further development of these promising and challenging materials.