Fundamentals of NaMnO2, the cathode material for Na-ion rechargeable batteries

Sodium manganese oxide (NaMnO2) is perhaps the most promising cathode material for Na-ion rechargeable batteries, owing to a reported charge capacity comparable to that of commercial LiCoO2. The combination of earth abundant and environmentally benign atomic constituents and the good thermodynamic and structural stability makes NaMnO2 a potential cornerstone for Li-ion replacement in batteries. At the fundamental level, NaMnO2 is characterized by a complex landscape of intertwined structural, electronic, and magnetic properties. Using a combination of advanced ab initio approaches, we reveal a nature far more intriguing than typically reported. Strikingly, while magnetic frustration is commonly believed to dominate the behavior of both α and β polymorphs, we find its impact largely suppressed due to the formation of MnO2 stripes patterned by a peculiar magnetic ordering at the intra-atomic length scale. This phenomenon underlies the extremely anisotropic magnetic interactions and establishes NaMnO2 as a paradigmatic case of magnetic frustration primarily relieved by electronic mechanisms, rather than by structural distortions.