Surface Engineering Stabilizes Rhombohedral Sodium Manganese Hexacyanoferrates for High‐Energy Na‐Ion Batteries

The rhombohedral sodium manganese hexacyanoferrate (MnHCF) only containing cheap Fe and Mn metals was regarded as a scalable, low-cost, and high-energy cathode material for Na-ion batteries. However, the unexpected Jahn-teller effect and significant phase transformation would cause Mn dissolution and anisotropic volume change, thus leading to capacity loss and structural instability. Here we report a simple room-temperature route to construct a magical Cox B skin on the surface of MnHCF. Benefited from the complete coverage and the buffer effect of Cox B layer, the modified MnHCF cathode exhibits suppressed Mn dissolution and reduced intergranular cracks inside particles, thereby demonstrating thousands-cycle level cycling lifespan. By comparing two key parameters in the real energy world, i.e., cost per kilowatt-hours and cost per cycle-life, our developed Cox B coated MnHCF cathode demonstrates more competitive application potential than the benchmarking LiFePO₄ for Li-ion batteries.