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

Abstract 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 Co x B skin on the surface of MnHCF. Benefited from the complete coverage and the buffer effect of Co x 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 Co x B coated MnHCF cathode demonstrates more competitive application potential than the benchmarking LiFePO 4 for Li‐ion batteries.