Chemical Design Principles for Managing the Capacity-Stability Trade-Off in High-Voltage Sodium Layered Cathodes

High-voltage layered transition-metal (TM) oxides are attractive for sustainable sodium-ion battery (SIB) cathodes, yet their development is constrained by the persistent capacity-stability trade-off. Here, we establish a functional-unit-based design framework by decomposing the layered oxide lattice into active, buffer, and skeletal units that govern redox capacity, structural accommodation, and framework stability. Through systematic evaluation of TMO₆ octahedra, NiO₆, MnO₆, and TiO₆ are identified as representative functional units and integrated into a series of NaNi_{x /10}Mn_{y /10}Ti_{z /10}O₂ (NMTxyz) oxides to validate functional cooperation. Binary and functionally mismatched ternary configurations exhibit poor or lopsided electrochemical behavior, whereas functionally matched architectures simultaneously deliver high capacity and durable stability, exemplified by NMT523 and NMT433 cathodes. To translate these insights into design guideline, two quantitative descriptors, active unit content and functional unit mismatch, are introduced to regulate the balance between capacity and cycling stability.