Rigid Flexible Pillaring via Synergistic Co‐Doping Stabilizes High‐Capacity Sodium Layered Oxide Cathode

O3‐type layered oxide cathodes offer high energy density but suffer from interlayer gliding and oxygen release at elevated voltages, leading to rapid capacity fade. Here, we propose a rigid–flexible coupled pillaring strategy, where Mg/Li co‐doping offers a versatile route to achieve excellent performance at high voltage. This stems from rigid Mg─O pillaring that suppresses slab gliding, together with compliant Li─O buffering that smooths the in‐plane potential. In tandem, closed‐shell Mg/Li weakens TM─O eg* antibonding and downshifts the O‐2p band center, promoting a tempered, reversible anionic‐redox reaction, thereby avoiding the usual trade‐off in which improvements in durability come at the expense of capacity. Consequently, this pair redirects the high‐voltage response from P3→OP2 to a P3 solid solution, cutting c‐axis breathing to ∼1.7%, and the cathode delivers a capacity of over 160 mAh g −1 at 4.2 V, retains 80% after 500 cycles at 5C. Furthermore, the cathode is scalable to kilogram batches. With excellent coin and Ah‐scale pouch‐cell performance, it shows guidance of design and commercial promise for high‐voltage O3‐type cathodes.