Dual-Site Doping Enables a Stress-Balanced and Single-Phase Cathode for Durable Sodium-Ion Batteries

P2-type Na 0.67 MnO 2 cathodes suffer from irreversible phase transitions caused by interlayer stress during sodium extraction/insertion. Here, we demonstrate that trace Ca 2+ doping in Na layers serves as structural pillars to modulate the stress evolution. The strong, electronegative Ca–O bonds enhance interlayer cohesion, while the fixed Ca 2+ cations shield the repulsive forces between the transition-metal layers. Combined with transition-metal (TM) site codoping that stabilizes TMO 6 octahedra, an ordered evolution of the a / b -axes during charge and discharge cycles. This dual-doping strategy enables near-zero c -axis variation (0.1% at 4.3 V) and maintains a single-phase reaction mechanism throughout cycling. This interlayer engineering approach yields enhanced structural stability and cycling performance, providing a general design principle for durable layered oxide cathodes in sodium-ion batteries.