Out-Of-Plane Symmetry Design Arrests Structural Evolution in Layered-Type Framework for Sustainable Sodium Shuttling

Sodium-ion batteries (SIBs) offer a compelling and sustainable solution to the challenges of the energy transition and lithium resource scarcity. However, the crystallographic diversity in sodium transition metal oxides (Na_<i>x</i>TM_<i>y</i>O₂) induces symmetry reconfigurations during de/sodiation, posing a formidable challenge to the stable operation of SIBs. Here, we elucidate the critical yet underexplored role of out-of-plane symmetry in mitigating structural evolution in Na_<i>x</i>TM_<i>y</i>O₂. By engineering a monoclinic distortion, we establish a distinctive out-of-plane symmetry enabling the coexistence of P- and O-type interstitial sites for Na ions, effectively preventing interlayer oxygen ion slipping and subsequent symmetry evolution, even in deeply desodiated states (with <0.2 Na⁺ per formula remaining). This symmetry engineering fundamentally addresses the intrinsic phase instability in P3-type cathodes and eliminates cumulative voltage hysteresis arising from irreversible structural evolution (average hysteresis of 0.16 V over 100 cycles), achieving stable cycling under high-energy-density operation (437.1 Wh kg^-1).