Unusual Dual-Site Substitution Adjusts the Degree of Stacking Disorder in Honeycomb Na₃Ni₂SbO₆ Cathode for Sodium-Ion Batteries

O′3-Na3Ni2SbO6 with a honeycomb cation order, as a potential cathode, presents simplified phase-transition steps and a high average voltage. To mitigate the intrinsic phase irreversibility, Mg, Zn, and Co have been introduced to displace part of the Ni, which inevitably reduces the theoretical capacity related to the Ni2+/Ni3+ redox reaction. In this work, an unusual dual-site substitution is carried out to increase the P′3–O′3 structure reversibility without sacrificing the practical capacity. In addition, it is found that special stacking faults along the c-axis direction can be induced by doping to result in incomplete Sb/Ni disorder, though the honeycomb order remains in every TM (transition-metal) layer. The codoped Na2.85Cs0.15Ni1.9Mg0.1SbO6 has a high degree of disorder, which breaks the ideal monoclinic symmetry (C2/m) and partly upgrades its structure to higher-symmetry models. Profiting from the influence of stacking disorder and doping ions on the coordination environment around Na, more gradual and smaller variations of the lattice parameters appear upon Na-ion extraction/insertion. Consequently, this cathode displays a high initial discharge capacity (120 mAh g–1), long-term cycling stability, and excellent rate performance (66 mAh g–1 under 10 C). These findings reveal that not only a full TM-disordered arrangement but also this incomplete stacking disorder can effectively improve the performance of a layered cathode.