Droplet-like Na/Vacancy Ordering Enables Ultrahigh-Na-Content P2-Type Oxide Cathodes

Layered P2-type transition-metal oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high specific capacity and rapid Na⁺ diffusion, but their Na-deficient nature would induce high-voltage phase transitions and limit the quantity of active sodium ions in full cells, impeding the practical implementation of such materials. Herein, we report a P2-Na_0.91Ni_0.18Cu_0.08Mn_0.74O₂ (H-Ni0.18) cathode with an ultrahigh Na content of 0.91 enabled by a "droplet-like" Na/vacancy ordering. This Na-layer superstructure ordering at such a high Na level, systematically confirmed by synchrotron X-ray techniques, neutron diffraction, and theoretical computations, effectively minimizes the electrostatic repulsion among Na ions and lowers the total system energy, thereby stabilizing the P2 framework during synthesis. Benefiting from this high-Na configuration, the H-Ni0.18 cathode demonstrates pure solid-solution reaction behavior within 2.0-4.3 V and excellent cycling performance in half-cells. More impressively, the H-Ni0.18 cathode can also act as an intrinsic self-sacrificial Na reservoir, enabling the assembled H-Ni0.18//hard carbon full cell to achieve a respectable cycling stability (82.8% capacity retention after 150 cycles), superior to its low-Na analogue. This unique ordering-structure engineering provides a new design paradigm for developing ultrahigh-Na-content P2-type cathode materials for high-performance SIBs.