Triple-Phase Synergistic Bulk-Surface Engineering Enables High-Rate and Long-Cycling P2/O3-Based Cathode for Na-Ion Batteries.

Layered oxide cathodes hold great promise for Na-ion batteries (NIBs) but suffer from rapid performance degradation under high voltage, primarily arising from severe lattice strain, transition metal migration, and detrimental surface degradation. Here, we report a triple-phase synergistic bulk-surface engineering (TP-SBSE) strategy that integrates a P2 phase, an O3 phase, and a surface disordered phase to address these challenges. In the bulk, guided by cationic potential control, we employ multielement composition modulation to construct a P2/O3 biphasic structure. The interlocking effect at the biphasic interfaces effectively suppresses structural strain and mitigates volume variations under high voltage. Meanwhile, surface reconstruction yields a protective disordered phase that not only suppresses electrode-electrolyte reactions but also alleviates anisotropic lattice strain, effectively inhibiting particle microcrack initiation and preserving bulk structural integrity. Owing to the TP-SBSE strategy, the resulting P2/O3-Na0.75Zn0.05Ni0.23Fe0.18Mn0.49Ti0.05O2 (P2/O3-NZNFMTO) cathode exhibits exceptional electrochemical performance. It delivers 80% capacity retention after 600 cycles at 1C and over 75% after 1000 cycles at 3C within 2.0 to 4.3 V, demonstrating a high rate capability and long-term cycling stability. Moreover, this material exhibits enhanced thermal and air stability. This work presents a TP-SBSE strategy and provides fundamental insights into developing high-voltage, long-life layered oxide cathodes for NIBs.