Exposed {010} Crystal Surfaces Drive High Rate Performance and Cyclability in Air Stable P2-Type Cathode for Na-Ion...

Particle morphology and synthesis conditions play a major role in determining the electrochemical behavior of layered oxide cathodes. Herein, various structures (P2, O3, and P2/O3 biphasic) are achieved for dual pillar-ion doped Na_xMn_0.47Ni_0.33Ti_0.1Al_0.1O₂ by varying sodium content and calcination temperature. P2-type sample with 0.77Na exhibits excellent electrochemical properties, with a reversible specific capacity of 141.5 mAh g^-1 at 0.1C (1.5-4.1 V) and capacity retention of 93% after 100 cycles. Owing to the larger Na⁺ conducting lateral {010} surfaces, this cathode achieves the best rate-performance among all compositions, with about 84% capacity at 2C relative to 0.1C, as confirmed by galvanostatic intermittent titration technique and in situ electrochemical impedance spectroscopy (EIS) data. The distribution of relaxation times (DRT) analysis of temperature-dependent EIS confirmed a much lower charge-transfer activation energy (≈0.54 eV) in the P2-Na_0.77Mn_0.47Ni_0.33Ti_0.1Al_0.1O₂ cell as compared to the O3-analogous cell (≈0.97 eV). Ex situ XRD measurements showed that while O3-Na_1.00Mn_0.47Ni_0.33Ti_0.1Al_0.1O₂ undergoes a determinantal O3↔P3 phase transformation, Na0.77-850 cathode maintained a P2 dominant framework during charge-discharge cycling in the 2.0-4.0 V range with low lattice strain. A Na0.77-850||hard-carbon full cell showed 85% capacity retention after 100 cycles, highlighting its potential for practical applications.