Microstructure Regulation of O3/P3 Biphasic Layered Oxide Cathodes for Stable and Fast-Charging Sodium-Ion Batteries

The O3-type sodium cathodes hold great promise for energy storage. However, the complex phase transition and structural stability issues, especially at high voltage, heavily hinder their application, yet a precise phase/microstructure regulation for cathodes remains elusive. Herein, we propose a biphasic engineering strategy in the layered oxide cathodes (Na_0.67Mn_0.4Co_0.3-xCu_0.15Li_0.15Fe_xO₂, NMCCLF) with purposely designed O3/P3 biphasic integration for high-performance sodium-ion batteries (SIBs). By controlling the content of Fe in NMCCLF cathodes, the phase composition combined with the cationic potentials of cathodes can be precisely regulated. The optimized O3/P3 biphasic cathode exhibits the features of P-type and O-type materials, including the expanded interlayer spacing, accelerated Na⁺ diffusion kinetics, and increased oxygen vacancy concentration, thereby enabling attractive structural stability and fast-charging performance. Meanwhile, the designed biphasic cathode demonstrates a continuous and gentle phase transition process, especially at high voltages, thus showing superior electrochemical stability. Consequently, the optimized biphasic cathode delivers an attractive reversible capacity and cycling stability under an elevated charging cutoff voltage of 4.5 V, achieving a reversible capacity of 151.6 mAh g^-1 at 0.1C and ultrahigh capacity retention of 93.91% after 200 cycles at 1C. Additionally, the optimized cathodes retain approximately 60 mAh g^-1 even at a high C-rate of 10C, verifying their superior fast-charging performance. The hard carbon∥NMCCLF5 full cells also display a reversible capacity of 118.16 mAh g^-1 and a slight capacity decay for 100 cycles. This study provides insights for designing highly stable and fast-charging layered oxide cathodes for SIBs.