Ti4+ doping regulates cationic potential to construct a P2/O3 biphasic structure for synergistic optimization of cathode materials for sodium-ion batteries

Prime Novelty Statement • Ti 4+ doping successfully induces the transformation of the material from a single P2 phase to a more stable P2/O3 biphasic coexistence structure. • The stabilization mechanism of Ti 4+ doping is elucidated from a thermodynamic perspective using the total cationic potential and high-entropy design theories. • The optimized material achieves a ∼41% reduction in volume strain, significantly boosting structural reversibility and cycling stability. Layered oxides are ideal cathode materials for sodium-ion batteries (SIBs), but their practical application is limited by sluggish ionic kinetics and detrimental phase transitions under high potential. To address this issue, this study proposes a bulk doping modification strategy to optimize the Na 1.1 Ni 2/9 Fe 1/3 Mn 1/3 Cu 1/18 Mg 1/18 O 2 (NaNFMCM) cathode material via Ti 4+ doping. The results demonstrate that an appropriate amount of Ti 4+ doping (x = 0.1) can effectively regulate the total cation potential ( Φ cation ), driving the material to form a stable P2/O3 biphasic coexistence structure with uniform layered flake-like morphology and homogeneous elemental distribution. This structure thermodynamically suppresses irreversible phase transitions, enhances interfacial kinetics by reducing charge transfer resistance, and significantly reduces the lattice volume strain during Na + deintercalation/intercalation (with Δ V max decreasing from 11.48% to 6.79%). Benefiting from the enhanced crystal structural stability, the modified material exhibits a reversible capacity of 147.3 mAh/g at 0.1C. After cycling at various rates, it still delivers 139.6 mAh/g when the rate is returned to 0.1C, corresponding to a capacity retention of 94.8% relative to its initial 0.1C capacity. Combined with density functional theory calculations, this work elucidates the intrinsic mechanism by which doping modulates the crystal structure and optimizes electronic conduction and ion diffusion kinetics at the electronic level. From the perspectives of thermodynamics and structural evolution, this study reveals the stabilization mechanism of Ti 4+ doping, providing a clear design principle for the development of high-performance and high-stability layered oxide cathode materials for SIBs. The Na 1.1 Ti 0.1 (Ni 2/9 Fe 1/3 Mn 1/3 Cu 1/18 Mg 1/18 ) 0.9 O 2 exhibits excellent electrochemical performance: it delivers a reversible capacity of 147.3 mAh/g at 0.1C and maintains a capacity retention rate of 94.8% after 40 cycles. Download: Download high-res image (227KB) Download: Download full-size image