Dual-Gradient Structural Design for Synchronized Phase Transitions and Stress-Resilient O3-Type Sodium Cathodes

O3-type layered oxides with secondary particle morphology are considered highly promising cathode materials for sodium-ion batteries due to their low cost and high capacity. However, uneven Na⁺ distribution during (de)sodiation often triggers inhomogeneous phase transitions and microcrack formation, leading to rapid capacity decay and severely limiting practical application. In this study, leveraging the principle that elements with low diffusion rates and strong transition metal-oxygen (TM-O) bonds can effectively reduce the surface energy of primary particles and suppress elemental diffusion during high-temperature sintering, an O3-type Na_0.99La_0.01[Ni_1/3Fe_1/3Mn_1/3]_0.99Ti_0.01O₂ cathode material with a dual gradient distribution of La/Ti both at primary and secondary particle levels was successfully synthesized. This dual gradient structure not only promotes the formation of a robust framework within the secondary particles but also constructs a fast Na⁺ diffusion network, which effectively mitigates the inconsistency of phase transitions and suppresses crack formation caused by uneven Na⁺ distribution during (de)sodiation. This study provides mechanistic insights into improving the structural stability and long-cycle performance of O3-type layered cathode materials.