Mitigating irregular stress release in cathode materials to enable long-term cycling of Ah-level sodium-ion pouch cells

The O3-type NaTMO₂ (where TM represents a transition metal) cathode material is highly promising for sodium-ion batteries due to its high theoretical specific capacity and cost-effectiveness. However, its industrialization and commercialization have been hindered by complex phase transitions, sluggish sodium-ion diffusion kinetics, and inhomogeneous stress release within the transition metal layers. In this study, we employ a Zr-doped O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFMZ) cathode material to address these challenges. The incorporation of Zr promotes the growth of the (003) crystal plane, accelerates the kinetic process, increases the sodium-ion diffusion coefficient, and enhances the TM-O bond energy, thereby improving phase transition reversibility and enabling a rapid O3-to-P3 transformation. When assembled into Ah-level pouch-type full-cells with a hard carbon anode, the NFMZ cathode delivers an excellent discharge capacity of 1.76 Ah within a voltage range of 1.5–3.9 V at 0.5 C, and retains 95.63% of its initial capacity after 1000 cycles. More importantly, this work elucidates the mechanism by which Zr doping enhances the electrochemical performance of NFM. The NFMZ cathode also exhibits an enhanced stress release mechanism from the interior to the exterior, effectively mitigating stress accumulation and enabling the stable operation of Ah-level pouch-type full-cells.