Native Defect Elimination and Lattice Framework Reinforcement Toward Ultra-Stable Sodium-Ion Layered Cathodes.

Abstract

Structural and performance degradation in layered transition metal oxide (TMO) cathode materials is often attributed to phase transition induction during sodium de-embedding, while the significance of native defects during complex synthesis is frequently overlooked. Here, the role of native surface remodeling in progressive capacity degradation in P2-type Na2/3Ni1/3Mn2/3O2 is emphasized, where lattice mismatches and elemental distortions are found on the surface of the particles and result in the accumulation of low-valent TMs. Interestingly, the accumulation gradually became the center of cathodic degradation rather than phase transition induction. Given the apparent spatiality of the primary defects and recognizing the importance of the surface state, the stripping repair of the defects and gradient introduction of La can be manipulated. The unique LaO6 configuration enhanced the rigid framework of TMO6 and suppressed the emergence of low-valent TMs, resulting in surface-corrected and reinforced particles, which can be explained by generalized functional density calculations and ex-situ hard X-ray absorption spectroscopy. As a result, the reinforced cathode brought about a capacity retention of up to 98% for 500 cycles at 2 C and 87% for 4000 cycles at 10 C and stable electrochemical performance over a wide temperature range (-20 °C-60 °C).