Failure Acceleration of O3-Type Na-Layered Oxides: Role of Spatial Heterogeneity in Single Crystals vs Polycrystals

Despite the fact that single-crystalline O3-type Ni/Fe/Mn-based oxide cathodes (O3-NFM) for sodium-ion batteries (SIBs) demonstrate enhanced structural integrity and outstanding air stability, compared to polycrystalline counterparts, the diversity of failure mechanism caused by the elongated ion-diffusion pathways and the increased long-range order has not drawn adequate attention. In this paper, single-crystalline and polycrystalline O3-NFM are successfully synthesized and the failure discrepancies between them are uncovered. Compared with polycrystalline O3-NFM, the phase transformations and redox reactions of single-crystalline O3-NFM display more significant spatial heterogeneity, leading to a more rapid chemo-mechanical failure of single crystals. Specifically, owing to the higher oxidation state on the surface of single crystals, a substantial number of Jahn-Teller (J-T) active Fe⁴⁺ ions are generated on the surface, triggering a severe surface J-T effect, which causes a rapid deterioration of the surface layered structure, the formation of intragranular cracks and the drastic dissolution of Fe. As a result, under conditions of fast charging, high-voltage, and an iron-rich sample, the acceleration of failure caused by the exacerbated spatially heterogeneous reactions (SHR) becomes even more prominent. This paper offers valuable theoretical insights for deciphering the reaction mechanism of single-crystalline materials and for the design of high-performance single-crystalline materials.