Highly stabilized single-crystal P2-type layered oxides obtained via rational crystal orientation modulation for sodium-ion batteries

The development of P2-type Na–Ni–Mn oxides as high-voltage cathode materials for sodium-ion batteries is being extensively researched. However, achieving good electrochemical reversibility for such oxides at high voltage remains challenging. Herein, highly dispersed hexagonal-prism-like single-crystal P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 (MC-NNZM) with a high proportion of {0 0 1} planes is synthesized through a combined coprecipitation and molten-salt method. The presence of molten Na2SO4 and the low surface energy of {0 0 1} planes are critical in forming the anisotropic single-crystal structure. In the 2.0–4.4 V voltage window, MC-NNZM exhibits a reversible capacity of 122.1 mAhg−1 with a median discharge voltage of 3.5 V at 10 mA g−1. Moreover, the capacity retentions of MC-NNZM reach 95.8% and 98.3% in the 2.0–4.4 V and 2.0–4.3 V voltage ranges after 100 cycles at 100 mA g−1, respectively. Electrochemical in situ X-ray diffraction patterns reveal that, because of the {0 0 1}-dominated single-crystal structure, the gliding of transition metal oxide slabs in the high-voltage region is effectively restrained; subsequently, this significantly mitigates the volume change of MC-NNZM during cycling. Consequently, unlike layered oxides with random crystal orientation, MC-NNZM is effectively resistant to mechanical fracture without the irreversible formation of intragranular cracks and dislocations after extensive cycling, thus avoiding continuous electrolyte decomposition on the material surface and stabilizing efficient ionic/electronic transport paths in the cathode. This work paves a new avenue for improving the cycling stability of P2-type layered oxides with a high upper-cutoff voltage based on a single-crystal structure formed via rational crystal orientation modulation.