Revealing the origins of superior ion diffusion in biphasic layered oxide cathode for sodium-ion batteries

Abstract

P2/O3 biphasic materials have emerged as competitive candidates for high-performance sodium-ion battery cathodes, and a thorough understanding of the ion diffusion behavior in biphasic structures particularly at phase interface is critical for unlocking their full potential. Herein, the Na_xZn_0.07Ni_0.30Mn_0.53Ti_0.10O₂ cathodes with finely-tuned P2/O3 phase ratios are designed and their ion diffusion mechanism is revealed through in-depth structural-electrochemical investigation. Experiments verify that a balanced phase composition of P2/O3-Na0.82 with 52.83 % P2 and 47.17 % O3 can maximize the coupling advantages of the biphasic structure and exhibit excellent Na⁺ diffusion kinetics, delivering a remarkable rate capability (143.0 mAh g⁻¹ at 0.2 C, 100.2 mAh g⁻¹ at 10 C with 70.1 % retention), outperforming most reported P2/O3 biphasic cathodes. High-resolution transmission electron microscopy and X-ray absorption fine structure results indicate that the coordination environment of Ni-O and Ni-TM paths undergoes conspicuous local symmetry breaking, driving the P2/O3-Na0.82 interface structure distortions, which exhibit unique characteristics distinct from single-phase systems. Theoretical analyses reveal that the interfacial distortions structures facilitate the overlap of Na⁺ energy distributions and create interconnecting bridges across different Na⁺ sites, ultimately promoting low-energy-barrier Na⁺ diffusion. These findings establish an atomic-level insight into the interface-induced ion diffusion acceleration mechanism in biphasic materials.