Atomically engineered cobaltite layers for robust ferromagnetism

Interfacial Oxygen Octahedral Coupling-Driven Robust Ferroelectricity in Epitaxial Na0.5Bi0.5TiO3 Thin Films

The oxygen octahedral rotation (OOR) forms fundamental atomic distortions and symmetries in perovskite oxides and definitely determines their properties and functionalities. Therefore, epitaxial strain and interfacial structural coupling engineering have been developed to modulate the OOR patterns and explore novel properties, but it is difficult to distinguish the 2 mechanisms. Here, different symmetries are induced in Na_0.5Bi_0.5TiO₃ (NBT) epitaxial films by interfacial oxygen octahedral coupling rather than epitaxial strain. The NBT film grown on the Nb:SrTiO₃ substrate exhibits a paraelectric tetragonal phase, while with La_0.5Sr_0.5MnO₃ as a buffer layer, a monoclinic phase and robust ferroelectricity are obtained, with a remanent polarization of 42 μC cm^-2 and a breakdown strength of 7.89 MV cm^-1, which are the highest record among NBT-based films. Moreover, the interfacial oxygen octahedral coupling effect is demonstrated to propagate to the entire thickness of the film, suggesting an intriguing long-range effect. This work provides a deep insight into understanding the structure modulation in perovskite heterostructures and an important avenue for achieving unique functionalities.

Reducing Co/O Band Overlap through Spin State Modulation for Stabilized High Capability of 4.6 V LiCoO2

High-voltage LiCoO₂ (LCO) attracts great interest because of its large specific capacity, but it suffers from oxygen release, structural degradation, and quick capacity drop. These daunting issues root from the inferior thermodynamics and kinetics of the triggered oxygen anion redox (OAR) at high voltages. Herein, a tuned redox mechanism with almost only Co redox is demonstrated by atomically engineered high-spin LCO. The high-spin Co network reduces the Co/O band overlap, eliminates the adverse phase transition of O3 → H1-3, delays the exceeding of the O 2p band over the Fermi level, and suppresses excessive O → Co charge transfer at high voltages. This function intrinsically promotes Co redox and restrains O redox, fundamentally addressing the issues of O₂ release and coupled detrimental Co reduction. Moreover, the chemomechanical heterogeneity caused by different kinetics of Co/O redox centers and the inferior rate performance limited by slow O redox kinetics is simultaneously improved owing to the suppression of slow OAR and the excitation of fast Co redox. The modulated LCO delivers ultrahigh rate capacities of 216 mAh g^-1 (1C) and 195 mAh g^-1(5C), as well as high capacity retentions of 90.4% (@100 cycles) and 86.9% (@500 cycles). This work sheds new light on the design for a wide range of O redox cathodes.