Chen Xi, Hwang Sooyeon, Chisnell Robin, Wang Yichao, Wu Fan, Kim Sooran, Lynn Jeffrey W, Su Dong, Li Xin
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
Center for Functional Nanomaterials, Brookhaven National Laboratory Upton, NY 11973, USA.
Adv Funct Mater. 2018;28(39). doi: https://doi.org/10.1002/adfm.201803896.
Layered sodium transition metal oxides of NaTMO (TM = 3d transition metal) show unique capability to mix different compositions of Fe to the TM layer, a phenomenon that does not exist in LiTMO. Here, a novel spontaneous TM layer rippling in the sodium ion battery cathode materials is reported, revealed by in situ X-ray diffraction, Cs-corrected scanning transmission electron microscopy, and density functional theory simulation, where the softening and distortion of FeO octahedra collectively drives the flat TM planes into rippled ones with inhomogeneous interlayer distance at high voltage. In such a rippling phase, charge and discharge of Na ions take different evolution pathways, resulting in an unusual hysteresis voltage loop. Importantly, upon discharge beyond a certain Na composition, the rippling TM layer will go back to flat, giving the reversibility of such structural evolution in the following cycles.
NaTMO（TM = 3d过渡金属）层状钠过渡金属氧化物表现出独特的能力，可将不同组成的铁混入TM层，这一现象在LiTMO中不存在。在此，通过原位X射线衍射、Cs校正扫描透射电子显微镜和密度泛函理论模拟揭示了一种新型的钠离子电池阴极材料中TM层自发波动现象，其中FeO八面体的软化和畸变共同驱动平坦的TM平面在高电压下转变为具有不均匀层间距的波纹状平面。在这种波纹状相中，钠离子的充放电遵循不同的演化路径，导致出现异常的滞后电压环。重要的是，当放电超过一定的钠组成时，波纹状的TM层将恢复平坦，使得这种结构演化在随后的循环中具有可逆性。
