Zhang Yi, Wu Miaomiao, Teng Wei, Ma Jiwei, Zhang Renyuan, Huang Yunhui
Institute of New Energy for Vehicles, Shanghai Key Laboratory for Development and Application of Metallic Functional Materials, School of Materials Science and Engineering, Tongji University, Shanghai 201804, P. R. China.
State Key Laboratory for Pollution Control, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, P. R. China.
ACS Appl Mater Interfaces. 2020 Apr 1;12(13):15220-15227. doi: 10.1021/acsami.0c00386. Epub 2020 Mar 23.
Most of sodium-layered oxide cathodes are unstable under moisture conditions. As a unique water-stable cathode, NaNiMnO (NNM) usually becomes vulnerable to water molecules after element substitution treatment to suppress the Na vacancy ordering arrangement, which causes limited Na diffusion kinetics. Herein, we show that these issues can be addressed simultaneously by rational designing the transition-metal (TM) layer to achieve both water-stable and Na vacancy disordering structures. Density functional theory calculations reveal that the water-stability of the layered oxide cathode can be correlated to the surface adsorption energy of HO molecules. In the TM layer, the Co/Mn and Fe/Mn units exhibit a much lower adsorption energy than that of the Li/Mn unit, and hence the HO molecule prefers to be absorbed on Co/Mn and Fe/Mn units rather than Li/Mn. Moreover, the Li/Mn unit in the TM layer can suppress the Na vacancy ordering structure in NNM to improve the Na diffusion kinetics. As a consequence, the well-designed NaLiNiMnO cathode can not only maintain its original crystal structure and electrochemical property after water soaking treatment but also exhibit high rate capability (78% capacity retention at 20 C) and excellent cycling stability (87% capacity retention after 1000 cycles).
大多数钠层状氧化物阴极在潮湿条件下不稳定。作为一种独特的水稳定阴极，NaNiMnO（NNM）在进行元素取代处理以抑制钠空位有序排列后，通常会变得易受水分子影响，这导致钠扩散动力学受限。在此，我们表明通过合理设计过渡金属（TM）层以实现水稳定和钠空位无序结构，可以同时解决这些问题。密度泛函理论计算表明，层状氧化物阴极的水稳定性与HO分子的表面吸附能相关。在TM层中，Co/Mn和Fe/Mn单元的吸附能比Li/Mn单元低得多，因此HO分子更倾向于吸附在Co/Mn和Fe/Mn单元上而不是Li/Mn上。此外，TM层中的Li/Mn单元可以抑制NNM中的钠空位有序结构，以改善钠扩散动力学。因此，精心设计的NaLiNiMnO阴极不仅在水浸泡处理后能保持其原始晶体结构和电化学性能，还表现出高倍率性能（在20 C下容量保持率为78%）和出色的循环稳定性（1000次循环后容量保持率为87%）。
