Shimizu Masahiro, Tsushima Yuji, Arai Susumu
Department of Materials Chemistry, Faculty of Engineering, and Institute of Carbon Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan.
ACS Omega. 2017 Aug 8;2(8):4306-4315. doi: 10.1021/acsomega.7b00950. eCollection 2017 Aug 31.
Electrical conductivity is one of the properties required for an active material, and it is extremely essential to exert its potential. In the present study, the strategy of coating a metal at a single particle level by an electroless deposition method was applied to enhance the cycling performance of phosphorus-based negative electrodes for Na-ion batteries. The deposition morphology and composition of the Ni coating layer were characterized by field-emission scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction. In the 5 wt % Ni coating, an amorphous Ni layer of several nanometers thickness was homogeneously formed on the phosphorus surface, whereas a shell layer having a 200 nm thickness was formed in the order of NiP, NiP, NiP, and metallic Ni from the surface toward the center in the 30 wt % Ni coating. Electrochemical impedance spectroscopic measurements clarified that the good electron transport proceeded throughout the developed conduction pathway to promote the phase transition to trisodium phosphide (NaP), leading to a high reversible capacity for phosphorus; the as-prepared black phosphorus showed only a reversible capacity of 140 mA h g at the 60th cycle, whereas the 30 wt % Ni-coated composite delivered a relatively high capacity of 780 mA h g. In addition, the expansion ratio of the electrode after the 30th desodiation was the lowest among the three kinds of electrodes. By contrast, cracks and exfoliation of the active material layer from the current collector were confirmed in the as-prepared black phosphorus. These results demonstrate that the upgraded performance accomplished using the 30 wt % Ni-coated composite with the Ni/Ni-P layer is due to the synergetic effects of the electron conduction channel and a buffer matrix against a large volumetric change (∼400%) in phosphorus during the charge-discharge reactions.
电导率是活性材料所需的性能之一，对于发挥其潜力极其重要。在本研究中，采用化学沉积法在单颗粒水平上包覆金属的策略，以提高钠离子电池磷基负极的循环性能。通过场发射扫描电子显微镜、扫描透射电子显微镜和X射线衍射对镍涂层的沉积形态和成分进行了表征。在5 wt%的镍涂层中，磷表面均匀形成了厚度为几纳米的非晶态镍层，而在30 wt%的镍涂层中，从表面向中心依次形成了厚度为200 nm的壳层，其顺序为NiP、NiP、NiP和金属镍。电化学阻抗谱测量表明，良好的电子传输在整个形成的传导路径中进行，促进了向三钠磷化物（NaP）的相变，从而使磷具有高可逆容量；制备的黑磷在第60次循环时仅显示140 mA h g的可逆容量，而30 wt%镍包覆的复合材料提供了相对较高的780 mA h g的容量。此外，在三种电极中，第30次脱钠后电极的膨胀率最低。相比之下，在制备的黑磷中证实了活性材料层与集流体之间出现了裂纹和剥落。这些结果表明，使用具有Ni/Ni-P层的30 wt%镍包覆复合材料实现的性能提升，是由于电子传导通道和缓冲基质对充放电反应过程中磷的大体积变化（约400%）的协同作用。
