Monday, 14 May 2018: 11:40
Room 604 (Washington State Convention Center)
β-Ni(OH)2 is commonly used as the cathodic material in rechargeable aqueous battery systems due to its well-defined electrochemical redox activity, high electric potential, high theoretical specific capacity (289 mAh/g) and relatively low cost. However, β-Ni(OH)2/β-NiOOH cathodes are also known to have inherently low electrical conductivity, which often results in incomplete discharge, low Coulombic efficiency and slow rates of charge and especially discharge. In this study, Ni(OH)2 nanoparticles (100x20 nm platelets) were synthesized and coated with epitaxial Co(OH)2 shells and a series of Ni(OH)2/Co(OH)2 core/shell nanoplatelets with varying shell thickness (0.5 to 4.1 nm) were systematically investigated with a combination of scanning electron microscopy (SEM) with energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), in situ and ex situ x-ray absorption fine structure spectroscopy (XAFS), and electrochemical tests. Structure-property correlations revealed that electrochemical behavior and reversibility of Co(OH)2 redox conversion depends non-linearly on the shell thickness, with the best performance (99.6% of theoretical capacity of the composite material, 10% improvement over the performance of pristine Ni(OH)2 nanoparticles) achieved at shell thickness of 1.9±0.3 nm. We suggest that in addition to the improvement in electrical conductivity, such thickness dependent Co(OH)2 shell behavior and the superior performance are due to a lattice templating effect, and the galvanic coupling of core and shell materials. Homogeneous deposition of the shell is confirmed with XRD, SEM and EDX, while lattice templating effect was suggested from XAFS results showing that Co-M and Co-O distances are close to those of the Ni(OH)2 lattice in thin shells and shift gradually towards values of bulk Co(OH)2 as the shell thickness increases. From a combination of electrochemical and structural characterization of these composite nanomaterials, including in situ XAFS, galvanic coupling between the shell and core is proposed as a material activation mechanism, which explains the limited reversibility of the Co(II)/Co(III) oxidation in some cases. The prospect of applying this nanostructuring to other combinations of materials such as MnO2/Fe2O3 and Fe2O3/MnO2 will also be discussed.