Role of crystal lattice templating and galvanic coupling in enhanced reversible capacity of Ni(OH)2/Co(OH)2 core/shell battery cathode

Abstract

A series of Ni(OH)2/Co(OH)2 core/shell nanoplatelets with varying shell thickness (0.5–4.1 nm) are 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-properties correlations reveal that electrochemical behavior and reversibility of Co(OH)2 redox conversion depends non-linearly on the average 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) is achieved at shell thickness of 1.9 ± 0.3 nm. Two fundamental phenomena are suggested to be responsible for the superior performance: templated shell deposition and 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. Proposed performance enhancement mechanisms are applicable for design of other core/shell electrode materials.