Anisotropic strain induced fracture of Ni(OH)2 nanosheet electrodes for supercapacitors through a charge-ordered mixed-valent intermediate

Abstract

Beta nickel hydroxide (β-Ni(OH)2) is a popular electrode material for manufacturing Ni-H batteries and pseudocapacitors. The cycle stability of β-Ni(OH)2 electrodes is limited owing to their volumetric changes during charge–discharge process, in which, internal charge distribution of β-Ni(OH)2 changes with the external swelling. In this paper, β-Ni(OH)2 nanosheets synthesized by HCl etching have been taken as an example to investigate details of the morphology and structure variations during capacity fading of these electrodes. The findings shows that the β-Ni(OH)2 nanosheets form a wrinkled surface and become fractured after the charge–discharge cycles. Fractured nanoparticles with diameters around 10 nm could be collected by the polyaniline coating on the β-Ni(OH)2 nanosheets. Charge-ordering of Ni2+/Ni3+ has been observed in β-Ni(OH)2 nanosheet via a sluggish cyclic voltammetry cycling. Although the charge-ordering state may lower the strain penalty involved in charging-discharging, the increased anisotropic strain in the β-Ni(OH)2 finally leads to the fracture and capacity fading of the nanosheet electrodes.