Modification of lutetium hydroxide for the structural and electrochemical stability of Ni-Al layered double hydroxide

Abstract

The modification of lutetium hydroxide on a layered double hydroxide (LDH), [Ni₄Al(OH)₁₀]NO₃, is carried out by coprecipitation and thereafter hydrothermal treatment. When the content of Lu is less, only X-ray diffractions (XRD) due to [Ni₄Al(OH)₁₀]NO₃ are found. When the content is 2.89 wt% or more in the product, a series of low-intensity diffractions is identified, which can be indexed with a triclinic cell, space group $P\overline{1}$. After the modification, the a lattice parameter for Ni-Al layered double hydroxide does not show any considerable changes, which means Lu³⁺ could not be incorporated into M(OH)₂ (M=Ni, Al) layers because of its larger ion radius (85 pm). According to the observations by scanning electron microscopy (SEM) or transmission electron microscopy (TEM), with low Lu content, single nanorods appear among the disks of [Ni₄Al(OH)₁₀]NO₃ and turn into bundles with high Lu content. The modification of Lu prevents the structural transformation of the Ni-Al LDH into β-Ni(OH)₂. It can be found that the transformation is stopped by 0.9 wt% Lu when being soaked in 7.0 mol L⁻¹ potassium hydroxide (KOH) for 72 days at 60 °C, or by 1.29 wt% Lu after the 25th charging/discharging cycle at 60 °C in the same alkali solution. The modification also improves the high-temperature performances of the electrode. At a high temperature of 60 °C, the 2.89 wt% Lu-modified [Ni₄Al(OH)₁₀]NO₃ has a more slowly reduced capacity of 265.8 mAh g⁻¹ after 25 charge/discharge cycles under a current density of 800 mA g^-1 in 7.0 mol L⁻¹ KOH, while the [Ni₄Al(OH)₁₀]NO₃ has a sharply reduced capacity of 202.7 mAh g⁻¹ under the same condition.