Low humid transport of anions in layered double hydroxides membranes using polydopamine coating

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Journal Article
Journal of Membrane Science, 2021, 624
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Lithium carbonate and lithium hydroxide are two compounds used for producing battery cathodes, however, lithium hydroxide is favoured in producing battery compounds as it possesses very high electrochemical potential and low density. As a result, there is substantial demand for development of anion-selective membranes capable of selecting hydroxide over carbonate. Although single-layer nanosheets of Layered Double Hydroxides (LDHs) have shown anion conductivity, the reassembling of the building blocks in a membrane-like morphology will not result in a conductive membrane. Here we show that an appropriate size of LDH nanosheets with post-treatment with polydopamine (PDA) can form a highly conductive hydroxide-selective membrane. Our experimental results show that the in-plane ion conductance of the membrane for different ions is OH− > Br− > I− > NO3− > Cl− > CO32− with corresponding selectivity ratios of OH− to Br−, I−, NO3−, Cl−, CO32− found to be 2.30, 2.83, 2.99, 3.11 and 4.38 respectively, showing the membrane's excellent hydroxide selectivity. Molecular dynamics (MD) simulations revealed that the small nanochannels of LDH increase its diffusion barrier against ion permeation and cause the partial dehydration of ions. We showed that ions move inside the LDH nanochannels in a semi-dry transport manner so that they attract to the surface more than water molecules inside the nanochannels. Selectivity predictions from MD simulations were in excellent agreement with the experimental data, confirming our hypothesis about ion transport in LDH nanochannels. This work takes advantage of the semi-dry transport mechanism of anions in the narrow nanochannels of LDHs to produce a hydroxide selective membrane applicable in the production of LiOH for the new generation of Li-ion batteries.
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