'til membranes do us part: neutrons reveal the molecular mechanism behind sustainable lithium extraction

Researchers designed novel membranes to extract precious lithium ions from brine solutions and used neutrons to obtain a deeper understanding of their mechanism

From mobile phones to car batteries, technology all over the world relies heavily on lithium (Li⁺) ion batteries. Lithium, an alkali metal, needs to be sourced from the environment. In a quest to minimise the deleterious effects of this process, a sustainable option of Li extraction is based on a mechanism known as selective electrodialysis from salt lake brines. In a nutshell, electrodialysis transports Li⁺ ions to an ion-selective membrane within an electric field.

A crucial part of Li⁺ extraction is its separation of its monovalent ion (carrying one positive charge) from other ions with 2 or more positive charges (referred to as divalent or multivalent ions). This process requires a highly selective membrane. A material which is particularly suited for this purpose are so-called polymers of intrinsic microporosity (PIMs for short). PIMs are known to easily lend themselves to the manufacturing of ion-selective membranes.

"Our hypothesis was that PIMs would be an excellent base for the manufacturing of membranes capable of separating mono- from divalent ions", says Dingchang Yang, a PhD student at Imperial College London, the first author of an interdisciplinary study elucidating ion transport through novel PIM membranes.  

To put their idea into practice, the research team developed PIM membranes consisting of rigid, contorted polymer chain containing hydrophilic ("water-loving") chemical groups. The advantage of these groups is twofold: not only do they lead to ion-pore interactions which are essential to regulating ion transport, but they also reinforce the rigidity of the networks by intra- and interchain hydrogen bond networks. This gives rise to tiny, sub-nanometric ion channels which ensure a confined diffusion of ions across the membrane. Structures known as dynamic pore gates restrict the diffusion of both water and ions in between the channels, with the degree of restriction depending on the type of polymer used. Diffusion through the channels themselves, on the other hand, happens rather quickly.

When analysing the diffusion of different ions across the membrane, the team found that the comparatively large Mg2+ ions diffused much more slowly than the smaller, monovalent Li+ ions. In addition to sterical hindrance within the small channels, the large hydration shell of Mg2+ also hindered their diffusion. Indeed, the motion of water molecules is an important parameter determining the diffusion of ions across porous membranes and was studied using inelastic neutron spectroscopy on ILL's instrument WASP.

 "We observed a remarkable decrease in water diffusion compared to bulk water as a function of pore size", explains Peter Fouquet of the WASP team at ILL. Fabrizia Foglia from University College London adds, "Using neutron spectroscopy, we were able to investigate molecular motions in detail, providing - alongside NMR and computer simulations - an excellent understanding that is critical for further optimising ion-selective membranes." The neutron data agreed very well with complementary methods such as NMR and computer simulations.

The researchers tested the novel membranes using a simulated salt-lake brine containing a mixture of different ions. The resulting ion selectivity was excellent, indicating that the interactions between the ions and the hydrophilic functional groups the team had introduced lead to efficient lithium extraction. Indeed, the performance of the PIM membranes allowed to produce battery-grade lithium carbonate.

"We are delighted to have the opportunity to collaborate with experts at ILL to perform in-depth study of the membrane transport mechanism, which help us gain a better understanding of the structure-property relationships and design new membranes", says Qilei Song from Imperial College London, the principal investigator of the study.

This multimethod study is therefore an excellent example showcasing how neutrons can help design tomorrow's sustainable technology. This international collaboration team, including researchers from UK, France and China, will carry on using the neutron spectroscopy to study new membranes for broad sustainable applications.