Design principles of ion selective nanostructured membranes for the extraction of lithium ions

REVIEW ARTICLE https://doi.org/10.1038/s41467-019-13648-7 OPEN Design principles of ion selective nanostructured membranes for the extraction of lithium ions 1234567890():,; Amir Razmjou 1,2*, Mohsen Asadnia 3, Ehsan Hosseini4, Asghar Habibnejad Korayem 4 & Vicki Chen1,5 It is predicted that the continuously increasing demand for the energy-critical element of lithium will soon exceed its availability, rendering it a geopolitically significant resource. The present work critically reviews recent reports on Li+ selective membranes. Particular emphasis has been placed on the basic principles of the materials’ design for the development of membranes with nanochannels and nanopores with Li+ selectivity. Fundamental and practical challenges, as well as prospects for the targeted design of Li+ ion-selective membranes are also presented, with the goal of inspiring future critical research efforts in this scientifically and strategically important field. L ithium consumption has been increasing substantially worldwide from 265,000 tons in 2015 (based on Li2CO3) to an estimated 498,000 tons in 2025 (ref. 1). This sharp increase in Li demand is predominantly due to the extensive use of Li-ion batteries (LiBs) or electronic devices. Indeed, over the course of five years from 2010 to 2015, the consumption of LiBs leaped from 4.6 to 7 billion units1. Currently, the main sources of Li+ supply are brine deposits and lithium ores which are reported to amount to approximately 34 million tons worldwide2. While these Li+ reserves are sufficient to address the current market demands, the conventional technologies to extract Li+ from the resources are either difficult or require high-cost investment3, and will struggle to meet future market demands. Furthermore, a major bottleneck is the distribution of conventional resources of Li+ around the globe, many in less accessible regions. In comparison, the Li+ reserves in seawater are estimated at 230,000 million tons4. This unconventional resource of Li+ is not limited to a geographic boundary. However, Li+–seawater processing is complicated due to the low concentration of Li+ (0.1–0.2 ppm4) and the coexistence of chemically similar ions such as Na+ and K +. Therefore, the development of new processing technologies with enhanced product yields are urgently needed. The concentration of lithium in brine ranges from 200 to 700 ppm, and can be categorized into various types based on the nature of the salt ions3. Although Li+ concentration in brine is higher than that of seawater, the rapid depletion of ground resources and its demand projection is the current concern. Innovative brine enrichment processes that can be applied to selectively enrich and extract the Li+ from brines are urgently required. The conventional approach to extract Li+ from aqua-based resources such as brine and seawater consists of three main stages, namely (i) enrichment (solar evaporation, adsorption, and diffusion dialysis), (ii) purification (solvent extraction, ion exchange, or adsorption), and (iii) Li+ precipitation (predominately by 1 UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia. 2 Department of Biotechnology, Faculty of Biological Science and Technology, University of Isfahan, Isfahan, Iran. 3 School of Engineering, Macquarie University, Sydney, NSW 2109, Australia. 4 School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran. 5 School of Chemical Engineering, University of Queensland, St. Lucia, QLD 4072, Australia. *email: NATURE COMMUNICATIONS | (2019)10:5793 | https://doi.org/10.1038/s41467-019-13648-7 | www.nature.com/naturecommunications 1 REVIEW ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-13648-7 adding Na2CO3). Pankaj et al.1,3 recently reviewed the conventional Li+ extraction methods, and therefore they are not reviewed here. Membrane technologies including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) have been used for water and wastewater treatment including desalination for many years5. Membranes can be made from organic and inorganic materials, and their structures vary from the finely porous structures to nonporous. Membranes can separate contaminants such as bacteria and protozoa (1–10 µm), down to gases and ions (0.1–1 nm). They can be classified according to their driving force including concentration difference ΔC (forward osmosis (FO), dialysis and pervaporation), potential difference ΔE (Electrodialysis), pressure difference ΔP (including MF, UF, NF, and RO), and temperature difference ΔT (membrane distillation, MD). The most popular materials used for membrane fabrication are polymers, due to their chemical and thermal stabilities, high mechanical strength as well as ability to form into different morphologies such as flat sheets or hollow fibers. For commercial membrane production, the phaseinversion process and interfacial polymerization are the two most widely used preparation techniques6. Among the techniques of the membrane extraction, the nanofiltration processes have been widely used for preconcentration and Li extraction from a lithium-bearing brine. Lithium brines usually contain high Li+ concentrations (>5.0 wt. %). Practically, the maximum salt concentration that the membrane processes can achieve is a function of the osmotic pressure and NF membrane selectivity. It is well-known that in NF the combination of Donnan exclusion, steric hindrance, and dielectric exclusion controls the mass transfer7. Nanofiltration, having smaller pores (molecular weight cut-off, MWCO, ranging from 0.2 to 1 kDa), is more suited for Li+ ion separation purposes. Unfortunately, due to the poor monovalent selectivity of nanofiltration, it can only be used to concentrate Li+ by removing all the divalent ions mostly Mg2+. It should be pointed out here that the conventional NF processes suffer from severe inorganic fouling and scaling; they cannot efficiently extract Li without a heavy pre-treatment stage, and they require diluting the brine with a large quantity of freshwater. For example, Somrani et al8. used negatively charged NF90-2540 membranes for the extraction of Li with Mg/Li mass ratio of 56.76. Although their study showed 100% and 15% rejections for Mg2+ and Li+, the separation of Li+ from Na+ was unsatisfactory. In addition, they found that NF90-2540 suffers significantly from fouling (50% reduction in pure water flux). It is also reported that positively charged NF membranes have shown higher Li selectivity than negatively charged ones. According to Donnan effect, the negatively charged NF membranes are suitable for the rejection of anions while they exhibited unsatisfactory separation permanence when treating solutes with positive charges9. A variety of positively charged organic molecules such as polyethyleneimine9, Ethylenediaminetetraacetic acid (EDTA)10, and 1 (...truncated)