An efficient lithium extraction pathway in covalent organic framework membranes

Lithium extraction is one of the most important concerns in the energy storage field, since the methodologies used play a crucially impactful role in the sustainable development of lithium batteries. Recently in Matter, Ma, Sun, and colleagues developed a COF-based membrane implanted with oligoether side chains to provide a selective lithium ion-diffusion pathway. Thanks to the precise control over the pore structure and surface chemistry, the oligoether functionalized COF membrane allows efficient transport of Li+ while obstructing the pass- through of other ions, leading to an improved Li+/Mg2+ separation factor up to 64. This work highlights the significance of pore-environment engineering for developing customized membrane materials for ion sieving and lithium ion extraction. Lithium extraction is one of the most important concerns in the energy storage field, since the methodologies used play a crucially impactful role in the sustainable development of lithium batteries. Recently in Matter, Ma, Sun, and colleagues developed a COF-based membrane implanted with oligoether side chains to provide a selective lithium ion-diffusion pathway. Thanks to the precise control over the pore structure and surface chemistry, the oligoether functionalized COF membrane allows efficient transport of Li+ while obstructing the pass- through of other ions, leading to an improved Li+/Mg2+ separation factor up to 64. This work highlights the significance of pore-environment engineering for developing customized membrane materials for ion sieving and lithium ion extraction. Due to the wide application of lithium in many fields, such as ceramics, pharmaceuticals, nuclear industries, and the rapidly developing lithium-ion batteries in electronic devices and vehicles, modern society and industry require a continuously growing amount of lithium resource, the total demand for which is estimated to be 498,000 tons in 2025.1Choubey P.K. Chung K.S. Kim M.S. Lee J.C. Srivastava R.R. Advance review on the exploitation of the prominent energy-storage element Lithium. Part II: From sea water and spent lithium ion batteries (LIBs).Miner. Eng. 2017; 110: 104-121Crossref Scopus (142) Google Scholar Currently, the main lithium sources on the earth are widely distributed in lithium ores and brine deposits.2Choubey P.K. Kim M.S. Srivastava R.R. Lee J.C. Lee J.Y. Advance review on the exploitation of the prominent energy-storage element: Lithium. Part I: From mineral and brine resources.Miner. Eng. 2016; 89: 119-137Crossref Scopus (247) Google Scholar Compared with the limited lithium reserves on the land, the ocean stores a huge amount of hundreds of billion tons of lithium.3Diallo M.S. Kotte M.R. Cho M. Mining Critical Metals and Elements from Seawater: Opportunities and Challenges.Environ. Sci. Technol. 2015; 49: 9390-9399Crossref PubMed Scopus (90) Google Scholar Unfortunately, lithium extraction from seawater is complicated and challenging, because of not only the low concentration (0.1–0.2 ppm) of lithium ions but also the coexistence of many other chemically similar monovalent and divalent ions, such as sodium, potassium, magnesium, etc.4Razmjou A. Asadnia M. Hosseini E. Habibnejad Korayem A. Chen V. Design principles of ion selective nanostructured membranes for the extraction of lithium ions.Nat. Commun. 2019; 10: 5793Crossref PubMed Scopus (161) Google Scholar Noteworthily, the conventional lithium extraction technologies including solvent extraction, ion exchange, adsorption, and precipitation suffer from either low efficiency or intensive energy use at the cost of heavy investment or environment pollution.4Razmjou A. Asadnia M. Hosseini E. Habibnejad Korayem A. Chen V. Design principles of ion selective nanostructured membranes for the extraction of lithium ions.Nat. Commun. 2019; 10: 5793Crossref PubMed Scopus (161) Google Scholar,5Yang S.X. Zhang F. Ding H.P. He P. Zhou H.S. Lithium Metal Extraction from Seawater.Joule. 2018; 2: 1648-1651Abstract Full Text Full Text PDF Scopus (121) Google Scholar Despite the enormous efforts in developing membrane-based materials for large-scale energy-efficient separation, the state-of-the-art nanofiltration membranes show a rather low Li+/Mg2+ separation factor of around 10, which is still far from satisfactory.6Somrani A. Hamzaoui A.H. Pontie M. Study on lithium separation from salt lake brines by nanofiltration (NF) and low pressure reverse osmosis (LPRO).Desalination. 2013; 317: 184-192Crossref Scopus (195) Google Scholar The recently developed porous materials with nanofluidic channels provide an ideal platform with which to strategically design novel selective membranes.7Bocquet L. Nanofluidics coming of age.Nat. Mater. 2020; 19: 254-256Crossref PubMed Scopus (139) Google Scholar Among those, two-dimensional (2D) covalent organic frameworks (COFs) demonstrate exceptional advantages for efficiently extracting lithium with high selectivity and permeability, benefitting from their high porosity, precisely controllable pore structure, and rich surface property.8Lohse M.S. Bein T. Covalent Organic Frameworks: Structures, Synthesis, and Applications.Adv. Funct. Mater. 2018; 28: 1705553Crossref Scopus (632) Google Scholar Moreover, due to the strong π-π interactions between adjacent layers, one-dimensional (1D) channels with low tortuosity and lithiophilic functionalities could be formed and function as the highways for rapid lithium ion transport. The oligoether moieties have been previously reported to undergo reversible coordination with lithium ions and thus enhance the transport efficiency of them.9Webb M.A. Jung Y. Pesko D.M. Savoie B.M. Yamamoto U. Coates G.W. Balsara N.P. Wang Z.-G. Miller 3rd, T.F. Systematic Computational and Experimental Investigation of Lithium-Ion Transport Mechanisms in Polyester-Based Polymer Electrolytes.ACS Cent. Sci. 2015; 1: 198-205Crossref PubMed Scopus (126) Google Scholar Inspired by this, Ma, Sun, and colleagues recently developed a selective lithium diffusion pathway by modifying the pore surface of COF membranes with oligoether side chains, in which the transport of lithium ions is accelerated while the diffusion of other ions is hampered (Figure 1A), giving rise to an efficient differentiation of lithium ions from other ions with a Li+/Mg2+ separation factor up to 64.10Bing, S., Xian, W., Chen, S., Song, Y., Hou, L., Liu, X., Ma, S., Sun, Q., and Zhang, L et al.Biol.-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction.Matter. 2021; 4: 2027-2038Abstract Full Text Full Text PDF Scopus (18) Google Scholar In this study, the Schiff-base condensation reaction between amine-ended 1,3,5-tris(4-aminophenyl)benzene (TAB) monomer and aldehyde monomer containing oligoether side chain of 4EO (2,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)terephthalaldehyde) was employed to prepare the stable COF membrane. As-synthesized COF membrane characteristically has a thickness of ~1 μm and a pore size of 2.34 nm, which is deposited on the hydrophilic and negatively charged polyacrylonitrile (PAN) support (denoted as COF-4EO-PAN, Figure 1B). The cross-sectional scanning electron microscopy (SEM) image reveals a regular and lamellar structure, suggesting the formation of a highly ordered and oriented membrane (Figure 1B). Such a configuration is particularly beneficial for permeability, owing to its regular and low-tortuosity nanochannels that considerably reduce the mass transfer resistance. Notably, the diversity of organic side chains endows the COF membrane with a high degree of controllability of the pore size and chemical surface property, allowing precise modulation of the ion sieving behavior of channels. For instance, two other monomers of 2,5-bis(heptyloxy)terephthalaldehyde (OHep) and 2,5-dimethoxyterephthalaldehyde (OMe) were also chosen to synthesize COF membranes with varied pore size and pore environment, denoted as COF-OHep and COF-OMe, respectively. Theoretically, this strategy may be applied to incorporate any chemically reactive sites, which offers the unlimited versatility of COF membranes with customized pore environment. To demonstrate the activity of the ether-mediated transport of lithium ions, the reversal potential was first used to determine the selective ion transport in a bi-ionic system separated by COF membranes. MgCl2 solution was filled on one side of the COF membrane, and other metal chlorides were placed on the other side with a fixed Cl− concentration of 1 mM, separately. Figure 1C summarizes that the relative permeability decreases in the order of Li+ > K+ > Na+ > Ca2+ > Mg2+ for COF-4EO-PAN membrane. As a comparison, COF-OHep-PAN exhibits an intrinsic ion transport efficiency trend of K+ > Na+ > Li+ > Mg2+ > Ca2+. Given the similar chain length of 4EO and OHep, the accelerated transport of Li+ through COF-4EO-PAN membrane is mainly ascribed to the chemical coordination between Li+ and the oligoether moieties rather than the sieving effect of pore size. Furthermore, the ion transport kinetics was surveyed in a homemade diffusion cell in which the COF membrane was sandwiched between two chambers. 0.1 M LiCl or MgCl2 solution was separately filled in the feed compartment, and ion chromatography was used to determine the ion concentration of the permeate side filled with deionized water. A Li+/Mg2+ separation factor of 12 was obtained for COF-4EO-PAN, while COF-OHep-PAN afforded a Li+/Mg2+ separation factor of only 3. To further shed light on the separation mechanism, quantum density functional (DFT) computation was adopted to investigate the chemical basis of binding selectivity toward Li+ over Mg2+. The oligoether moiety was determined to possess a higher binding affinity toward Li+ over Mg2+ by 55.5 kJ mol−1 in aqueous solution. The coordination interaction between Li+ and the oligoether was proved by X-ray photoelectron spectroscopy (XPS), and the binding energy of lithium species in Li+@COF-4EO (55.9 eV) was lower than that in LiCl (56.6 eV), revealing the electron transfer from oligoether to Li+. Considering the fact that single Li+ would be tightly bound in the membrane through the specific coordination interaction, the enhanced transmembrane movement of Li+ could be ascribed to mutual repulsion among the densely crowed Li+ in pore channel, which promotes the transport of Li+ along the concentration gradient. Distinct from lithium ions showing obviously facilitated transport behavior in the oligoether functionalized pores, the permeation of Mg2+ displays strong dependence on the size of the channel with the trend of COF-OMe-PAN > COF-OHep-PAN > COF-4EO-PAN, further suggesting that the accelerated transport of lithium ions originates from the densely arranged lithiophilic moieties in the COF-4EO-PAN membrane. The lithium extraction efficiency of COF-4EO-PAN membrane in a binary mixture of LiCl (0.1 M) and MgCl2 (0.1 M) was next investigated, showing a maximum Li+/Mg2+ separation factor of 64 (Figure 1D) and a high stability over at least 40 h with only slight decrease in selectivity. The much higher Li+/Mg2+ selectivity in the binary mixture than the ideal selectivity mainly originates from the competitive interaction between the oligoether and Li+/Mg2+. Importantly, the flux of Li+ through COF-4EO-PAN increases from 6.8 mmol m−2 h−1 to 230 mmol m−2 h−1 upon the feed concentration from 0.01 M to 1.0 M while the selectivity is maintained, showing much improved separation efficiency. Overall, this study illustrates how the densely aligned lithiophilic oligoether moieties can be well integrated into COF membranes, and subsequently facilitate the transport of lithium ions while obstructing the other ions, on account of selective coordination with lithium ions to lower the ion transfer barrier; namely, the transport of lithium ions is accelerated by rapid and reversible coordination with the oligoether moieties, thereby differentiating lithium ions from other ions. The proof of concept marks an essential step toward an understanding of the ion transport process through the channels of COF materials with a flexible and versatile pore environment, which opens a bright venue toward the development of next-generation membranes for ion sieving and separation. Despite the significant progress in fabrication of oligoether-COF membranes for Li+/Mg2+ separation, many efforts are still needed to construct membranes with monovalent ion selectivity to address the challenge of lithium extraction from chemically similar monovalent ions such as Na+ and K+. As to the ion permeability, although the oriented stacking structure of COF membranes is able to largely improve the mass transport efficiency, the microscale thickness of membranes inevitably results in high transport resistance. From this point of view, novel synthetic methodologies are highly desired for preparing ultrathin or even molecularly thin COF membranes with extremely low membrane resistance. Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extractionBing et al.MatterApril 7, 2021In BriefBy virtue of their tailorable pore environment and unique pore structure, 2D COFs serve as excellent candidates for exploring biomimetic ion channels for sophisticated separation. The 1D pore structure offers unidirectional pathways for swift ion diffusion, wherein the aligned lithiophilic oligoethers oriented in close proximity further facilitate Li ion transport but obstruct the other ions from entering the channel, resulting in a high Li+/Mg2+ separation factor of up to 64. Full-Text PDF Open Archive