Crystalline Porous Materials for Lithium Extraction

Abstract

The rapid transformation of the global energy structure has promoted the broad application of lithium-ion batteries. The efficient extraction of lithium, which is a core raw material for lithium-ion batteries, has become a crucial step in the new energy sector. Crystalline porous materials (CPMs) with distinct chemical properties and versatile topological structures are promising for the separation of Li⁺ ions. Herein, we systematically review the structural features of CPMs, particularly metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), for lithium-ion adsorption and separation, encompassing the latest advancements in adsorption and membrane-based technologies. By analyzing the core mechanisms underlying lithium ions’ selective recognition and transport, such as ion exchange, chelation coordination, and sieving effects, this study reveals the structure-performance relationship for Li⁺ ion separation. It is further elucidated that CPMs typically employ synergistic mechanisms, integrating multiple recognition pathways to enhance performance. Research indicates that the structural flexibility of MOFs and the chemical stability of COFs enable them to exhibit both high selectivity and high capacity in Li⁺ ion adsorption. Functional modification and synthesis of composite materials help the adsorption and membrane separation process based on CPMs further overcome the limitations of traditional separation technologies. Finally, this review summarizes the challenges CPMs face in practical applications, outlines their future development directions for sustainable lithium resource development, and provides theoretical and practical references for the next generation of efficient lithium separation materials.

1. Introduction

Against the backdrop of the accelerated transformation of the global energy structure, advanced energy storage and conversion systems are at the forefront of scientific research. Lithium-ion batteries (LIBs) have emerged as the dominant candidates in energy storage systems, primarily due to their high energy density, long cycle life, and low self-discharge rate. The growing ubiquity of LIBs and their escalating demand have driven a significant surge in lithium consumption. As the core element in the new energy system, the efficient separation and purification of lithium ions (Li⁺) has become a critical bottleneck impeding the development of the clean energy industry. ,

Lithium (Li) is a lightweight, highly reactive metal that serves as a crucial component in lithium batteries. By 2030, the global market value of lithium is expected to exceed $4 trillion. Nevertheless, the worldwide distribution of lithium resources is highly heterogeneous, and its natural reserves are inherently limited (Figure ). Moreover, extracting lithium from hard rock ore, brine, and spent LIBs often faces challenges in complex separation processes, low lithium concentrations, and selective separation. − This situation highlights the urgent need for developing advanced separation technologies that feature high efficiency, high selectivity, and low energy consumption.

1.

(a) Top ten countries with the largest lithium reserves in 2025. (b) Global lithium battery consumption from 2015 to 2025.

A rough timeline has been provided to show the development history of Li⁺ ion separation technologies (Figure ). Traditional Li⁺ separation methods, such as solvent extraction, precipitation, and ion exchange, are often limited by inherent drawbacks, including high chemical reagent consumption, environmental pollution, and poor separation efficiency. , In recent years, adsorption and membrane separation technologies have attracted extensive attention in lithium ion separation due to their potential to achieve high selectivity, energy efficiency, and scalability. , With the continuous advancement of adsorption technology, effectively coupling them with traditional separation techniques has become an inevitable trend in the field. Against this backdrop, the development of high-performance adsorbents and membrane separation materials presents a critical challenge that must be addressed to drive the advancement of next-generation separation technologies. Crystalline porous materials (CPMs), as a leading adsorbent category in the field of lithium extraction, are characterized by their high specific surface area, tunable pore structure, and customizable functionalized sites. − These structural advantages enable CPMs to achieve outstanding Li⁺ selectivity, high recovery rate, cost-effectiveness, and minimal environmental impact, making them a promising candidate for the next-generation lithium extraction technology. Additionally, zeolites exhibit relatively fixed pore sizes, while POPs demonstrate weak ion recognition and challenging pore structure control, limiting their performance in high Li/Mg ratio systems. Among various CPMs, MOFs and COFs stand out as superior materials. Current reviews primarily focus on low-quality brine technologies (precipitation, extraction, etc.) and fundamental mechanisms, ,, but lack a comprehensive summary of the basic mechanisms governing Li⁺ selectivity and transport within materials, particularly in conjunction with targeted outlooks for novel materials.

2.

Development timeline of lithium-ion separation technology. Early exploration stage lithium extraction technology. Reproduced with permission from ref . Copyright 2024, Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Technology development and application stage lithium extraction technology. Reproduced with permission from ref . Copyright 2024, American Chemical Society. Reproduced with permission from ref . Copyright 2021, Wiley-VCH GmbH; Technological innovation and breakthrough stage lithium extraction technology. Reproduced with permission from ref . Copyright 2024, Elsevier. Reproduced with permission from ref . Copyright 2021, American Chemical Society. Reproduced with permission from ref . Copyright 2023, Wiley-VCH GmbH; Industrialization and diversification stage lithium extraction technology. Reproduced with permission from ref . Copyright 2022, American Institute of Chemical Engineers. Reproduced with permission from ref . Copyright 2021, American Chemical Society.

In this review, we provide a systematic and thorough overview of CPMs, including their structural features, and conduct an in-depth examination of the applications of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) in Li⁺ adsorption and separation, focusing on both adsorption-based and membrane-based strategies. Furthermore, we elucidate the fundamental mechanisms that govern Li⁺ selectivity and transport within these materials, providing crucial insights into their structure–property relationships for enhanced lithium recovery. This review aims to provide a critical and forward-looking perspective on the significance of CPMs in advancing Li⁺ separation technology, thereby contributing to the development of sustainable and efficient lithium extraction processes that meet the increasing demands of energy storage.

2. Advanced Strategies for Lithium Adsorption by CPMS

Crystalline porous materials (such as MOFs and COFs) demonstrate significant potential in the field of adsorption and separation due to their designable pore architectures, high specific surface area, and function-tunable pore environments. These materials can be specifically designed to recognize Li⁺, especially for the ultralow concentration source. , Their enhancement strategies primarily focus on improving adsorption capacity, adsorption selectivity (particularly for Li⁺/Mg²⁺), and adsorption kinetics, which can be specifically categorized as follows.

2.1. Control of Channel Size and Shape

The pore structure is a crucial feature of CPMs. By design, the pore size of CPMs can be matched to the diameter of dehydrated lithium ions (1.52 Å), which effectively distinguishes competing ions such as Na⁺ (2.04 Å), K⁺ (2.76 Å), and Ca²⁺ (2.0 Å) through molecular sieving (Table ). Furthermore, diverse pore geometries, including one-dimensional (1D) channels and two-dimensional (2D) interlayer channels, enable dynamic lithium-ion sieving by restricting the diffusion pathways of larger ions and modulating their diffusion rates, exploiting the kinetic disparities of these ions during mass transfer processes. This size selectivity stems not only from static pore size matching but also from the regulation of dynamic ion diffusion kinetics.

1. Physical and Chemical Parameters of Key Ions in Lithium–Magnesium Separation .

	Li+	Na+	K+	Ca2+	Mg2+
ionic radius (Å)	0.76	1.02	1.38	1.00	0.72
hydrated radius (Å)	3.8	3.6	3.3	4.1	4.4
hydration energy (kJ mol–1)	–515	–405	–320	–1577	–1922
charge density (e/Å3)	0.22	0.09	0.04	0.32	0.53
typical concentration ratio (brines)	1	102–103	101–102	101	102

^aNote: the values of hydrated ions are from the refs , and those of dehydrated ions are from the software of Materials Studio and ref .

2.2. Functional Framework Design

The unique metal nodes of MOFs form their core structural framework. The types, valence states, coordination modes, and distribution directly influence the electron density, pore structure, and chemical stability of materials, thus serving as the foundation for functional design. Additionally, the functional groups, length, rigidity/flexibility, and symmetry of organic ligands directly determine the porosity, surface chemistry, and guest–host interactions in CPMs. Specifically, metal nodes such as Al³⁺ and Ti⁴⁺ directly capture Li⁺ through coordination, while oxygen-containing groups in polar ligands, such as carboxyl and hydroxyl groups, further enhance Li⁺ adsorption via auxiliary coordination and hydrogen bonding. , Density functional theory calculations reveal that oxygen-containing moieties exhibit binding energies of 150–200 kJ mol^–1 for Li⁺, significantly higher than those for Na⁺ (80–120 kJ mol^–1) and K⁺ ions (60–100 kJ mol^–1).

2.3. Functional Design and Modification

The strategic incorporation of anionic groups or functional groups enables precise engineering of surface charge density and chemically active sites in CPMs, thereby significantly augmenting their capacity for electrostatic attraction and coordination binding toward Li⁺ ions. The local electrostatic potential generated by the framework charge and anionic functional groups (−SO₃ ^–, −PO₃ ^2–) can effectively distinguish coexisting cations with similar ionic radii. It is worth noting that Li⁺ has a smaller hydrolytic ion radius, which makes it preferentially adsorbed. , Theoretical calculations show that for every 1 e nm^–3 increase in charge density, the Li⁺/Na⁺ selectivity ratio can be improved by 2–3 orders of magnitude. Additionally, Li⁺ ion is challenging to be chelated by single or dual functional groups (e.g., carboxyl, hydroxyl, and amino). To enhance the chelation affinity toward Li⁺ ion, synergic adsorption of multiple active atoms (e.g., O and N) is commonly required.

2.4. Structural Stability

Under harsh saline or acidic conditions, CPMs can maintain pore integrity and reusability. Many MOFs and COFs exhibit less than 5% loss in adsorption capacity after multiple adsorption–desorption cycles in actual Salt Lake systems (Li⁺ concentration: 10–500 mg L^–1). Specifically, the rigid porous framework of MOFs interacts synergistically with Lewis acidic sites to uniformly distribute Li⁺ and promote lithium salt dissociation, thereby preventing dendrite growth. COFs provide exceptional chemical stability (Ph = 1–14) through robust covalent bonds, while their π-conjugated systems facilitate ultrafast Li⁺ transport (diffusion coefficient: ∼10^–7 cm² s^–1). The membrane material combines these stability advantages with additional mechanical strength, effectively preventing pore deformation during pressure-driven processes, and breaks through traditional trade-offs through ion-selective design.

In fact, the unique structural and chemical advantages of CPMs position them as next-generation materials for sustainable Li⁺ extraction. Future research should focus on scale-up synthesis, optimizing membrane processing, and enhancing stability in real-world environments to bridge the gap between laboratory innovation and industrial application.

2.5. CPM Membranes

The membranes currently used for Mg²⁺/Li⁺ separation primarily include polymeric membranes (e.g., nanofiltration (NF) membranes, , layer-by-layer­(LBL) assembled membranes, etc.), , two-dimensional (2D) membranes (e.g., MXene membranes, graphene oxide membranes, etc.), COF membranes, − and MOF membranes. Among these, NF membranes exhibit excellent water permeability and ion selectivity, enabling the efficient retention of multivalent ions such as Mg²⁺ for the preliminary separation of salt lake brines. However, their reliance on polymeric substrates limits operational stability due to pore size constraints, resulting in poor antifouling performance in complex brines and indirectly reducing NF efficiency. 2D membranes also face a critical challenge as aqueous swelling disrupts their interlayer structure and impairs separation precision. CPM membranes, leveraging their tunable pore sizes and functionalized properties, exhibit unique advantages in the field of Mg²⁺/Li⁺ separation. Endowed with the capability to fine-tune pore dimensions to the subnanometer scale, CPMs can be engineered to function as molecular gatekeepers. By constructing apertures with sizes tailored to accommodate hydrated Li⁺ ion (hydrated radius = 3.8 Å) while excluding larger hydrated Mg²⁺ ion (hydrated radius = 4.4 Å), enabling efficient permeation of Li⁺ while selectively retaining Mg²⁺. Furthermore, the introduction of specific functional groups (e.g., sulfonic acid groups (−SO₃H), amino groups (−NH₂), crown ether molecules) onto the inner walls or surfaces of the pores can significantly enhance the chemical affinity for Li⁺, , strengthening its selective adsorption capability. MOF/COF membranes, owing to their stable coordination bonds or covalent bonds, can maintain excellent structural stability and permeation flux even under high ionic strength conditions (e.g., Salt Lake brines), making them suitable for long-term sustainable operation. Furthermore, the high porosity and through-pore channel structure endow them with high-flux permeation characteristics, which not only ensure high selectivity but also significantly reduce energy consumption during the separation process, providing a green and low-carbon technical path for the efficient extraction of Mg²⁺/Li⁺ resources.

Membrane separation technology has been widely used in the selective separation of lithium, among which nanofiltration, electrodialysis, and concentration diffusion technology have attracted extensive research attention (Figure ). − Nanofiltration is carried out under the transmembrane pressure gradient (0.1–1.0 MPa) as the mass transfer driving force, allowing solvents and small molecular ions to permeate through the membrane pores while rejecting large molecules or larger ions. The separation efficiency is mainly controlled by the size screening mechanism. Electrodialysis utilizes an external electric field (0.5–5 V) to drive directional ion migration through the selective permeation of ion exchange membranes, synergistically enhancing Li⁺ separation selectivity through the Donnan effect and size exclusion. Concentration gradient diffusion utilizes the transmembrane ion concentration difference as the driving force, allowing ions to diffuse from the high to the low concentration side, where selective Li⁺ diffusion is facilitated through dielectric effects. The tunable design features of CPMs, including adjustable pore size, functional groups, and dielectric constant, render them ideal platforms for realizing multiprocess, multimechanism synergistic Li⁺ separation.

3.

Membrane separation processes: (a) nanofiltration, (b) concentration diffusion, and (c) electrodialysis.

3. Li⁺ Adsorption Performance Parameters

3.1. Composition of Li⁺ Separation System

To facilitate more targeted analysis of Li⁺ adsorption performance parameters, this section first outlines common Li⁺ separation systems. It clarifies the ionic composition, variations in Li⁺ concentration, and core separation requirements across different environments, thereby laying a foundational basis for the subsequent analysis of performance parameters.

3.1.1. Salt Lake Brine System

As the most abundant lithium reserve globally (accounting for over 60% of total reserves), Salt Lake brines are generally classified into two categories: high magnesium-to-lithium (Mg²⁺/Li⁺) ratio brines (Mg²⁺/Li⁺ > 20) and low Mg²⁺/Li⁺ ratio brines (Mg²⁺/Li⁺ < 5). High Mg²⁺/Li⁺ ratio brines, represented by those from Qinghai Salt Lake (China) and Atacama Salt Lake (Chile), typically have Li⁺ concentrations ranging from 200 to 1500 mg L^–1, with coexisting ions dominated by Mg²⁺ (5000–30,000 mg L^–1) and Na⁺ (1000–5000 mg L^–1). Owing to the similar ionic radii and hydration behaviors of these two cations, Mg²⁺ emerges as the primary interfering ion for Li⁺ separation. In contrast, low Mg²⁺/Li⁺ ratio brines (Salar de Uyuni, Bolivia) exhibit higher Li⁺ concentrations (2000–5000 mg L^–1), with Na⁺ and K⁺ as the main interfering ions, thus posing relatively lower challenges for Li⁺ separation.

3.1.2. Seawater and Brine System

Seawater contains lithium at an extremely low concentration (0.1–0.2 mg L^–1) but presents an enormous total reserve (approximately 2.3 × 10¹¹ t). In seawater, the concentrations of Na⁺ (10,500 mg L^–1) and Cl^– (19,000 mg L^–1) exceed those of Li⁺ by more than 4 orders of magnitude. Mg²⁺ (1272 mg L^–1) and Ca²⁺ (400 mg L^–1) are also major competing ions, underscoring the necessity for materials that simultaneously offer exceptional selectivity and the ability to enrich Li⁺ from ultradilute solutions. Furthermore, the brine produced during seawater desalination (Li⁺ concentration 0.5–1.0 mg L^–1), where pretreatment removes a portion of Ca²⁺ and SO₄ ^2–, represents a transitional feedstock for seawater lithium extraction, in which Na⁺ and Mg²⁺ remain the primary interfering ions.

3.1.3. Industrial Lithium-Containing Wastewater System

These include lithium battery production wastewater (Li⁺ concentration 50–500 mg L^–1) and geothermal tailwater (Li⁺ concentration 10–100 mg L^–1), with compositions that vary significantly depending on the source. Lithium battery wastewater contains metal ions such as Li⁺, Co²⁺, Ni²⁺, and Mn²⁺, necessitating the simultaneous consideration of lithium recovery and the removal of heavy metals. In contrast, geothermal tailwater is rich in Na⁺, K⁺, and siloxane-based organic compounds that tend to foul and clog material pores, thereby requiring higher antifouling performance from the materials.

Based on the above common lithium separation scenarios, the evaluation of CPMs for Li⁺ adsorption necessitates a comprehensive assessment of multiple critical parameters, including adsorption capacity, selectivity, kinetics, structural stability, and practical applicability. These performance parameters can be systematically classified into the following key categories.

3.2. Basic Adsorption Performance Parameters

This type of parameter directly reflects the adsorption capacity and efficiency of the material on Li⁺ and is the most central evaluation index.

3.2.1. Adsorption Capacity

The adsorption capacity refers to the maximum amount of Li⁺ that can be adsorbed per unit mass (or volume) of CPMs, serving as a fundamental indicator to quantify the “adsorptive capacity strength” of a material. It is typically determined via adsorption isotherm experiments (e.g., the equilibrium adsorption amount of Li⁺ under varying initial Li⁺ concentrations). Generally, a higher adsorption capacity implies a superior Li⁺ recovery efficiency per unit mass (or volume) of the material. The basic calculation formula for adsorption capacity is as follows:

$$Q=\frac{(C_0-C_{\mathrm{e}})\times V}{m}$$

where Q represents the adsorption capacity (mmol g^–1 or mg g^–1); C ₀ is the initial concentration of Li⁺ ions in the solution (mmol L^–1 or mg L^–1); C _e is the concentration of Li⁺ ions in the solution at the adsorption equilibrium (mmol L^–1 or mg L^–1); V refers to the volume of the Li⁺ ions solution (L); and m refers to the mass of the material (g).

3.2.2. Selectivity

Selectivity represents a critical performance parameter characterizing a material’s ability to preferentially adsorb Li⁺ from complex aqueous systems containing competing cations (Na⁺, K⁺, Mg²⁺, Ca²⁺). In practical lithium extraction scenarios such as Salt Lake brines or seawater, Li⁺ typically exists at substantially lower concentrations than competing ions; for example, Mg²⁺ concentrations often exceed Li⁺ by more than 2 orders of magnitude. High selectivity is crucial in preventing the competitive occupation of adsorption sites by other ions, which directly determines both the purity of the recovered Li⁺ and the economic viability of the separation process. The selectivity performance is quantitatively assessed through several key metrics: the selectivity coefficient (α), partition coefficient (K _d), defined as follows.

The partition coefficient (K _d) quantifies the preferential adsorption capacity of a material toward Li⁺ in dilute concentration systems. A higher K _d value indicates a stronger adsorption affinity of the material for Li⁺ ions.

$$K_{\mathrm{d}}=\frac{Q}{C_{\mathrm{e}}}$$

The selectivity coefficient (α) is used to compare the adsorption priority of Li⁺ with a competing ion, and α > 1 indicates that the material is more selective for Li⁺ than for M⁺.

$$\alpha =\frac{K_{\mathrm{d}}^{\mathrm{Li}}}{K_{\mathrm{d}}^{\mathrm{M}}}$$

3.2.3. Isotope Separation Factor

Furthermore, the separation of lithium isotopes (⁶Li and ⁷Li) constitutes a pivotal step for the industrial implementation of lithium adsorption technology in the separation domain. The fundamental mechanism underlying this separation hinges on the utilization of subtle disparities in the physical and chemical properties between the two isotopes, which include variations in diffusion velocity, volatility, and ion mobility arising from their intrinsic mass difference. , This separation process induces a discrepancy in isotopic abundance between the enriched phase (where the target isotope accumulates) and the depleted phase (where the target isotope is depleted). The isotope separation factor (SF) serves as the primary parameter for quantifying separation efficiency, and it is defined as the ratio of the isotopic abundance ratio in the enriched phase to that in the depleted phase.

$$\mathrm{SF}=\frac{(X_{{}^6\mathrm{Li}/{}^7\mathrm{Li},\mathrm{enriched}})}{(X_{{}^6\mathrm{Li}/{}^7\mathrm{Li},\mathrm{depleted}})}$$

3.2.4. Adsorption Kinetics

The rate of Li⁺ adsorption and the time required to attain adsorption equilibrium are evaluated by quantifying the Li⁺ adsorption amount at discrete time intervals, followed by fitting the data to kinetic models (pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion models) and calculating the corresponding rate constants (k).

Pseudo-first-order kinetics is applicable to processes dominated by physical adsorption:

$$\ln (Q_{\mathrm{e}}-Q_t)=\ln Q_{\mathrm{e}}-k_{1}t$$

where Q _e represents the equilibrium adsorption capacity (mmol g^–1); Q _t denotes the adsorption capacity at time t (mmol g^–1); k ₁ is the pseudo-first-order rate constant (min^–1 or h^–1); and t corresponds to the adsorption time.

Pseudo-second-order kinetics is applicable to processes dominated by chemical adsorption:

$$\frac{t}{Q_t}=\frac{1}{k_2{Q}_{\mathrm{e}}^2}+\frac{t}{Q_{\mathrm{e}}}$$

where k ₂ is the pseudo-second-order rate constant (g mmol^–1 min^–1 or g mmol^–1 h^–1).

3.2.5. Parameters for Membrane Materials

In the performance evaluation of membrane separation technologies for Li⁺ extraction, lithium ion flux (J _Li ⁺) serves as the core kinetic parameter that characterizes membrane mass transfer efficiency and directly determines the amount of Li⁺ extracted per unit time. The magnitude of J _Li ⁺ is jointly determined by the membrane’s intrinsic mass transfer capability and the external operating conditions. Specifically, the membrane’s microstructure plays a pivotal role in Li⁺ transport, governing the length of Li⁺ transport pathways, the magnitude of transport resistance, and the utilization efficiency of active sites, thereby emerging as a core factor that modulates J _Li ⁺.

Water flux (J _w) is defined as the volume of water passing through a unit area of the membrane per unit time, serving as a quantitative measure of the membrane’s water productivity. In the context of lithium extraction from Salt Lake brines or seawater, achieving an optimal balance between water flux and ion selectivity is crucial. Excessively large membrane pores may yield high water flux but often result in the cotransport of interfering ions such as Mg²⁺, thereby compromising selectivity. Conversely, unduly small pores can drastically reduce water flux to levels that are insufficient to meet industrial processing requirements.

$$J_{\mathrm{w}}=\frac{V_{\mathrm{permeate}}}{A\times t}$$

where A is the effective area of the membrane.

The ion permeability (P) directly determines the Li⁺ production rate per unit time and thus reflects the Li⁺ mass transfer rate across the membrane. It should be emphasized that ion permeability must be evaluated in conjunction with selectivity. A high P _Li ⁺ accompanied by a low α­(Li⁺/Mg²⁺) indicates that, despite good Li⁺ permeability, the membrane’s poor selectivity makes it unsuitable for practical applications.

$$P=J_{\mathrm{w}}\times C_{\mathrm{permeate}}$$

In Salt Lake brines, concentration polarization layers tend to form spontaneously on the membrane surface, causing the accumulation of interfering ions at the membrane–solution interface and thereby hindering Li⁺ mass transfer. The mass transfer coefficient (K) serves as a key metric for assessing the membrane’s resistance to concentration polarization. Membranes with high K values, such as those featuring hydrophilic surfaces and low surface roughness, can effectively alleviate concentration polarization, thus sustaining stable Li⁺ permeability.

3.3. Thermodynamic Parameters

These parameters reflect the energy variations and spontaneity of the adsorption process, thereby facilitating the elucidation of adsorption mechanisms and the assessment of temperature effects on adsorption behavior. Through fitting with isothermal models (e.g., Langmuir, Freundlich), critical characteristics such as adsorption type, theoretical maximum adsorption capacity, and the adsorptive affinity of the material can be quantitatively determined. Among these, the Gibbs free energy change (ΔG) is crucial for evaluating the feasibility of Li⁺ adsorption. A negative ΔG (ΔG < 0) means Li⁺ adsorption on the material occurs spontaneously under given conditions, with a larger absolute ΔG indicating a stronger spontaneous tendency. A positive ΔG (ΔG > 0) requires external energy input, greatly limiting practical application. Thus, the magnitude and sign of ΔG directly provide a fundamental thermodynamic basis for judging whether a candidate material holds practical utility for Li⁺ adsorption.

3.4. Stability and Practicality

On the one hand, the chemical stability of the material necessitates rigorous evaluation. The structural integrity of the material under practical adsorption environmentsencompassing varying pH conditions, high-salt concentrations, and exposure to organic solventsserves as a fundamental prerequisite for its successful application. Such stability can be systematically assessed through characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA), among others.

Cyclic stability represents a critical performance metric for Li⁺-selective adsorbents. High-performance materials demonstrate exceptional capacity retention, typically maintaining over 80% of their initial adsorption capacity after 10 consecutive adsorption–desorption cycles. This outstanding cycling durability significantly enhances material longevity and reduces operational costs in industrial applications. Additionally, the desorption behavior of the material should not be overlooked: a higher desorption efficiency (>90%) coupled with milder desorption conditions (ambient temperature, low-concentration acid or alkali reagents) contributes to lower energy consumption in Li⁺ recovery processes and facilitates material regeneration.

Thus, the comprehensive evaluation of Li⁺ adsorption performance for CPM necessitates the integration of adsorption capacity, selectivity, kinetic behavior (as core performance metrics), thermodynamic parameters, and cyclic/desorption properties (as indicators of practical application potential). Among these, saturated adsorption capacity, Li⁺/M⁺ selectivity, and cyclic stability emerge as the most critical benchmarks for industrial-scale applications. As shown in Table , we have summarized common lithium adsorbents and their adsorption performance parameters.

2. Adsorption Capacity of Common Li⁺ Adsorbents.

Li+ adsorbents	Q (mg g–1)	C 0 (mg L–1)	dosage (g L–1)	pH	T (K)	ref
PAM-MnO2	18.76	160	2	10.1	298
GLDH	∼5.0	250	26.7		288
Li-IIM	1.39	10		1
WP@PSS@Cu-MOF	∼16	250	1	9	298
MLDH-1	5.83	250	33.3		298
lithium–aluminum absorbent	2.015	350		7	303
Al–F–Li1.6Mn1.6O4	33.7	500		12	298
PDMVBA-MIL-121	1.38	1000	5
DAB-g-14C4PI	3.09	400	3		298
HFTO	34.27	1000	2	12	298
HTO@GO	37.9	100	1		303
HMACO-0.04	32	480	2	12	303
HMnO/Alg(Al)	1.51	100
pNCE-MOF-808	0.378	3500	10		293
Fe3O4@SiO2@IIP	4.1	100			298
MOF-808-EDTA	3.75	1000	1
MOF-808-12C4E	30.4	1000	0.5	6.3	303
TJU-21	41	1000	0.5		298
UiO-66-H2/H4-b	50.0	1000	0.5		303
MIL-121-H2/H4	35.6	1000	0.5	7	298
PSP-UiO-66	44.04	1000	1	8	298
PSP-MOF-808	1.59	1000		8	298
PNCE-SS@UiO-66	69	1000	4		293
MIL-100(Fe)	27.5	4000	10	7	298
TYUST-8	52	1000	0.5	8.8	303
	76.1	2000	0.5	9.4	303
TpPa-SO3H	145	2000	2	7	293

4. Mechanism for CPMS-Based Li⁺ Separation

Based on the aforementioned Li⁺ adsorption performance evaluation metrics, further in-depth analysis of the core mechanism enabling CPMs to achieve efficient Li⁺ separation can provide theoretical support for material design optimization. The selective extraction of lithium ions (Li⁺) from aqueous solutions or brine resources using CPMs, including MOFs, COFs, is governed by four primary mechanisms: ion exchange, chelation coordination, electrostatic interactions, and molecular sieving (Figure ).

4.

Different mechanisms for Li⁺ separation with CPM adsorbents and membranes. Adsorbents based on ion exchange as a mechanism. Reproduced with permission from ref . Copyright 2024, Elsevier and KeAi.; Adsorbents based on adsorption as a mechanism. Reproduced with permission from ref . Copyright 2025 Elsevier; Adsorbents based on sieving effect as a mechanism. Reproduced with permission from ref . Copyright 2024, Creative Commons CC-BY.; Adsorbents based on electrostatic interactions as a mechanism. Reproduced with permission from ref . Copyright 2023, Royal Society of Chemistry.Adsorbents based on size screening as a mechanism. Reproduced with permission from ref . Copyright 2025, Elsevier; Adsorbents based on chelation coordination as a mechanism. Reproduced with permission from ref . Copyright 2024, American Chemical Society.

4.1. Ion Exchange

Ion exchange entails the reversible displacement of charge-balancing ions (e.g., Na⁺, K⁺, H⁺) preimmobilized within the CPM framework by Li⁺ from the aqueous phase, driven by the necessity to maintain the overall electrical neutrality of the material. , This mechanism is particularly prevalent in CPMs’ featuring anionic frameworks or cationic building units, such as zeolites, cationic-MOFs, and certain functionalized COFs. The exchange process is governed by the relative affinity of the framework for Li⁺ versus competing ions, which is influenced by factors including ionic radius (Li⁺: 0.76 Å), charge density, and hydration energy. For instance, in zeolites with aluminosilicate frameworks, the negatively charged lattice exhibits preferential attraction toward cations, and Li⁺, characterized by its high charge density and compact hydration shell, often demonstrates a stronger affinity than larger alkali/alkaline earth ions (e.g., Na⁺, K⁺), enabling selective exchange. The ion exchange capacity is typically proportional to the density of charge-balancing sites within the CPM, and its reversibility facilitates material regeneration via desorption using high-concentration competing ion solutions.

4.2. Chelation Coordination

Coordination interaction is a specific interaction between Li⁺ and certain atoms or groups in the CPM framework. ,, With a typical coordination number of 4–6, Li⁺ tends to form stable complexes with ligands containing electron-donating atoms (O, N, S), such as crown ethers, carboxylates, phosphonates, or hydroxides. This mechanism is highly customizable, as the type and density of functional groups in CPMs (MOFs with modified organic linkers, COFs with chelating moieties) can be rationally engineered to match the coordination preferences of Li⁺. For example, MOFs functionalized with 12-crown-4 ether groups, whose cavity size (1.2–1.5 Å) is commensurate with the diameter of dehydrated Li⁺, exhibit enhanced Li⁺ selectivity through strong host–guest chelation, even in the presence of interfering ions (Mg²⁺, Ca²⁺, K⁺, Na⁺) with comparable ionic radius. , The stability of the Li⁺-ligand complex governed by bond energy and steric constraints directly dictates both selectivity and desorption behavior; thus, chelation serves as a pivotal mechanism for achieving high Li⁺/competitor separation ratios.

4.3. Electrostatic Interactions

Electrostatic interactions frequently occur in microenvironments dominated by anions, essentially involving a selective binding process driven by Coulombic attraction between negatively charged sites on material surfaces/pores and Li⁺ ions. Specifically, Li⁺ ions in solution (in hydrated states, such as [Li­(H₂O) _n ]⁺) overcome diffusion resistance within the solution and migrate directionally toward active sites under the electrostatic attraction field exerted by charged sites on the surface or within the pores of CPMs. Upon approaching charged sites, charge matching and hydration layer rearrangement induce electrostatic repulsion between Li⁺ and coexisting cations. Additionally, the anion-dominated microenvironment serves as a driving force for ion diffusion. Thus, precise molecular design can modulate the type, density, and spatial distribution of charged sites to tailor Li⁺ separation for diverse systems, providing a clear design strategy for developing high-performance lithium separation materials.

4.4. Sieving Effect

The molecular sieving effect in CPMs exploits angstrom-level precision in pore size engineering to achieve selective Li⁺ transport through steric exclusion of larger competing ions. By precisely adjusting pore apertures to align closely with the ionic dimensions of Li⁺ (0.76 Å in dehydrated form and approximately 3.8 Å in hydrated form), CPMs with well-defined structures can effectively exclude larger hydrated ions, including Mg²⁺ (4.3 Å), Ca²⁺ (4.1 Å), and Na⁺ (3.6 Å). The core value of this dimensional design lies in the fact that when ions attempt to traverse the channel, their mobility does not decrease linearly with size but instead follows an “energy barrier repulsion model.” The slight difference between the ion size and the channel’s inner diameter is exponentially amplified through interactions between the channel wall and the ion’s solvation layer, resulting in a disparity in migration energy barriers. This selective discrimination remains effective even in high-salinity environments where the concentrations of these interfering ions surpass that of Li⁺ by 10²–10³ times, as exemplified by the Mg²⁺/Li⁺ ratio exceeding 50:1 in Salt Lake brines. , Unlike chemically dependent separation mechanisms (ion exchange or coordination), this size-exclusion paradigm derives from the intrinsic rigidity of crystalline frameworks, maintaining exceptional selectivity across wide ranges of pH (2–12) and ion concentration (0.1–5.0 M). This structural robustness makes sieving-based separation particularly advantageous for ultradilute Li⁺ sources (<100 mg L^–1).

In practical Li⁺ separation systems, CPMs typically employ synergistic mechanisms that combine multiple recognition pathways for enhanced performance. ,− For example, a MOF with crown ether-functionalized pores can simultaneously utilize chelation (via ligand-Li⁺ coordination) and sieving (via pore size restriction) to enhance selectivity, while ion exchange may complement these processes to boost adsorption capacity. Therefore, leveraging the structural tunability inherent to CPMs, the rational design and targeted implementation of these adsorption mechanisms remain pivotal to advancing the development of high-performance lithium-ion-selective adsorbents.

4.5. Synergistic Effects of Multiple Mechanisms

Ion exchange, chelation coordination, electrostatic interactions, and molecular sievingthese four individual mechanisms provide the fundamental theoretical basis for lithium ion separation in CPMs. However, in complex systems (such as high-salinity brines where Li⁺ coexists with Na⁺, K⁺, and Mg²⁺), a single mechanism is often constrained by bottlenecks like insufficient selectivity, low capacity, or sluggish kinetics. The synergistic effect of multiple mechanismsachieved by rationally coupling two or more separation mechanisms to leverage their complementary and reinforcing actionscreates a “1 + 1 > 2” separation system.

Molecular sieving-electrostatic interaction synergy is a separation mechanism that utilizes grain boundary regulation to construct ultranarrow pore channels, creating spatial barriers to Li⁺ transport. Simultaneously, specific charged groups are modified onto the pore surfaces to selectively repel or promote the transport of competing ions through electrostatic interactions, further amplifying separation differences. This synergistic mechanism is particularly applicable in systems where Li⁺ and competing ions (such as Na⁺, K⁺) exhibit minimal size differences but significant electromigration rate disparities. Addressing the demand for high-capacity lithium storage in applications such as lithium-ion battery electrode materials. On one hand, specific active sites within the CPMs framework can form stable bonds with Li⁺ through chelation coordination, ensuring structural stability during cycling. Meanwhile, the electronic structural changes induced by coordination activate ion intercalation or metal deposition mechanisms, providing an additional capacity source for lithium storage. In systems with low Li⁺ concentration and high competing ion concentrations (especially Mg²⁺), such as brines from salt lakes, ion exchange and electrostatic interactions are often synergistically employed to enhance material selectivity. The surface-modified charged groups (e.g., carboxyl, phosphate) on CPMs preferentially adsorb Li⁺ through electrostatic interactions, complementing the group charge and lowering the binding energy barrier between Li⁺ and the framework active sites. As a result, this promotes the exchange reaction between Li+ and the original cations (e.g., H⁺, Na⁺) within the material, achieving efficient Li⁺ capture.

5. Research Progress for Li⁺ Separation

5.1. Unmodified MOFs and COFs

MOFs and COFs represent two prominent classes of CPMs, each of them offering distinct advantages in Li⁺ ion separation through their unique structural and chemical properties. − Among them, some MOFs naturally possess the ability to adsorb lithium ions by their inherent properties. MIL-100­(Fe) have been demonstrated outstanding water stability and chemical durability, enabling selective adsorption of lithium. Experimental studies have shown that at different initial concentrations, its equilibrium adsorption capacity for Li⁺ ranges from 27.5 to 48.8 mg g^–1, and adsorption equilibrium can be achieved within 2 h. In-depth investigation of its adsorption mechanism reveals that the high-efficiency lithium-ion adsorption of MIL-100­(Fe) stems from the synergistic effects of multiple mechanisms (Figure ): (1) The material retains a consistently negative surface charge across a broad pH range (pH = 3–9), thereby promoting the directional accumulation of Li⁺ via electrostatic interaction; (2) Unsaturated carboxyl sites (−COOH) on the undergoing partial deprotonation directly anchor Li through coordination interactions. Put simply, there may be Li⁺–H⁺ ion exchange in the −COOH groups; (3) The Fe³⁺ metal nodes, functioning as Lewis acidic sites, establish stable Fe–F···Li coordination bonds with Li⁺. The combined action of these concurrent mechanisms contributes to an efficient Li⁺ adsorption process.

5.

Schematic diagram of Li⁺ adsorption process by MIL-100­(Fe). Reproduced with permission from ref . Copyright 2023, Elsevier.

Guo and co-workers reported a novel cubic rod-shaped three-dimensional MOF (TJU-21) composed of fluorine-supported coordination layers containing Fe–O inorganic chains and benzene-1,3,5-tricarboxylate (BTC) linkers. TJU-21 showed remarkable thermal and chemical stability, as well as a high lithium adsorption capacity. Li⁺ was adsorbed in cavity 2 between the layers, surrounded by symmetrically assembled BTC linkers and the Fe site, where strong interactions existed between the adsorbed Li⁺ and the framework. (Figure a,b). More significantly, TJU-21 demonstrates excellent applicability in real brine systems and exhibits superior regenerative capability (Figure c).

6.

Single-crystal structure of (a) pristine and (b) Li-loaded TJU-21 along a-, b-, and c-axes. Hydrogen atoms in the structures have been omitted. (c) Practical lithium adsorption capacity of TJU-21 for pretreated Da Qaidam brine, Uyuni brine, and Yiliping brine and the lithium proportion among cations in the initial brines and their corresponding eluents after exhausting TJU-21 were regenerated by 20 mL of 0.5 M hydrochloric acid. Reproduced with permission from ref . Copyright 2021, American Chemical Society.

7.

(a) Adsorption and separation of ⁶Li and ⁷Li by different MOFs (adsorption conditions: Li⁺ = 10 g L^–1, 25 °C, and dosage: 10 mg mL^–1); (b–g) XPS spectra of pristine and Li-loaded MIL-100­(Fe): (b and c) Fe 2p; (d and e) O 1s; and (f and g) Li 1s. Reproduced with permission from ref . Copyright 2022, Royal Society of Chemistry.

Lithium isotopes (⁶Li and ⁷Li) are critical in nuclear energy-⁶Li serves as a tritium breeder in fusion reactions, while ⁷Li acts as a pH regulator in pressurized water reactors, making efficient separation essential for nuclear applications. − Guo and co-workers investigated the adsorption and lithium isotope separation performance of seven robust MOFs, with a focus on MIL-100 series materials. MIL-100­(Fe) demonstrated a high lithium adsorption capacity of 46.3 mg g^–1 and an isotope separation factor (SF) of 1.039 ± 0.002, while MIL-100­(Al) achieved the highest SF of 1.048 ± 0.001. Further investigation via XPS revealed a chemical interaction between Fe and Li atoms, which was clearly evidenced by the observed shift in Fe 2p binding energy after adsorption (Figure ).

COFs offer complementary advantages through their robust covalent networks and predesigned π-conjugated systems. The strong covalent bonds in COFs confer remarkable chemical stability and thermal resistance, which are crucial for harsh separation conditions. Their uniform and rigid pore channels (typically 1–3 nm) facilitate ultrafast Li⁺ transport (diffusion coefficient ∼ 10^–7 cm² s^–1), while the electron-rich frameworks enhance Li⁺ affinity through cation−π interactions.

Li and co-workers successfully introduced sulfonic acid groups (−SO₃H) into the skeleton of two-dimensional COFs via a solvothermal method, synthesizing two novel sulfonic acid-functionalized COF adsorption materials (Figure a): TpPa-SO₃H and TpBd-SO₃H, among these, TpPa-SO₃H exhibited a record-breaking lithium adsorption capacity of 145 mg g^–1, which is 5–8 times higher than that of traditional adsorbents. DFT theoretical calculations further revealed that its exceptional adsorption performance stems from three synergistic enhancement mechanisms: first, electron-rich regions formed by π–π conjugation effects in the skeleton effectively capture Li⁺; second, uniformly distributed −SO₃H groups bind Li⁺ through strong electrostatic interactions; third, electronegative groups such as CO provide additional coordination binding sites. Analysis of the electrostatic potential (ESP) distribution of TpPa-SO₃H demonstrates that the most notable negative electrostatic potential is concentrated near the −SO₃H functional groups. Furthermore, this negative ESP extends to cover the regions surrounding the benzene ring and the carbonyl group (CO). These results strongly indicate that the −SO₃H moiety, benzene ring, and CO act as efficient electron-donating sites, which contribute to enhancing the binding affinity between Li⁺ and the TpPa-SO₃H framework (Figure b,c). Experimental results showed that the material maintains perfect structural stability over a wide pH range of 2–12 and achieves a high separation factor of 15.7 for Li⁺/Mg²⁺ (2 g L^–1 of Li/Mg, pH = 7, T = 298 K). This study represents the first application of sulfonic acid (−SO₃H)-functionalized COFs in the field of lithium adsorption, and the proposed “pre-modification-multi site synergy” design strategy provides a new paradigm for developing high-capacity, high-selectivity lithium adsorption materials.

8.

(a) Schematic illustration of the fabrication process of COF frameworks. (b) Atomic configuration of the TpPa-SO₃H fragment (gray: C; pink: H; red: O; blue: N; yellow: S). (c) ESP distribution map of the TpPa-SO₃H model fragment (red represents negative value, white represents positive value). Reproduced with permission from ref . Copyright 2025, Elsevier.

In 2023, Shi and co-workers reported three calix[4]­arene-decorated COFs with wave-like layered structures and AA-stacking configurations (CX4-BD, CX4-BD­(OH)₂, and CX4-DPT) synthesized via solvothermal method for lithium adsorption and isotope separation (Figure a). These COFs demonstrated an exceptional lithium adsorption capacity, reaching up to 94.66 mg g^–1 which is roughly double that of previously reported adsorbents (Figure b,c). This superior performance can be attributed to the abundant binding sites originating from calix[4]­arene units. Notably, they achieved a maximum lithium isotope separation factor of 1.053 ± 0.002, comparable to state-of-the-art solid-phase adsorbents, representing the first application of calixarene-based materials and COFs in lithium isotope separation. Characterization and DFT calculations revealed that calix[4]­arene plays a key role in lithium adsorption through Li⁺–π interactions with benzene rings and Li–O coordination bonds, while aromatic amine linkers influence isotope separation performance (Figure d–f).

9.

(a) Schematic illustrates the synthesis routes of CX4-BD, CX4-BD­(OH)₂, and CX4-DPT. (b) Lithium adsorption kinetic curve of CX4-BD and fitted results with different models. (c) Lithium adsorption capacity of CX4-BD as a function of concentration and the fitted isotherm curves with different models. (d) Atomic configuration of the calix[4]­arene monomer. (e) Average adsorption energy of lithium atoms on the sites of the monomer. (f) Adsorption configuration of the calix[4]­arene monomer with different numbers of lithium atoms on it. Reproduced with permission from ref . Copyright 2023, American Chemical Society.

Unmodified MOFs and COFs achieve Li⁺ separation through their inherent structural properties. MOFs synergistically adsorb Li⁺ through metal nodes, carboxyl groups, and multiple mechanisms, with some capable of separating lithium isotopes. COFs enhance ion transport efficiency and adsorption capacity via stable covalent networks and π-conjugated systems. Both structures offer stability, combining adsorption capacity with selectivity to provide high-performance foundational materials for Li⁺ separation. This validates the application potential of unmodified CPMs and establishes a reference for future optimization.

5.2. Design and Optimization

In the design and optimization of Lithium-adsorption-type CPMs, the introduction of Lithium-affinity functional groups is crucial for overcoming the inherent limitations of the original CPMs, such as insufficient specific binding sites, weak Li⁺ affinity, and poor selectivity against coexisting ions (Na⁺, K⁺, Ca²⁺, Mg²⁺). These groups address these issues by creating specific recognition sites, amplifying active site density, and enhancing selectivity via size matching or preferential coordination. Common functional groups include crown ethers (12-crown-4, with cavity sizes matching Li⁺ diameter and O atoms enabling chelation), phosphonic acid groups (via multiple O atoms for strong electrostatic interactions), amide/hydroxamic acid groups (dual electron donors for coordination), and hydroxyl-rich moieties (forming bidentate/tridentate bonds with Li⁺).

Wang and co-workers reported a thermoresponsive amphoteric MOF material, PDMVBA-MIL-121, synthesized by introducing poly­(N,N’-dimethylvinyl-benzylamine) (PDMVBA) with tertiary amine groups into the cavities of water-stable MIL-121 (which contains carboxylic groups). This integration endows the material with both cation-binding carboxyl moieties and anion-binding tertiary amine sites within a single cohesive framework (Figure a). This amphoteric MOF exhibited efficient and reversible adsorption of multiple salts from water. At room temperature, it can adsorb monovalent salts such as LiCl (0.56 mmol g^–1) and NaCl (0.92 mmol g^–1), as well as divalent salts including MgCl₂ (0.25 mmol g^–1) and CaCl₂ (0.39 mmol g^–1). At elevated temperature, the adsorbed salts can be effectively released, enabling chemical-free regeneration (Figure b,c).

10.

(a) Schematic illustration of the synthesis of amphoteric-functionalized MIL-121 (PDMVBA-MIL-121). (b) NaCl adsorption capacity of PDMVBA-MIL-121 is much greater than that of individual PDMVBA and MIL-121-300 °C. (c) XRD patterns of as-prepared MIL-121, MIL-121-300 °C, and PDMVBA-MIL-121-300 °C before and after NaCl adsorption. Reproduced with permission from ref . Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

MOF-808 has a high specific surface area, great water stability, and replaceable formic acid groups, a lot of research has selected it as the base MOF for grafting. , Zhang and co-workers synthesized a thermally regenerable adsorbent MOF-808-EDTA by grafting ethylenediaminetetraacetic acid (EDTA) onto MOF-808 (Figure a). In MOF-808-EDTA, the EDTA groups exist in a zwitterionic state in neutral water, simultaneously bearing positively charged amine groups and negatively charged carboxyl groups. These groups respectively serve as cation-binding sites and anion-binding sites, facilitating the adsorption and enrichment of anions (e.g., Cl^–) and cations (e.g., Na⁺) in aqueous solutions, thereby enabling desalination of the water. Upon heating (80 °C), the EDTA groups transition from the zwitterionic state to a molecular state: protons on the amine groups transfer to the carboxyl groups, causing both groups to lose their charges. This process releases the previously adsorbed and enriched cations and anions, achieving reversible regeneration of the desalination adsorbent. As shown in Figure b,c, MOF-808-EDTA exhibits excellent applicability for the removal of multiple salts in aqueous solutions (for lithium ions, C ₀ = 10,000 mg L^–1 Q _e = 4.34 mmol L^–1) and demonstrates superior cyclic stability.

11.

(a) Schematic illustration of the structures of MOF-808 and MOF-808-EDTA. (b,c) Multiple-salt adsorption capacity and cycling performance measurement of MOF-808-EDTA. Reproduced with permission from ref . Copyright 2021, American Chemical Society.

Wu et al. proposed an MOF-based ion sieving adsorption method. By integrating the ion sieving functionality of subnanometer pore-sized MOFs with the photosensitive adsorption sites of polyspiropyran (PSP), they designed a series of sunlight-regenerable lithium adsorbents, termed PSP-MOFs (Figure a). Among these, PSP-UiO-66 with a pore window size of 6.0 Å exhibited the highest Li⁺ ion adsorption capacity and effectively achieved Li⁺/Mg²⁺ separation. Experimental and theoretical calculations elucidated the influence of MOF pore window dimensions on ion adsorption behavior: First, the angstrom-scale pores of MOFs enabled effective sieving of monovalent and divalent ions in water, with the Li⁺/Mg²⁺ selectivity decreasing as the MOF pore window size increased. Second, the synergistic effect of MOF pore sieving and PSP chemical affinity endowed PSP-UiO-66 with the capability for selective Li⁺ ion adsorption (Figure b–d). PSP-UiO-66 reached adsorption equilibrium within 30 min under dark conditions and rapidly desorbed within 6 min under sunlight irradiation, demonstrating excellent cycling performance. This work proposed a novel design method for ion-selective adsorbents and validated the ion sieving capability of MOFs in aqueous systems.

12.

(a) Schematic illustration of the mechanism of selective lithium salt adsorption by PSP-UiO-66 in the dark and desorption under sunlight irradiation. (b) The chemical structure and window size of UiO-66­(Zr), MIL-53­(Al), and MOF-808­(Zr). (c) Adsorption capacity of PSP-MOFs in 10,000 mg L^–1 individual salt solutions. (d) Ion selectivity of PSP-MOFs in 10,000 mg L^–1. Reproduced with permission from ref . Copyright 2024, PNAS.

Crown ethers (CEs) such as 12-crown-4 ether and 14-crown-4 ether exhibit lithium-ion targeting affinity due to their rich O sites and cavity sizes matching the diameter of lithium ions. However, poor water stability makes them typically loaded onto solid matrices to construct adsorbents. Zhang et al. reported a building block design strategy for fabricating highly selective and thermally regenerable 12-crown-4 (12CE4) based lithium adsorbent. Building upon the selective lithium-ion adsorption characteristics of 12C4E, integrating the synergistic coordination effect of sulfonic acid groups, size-screening effect of UiO-66, and the temperature-dependent regulation of the skeleton hydrophilic–hydrophobic properties by thermosensitive molecules (Figure a), the pNCE-SS@UiO-66 enables high-capacity and selective adsorption of lithium, and more importantly, it can achieve complete regeneration in 40 °C warm water (Figure b–e).

13.

(a) Schematic illustration of lithium-selective adsorption at room temperature and thermosensitive desorption under mild temperature by pNCE-SS@UiO-66. (b) Adsorption capacity of UiO-66, pNIPAM@UiO-66, pNCE@UiO-66, and pNCE-SS@UiO-66. (c) Progressive optimization of the Li⁺/Mg²⁺ selectivity. (d) Adsorption capacity and (e) selectivity of pNCE-SS@UiO-66 tested in individual MCl solutions at 1000 and 10,000 mg L^–1. Reproduced with permission from ref . Copyright 2021, Elsevier.

During the preparation of composites with MOFs, the loading capacity and exposure rate of crown ether molecules are critical. Huang and co-workers synthesized a crown-functionalized metal–organic framework (MOF-808-12C4E) through immobilization of carboxybenzo-12-crown-4-ether (CB-12C4E) ligands within a highly porous MOF-808 substrate (Figure a). MOF-808 was successfully functionalized with dense 12C4E units (1.47 mmol g^–1) by leveraging its abundant exchangeable framework sites. These 12C4E units were firmly anchored to the Zr nodes through carboxylate coordination with the Zr–O clusters. The resulting MOF-808-12C4E material exhibited excellent Li⁺ adsorption performance, achieving a capacity of 30.4 mg g^–1, rapid equilibrium within 15 min, and high selectivity for Li⁺ over competing ions. This superior performance can be attributed to the material’s spacious ion diffusion channels and high concentration of active sites. Both experimental data and density functional theory (DFT) calculations confirm the critical role of the 12C4E units in enabling this selective Li⁺ adsorption behavior (Figure b,c).

14.

(a) Reaction route from MOF-808-FA to MOF-808-12C4E. (b) IGM image and differential charge diagram of Li⁺-adsorbed MOF-808–12C4E. (c) Adsorption isotherm of MOF-808-12C4E for Li⁺. Reproduced with permission from ref . Copyright 2024, Elsevier.

Li⁺ ions have saturated outer electron orbitals (1s²) and exhibit weak affinity for traditional adsorption sites containing lone pairs of electrons. Their effective capture typically requires the synergistic action of multiple active atoms at the angstrom scale (such as oxygen-rich units). Most MOFs still have limitations in terms of lithium adsorption capacity and selectivity due to the insufficient number of active groups in the adsorption microenvironment. It has been observed that optimizing the density of free −COOH is an effective target for adjusting the adsorption performance toward alkali/alkaline earth metal ions. Zhao and co-workers designed a series of MOF via optimizing the ratio of mixed ligands of UiO-66 (The H₄ linker will elevate −COOH concentration locally to create an ion trap enriched with O atoms) (Figure a). Among them, UiO-66-H₂/H₄-b has been confirmed as a potential adsorption trap for Li⁺ ions due to the suitable window size (4.6–6 Å) (Figure b) and 4O stronger host–guest interactions between the adsorbent and Li⁺ (Figure c–f), moreover, experiment have confirmed the existence of ion exchange. UiO-66-H₂/H₄-b exhibits higher adsorption capacity (50.0 mg g^–1 at 303 K) and excellent selectivity. Additionally, it maintained effective adsorption in simulated Salt Lake brine.

15.

(a) Schematic diagram of the adsorption traps in UiO-66-H₂ and UiO-66-H₂/H₄ (color scheme: Zr, pink; C, gray; O, red; H, white). (b) Window topology from theoretical calculations. Optimized configurations for Li⁺ adsorption in (c) UiO-66-H₂ and (d) UiO-66-H₂/H₄-b. Isosurface plots of charge density difference for Li⁺ adsorption in (e) UiO-66-H₂ and (f) UiO-66-H₂/H₄. Reproduced with permission from ref . Copyright 2023, Royal Society of Chemistry.

MOFs can be classified into four distinct structural categories based on their dimensional characteristics: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) configurations. , Among different dimensions, 1D nanostructures exhibit prominent advantages in nanotechnology applications, primarily owing to their ability to serve as efficient channels for the rapid transport of both ions and electrons. − Huang and co-workers by tailoring carboxyl-rich microenvironment of MIL-121 via mixed linkers strategy for enhanced adsorption of lithium ions, the addition of H₄ ligand does not cause channel blockage; instead, it increases the density of local carboxyl groups (Figure a). MIL-121-H₂/H₄ exhibits a high adsorption capacity and rapid adsorption rate. The systematic experimental characterizations and theoretical calculations proved the vital contributions of 4-O traps (Figure b–e).

16.

(a) Synthesis strategies of MIL-121-H₂ and MIL-121-H₂/H₄ materials (color: Al, pink; C, gray; O, red; H, white). The DFT-optimized configuration of the interaction of Li⁺ with (b) MIL-121-H₂ and (d) MIL-121-H₂/H₄. The charge density differences before and after the interaction of Li⁺ with (c) MIL-121-H₂ and (e) MIL-121-H₂/H₄. Reproduced with permission from ref . Copyright 2025, Elsevier.

Besides facilitating ion diffusion, the nanoscale pores of 1D materials can also achieve molecular-level sieving. Huang and co-workers designed a novel ultrastable MOF material, TYUST-8, which innovatively constructed an array of O-rich angstrom-scale ion pockets on 1D channel walls for efficient Li⁺ sieving (Figure a). This unique structure avoids pore blockage by preadsorbed ions and accelerates ion diffusion. The size of the 1D channel is 7.1 × 7.5 Å, similar to the diameter of hydrated lithium ions diameter, which can effectively distinguish Li⁺ from other large-sized ions; The tetrahedral pockets (in wall B) were exhibited distinct electronegativity and strong chelation for Li⁺ (Figure b). The synergy sieving between the 1D channel structure and oxygen-rich ionic pockets enables TYUST-8 to achieved a record-high adsorption capacity of 76.1 mg g^–1 and short equilibrium time (Figure c,d). TYUST-8 overcomes the limitations of traditional MOF materials in lithium adsorption, including insufficient selectivity, slow diffusion, and poor stability, and provides a novel material design concept for high-efficiency lithium resource separation.

17.

(a) Schematic diagram of Li⁺ ion adsorption situations in the blocked channel and unobstructed 1D channel. (b) Schematic diagrams for the framework structure of TYUST-8. (c,d) Adsorption amounts of Li⁺ at various initial concentrations and contact times. (e) Adsorption capacities of various ions in binary solutions. Reproduced with permission from ref . Copyright 2024, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A review of the above materials reveals that strategies such as introducing lithium-affinity functional groups, regulating pore structures, and adjusting ligand ratios can overcome the limitations of the original materials. The optimized materials exhibit richer active sites, enhanced selectivity and regenerability, and adaptability to complex systems. The functionalized materials have broken through the performance bottlenecks of the base materials, providing an engineered design approach and technical support for high-efficiency, low-energy-consumption lithium extraction.

5.3. Membrane Separation

Traditional membranes (such as polymer membranes, inorganic ceramic membranes, molecular sieve membranes, etc.) commonly face a “selectivity flux trade-off” in fields like lithium separation. Achieving high selectivity requires reducing pore size, increasing active site density, or enhancing interactions with target ionsall of which drastically increase mass transfer resistance and significantly reduce flux. Conversely, boosting flux necessitates enlarging pores or thinning the membrane, which sacrifices selectivity. Leveraging molecular level precision-engineered crystal structures and multimechanism synergistic separation properties, CPM membranes overcome the traditional membrane bottleneck, achieving where high selectivity and high flux cannot be achieved simultaneously.

MOF membranes leverage their ordered and tunable pore structures along with surface functionalization to separate Li⁺. This separation is underpinned by fundamental mechanisms such as size-based exclusion and specific chemical interactions. In contrast to single MOFs that necessitate periodic adsorption–desorption cycles for operation, MOF membranes can achieve steady-state Li⁺ permeation. Moreover, the incorporation of thick and multiporous architectures shortens the diffusion paths for Li⁺, thereby reducing mass transfer resistance.

Zhang and co-workers employed a back-diffusion strategy to grow UiO-66-(SH)₂ within the nanochannels of a polymer substrate, successfully fabricating thiol-functionalized MOF channel membranes (MOFCMs) capable of efficient direct lithium extraction under harsh operational conditions (Figure a). These membranes exhibited remarkable selectivity for monovalent over divalent ions, with Li⁺/Mg²⁺ selectivity reaching up to 10³. Due to the robust binding interaction between −SH groups and Mg²⁺, along with the dehydration effect, the Li⁺/Mg²⁺ selectivity decreases from 1516 to 19 under binary ion diffusion systems as the molar ratio of Mg²⁺/Li⁺ in the feed solution rose from 0.2 to 30. In multi-ion diffusion systems comprising Mg²⁺, K⁺, Na⁺, and Li⁺, the Li⁺/Mg²⁺ selectivity reached 1114 at an Mg²⁺/M⁺ (where M⁺ = K⁺, Na⁺, and Li⁺) molar ratio of 1:1 and remained 19 even when this ratio increased to 30:1. This work innovatively enhances the blocking capability toward divalent ions through thiol functionalization, thereby providing a viable strategy for direct lithium extraction from brines with Mg²⁺ concentrations as high as 3.5 M (Figure b–f).

18.

(a) Fabrication of the UiO-66-(SH)₂/PET membrane. (b) SEM images of the membrane tip side, base side, and cross section with Zr mapping. Ion transport, conductance, and selectivity performance of the UiO-66-(SH)₂ membrane at (c,d) 0.1 M salt solutions and (e,f) varying concentrations of salt solutions. Reproduced with permission from ref . Copyright 2024, American Chemical Society.

With the assistance of charge properties and appropriate pore size, advanced ion transport channels serve as the foundation for achieving precise ion separation. Guo and co-workers pioneered the development of a MOF composite membrane featuring a positively charged surface and subnanometer channels. This membrane was fabricated via interfacial polymerization of UiO-66-NH₂ with trimethylolpropane trichloride (TMC), followed by subsequent modification with amino-functionalized polyethylenimine (PEI). The intrinsic pores and interparticle voids within the MOF framework provided rapid transport channels for Li⁺. Meanwhile, PEI modification increased the surface charge density and effectively reduced the pore sizes. These combined actions synergistically intensified the electrostatic repulsion and size-exclusion effects against Mg²⁺. This optimization process resulted in a membrane that exhibited exceptional Li⁺ permeability (0.68 mol m^–2 h^–1), which was 1 order of magnitude higher than that of the state-of-the-art MOF membranes. Additionally, it achieved a Li⁺/Mg²⁺ selectivity of 13.2, marking a 6.6-fold improvement compared to the unmodified UiO-66-NH₂-TMC membrane (Figure ).

19.

(a) Preparation of the PEI@UiO-66-NH₂-TMC membrane. (b) Zeta potentials of synthesized membranes under various pH. (c) Effective pore size distribution of UiO-66-NH₂-TMC and PEI@UiO-66-NH₂-TMC-1 membranes. (d) The ion conductivity on the permeate side over time was studied for the PES substrate, UiO-66-NH₂-TMC, and PEI@UiO-66-NH₂-TMC-1 membranes. Reproduced with permission from ref . Copyright 2023, Elsevier.

COF membranes leverage their rigid covalent frameworks and precisely engineered functional groups (ether-oxygen chains, quaternary ammonium moieties et al.) to achieve synergistic Li⁺ separation through combined size-exclusion and chemical affinity mechanisms. Compared to COF-based adsorption systems, these membranes demonstrate superior performance in two critical aspects: (1) Unparalleled ion discrimination capabilities in high-salinity environments (exceeding 3 M NaCl). Their structurally stable pores effectively resist performance degradation caused by swelling, which is a prevalent limitation for traditional adsorption frameworks; (2) Enhanced energy efficiency, as pressure-driven or electro-driven separation processes eliminate the need for aggressive chemical regenerants required in adsorption cycles, reducing energy consumption by 40–60% relative to conventional adsorption.

Precisely controlling the pore size of COF membranes and constructing Mg²⁺ energy traps within the channels enables high-resolution Li⁺/Mg²⁺ discrimination, a strategy proven effective for extracting high-purity lithium. Zhang and co-workers developed a bioinspired ion-channel approach based on triazine-based COF membranes, achieving highly selective Li⁺/Mg²⁺ separation via dehydration-enhanced ion recognition. Under electrodialysis conditions, the membrane exhibited an ultrahigh Li⁺ permeation rate of 0.56 ± 0.03 mol m^–2 h^–1 with undetectable Mg²⁺ (below the instrument detection limit). Further mechanistic studies revealed that the tailored pore size and pore environment induce Mg²⁺ dehydration, trapping it in energy wells formed by triazine and sulfonic acid groups. In contrast, Li⁺ retains its hydration shell and undergoes rapid hopping transport through sulfonic acid side chains in the hydrophilic channel environment. The TAT-TP-P membrane achieved an ideal Li⁺/Mg²⁺ selectivity of 474, which even increased to 823 under high-salinity conditions (Figure ).

20.

(a) Electrostatic potential (ESP)-mapped surfaces of TAT-TP and TAB-TP. (b) The permeation rates of Li⁺ and Mg²⁺ across the TAT-TP-P during electrodialysis at different concentration gradients in binary salt systems. (c,d) The ion permeation rate and Li⁺/Mg²⁺ selectivity of TAT-TP and TAT-TP-P membranes at different concentration gradients in a single-salt system. (e) Performance comparison of TAT-TP-P with other membrane materials. Reproduced with permission from ref . Copyright 2025, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Recently, addressing the significant decline in ion selectivity in traditional membrane materials within mixed electrolyte systems, Zhang and co-workers proposed a “dual nano-confinement” strategy. By constructing COF membranes featuring acidic groups and precisely controlled pore sizes (1.2–2.2 nm), they achieved ultrahigh selectivity in the separation of mixed monovalent/multivalent ions (Figure ). These acidic groups preferentially engage in electrostatic interactions with multivalent ions (such as Mg²⁺), confining them near the pore walls. Meanwhile, they shield the electrostatic interactions between the acidic groups and monovalent ions (such as Li⁺), thereby creating a free central channel that enables rapid transport. In the mixed system, the Li⁺/Mg²⁺ selectivity exceeds 1300.

21.

(a) Schematic diagram of the structural design of the COF membrane. (b) Ion flux in a single-ion system. (c) Interaction energy of different ions with the COF unit. (d) The ion flux in single- and mixed-ion systems. (e) The cation selectivity in single- and mixed-ion systems. Reproduced with permission from ref . Copyright 2025, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Pure MOF or COF membranes often exhibit limited performance owing to their reliance on a single separation mechanism, a constraint that has prompted researchers to explore alternative strategies to unlock the full potential of these materials. Wu et al. developed a hybrid bilayer membrane (ZIF-8/TpPa-SO₃H/nylon) by integrating MOF and COF layers on a nylon substrate via continuous liquid–liquid interfacial polymerization. This fabrication approach established robust chemical bonding between the MOF and COF layers, thereby constructing efficient ion transport channels (Figure ). The ZIF-8 layer (MOF component) that exerted strong electrostatic repulsion toward cations was combined with the TpPa-SO₃H layer (COF component) with strong cation-coordinating abilities. This synergistic effect produced a synergistic effect that significantly improved ion separation performance. The bilayer membrane demonstrated an outstanding Li⁺/Mg²⁺ separation ratio of 501. This value was 400 times and 200 times higher than those of the pristine ZIF-8/nylon (1.3) and TpPa-SO₃H/nylon (1.6) membranes, respectively. It attained a high Li⁺ permeation rate of 23.2 mol m^–2 h^–1 while limiting the Mg²⁺ permeation rate to 0.2 mol m^–2 h^–1.

22.

(a) Schematic outline for the synthesis of the ZIF-8/TpPa-SO₃H/nylon membrane. (b) Ionic conductivity of the nylon substrate and ZIF-8/TpPa-SO₃H/nylon membrane. (c) Cation selectivity of the nylon substrate and ZIF-8/TpPa-SO₃H/nylon membrane. (d) ZIF-8/TpPa-SO₃H/nylon ion transport mechanism diagram. Reproduced with permission from ref . Copyright 2024, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Research on CPM membranes including MOF membranes, COF membranes, and MOF/COF composite membranes for Li⁺ separation has progressed steadily and exhibits complementary developments. MOF membrane early studies relied on pore size matching to achieve basic selectivity, followed by performance enhancement via ligand functionalization, and finally the use of mixed matrix membrane (MMM) strategies to balance material stability and separation selectivity. For COF membranes, initial research focused on the synergistic effect between well-defined pores and acidic functional groups; a key breakthrough came with the discovery that randomly oriented COF membranes can achieve efficient Li⁺ retention through unique grain boundary structures, and coupling with electrodialysis has validated their applicability in high-salinity systems. In contrast, research on MOF/COF membranes remains in its nascent stage. Their core advantage lies in the functional complementarity of “MOF active sites and COF mass transport channels”, which significantly enhances Li⁺ separation efficiency.

6. Conclusions and Outlook

This review provides a comprehensive analysis of the advancements in CPMs, encompassing MOFs and COFs, for lithium-ion adsorption and membrane-based separation. It elucidates the synergistic effects of core separation mechanisms, including ion exchange, coordination chelation, and molecular sieving, while integrating key strategies for material design optimization and process innovation. Nevertheless, the development of CPMs in lithium-ion separation is still encumbered by several critical challenges:

• 1.Synthetic and scalability constraints: The reliance on costly metal precursors and organic ligands, coupled with intricate synthesis protocols (requiring high-temperature/pressure conditions and inert atmospheres), hampers the industrial-scale production of CPMs. Additionally, the uniformity and structural stability of crystals are challenging to control during bulk production, which directly affects the reproducibility of separation performance.
• 2.Insufficient stability under extreme conditions: The strongly acidic or alkaline, high-ionic-strength environments of actual lithium resources (such as high-salt Salt Lake brine and spent battery leachate) can cause CPM frameworks to collapse or functional sites to deactivate. Some MOFs exhibit capacity loss rates exceeding 20% after more than 10 adsorption–desorption cycles.
• 3.Inadequate selectivity in practical environments: The lithium concentration in seawater is merely approximately 0.1–0.2 mg L^–1, whereas the concentrations of coexisting ions can be several thousand times higher. This extreme ion composition imposes dual requirements on adsorbents, including efficient Li⁺ enrichment under ultralow lithium concentration conditions and robust selectivity in environments with intense competitive ions. However, most existing adsorbent materials suffer from insufficient utilization of active sites, which weakens the mass transfer driving force at low concentrations and thereby limits both adsorption capacity and kinetics.
• 4.Incomplete mechanistic understanding: The dynamic processes involving multimechanism synergistic effects (such as the coupling of chelation and molecular sieving effects) lack direct characterization methods. The transmission pathways and rate regulation mechanisms of lithium ions within pore channels remain unclear, limiting precise material design.

The global expansion of the new energy industry has significantly exacerbated the lithium supply–demand imbalance. With China importing more than 70% of its lithium, seawater lithium reserves of 23 billion tons offer a critical strategic supplement. A demonstration project in Qingdao has recently extracted lithium from desalination-derived concentrated brine and, through adsorption and membrane separation, successfully produced battery-grade lithium carbonate, validating the engineering feasibility of this approach. Consequently, developing high-performance materials will be a key research focus moving forward.

The prospects of CPMs in lithium adsorption are centered on simultaneously enhancing selectivity and capacity through precision functionalization strategies (e.g., bioinspired ion-channel design) and synergistic multimechanism integration (including size sieving, electrostatic interactions, and chemical recognition). Concurrently, efforts will focus on developing high-stability frameworks (e.g., Zr-based MOFs, fully conjugated COFs) and implementing composite/coating strategies to bolster their chemical and mechanical stability in real brine environments. Future research will prioritize performance validation in authentic complex systems (e.g., natural Salt Lake brines) and advance green, low-cost synthesis routes alongside scalable preparation technologies. Ultimately, by coupling these materials with electrochemical processes and other advanced separation platforms, highly efficient, low-energy integrated systems will be established, offering a transformative solution for achieving sustainable lithium resource extraction.

The authors declare no competing financial interest.

Published as part of Chem & Bio Engineering special issue “Framework Materials”.