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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 10: 1519 - 1533

DOI: https://doi.org/10.7536/PC230214

Review

Preparation and Extraction Application of Lithium Ion Selective Adsorption Materials

  • Xinyi Chen 1 ,
  • Kaisheng Xia , 1, * ,
  • Qiang Gao 1 ,
  • Zhen Yang , 2, * ,
  • Yudie Li 1 ,
  • Yi Meng 1 ,
  • Liang Chen 3 ,
  • Chenglin Liu 2
Expand
  • 1 Faculty of Materials Science and Chemistry, China University of Geosciences,Wuhan 430078, China
  • 2 School of Earth Resources, China University of Geosciences,Wuhan 430074, China
  • 3 School of Physics, Huazhong University of Science & Technology,Wuhan 430074, China
*Corresponding author e-mail: (Kaisheng Xia);
(Zhen Yang)

Received date: 2023-02-14

  Revised date: 2023-07-01

  Online published: 2023-08-07

Supported by

Science and Technology Major Projects of Xinjiang Autonomous Region(2022A03009)

National Natural Science Foundation of China(21975228)

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Abstract

In recent years, with the rapid advancement and large-scale application of lithium battery technology and electric vehicle, the market demand for lithium resource is growing sharply. However, due to insufficient mining degree and extraction technology, the total production capacity of ore lithium and brine lithium resources is far below the actual market demand. Extracting lithium from surface salt lake brine, deep brine and other liquid resources has the advantages of large resource potential and low extraction cost, which presents an important research direction in the lithium resource extraction field. Among available lithium extraction technologies, adsorption method is suitable for extracting lithium from low concentration and large volume liquid brine resources in China, and selective lithium ion adsorption materials are the core of adsorption method. In this review, we focus on the preparation and application of lithium ion selective adsorption materials for lithium extraction from brine. The preparation methods, adsorption properties and adsorption mechanisms of organic (crown ether), inorganic (aluminum-, manganese- and titanium-based adsorbents) and composite selective lithium adsorption materials are reviewed. This review provides a brief prospect for the design and development of new lithium adsorption materials, which may push forward the efficient extraction and utilization of lithium resources from salt lake brine.

Contents

1 Introduction

2 Crown ether adsorbents

2.1 Preparation of crown ether adsorbent

2.2 Selective lithium extraction performance

2.3 Selective lithium extraction mechanism

3 Alumina-based materials

3.1 Preparation of aluminum adsorbent

3.2 Selective lithium extraction mechanism of aluminum adsorbent

3.3 Selective lithium extraction performance of aluminum-based adsorbent

4 Lithium ion sieve adsorbent

4.1 Preparation of ion sieve adsorbent

4.2 Lithium ion insertion/extraction mechanism

4.3 Selective lithium extraction performanc of lithium ion sieve

4.4 Molded lithium ion sieve adsorbent

5 Other types of adsorbents

6 Conclusion and outlook

Key words： brine; lithium extract; adsorbent; ion sieve

Cite this article

Xinyi Chen , Kaisheng Xia , Qiang Gao , Zhen Yang , Yudie Li , Yi Meng , Liang Chen , Chenglin Liu . Preparation and Extraction Application of Lithium Ion Selective Adsorption Materials[J]. Progress in Chemistry, 2023 , 35(10) : 1519 -1533 . DOI: 10.7536/PC230214

1 Introduction

As the lightest alkali metal, lithium has many excellent physical and chemical properties, such as high electrochemical activity, high specific heat capacity and high redox potential. These characteristics make lithium and its compounds have attracted much attention. In recent years, lithium has been widely used in batteries, electronic information equipment, ceramics, aerospace, metallurgical engineering and many other high-tech fields, and is considered to be an important strategic resource for people's livelihood, economy and national defense construction in the 21st century[1].
With the booming development of the new energy industry, the global consumption of lithium resources has rapidly increased from 228,000 tons of LCE (lithium carbonate equivalent) in 2016 to 780,000 tons of LCE in 2022, and the consumption in 2023 is expected to reach about 1.07 million tons of LCEs[2][3]. In the long run, the demand for lithium resources will continue to grow rapidly in the future, but due to the limitations of lithium mining conditions and lithium extraction technology, the current situation of high demand and low supply in the global lithium resources market is inevitable. According to the survey, continental brine is the largest resource of lithium occurrence (64%), followed by hard rock (36%)[4,5]. Therefore, the recovery of lithium from water resources is extremely necessary and has become a new trend in the industry. It is worth noting that China is the world's largest consumer of lithium, accounting for 6.31% of the world's lithium resources, ranking fifth in the world, but two-thirds of its reserves are in low-grade salt lake brine, where high magnesium-lithium ratio and coexistence of a large number of alkali metals and alkaline earth metals are prevalent, making it difficult to extract lithium[6][7]. Moreover, the extreme imbalance between supply and demand of lithium resources has led to the price of battery-grade lithium carbonate (Li2CO3) rising to 595000 yuan/ton at the end of 2022 and falling back to 224000 yuan/ton in 2023. At present, it shows a trend of stopping falling and rising, which affects the healthy development of the industry[8]. Therefore, it is of great theoretical and practical significance to seek environmentally friendly, low-cost and highly selective lithium extraction technologies and to establish a lithium resource recovery system.
At present, the common methods of lithium recovery from brine include solvent extraction, precipitation crystallization, adsorption, membrane separation and so on[9][10][11,12][13]. Among them, the precipitation crystallization method has been used earlier in the extraction of lithium, and its mature precipitation technology is suitable for the system with low magnesium-lithium ratio. However, this method has poor selectivity and is not suitable for the salt lake system with high ratio of magnesium to lithium in China. Solvent extraction is still one of the main methods for selective separation and metal extraction because of its mature application foundation in hydrometallurgy and its wide application in extracting lithium from salt lake system with high magnesium-lithium ratio. However, solvent extraction is easy to produce three phases, the organic solvent used will corrode the process equipment, and the leakage of solution will pollute the environment, which seriously hinders the popularization and application of this method. Membrane separation technologies, including selective electrodialysis (SED), nanofiltration (NF), ion-imprinted membrane (IIM) and membrane capacitive deionization (MCDI), have the advantages of high efficiency and environmental protection, and show great potential in the field of lithium recovery, but their high energy consumption and unsatisfactory membrane durability limit their industrialization[14][15][16][17].
Compared with the above lithium extraction technologies, the adsorption method has been developed rapidly in recent years because of its outstanding advantages such as low cost, simple operation, green environmental protection, high selectivity and wide application range, especially for the liquid phase system with large volume and low concentration of target ions in China. The adsorption method is to achieve the selective separation and recovery of lithium ions by selecting or designing and synthesizing adsorption materials with recognition ability to Li+ and using specific physical or chemical effects. Adsorption material is the core of lithium extraction by adsorption. Generally speaking, an ideal adsorption material should have the following characteristics: 1) high selectivity, which can selectively capture Li+;2) from solutions with complex elemental composition and high salinity, and good stability, which can maintain good structural and mechanical stability during high salinity and repeated adsorption/desorption processes; 3) Sufficient adsorption capacity, that is, unit mass of adsorption material can capture as much lithium ions as possible. At present, adsorption materials can be generally divided into organic, inorganic and composite adsorption materials (Fig. 1). Among them, organic adsorption materials include crown ethers, calixarenes, etc. The selectivity of the former mainly depends on the control of hole size and target ion matching, as well as the type of heteroatoms on the ring and other factors. The latter is closely related to the coordination and solvent of Li+. Because of its limited solubility in water, the research on alkali metal selectivity is mainly focused on methanol, tetrahydrofuran and other solutions[18]. Inorganic adsorption materials are mainly divided into aluminum-based adsorbents, lithium ion sieves, natural ores and carbon materials. The surface active sites of natural ores and carbon materials can combine with most metals, and have poor selectivity for Li+ and low adsorption capacity, which limits their application in lithium extraction from brine. Lithium ion sieve adsorbents have good adsorption efficiency, adsorption selectivity and chemical stability, mainly including lithium manganese oxide (LMO) and lithium titanium oxide (LTO). The development of lithium manganese oxide is limited by high dissolution loss, poor cycle stability and slow equilibrium rate in the acid leaching process. The performance of lithium manganese oxide can be improved by metal doping, structural morphology change and improvement of preparation methods[19][20][21]. Compared with the lithium manganese oxide, the lithium titanium oxide has the advantages of low metal dissolution loss rate, good stability and good application prospect. Aluminum-based adsorbents have been commercialized, which have the advantages of low cost and stable adsorption performance. Taking the aluminum-based adsorbent as an example, the overall lithium extraction process of the adsorbent material is roughly as follows: after the :Li+ is adsorbed and saturated, the adsorbent is soaked in water to separate lithium from aluminum. And removing impurities such as magnesium, calcium and the like in the lithium-containing solution by using a precipitator, adding sodium carbonate after evaporation and concentration to carry out a lithium precipitation reaction, and realizing the production of a lithium carbonate product. In this paper, the research progress of different types of adsorbents is reviewed, and the preparation methods, adsorption properties and adsorption mechanisms of different adsorbents are compared, which provides a useful reference for the subsequent study of new adsorbents for lithium extraction, the optimization of preparation technology and the promotion of the application of lithium extraction technology in salt lake brine.
图1 用于锂回收的吸附剂的分类

Fig.1 Classification of adsorbents for lithium recovery

2 Crown Ether Adsorbent

2.1 Preparation of Crown Ether Adsorbent

Crown ether (CE), also known as macrocyclic ether, is a cyclic organic compound composed of multiple oxygen atoms and carbon atoms. CE with an electron-rich cavity can form very stable complexes with specific metal ions due to its "macrocyclic effect."[22]. Crown ethers have precise size selectivity for cations, and the accuracy depends on the proximity of the cavity size and the diameter of the metal ion. In general, the closer the cavity size and the target ion diameter are, the higher the probability that the target ion is selectively trapped. A large number of studies have shown that crown ethers with 12-14 membered rings have strong binding affinity to Li+[23]. However, crown ethers are expensive and difficult to recycle, and they are often grafted or polymerized onto fibers, organic resins, porous macromolecular materials and other matrices to form organic ligand composite adsorbents. Accord to that composite adsorbent, the specific recognition capability of the ligand on the Li+ can be maintained, and the composite adsorbent has the property of high stability, acid resistance, easiness in solid-liquid separation and the like of a matrix, so that the contactability, recycling and other properties of CE can be remarkably improved. The matrix materials involved are silicon materials, magnetic particles, porous materials and so on[24][25][26]. The matrix material is used as the skeleton of the adsorbent, and the crown ether ligand is connected to realize the Li+ coordination. From the point of view of practical application, the substrate material must have the advantages of high mechanical strength, acid washing resistance, good stability and easy grafting modification.
Bai et al. Prepared porous polymers (PolyHIPEs) by high internal phase emulsion (HIPEs) template method, and used ultraviolet (UV) to initiate surface polymerization to introduce 2- (allyloxy) methyl-12-crown-4 (2AM12C4) to synthesize functionalized polymer brushes (PVBC-g-PCE)[27]. PVBC-g-PCE is a good adsorbent for selective capture of Li+ due to its high physical strength, high density of accessible binding sites, and highly permeable pores. Tachibana et al. Prepared crown-ether-type organic composite adsorbents embedded in high-porous silica beads, such as benzo-15-crown-5 (BC15) and benzo-18-crown-6 (BC18), which have relatively high adsorption capacity for lithium ions in seawater[24]. Cheng et al. Used a green low-temperature phase separation method to prepare chitosan (CS) nanofibrous membranes, and selected 2-hydroxymethyl-12-crown-4 (2H12C4) with a unique cavity structure to graft onto the surface of CS to prepare crown ether modified chitosan nanofibrous membranes (CS-CE)[26]. This kind of material has large adsorption capacity, good cycle stability and good selectivity, and shows good application potential in lithium ion extraction. Jo et al. Incorporated a macrocyclic chelating ligand (12C4) molecule as an ion acceptor into graphene of polyethersulfone (PES) nanofiber membrane (CGPNF), and the CGPNF membrane showed a maximum adsorption capacity of 86.3 mg/G and retained more than 93% of its initial adsorption capacity after 10 regeneration cycles[28]. Ding et al. Grafted 2-methyl-12-crown-4 (2M12C4) onto nanoscale SiO2 by chemical grafting and electrospinning to synthesize nanoscale adsorption material PAN-CE@SiO2[29]. The material has low cost, good hydrodynamic properties, and is proven to selectively adsorb Li (I).

2.2 Selective extraction of lithium

The selectivity, adsorption capacity and cycle stability of crown ether adsorbents have attracted the most attention of researchers in the Li+ of water resources such as seawater and salt lake brine.
1) Different crown ether monomers have different recognition effects on lithium ions. Dixit et al. Bonded 12-crown-4 with three different inositol substituents, and found that the recognition performance of different inositol substituents on Li+ was significantly different, and the relative binding affinity between metal ions and crown ether could be adjusted by changing the relative orientation of crown ether oxygen atoms[30]. Tachibana et al. Realized the synthesis of crown ether-type organic composite adsorbents embedded in highly porous silica beads for the simultaneous recovery of lithium and uranium from seawater[24]. The synthesized crown ether resins include benzo-12-crown-4 (BC12), dibenzo-14-crown-4 (DBC14), benzo-15-crown-5 (BC15), benzo-18-crown-6 (BC18), etc. It is found that the adsorption properties of different crown ether monomers are different. BC15 and BC18 resins have high selective adsorption capacity for Li+ in seawater, and their adsorption behavior is strongly dependent on the cavity size and hydrophobicity of these crown ethers. Park et al. Used mesoporous silica materials partially functionalized with methyl crown ether (AC-SBA-15) and azacrown ether (HMC-SBA-15), and the materials functionalized with different crown ether groups could achieve highly selective adsorption of Li+ in artificial seawater[31].
2) The type of matrix material, specific surface area, hydrophilicity, and the number of adsorption sites have important effects on the selective adsorption of lithium ions by crown ethers. Zheng et al. Successfully prepared a series of nanofiber membranes by functionalizing graphene oxide (GO), chitosan and polyvinyl alcohol (PVA) with crown ether, and the maximum adsorption capacity could reach 168.50 mg/G[32]. In the selective adsorption experiment, the nanofiber membrane showed high selectivity for Li+ in salt lake brine. After five cycles, the material still maintained 88.31% of the adsorption capacity. Although organic composite adsorption materials have obvious selectivity and stability, there is a contradiction between the high mass of active materials and the low relative atomic mass of lithium, and the organic polymer cross-linked framework embeds internal active sites, which limits the development of adsorption capacity. In addition, SiO2, Fe3O4, carbon nanotubes and glass fiber mats are good matrix materials for loading CE[33]. The results show that the substrate with high specific surface area can accelerate the mass transfer rate, thereby enhancing the adsorption performance of the adsorbent[34]. Wang et al. Used the Pickering emulsion template method to introduce a crown ether monomer into the emulsion to synthesize a crown ether functionalized porous matrix material for the selective separation of lithium ions[35]. The porous nature of the developed matrix material and a large number of active sites are conducive to improving the adsorption capacity of the adsorption material. Alexandratos et al. Quantitatively analyzed the selectivity of Li (I) in aqueous solution by immobilizing 14-crown-4 on the surface of poly (glycidyl methacrylate) matrix by chemical grafting[36]. The experimental results show that the degree of complexation of Li (I) by crown ethers is affected by the polarity of the polymer matrix, and the complexation ability of crown ethers to Li (I) increases with the increase of the hydrophilicity of the matrix. The properties of some of the above crown ether adsorption materials are summarized in Table 1.
表1 冠醚配体复合吸附材料的性能

Table 1 Performance of crown ether ligand composite adsorption materials

Crown ether ligands Matrixes Specific surface area
(m2/g)		 pH Adsorption capacity
(mg/g)		 Selectivity
α(Li/Na)		 Cycling stability ref

Aminoethylbenzo-12-crown-4		 Polymer nanosheets
(PMBA-PMA)		 / 7 14.67 / 90%
(Five cycles)		 30

1-aza-12-crown-4		 mesoporous silica
SBA-15
3-Aminopropyltriethoxy
4-ylsilane		 578 8 7.63 × 10-3 / / 31

2- ( hydroxymethyl ) 12-crown-4		 graphene oxide
chitosan
polyvinyl alcohol		 101.5 7 168.50 2.51 88.31%
(Five cycles)		 32

Aminoethylbenzo-12-crown-4		 Porous polymer substrate
(PVBC)		 / 7 4.22 6.59 95.0%
(Five cycles)		 35

Octamethyl 14-crown-4		 methacrylate polymer / / 3.05 / / 36

1) 选择性系数α,也称分离因子,是表示某一单元分离操作或某一分离流程将两种物质分离的程度,其计算方法为 α M e L i=Kd(Li+)/Kd(Me)。Me: K+、Na+、Ca2+、Mg2+。

2) Kd为吸附分配系数,是指一定温度达到反应平衡时,组分在固定相中的质量分数与流动相中的质量浓度之比,其大小反映离子在固液两相中的移动与分离能力。

2.3 Mechanism of selective extraction of lithium

2.3.1 The matching degree between the hole size and the ion size of crown ether

The selectivity of crown ether adsorbents is affected by the matching degree between the size of the crown ether pore cavity and the size of the ion. The host can coordinate with the guest ion due to the electron donation of the oxygen atom in the ether bond. The closer the cavity size is to the diameter of the target ion, the more likely it is to enter the crown ether hole. The cavity size of small cyclic crown ether (C*-C*) is usually between 0.122 and 0.153 nm, while the size of lithium ion is 0.118nm,Li+, which perfectly fits the cavity size to form trapping, and can form a strong electrostatic mutual attraction with the ether bond in the cavity. However, the ion diameters of K+, Na+ and Ca2+ are much larger than that of Li+, which makes it difficult for them to combine with small cyclic crown ethers stably. Some of the alkali metal ion diameters and crown ether dimensions are listed in fig. 2[37].
图2 碱金属离子和冠醚尺寸

Fig.2 Size of alkali metal ions and crown ethers

Valente et al. Found that the separation factor changed little with the increase of the number of oxygen atoms on the crown ether from 4 to 12,Li+, while the separation factor for other alkali metals K+, Rb+, Cs+ increased significantly[38]. The crown ether containing four oxygen atoms can saturate the first coordination sphere of Li+, while K+, Rb+ and Cs+ need more electrons and need more oxygen to provide electrons. Therefore, the selection of small ring crown ether will be conducive to the selective separation of Li+. It is worth mentioning that metal cations with larger diameters will coordinate above the hole plane formed by oxygen atoms to form a pyramidal structure. In the other case, the cation is located above the cavity, and the cation bridges the two crown ethers to form a sandwich structure, as shown in Figure 3. Compared with the pyramidal structure, the cation coordination of the sandwich structure is not close and the stability is poor[39]. When the metal ion is small, the ligand distorts and encloses the cation to form an inclusion complex to reduce the solvent entry, but the ion is far away from the ligand atom and the electrostatic attraction is small. Therefore, when the metal ion and the crown ether with a suitable cavity size form a stereo match, the adsorption material has outstanding specific selectivity.
图3 15-冠醚-5(a)、K+与15-冠醚-5形成“金字塔结构”(b)、K+与15-冠醚-5形成夹层结构”(c)的平衡几何图形

Fig.3 The equilibrium geometry of 15-crown-5 (a), K+ and 15-crown-5 forming a ‘pyramid structure’ (b) and K+ and 15-crown-5 forming a ‘ sandwich structure ’ (c)

2.3.2 Softness of crown ether molecule

The softness of the crown ether molecule also has an effect on the selectivity. When the crown ether molecule is relatively rigid, the crown ether is highly specific and selective for only a single ion. However, in fact, most of the crown ether molecules reported in the literature are relatively soft, and their hole size is easy to change, thus adapting to a variety of metal cations in a certain size range[40]. Taziaux et al. Reported the photophysical and complexation properties of a series of fluorescent ionophores based on monoazacrown ether linked coumarin 343. In previous experiments, the photophysical and complexation properties of fluorescent ionophores composed of coumarin C343 linked to 1-aza-15-crown-5 (C343-crown) through an amide bridge were studied, and it was found that C343-crown had better selectivity for alkaline earth metal cations[41]. In order to change the selectivity of the fluorescent probe, monoaza-15-benzo-crown-5 and monoaza-15-dibenzo-crown-5 were used instead of monoAza-15-crown-5. The results show that the selectivity of alkaline earth metal ions for Li+ detection is greatly improved by the introduction of benzo groups to rigidify the complexation cavity. Babujohn et al. Reported a novel triphenylethylene-based aromatic linkage to form highly crystalline COFs/COPs in one step through oxidative trimerization of flexible benzene-crown units[42]. Oxidative coupling of commercial dibenzo-18-crown-6 and synthetic dibenzo-24-crown-8 was employed, and the corresponding covalent organic frameworks (COFs)/covalent organic polymers (COPs) obtained were denoted as COP-TPC6 and COP-TPC8, respectively. The flexibility of the crown ether monomer is expected to provide additional advantages such as reversible structural dynamics upon guest binding. Considering the strong affinity of crown ethers for gold ions via O-Au interaction, COP-TPC8 and COP-TPC6 were explored as adsorbents for the recovery of Au3+ ions from solution. Adsorption studies showed that COP-TPC8 and COP-TPC6 had good adsorption capacity and high selectivity for Au3+.

2.3.3 Donor heteroatom and substituent

In addition to the hole size and the softness of the crown ether molecule, the type and number of coordinating heteroatoms in the crown ether ring, the type of substituents on the ring, and the charge-to-size ratio of the ion also affect the interaction between the host crown ether and the guest metal cation. Liang et al. Studied Si-doped 15-crown-5 based on density functional theory, and found that Si could increase the size of crown ether and enhance/weaken the Li+ recognition ability through different doping methods[43]. According to the natural population analysis, the Si — O — Si bond in the crown ether ring should be avoided as much as possible, which makes it difficult for O to polarize the electrons of Si, and the distance between the positively charged Si and the Li+ is very close, resulting in electrostatic repulsion, which is not conducive to the complexation of Li+ by crown ether. Torrejos et al. Used carboxyl (— COOH) as the functionalization site of crown ether epoxy-terminated linker, and found that it could enhance the selective adsorption capacity of Li+[44]. Based on this, they prepared a crown ether solid-supported adsorbent with a carboxyl group as a side arm. Among them, type 1 is neutral hydroxy-dibenzo-14-crown-4 ether (HDB14C4) immobilized multi-walled carbon nanotubes (MWCNTs), and type 2 is MWCNTs with HDB14C4 — COOH side arms. They found that the presence of the — COOH side arm in type 2 significantly enhanced the adsorption capacity for Li+ at pH = 7 and exhibited Li+>Na+>Mg2+>Ca2+>K+>Sr2+ selectivity in a competitive environment.

3 Aluminium-based adsorbent

3.1 Preparation of aluminium-based adsorbent

Aluminum-based adsorbents are a special class of metal-based adsorbents. The basic skeleton of the adsorbent is mainly composed of aluminum hydroxide, so it is called aluminum-based adsorbent, which is the only adsorption material put into industrial application at present. Among them, Li-Al layered double hydroxides (LiAl-LDHs) are widely studied aluminum-based adsorbents with outstanding comprehensive properties, including negligible elution damage, simple preparation process, green economy, and stable adsorption performance.
Aluminum salt adsorbents can generally be represented by LiX·2Al(OH)3·nH2O, where the anion represented by X is usually Cl-, so it can be represented by LiCl·2Al(OH)3·nH2O. In order to obtain the LiCl·2Al(OH)3·nH2O or the like, preparation methods including a soaking method, a mechanochemical method, a precipitation method and the like are generally adopted[45][46][47]. In addition, some researchers have combined aluminum-based adsorbents with polymers or inorganic materials to form composite adsorption materials[48].
At present, the common adsorption characterization method is mainly static adsorption/desorption, which means that the quantitative adsorbent and the quantitative solution reach adsorption equilibrium after a long time of full contact under the condition that the solution does not flow, and the equilibrium adsorption capacity is obtained; Dynamic adsorption/desorption means that a solution with a certain concentration flows through an adsorption column filled with a certain mass of adsorbent at a certain flow rate and temperature, so as to obtain the permeation adsorption capacity and the equilibrium adsorption capacity.
Guo et al. Prepared aluminum-based adsorbents by one-step method and immersion method respectively, and studied the static adsorption/desorption and dynamic adsorption/desorption performance of Li+, and found that the adsorbents had good adsorption performance in both cases[49]. The order of selectivity is Li+>Mg2+>Na+>K+, the structure is stable before and after adsorption/desorption, and the matrix itself is not dissolved. Liu et al. Prepared LiCl·2Al(OH)3·xH2O aluminum-based material by mechanical ball milling, which has a lithium precipitation rate of 78.3% and can effectively separate lithium from magnesium, and the mass ratio of Mg/Li in the precipitate is only 0.02[50]. Zhong et Al. Prepared Li/Al LDH with a regular two-dimensional hexagonal plane by a simple coprecipitation method, which had a strong selective adsorption ability for Li+. After 12 adsorption-desorption cycles, the adsorption capacity of Li/Al LDHs was still more than 7.0 mg/G[51]. Dong Qian et al. Prepared Al(OH)3 crystals containing specific holes by acid conversion after soaking Al(OH)3 in LiOH[52]. The adsorbent has a lithium adsorption capacity of 0. 6-0. 9 mg/G, has high selectivity for Li+ in brine, and basically does not adsorb alkali metal elements such as Mg2+ and K+.

3.2 Mechanism of selective extraction of lithium from aluminum-based adsorbent

Among aluminum-based adsorbents, Li-Al layered double hydroxides (LiAl-LDHs) have been widely studied, which are two-dimensional aluminum hydroxide layered structures connected by hydrogen bonds, electrostatic interactions and Van der Waals forces.It consists of an edge-shared octahedron with the metal ion at the center and the hydroxide ion at the apex, with two-thirds of the octahedral center site occupied by Al3+ and one-third of the site occupied by Li+. The bulk layer is positively charged, and for charge compensation, the interlayer region is filled with water molecules or anions, and the schematic diagram of LiAl-LDHs recycling Li+ is shown in Fig. 4[53].
图4 氯离子插层LiAl层状双金属氢氧化物(LDHs)的结构模型

Fig.4 The structural model of chlorine-ion-intercalated LiAl-layered double hydroxides (LDHs)

Before aluminum-based adsorbents can be used for lithium adsorption, a portion of Li+ needs to be removed from the original structure to create vacancies as adsorption sites. These vacancies tend to bond with the introduced Li+ to form the most suitable crystal configuration, thus exhibiting specific Li+ adsorption selectivity. In addition, the presence of steric hindrance prevents competitive cation entry. Therefore, the aluminum-based adsorbent shows good selectivity in salt lakes with high Mg/Li ratio. The adsorption and desorption mechanism of LDHs can be expressed by the following equation:[54]
$ \mathrm{LiCl}+(1-x) \mathrm{LiCl} \cdot m \mathrm{Al}(\mathrm{OH})_{3}+(n+1) \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \\ \mathrm{LiCl} \cdot m \mathrm{Al}(\mathrm{OH})_{3} \cdot n \mathrm{H}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O}$

3.3 Selective extraction of lithium from aluminum-based adsorbent

Aluminum hydroxide-based adsorbents have the characteristics of mature preparation technology, excellent stability and recyclability, and green economy. Compared with titanium and manganese adsorbents, Al-based adsorbents have the advantages of no elution damage, high technical maturity, high possibility of industrial scale application and desorption effect with deionized water. However, the lack of adsorption capacity and selectivity is one of the bottlenecks limiting its development. Table 2 compares the relevant properties of the three types of metal-based adsorbents.
表2 三种金属基吸附剂性能比较

Table 2 Performance comparison of three metal-based adsorbent

Performance Al-based Mn-based Ti-based
Li+ Adsorption Capacity √ √√ √√√
Li+ Selectivity √ √√√ √√
Technology Maturity √√√ √√ √
Stability and Regeneration
Ability		 √√√ √ √√
Facile Operation
Conditions		 √√√ √ √√
Environmental Safety √√√ √ √√
Low Preparation Cost √√√ √ √√
Sun et Al. Used a reaction-coupled separation technique to separate sodium and lithium ions from high Na/Li ratio brine (Na/Li = 48.7, w/w), and Li-Al layered double hydroxide (LiAl-LDH) was used to extract lithium. The effect of Li+ combined with LiAl-LDH structure vacancy was better, and the loss of lithium was as low as 3.93% under the optimal separation conditions[55]. Chen et al. Synthesized magnetic lithium-aluminum layered double hydroxides (MLDHs) with different contents of Fe3O4 nanoparticles by a stepwise chemical coprecipitation method, and the MLDHs showed excellent Li+ selectivity, and the Mg/Li mass ratio of the desorption solution was significantly reduced to less than 7.0[56]. Heidari et al. Used a co-precipitation method to prepare aluminum-based adsorbents, and aluminum hydroxide was prepared by adding NaOH and AlCl3·6H2O to the salt lake at a Al3+/Li+ molar ratio of 5[57]. The recovery of Li+ was 76.4% at 30 ℃ and pH = 7.5. Cheng Penggao et al. Prepared aluminum-based adsorbent by one-step precipitation method and applied it to the extraction of lithium from Taihe underground brine[58]. The results show that the average adsorption capacity of aluminum-based lithium adsorbent is up to 15. 06 mg/G, the average desorption capacity is 14. 11 mg/G, and the desorption efficiency is 93. 69%. Paranthaman et al. Reported a three-stage bench-top column extraction process for the selective extraction of lithium chloride from geothermal brine with an excellent Li apparent selectivity of 47.8 compared to Na+ and 212 compared to K+[59]. The performance of aluminum-based adsorbents in the above literature is summarized in Table 3.
表3 铝基吸附剂的合成方法与性能

Table 3 Synthesis methods and properties of aluminum-based adsorbents

Adsorbent Source Method Li+ adsorption
capacity
(mg/g)		 pH Selectivity
(α)		 Recovery rate ref
LiAl-LDHs AlCl3·6H2O
NaOH Na2CO3		 Reaction coupling
separation technology		 / / / 96.07% 55
MLDH
(Fe3O4 doped LiAl-LDHs)		 FeCl3·6H2O
AlCl3·6H2O
LiCl·H2O
NaOH
FeCl2·4H2O		 Sectional chemical co-precipitation method 5.83 7 α(Li/Mg)=
362.68		 / 56
Al(OH)3 AlCl3·6H2O
NaOH brine		 co-precipitation method / 7.5 / 76.4% 57
LiOH/Al(OH)3 NaOH
anhydrous aluminum chloride
anhydrous lithium		 single step
co-precipitation		 15.06 6~7 / / 58
Li/Al-LDHs Al(OH)3
LiOH·H2O		 hydrothermal method / / α(Li/Na)=
47.80		 91% 59

4 Lithium ion-sieve adsorbent

4.1 Preparation of ion-sieve adsorbent

The ion-sieve adsorbent is prepared by introducing Li+ into the compound, and the ion-sieve precursor [Li (IS)] is prepared by chemical synthesis. After acid wash treatment, Li+ was eluted from the crystal structure to form lithium ion sieve [LIS (H)]. Lithium ion-sieve can selectively adsorb Li+ in lithium-rich solution. Such adsorbents tend to bind target ions (Li+) to form stable crystal structures during application. Due to the size effect and steric hindrance, the narrow active site rejects the occupation of other ions with different ionic radii, so it has a specific memory and sieving effect on the target ion in the coexistence of multiple heteroatoms.
Ion sieve adsorbents mainly include manganese adsorbents and titanium adsorbents. The synthesis and application of manganese-based adsorbents have been widely studied. They have outstanding adsorption rate and adsorption capacity, but there is a problem of solution loss in the adsorption/desorption process. The research of titanium lithium ion-sieve started relatively late, but it has the advantages of good acid resistance, high structural stability, considerable adsorption capacity and so on, so it has a broad application prospect.

4.1.1 Preparation of Li-Mn-O Ion-Sieve Adsorbent

1) Synthesis of LMO Precursor Traditional solid phase synthesis methods include solid phase calcination and redox precipitation. Shi et al. Synthesized the precursor Li1.6Mn1.6O4 by a two-step solid phase method at a calcination temperature of 470 ℃ for 6 H. After acid elution, the adsorption capacity of the material was 2. 65 mmol/G[60]. Subsequently, mechanochemical method and room temperature solid phase coordination method were developed[61][62]. Mechanochemical methods are mechanically activated methods for the synthesis of highly dispersed compounds at room temperature or lower temperatures. Kosova et al. Synthesized a highly dispersed series of stoichiometric and non-stoichiometric LixMn2O4 spinel compounds starting from different manganese (MnO2, Mn2O3, MnO) and lithium (LiOH, LiOH·H2O, Li2CO3)[61]. The product has the characteristics of high specific surface area, uniform reaction phase and the like. Huang et al. First successfully synthesized LiMn2O4-yBry nanoparticles via a solid state coordination method at room temperature[62]. Lithium acetate, manganese acetate and lithium bromide are used as raw materials, citric acid is used as a chelating agent, the raw materials are respectively ground into powder, ethylene glycol 400 (used as a dispersing agent) is used for mixing, and then the LiMn2O4-yBry powder is prepared by stage heating. After characterization, it was confirmed that the LiMn2O4-yBry powder was a well-crystallized pure spinel phase composed of small and uniform nanoparticles.
2) Soft chemical synthesis is also a common method for the preparation of ion sieves, including solvent-gel method, hydrothermal method, coprecipitation method, etc. The sol-gel method is a method in which the reaction is carried out at high temperature and high pressure in a sealed pressure vessel with water as the solvent. Takada et al. Synthesized well-crystallized Li4Mn5O12 powder by mixing lithium acetate (LiOAc) and manganese nitrate (Mn(NO3)2) at 700 ° C for 1 – 3 days in a O2 atmosphere using a sol-gel method[63]. The hydrothermal method can obtain products with different morphologies such as nanowires, nanospheres, nanosheets, and nanocubes by controlling the hydrothermal conditions[64,65]. Zhang et al. Combined hydrothermal method and low temperature solid state reaction to synthesize b-MnO2, spinel Li4Mn5O12 and pure cubic phase MnO2 nanorods[66]. Compared with the traditional high-temperature calcination process, it is more beneficial to control the nanocrystalline structure with good pore size distribution and high surface area. When the initial lithium concentration was only 5. 0 mmol/L, the adsorption capacity was significantly increased to 6. 62 mmol/G. In addition, there are molten salt method, biological analysis method and so on[67][68].

4.1.2 Preparation of Li-Ti-O system ion-sieve adsorbent

The lithium-titanium-based ion sieves commonly reported in the literature usually include Li4Ti5O12 and Li2TiO3. Their synthesis methods are the same as those of manganese-based lithium ion sieves, including solid phase sintering, hydrothermal method, sol-gel method, etc. Zhang et al. Used the sol-gel method to synthesize Li2TiO3,Li2TiO3, the Li+ removal rate reached 78.9%, and the dissolution rate of titanium ions was as low as 0.07%[69]. Zhao et al. Synthesized Li4Ti5O12 nanorods by hydrothermal method, and the maximum adsorption capacity of H4Ti5O12 nanorods reached 23.20 mg/G in 24 mmol/L LiCl solution[11]. Li et al. Prepared ultrathin nanosheet-assembled Li4Ti5O12 porous microspheres with hierarchical mesoporous structure by hydrothermal method[70]. Their high specific surface area (>180 m2/g) exposes more adsorption sites and significantly accelerates the adsorption rate (reaching equilibrium within 1 H). Moreover, the H4Ti5O12 microspheres showed an adsorption capacity of up to 43.20 mg/G. Shi et al. Prepared Li2TiO3,Li+ by high temperature solid phase synthesis with an extraction rate of 98.86% and a dissolution loss rate of titanium of only 0.17%[71]. In LiOH solution containing 694.1 mg/L lithium ion, the equilibrium adsorption capacity reached 39.8 mg/G.

4.2 Intercalation/deintercalation mechanism of lithium ion

The intercalation/delamination of Li+ in the interior of lithium ion-sieve is usually explained by redox, ion-exchange, and recombination mechanisms[72].

4.2.1 Redox mechanism

According to the Mn/Li ratio and crystal structure, lithium-manganese ion-sieves can be divided into three types: LiMn2O4, Li1.33Mn1.67O4(Li4Mn5O12) and Li1.6Mn1.6O4(Li2Mn2O3). The first two spinel-based precursors exhibit a cubic crystal structure, with Li and Mn occupying the tetrahedral and octahedral sites of LiMn2O4, respectively, and the structural model of Li2Mn2O5 has not been clearly reported. In Hunter's early studies, adsorption/desorption was considered to be a redox process[73]. At the same time of deintercalation of Li+, the disproportionation reaction of Mn3+ occurs in acidic environment, which is converted into Mn4+ and Mn2+,Mn2+ and dissolved in the solution, resulting in material loss. At the same time, lithium ions diffuse from the interior of the crystal to the surface and dissolve in the acidic solution. The reaction formula is as shown in (1). Ooi et al. Demonstrated that during the adsorption of Li+ into the tetrahedral site of the λ-MnO2,The two-step reduction of Mn4+ to Mn3+ and oxidation of OH- to O2,Li+ and electron migration are independent of each other, and the reaction formula is as shown in (2)[74]. The Mn of Li1.33Mn1.67O4 and Li1.6Mn1.6O4 belongs to + 4 valence in theory, and the solution loss in the adsorption/desorption process is relatively small, and the proportion of Li element is high, so the theoretical adsorption capacity is large. However, the redox mechanism cannot explain the positive effect of pH on the adsorption capacity of Li+.
4(Li)[Mn(Ⅲ)Mn(Ⅳ)]O4+8H+ →3(□)[Mn2(Ⅳ)]O4+4Li++2Mn(Ⅱ)+4H2O
4()[Mn2(Ⅳ)]O4+4Li++4OH-→4(Li)[Mn(Ⅲ)Mn(Ⅳ)]O4+2H2O+O2
In the above equation, (), [], □ represent the 8 a tetrahedral site, 16 d octahedral site, and cavity site of the spinel structure, respectively. The specific process is shown in Figure 5.
图5 尖晶石LMO吸附剂中Li嵌入和脱嵌机制的示意图

Fig.5 Mechanism of Li intercalation and deintercalation in spinel LMO adsorbent

4.2.2 Ion exchange mechanism

1) Absorption/extraction mechanism of spinel structure
In an earlier study by Shen et al., it was shown that ion exchange between H+ and Li+ provides Li+ vacancies rather than disproportionation of Mn ions[75]. They prepared manganese oxides by electrolysis in the temperature range of 10 ~ 95 ℃, and showed different exchange behaviors of manganese atoms and protons at different temperatures. In addition, the Li+ exchanged solid forms spinel LiMn2O4 upon heating and is converted to HMn2O4 upon treatment with dilute acid. Sato et al. Used XPS to characterize the valence state of Mn ions on the surface and on the whole of the spinel structure of manganese oxides with Li+ extraction and insertion, and found that the valence state of Mn ions remained unchanged in the process of extraction and insertion under the action of Li+[76]. This indicates that Li+ and H+ ion exchange reactions occur on the surface regardless of the solution pH. Koyanaka et al. Verified that the lithium adsorption capacity was proportional to the hydrogen ion content by analyzing the correlation between the composition and adsorption capacity of several spinel λ-MnO2[77]. They suggested that the reason for the selective adsorption of lithium ions in λ-MnO2 is not based on the so-called ion-sieve effect, but rather a suitable ion-exchange reaction that occurs only between lithium ions and protons.
The ion exchange mechanism illustrates the Li+ extraction process of spinel Li4Mn5O12(Li1.33Mn1.67O4) (the process is shown in Fig. 6), as shown in Equation (3):
L i 4 M n 5 O 12 → H 4 M n 5 O 12
图6 LIS中Li+提取和插入流程示意图

Fig.6 A schematic diagram of Li+ extraction and insertion process in LIS

Ammundsen et al. Performed a neutron diffraction study on the H+ exchange and back reaction of Li1.33Mn1.67O4 (Li+ exchange in H1.33Mn1.67O4), and the results indicated an ion exchange mechanism in the process of Li+ extraction and reinsertion[78]. Kim et al. Compared the bonding properties between Li1.33Mn1.67O4 tetrahedral and octahedral Li sites[79]. Li in manganites is highly ionized at both sites, but the net charge of Li at the tetrahedral site is larger than that at the octahedral site. These calculations indicate that the tetrahedral site has a higher Li+/H+ exchange capacity than the octahedral site, which is more favorable for the selective adsorption of Li ions.
According to this mechanism, the Li+ in the oxide can be completely replaced by the H+,However, Mn4+ and Mn3+ do not change in the process of Li+ and H+ exchange, and maintain a good spinel structure. Therefore, the protonated lithium ion-sieve exhibits high selectivity and reversibility for Li+, which also explains the positive effect of pH on adsorption capacity and rate. However, this theory can not explain the dissolution of elements and the decrease of adsorption capacity.
Wei et al. Designed a general strategy combining electrospinning and calcination techniques to fabricate electrospun porous spinel-structured lithium titanate nanofibers (P-LTO-NF)[80]. The chemical composition and crystal structure of P-HTO-NF before and after Li adsorption were determined by XPS and XRD. It is considered that the adsorption of Li on P-HTO-NF is an ion exchange reaction. Moazeni et al. Synthesized the spinel structure ion-sieve H4Ti5O12 by two-step hydrothermal method and acid treatment[81]. The adsorption process is analyzed as a simple ion exchange process, as shown in equations (4) to (8).
L i b u l k + → L i s u r f a c e +
L i s u r f a c e + → L i i n s e r t +
L i i n s e r t + + H 2 T i n O 2 n + 1 → L i H T i n O 2 n + 1 + H i n s e r t +
H i n s e r t + → H s u r f a c e +
H i n s e r t + → H s u r f a c e +
The reaction equation can well explain the properties of titanium-based ion-sieve, such as stable structure and small solution loss during adsorption/desorption.
2) absorption/deintercalation mechanism of layered structure
According to Chitrakar et al., the absorption/deintercalation process of layered structure materials is relatively simple compared with that of spinel structure[82]. Because of the restricted active site which can only exchange Li+ and H+ and the small structural gap,Layer-structured H2TiO3 can selectively adsorb lithium ions efficiently in the presence of a large number of competitive ions such as K+, Na+, Mg2+, Ca2+, etc. H2TiO3 can be regenerated and reused for lithium exchange in brine, and the exchange process is very similar to that of the original H2TiO3.
Hosogi et al. Treated the layered compound Li2TiO3 with molten AgNO3(573 K,5 h), and found that the morphology of Li2TiO3 was similar to that of Ag[Li1/3Ti2/3]O2, indicating that the crystal framework structure of Li2TiO3 did not change in the experiment[83]. Ag+ cannot undergo Li+ exchange at the (LiTi2) layer, but it can exchange at the (Li) layer forming the layered material Ag[Li1/3Ti2/3]O2. Thus, Li+ in the (Li) layer is first exchanged to form H[Li1/3Ti2/3]O2, followed by further exchange of Li+ in the (LiTi2) layer to form the fully exchanged phase H[H1/3Ti2/3]O2. This process illustrates that only ion exchange occurs before and after the exchange of Li+ and H+, and the displaced HTi2 may theoretically change back to LiTi2.

4.2.3 Recombination mechanism

Although redox mechanism and ion exchange mechanism can explain many phenomena in the adsorption/desorption process of ionic sieve in aqueous solution, there are still limitations, so on this basis, researchers further proposed a composite mechanism. Ooi et al. Prepared and serially characterized three spinel manganese oxides and found that the insertion sites could be divided into three groups: redox sites, Li+ specific ion exchange sites, and nonspecific ion exchange sites[84]. The proportion of confirmed sites varied according to the preparation conditions of manganese oxide. And changes in the oxidation state of manganese in the precursor correlate with the formation of different types of sites. Feng et al. Found that the Li+ extraction/insertion site can be divided into oxidation-reduction type and ion-exchange type[85]. In general, Li+ ions are preferentially extracted/inserted from the ion exchange sites unless the amount of Mn (Ⅲ) in the crystal is increased, and the number of both sites is related to the amount of trivalent Mn ions and Mn defects, respectively, and varies with the preparation conditions of lithium manganese oxide spinel (heat treatment temperature and Li/Mn molar ratio of starting materials, etc.).

4.3 Performance of lithium ion sieve for selective extraction of lithium

The crystal structure and particle morphology of ion-sieve oxide materials are affected by raw materials, precursor types, doping conditions and heat treatment methods, and then the adsorption properties (adsorption capacity and adsorption rate) of ion-sieve are affected. In the static and dynamic adsorption/desorption experiments, the pH value, the concentration of the enriched lithium solution, and the temperature can also change the adsorption performance of the optimized ion-sieve. In the final stage of industrial application, the adsorption capacity is closely related to the molding process and material selection. The following mainly focuses on four aspects that affect the performance of lithium extraction.
1) Effect of composition
In the preparation of ion-sieve precursor, Li source, Mn source, Ti source, molar ratio of raw materials, doping elements and so on will affect the adsorption performance of ion-sieve. Zandevakili et al. Studied the effects of six effective parameters, including lithium salt compound, manganese salt compound, oxidant, calcination temperature, heating time and Li/Mn molar ratio, on the synthesized ion-sieve through orthogonal experiments, and found that although all the mentioned parameters had a significant effect on the lithium absorption capacity, the oxidant and Li source were the most effective factors[86]. In order to suppress the agglomeration during the solid-state reaction, Gu et al. First used C2H3LiO2·2H2O as a lithium source to synthesize the precursor Li2TiO3[87]. The separation factor α (Li/Mg) of HTO obtained in the actual salt lake brine reached 5441.17. Tomita et al. Synthesized LiMxMn2-xO4(M=Cu, Mg, Zn; X ≤ 0.5) and studied the effect of substitution on crystal structure and conductivity[88]. With the increase of doping elements, the Mn — O bond becomes shorter and the Li — O bond becomes longer. The XPS results show that with the increase of the content of Mg, Zn or Cu in the compound, the Mn3+ is transformed into Mn4+, and then the stability of Mn element is enhanced and the solution loss is reduced. Tian et al. Synthesized magnesium (II) -doped spinel lithium manganese oxide (LMS) by soft chemistry method, and prepared nanoscale ion-sieve manganese oxide (HMS) by extracting lithium and magnesium from LMS[89]. It was found that Mg doping improved the stability of HMS, and the adsorption capacity remained above 95% after four times of reuse, and up to 99.2% of Li+ could be recovered from the solution within 24 H. The elements such as Mg, Fe, Ni and the like which can form strong ionic bonds with O are introduced into the structure of the lithium ion sieve, so that the dissolution rate of Mn during acid leaching can be reduced, and the practical application ability can be improved.
2) Effect of texture
The adsorption/desorption rate can be increased by changing the texture of the ion sieve, such as increasing the surface area of the ion sieve, reducing the length of the internal diffusion path, changing the pore size channel and structure morphology. High specific surface area will have a large number of adsorption sites, which is conducive to the insertion and migration of Li+. Zhang et al. Prepared spinel Li4Mn5O12 nanorods with a diameter of 20 – 140 nm and a length of about 0.8 – 4 μm by hydrothermal method, the specific surface area of which reached about 70 m2/g and the adsorption equilibrium could be reached in 5 H[65]. The porous and macroporous structure of the ionic sieve can improve the permeation, exchange and equilibrium rate of the Li+ in the ionic sieve. Li et al. Synthesized a three-dimensional macroporous mesoporous lithium ion-sieve (3DM-H4Ti5O12), and compared with its nonporous counterpart (1.12 mmol/G), 3DM-H4Ti5O12(5.51 mmol/g) showed superior Li+ adsorption performance, which was attributed to the relatively less mass transfer resistance of Li+ in the highly interconnected porous channels[90]. The morphologies of ion-sieve include nanowires, nanospheres, nanosheets, etc. Different morphologies of adsorbents have different adsorption properties. Moazeni et al. Synthesized lithium titanate spinel in nanotube morphology by a two-step hydrothermal process[81]. Li4Ti5O12 spinel ternary oxide nanotubes with a diameter of about 70 nm and a length of 2 μm were then prepared by treatment with dilute acidic solution. 39.43 mg/G of Li+ in the initial 120 mg/L lithium solution can be recovered. Han Hongjing et al. Synthesized aluminum-doped manganese-based ion-sieve H1.6(Mn1-xAlx)1.6O4 with large specific surface area and uniform and smooth nanosheet polyhedron morphology[91]. The adsorption capacity reach 32.32 mg/L in that solution with the initial Li+ concentration of 80 mg/L, and the adsorption capacity is large.
3) Effect of interfacial structural properties
According to the research, the wettability and exposed surface of lithium ion sieve are closely related to its adsorption performance. The wettability of lithium ion-sieve has a great influence on its adsorption capacity. Li+ often exists in aqueous solution in the form of hydrated ions, which has better wetting ability, and can make the ion sieve more fully contact with Li+ in brine, thus achieving efficient ion exchange. Li et al. Studied the effect of H2TiO3 lithium ion-sieves (HTO) prepared from different crystalline phase TiO2 precursors (amorphous, anatase and rutile) on their adsorption properties[92]. The anatase TiO2 derived HTO-400 ion sieve showed the smallest contact angle of 19 °, indicating the excellent hydrophilicity of the substrate, and Li+ could be easily transported and exchanged with H+. Adsorbents with different exposed facets can affect the surface dehydration process and Li+ adsorption behavior. Zhao et al. Synthesized octahedrally assembled nanospheres with main (111) face Li4Ti5O12(LTO-OS) and nanosheet-assembled microspheres with main (01-1) face precursor Li4Ti5O12(LTE-NS)[12]. These corresponding H4Ti5O12 adsorbents (HTO-OS and HTO-NS) were obtained by acid treatment. HTO-NS (35.5 mg/G) had higher adsorption capacity and faster adsorption rate (equilibrium time < 2 H) than HTO-OS (31.2 mg/G) due to different dehydration processes and exposed surfaces.

4.4 Molded lithium ion sieve adsorbent

Lithium ion sieve adsorbents are usually powdery, with poor fluidity and permeability, and are not easy to recover, which can not meet the needs of practical industrial application. Therefore, it must be shaped to meet specific operational needs. Forming methods mainly include granulation and membrane making. Material forming can improve the structure stability and cycle ability to a certain extent, but the mass transfer rate and adsorption capacity of Li+ are affected[93][94].
Hong et al. Pelletized LMO using chitosan as a binder[95]. The prepared chitosan LMO particles are mesoporous materials with pore sizes ranging from 6.5 to 30.0 nm. The stability of chitosan LMO particles was improved from 88% to 100%. When the concentration of Li+ was 30 mg/L, HMO powder and chitosan HMO particles showed similar adsorption capacity of 10 mg/G, but the adsorption/desorption equilibrium time was greatly increased after granulation. Zhou et al. Synthesized a novel Zr-doped Ti-LIS (HZrTO) by a simple calcination method[96]. Cylindrical EP/HZrTO composites were obtained by pelletizing HZrTO with epoxy resin (E-12). The Li+ adsorption capacity decreased before and after molding, but showed excellent stability and high selectivity for Li+. Xie Lixin et al. Made membranes by hybridization of Li1.6Mn1.6O4 and polymer resin PVDF, the extraction rate of Li+ in acid solution reached 95%, and the solution loss of Mn was about 3.5%[97]. The adsorption equilibrium is reach after 12 hour in lithium-rich solution, and that membrane has good selectivity for Li+. Ma et al. Prepared lithium manganese oxide (LMO) foam with three-dimensional intercommunicating network structure by polyurethane template method, and crosslinked asphalt was used as carrier and adhesive[98]. The adsorption capacity of the lithium sub-sieve is about 37. 22 mg/G before it is made into composite foam, and it is 8. 83 mg/G after it is formed. The adsorption capacity decreases, and the binding capacity between asphalt and ion-sieve becomes weak after lithium extraction.
The properties of some of the above lithium ion sieves are summarized in Table 4 and Table 5.
表4 不同类型钛基离子筛合成方法与性能

Table 4 Synthesis methods and properties of different titanium-based ion sieves

Precursor Source Method Li+ adsorption
Capacity (mg/g)		 pH Selectivity
(α)		 Cycling stability ref
Li4Ti5O12 TTIP
LiOH·H2O		 solvothermal reaction 35.5 13 / 92.5%
(Five cycles)		 12
Li4Ti5O12 Ti3AlC2
LiOH		 two-step hydrothermal method 43.20 12.1 α(Li/Mg)
=269.00		 93%
(Twenty cycles)		 70
Li4Ti5O12 TiO2LiOH Soft hydrothermal method 39.43 / / / 81
Li2TiO3 C2H3LiO2·2H2O
TiO2		 high-temperature calcination 40.16 10 α(Li/Mg)
=5441.17		 98%
(Five cycles)		 87
3DM-Li4Ti5O12 CH3COOLi
C12H28O4Ti		 hydrothermal method
low temperature calcination method		 38.24 / α(Li/Mg)
=30.00		 80%
(Six cycles)		 90
Li2TiO3 Ti(OBu)4
Li2CO3		 solid state reaction 34.2 12 α(Li/Na)
=19.96		 90.6%
(Eight cycles)		 92
表5 不同类型锰基离子筛合成方法与性能

Table 5 Synthesis methods and properties of different manganese ion sieves

Precursor Source Method Li+ adsorption capacity pH Selectivity Cycling stability ref
1-D Li4Mn5O12 MnSO4LiNO3 hydrothermal method
low-temperature solid-phase reaction		 6.62 mmol/g / α(Li/Mg)
=599.12		 / 66
LiMg0.56Mn1.50O4 MnCl2·4H2O
Mg(NO3)2·6H2O
LiOH		 soft chemical method 37.4 mg/g 12 / 95%
(Four cycles)		 89
LiMxMn2-xO4(M=Mg,Cu and Zn) Li2CO3
CuO ZnO
MgCO3MnO		 high-temperature calcination / / / / 88
Li1.6(Mn0.7Al0.3)1.6O4 MnO2LiCl
AlCl3		 hydrothermal method 32.32 mg/L / / 95%
(Five cycles)		 91
Li1.33Mn1.67O4 Li2CO3MnCO3 solid-phase synthesis method 10.00 mg/g / / / 95
Li1.6Mn1.6O4 KMnO4LiOH hydrothermal method
Solid high
temperature sintering method		 41 mg/g / C F L i +
=474.46		 85.37%
(Five cycles)		 97

5 Other types of adsorbent

In addition to the above adsorbents, there are other adsorption methods such as layered metal acid salt adsorbents, Li+ imprinted polymers, natural ores and intelligent adsorption materials. Layered adsorbents are generally + 4 metal acid salts. Because lithium has the smallest radius among the metal elements, the smaller the interlayer spacing is, the more large-diameter metals can be excluded, and the higher the selectivity to lithium is. The acid salt of that + 4 metal generally include phosphate and arsenate. The crystal structure of thorium arsenate is very compact, and only the diameter of lithium ion matches its gap, so it can freely enter the interior for replacement, while other ions are blocked by size and steric hindrance, so it shows high selectivity[99]. Li+ imprinted polymer is a hot adsorption material recently, in which Li+ is introduced as a template in the preparation process, and the coordination structure ( "imprinted hole") and position are unchanged after lithium removal, so it has affinity for lithium. Liang et al. Prepared a magnetic carbon-based lithium-ion imprinted material (Li+-IIP-Fe3O4@C)[100]. The maximum adsorption capacity of Li+ was 22.26 mg/G. Li+-IIP-Fe3O4@C decreased by only 8.8% after six adsorption-desorption cycles, showing excellent regeneration ability, making it very useful for lithium recovery. Natural ores and carbon adsorption materials, such as activated carbon, adsorb Li+ through the abundant oxygen-containing groups (— OH, — COOH, etc.) on their surfaces. Park et al. Used activated carbon (AC) to introduce functional groups on its surface through chemical treatment, which improved the recovery of lithium ions[101]. It showed good stability after five adsorption-desorption cycles, and pointed out that the adsorption of Li+ was due to the electrostatic interaction of positive and negative charges. Huang et al. Designed novel smart photo-controlled lithium ion imprinted polymers (P-IIPs) based on the surface of mesoporous C3N4 using a mixture of crown ether and azobenzene derivatives as functional monomers[102]. The results showed that UV irradiation led to the Li+ desorption of P-IIPs, while visible light irradiation promoted their adsorption. Under visible light irradiation, the maximum adsorption capacity of 400 mg/L solution reached 3280.5 μmol/G. The adsorbent regenerated by ultraviolet lamp provides a green and feasible strategy for the recovery of lithium resources.

6 Conclusion and prospect

In recent years, with the explosive growth of lithium terminal products, the demand for lithium in lithium power and other industries is bound to continue to grow for a long time in the future. Extracting lithium from brine, seawater and other water resources will be the core of obtaining lithium resources in the future, and is expected to lead the development of lithium industry. Although the selectivity and adsorption capacity of aluminum-based adsorbent are poor compared with titanium manganese oxide adsorbent, its technology maturity, simple process and low price make it the first to realize industrial application. Organic adsorbents are easy to be chemically coupled and modified, and have good selectivity, but their price is high, the synthesis process is complex, and the complexing ability of monomers to lithium ions is affected by many factors. At present, it tends to develop in the direction of lithium extraction application materials with high selectivity ligand and high stability matrix. The inorganic adsorbent has the advantages of simple synthesis, low cost, good selectivity and adsorption capacity for Li+ and the like, and has good application prospect. However, both manganese-based and titanium-based adsorbents have problems that need to be improved. Manganese-based materials are easy to dissolve and have poor stability, which is the biggest problem limiting their industrial application. It is very important to select the matrix material by loading the matrix material for film formation, granulation and foaming to alleviate the dissolution problem and facilitate recycling. The high bond energy of Ti-O in titanium-based materials improves the stability of molecular structure, but there are some problems such as difficult desorption, slow solid phase mass transfer, and lower adsorption capacity compared with manganese-based materials. Only by using appropriate materials to solidify lithium ion sieve adsorbents, doping elements, changing the structure and morphology of ion sieve and preparation methods, and solving the problems of dissolution loss, slow adsorption rate and low actual adsorption capacity, can the industrialization of lithium ion sieve adsorbents be greatly promoted. Although other types of adsorbents are not the mainstream at present, they also open up new directions for the field of adsorbents. The advantages and disadvantages of crown ether, aluminum-based, lithium-ion sieve and other adsorbents in this paper are summarized in Table 6. The ultimate goal of all adsorbents is to expand their functions to actual plant applications, and it is also an inevitable choice for energy development. Therefore, the future development of inorganic metal-based adsorbents should focus on: 1) high stability, which can achieve long-term stable recycling. 2) Economical, which can meet the actual requirements of green environmental protection and large-scale production. 3) Optimize the preparation process, explore the preparation method, raw materials, temperature, etc., and obtain the adsorbent with low cost and excellent performance. Under the background of new energy, it is expected to prepare high-quality adsorbents for industrial application, promote the construction of recycling system of lithium resources at upper and lower ends, and have a comprehensive impact on the low-carbon application of lithium resources in the future[103,104,105].
表6 冠醚、铝基、锂离子筛型以及其他类型吸附剂的优缺点总结

Table 6 Summary of advantages and disadvantages of crown ether, aluminum based, lithium-ion sieve type, and other types of adsorbents

Performance Crown ether Alumina-based adsorbent Lithium ion-sieve Others
Adsorption Capacity ★★ ★ ★★★ ★★
Selectivity ★★ ★ ★★★ ★★
Technology Maturity ★ ★★★ ★★ ★
Stability and
Regeneration		 ★★ ★★★ ★ ★★
Cost ★ ★★★ ★★ ★

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