LU Qingqiang 1 , CHEN Linlin 2 , CHAO Yanhong 3 , LI Xiaowei 2 , JIANG Lei 4 , ZHU Wenshuai
(1 School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China; 2 School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China);3 School of Pharmacy, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China; 4 Jiangsu Tefeng New Material Technology Co.)
Abstract: With the rapid development of new energy industry in the world, lithium metal is widely used because of its high energy density and other advantages, and the extraction of lithium from the brine of saline lakes, which contains a large amount of lithium resources, is an important direction to obtain lithium resources. The methods of lithium extraction from salt lake brine include carbonization, precipitation, ion sieve adsorption, and electrochemical auxiliary methods. Ion sieve adsorption method is suitable for selective recovery of lithium from the liquid phase with low concentration, among which, titanium lithium ion sieve has become a hot spot in the research of adsorption method because of its strong stability and large adsorption capacity. This paper takes the titanium lithium ion sieve technology as a foothold, analyzes the current distribution of global lithium resources, the mechanism of lithium extraction by titanium lithium ion sieve, and reviews the current synthesis methods of titanium oxide lithium ion sieve, molding methods, existing problems, etc., to provide reference for the subsequent development of new titanium lithium ion sieve, and to improve saturation adsorption capacity, etc..
Lithium metal and its compounds are extensively utilized in defense, energy, battery production, glass manufacturing, and metallurgy, owing to their superior physical and chemical properties [1-5]. Lithium batteries, compared to traditional manganese-zinc and lead-acid batteries, offer high energy, a long lifespan, high rated voltage, substantial power capacity, and a low self-discharge rate [6-9]. Given the rapid increase in lithium demand [10-12], it is crucial to develop an efficient and stable lithium extraction method. Lithium resources naturally occur in solid and liquid forms [13-15]: solid lithium is primarily found in pegmatite and sedimentary rock-type ores, while liquid lithium mainly resides in the brines of saline lakes, underground reservoirs, and seawater [16-19]. Currently, all commercially available lithium is derived from land-based ores and brines. According to this year’s latest survey, these sources total approximately 14 million tons, primarily located in Chile, Argentina, China, and Australia. The reserves of liquid lithium in seawater amount to about 230 billion tons, nearly 2,000 times the total land-based lithium resources [20-22]. The vast reserves of liquid lithium in seawater and lake brines dwarf the annual consumption of lithium for human development. Thus, these sources could provide a virtually inexhaustible supply of lithium resources [23].
The traditional lithium mining industry relies on a chemical recovery process to extract lithium products from pegmatite-type lithium ores. This process is complex, involves harsh reaction conditions, consumes a significant amount of energy, and has a low lithium ore utilization rate [24-26]. This inefficiency fails to meet the growing demand for lithium in societal development. To optimize the use of lithium resources in salt lakes, researchers worldwide have developed various technologies to extract lithium from these lakes. The primary methods for lithium extraction from brine include evaporation crystallization [27], co-precipitation [28-30], solvent extraction [31], and ion exchange [32], with evaporation crystallization being the most prevalent. This method employs solar energy to evaporate and concentrate saline lake water to produce a lithium-rich mother liquor, which is then treated with carbonate to precipitate lithium and produce lithium carbonate. However, this process is time-consuming, generates considerable waste, and requires substantial amounts of carbonate, substantially increasing production costs.
The co-precipitation method involves adding agents like AlCl3 to concentrated brine and adjusting the pH with NaOH to precipitate LiOH. This method demands precise control over temperature and pH, making it unsuitable for large-scale industrial production. The solvent extraction method offers high purity and a straightforward process but suffers from high dissolution rates and severe pollution risks, posing corrosion risks to equipment. Conversely, the ion exchange method uses ion exchange adsorbents to extract lithium ions from brine, offering low energy consumption, cost-effectiveness, and environmental benefits. It is thus considered one of the most promising methods for lithium extraction. Among these, the lithium ion sieve adsorbent is crucial and a focal point of research for recovering lithium from brines in salt lakes. The most studied adsorbents so far include manganese lithium oxide and titanium lithium oxide ion sieves. Research on manganese lithium oxide ion sieves began earlier, with Soviet scientists Volkhin et al [33] first synthesizing a spinel structure of manganese oxides in 1970 that selectively adsorbs Li+. Subsequent studies revealed that these ion sieves have high adsorption capacity and selectivity, although the Mn3+ in the sieve structure tends to dissolve during Li+ elution, worsening with more cycles and significantly impairing the performance of the sieves. The first studies on lithium titanium oxide ion sieves were conducted in 1988 by Japanese researchers Onodera et al [34], who synthesized Li2TiO3 using a solid-phase method. They found that the acid-washed product, H2TiO3, effectively adsorbs Li+ in liquid phase. Further research introduced another spinel structure, lithium titanate oxide Li4Ti5O12, which shows high adsorption capacity, excellent selectivity, and, due to the strong titanium-oxygen bonds, remarkable cyclic stability. As a result, lithium titanate oxide ion sieves are viewed as highly promising adsorbents for industrial applications and have become a research focus for leading laboratories and lithium companies [35]. This paper reviews the lithium extraction mechanism, synthesis methods, molding techniques, and ongoing research challenges from the perspective of lithium titanate oxide ion sieves, aiming to provide insights for future developments and performance improvements in this field.
1 Types of Lithium Ion Sieves
Ion sieves are typically prepared by first synthesizing a compound, followed by introducing target ions into the compound using specific physicochemical methods to form a complex oxide. Subsequently, these target ions are extracted without altering the structure, resulting in ion sieves with selective ion adsorption properties. As illustrated in Figure 3, due to size effects and sieving capabilities, ion sieves can efficiently adsorb specific ions even in the presence of multiple competing ions.
To date, the main types of ion sieves developed include aluminum ion sieves (references [38-39]), manganese lithium ion sieves (references [40-42]), and titanium lithium ion sieves (references [43-46]). Ion sieves are generally classified into four categories: aluminum, manganese lithium, titanium lithium, and composite ion sieves (references [47-50]).
1.1 Aluminum Lithium Ion Sieve
Aluminum lithium ion sieves are commonly produced by embedding Li+ ions into Al(OH)3 to form [LiAl2(OH)6]x – mH2O. The Li+ ions are then eluted from this compound. These sieves are primarily used as industrial adsorbents for extracting lithium from salt lake brines, offering benefits such as high selectivity, reversibility, and rapid adsorption rates. Tang Na and colleagues (reference [38]) utilized anhydrous lithium chloride and anhydrous aluminum chloride as raw materials and employed the co-precipitation method to synthesize an aluminum lithium ion sieve. They conducted adsorption tests in an artificially simulated salt lake brine containing Na+, Li+, K+, Mg2+, and Ca2+. The results indicated that this lithium ion sieve had an adsorption capacity for Li+ of 8.66 mg/g, with the equilibrium adsorption coefficient for Li+ being over a hundred times higher than that for other coexisting cations. However, the aluminum lithium ion sieve is only effective in neutral environments, as alkaline or acidic conditions can degrade its structure.
1.2 Manganese Lithium Ion Sieves
Manganese lithium ion sieves typically consist of spinel-structured manganese-lithium oxides, LixMnyO2, which are used for the elution of lithium. These sieves exhibit high selectivity for lithium ions, primarily through the three-dimensional channels within the spinel structure that facilitate the insertion and removal of lithium ions. LixMnyO2 can remove almost all lithium ions from its structure under the influence of a lithium removing agent, thereby demonstrating a high adsorption capacity for lithium ions.
Marthi et al. prepared MnO2@DE precursors using potassium permanganate (KMnO4), ammonium persulfate ((NH4)2S2O8), and diatomaceous earth (DE) through a process combining hydrothermal treatment and solid-state roasting. They further calcined these precursors with lithium carbonate (LiCO3) at 650°C to produce LMO@DE. Subsequent treatment with ammonium persulfate yielded a diatomaceous earth-type manganese ion sieve (DLMO@DE), which showed high selectivity and adsorption capacity for lithium ions in adsorption tests.
Manganese ion sieves are user-friendly and have rapidly developed due to their high adsorption capacities. However, during the lithium elution process, Mn3+ ions in the crystal structure of the manganese-based lithium ion sieves may dissolve, which can lead to partial framework dissolution and significantly impair the cycling performance of the sieve. To combat these challenges, researchers have employed doping and other methods to stabilize the structure and have also developed titanium-based lithium ion sieves.
1.3 Titanium Lithium Ion Sieves
Titanium lithium ion sieves are primarily synthesized using TiO2 as the titanium source and Li2CO3 as the lithium source. This synthesis involves the solid-phase synthesis of precursors, followed by acid washing. The titanium lithium ion sieve exhibits a higher titanium-oxygen bonding energy within the titanium system, enhancing its structural stability and resistance to dissolution. Currently, three distinct crystal forms of TiO2 are recognized: anatase, plagioclase, and rutile. Precursors derived from these different crystal forms of TiO2 vary in structure and performance. This variability endows titanium lithium ion sieves with diverse properties that significantly enhance the efficiency of lithium extraction.
Jiang[43] synthesized Li2.5Ti1.375O4 by combining ball milling and the solid-phase method, using Li2CO3 and TiO2 at a Li/Ti ratio of 2.5:1.375. This compound was eluted with HNO3 and then subjected to an adsorption test. The results demonstrated that the lithium ion sieve exhibits excellent selective adsorption for Li+ ions, with an adsorption capacity reaching up to 8 mmol/g. Zhang et al.[44] synthesized a mesoporous TiO2 material via a hydrothermal method, using it as a titanium source. After high-temperature calcination with Li2CO3, they obtained a precursor to the lithium ion sieve, which was then treated with acid to produce the final TiO2 ion sieve.
The results indicated that the TiO2 ion sieves exhibited outstanding selective adsorption performance for lithium. These sieves were utilized to adsorb LiCl solutions containing hetero ions such as Na+, K+, Mg2+, and Ca2+, demonstrating their exceptional selectivity for lithium. The straightforward hydrothermal method and the use of a recyclable alkali solution render the TiO2 ion sieves highly promising for future applications.
Zhang Lifen and colleagues synthesized Li2TiO3 from TiO2 and Li2CO3 using a solid-phase method. They then eluted it with hydrochloric acid to produce the metatitanate-type lithium adsorbent, H2TiO3. This adsorbent showcased an impressive lithium elution rate of 98.86%, with negligible titanium solubility, and an adsorption capacity of 39.8 mg/g. Its excellent structural stability and high saturation capacity have made metatitanate-type lithium ion sieves a focal point of research in the lithium extraction industry from salt lakes. Chen and his team adopted the solid-phase technique, mixing dried TiO2 with Li2CO3 in a Li/Ti ratio of 0.5/1, and calcinated the mixture at 1123 K for 36 hours to produce the titanate lithium ion sieve precursor, LiTi2O4. This precursor was further processed under the same conditions to enhance its properties. Subsequent elution with 0.3 mol/L HCl yielded the titanate-based lithium ion sieve, HTi2O4, which demonstrated a maximum lithium adsorption capacity of 9.5 mg/g.Zhang et al. modified commercial grade TiO2 by impregnation, roasting, acid washing, and drying to obtain the LiTi2O4 precursor. It was then eluted with 0.3 mol/L HCl to produce HTi2O4.
The ion sieve was synthesized through a Li/Ti mixing process in a 0.8/1.2 ratio with LiOH. Using a solid-phase method, we obtained the precursor of the titanium lithium ion sieve, Li1.33Ti1.66O4. This ion sieve, after undergoing acid leaching, yielded a titanium-acid phase of TiO2. The lithium ion sieve displayed a saturated adsorption capacity for Li+ of 0.7 mmol/g.
1.4 Doped Composite Lithium Ion Sieves
The emergence of titanium lithium ion sieves mainly addresses the limitations of manganese ion sieves. In comparison, titanium lithium ion sieves offer several advantages, including low loss of backbone elements, robust periodicity, and superior structural stability. Consequently, they have become a focal point in research on lithium extraction from salt lake brines.
In response to the rapid societal demand and the scarcity of lithium products, researchers have been innovating ion sieve technology to enhance properties such as acid stability, adsorption capacity, and rate. One effective strategy has been to modify ion sieves with specific ion doping, which enhances the valence state of manganese, minimizes disproportionation reactions during lithium leaching, reduces solubility loss, and bolsters the structural stability. Common dopants include metal cations (such as Al3+ and Ni2+), non-metal ions (like S2- and F-), rare earth elements, and combinations of metal and non-metal ions.
Wang et al. prepared Fe-doped titanium lithium ion sieves (Ti-Fe-O) by mixing LiOH-H2O, TiO2, and Fe2O3 in a molar ratio of Li:Ti:Fe=2:1:0.15 and subsequently heating the mixture to 600°C. Electron microscopy was utilized for characterization, as depicted in Figure 5. Using an inductively coupled plasma spectrometer (ICP), the adsorption capacities for Li+, Na+, K+, Ca2+, and Mg2+ were measured. The results revealed that Li+ had an adsorption capacity of 34.8 mg/g, while Na+, K+, Ca2+, and Mg2+ showed capacities of 1.29 mg/g, 1.13 mg/g, 1.67 mg/g, and 2.26 mg/g, respectively. This demonstrates the Fe-doped titanium lithium ion sieve’s high adsorption capacity and selectivity for lithium ions.
Dong Dianquan et al. [48] synthesized the spinel LiAlTiO4 using a co-precipitation/thermal crystallization method and subsequently produced the Al-doped lithium titanium ion sieve, HAlTiO4, following acid treatment. The findings indicated that the Li+ elution rate from this lithium titanium ion sieve reached up to 72%, while the dissolution loss rate of Al3+ remained below 6.8%; its saturation exchange capacity was measured at 30.03 mg/g. Qian Fangren et al. [49] employed the hydrothermal method to synthesize LiMnO2 using MnCl2·4H2O and LiOH·H2O as precursors. They then mixed the resulting LiMnO2 with MgCl2·6H2O to create Mg2+-doped Li1.6Mn1.6O4. These Mg2+-doped spinel lithium ion sieves, produced via hydrochloric acid elution, exhibited unchanged saturation adsorption capacities, but the dissolution loss rate of Mn was significantly reduced under identical experimental conditions compared to the pre-doped sieves. Dong Dianquan et al. [50] synthesized LiNi0.05Mn1.95O4 with a spinel structure using acetate and citric acid as raw materials via a sol-gel method, and modified it with 0.5 mol/L ammonium persulfate to create LiNiMn-H, achieving a saturated adsorption capacity of Li+ at 5.2 mmol/g under the same conditions as before doping. The doped composite lithium ion sieve maintains a high adsorption capacity while greatly increasing structural stability, providing a new avenue for research aimed at enhancing the resistance of lithium ion sieves to solvation damage.
2 Mechanism of Lithium Extraction by Titanium Lithium Ion Sieve
The structure of titanium-based lithium ion sieves is generally categorized into two types: H2TiO3 with a layered structure and H4Ti5O12 with a spinel type.
2.1 Layered Structure H2TiO3
The structure of the layered titanium oxide ion sieves is well-defined. It is generally assumed that the main ion exchange process occurs during adsorption. Chitraker et al. [51] synthesized a layered H2TiO3 lithium ion sieve and analyzed it. They found that the structural gap of the H2TiO3 is very narrow, which allows only Li+ and H+ ions to pass through, whereas larger ions, such as K+, Na+, and Mg2+, are unable to pass. Therefore, in a solution containing various ions, the ion sieve with a layered structure only facilitates the exchange of Li+ and H+ ions, without other reactions. The lithium extraction process is illustrated in Figure 6.
Hosogi et al. [52] produced AgLi1/3Ti2/3O2 by mixing and heating AgNO3 and Li2TiO3 in a photocatalytic experiment. Observations of Li2TiO3 and the surface morphology of AgLi1/3Ti2/3O2 revealed that the two particles have similar morphologies, indicating that the crystal structure of Li2TiO3 remains unchanged during the experiment. Thus, Hosogi et al. suggested that the reaction involves a Li+ ion exchange, where Ag+ can replace Li+ in the Li layer but not in the LiTi2 layer, leading to the formation of AgLi1/3Ti2/3O2. This supports the hypothesis that during the elution of Li2TiO3 with a layered structure, H+ first replaces Li+ in the Li layer to form H[Li1/3Ti2/3]O2, and subsequently replaces Li+ in the LiTi2 layer to form H[H1/3Ti2/3]O2. The specific lithium extraction process is shown in Equations (1) to (3) [53].
Li[ Li 1/3Ti2/3]O2 + H+ —→H [ Li 1/3Ti2/3]O2 + Li+ ( 1)
H [ Li 1/3Ti2/3]O2 + H+ —→H [ H 1/3Ti2/3]O2 + Li+ (2)
H [ Li 1/3Ti2/3]O2 + xLi+ —→H 1 – x Lix [ H 1/3Ti2/3]O2 + xH+ (3)
The description above clearly outlines the exchange process between H+ and Li+ ions. Since this involves only an ion exchange reaction, in theory, the HTi2 layer formed should be reversible to the original LiTi2 layer through a replacement reaction with Li+. Consequently, the adsorption capacity is expected to remain consistent before and after acid washing. However, in practice, the adsorption capacity of the lithium titanium sieve significantly decreases after acid washing. To investigate this issue, Zhang et al. [54] synthesized H2TiO3 using titanium sulfate and lithium acetate. They concluded that Li+ tends to form ionic bonds, whereas H+ typically forms covalent bonds. During the pickling process, H+ replaces Li+, maintaining electroneutrality. Nevertheless, the manner in which H+ integrates into the crystal lattice differs from Li+; Li+ is bonded ionically in the precursor, whereas H+ primarily associates through electrostatic attraction. This difference explains the substantial variation in adsorption capacity before and after elution.
By integrating the analysis from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) of the lithium ion sieve before and after adsorption, it has been determined that the lithium extraction mechanism of this sieve operates through an ion exchange process.
Moazeni et al. [56] synthesized Li4Ti5O12 using a two-step hydrothermal method. The lithium ion sieve was obtained following acid washing, and the adsorption process was analyzed. It was concluded that the lithium extraction process from the lithium ion sieve is a straightforward ion-exchange process, as illustrated by Equations (4) to (8). This mechanism also effectively explains the low dissolution rate and high stability of the titanium lithium ion sieve.
Dong Dianquan and colleagues [57] synthesized spinel-structured H4Ti5O12 using the sol-gel method and successfully analyzed the kinetics of Li+ exchange in this ion sieve with the shrinkage model. They discovered that particle diffusion controls the adsorption process in this lithium ion sieve. Additionally, they briefly explained that the adsorption process of the spinel-structured H4Ti5O12 is an ion-exchange reaction.
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Synthesis of Titanium-based Lithium Ion Sieves
Titanium and lithium sources are synthesized into lithium ion sieve precursors through acid elution, yielding titanium-based lithium ion sieves with excellent lithium selectivity. The current methods for synthesizing lithium ion sieve precursors include the solid-phase method, the liquid-phase method, and the mold plate method.
3.1 Solid Phase Method
The solid-phase method typically involves uniformly mixing lithium sources (such as LiOH, Li2CO3, LiNO3, C2H3LiO2) with a titanium source (generally TiO2). This mixture is then calcined under specific temperature and gas conditions to produce the titanium lithium ion sieve precursor. Subsequent acid washing extracts lithium, resulting in the final titanium lithium ion sieve. The solid-phase method offers advantages such as simplicity, convenience, and ease of industrial application. However, it requires precise control of temperature and gas conditions and is energy-intensive. Additionally, inconsistent mixing of raw materials can result in impurities, uneven particle sizes, and significant agglomeration.
Li2TiO3 with a lamellar structure is commonly synthesized using the solid-phase method. This approach gained attention after Onodera used TiO2 and LiOH to synthesize Li2TiO3, discovering that H2TiO3, after acid washing, exhibits notable lithium extraction performance.
Research on titanium lithium ion sieves in China was initially conducted by Yan Shuwang et al., who utilized rutile TiO2 and Li2TiO3 as raw materials. These were mixed and milled in specific ratios, heated to obtain the precursor, and then washed with 0.5 mol/L HCl solution. The resulting titanium lithium ion sieves demonstrated an adsorption capacity of 29.4 mg/g, with a remarkably low titanium loss solubility rate of only 0.1%. Wang et al. applied the traditional solid-phase method to prepare iron-doped lithium ion sieves, using LiOH-2H2O, TiO2, and Fe2O3 as raw materials. The materials were mixed, calcined at high temperatures, and the iron-doped sieves were treated with 0.5 mol/L HCl at 30°C for 24 hours. Adsorption tests conducted in a LiOH solution with a pH of 12 showed that the adsorption capacity of Fe/Ti-doped titanium lithium ion sieves reached 53.3 mg/g.
Gu et al. [59] enhanced the solid-phase method by mixing and grinding C2H3LiO2-2H2O with TiO2 in a Li/Ti molar ratio of 2:1 and calcining at a specific temperature to obtain an ion sieve precursor. Compared to the lithium ion sieve produced by the traditional solid-phase method, the improved method significantly reduced precursor agglomeration, with an average particle diameter of 70 nm (as shown in Fig. 8). This method achieved an adsorption capacity of 24.5 mg/g in the brine of Xitai Genel Salt Lake, characterized by a high magnesium-to-lithium ratio. Given the high Mg/Li ratios in most Chinese salt lakes, this synthesis method could substantially benefit China’s lithium extraction industry from salt lakes.
Yan Hui et al. [60] accurately weighed TiO2 and Li2CO3 to maintain a strict lithium-to-titanium ratio, mixed them evenly, and dried the mixture at 95°C for 5 hours. After cooling to room temperature in a desiccator, the mixture was ground in a ball mill to a particle size of less than 150 µm and calcined at 700–850°C for 12 hours. The precursor, Li4Ti5O12, was then treated with hydrochloric acid to obtain a titanium-based lithium ion sieve, H4Ti5O12, with a notable saturated adsorption capacity. This synthesis method for the titanium-based lithium ion sieve is advantageous due to its low energy consumption and simplicity compared to other reported ion sieve adsorbents.
3.2 Liquid Phase
Liquid-phase methods involve reacting raw materials in a solution to produce a target product or precursor. The most commonly used techniques in this category are solvothermal and sol-gel methods.
3.2.1 Solvothermal Method
The solvothermal method involves synthesizing target products through a series of physicochemical changes. This process is conducted by dissolving reaction materials in water or organic solvents and then placing them in a reactor under specific temperatures and pressures. Since the reaction unfolds in a solution, it ensures uniform heating of the raw materials, leading to faster reactions and the production of target products with fewer impurities, smaller particle sizes, and minimal agglomeration.
Lin et al. [61] conducted an experiment where a suspension of anatase TiO2 and LiOH-2H2O was placed in an autoclave, heated to 433 K with stirring, and maintained for 24 hours. It was then removed, filtered to obtain a filter cake, and processed in a tube furnace at 733 K for 2 hours to produce the precursor. After acid washing, titanium lithium ion sieves were obtained, exhibiting a saturated adsorption capacity of 12.84 mg/g with a 20 mg/L Li+ solution.
Wang et al. [62] conducted a separate experiment by placing TiO2 and LiOH-2H2O in a crucible and heating it to 500 °C at a rate of 4 °C/min for 2 hours. The sample was then washed to neutrality and cooled at 90 °C for 24 hours to obtain c-Li2TiO3, and c-H2TiO3 was acid washed with HCl solution. Weighing c-H2TiO3 and TBA (tetrabutylamine) dissolved in LiOH solution, after stirring and sonication, the mixture was transferred into an autoclave and maintained at 170 ℃ for 24 hours. The final product, β-Li2TiO3, was acid washed to achieve an adsorption capacity of 76.7 mg/g in a 2g/L LiOH solution, the highest reported to date.
Moazeni et al. [56] innovatively applied a two-step hydrothermal method to synthesize Li4Ti5O12. Initially, the precursor was synthesized hydrothermally using TiO2 and NaOH as raw materials, followed by acid washing of the resultant titanium nanotubes to achieve titanium nanotubes with a diameter of 1.2 mm. These titanium nanotubes were then used to synthesize Li4Ti5O12 by reacting with LiOH, followed by another acid washing. The resulting H4Ti5O12, in nanotube form at 100 nm, demonstrated an excellent lithium adsorption capacity of 39.43 mg/g in a 0.5 mol/L LiCl solution, highlighting the potential of low-dimensional TiO2 in low concentration seawater and brine applications.
3.2.2 Sol-gel Methods
The sol-gel method typically involves dissolving metal salts in a solvent, forming a sol through hydrolysis or polymerization, and then converting this sol into a gel by heating. This gel is subsequently dried and fired to produce ultrafine particles. This technique has been effectively employed in the synthesis of lithium ion sieves. The process begins by selecting organic lithium and titanium sources as the raw materials, which are mixed and cross-linked in specific solvents to form a gel. This gel is then calcined to create precursor materials. These precursors are characterized by their small, uniform particle size, resistance to agglomeration, and exceptional lithium adsorption properties.
Zhang et al. used CH3COOLi and Ti(OC4H9)4 as lithium and titanium sources, respectively, to synthesize Li2TiO3 by the sol-gel method. This compound was then treated with hydrochloric acid to produce H2TiO3, a lithium adsorbent with a saturated adsorption capacity of 49.38 mg/g. During acid washing, the elution rate of Li+ reached 78.9%, while the dissolution loss of Ti was minimal at only 0.07%, indicating the robust stability of this lithium ion sieve.
Li et al. prepared TiO2 microspheres via the sol-gel method by dispersing titanium sulfate in poly(vinylpyrrolidone). These were combined with LiOH to create Li4Ti5O12 microspheres. A core-shell structure of C@Li4Ti5O12 was achieved by integrating carbon balls, which after acid washing, led to the formation of a TiO2 lithium ion sieve with surface voids replacing the carbon core. This sieve demonstrated an adsorption capacity of 28.46 mg/g under alkaline conditions, significantly surpassing the performance of standard TiO2 lithium ion sieves (3.68 mg/g).
Inspired by Dong Dianquan et al., who used Ti(OC4H9)4, CH3COOLi, and citric acid as a complexing agent to produce a dry gel that was roasted at 750°C for 24 hours to yield Li4Ti5O12, and subsequently acid-washed to obtain lithium ion sieve H4Ti5O12, Chen Nian also utilized these materials. However, Chen altered the proportions to achieve a 1:1 ratio of Li to Ti, resulting in the formation of Li2TiO3 after acid washing. This material exhibited a saturated adsorption capacity for lithium ions of 18.62 mg/g, with a Ti dissolution rate of less than 0.02% during the cycling process, enhancing the structural stability of the lithium ion sieve and promoting the industrialization of metatitanate-type H2TiO3.
3.3 Template Method
To enhance the specific surface area and material homogeneity of lithium ion sieve particles, researchers have explored the use of materials with organized structures as templates. Commonly used polymers for templates include polymethyl methacrylate (PMMA) and polystyrene (PS), chosen for their excellent mechanical properties and high flexibility. Zheng [67] utilized lithium acetate, zirconium nitroxide hydrate, and tetrabutyl titanate as metal sources, citric acid as a chelating agent, and F-127 as a soft template. The components were dissolved in anhydrous ethanol, then introduced into a PMMA hard template (Fig. 10). This mixture was filtered under reduced pressure, dried at a constant temperature, and subjected to controlled heating to produce the titanium-zirconium composite adsorbent precursor, Li4Ti5-xZrxO12. This precursor exhibited a robust structure. The lithium ion sieve, H4Ti5-xZrxO12, was produced by acid washing with 0.2 mol/L hydrochloric acid, achieving a lithium ion elution rate of about 88%. The maximum lithium exchange capacity measured at 50.61 mg/g in 0.05 mol/L LiCl solution, represented 79.7% of the theoretical capacity. The solubility loss rate of titanium and zirconium in the composite produced by the PMMA template was below 6%, showing excellent adsorption selectivity and capacity for Li+.
Dong Tianquan et al. [68] employed PS microspheres as templates and used CH3COOLi – 2H2O, Ti(OH)4, and H2C2O4 as raw materials to create a three-dimensionally ordered macroporous precursor material (Li4Ti5O12). Following acid washing, the titanium lithium ion sieve (H4Ti5O12) was obtained. The crystal structure of this product, characterized by a spinel type arranged neatly and orderly, was significantly superior in terms of ion exchange performance compared to sieves synthesized by other methods. Additionally, its selectivity for Li+ surpassed that of ion sieves produced using alternative approaches.
Xu et al. [69] utilized polystyrene (PS) as a template to synthesize Li2TiO3 with a layered structure using LiNO3 and [Ti(OC2H5)3CH3]4. After acid washing, they obtained the titanium lithium ion sieve H2TiO3, which demonstrated effective Li+ adsorption in strongly alkaline solutions with multiple coexisting cations. Its saturation capacity reached 76.3 mg/g, maintaining a high adsorption capability for Li+ over subsequent cycles.
In summary, the production of titanium-based lithium ion sieve precursors presents various advantages and disadvantages. The solid-phase synthesis process is simpler and easier to operate, but it typically yields sieves with larger and unevenly distributed particles, significantly impairing their performance and adsorption capabilities. In contrast, the liquid-phase method produces smaller, more uniformly distributed particles, enhancing the adsorption capacity for lithium ions. However, this method requires high temperatures and pressures, involves the addition of numerous reagents, and demands stringent environmental controls, which complicates its operation and hinders its industrial application. On the other hand, the template method results in materials with small, uniformly distributed particles, a large specific surface area, and high selectivity and adsorption capacity for lithium ions. Nonetheless, this method imposes strict requirements on the template material, which must be regular and pliable. The necessity to remove the templating agent poses a significant challenge to the development of the template method. The primary challenge now is to identify templates that are both highly regular and easily removable.
Molding Methods for Titanium-based Lithium Ion Sieves
Titanium-based lithium ion sieves are primarily available in powder form, characterized by poor flowability and permeability. These properties make it challenging to use them for lithium extraction in salt lakes. Additionally, the high energy consumption associated with the operation of adsorption towers greatly restricts the industrial application of these materials. To address this issue, researchers have explored various methods for processing and molding the powdered lithium ion sieve materials. The most commonly adopted molding processes for lithium ion sieves include granulation, foaming, and film formation.
4.1 Granulation
Granulation is a widely used technique that transforms powders into granules or pellets using organic or inorganic binders. Liu et al. successfully produced PVC-lithium ion sieve microspheres by bonding the precursor Li1.5Ti1.625O4 with an N,N-methylformamide (DMF), polyvinyl chloride (PVC), and water system. These microspheres were then used to extract lithium from oilfield water. Notably, lithium ion sieves with a 15% PVC addition demonstrated a high lithium ion exchange capacity of 11.54 mg/g and exhibited effective ion sieving capabilities for lithium ions. While granulation effectively addresses the challenges related to powder flowability, it faces issues such as difficulty in precise control over particle size and a decrease in adsorption capacity post-granulation, which have hindered the advancement of this method.
4.2 Foaming
Foaming is another commonly used method in powder molding, employed to transform powdered lithium ion sieves into solid forms with robust mechanical properties for lithium extraction from salt lakes. Typically, foamed lithium ion sieves exhibit superior mechanical strength, exceptional lithium selectivity, and excellent regenerative capabilities.
Zhang et al. [71] employed a foaming technique to impregnate a titanate-type lithium adsorbent onto a foamy lithium sieve made of cordierite-based foam ceramics, designated as H2TiO3. The adsorption capacity of this foamy lithium sieve reached 34.87 mg/g using a 0.5 g/L Li+ solution. It was also noted that this foamy lithium ion sieve could effectively enrich Li+ even at low concentrations.
4.3 Film Formation
In comparison to granular forms, membrane lithium ion sieves offer several advantages. Due to the flexibility of the polymer membranes, they can be bent, wrapped, or stacked, enhancing the contact area with brine and significantly reducing ion diffusion resistance. This makes the membrane lithium ion sieve highly promising for industrial applications. Commonly used film-forming agents include PVC, polyalum, polyvinylidene fluoride, and porous ceramics [72].
Mani et al. [73] utilized a sol-gel process to synthesize lithium-containing titanium isopropoxide gels by adding LiNO3 to titanium isopropoxide. These gels were then spin-coated onto silicon oxide or quartz glass to create Li-Ti-O films.
While lithium ion sieve membranes can increase the surface area in contact with salt lake water and are conveniently applied, they tend to have reduced adsorption capacity and rate. Additionally, these membranes are prone to damage such as tearing or powder detachment during use, which can lead to contamination and thus impede their commercialization.
Challenges in the Application of Titanium-Based Lithium Ion Sieves
Titanium lithium ion sieves have emerged as a new type of adsorbent for lithium extraction from salt lake brines, earning recognition from the industry for their stable and efficient performance. However, the industrial application of these sieves has been progressing slowly, primarily due to several key issues:
- Mechanism of Lithium Extraction
Currently, there are two main theories regarding the mechanism of lithium extraction by titanium ion sieves. The first theory suggests that the structure of titanium ion sieves contains numerous “pores,” and these pores facilitate the selective exchange of Li+ ions through the spatial potential resistance effect. The second theory proposes that the selective attachment of Li+ ions is achieved via surface complexation of titanium oxides in aqueous solutions. Since the kinetics of inorganic titanium ion separation materials have not been thoroughly studied domestically or internationally, the precise mechanism behind the lithium extraction capabilities of titanium lithium ion sieves remains unresolved.
- Challenges with Synthesizing Lithium Ion Sieves
Lithium ion sieve adsorbents are synthesized using a solid-phase method, which involves mixing lithium and titanium sources and then initiating a solid-phase reaction at high temperatures. However, this method faces challenges in ensuring the homogeneous mixing of raw materials. Additionally, the diffusion of titanium ions within the solid phase is slow at high temperatures. This slow diffusion necessitates a longer reaction time, consumes significant energy, and often results in precursors—such as titanium lithium ion sieves—having low crystallinity and defective crystal structures. These defects compromise the recyclability of lithium ion sieves, thereby impeding progress in the research and development of lithium titanium ion sieves for recovering liquid lithium resources.
- Insufficient Research on Lithium Ion Sieve Precursors
TiO2 is found in three crystalline forms: anatase, plagioclase, and rutile. Each form of crystalline TiO2 possesses distinct physical and chemical properties, which in turn affect the structure and characteristics of the synthesized lithium ion sieves. The limited understanding of the crystallization process of lithium ion sieves makes it difficult to control the crystalline form of TiO2 during production. This is a fundamental issue leading to the poor stability and limited capacity of inorganic titanium separator materials in practical applications.
Conclusion and Outlook
Titanium lithium ion sieves are efficient, eco-friendly, and stable adsorbents for lithium extraction from salt lake brines, showing significant potential for industrial development. These sieves stand out from traditional adsorbents due to their high efficiency, excellent processability, and superior adsorption capacity. The advancement of titanium lithium ion sieves, particularly their integration into the lithium extraction industry of salt lakes, could play a crucial role in addressing the global lithium shortage. Researchers worldwide are striving to develop titanium lithium ion sieves with a stable structure, low solubility loss, high adsorption capacity, and robust cycling performance to facilitate their industrialization.
Given the current research progress, several strategies should be considered to enhance the research and application of titanium lithium ion sieves:
- Thoroughly investigate the lithium insertion/extraction processes in ion sieves, covering aspects like adsorption kinetics, isothermal adsorption characteristics, and the underlying mechanisms of lithium insertion/extraction.
- Develop new synthetic methods since the commonly used solid-phase, liquid-phase, and templating approaches often result in uncontrollable reactions and crystalline defects in the sieves, which adversely affect their adsorption performance. Innovating synthesis methods is thus essential.
- Optimize the material molding techniques for lithium ion sieves. Presently, most sieves are produced in powder form, which poses challenges in terms of mobility and permeability. Common molding methods tend to significantly reduce the material’s adsorption capacity and lead to detachment issues. Therefore, improving molding techniques to maintain adsorption efficiency after processing is crucial for the practical deployment of titanium-based lithium ion sieve adsorbents in the industry.
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