The Technologies of Electrochemical Extraction Lithium Process from Lithium-Containing Solutions

1. Introduction

With the continuous advancement of the new energy sector, lithium-ion batteries, which offer advantages such as long service life and mature technology, have been applied in various aspects of daily life [1,2,3]. Particularly in recent years, following the rapid growth of the new energy sector and the continuous expansion of the electric vehicle market, the consumption of lithium resources has been escalating annually [4,5,6]. In light of the Statistical Multifragmentational Model’s (SMM), the worldwide demand for lithium batteries witnessed a year-on-year growth of 52% in 2022, while the supply of lithium resources reached as high as 840 Kt [7]. From a global perspective, a notable mismatch in the regional supply and demand of lithium resources is evident. Lithium ore exploitation resources in the upstream of the supply chain are mainly distributed in Australia, Chile, Argentina, and other regions in South America [8,9]. Thus, the extraction of lithium resources from secondary resource lithium becomes the focus of researchers. Secondary lithium resources are generated along with the slag from lithium ore [10], hydrometallurgical leaching the lithium from waste lithium batteries [11]. With the continuous expansion of the scale of lithium ore production and the recycling of waste lithium-ion batteries, the total amount of lithium-rich solutions shows an increasing trend year by year [12,13]. Therefore, the efficient and convenient lithium extraction technology is worthy study in the future.

Currently, a wide array of lithium extraction technologies are employed to recover lithium from lithium-containing solutions, including hydrometallurgy [14,15] and electrochemical recovery processes [16,17]. The main technology is hydrometallurgy method, through the co-precipitation method to recover the metal elements in the lithium solution, and reproduced as chemical raw materials to achieve the recycling of lithium resources. Electrochemical extraction technology is an efficient lithium extraction technology that has rapidly developed in recent years [17,18]. The primary technology used is the hydrometallurgy method, which utilizes co-precipitation to recover metal elements from lithium solutions and reproduce them as chemical raw materials, thereby achieving the recycling of lithium resources [19,20,21]. The electrochemical lithium extraction technology features high efficiency, environmental friendliness, and low energy consumption, with extensive prospects in the future [22,23]. This review aims to analyze the properties of various electrochemical lithium extraction methods from lithium-containing solutions, outline the current research status and technical characteristics, and promote the development of more efficient and environmentally friendly electrochemical lithium extraction technologies to meet the increasing demand for lithium extraction.

2. Different techniques of electrochemical lithium extraction

2.1. Electrochemical Deintercalation Method

LiFePO₄ is a commonly used anode materials of lithium ion battery. LiFePO₄ has a typical olivine-type crystal structure [23,24]. The crystal structure of LiFePO4 is composed of [LiO6] octahedra and [PO4] tetrahedra, which facilitate the migration of lithium ions and electronic conduction through the Fe-O-Fe lattice planes. The chemical reactions involved in the removal and insertion of Li+ in the LiFePO4 crystal structure are depicted in equations (1) and (2) [25] (Figure 1).

LiFePO₄ - xLi⁺ - xe^- → Li_(1-x)FePO₄

(1)

Li_(1-x)FePO₄ + xLi⁺ + xe^- → LiFePO₄

(2)

Based on above electrochemical delithiation theory, lithium iron phosphate is widely used for lithium extraction and lithium enrichment. Zhao et al. [26,27,28,29] have conducted extensive research on lithium extraction using the LiFePO₄ electrode. They developed a "rocking chair" electrode system, which alternates between a lithium-rich LiFePO₄ electrode and a lithium-poor Li(1-x)FePO₄ electrode. This system is divided by an anion-exchange membrane, with the electrolytic cell containing initial liquid on one side and lithium-rich liquid on the other. Under a direct current electric field, the lithiation reaction occurred at the lithium-poor electrode (FePO₄ cathode) where lithium ions were absorbed from initial liquid, while the delithiation reaction occurred at the lithium-rich electrode (LiFePO₄ anode), diffusing lithium ions into the lithium-rich liquid. Anion exchange membranes are used, allowing Cl^- ions to migrate from the cathode to the anode side to maintain charge balance. By switching the positions of the two electrodes and repeating the process, selective enrichment of lithium ions is achieved. Experimental results indicated that the maximum amount of lithium embedded reaches 38.93 mg/g after 10 h, while the maximum amount of embedded Mg²⁺ is 5 mg/g in MgCl₂ solution (220 mg/L). The selective enrichment of lithium ions has been confirmed in a simulated brine composed of LiCl and MgCl₂ solutions. The initial concentration of Li⁺ was 220 mg/L, and the final Mg/Li ratio decreased from 60 to 45, demonstrating effective lithium ion selectivity in the process..

Deng et al. [30] carried out electrochemical extraction lithium using a LiFePO₄/FePO₄ electrochemical system. The electrode was modified using PEG-6000 as a poregen to obtain abundant pore structures of about 5–20 μm formed on the surface of electrode. This modification increased the adsorption capacity to 17.10 mg g⁻¹, a 57.02% improvement. In this electrochemical system, delithium occurred at the LiFePO₄ electrode, while the lithiation reaction took place at theFePO₄ electrode. The system was effectively employed to recover lithium resources from geothermal water with low lithium concentration. When the modified LiFePO₄ and FePO₄ electrodes were utilized in an H-type electrolytic cell with operated voltage of 1 V, the retrieval rate of Li⁺ reached 90.65% after 8 cycles in the geothermal water, and the electrode structure of both anode and cathode maintained stable (Figure 2).

The electrochemical deintercalation method is commonly utilized for lithium extraction. Researchers have observed that the diffusion of anions is driven by the concentration differences of cations between the anode and cathode, leading anions to migrate towards the opposite electrode [31]. As a result, a small number of anions and impurity ions pass through anion-exchange membrane under the driving force of concentration difference, and the separation rate of Li⁺ is declined. Specifically, in the treatment of Li2SO4 solutions, the relatively large ionic volume of SO₄^2- increases migration resistance. This not only necessitates higher voltages for the electrochemical adsorption reaction, prompting numerous side reactions, but also results in substantial electrical resistance and high energy consumption during the process [32].

While the electrochemical deintercalation method is straightforward and easy to operate, it has several drawbacks that require improvement. Specifically, the ionic conduction ability of the membrane and the stability of the electrode material need significant enhancements to optimize the process.

2.2. Electrochemical ion pump

The principle of electrochemical ion pump involves extraction lithium under inverse concentration gradient. The Lithium ions are captured selectively by lithium absorption materials, while the counter electrodes (such as Pt and Ag) are adopted to capture anions [33,34,35]. Initially reported in the early 1990s by Kanoh and his team, the electrochemical ion pump method utilized λ-MnO₂ as the working electrode and a Pt wire as the counter electrode to extract lithium ions from a polycationic solution [36,37]. Subsequently, La Mantia and Yoon’s group innovated on this by replacing the Pt electrode with an Ag electrode, and its working principle is shown in Figure 3. The λ-MnO₂ serves as the intercalating electrode, adsorbing and desorbing lithium ions, while the Ag electrode functions as the counter electrode, capturing and releasing chloride ions,, thereby significantly reducing the power consumption during the electrochemical reaction [38].

In recent years, with continuous advancements in materials manufacturing technology, the scope of electrochemical ion pumping applications has notably expanded. Deng et.al introduced a green and innovative FePO₄/K₂NiFe(CN)₆ [FPO/KNiFC] electrochemical ion pump technique to recover lithium from low-concentration sources in geothermal water (Figure 4). The performance of the LiFePO₄ electrode was enhanced by modifying it with multiwalled carbon nanotubes, improving both the insertion capacity and rate. Although the single insertion capacity stands at 14.88 mg/g, the recovery rate can be increased through cycle insertion-release processes [39]. Moreover, Kazuya Sasaki et al. developed a new electrochemical pumping technology for the extraction/recovery of lithium. This system comprises two power supplies, three electrodes, and a La0.57Li0.29TiO3 (LLTO) electrolyte [40] (Figure 5). The distinctive structure enables the positive voltage of the third electrodes in the cathode solution to be applied to the conventional battery through a secondary power supply. LLTO serves as an ideal solid electrolyte for high-purity lithium extraction and recovery, effectively blocking the permeation of cations other than lithium ions. Notably, this newly designed system demonstrates flexibility in adapting to a variety of commercial electrolytes and capability of significantly enhancing the lithium collection rate of the mentioned main circuit [41]. Furthermore, an additional subcircuit provides effective control, accelerating the lithium absorption process by at least 464 times compared to traditional systems.

Ion pump lithium-extraction technology surpasses other electrochemical methods in selectivity, cycling stability, and energy efficiency, offering significant advantages for practical applications. The success of this technology hinges on electrode materials that exhibit high selectivity and a robust ion exchange capacity. Currently, the use of ion pump technology for lithium recovery remains predominantly within the realm of laboratory research. For it to transition to industrial applications, extensive further research is essential to address practical implementation challenges and scale-up processes.

2.3. Electrodialysis method

Electrodialysis is an electrochemistry separation technology based on selectivity membrane, which was originally developed in the field of seawater desalination [42,43,44]. And with the development of membrane materials, electrodialysis began to be applied for lithium recovery from saline and other lithium solution. The principle of electrodialysis technology is that a voltage is applied between the two electrodes, driving ions migration across the membranes to achieve the enrichment of target ions [45,46]. The working principle of the electrodialysis system is schemed in Figure 6. The electrodialysis system consists of repetitive units including cation-exchange membrane (CEM) and an anion-exchange membrane (AEM), which separate concentration and dilution chambers alternately. The monovalent ions (e.g., Li⁺, Na⁺, K⁺, and Cl^-) could permeated through these chambers and pass through the CEM and the AEM into the concentration chamber, while the divalent ions (e.g., Mg²⁺, Ca²⁺, SO₄^2-) are blocked by the ion exchange membrane and retained in the desalination chamber.

Abuliti Abudula et. al. developed a novel lithium-ion separation composite membrane composed of H_1.6Mn_1.6O₄ nanoparticles and reduced graphene oxide (rGO) by vacuum filtration technology [47]. The membrane's effectiveness in selectively extracting low-concentration lithium ions from aqueous solutions is attributed to the selective adsorption properties of H_1.6Mn_1.6O₄ and the excellent conductivity of rGO (Figure 7). The results showed that the lithium ion adsorption capacity of H_1.6Mn_1.6O₄/rGO composite membranes reached an equilibrium of 38.78 mg/g after 5 h. Impressively, the adsorption capacity of these composite membranes was maintained at 99% even after 5 cycles.. Specifically, during selective adsorption experiments with initial molar ratios of Li⁺/Na⁺ and Li⁺/Mg²⁺ set at 1:1, the adsorption ratios achieved were 10.39 for Li⁺/Na⁺ and 10.23 for Li⁺/Mg²⁺, respectively, demonstrating significant selectivity in ion exchange.

Shao et al. [48] developed a multistage selective electrodialysis (S-ED) process utilizin high-performance monovalent cation exchange membranes (MCEMs) as shown in Figure 8. These MCEMs were tailored to treat simulated osmotic water produced by a one-stage nanofiltration (NF) process. The mussel-inspired gallic acid/polyethyleneimine assembled membrane (M-GA/PEI) its unique specific architecture and charge properties demonstrated better separation performance than CSO (a commercial MCEM) during the multi-stage S-ED process. Leveraging its specific architectural and charge characteristics, the M-GA/PEI achieved a Li⁺-rich solution with a high concentration (approximately 8.33 g/L) and purity over 96.4% simultaneously, under a volume ratio of 10:1 (under DC:CC converter) during both the first and second-stage S-ED processes.

Liu et al. introduced an innovative electrodialysis technology which utilized Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), a lithium superionic conductors, and incorporates an assisted charge balance (ACB) system [49]. The electrodialysis technology was operated all under low voltage and ultra-low current conditions (Figure 9). The excellent ion selectivity of LATP, coupled with the improved recovery of ACB, enable this technology to achieve exceptionally high separation efficiencies and significantly reducedenergy consumption. By implementing a two-stage extraction process on aged brine, the application of directly producing battery-grade Li₂CO₃ with a purity of 99.93%. Remarkably, when applied to pristine brine,, the technology achieves an impressive Li/Mg separation coefficient as high as 5924. This simplified production process is expected to significantly reduce the cost of Li₂CO₃ production.

The electrodialysis method offers significant advantages in terms of cost-efficiency, environmental sustainability, and effective separation of lithium ions from bivalent cations. However, its effectiveness in separating lithium from sodium ions is limited. Additionally, the presence of specific anions in the solution, particularly high concentrations of SO₄^2-, can dramatically reduce the lifespan of the electrodialysis membranes [50,51]. Another challenge with this method is the generation of substantial amounts of wastewater that require further treatment. Moreover, the migration rate of lithium ions during electrodialysis is relatively slow, leading to lengthy processing times. Consequently, while electrodialysis presents potential benefits, its application in lithium extraction requires more in-depth study to overcome these operational challenges.

3. Conclusions

Electrochemical technology holds promising prospects in lithium extraction and adsorption, encompassing methods such as electrochemical deintercalation, ion pumps, and electrodialysis. These techniques offer advantages such as low cost, environmental sustainability, and reusability. Despite their potential, many of these technologies are still under development, facing significant barriers to commercialization. To advance the field of electrochemical lithium extraction, further research should focus on the following aspects:

(1) Electrochemical materials play a crucial role in determining the performance of lithium extraction systems. The effectiveness of both the anode and cathode materials in the electrochemical system, the ion exchange active materials in electrically controlled setups, and the membrane materials in electrodialysis processes directly influences the efficiency of lithium extraction. Therefore, there is a pressing need to enhance basic research into the development of new materials. Challenges such as low ionic mobility and suboptimal cycling stability persist in current materials. Addressing these issues could involve doping and coating materials, as well as engineering electrodes with perforations to modify their microstructures and chemical compositions, thereby improving their functional properties. In electrodialysis, the choice of membrane materials is critical for effectively separating monovalent from polyvalent ions. Advancements in this area should focus on increasing the ion conductivity, minimizing electrochemical impedance, and enhancing the chemical stability of these membranes. Such improvements will not only boost the performance of lithium extraction but also enhance the overall sustainability and efficiency of the electrochemical methods employed.

(2) Optimization of electrochemical processes. In the past, the industrial production of lithium primarily focused on generating industrial-grade Li₂CO₃. However, advances in electrochemical production technology are now paving the way for the direct one-step production of battery-grade lithium compounds such as Li₂CO₃, LiOH, and LiCl. This shift enables not only a transition from single to multiple electrochemical processes but also a transformation of the industry towards refining chemicals and achieving battery-grade production through process optimization. Electrochemical lithium extraction optimally utilizes lithium-containing solutions and recovers lithium from low-concentration and economically unviable brines efficiently. As global efforts to reduce carbon emissions continue to evolve, industrial practices must adapt to minimize environmental impact, carbon emissions, energy, and freshwater consumption. Upgrades in electrochemical lithium extraction technology are essential for significantly improving these parameters, which in turn will accelerate the rapid industrialization and commercialization of lithium extraction processes. This evolution is critical for supporting the growing demand for lithium in various high-tech applications, including electric vehicles and renewable energy storage solutions.