Recent Advances in Lithium Extraction Using Electrode Materials of Li-Ion Battery from Brine/Seawater

Abstract

With the rapid development of industry, the demand for lithium resources is increasing. Traditional methods such as precipitation usually take 1–2 years, and depend on weather conditions. In addition, electrochemical lithium recovery (ELR) as a green chemical method has attracted a great deal of attention. Herein, we summarize the systems of electrochemical lithium extraction and the electrode materials of the Li-ion battery from brine/seawater. Some representative work on electrochemical lithium extraction is then introduced. Finally, we prospect the future opportunities and challenges of electrochemical lithium extraction. In all, this review explores electrochemical lithium extraction from brine/seawater in depth, with special attention to the systems and electrode of electrochemical lithium extraction, which could provide a useful guidance for reasonable electrochemical-lithium-extraction.

1. Introduction

Rapid industrial growth and the increasing demand for raw materials require accelerated mineral exploration and mining to meet production needs [1,2,3,4,5,6,7]. Among some valuable minerals, lithium, one of important elements with economic value, has the lightest metal density (0.53 g/cm³) and the most negative redox-potential (−3.04 V), which is widely used in battery technologies for portable electronic devices, new energy vehicles and power storage [8,9,10]. Recently, along with the development of electric vehicles, a large number of lithium-ion batteries are being used to power electric vehicles, and the consumption of lithium for battery production is increasing, accounting for 65% of the lithium-ion end market in Figure 1 [11,12]. Correspondingly, it is particularly important to find a suitable method to extract lithium. At present, lithium is mainly derived from hard-rock sources and lithium-containing solution including brines, seawater and waste lithium electrolyte. The process of lithium extraction from hard-rock sources has a high energy consumption, high cost and causes serious pollution. In addition, the content of lithium in brine/seawater is very rich, accounting for approximately 80% of the total known reserves [13,14]. Moreover, as an important potential source of lithium, the total inventory of lithium in the world’s oceans is approximately 230 billion tons, so that seawater also has received a lot of attention [15,16]. Therefore, the development of an economic and environmental-protection method for lithium recovery from brine/seawater will be a trend for lithium recovery in the future.

Currently, many methods such as precipitation [17,18,19], adsorption [20,21,22,23], extraction [24], ion exchange [25,26] and electrodialysis [27,28,29] are reported for lithium recovery from brine/seawater. Among them, precipitation is a mature and reliable method, and the first method used in industry for lithium extraction from salt lakes. However, it requires a pre-concentration process, which takes a long time to complete, and is subject to weather, while the addition of lime to separate lithium and magnesium increases the cost and generates waste, which puts great pressure on the environment. Studies also have shown clearly that precipitation can be used for separation when the ratio of magnesium to lithium is greater than six. However, since most brines have Mg/Li ratios lower than six, simple chemical precipitation is not applicable [13]. In addition, Mg and Li have similar radii and similar properties [30]. There is an urgent need to develop new methods to obtain lithium. Recently, electrochemical lithium-extraction as a highly efficient, environmentally friendly and simple operation-method without generating environmental pollutants has received a lot of attention from researchers [31,32,33,34]. In fact, the electrochemical-lithium-extraction method mainly realizes the embedding and removal of Li⁺ in the electrode material by controlling the potential, so as to achieve the purpose of lithium extraction from brine/seawater. Recently, with the rapid development of nanomaterials, great efforts have been made to design lithium-selective electrodes for lithium recovery from brine/seawater. However, there are still relatively few reviews on the acquisition of electrochemical lithium.

Herein, in this review, the recent research into electrochemical lithium extraction from brine/seawater is introduced in detail (Figure 2). Firstly, systems of electrochemical lithium extraction are summarized. Some representative electrode materials for electrochemical lithium extraction are then introduced. Finally, the future opportunities and challenges of electrochemical lithium extraction are prospected. This review provides some guidance for reasonable electrochemical-lithium-extraction.

2. Electrochemical Lithium-Recovery Systems

To achieve efficient electrochemical Li acquisition, many electrochemical-Li-recovery systems including the water-splitting battery system, asymmetric battery system, hybrid supercapacitor system and symmetric rocking chair battery-liked system have been reported. Therefore, in this section, we summarized the electrochemical Li recovery systems in detail.

2.1. Water-Splittig Battery System

The water-splitting battery system, as the earliest electrochemical-Li-recovery (ELR) system, is combined with the working electrode (WE) and counter electrode (CE), wherein the capture of Li⁺ and water-splitting process occur simultaneously [35]. Typically, such CE can not only facilitate the formation of closed circuits, but also ensure the electrical neutrality of the system. Kanoh et al. reported a λ-MnO₂/Pt system, which could realize Li insertion by the cycle transformation of λ-MnO₂ and LiMnO₂ [36]. Despite the high selectivity of Li ions, the water-splitting that occurred on the Pt electrode increased energy consumption, as shown in Equation (1), and resulted in reduced efficiency of Li extraction. On the other hand, the pH conditions should be maintained in the process of Li recovery.

$$\frac{1}{2}{H}_{2}O\to \frac{1}{4}{O}_2+e^{-}+H^{+}$$

(1)

2.2. Asymmetric-Battery System

The asymmetric battery is considered to be an efficient system for the extraction of Li, and features stable solution-acidity and alkalinity. For instance, Lee et al. coupled the λ-MnO₂ electrode with silver for a rechargeable battery which exhibited good reversibility [37]. Typically, this type of $\mathsf{\lambda }$-MnO₂–Ag cell can realize the recovery of Li ions without the interference of other cations in the brine, something which is attributed to the Li ion-sieve property caused by the electrode crystal structure. However, the silver electrodes, with high cost and poor reversibility, restrict the large-scale application, and suitable alternatives have attracted wide attention. Polyaniline, as a polymer compound, has been considered as an efficient electrode material for Li-recovery systems due to its superior reversibility and chemical and environmental stability. For example, Zhao et al. constructed a high-efficiency, low-cost and unpolluted Polyaniline/Li_xMn₂O₄ cell, which could extract LiCl via significant Li ion selectivity and with remarkable ability to trap chloride ions, in Figure 3 [38]. Typically, the reaction occurred on the PANI electrode and is based on the p-doping/de-doping, as shown in the Equation (2). In addition, Prussian blue analogues with an open framework are a superb alternative to Ag, due to their charge-retention capability [39,40]. For example, La Mantia et al. replaced the Fe³⁺ with Ni²⁺ to form nickel hexacyanoferrate (NiHCF), which was used as the CE by virtue of the selectivity for K⁺ and Na⁺. Typically, such an environmentally friendly NiHCF could achieve low energy consumption and inexpensive bulk synthesis. Similarly, this asymmetric battery system can selectively recover Li+ using λ-MnO₂ electrodes.

$$PANI+C{l}^{-}-e^{-}\leftrightarrows PAN{I}^{+}C{l}^{-}$$

(2)

2.3. Hybrid Supercapacitor System

The hybrid supercapacitor system, with good rate performance, has attracted wide attention and draws not only on the advantages of Li-ion batteries but also the double-layer capacitors [41]. In general, a negative electrode can effectively adsorb and desorb ions in the electrolyte, and works as a non-Faradaic capacitive cathode. Typically, the activated carbon that is suitable for aqueous solutions containing many different anions is widely utilized in the hybrid supercapacitor system, due to its environmental friendliness, economic feasibility and stability [42]. For instance, Kim et al. reported a hybrid supercapacitor, in which $\mathsf{\lambda }$-MnO₂ and activated carbon were matched, in Figure 4 [43]. Considering the adsorption of cations on activated carbon, it was necessary to arrange an anion exchange membrane in the system. Meanwhile, it should be noted that the amount of extracted Li ion variation remained stable after 50 operation cycles, and exhibited excellent long-term stability. To avoid the corrosive acidic solution used in the conventional Li-recovery process, Ryu et al. presented a modified membrane-capacitive-deionization process via an electric-field-assisted desorption system, which was composed of LiMn₂O₄ as the WE, and activated carbon [44]. Compared with the conventional desorption process of Li, the electric-field-assisted desorption system was expected to be a practical process for Li recovery, due to its low-cost and low-pollution properties. Nevertheless, such a system is faced with the dilemma of low resolution, which should be addressed.

2.4. Symmetric Rocking-Chair-Battery System

Rechargeable Li ion batteries that are used in the aqueous electrolyte have been considered as a safe and low-cost electrochemical Li-extraction system [45,46]. Typically, Li can be moved from brine to the recovery solution by insertion and release at the electrode under the condition of applied voltage, which is the origination for the name of rocking-chair battery [45,47]. Currently, the representative symmetric rocking-chair battery-like systems are LiFePO₄/FePO₄, wherein the electrodes maintain Li saturation- and Li deficiency-states. For instance, Zhao et al. used the LiFePO₄/FePO₄ system to achieve effective Li extraction from brine with a high Mg/Li ratio [48]. Typically, the insertion and release of Li occurred on the electrodes which were based on the oxidation and reduction processes of iron. In addition, like LiFePO₄/FePO₄, LiMn₂O₄/Li_1−xMn₂O₄, as a rocking-chair electrode system, can be used for Li extraction. For example, Guo et al. fabricated a LiMn₂O₄/Li_1−xMn₂O₄ system, wherein the adsorption and desorption of Li could be performed simultaneously, in Figure 5 [49]. It should be noted that the effect of heteroatomic ions on the Li-extraction process is systematically discussed. It was demonstrated that the chloride ion had a positive effect on the Li-extraction process while the magnesium ion had a negative effect. Meanwhile, the difference of ion concentration near the cathode boundary-layer caused an increase in spatial potential-resistance, which reduced the efficiency of Li extraction. Typically, stirring the source solution could effectively reduce such a negative effect. In addition, Zhao et al. reported a rocking-chair-battery-like system that consisted of LiMn₂O₄ and Li_1−xMn₂O₄, and the influencing conditions for Li extraction were discussed in detail [50]. The coexisting ions, such as Mg²⁺, Na⁺, Ca²⁺, K⁺, which are present in the electrolyte, have a negative effect on Li extraction, wherein the magnesium ion has the greatest effect. Meanwhile, the voltage of 1.2 V could result in a maximum Li-extraction capacity; however, it also had the negative impact of high energy-consumption. Temperature also had a positive effect on the increase in the Li-extraction rate, due to the change in ion-migration rate. Considering the influence of the Li-extraction rate and specific energy consumption, 25 °C was chosen as the optimal temperature.

Although the above symmetric rocking-chair-battery-like system provides great flexibility in Li extraction from seawater, high ion-resistance caused by the anion exchange membrane leads to the high energy-consumption of the system, which needs to be further optimized.

3. Electrode Materials

3.1. Working Materials

Electrochemical Li-extraction is achieved by using the principle of Li⁺ transfer between electrode and electrolyte, during the charging and discharging process of the Li-ion battery (LIBs) [9,51]. Theoretically, any cathode or anode material that can be used for rechargeable LIBs may be used to extract Li from brine/seawater. As shown in Figure 6, the main electrode materials that can be used for aqueous LIBs are shown. Considering the stability, ease of preparation, low cost, and environmental friendliness, many electrode materials can be chosen as the electrode material for Li recovery. This part mainly introduces the new development of electrochemical lithium extraction technology in recent years, and summarizes and analyzes the suggestions of electrode materials such as LiFePO₄ (LFP) and LiMn₂O₄ (LMO).

3.1.1. LiFePO₄-Based Materials

LiFePO₄ (LFP) has an orthorhombic lattice structure with a space group of Pnma. Its lattice parameters satisfy a = 10.33 Å, b = 6.01 Å and c = 4.69 Å. Its structure can be simplified to consist of FeO₆ octahedra and LiO₆ octahedra, which are connected together by PO₄ tetrahedra in Figure 7 [52]. Therefore, when the FePO₄ is formed by delithiation, the skeleton structure is not changed [53]. Moreover, it has been shown that LFP has a high stability, due to the strong interaction between the O atoms and Fe and P atoms, which can remain stable at high temperatures up to 400 °C [54]. In turn, LFP exhibits high cyclic-stability.

Recently, LFP as a transition-state salt of Li phosphate is proved to have the capability of inserting/detaching a Li⁺ between each iron atom, reversibly. This has become the most attractive electrode material for Li⁺ extraction [55,56]. The reaction equations for LFP in the extraction process of Li⁺ is shown in Equation (3).

$$LiFeP{O}_4\leftrightarrow FeP{O}_4+L{i}^{+}+e^{-}$$

(3)

Based on the possibility of crystal-phase transition, Pasta et al. pioneered the use of LFP as an electrode material for selective Li⁺ extraction from seawater [31]. Through a single charge–discharge cycle, the battery can convert sodium-rich brine (Li⁺:Na⁺ = 1:100) into Li-rich (Li⁺:Na⁺ = 5:1) solution. As a comparison, the authors calculated the energy consumption of the electrochemical-Li-extraction process to be only 33 W h mol⁻¹ and exhibited low energy-consumption, based on the method proposed by Kanoh et al., and which is higher than the previously reported literature. However, Ag was used as the anionic material in Pasta’s experiment, which lead to an expensive cost, and the presence of Cl⁻ in seawater accelerated the irreversible precipitation of the cathode electrode. Therefore, in order to reduce the cost and improve the stability of the reaction, Zhao et al. used FePO₄ instead of Ag as the anionic material, to extract Li⁺ [48]. Due to the distinct layered structure in FePO₄, Li⁺ can be easily inserted into the FePO₄ electrode from the Li-containing reserve solution. In addition, due to the reversible redox reaction of Li⁺ detached from LFP materials and Li⁺ inserted into FePO₄ materials, the battery voltage was successfully reduced, and the theoretical cell voltage of the cell LFP/FePO₄ was calculated as zero. The result shows that the inserted capacity of lithium could reach 41.26 mg·(1 g LFP)⁻¹ in the pure lithium solution, which is 93.78% of its theoretical value. To solve the worldwide problem of Li extraction from brines with high magnesium-to-Li ratios, Zhao’s group also used LFP/FePO₄ as the electrode materials to extract Li [57]. Figure 8 shows the Li-extraction efficiency under different Mg²⁺:Li⁺ ratios (=10, 20, 30, 60) using electrochemical methods. The results show that even in the high-Mg^2+-content solution (Mg²⁺:Li⁺ = 10), as long as the low voltage (below 0.75 V) was controlled during the Li extraction process, the extraction rate could still be guaranteed up to 83%.

Although LFP/FePO₄ could be used as an electrode material for efficient Li acquisition, current studies have focused on high-Li-concentration brines and Li and Na concentrations ranging from 0.001 to 1. However, real seawater with a very low Li concentration has rarely been tested. Therefore, on the basis of using FePO₄ material, Liu et al. plated a layer of hydrophilic TiO₂ coating on the electrode surface to reduce the intercalation overpotential, further improving the Li-extraction efficiency in seawater with an extremely low Li/Na ratio [34]. Even when the initial Li⁺/Na⁺ ratio reached 1.6 × 10⁻³, the recovery ratio of lithium to sodium reached more than 50:1 in 10 cycles. Meantime, the hydrophilic TiO₂ coating increased the contact area between the electrode and electrolyte. In 10 Li-acquisition cycles, the material could still achieve a 1:1 Li-to-sodium ratio with a molar-ratio selectivity of up to 1.8 × 10⁴. At the same time, their team introduced the pulse-rest method and pulse-rest-reverse method in the test, to improve the stability of the electrode structure and prolong the life of the electrode. This is because the hydrophilic interface-coating acts as a barrier, preventing Na⁺ from invading the electrode.

Moreover, the carbon-material coating on LFP could be also very important. Armand et al. found that carbon material always grew on the surface of LFP when it was coated in a reducing atmosphere, thus setting a precedent for carbon-coated LFP [58]. Sara et al. adopted an almost symmetrical battery design, and conducted Li-extraction experiments with LFP/Li_0.25FePO₄ electrodes [59]. The SEM image after the reaction showed that the carbon coating on the surface of the LFP electrode was almost non-dissolved, due to the existence of the symmetrical electrode. They found that the dissolution of LFP electrodes in water was one of reasons for electrode capacity-fading. Several cycles of Li-extraction experiments were carried out on LFP loaded with or without carbon coating, and from SEM images of two, it could be seen that the electrode with carbon coating effectively slowed down the dissolution of the LFP electrode, so as to ensure electrode Li-extraction efficiency. Meantime, this was the first reported capacity retention: from 79% (without dilution) to 89% (with dilution) after 50 cycles.

3.1.2. LiMn₂O₄-Based Materials

LiMn₂O₄ (LMO) has a spinel structure with the space group Fd3¯m, where its structure can also be understood as Li and Mn occupying the 8a tetrahedral and 16d octahedral positions of cubic dense-row oxygen ions in Figure 9, respectively [60]. At the same time, due to the highly stable structure of its edge-sharing octahedral Mn₂O₄, LMO has a series of intersecting tunnels formed by the face-sharing of tetrahedral Li (8a) sites and empty octahedral (16c) sites. This tunneling allows three-dimensional diffusion of Li, which in turn generates λ-MnO₂. Therefore, LMO-based materials are widely used as another common electrode material for Li⁺ extraction from seawater.

As early as the 1990s, Kanoh et al. first reported the research on the extraction of Li⁺ from seawater using electrochemical methods [35]. In their study, spinel-type manganese oxide (λ-MnO₂) was used as the WE and Pt as the CE. The Li^+- enrichment process was completed in various solutions, which examined the feasibility of recovering Li from geothermal water. In the selection of electrodes, Leandro et al. used an electrode system composed of LMO/λ-MnO₂ and polypyrrole (PPy) to successfully realize the recovery of LiCl [61]. The electrode system remained stable in the Li⁺ recovery test ,over 200 cycles. The crystal structure of LMO limited the insertion of Na⁺, so that the recovery rate of LiCl reached more than 50%. Compared with the Pt electrode, the use of PPy materials also had a reduced cost. However, in practical applications, LMO had the disadvantages of fast capacity-decay and low rate, due to the dissolution of Mn [62,63] and the Jahn–Teller effect [64]. To solve these problems, Xu et al. created a novel Li-ion battery, and LMO was used as the cathode material and the anode material was Li_1−xMn₂O₄ (0 < x < 1) [65]. In the Pourbaix diagram of the Li-Mn-H₂O system, the excellent cycle performance of the EID system was demonstrated; the recovery rate of Li⁺ reached 83.3%, and the dissolution rate of Mn per cycle was < 0.077% (Figure 10a,b). After one hundred cycles, the material’s intercalation capacity remained at 91%, compared with the initial capacity, in Figure 10c. Meantime, after 100 cycles, the peak area-ratio of Mn⁴⁺ to Mn³⁺ in high resolution Mn 2p was approximately 1, indicating that the anode electrode always maintained lithium saturation and high lithium-acquisition stability, in Figure 10d.

For the purpose of avoiding manganese dissolution, Guan et al. used atomic layer deposition to deposit nanoscale Al₂O₃ on LMO for the first time [62]. Compared with the original electrode, the coated-cathode material significantly improved the cyclability and avoided the dissolution of LMO. It could be seen that cathodes coated with Al₂O₃ exceeded the capacity of bare cathodes after 44 cycles. In the selection of coating materials, there was a recent report of Al₂O₃-ZrO_2-coated LiMnO₂ [66]. In this report, an Al₂O₃-ZrO₂ coating of the LiMnO₂ (AlZr-LMO) electrode was prepared and operated for the recovery of Li⁺. The Al₂O₃-ZrO₂ hybrid film on LMO effectively improved surface stability, increased active centers and reduced electrode polarization. Compared with the single LMO-electrode system, AlZr–LMO demonstrated evident electrochemical stability and cycle life. After 30 consecutive cycles, the extraction capacity of Li⁺ increased from 29.21% to 57.67%.

3.1.3. Multi-Component Composite Materials

Although LFP and LMO materials are currently the mainstream choices for Li extraction, there are other materials with excellent Li-extraction performance. In all, the overall trend of electrode materials is to be cheap and stabilized. LiNi_0.5Mn_1.5O₄ (LNMO), is a cubic spinel structure, and this nickel-substituted spinel-type LNMO has higher capacity and operating voltage than pure LMO, in Figure 11a [67]. LNMO involves two redox reactions of Ni²⁺/Ni⁴⁺ and Mn³⁺/Mn⁴⁺ in the process of Li extraction, which accelerates the intercalation/detaching of Li [68]. Based on the above characteristics, Lawagon et al. designed a recycling device with Li_1−xNi_0.5Mn_1.5O₄ as the Li⁺ capture electrode; the advantage of this electrode was that it could effectively distinguish Li⁺ from the Na⁺, Mg²⁺, K⁺ and Ca²⁺ in seawater, and released it into the electrolyte in the form of LiCl [69]. Meantime, in the continuous cycle, the charge of ΔE during the cycle (Figure 11b) could be ignored, and the adsorption capacity reached 1.259 mmol/g by optimizing the conditions in Figure 11c,d. Zhu’s group extracted Li using an asymmetric hybrid capacitor composed of a heterostructured Li-rich cathode and nano-bismuth anode [70]. In their research, a heterostructured Li_1.16Mn_0.6Ni_0.12Co_0.12O₂ material (LSNCM) was designed as a Li insertion-electrode, which could improve the diffusion of Li ions and the stability of the layered structure by destroying the long-range order of the Mn Jahn–Teller distortion inside the particles. Nanocrystalline bismuth (ncBi) was paired with LSNCM as a Cl⁻ storage electrode to construct an asymmetric Li capacitor. The LSNCM/ncBi system reached 99% Li-recovery in optimal conditions (I = ±0.75 mA). Recent study reported a novel material from the layered cobalt oxide family that can achieve highly selective Li extraction by structural design [71]. The layered-cobalt-oxide family had a large difference in stabilization energies between Li and Na, with favorable selectivity for Li. (NaLi)_1−xCoO₂ was one of the representative substances, and was significant in governing a high Li-selectivity with stable co-intercalation, because the existence of the core Li phase and the shell limited the expansion of the layer. The (NaLi)_1−xCoO₂ /Ag electrode achieved a recovery solution with a Li:Na ratio of 7.6:1 in low-Li-concentration solution (Li:Na = 1:20,000). And

Table 1. Comparison of electrochemical Li recovery methods.

Source	Electroactive Materials	Energy Consumption (Wh/mol)	Extraction Capacity (mg/g)	Reference
Simulated brine	λ-MnO2/Ag	3.07	10.1	[33]
Simulated brine	LMO/L1−xMO	18	22	[50]
Simulated seawater	LMO/L1−xMO	18.6	21	[50]
Salt Lake brine	LiMn2O4/Li1−xMn2O4	16	15-16	[65]
Simulated brine	LNMO/Ag	13.32	8.74	[69]
Simulated brine	L1−xN1/3C1/3M1/3O2/Ag	2.6	10.83	[72]
Simulated brine	PPy/Al2O3/LiMn2O4	1.41	12.84	[73]
Simulated brine	λ-MnO2/LiMn2O4-Pt	23.38	75.06	[74]
Simulated brine	LiNi0.038Mo0.012Mn1.95O4	7.91	14.4	[75]
Simulated Brine	λ-MnO2/BiOCl@PPy	1.007	10.88	[76]

showed comparison of electrochemical Li recovery methods in simulated brine/seawater.

3.2. Counter Electrodes

In addition to the efficiency of electrochemical Li extraction, other factors such as energy consumption and equipment cost of Li extraction should also be comprehensively considered. Therefore, there are also requirements for the selection of CEs. In this part, we comprehensively summarize the researchers’ exploration opinions of different pairs of electrodes.

3.2.1. Metal Materials

Pt electrode, as the earliest reported CE for Li extraction by Kanoh in 1993, was selected as CE for a period of time [35]. However, as an inert metal, only the electrolysis reaction occurs on the electrode surface during the Li-extraction process, which increases the energy consumption of the reaction. In order to avoid the electrolyzed water reaction on the CE, some CEs that can capture anions have been developed, and the Ag/AgCl electrode is a typical representative. The difference with inert electrode is that Ag can combine with Cl⁻, which means that the Ag/AgCl electrode has a stable redox-potential. Pasta et al. used Ag as a CE to capture Cl⁻ in 2012, and LiFePO₄/Ag showed high selectivity for Li [31]. Moreover,, it could convert a sodium-rich brine (Li:Na = 1:100) into a lithium-rich solution (Li:Na = 5:1). In addition, Jaehan and Lawagon et al. completed Li recovery using Ag as a CE in 2013 and 2018. Jaehan et al. constructed λ-MnO₂/Ag [37], which could produce 96.4% pure Li⁺ from brine by expending 2.60 W·h mol⁻¹ Li⁺. In addition, Lawagon used Ag CE with a Li_1−xNi_1/3Co_1/3Mn_1/3O₂ electrode, consuming 1.0 W h mol⁻¹ of the Li⁺ recovered [72]. In these studies, the high energy-consumption of water electrolysis was perfectly avoided. Nevertheless, Ag is a kind of noble metal as well, which limits the application and development of this kind of electrochemical Li-extraction system. As a result, other more economical CE systems are gradually being developed. The Zn electrode has a cheap price compared with Ag material, and also has a relatively stable redox-potential, which is attracting attention from researchers. In 2017, Kim et al. built a LiMn₂O₄/Zn system to realize the selective recovery of LiCl (Figure 12) [77]. In one recovery cycle, there was no side reaction and weight loss in the whole process, due to the reversible redox of the Zn electrode.

3.2.2. Non-Metallic Material

The activated carbon electrode (AC) has the characteristics of low price and excellent chemical stability, suitable for a variety of brine solutions. Based on the above advantages of AC electrodes, Kim et al. replaced the Ag with AC materials, and designed a λ-MnO₂/AC electrochemical Li-extraction system [43]. The system had lithium selectivity similar to that using silver CE, and showed low energy-consumption (4.2 W h mol Li⁻¹). Eunhyea et al. used LMO electrodes and AC to recover Li from practical solutions [78]. This work established a simple electrochemical-separation step for the direct extraction of elemental Li. The maximum lithium-recovery capacity was 3.51 mmol g⁻¹, and the energy consumption was 13.6 kJ mol⁻¹. In the study with AC as the CE, multiple cycle experiments showed that there was no significant decrease; the electrode exhibited high selectivity to Li⁺ (Figure 13) and had excellent cycle-performance. However, the biggest problem with AC materials is that, compared with other CE materials, AC can only capture a small amount of Cl⁻ and cannot completely prevent the water electrolysis reaction. Zhao’s work had successfully constructed the AC Li-extraction system [43,75]. However, limited by the battery capacity, five times the volume of the AC material was required to achieve the same amount of Li extraction as other counter materials. Chloride reversible PPy is another common CE material capable of absorbing chloride ions to counteract excess positive charges while an oxidation reaction occurs [79]. Calvo developed the LMO/PPy electrode to achieve LiCl recovery [61]. In Calvo’s work, the potential during the reaction was below 1 V, and the Li-recovery efficiency remained at 50% after 200 cycles. Polyaniline (PANI) is also considered as a CE that can complete Li recovery with high efficiency. Zhao et al. reported that the PANI/Li_xMn₂O₄ system could recover Li⁺ from brine containing high-impurity cations (K⁺, Na⁺, Mg²⁺) [80]. The average current-efficiency of the electrode reached 95%, and the electrode capacity remained at 70.8% after more than 200 cycles. In addition, nickel hexacyanoferrate (NiHCF), a novel open-framework material with a Prussian blue crystal-structure serves as a candidate material. The results showed that the use of low-cost, environmentally friendly NiHCF as an anion-trapping electrode had similar energy consumption as when silver was used as the Cl-capture electrode, and could greatly shorten the time required for Li extraction.

4. Conclusions and Prospect

In recent years, many works have been widely used in new-energy storage and conversion [81,82,83]. The electrochemical Li-recovery technology has also been proved to be effective in the extraction of Li⁺ from brine and seawater, and can significantly improve the productivity of Li⁺, with low energy-consumption and an environmentally friendly and simple operation. Therefore, the electrochemical extraction of Li needs further research. In this review, the mechanism and systems of electrochemical Li extraction were summarized, and some representative works on electrochemical Li-extraction were systematically discussed. Although many works have been applied in optimizing the electrochemical Li-extraction process, there are many approaches to be further studied.

4.1. Electrochemical Li-Recovery Systems

To achieve highly efficient electrochemical-Li-recovery from brine/seawater, many electrochemical Li-recovery systems have been designed. However, due to the different ion concentrations in brine/seawater, this requires us to choose the right Li-acquisition method and design different reaction systems. Currently, to reduce energy consumption, salt-capture cell systems and hybrid capacitor systems offer greater advantages, by not requiring the use of expensive ion-exchange membranes. However, industrialized production also requires us to understand the impact of some more specific parameters, such as brine flow, electric-field distribution, distance between electrodes and membrane, and the energy efficiency and cycle life of the system. In addition, it is also important to optimize the voltage, brine concentration and recovery-solution concentration of the electrochemical Li-extraction system to obtain the best recovery conditions for the specific situation.

4.2. Electrode Materials

In addition to optimizing the electrochemical Li-recovery system, the design of electrode materials is also important to achieve efficient Li acquisition. In addition to selecting environmentally friendly and cheap electrode-materials, electrochemical Li-acquisition requires electrode materials with high Li-capture- and release-capacity and high stability. However, the high Li-capture-capacity easily leads to the lattice expansion of electrode materials, which will affect their activity and stability. Therefore, we are required to design various strategies to optimize the WE. Moreover, optimization of the CEs is also essential. Meantime, due to the highly corrosive brine/seawater, electrode stability will be seriously affected. On the one hand, magnesium ions with a similar ionic radius and properties will affect the embedding/de-embedding of Li ions in the WE. On the other hand, other metal ions may damage the structure of the WE, reducing the stability of the reaction. Therefore, a reasonable electrode material needs to be designed and chosen to achieve efficient Li-acquisition.

In summary, with the development of advanced technologies, characterizations and relevant simulation calculations, we could deeply understand the mechanism of electrochemical Li-acquisition in brine/seawater and further design effective electrochemical Li-acquisition systems. In addition to improving Li recovery and selectivity via optimizing the electrochemical Li-recovery systems and electrode materials, further consideration should be given to the utilization and development of other associated resources, to develop sustainable Li-extraction technology from brine/seawater.