Electrochemical lithium recovery process via state-of-charge control (SoC) for efficient lithium recovery from concentrated seawater

Article information

Environmental Engineering Research. 2025;30(3)

Publication date (electronic) : 2024 October 9

doi : https://doi.org/10.4491/eer.2024.440

Department of Biological and Chemical Engineering, College of Science and Technology, Hongik University, 2639 Sejong-ro, Sejong-si 30016, Republic of Korea

^†Corresponding author: E-mail: ljh0322@hongik.ac.kr, Tel: +82-44-860-2514, Fax: +82-44-866-6940, ORCID: 0000-0002-8191-1396

Received 2024 July 18; Revised 2024 October 8; Accepted 2024 October 8.

Abstract

With the increasing demand for lithium driven by its widespread use in various industries, researchers are exploring ways to expand the lithium supply. This study proposes an electrochemical lithium recovery (ELR) system using the state-of-charge (SoC) control for extracting lithium from the actual concentrated seawater from the salt manufacturing process. In cyclic stability tests where the 60% SoC level of the LiMn₂O₄(LMO) electrode was adjusted, the charge-discharge retention rate showed approximately two times higher performance compared to 100% SoC after 30 cycles. In particular, it was observed that the amount of extracted lithium from source water at 60% SoC is 8 times higher, consuming 89% less energy than at 100% SoC. The study also confirmed the physical stability of the LMO electrodes by X-ray diffraction analysis, showing relatively better results at 60% SoC operation. Based on these results, the SoC control operation in the ELR system has the capacity to enhance both the stability and efficiency of lithium extraction from concentrated seawater.

Abstract

Graphical Abstract

Keywords: Concentrated seawater; Electrochemcial lithium recovery; Lithium manganese oxide; State-of-Charge control

1. Introduction

With the growing focus on energy and environmental issues, lithium has become an increasing concern since it provides a key part in green energy technology [1–3]. The widespread adoption of lithium-ion batteries, particularly in electric vehicles, has had a substantial positive impact on the lithium market [4–8]. Consequently, ensuring a reliable supply of lithium is crucial for the advancement and sustainability of green technologies [9].

Despite the increasing demand and limited stocks, researchers have been striving to increase the lithium source [10, 11]. While the majority of the world’s lithium deposits are found in salt lakes, efforts to secure lithium resources have led to extensive exploration of seawater as a potential source [3, 7]. Seawater contains a staggering 16,000 times more lithium (230,000 MT) than salt lakes or ores (14 MT), making it an attractive option [12]. However, the process of extracting lithium from seawater is a complex challenge due to the low concentration of lithium ions and the presence of numerous other cations with similar properties [7, 13–15]. To increase the lithium-ion concentration, the approach involved attempting to extract lithium from concentrated seawater that is a by-product from salt manufacturing companies, which typically contains lithium-ion concentrations between two and one hundred times higher than seawater [16]. These solutions contain ions such as Ca²⁺, Mg²⁺, K⁺, and Na⁺, especially highly concentrated Li⁺ (approximately 20 mg/L) compared to ordinally seawater (0.17 mg/L) [17]. Therefore, the solution could be considered one of the potential lithium sources.

However, the solution is typically discharged or used in food production and discarded approximately 100,000 tons of concentrated seawater annually in South Korea [18]. Therefore, an efficient method of recovering lithium from concentrated seawater is required. Some previous research has proposed absorption methods for recovering lithium from seawater [19–25]. The spinel lithium manganese oxide or analogue adsorbents have been reported to extract lithium from source water via ion exchange [26]. Although these adsorbents have high selectivity for lithium, the method is time-consuming and hard to reuse since manganese ions can be released during acid treatment [27].

Recently, an electrochemical lithium recovery (ELR) system using λ-MnO₂ has been considered a promising technology due to its rapid, energy saving, and eco-friendly characteristics [28, 29]. However, λ-MnO₂ electrodes have limited stability, resulting in rapid electrode degradation from manganese dissolving after repeated charging and discharging processes [30, 31]. In particular, recovery lithium at extremely low lithium concentration significantly reduces the stability of the electrode, so it is a big challenge to extract lithium from concentrated seawater by the ELR system [15, 32]. In this study, we present a method to improve the stability of ELR systems using a λ-MnO₂ electrode to increase the efficiency of lithium recovery from the actual concentrated seawater from the salt manufacturing process (Fig. 1). In order to demonstrate the lithium-ion recovery performance of the LiMn₂O₄ electrode, we employed an Ag/AgCl electrode as the counter electrode due to the high capacity of the Ag electrode (theoretical value: 250 mAh/g), which is greater than that of the LiMn₂O₄ electrode (theoretical value: 148 mAh/g) and its ability to capture chloride ions effectively. By applying the state-of-charge (SoC) level during the operation, we effectively improved lithium-ion selectivity and cycling ability of the LiMn₂O₄ electrode. This approach has significantly enhanced both the stability and efficiency of the system [33–35]. The results of the energy efficiency and lithium-ion selectivity were confirmed by conducting lithium recovery tests in actual concentrated seawater under different SoC level operations. Furthermore, constant current cycling experiments were conducted using the SoC control method for evaluating the system’s stability.

Fig. 1

Scheme diagram of the lithium recovery system in concentrated seawater from a salt manufacturing company.

2. Materials and Methods

2.1. Material Synthesis

For the positive electrode, LiMn₂O₄ (LMO) (TOB New Energy, China) as an active material, carbon black (Super P, Timcal, Switzerland) as a conductive material, and Polytetrafluoroethylene (PTFE, Sigma-Aldrich, USA) as a binder were produced with a mass percentage (wt%) of 80:10:10. The electrode dough was made to have a thickness of 200 μm using a roller compressor and then stored in a vacuum oven at 60°C for 12 hours to remove ethanol and moisture in the electrode Similar to the positive electrode, we mixed the Ag powder, the conductive material Super P, and the binder PTFE at a mass percentage (wt%) of 80:10:10.. The Ag electrode attached to the graphite sheet was charged at 0.5 V for 15 minutes under a NaCl 1.0 M solution condition to form silver/silver chloride.

2.2. Lithium Recovery Tests

For the lithium recovery experiment, the positive and negative electrodes are attached to a titanium plate having a size of 2.0 cm ×2.0 cm using carbon paste and then set so that the distance between the two electrodes is 1.0 cm in a 65 mL volume cell. First, the cell is precharged at 30 mM KCl solution (25 mL), and the solution is exchanged to 25 mL of the actual concentrated seawater (Hanjusalt, Republic of Korea) for the discharge step. Finally, the cell is charged again with 25 mL of recovery solution (30 mM KCl). The concentration of concentrated seawater was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 8300, PerkinElmer, USA), and the concentration changes of cations were analyzed through the result values of ion chromatography (Dionex Aquion, Thermo Fisher Scientific, USA).

2.3. Electrode Analysis

A battery cycler (WBCS3000, Wona tech, Republic of Korea) was used for electrochemical analysis. The LMO electrode has a diameter of 9 mm, the Ag electrode has a diameter of 18 mm, and an Ag/AgCl (KCl sat’) reference electrode was used to assemble an electrochemical three-electrode cell. The cyclic voltammetry (CV) measurement was carried out in the five solutions of LiCl, NaCl, KCl, MgCl₂, and CaCl₂, each at a concentration of 1.0 M. The galvanostatic charging discharging (GCD) tests were conducted using a two-electrode system in actual concentrated seawater to verify the performance and stability of the LMO electrode depending on the voltage range. The changes of crystal structure were analyzed by X-ray diffraction (XRD, Rigaku D-MAX2500, Japan) in a scan range from 0° to 80°.

2.4. Calculations and Analytical Methods

Three equations were used to compare the lithium recovery performance and efficiency of the LMO/Ag ERL system: Capacity (1), Coulomb efficiency (2), capacity retention (3), energy consumption (4)

The capacity was determined through charging/discharging time, current, and weight of the active material as shown in Eq. (1):

(1) Q (mAhg)=Time[sec]×I[A]×1mAh3.6C×1Active material weigh[g]

The coulomb efficiency was calculated by dividing the charging capacity by the discharge capacity using Eq. (2):

(2) Coulombic efficiency(%)=QDischargeQCharge×100

The capacity retention rate was calculated by dividing the capacity of the n^th cycle by the capacity of the first cycle as Eq. (3).

(3) Capacity retention(%)=QnQ1×100

The energy consumption during the lithium recovery system was calculated by the circular integration of the plot. (W is the energy (J), ΔE is the cell voltage (V), q is the amount of charge (C)) as shown in Eq. (4) [36].

(4) W=-∮CΔEdq

3. Results and Discussion

Cyclic voltammetry (CV) was performed to confirm the electrochemical properties of LMO at different cations. Fig. 2 shows the results of CV curves according to the potential change from 0 V to 1.2 V at a scan rate of 0.5 mV s⁻¹. Under LiCl 1.0 M conditions, the two redox peaks attributed to the spinel structure of LMO can be verified with clarity [37–40]. In contrast, the significant peak could not be identified in the other four solutions, indicating that the LMO electrode has a strong affinity for Li ion and is capable of recovering lithium under a cation-mixed electrolyte, as shown in previous research [34].

Fig. 2

Cyclic Voltammetry (0.5 mV s⁻¹) of LiMn₂O₄ (LMO) electrode in 1.0 M of different electrolytes (LiCl, NaCl, KCl, MgCl₂, CaCl₂).

Table 1 shows the chemical composition of concentrated seawater obtained from a salt manufacturing company (Hanjusalt). The lithium-ion concentration is 18 mg L⁻¹, which is about 100 times higher than seawater (0.17 mg L⁻¹); however, the solution is highly concentrated with ions other than lithium. For example, the mass ratios of sodium, potassium, and magnesium ion over lithium ion are 2000, 2570, and 2210, respectively. When comparing the concentrations of other ions focusing on the lithium-ion concentration, the concentrations of other ions were remarkably mixed at a high concentration compared to the lithium-ion concentration.

Table 1

The chemical composition of actual concentrated seawater obtained from the salt manufacturing company (Hanjusalt Co., Republic of Korea).

Fig. 3(a) shows the potential change with charging time for LMO electrodes during the lithium recovery process. The potential curves show that the charging time is similar for 100% SoC and 60% SoC; however, the voltage is gradually increasing at 60% SoC. This result indicates that 60% SoC has a more stable charging process, extending operation time and increasing the amount of lithium recovery. Fig. 3(b) shows the concentration changes of each ion in the recovery solution after the recovery process. The concentration changes of other ions, except lithium, were similar between 100% SoC and 60% SoC. However, focusing on the change in lithium ions, it indicates that using SoC control at 60% (1.99 mM) had a better recovery rate compared to the general method at 100% SoC (0.23 mM). In terms of energy, the energy consumption to recover 1 mole of lithium with 60% SoC is 27 Wh. Compared to 100% SoC (243 Wh/Li⁺mole), this value is about 89% lower, suggesting that SoC control significantly reduces the energy required for lithium recovery. In the case of 100% SoC (cell voltage range: 0 to 1.0 V) during a charging step, an oxygen evolution reaction (OER, 0.817 V vs. NHE at pH 7) may occur more actively compared to 60% SoC level operation, and hydrogen ions generated by the reaction (2H₂O → O₂ + 4H⁺ + 4e⁻) create an acidic condition. In particular, the LiMn₂O₄ electrode would be damaged by an acidic electrolyte as Mn³⁺ convert to Mn²⁺owing to the disproportionation process [41]. As a result, the degradation of the LiMn₂O₄ electrode accelerates under 100% SoC operation, leading to changes in the crystal structure, which decreases the lithium-ion selectivity and reduces the repetitive stability. As shown in Fig. 3(c) , the operation at 60% SoC leads to significantly reduced OER and Mn dissolution compared to 100% SoC during a charging step. This reduction in undesirable side reactions is a key reason why we concluded that the 60% SoC operation is more advantageous for the stability and efficiency of the system. By limiting the SoC to 60%, the side reactions are minimized, reducing the structure change of the LiMn₂O₄ electrode and enhancing the overall lithium recovery performance. Consequently, the SoC control strategy enables more stable and energy-efficient lithium recovery from concentrated seawater.

Fig. 3

(a) The voltage profiles of LMO and Ag electrodes during the electrochemical lithium recovery process in the LMO-Ag system. (b) The concentration changes of the reservoir solution in the electrochemical lithium recovery process with the LMO-Ag system. (c) Schematic diagram of side reactions during a charging step at 100% and 60% SoC level operations.

A galvanostatic charge-discharge method (GCD) was performed with a two-electrode cell in concentrated seawater, and the stability of the electrode was checked during charging and discharging for 30 cycles. Fig. 4(a) shows the results of the experiment under general conditions (100% SoC) as a charge/discharge curve. When it was 100% SoC, the experiment was performed at 0.05 A/g from 0 V to 1.0 V. The discharge capacity was 45.7 mAh/g in the first cycle, however, the capacity decreased sharply to 10.8 mAh/g after 30 cycles.

Fig. 4

The voltage profile during 30 cycles in a λ-MnO₂ – Ag cell (a) 100% SoC level (b) 60% SoC level (current density: 0.05 A/g) and (c) the battery cell voltage (V) vs. specific capacity (mAh/g) at 30^th charge-discharge cycle.

Fig. 4(b) shows the 60% SoC result. The 60% SoC control was performed in a voltage range from 0 V to 0.8924 V, using a voltage value when the experimental value was 52.5 mAh/g obtained at 100% SoC. The discharging capacity was 40.5 mAh/g in the first cycle and decreased to 16.3 mAh/g after 30 cycles. Although the capacity decreased in both cases, comparing the capacity difference between the first and last cycles showed that 60% SoC (24.2 mAh/g) reduced the capacity difference less than 100% SoC (34.9 mAh/g). This indicates that 60% SoC provides better cyclic stability, demonstrating that 100% SoC causes more damage to the electrode and less stability than 60% SoC.

Fig. 4(c) shows the specific capacity of the electrodes of SoC 100% and SoC 60% after 30 cycles of charge and discharge in concentrated seawater. Generally, SoC 100% usually shows a higher specific capacity because the battery can store more energy due to the longer charging time, however, this figure shows that SoC 60% has a higher capacity. This can be attributed to the fact that the overall capacity of SoC 100% has decreased over multiple charge and discharge cycles, and the multiple side reactions that occur at full charge have further reduced the capacity of the battery. Furthermore, the average coulombic efficiency at 100% SoC was 88.7%, whereas at 60% SoC, it recorded a higher average value of 98.5%. These results indicate that maintaining a 60% SoC contributes to reduced side reactions and improved stability.

The crystal structure of the LMO electrodes before and after 30 cycles was analyzed using X-ray diffraction (XRD) to confirm the changes in electrode structure, and the results are shown in Fig. 5. After 30 cycles were performed under the same conditions as in Fig. 4, surface analysis of pristine, 100% SoC, and 60% SoC was conducted. LiMn₂O₄ XRD analysis from 100 to 800 showed characteristic peaks of [111], [311], [222], [400], [331], [511], [440], and [531] due to the spinel structural characteristics [42, 43]. Comparing the 511 and 440 peaks between 58° and 66° with pristine, there was a noticeable change in the peak positions under full charge/discharge conditions (100% SoC). However, after controlling the SoC level, the peak of the electrode did not change significantly compared to pristine electrode. According to the results of our previous studies and the experiment, controlling the SoC level improves the stability of the LMO electrode for lithium recovery from concentrated seawater.

Fig. 5

XRD patterns of LiMn₂O₄ electrodes before cycling and after 30 cycles for different SoC levels (100% and 60%).

4. Conclusions

This research developed a process technology to enhance the stability of lithium recovery from concentrated seawater using an ELR system by incorporating state-of-charge (SoC) control. The feasibility of lithium recovery was verified under the actual concentrated seawater conditions, where various ions are present in high concentrations. In the lithium recovery tests, the 60% SoC configuration selectively recovered more lithium and consumed 89% less energy per mole of lithium recovered (27 Wh/Li⁺mole) compared to the 100% SoC configuration (243 Wh/Li⁺ mole). Further stability analysis by GCD results showed superior cyclic stability and higher capacity at 60% SoC. When comparing the average Coulombic efficiency over 30 charge and discharge cycles, the 60% SoC maintained a remarkable efficiency of 98.5%, compared to 88.5% for the 100% SoC. Furthermore, the XRD analysis revealed that the 60% SoC minimized structural changes in the LMO. According to these results, the SoC control operation in the ELR system has the potential to improve both the stability and efficiency of lithium recovery from concentrated seawater.

Acknowledgments

This work was supported by 2024 Hongik University Innovation Support Program Fund, Republic of Korea; 2024 Hongik University Research Fund, Republic of Korea.

Notes

Author Contributions

H.P. (PhD student): Investigation, Writing – Original draft. J.L. (Professor): Supervision, Resources, Writing – review & editing.

Conflict-of-Interest Statement

The authors declare that they have no Conflict of interest.

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Ions	ppm (mg L⁻¹)	Mole (mol L⁻¹)	Mass ratio [M]/[Li]	Mole ratio [M]/[Li]
Li⁺	18	0.0026	1	1
Na⁺	36,000	1.57	2000	604
K⁺	46,200	1.18	2570	454
Mg²⁺	39,700	1.63	2210	447
Ca²⁺	19,300	0.48	1070	185