New Insights into the Application of Lithium‐Ion Battery Materials: Selective Extraction of Lithium from Brines via a Rocking‐Chair Lithium‐Ion Battery System

Abstract Lithium extraction from high Mg/Li ratio brine is a key technical problem in the world. Based on the principle of rocking‐chair lithium‐ion batteries, cathode material LiFePO4 is applied to extract lithium from brine, and a novel lithium‐ion battery system of LiFePO4 | NaCl solution | anion‐exchange membrane | brine | FePO4 is constructed. In this method, Li+ is selectively absorbed from the brine by FePO4 (Li+ + e + FePO4 = LiFePO4); meanwhile, Li+ is desorbed from LiFePO4 (LiFePO4 − e = Li+ + FePO4) and enriched efficiently. To treat a raw brine solution, the Mg/Li ratio decreases from the initial 134.4 in the brine to 1.2 in the obtained anolyte and 83% lithium is extracted. For the treatment of an old brine solution, the Mg/Li ratio decreases from the initial 48.4 in the brine to 0.5 and the concentration of lithium in the anolyte is accumulated about six times (from the initial 0.51 g L−1 in the brine to 3.2 g L−1 in the anolyte), with the absorption capacity of about 25 mg (Li) g (LiFePO4)−1. Additionally, it displays a great perspective on the application in light of its high selectively, good cycling performance, effective lithium enrichment, environmental friendliness, low cost, and avoidance of poisonous organic reagents and harmful acid or oxidant.

In recent years, the increasing demands for portable electronic devices, hybrid and plug-in electric vehicles, and storage of electricity from wind/solar energy promote the development of Li-ion batteries. Meanwhile, the production of lithium and its compounds for lithium-ion batteries has attracted more and more attention in the global lithium industry. [1][2][3] Salt lake brines are considered as valuable resources having great potential for the development of the lithium industry in the operated by an external circuit. But the cycling performance of LiMn 2 O 4 battery is much more excellent than that of the traditional absorption process. So, if we use the brines instead of the lithium electrolyte, it is possible to develop a new method for lithium extraction based on this battery's principle.
In theory, any cathode or anode material that can be used in rechargeable lithium-ion batteries can possibly be used for the lithium extraction from brines. Figure 1 and Tang et al [14] show the main electrode materials that can be used for aqueous lithium-ion batteries. Taking into consideration the stability, easy preparation, low cost, and environmentally friendly of LiFePO 4 , it can be chosen as the electrode materials for lithium recovery. More importantly, recent research studies have verified that LiFePO 4 has a great cycling performance in aqueous lithium-ion batteries and its capacity retention is about 90% after 1000 cycles. [13] Additionally, to reduce the cell voltage, FePO 4 , which is prepared by Li + deintercalation from LiFePO 4 , can be used as the cathode, because the reaction of Li + deintercalation and intercalation from/into the LiFePO 4 /FePO 4 structure is a reversible redox couple (Li + + e + FePO 4 ↔ LiFePO 4 , E ⊖ = 0.45 V (vs SHE)), so the theoretical cell voltage of the cell LiFePO 4 /FePO 4 is zero.
The schematic of the electrolytic cell for lithium extraction from brines is shown in Figure 2. Based on this electrochemical system of "LiFePO 4 (anode) | supporting electrolyte | anion-selective membrane | brine | FePO 4 (cathode)," Li + can be selectively absorbed from the brine in the cathode chamber (Li + + e + FePO 4 = LiFePO 4 ); meanwhile, Li + is desorbed to the anode chamber from LiFePO 4 (LiFePO 4 − e = Li + + FePO 4 ). The anion-exchange membrane prohibits the passage of Li + from the anode chamber to the cathode chamber, so Li + can be enriched in the supporting electrolyte. At the end of a cycle, exchanging the negative and positive electrodes and then restarting the electrolytic process, lithium can be extracted continually from the brine and be enriched in the anolyte (filled with a supporting electrolyte).
Different from the traditional electrolytes of Li-ion batteries (in nonaqueous lithium-ion batteries, the electrolyte is usually an LiPF 6 solution with a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate; and for the aqueous lithiumion batteries, it is always a pure lithium salt solution, such as LiNO 3 , Li 2 SO 4 , and LiCl); however, salt-lake brines contain much higher concentrations of impurities like Na + , Mg 2+ , and K + . Therefore, to verify the selectivity of LiFePO 4   in the 0.5 mol L −1 LiCl solution, and it corresponded to the deintercalation and intercalation of Li + ion from/into the spinel LiFePO 4 /FePO 4 structure ( Figure 3a). While in the 0.5 mol L −1 NaCl solution, except an anodic peak situated at 0.29 V (vs saturated calomel electrode (SCE)) which matched the deintercalation of Li + from LiFePO 4 structure in the first anodic sweep process, the other redox peaks reflected the de-intercalation/intercalation of Na + from/into the NaFePO 4 /FePO 4 structure ( Figure 3b). In the cases of 0.5 mol L −1 KCl and 0.5 mol L −1 MgCl solutions, except an anodic peak of Li + de-intercalation from LiFePO 4 was found, no K + and Mg 2+ de-intercalation and intercalation peaks were discovered (Figure 3c,d).
Comparison with the intercalation behaviors of Li + , Na + , K + , and Mg 2+ , it can be seen that K + and Mg 2+ almost cannot insert into the FePO 4 structure and Na + is much more difficult than Li + , because the cathodic intercalation potential of Na + is much lower than that of Li + . This phenomenon just reveals the embodiment of the ion-sieve effect, i.e., (1) FePO 4 is prepared from LiFePO 4 by an Li + deintercalation process, so the vacant site of FePO 4 is just suitable for the cation whose ionic radius is close to that of Li + . The ionic radii of Na + (1.02 Å) and K + (1.38 Å) are bigger than that of Li + (0.76 Å), so it is difficult for them to insert. (2) Although ionic radius of Mg 2+ (0.72 Å) is smaller than that of Li + , the vacant sites are fit for the monovalent cation, so Mg 2+ will suffer due to too strong Coulomb repulsion to insert into it.
To further investigate the different intercalation and deintercalation behaviors between Li + and the impurity ions, chronopotentiometry was used (Figure 4). Consistent with the results of cyclic voltammetry shown in Figure 3, FePO 4 shows a high absorption selectivity for lithium ions and the anodic intercalation sequence is Li + >> Na + > K + > Mg 2+ (Figure 4a). Conversely, the cathodic deintercalation sequence is Li + < Na + < K + < Mg 2+ (Figure 4b). In fact, it has a positive effect on the cycling performance, because if some impurity ions are unfortunately intercalated into the FePO 4 structure, it can be easier to be deintercalated from the structure, rather than block the channel and hinder the movement of Li + in LiFePO 4 and/or FePO 4 .
To investigate the feasibility of this new lithium recovery method, the Yiliping salt lake brine (chemical compositions Li 97.5 mg L −1 , Mg 13.1 g L −1 , K 3.5 g L −1 , Na 100.4 g L −1 , Mg/Li = 134.4) was treated via an electrochemical separation system "LiFePO 4 (anode) | NaCl | anion-selective membrane | Yiliping brine | FePO 4 (cathode)," and the results are shown in Figure 5. It can be seen that the recovery ratio of lithium increased with the electrolysis time, and it was raised to about 83% after four cycles (Figure 5a). The lithium concentration in the brine was reduced to about 18 mg L −1 ; meanwhile, its concentration in the anolyte was enriched to about 80 mg L −1 . Especially, the Mg/Li ratio was decreased from the initial 134.4:1 in the brine to 1.  Based on the experiences shown in Figure 5, a membranestacked electrolytic bath was designed for the extraction of lithium from an old brine solution of China West Taijinar (Li 0.51 g L −1 , Mg 24.7 g L −1 , Mg/Li = 48.4), and the device and results are shown in Figure 6. It can be seen that the lithium concentration in the anolyte increased with the cycling, and it reached to about 3.2 g L −1 after eight cycles (lithium was enriched about six times). In addition, the Mg/Li ratio was almost maintained at 0.5-0.6:1, which was much lower than that in the initial old brine solution at 48.4:1. Obviously, the new method expressed an excellent characteristic for Mg/Li separation and lithium enrichment. Figure 7 shows the cycling performance of the lithium extraction system in a mixed electrolyte of Li + 2 g L −1 , Mg 2+ 40 g L −1 , NaCl 116 g L −1 under the conditions of current density 0.3 mA cm −2 , cutoff value 0.2 V. For the first cycle, the absorption capacity of lithium was about 32 mg (Li) g (LiFePO 4 ) −1 , and ≈84% of the maximal absorption capacity was maintained after 50 cycles. In addition, the cell voltage platform was about 0.05 V for each cycle; it was closed to the theoretical cell voltage of 0 V. Meanwhile, we also noticed that lithium absorption capacity faded slightly. We speculated that the reason can be attributed to the swelling and degumming of electrodes. Fortunately, this capacity fading can be ameliorated by adjusting the composition, i.e., the mixing ratio of LiFePO 4 , carbon black, and polyvinylidene fluoride (PVDF), to enhance the agglutinative property of the particle-to-particle and particle-to-carbonfiber cloth.
In summary, an electrochemical system of "LiFePO 4 (anode) | supporting electrolyte | anion-selective membrane | brine | FePO 4 (cathode)" was constructed for selective extraction and concentration of lithium from the brine by using the Li-ion battery materials. It is a promising technology for lithium recovery from solutions in light of its high selectively, environmental friendliness, and low cost (inexpensive materials and easy assembly), and it avoids the use of poisonous organic reagents and harmful acid or oxidant. In addition, 83% lithium was recovered for the Yiliping brine by this method, with the Mg/Li ratio decreasing from the initial 134.4:1 in the brine to 1.   lithium increased to about six times, corresponding with the lithium absorption capacity of about 25 mg (Li) g (LiFePO 4 ) −1 . More importantly, this approach provides a new dimension to establish the other viable, practical, or more efficient electrochemical separation systems for lithium extraction by selecting more suitable battery materials as the positive and negative electrodes.

Experimental Section
Preparation of LiFePO 4 /FePO 4 Electrodes: LiFePO 4 electrode was prepared as follows: (1) mixed LiFeO 4 /C, carbon black, and PVDF thoroughly in N-methyl-2-pyrrolidone at the mass ratio of 8:1:1; (2) coated the prepared mixture onto a carbon fiber cloth (material density was about 40 mg (LiFePO 4 ) cm −2 ); and (3) dried the electrode in a vacuum oven at 100 °C for 10 h. FePO 4 was prepared by using the prepared LiFePO 4 and carbon fiber cloth as the anode and the cathode, respectively. Both of them were placed into an electrolytic cell, which was filled with 0.5 mol L −1 NaCl solution, and a constant electrolysis potential of 1.0 V was applied until the current density was less than 0.05 mA cm −2 .
Experiments for Lithium Extraction from Brines: The prepared LiFePO 4 and FePO 4 electrodes were used as the cathode and the anode, respectively, IONAC MA-3475 was used as the anion-selective membrane. For the extraction of lithium from the Yiliping raw brine solution (chemical compositions Li 97.5 mg L −1 , Mg 13.1 g L −1 , K 3.5 g L −1 , Na 100.4 g L −1 , Mg/Li = 134.4), the size of both electrodes was 6 × 6 cm 2 . The cathode chamber and the anode chamber were filled with 600 mL of the brine solution and 600 mL 0.5 mol L −1 NaCl (supporting electrolyte). For the extraction of lithium from an old brine solution of the China West Taijinar salt lake (chemical compositions Li 0.51 g L −1 , Mg 24.7 g L −1 , K 5.4 g L −1 , Na 54.6 g L −1 , Mg/Li = 48.4), the size of both electrodes was 28 × 16 cm 2 , and the membrane-stacked electrolytic bath comprised five pieces of LiFePO 4 and five pieces of FePO 4 electrodes. The volume of the brine and NaCl solutions was the same (5.5 L). The separation process was carried out under a constant cell voltage of 0.2 V until the current density reached to the cutoff value of 0.05 mA cm −2 . At the end of each cycle, the negative and positive electrodes were exchanged and then the electrolytic process was restarted. All the experiences were performed at room temperature.
Cycling Performance: The lithium extraction system was tested with Abin instrument BT-2000 in a mixed electrolyte of Li + 2 g L −1 , Mg 2+ 40 g L −1 , NaCl 116 g L −1 under the conditions of current density 0.3 mA cm −2 ,