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High-efficiency method for recycling lithium from spent LiFePO4 cathode

The extraction of Li from the spent LiFePO4 cathode is enhanced by the selective removal using interactions between HCl and NaClO to dissolve the Li+ ion while Fe and P are retained in the structure. Several parameters, including the effects of dosage and drop acceleration of HCl and NaClO, reaction time, reaction temperature, and solid–liquid ratio on lithium leaching, were tested. The Total yields of lithium can achieve 97% after extraction process that lithium is extracted from the precipitated mother liquor, using an appropriate extraction agent that is a mixture of P507 and TBP and NF. The method also significantly reduced the use of acid and alkali, and the economic benefit of recycling is improved. Changes in composition, morphology, and structure of the material in the dissolution process are characterized by inductively coupled plasma optical emission spectrometry, scanning electron microscope, X-ray diffraction, particle size distribution instrument, and moisture analysis.


Introduction
LiFePO 4 is one of the main cathode materials in rechargeable lithium-ion batteries in China [1]. However, one of the values of LiFePO 4 , compared with other cathode materials such as Li 2 CoO 2 [2], is that it does not contain high recovery value elements such as Mn, Ni, and Co [3]. Given this, China will have 9,400 tons of scrap LiFePO 4 batteries by 2021 [4]. The lithium content accounts for about 200 tons, and projections of possible lithium scarcity point to clear environmental and economic benefits from lithium recycling in the future [5].
Currently, there are several methods for treating waste cathode materials [6], including the fire method, the wet method, and a combined method [7]. Many companies used high-temperature method to recycle lithium battery cathode material, such as Toxco Inc. (USA), SONY Corp (Japan), and Umicore (Belgium). These methods are rather inefficient for recovery as mostly lithium is lost in metallurgical slag during refinement and is not recycled [8,9]. These high-temperature processes have high energy consumption and produce waste gas and waste residue pollution and have pushed methods toward wet extraction of lithium in the industry and academic research [10].
Through a direct regeneration process, Li et al. [11] obtained high purity cathode material mixture (LiFePO 4 þ acetylene black), anode material mixture (graphite þ acetylene black), and other by-products (shell, Al foil, Cu foil, electrolyte solvent, etc.) from scrapped LiFePO 4 batteries. Subsequently, recycled cathode material mixture without acid leaching is further regenerated with Li 2 CO 3 directly. Batteries using recycled LiFePO 4 confronted many problems, such as safety and cycling stability. Based on the concept of zero waste, Dutta et al. [12] proposed a complete process for the recycling of LIBs, in which the metals and materials as value added products were obtained. Although the leaching rate of Li + ions were 99.9%, the recovery rate of lithium indeed was not high, because the consumption of oxidant H 2 O 2 is relatively high, except that the battery grade Li 2 CO 3 cannot be obtained. Zhang et al. [13] studied the selective recovery of lithium from spent LiFePO 4 batteries. With the help of sodium persulfate (Na 2 S 2 O 8 ), the LiFePO 4 were oxidized to FePO 4 , and more than 99% of Li in the material can be selectively leached in 20 min at room temperature. But the sodium persulfate (Na 2 S 2 O 8 ) that has the chemical properties of highly oxidizable and toxic is expensive, and this method is difficult to realize industrial production.
Yang et al. [14] also proposed a method to extract lithium from spent LiFePO 4 cathode; the wet extraction of Li from LiFePO 4 cathode typically involves H 2 SO 4 and H 2 O 2 to dissolve the cathode material, adjusting pH to precipitate Fe 3+ , filtering the precipitate, and precipitating Li with Na 2 CO 3 solution. This approach is marred by relatively low lithium recovery, excessive use of acid and alkali, and high energy consumption with relatively poor Li 2 CO 3 purity. Economically, the market value is limited without a more efficient method for the recovery of lithium [15].
The method shown herein is based on the wet process [16]. The refined approach studies the structural change of LiFePO 4 by hydrometallurgy to achieve preferred dissolution of lithium, thereby minimizing the reaction volumes and retaining the solid structure of FePO 4 [17].
Careful control of HCl and NaClO additions establishes a chemical equilibrium, and lithium is efficiently extracted from the structure with a higher yield [18]. This dramatically reduces the consumption of acid and alkali [19,20]. Recycling lithium of the mother liquor by extraction technology reduces energy consumption [21]. The unique reverse sedimentation process prepares batterygrade Li 2 CO 3 for the increased economic benefit [22,23]. Keeping in view of stringent environmental regulations, limited natural resources, and energy crisis, adopting recycling will not only protect the environment and pacify the gap between demand and supply but also conserve the natural resources [24,25].

Materials
The waste batteries used in this experiment were provided by the Lithium Battery Power Research Institute of Jiangxi University of Science and Technology. All batteries were discharged in saturated NaSO 4 solution and disassembled by hand [26]. The removed positive electrode was washed with DMC (dimethyl carbonate) and was crushed mechanically with a grinder and screened to obtain LiFePO 4 material, with a mass fraction of lithium in the LiFePO 4 positive electrode material of 3.96% [27]. All the chemicals used in the experiment were analytically pure, and the solutions were prepared with ultrapure water.

Lithium preferential extraction
LiFePO 4 was mixed with water, adding HCl and NaClO slowly over time with heated stirring and careful pH monitoring. Controlled drip rates for HCl and NaClO prevent the production of Cl 2 gas [28,29].

High-efficiency lithium recovery 2.3.1 Purification
The pH of lithium-containing liquid for a certain volume was adjusted to a range of 9-10 with alkali, stirred at 85℃ for 1 h to remove impurities of Fe, Cu, Al, and others in the solution [30]. After filtration, the lithium-containing liquid was purified further by the addition of a sodium salt at a mass ratio of 0.3% and EDTA at a mass ratio of 0.1% as a complexing agent to adjust Ca 2+ to 60 μg mL −1 [31].

Precipitation of lithium
The lithium solution was heated to 70-80℃, and Na 2 CO 3 solution was added to the reaction dropwise to produce sediments of Li 2 CO 3 [32]. This precipitate was vacuum filtered, washed with high purity water several times, and dried under vacuum to obtain high purity battery grade Li 2 CO 3 [33].

Extraction
The solubility of lithium is 1.2-2.0 g L −1 in the precipitated mother liquor and accounts for 20-25% of the total lithium content [34]. Lithium extraction was performed using a mixed extraction agent (P507:TBP:NF = 10:2:1) in this experiment. The residual lithium was less than 50 μg L −1 in the final remaining liquid, giving a recovery rate of 98% [35,36].

Analytical equipment and characterization of materials
The solid samples were digested in concentrated hydrochloric acid for analysis using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher ICAP 7400). X-ray diffraction (XRD, D8 Advance, Bruker, Germany) was used to characterize the relative crystal phases during the experimental process and identify any structural changes in the samples in the reaction [37]. Scanning electron microscope (SEM) (Flex SEM 1000, Hitachi, Japan) was used to image the morphology of the products [38,39]. Particle size and the water content of the lithium products were measured by a particle size analyzer and moisture meter to determine if the Li 2 CO 3 product meets the standards for cathode material synthesis [40,41]. The addition of NaClO oxidizes Fe 2+ → Fe 3+ and facilitates the extraction of lithium from its structure. From simple stoichiometry, the moles of NaClO should equal the moles of lithium extracted, and the moles of HCl should be less than the moles of lithium if Fe 3+ remains were intact. The rate of addition for HCl and NaClO should maintain a pH of 1.5-2; otherwise, Fe 3+ will leach into the solution. Figure 1(a) shows the influence that the amount of NaClO and HCl has on the lithium dissolution rate, and "oxidant/Li and acid/Li molar ratio" expresses the ratio of NaClO and HCl to the molar amount of lithium, respectively. The concentration of Li + ion in solution was measured by ICP. The calculated recovery rate of lithium was 99% using excess NaClO. The recovery rate was 100% with excess HCl; however, unwanted Fe 3+ also dissolved. Figure 1(b) clearly shows that when the molar ratio of HCl and Li is greater than 0.8 (pH ∼ 0.5), the solution rate of Fe 3+ increases linearly and further introduces impurities during the extraction of lithium. "Fe leaching efficiency" refers to the mass of the dissolved Fe ion to total iron in LiFePO 4 , again measured in solution by ICP. Figure 1(c) shows the influence of temperature and time on extraction. Exceeding 60 minutes, the temperature has no effect and the leaching effect of lithium is the best. Figure 1(d) shows that with optimal conditions, the size of the solid-liquid ratio has little influence on extraction above a 1:1 ratio. Figure 1(e) shows the influence of pH during the experiment by HCl addition and the effect on lithium yield. It can be seen from Figure 1(f) that when pH ≤ 2.0 (the value measured with a pH strip and related to the amount of HCl added), marking a small amount of HCl, the lithium yield is greater than 99% [40]. Figure 2 shows the diffraction patterns phase after the extraction of lithium. LFP refers to LiFePO 4 , LFP-1 refers to the slag that NaClO is used only 70% in the process of extracting lithium, LFP-2 and LFP-3 refers to the slag that the addition amount of NaClO and HCl is optimal. The two reflections (121) and (131) are intense peaks seen in LFP, whereas the leached samples show a significant reduction or absence. The outlined sections A and B of complete patterns correspond to Figure 2(a and b). There are new peaks near the (131) peaks in the XRD pattern of LFP-1 and still retains intensity at (131). However, there are no (131) peaks in the XRD pattern of LFP-2/3.

Results and discussion
These patterns indicate that a small amount of LiFePO 4 remains in the LFP-1 but is completely nonexistent in the LFP-2/3 samples. It can be seen that the structure of LiFePO 4 changed in the process of preferential extraction of lithium in the XRD pattern. The main peak (011) and (111) of LiFePO 4 did not change with the dissolution of lithium, indicating that only part of the raw material structure changed in the optimal solution process to realize the preferential extraction of lithium. Figure 3(a-f) shows the morphology of the LiFePO 4 cathode material and FePO 4 residues. It comprises many primary and secondary particles (Figure 3a), and magnified images in Figure 3(b and c) show spherical agglomerates and much smaller spherical primary particles. Figure 3d shows the particle morphology of post-extraction FePO 4 residues. There is an absence of the previously seen secondary agglomerated particles. In Figure 3(e and f), the primary particles are much smaller irregular shapes with some agglomeration. The extraction caused the secondary particles of LiFePO 4 to break apart, possibly explained by the structural stress as Li + ion is extracted and the crystal structure has volumetric changes from LiFePO 4 → FePO 4. Figure 4 shows the XRD diagrams of battery-grade Li 2 CO 3 products; compared with the standard card of Li 2 CO 3 , each peak of the two products is perfectly  consistent with the standard peak, indicating that Li 2 CO 3 -1 and Li 2 CO 3 -2 are both Li 2 CO 3 products. Figure 5 shows the morphology of battery-grade Li 2 CO 3 -1 and Li 2 CO 3 -2 of Figure 4. The lithium-extracted solution was reacted with NaCO 3 after impurity removal. The dropwise addition of the Na 2 CO 3 solution permits particle ripening over nucleation to produce larger robust particles. This can be seen from the two batches of batterygrade Li 2 CO 3 products in Figure 5 (Li 2 CO 3 -1 and Li 2 CO 3 -2). Figure 6 plots the impurity fractions in the Li 2 CO 3 products measured by the ICP analysis. Line A represents the impurity standard for battery-grade Li 2 CO 3 , and Line B and Line C represent the impurity content of two batches of Li 2 CO 3 products respectively. Table 1 presents the purity, moisture content, and particle size ranges for battery-grade Li 2 CO 3 compared with the products from our Li extraction and precipitation methods. The products meet or exceed the standards for Li 2 CO 3 in battery material synthesis.
The lithium concentration ranged from 1.2 to 2.0 g/L in the mother liquid, and this concentration accounts for 20-25% of the total lithium. The extraction of lithium from the mother liquid was performed using a mixed extraction agent (P507:TBP:NF = 10:2:1). Several mother solutions (numbers 1-6) with different lithium content were selected [41], and the residual lithium after extraction was measured with ICP, as presented in Table 2. The remaining lithium content was less than 50 µg/mL in the mother liquor, yielding a 98% recovery [42]. Combining these results with a leaching efficiency near 99% for lithium from LiFePO 4 , the total yields for lithium recycling averaged near 97% [43].

Conclusion
A refined method has been tested for lithium extraction from the peridotite structure of LiFePO 4 by controlling the addition of NaClO and HCl. Mole ratios of NaClO and HCl   to moles of lithium at 1 and 0.8 produced the best leaching with minimized impurities. Compared with the traditional total solution process, the total acid and alkali needed are reduced to improve the economic and environ mental factors in the recycling process. A pH of 1.5-2 gave an optimal dissolution recovery rate (99%) and limited the dissolution of Fe 3+ to below 2%.
High-quality battery-grade Li 2 CO 3 was prepared by extraction using a mixed extraction agent (P507:TBP:NF = 10:2:1) with a recovery that gave a total yield of lithium above 97%. The economic benefits such as the efficiency of energy savings and consumption reduction also increased. This modified method showed an obvious advantage compared to traditional lithium extraction processes. It was helpful to establish recycled lithium as a viable pipeline for future resource and energy demands.