Separation and Efficient Recovery of Lithium from Spent Lithium-Ion

metals

Article Separation and Efﬁcient Recovery of Lithium from Spent Lithium-Ion Batteries

Eva Gerold *, Stefan Luidold and Helmut Antrekowitsch

Chair of Nonferrous Metallurgy of Montuniversitaet Leoben, 8700 Leoben, Austria; [email protected] (S.L.); [email protected] (H.A.) * Correspondence: [email protected]; Tel.: +43-3842-402-5207

Abstract: The consumption of lithium has increased dramatically in recent years. This can be primarily attributed to its use in lithium-ion batteries for the operation of hybrid and electric vehicles. Due to its specific properties, lithium will also continue to be an indispensable key component for rechargeable batteries in the next decades. An average lithium-ion battery contains 5–7% of lithium. These values indicate that used rechargeable batteries are a high-quality raw material for lithium recovery. Currently, the feasibility and reasonability of the hydrometallurgical recycling of lithium from spent lithium-ion batteries is still a field of research. This work is intended to compare the classic method of the precipitation of lithium from synthetic and real pregnant leaching liquors gained from spent lithium-ion batteries with sodium carbonate (state of the art) with alternative precipitation agents such as sodium phosphate and potassium phosphate. Furthermore, the correlation of the obtained product to the used type of phosphate is comprised. In addition, the influence of the process temperature (room temperature to boiling point), as well as the stoichiometric factor of the precipitant, is investigated in order to finally enable a statement about an efficient process, its parameter and the main dependencies.

Citation: Gerold, E.; Luidold, S.; Keywords: recycling; lithium-ion batteries; lithium recovery; precipitation; hydrometallurgy; criti- Antrekowitsch, H. Separation and cal element Efficient Recovery of Lithium from Spent Lithium-Ion Batteries. Metals 2021, 11, 1091. https://doi.org/ 10.3390/met11071091 1. Introduction Lithium differentiates itself from all other alkali metals due to its special properties, Academic Editor: Alexandre Chagnes such as the high energy density by weight and the electrochemical potential of −3.045 V, making this element useful for several applications, including rechargeable lithium-ion Received: 2 June 2021 batteries [1]. The lithium market has developed very dynamically in recent years. One of Accepted: 6 July 2021 Published: 8 July 2021 the reasons can be identified in the high expectations of the industry regarding the field of application of rechargeable batteries and in particular e-mobility. The storage of renewable

Publisher’s Note: MDPI stays neutral energy also plays an important role in this context [2]. Besides these main areas of usage, with regard to jurisdictional claims in lithium is also employed in ceramic and glass manufacture and in aluminum production published maps and institutional afﬁl- (see Figure1)[ 1]. Lithium carbonate (Li2CO3) is one of the most important compounds iations. of this alkali element and is often applied in medicine in addition to the areas already mentioned [3–8]. The consumption of lithium has increased dramatically in recent years [5,9–12]. This is primarily caused by its use in lithium-ion batteries for the operation of hybrid and electric vehicles [13–15]. Further developments in the battery sector, such as lithium-sulfur Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. batteries, continue to lead to an immense consumption of lithium in this area [16,17]. Due to This article is an open access article its speciﬁc properties, lithium will continue to be an essential component for rechargeable distributed under the terms and batteries in the next decades. Therefore, very high growth rates are expected in future conditions of the Creative Commons years. Depending on the scenario, the total demand for lithium is expected to double or Attribution (CC BY) license (https:// even more than triple by 2025 [2]. creativecommons.org/licenses/by/ An average LIB (lithium-ion battery) contains 5–20% Co, 5–10% Ni, 5–7% Li and 4.0/). 5–10% of other metals (Cu, Al, Fe, Cr, etc.) [2,18–21]. These values indicate that used

Metals 2021, 11, 1091. https://doi.org/10.3390/met11071091 https://www.mdpi.com/journal/metals Metals 2021, 11, 1091 2 of 14

rechargeable batteries are a high-quality raw material for lithium recovery. Currently, lithium is produced exclusively from brine [22–28] or minerals [29,30]. Traditionally, the primary extraction takes place from lithium-rich solutions, whereby a previous evaporation of the liquid is necessary [31]. The supply of lithium is currently determined by two countries. Together, Chile and Australia account for almost 80% of global mine funding. In 2025, this situation will shift in favor of Australia, Argentina and Canada. So far, any other resources have been employed to extract lithium, which means that hardly any technologies are available for the recovery from secondary materials. Until now, recycling, and with it the offer from the secondary sector, has not played a signiﬁcant role in the global range of lithium supply [5,11,32,33]. This is caused by large primary resources and reserves and the relatively cheap extraction. The dissipative distribution of lithium and technological requirements of purity for certain applications in the end products also play a major role in this context. Whether, and to what extent, alternative processing technologies for brine and bedrock deposits will be established in the future depends on several factors. In addition to environmental aspects, the long-term stability of the selected processes and production costs play an important role in this framework. It must be noted that lithium production from brine is a time-consuming process when using solar evaporation. Depending on the general conditions, a period of up to 16 months can elapse between brine extraction and further processing of the concentrated solution. Lithium is mainly traded in the form of Metals 2021, 11, x FOR PEER REVIEW 2 of 15 concentrates (spodumene, petalite) and the two most important intermediate products (lithium carbonate and lithium hydroxide) in different qualities [2].

Figure 1. Production volume of lithium by countries for 2020 (left),), and thethe most important application areas of lithium products (rightright).).

AnIn the average ﬁeld ofLIB metal (lithium recovery-ion battery) from used contains lithium-ion 5–20% batteries, Co, 5–10% the Ni, focus 5–7% is mostlyLi and on5– 10%valuable of other materials metals such (Cu, as cobaltAl, Fe, and Cr, nickeletc.) [2,18 [5]. Lithium–21]. These is often values recovered indicate as that a by-product used re- chargeableof poor quality batteries or is are not a retrievedhigh-quality at all.raw Duematerial to the for character lithium recovery. of this element, Currently it, often lith- iumgets is extracted produced in exclusively the last step from of hydrometallurgical brine [22–28] or minerals processes, [29,30]. while Tra itditionally, forms slags the in pri- the maryknown extraction pyrometallurgical takes place methods from lithium and is-rich therefore solutions, lost towhereby the raw a materialprevious cycleevaporation [34]. In ofhydrometallurgy, the liquid is necessary expensive [31]. andThe inefﬁcientsupply of lithium recycling is currently methods determined such as co-precipitation, by two countries.solvent Together, extraction Chile and and electrochemical Australia account processes for almost enable 80% only of aglobal minor mine recovery funding. [35,36 In]. 2025,The very this highsituation salt contentwill shift of in the favor solutions of Australia, and the Argentina low lithium and concentration Canada. So far, often anyprove other resourcesto be problematic have been [34 employed]. Classic to approaches extract lithium, such as which precipitation means that with hardly sodium any carbonatetechnolo- giesexhibit are inadequateavailable for results the recovery due to thefrom high seconda solubilityry materials. and the lowUntil purity now, ofrecycling, Li2CO3 asand a withproduct. it the In offer addition, from the consequential secondary sector, to the temperaturehas not played dependence a significant of role the precipitationin the global rangereaction, of lithium high energy supply consumption [5,11,32,33]. and This the is productioncaused by large of enormous primary amountsresources of and waste re- serveswater resultand the [37 relatively,38]. New cheap approaches extraction. work The with dissipative the precipitation distribution as of lithium lithium phosphate, and tech- with the choice of compounds based on sodium [39]. Nevertheless, the removal of the nological requirements of purity for certain applications in the end products also play a excess sodium is often challenging. However, there is still too few data available in this area major role in this context. Whether, and to what extent, alternative processing technolo- of hydrometallurgy and further research needs to be done. Additionally, the possibility of gies for brine and bedrock deposits will be established in the future depends on several factors. In addition to environmental aspects, the long-term stability of the selected processes and production costs play an important role in this framework. It must be noted that lithium production from brine is a time-consuming process when using solar evaporation. Depending on the general conditions, a period of up to 16 months can elapse between brine extraction and further processing of the concentrated solution. Lithium is mainly traded in the form of concentrates (spodumene, petalite) and the two most important intermediate products (lithium carbonate and lithium hydroxide) in different qualities [2]. In the field of metal recovery from used lithium-ion batteries, the focus is mostly on valuable materials such as cobalt and nickel [5]. Lithium is often recovered as a by-product of poor quality or is not retrieved at all. Due to the character of this element, it often gets extracted in the last step of hydrometallurgical processes, while it forms slags in the known pyrometallurgical methods and is therefore lost to the raw material cycle [34]. In hydrometallurgy, expensive and inefficient recycling methods such as co-precipitation, solvent extraction and electrochemical processes enable only a minor recovery [35,36]. The very high salt content of the solutions and the low lithium concentration often prove to be problematic [34]. Classic approaches such as precipitation with sodium carbonate exhibit inadequate results due to the high solubility and the low purity of Li2CO3 as a product. In addition, consequential to the temperature dependence of the precipitation reaction, high energy consumption and the production of enormous amounts of waste water result [37,38]. New approaches work with the precipitation as lithium phosphate, with the

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further processing by roasting with calcium oxide in order to obtain high-quality LiOH by subsequent leaching in water is exhibited. This processing enables the reuse of this material in lithium-ion batteries; however, due to the high quality standards for the battery production, this technology is currently not supplied [39]. This work is intended to compare the classic method of the precipitation of lithium from synthetic and real leaching liquors gained from lithium-ion batteries with sodium carbonate and with alternative precipitation agents in this area such as sodium phosphate and potassium phosphate. The comparison between a synthetic lithium sulfate solution and a sulfuric acid leaching liquid from the active material of lithium-ion batteries allows the determination of matrix effects in the process step of lithium precipitation by present anions and cations. Furthermore, the correlation of the obtained product to the used type of phosphate should be comprised. In addition to the variation of the process temperature (room temperature to boiling point), a comparison of different molar ratios of the precipitant is conducted in order to ﬁnally enable a statement about an efﬁcient process, its parameters and the correlating dependencies.

2. Materials and Methods 2.1. Sample Preparation and Analysis of the Input Materials To evaluate the selectivity and efﬁciency of lithium precipitation with sodium carbonate, sodium phosphate and potassium hydrogen phosphate, a synthetic lithium sulfate solution (Li2SO4·H2O, >99%, p.a., Carl Roth, Karlsruhe, Germany) with a concentration of 10 g·L−1 lithium was prepared. Additionally, active material from lithium-ion batteries was leached using 2-molar sulfuric acid for four hours at a temperature of 80 ◦C and a solid/liquid ratio of 100 g·L−1. The composition of the active material used was determined by means of sodium peroxide digestion and subsequent analysis by mass spectrometry with inductively coupled plasma (ICP-MS) and is listed in Table1.

Table 1. Composition of the pyrolyzed active material from lithium-ion batteries used for leaching experiments (determined by ICP-MS: mass spectrometry with inductively coupled plasma).

Element Li Mg Al Si Mn Fe Co Ni Cu Zn [Mass-%] 5.2 0.06 3.7 <0.1 7.3 0.01 6.5 22.0 4.8 0.7

Large amounts of cobalt, nickel, manganese, iron and aluminum could be removed from the leaching solution prepared by means of oxalic acid and sodium hydroxide precipitation, leaving lithium as the main component of the residual leachate (pH~9). The measurement was carried out using a pH meter (InLab Science, Mettler-Toledo, Vienna, Austria). The stoichiometric amount of precipitant was calculated based on the concentration of lithium in the leaching solution, which was determined via ICP-MS, whereby the corresponding results can be found in Table2. As shown there, the puriﬁed solution contains approx. 5 g·L−1 of lithium.

Table 2. Composition of the puriﬁed leaching solution after precipitation of Co, Ni, Mn and Al with oxalic acid and sodium hydroxide (measured by ICP-MS).

Element Li Mg Al Si Mn Fe Co Ni Cu Zn [g·L−1] 5.0 <0.01 <0.01 <0.1 <0.01 <0.01 0.03 0.23 <0.01 <0.01

For the further processing, the described precipitation media (Na2CO3 (>99.5%, p.a., ACS, anhydrous, Carl Roth, Karlsruhe, Germany), Na3PO4·12 H2O (>98%, p.a. ACS, Carl Roth, Karlsruhe, Germany)) and also K2HPO4·3 H2O (>99%, p.a., Carl Roth, Karlsruhe, Germany)) were added in solid form to both initial solutions. Metals 2021, 11, 1091 4 of 14

2.2. Design and Evaluation of Experiments The statistical experimental design and evaluation software (MODDE 12.1, Sartorius AG, Goettingen, Germany) was used for the experimental arrangement. Using a full factorial model, eleven individual experiments were carried out for each precipitant, with three center experiments being incorporated for the creation of the statistical model (see Table3). The variation of the temperature between 25 and 90 ◦C and the change of the added amount of precipitant related to the theoretically necessary stoichiometric consumption for the precipitation of the contained lithium (stoichiometric factor between 1 and 3) could thus be realized. The holding period of the precipitation process remained ﬁxed at one hour for the entire test series. The analysis of the residual solutions after vacuum ﬁltration to separate the precipitate was carried out using microwave plasma atomic emission spectrometry (MP-AES), since this measurement method also allows the lithium content in the liquid phase to be determined. With the aid of the statistical test evaluation program, a model could be created for each precipitant and corresponding initial solution. The results are compared with each other in order to define an optimum precipitation process for lithium.

Table 3. Parameters of executed experiments for each precipitation agent (with a ﬁxed holding period for each experiment of 60 min).

Temperature Stoichiometric Factor Experiment No. [◦C] [-] 1 25 1 2 60 1 3 80 1 4 90 1 5 25 3 6 60 3 7 80 3 8 90 3 9 60 1 10 60 1 11 60 1

2.3. Analysis of the Obtained Product In the course of the investigations, recordings of the obtained products could be obtained in a scanning electron microscope. Since no detection of lithium is possible by EDS analysis on the Jeol JSM-IT300 scanning electron microscope, Freising, Germany, which is used for this study, mappings are taken to check the distribution of detectable components such as sulfur and phosphorus. Additionally, X-ray diffraction (XRD) studies of the phosphatic precipitates were performed to determine the phase distribution in the lithium-containing product. The sample was analyzed by X-ray diffraction in parallel conﬁguration on a Bruker D8 Advance instrument, Vienna, Austria, in the diffraction angle range of 0 to 100◦. The step size was set to 0.01◦ with an acquisition time of 0.2 s per measurement point.

3. Results 3.1. Precipitation with K2HPO4

When potassium phosphate (K2HPO4) was used as precipitant, quite satisfactory levels of residual lithium concentrations in the solutions were obtained for certain parameter combinations. In principle, a reduction of the lithium concentration by 80 and 60% starting from the original concentration could be achieved for the synthetic and the authentic starting solution, respectively. For the precipitation process, large amounts of precipitating agent (stoichiometric factor > 2.5) proved to be particularly advantageous, since a considerable part of the lithium already precipitated at low temperatures. This Metals 2021, 11, x FOR PEER REVIEW 5 of 15

Metals 2021, 11, 1091 5 of 14 agent (stoichiometric factor > 2.5) proved to be particularly advantageous, since a considerable part of the lithium already precipitated at low temperatures. This statement applies tostatement both the appliessynthetic to and both the the authentic synthetic initial and thesolution, authentic although initial this solution, effect is although clearly more this pronouncedeffect is clearly in the more former pronounced (see Figure in the 2). former (see Figure2).

Figure 2. Statistical evaluation of the dissolved residual concentrations in mg·L−1 of lithium starting from the synthetic Figure 2. Statistical−1 evaluation of the dissolved residual−1 concentrations in mg·L−1 of lithium starting from the synthetic (left, 10,000 mg·L ) and authentic (right, 5000 mg·L ) leaching solution using the precipitant K2HPO4 as a function of (left, 10,000 mg·L−1) and authentic (right, 5000 mg·L−1) leaching solution using the precipitant K2HPO4 as a function of temperature and the stoichiometric factor of the precipitant. temperature and the stoichiometric factor of the precipitant. The models described for the precipitation process with potassium hydrogen phos- phateThe are models based ondescribed quadratic for modelthe precipitation equations. process These arewith given potassium in Equation hydrogen (1) for phos- the phatesynthetic are initialbased solutionon quadratic and inmodel Equation equations. (2) for These the authentic are given leach in Equation liquid. In (1 this) for case, the syntheticT[◦C] reﬂects initial the solution temperature, and in Equation x [-] the stoichiometric (2) for the authentic factor ofleach the liquid precipitant. In this and case, c(Li) T [°C]the residualreflects the concentration temperature, of lithiumx [-] the instoichiometric the leaching factor liquid of after the precipitation,precipitant and while c(Li) both the residualquadratic concentration and interaction of terms lithium were in considered.the leaching liquid after precipitation, while both quadratic and interaction terms were considered. c(Li) = 5480.8 + 59.5·T + 1349.5·x − 1.3·T2 − 948.8·x2 + 22.5·T·x (1) c(Li) = 5480.8 + 59.5·T + 1349.5·x − 1.3·T2 − 948.8·x2 + 22.5·T·x (1) c(Li) = 3342.8 − 10.7·T + 2033.9·x − 567.4·x2 − 6.1·T·x (2)

The evaluationc(Li) of = the3342 results.8 − 10. for7·T both+ 2033 the.9·x synthetic − 567.4·x and2 − 6 the.1·T·x authentic initial solution(2) provides a comparison of the dependencies on temperature and stoichiometric factor in the form of an interaction plot. Figure3 shows this relationship for the precipitation with K2HPOThe4 ,evaluation with the corresponding of the results for experimental both the synthetic results fromand the the authentic liquid samples. initial solution For the providescenter experiments a comparison (three of perthe testdependencies series), the on precipitation temperature of and lithium stoichiometric with K2HPO factor4 from in thethe form synthetic of an leachinginteraction liquid plot. resulted Figure 3 in shows a mean this value relationship of 4911 for mg the·L− precipit1 for theation residual with −1 Kconcentration2HPO4, with ofthe lithium corresponding in the solution experimental with a standardresults from deviation the liquid of 218 samples. mg·L For. Using the −1 centerthe authentic experiments starting (three solution, per test a series), mean valuethe precipitation of 4254 mg of·L lithiumin terms with ofK2 theHPO lithium4 from theconcentration synthetic leaching could be liquid obtained resulted with in a standarda mean value deviation of 4911 of mg· 88 mgL−1· Lfor−1 .the residual concentration of lithium in the solution with a standard deviation of 218 mg·L−1. Using the authentic3.2. Precipitation starting with solution, Na3PO a4 mean value of 4254 mg·L−1 in terms of the lithium concentration couldIn comparison, be obtained a different with a standard pattern emerges deviation for of the 88 precipitation mg·L−1. of lithium with sodium phosphate, especially considering the synthetic initial solution. The model visualizes a temperature independence; thus, only the stoichiometric factor of the precipitant remains as an influencing parameter. In this way, significant improvements can be made with regard to the energy efficiency of this process. When considering the results from the authentic leaching liquid, a dependence on the process temperature is, nevertheless, evident, albeit much less pronounced than in the case of precipitation with potassium hydrogen phosphate. With a stoichiometric factor of 3, the temperature exerts hardly any influence, while with lower precipitant quantities at least an experimental temperature of 60 ◦C is necessary to enable the efficient precipitation of the alkali metal (see Figure4).

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Metals 2021, 11, x FOR PEER REVIEW 6 of 15 Metals 2021, 11, 1091 6 of 14

Figure 3. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor (interaction plot of the model) based on the synthetic and authentic leaching solution for the precipitation with K2HPO4.

3.2. Precipitation with Na3PO4 In comparison, a different pattern emerges for the precipitation of lithium with sodium phosphate, especially considering the synthetic initial solution. The model visualizes a temperature independence; thus, only the stoichiometric factor of the precipitant remains as an influencing parameter. In this way, significant improvements can be made with regard to the energy efficiency of this process. When considering the results from the authentic leaching liquid, a dependence on the process temperature is, nevertheless, evident, albeit much less pronounced than in the case of precipitation with potassium hydrogen phosphate. With a stoichiometric factor of 3, the temperature exerts hardly any influ-

Figure 3. Relative residual concentrationence, while with of lithium lower in precipitant the samples q asuantities a function at of least temperature an experimental and stoichiometric temperature factor of 60 Figure 3. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor (interaction plot of the model)°C basedis necessary on the synthetic to enable and the authentic efficient leaching precipitation solution of for the the alkali precipitation metal (see with Figure K2HPO 4).4. (interaction plot of the model) based on the synthetic and authentic leaching solution for the precipitation with K2HPO4.

Figure 4. Statistical evaluation of the dissolved residual concentrations in mg·L−1 of lithium starting from the synthetic −1 Figure 4. Statistical−1 evaluation of the dissolved residual−1 concentrations in mg·L of lithium starting from the synthetic (left, 10,000 mg·L ) and authentic (right, 5000 mg·L ) leaching solution using the precipitant Na3PO4 as a function of (left, 10,000 mg·L−1) and authentic (right, 5000 mg·L−1) leaching solution using the precipitant Na3PO4 as a function of temperature and the stoichiometric factor of the precipitant. temperature and the stoichiometric factor of the precipitant. The singular dependencies for the model of the synthetic leaching solution can also be seen in the model equation (see Equation (3)), while both quadratic and interaction terms occur for the authentic leach liquid (see Equation (4)).

2 c(Li) = 420.7 + 3448.7·x − 1185.5·x (3)

c(Li) = 646.3 − 14.2·T + 641.2·x + 0.1·T2 − 186.0·x2 − 1.6·x·T (4)

Since the results for the precipitation of lithium with Na3PO4 starting from a synthetic leaching solution lead to temperature independence, only the interaction plot for the

authentic solution is shown in Figure5. For the center experiments (three per test series), Figure 4. Statistical evaluation of the dissolved residual concentrations in mg·L−1 of lithium starting from the synthetic the precipitation of lithium with Na3PO4 from the synthetic leaching liquid resulted in a −1 −1 (left, 10,000 mg·L ) and authenticmean (right value, 5000 of 3683 mg·L mg) ·leachingL−1 for thesolution residual using concentration the precipitant of Na lithium3PO4 as in a the function solution of with a temperature and the stoichiometric factor of the precipitant. standard deviation of 357 mg·L−1. Using the authentic starting solution, a mean value of

Metals 2021, 11, x FOR PEER REVIEW 7 of 15

The singular dependencies for the model of the synthetic leaching solution can also be seen in the model equation (see Equation (3)), while both quadratic and interaction terms occur for the authentic leach liquid (see Equation (4)).

c(Li) = 420.7 + 3448.7·x − 1185.5·x2 (3)

c(Li) = 646.3 − 14.2·T + 641.2·x + 0.1·T2 − 186.0·x2 − 1.6·x·T (4)

Since the results for the precipitation of lithium with Na3PO4 starting from a synthetic leaching solution lead to temperature independence, only the interaction plot for the authentic solution is shown in Figure 5. For the center experiments (three per test series), the Metals 2021, 11, 1091 precipitation of lithium with Na3PO4 from the synthetic leaching liquid resulted in a mean7 of 14 value of 3683 mg·L−1 for the residual concentration of lithium in the solution with a standard deviation of 357 mg·L−1. Using the authentic starting solution, a mean value of 506 −1 mg·506L− mg1 in· Ltermsin termsof lithium of lithium concentration concentration could could be obtained be obtained with witha standard a standard deviation deviation of −1 12of mg· 12L mg−1. ·L .

Figure 5. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor Figure 5. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor (interaction plot of the model) based on the authentic leaching solution for the precipitation with Na3PO4. (interaction plot of the model) based on the authentic leaching solution for the precipitation with Na3PO4. 3.3. Precipitation with Na2CO3 3.3. PrecipitationFinally, a comparisonwith Na2CO with3 the precipitation process, which corresponds to the state of theFina art inlly, the a comparison primary extraction with the of precipitation lithium, is possible. process, Therefore,which corresponds also for the to the precipita- state oftion the withart in sodium the primary carbonate extraction from theof lithium, synthetic is andpossible. authentic Therefore, starting also solution, for the statisticalprecipi- tationmodels with were sodium created carbonate and the from recovery the synthetic efficiency and was authentic evaluated. starting In this solution, process, s significanttatistical modelsdeviations were ofcreated the results and the for recovery the synthetic efficiency and authenticwas evaluated. initial In solution this process, are shown. significant When deviationslooking at of the the statistical results for evaluation the synthetic for the and synthetic authentic liquid, initial it cansolution be seen are that shown. high When temper- lookingatures leadat the to statistical sufficient evaluation precipitation for whenthe synthetic a largestoichiometric liquid, it can be factor seen of that the high precipitant tem- peraturesis applied. lead For to the sufficient authentic precipitation solution, a mediumwhen a large optimum stoichiometric range is found, factor with of the even precip- lower itantamounts is applied. of sodium For the carbonate authentic leading solution, to a similar medium results. optimum In general, range is however, found, with it must even be lowernoted amounts that very of low sodium precipitation carbonate efficiencies leading could to similar be achieved, results. especially In general, for however, the authentic it mustinitial be leachingnoted that liquid very low (maximum precipitation of 50%). efficiencies This may could be attributed be achieved, to theespecially relatively for highthe −1 ◦ authenticsolubility initial of Li2 leachingCO3 in aqueous liquid (maximum solutions (13.3 of 50%). g·L atThis 20 mayC). Forbe thisattributed reason, to only the arela- small tivelyfraction high can solubility be precipitated of Li2CO from3 in theaqueous low-concentrated solutions (13.3 lithium g·L−1 solutions. at 20 °C). TheFor processthis reason, could onlybe improveda small fraction by concentrating can be precipitated the lithium from content. the low- Furthermore,concentrated Figurelithium6 solutions.also shows The the expected fact that the solubility of lithium carbonate decreases with increased temperature (7.2 g L−1 at 100 ◦C), thus achieving a higher recovery efficiency. The observation of the statistical evaluation for the authentic initial solution clearly visualizes that a mere increase in the added precipitant amount does not guarantee an improvement of the precipitation process, but an increase of the temperature is indispensable. Moreover, in this case, a quadratic model equation considering the interaction parameters was used in both statistical models. Equation (5) represents the corresponding formula for the synthetic initial solution, while Equation (6) shows that of the authentic leaching liquid.

c(Li) = 2853.3 + 11.7·T + 3153.4·x − 715.8·x2 − 17.1·T·x (5)

c(Li) = 2951.5 − 40.9·T + 2277.9·x + 0.6·T2 − 384.9·x2 − 18.5·x·T (6) Metals 2021, 11, x FOR PEER REVIEW 8 of 15 Metals 2021, 11, 1091 8 of 14

Similar to the phosphatic precipitants, interaction plots representing the dependencies process could be improved by concentrating the lithium content. Furthermore, Figure 6 between temperature and stoichiometric factor can be generated for precipitation with alsosodium shows carbonate the expecte (seed Figure fact that7). For the the solubility center experiments of lithium (threecarbonate per test decreases series), thewith in- −1 creasedprecipitation temperature of lithium (7.2 withg L Na at 2100CO 3°C),from thus the achieving synthetic leaching a higher solution recovery resulted efficiency. in a The observationmean value of of the 4927 statistical mg·L−1 for evaluation the residual for concentration the authentic of initial lithium solution in the leaching clearly liquid visualizes thatwith a mere a standard increase deviation in the added of 912 mgprecipitant·L−1. Using amount the authentic does not starting guarantee solution, an improvement a mean − of thevalue precipitation of 2588 mg·L process,1 in terms but of an lithium increase concentration of the temperature could be obtained is indispensable. with a standard deviation of 41 mg·L−1.

Figure 6. Statistical evaluation of the dissolved residual concentrations in mg·L−1 of lithium starting from the synthetic Figure 6. Statistical evaluation−1 of the dissolved residual−1 concentrations in mg·L−1 of lithium starting from the synthetic Metals 2021(left, 11,, 10,000 x FOR PEER mg·L REVIEW) and authentic (right, 5000 mg·L ) leaching solution using the precipitant Na2CO3 as a function9 ofof 15 −1 −1 (left, temperature10,000 mg·L and) and the stoichiometric authentic (right factor, 5000 of the mg· precipitant.L ) leaching solution using the precipitant Na2CO3 as a function of temperature and the stoichiometric factor of the precipitant.

Moreover, in this case, a quadratic model equation considering the interaction parameters was used in both statistical models. Equation (5) represents the corresponding formula for the synthetic initial solution, while Equation (6) shows that of the authentic leaching liquid.

c(Li) = 2853.3 + 11.7·T + 3153.4·x − 715.8·x2 − 17.1·T·x (5)

c(Li) = 2951.5 − 40.9·T + 2277.9·x + 0.6·T2 − 384.9·x2 − 18.5·x·T (6)

Similar to the phosphatic precipitants, interaction plots representing the dependencies between temperature and stoichiometric factor can be generated for precipitation with sodium carbonate (see Figure 7). For the center experiments (three per test series), the precipitation of lithium with Na2CO3 from the synthetic leaching solution resulted in a mean value of 4927 mg·L−1 for the residual concentration of lithium in the leaching liquid with a standard deviation of 912 mg·L−1. Using the authentic starting solution, a mean value of 2588 mg·L−1 in terms of lithium concentration could be obtained with a standard −1 deviation of 41 mg·L . Figure 7. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor Figure 7. Relative residual concentration of lithium in the samples as a function of temperature and stoichiometric factor (interaction plot of the model) based on the authentic leaching solution for the precipitation with Na2CO3. (interaction plot of the model) based on the authentic leaching solution for the precipitation with Na2CO3. 3.4. Phase Distribution of the Phosphatic Products 3.4. Phase Distribution of the Phosphatic Products Figure8 shows the diffractogram for the best parameter combination (determined Figure 8 shows the diffractogram for the best parameter combination (determined by by the wet chemical analysis) in the precipitation with potassium hydrogen phosphate the wet chemical analysis) in the precipitation with potassium hydrogen phosphate for both the synthetic and the authentic initial solution. The standard peak positions of the individual phases are indicated by colored reference lines in the corresponding diagrams below. For the major part, phases could be assigned to the peaks in the complex diffractogram. It can be seen that, as expected, in the case of the synthetic initial solution, only lithium phosphate (Li3PO4) can be detected in the precipitation product. In the case of the authentic initial solution, any cobalt, nickel and manganese residues are found, but due to the previous precipitation with sodium hydroxide, sufficient Na+ ions are present to form sodium sulfate.

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for both the synthetic and the authentic initial solution. The standard peak positions of the individual phases are indicated by colored reference lines in the corresponding diagrams below. For the major part, phases could be assigned to the peaks in the complex diffractogram. It can be seen that, as expected, in the case of the synthetic initial solution, only lithium phosphate (Li3PO4) can be detected in the precipitation product. In the case of the authentic initial solution, any cobalt, nickel and manganese residues are found, but Metals 2021, 11, x FOR PEER REVIEW 10 of 15 due to the previous precipitation with sodium hydroxide, sufﬁcient Na+ ions are present to form sodium sulfate.

FigureFigure 8. 8.Diffractogram Diffractogram forfor thethe precipitationprecipitation products products of of the the synthetic synthetic ( top(top)) and and authentic authentic ( bottom(bottom)) starting starting solution solution using using potassiumpotassium hydrogen hydrogen phosphate phosphate as as precipitant. precipitant.

InIn comparison,comparison, the the diffractograms diffractograms (see (see Figure Figure9) for9) for the the products products from from the precipitationthe precipita- withtion sodiumwith sodium phosphate phosphate are to are be to examined be examined in order in order to analyze to analyze the differences the differ betweenences be- thetween phosphatic the phosphatic precipitants. precipitants. It can beIt can seen be in seen the in area the of area the of synthetic the synthetic initial initial solution solution that mainlythat mainly lithium lithium phosphate phosphate could could be detected. be detected. In addition, In addition, a further a further compound compound (lithium (lith- thiophosphate)ium thiophosphate) appeared appeared in small in amounts.small amounts. In contrast In contrast to the precipitationto the precipitation with potassium with po- hydrogentassium hydrogen phosphate, phosphate, sodium oxalate sodium was oxalate also detected was also in detected the precipitate in the originatingprecipitate fromorigi- thenating authentic from the starting authentic solution. startingIn solution. this case, In the this formation case, the of formation sodium sulfate of sodium and sodium sulfate oxalateand sodium can also oxalate be explained can also by be the explained previous by precipitation the previous steps pre withcipitation NaOH steps and oxalicwith NaOH acid. and Figureoxalic acid.10 shows the distribution for the precipitation with potassium hydrogen phosphate (left side). The fine precipitate appears very homogeneous, which can also be verified in the SEM, as no enrichments of species can be seen in individual areas. Compared to the phosphate product, a clearly different structure is seen for the precipitation with sodium carbonate. The platy structure also leads to enrichments of individual elements in defined areas, as can be seen in Figure 10 (right side). It follows accordingly that, due to this

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Metals 2021, 11, x FOR PEER REVIEW 11 of 15 inhomogeneity, the quality of the carbonate precipitate cannot be rated as high as that of the phosphate product.

Figure 9. Diffractogram for the precipitation products of the synthetic (top) and authentic (bottom) starting solution using MetalsFigure 2021, 9.11,Diffractogram x FOR PEER REVIEW for the precipitation products of the synthetic (top) and authentic (bottom) starting solution using12 of 15 sodiumsodium phosphatephosphate asas precipitant.precipitant.

Figure 10 shows the distribution for the precipitation with potassium hydrogen phosphate (left side). The fine precipitate appears very homogeneous, which can also be verified in the SEM, as no enrichments of species can be seen in individual areas. Compared to the phosphate product, a clearly different structure is seen for the precipitation with sodium carbonate. The platy structure also leads to enrichments of individual elements in defined areas, as can be seen in Figure 10 (right side). It follows accordingly that, due to this inhomogeneity, the quality of the carbonate precipitate cannot be rated as high as that of the phosphate product.

FigureFigure 10. 10.SEM/EDS SEM/EDS (Scanning (Scanning Electron Electron Microscopy/Energy Microscopy/Energy Dispersive Dispersive X-ray X-ray Spectroscopy) Spectroscopy) analysis analysis of the of products the products from from the precipitationthe precipitation processes processes with phosphatic with phosp (lefthatic) and ( carbonaticleft) and carbonatic (right) precipitants (right) precipitants and corresponding and corresponding element distribution element (mapping). distribution (mapping).

4. Discussion In summary, it can be stated that in the comparison of precipitants, sodium phosphate in particular stands out as a promising candidate. In this case, starting from both the synthetic and the authentic leaching solution, 95% of lithium could be recovered. This enables the implementation of a recycling process at a high selective level, with only a minimal amount of the alkali element remaining in the residual solution and thus escaping the raw material cycle. The specific properties of phosphate permit the high solubility of the precipitant in aqueous solutions and the rapid precipitation of the desired compound due to its poorly soluble character. Furthermore, a temperature independence of the precipitation process could be observed when using the synthetic starting solution, while this was not monitored in the case of the authentic leaching liquid. This shows a clear influence of the liquid matrix, since it is loaded with foreign ions such as sodium, oxalates and other impurities. Nevertheless, Figure 4 indicates that the temperature dependence is explicitly less pronounced at temperatures above 60 °C and again the stoichiometric factor of the precipitant exerts the main influence. This hydrometallurgical process of precipitation enables a feasible and efficient recovery of lithium. Since the recycling of lithium has not been practiced on a large scale to the present due to the low cost of primary resources, the approach shown allows a significant improvement in the availability of raw materials and the recyclability of this critical metal. However, due to the cost of chemicals used in the recycling process, it is currently necessary that regulatory restrictions, especially within the framework of the European Union, drive this field of research and thus promote the implementation of effective recycling processes for lithium. The usage of potassium phosphate as precipitant is shown to be an alternative method, although significantly lower recovery efficiencies were obtained compared to the sampled sodium-based phosphate compound. Matrix effects could also be observed in this case, as the recovery of lithium from the authentic starting solution is challenging, leaving higher residual concentrations of the alkali metal in the solution. Lithium recovery in the form of Li2CO3 cannot be achieved to the same extent and with the same success, as it is the standard with known state-of-the-art processes. The precipitation efficiency for the chosen parameters amounts to about 75% for the synthetic and 50% for the authentic starting solution. The use of sodium carbonate as a precipitating agent is, therefore, inefficient in the present series of tests with low lithium concentrations

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4. Discussion In summary, it can be stated that in the comparison of precipitants, sodium phosphate in particular stands out as a promising candidate. In this case, starting from both the synthetic and the authentic leaching solution, 95% of lithium could be recovered. This enables the implementation of a recycling process at a high selective level, with only a minimal amount of the alkali element remaining in the residual solution and thus escaping the raw material cycle. The specific properties of phosphate permit the high solubility of the precipitant in aqueous solutions and the rapid precipitation of the desired compound due to its poorly soluble character. Furthermore, a temperature independence of the precipitation process could be observed when using the synthetic starting solution, while this was not monitored in the case of the authentic leaching liquid. This shows a clear influence of the liquid matrix, since it is loaded with foreign ions such as sodium, oxalates and other impurities. Nevertheless, Figure4 indicates that the temperature dependence is explicitly less pronounced at temperatures above 60 ◦C and again the stoichiometric factor of the precipitant exerts the main influence. This hydrometallurgical process of precipitation enables a feasible and efficient recovery of lithium. Since the recycling of lithium has not been practiced on a large scale to the present due to the low cost of primary resources, the approach shown allows a significant improvement in the availability of raw materials and the recyclability of this critical metal. However, due to the cost of chemicals used in the recycling process, it is currently necessary that regulatory restrictions, especially within the framework of the European Union, drive this field of research and thus promote the implementation of effective recycling processes for lithium. The usage of potassium phosphate as precipitant is shown to be an alternative method, although significantly lower recovery efficiencies were obtained compared to the sampled sodium-based phosphate compound. Matrix effects could also be observed in this case, as the recovery of lithium from the authentic starting solution is challenging, leaving higher residual concentrations of the alkali metal in the solution. Lithium recovery in the form of Li2CO3 cannot be achieved to the same extent and with the same success, as it is the standard with known state-of-the-art processes. The precipitation efficiency for the chosen parameters amounts to about 75% for the synthetic and 50% for the authentic starting solution. The use of sodium carbonate as a precipitating agent is, therefore, inefficient in the present series of tests with low lithium concentrations in the liquid. Figure 11 shows a comparison of the correlation and efficiency of all tested precipitants, based on the authentic leaching liquid. Looking at the obtained products mainly from the processes with phosphatic precipitants, it can be seen that by-products such as sodium sulfate or sodium oxalate are also formed if the authentic leach liquid is used, which could not be detected in the processes with synthetic initial solutions. Thus, a certain load of impurities must also be expected in the recovered product, which, however, strongly depends on the preceding process. Furthermore, these results show that the solution purification process works efficiently, as any cobalt, nickel and manganese residues could be detected in the precipitated products. Metals 2021, 11, x FOR PEER REVIEW 13 of 15

in the liquid. Figure 11 shows a comparison of the correlation and efficiency of all tested Metals 2021, 11, 1091 precipitants, based on the authentic leaching liquid. 12 of 14

FigureFigure 11. Comparison 11. Comparison of the of precipitants the precipitants Na3PO Na4, 3 KPO2HPO4,K42 HPOand 4 Naand2CO Na3 based2CO3 based on the on authentic the authentic − leachingleaching solution solution from lithium from lithium ion batteries ion batteries (5000 mg· (5000L−1) as mg a· Lfunction1) as a of function temperature of temperature and the stoi- and the chiometricstoichiometric factor of the factor precipitant. of the precipitant.

Looking5. Conclusions at the obtained products mainly from the processes with phosphatic precipitants, it canIn be summary, seen that the by precipitation-products such of lithium as sodium from sulfate leaching or solutionssodium oxalate of lithium-ion are also batter- formedies if is the a thoroughly authentic leach complex liquid and is stillused, not which sufficiently could not researched be detected field. in Duethe processes to the complex with syntheticand frequently initial varyingsolutions. composition Thus, a certain of the load active of materialimpurities of lithium-ionmust also be batteries, expected technolo- in the recoveredgies for theproduct, recycling which, of specifichowever, components strongly depends must be on adapted the preceding and evaluated process.constantly. Fur- thermore,Often, these only results small amountsshow that of lithiumthe solut areion recovered purification in hydrometallurgical process works efficiently, processes, as or the any cobalt,alkali nickel metal and gets manganese completely residues removed could from be the detected raw material in the precipitated cycle. In the products. context of this research, precipitation with phosphatic compounds shows great promise, since only low 5. Conclusionslevels of lithium can be detected in the residual solution after the process. Optimization within the framework of a statistical test evaluation also made it possible to work out an In summary, the precipitation of lithium from leaching solutions of lithium-ion bat- optimum parameter combination for the precipitation process as a function of the tem- teries is a thoroughly complex and still not sufficiently researched field. Due to the com- perature and the stoichiometric factor of the precipitant. The clarification of the influence plex and frequently varying composition of the active material of lithium-ion batteries, of matrix effects shows that an efficient separation of interfering elements in advance technologies for the recycling of specific components must be adapted and evaluated con- is indispensable, but the occurring matrix effects can be well controlled and described. stantly. Often, only small amounts of lithium are recovered in hydrometallurgical pro- Furthermore, the precipitation product is homogeneously distributed and can be reused as cesses, or the alkali metal gets completely removed from the raw material cycle. In the a raw material after further processing. context of this research, precipitation with phosphatic compounds shows great promise, since Authoronly low Contributions: levels of lithiumConceptualization, can be detected E.G. in and the H.A.; residual methodology, solution E.G.; after software, the process. E.G.; valida- Optimizationtion, E.G., within S.L. and the H.A.; framework formal analysis, of a statistical H.A.; investigation, test evaluation E.G.; also resources, made it E.G.; possible data curation,to work E.G.;out an writing—original optimum parameter draft preparation, combination E.G.; for writing—review the precipitation and editing, process E.G.; as visualization,a function E.G.; of thesupervision, temperature H.A.; and projectthe stoichiometric administration, factor E.G. of All the authors precipitant. have read The and clarification agreed to the of publishedthe influenceversion of ofmatrix the manuscript. effects shows that an efficient separation of interfering elements in advanceFunding: is indispensable,This research but received the occurring no external matrix funding. effects can be well controlled and described. Furthermore, the precipitation product is homogeneously distributed and can be reusedInstitutional as a raw material Review after Board further Statement: processing.Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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