Precipitation and Crystallization Used in the Production of Metal Salts for Li-Ion Battery Materials: A Review

Li-ion battery materials have been widely studied over the past decades. The metal salts that serve as starting materials for cathode and production, including Li2CO3, NiSO4, CoSO4 and MnSO4, are mainly produced using hydrometallurgical processes. In hydrometallurgy, aqueous precipitation and crystallization are important unit operations. Precipitation is mainly used in the processes of impurity removal, separation and preliminary production, while controlled crystallization can be very important to produce a pure product that separates well from the liquid solution. Precipitation and crystallization are often considered in the development of sustainable technologies, and there is still room for applying novel techniques. This review focuses on precipitation and crystallization applied to the production of metal salts for Li-ion battery materials. A number of novel and promising precipitation and crystallization methods, including eutectic freeze crystallization, antisolvent crystallization, and homogeneous precipitation are discussed. Finally, the application of precipitation and crystallization techniques in hydrometallurgical recycling processes for Li-ion batteries are reviewed.


Introduction
Aqueous precipitation and crystallization refer to unit operations that generate a solid from a supersaturated solution [1], wherein supersaturation generates a chemical potential that drives the reaction. The non-equilibrium, supersaturated condition can be induced in a variety of ways, including removal of solvent by evaporation, addition of another solvent, change of temperature or pressure, addition of other solutes, oxidation-reduction reactions, or combinations of these [2]. In hydrometallurgy, precipitation is mainly used as a process for impurity removal, separation and preliminary production, while crystallization is often used to produce a pure product and one that separates well from the solution [3]. The development of precipitation and crystallization technologies with high selectivity and low energy consumption is one of the challenges in hydrometallurgy.
Recently, Li-ion battery materials, especially the cathode materials, have become a research hotspot. Metal salts of Li, Ni, Co and Mn are the most important raw materials for the cathode materials [4,5]. The production of metal salts for Li-ion battery materials usually involves a partial or complete hydrometallurgical process, including precipitation and crystallization processes [6][7][8][9]. This review focuses on the processes and advances of precipitation and crystallization as used in the production of these metal salts for use in the manufacturing of Li-ion battery materials.

Methods and Applications of Aqueous Precipitation and Crystallization
The terminology of precipitation and crystallization processes is often unclear, with the terms often used loosely. The distinction between the terms precipitation and crystallization is generally based on the rate of the process and on the form of the solid particles produced. Crystallization is the formation of solids with a crystalline structure from a liquid phase. Precipitation as used within the current context is the formation of solids (not necessarily crystalline, could also be amorphous) from a liquid phase, under conditions of very high supersaturation. The term precipitation often tends to be applied to processes involving chemical reactions between added reagents and/or other species in the solution, where the products have very low solubility. However, the terminology sometimes overlap. For example, the production of Li 2 CO 3 by the reaction of Na 2 CO 3 and Li 2 SO 4 in solution is usually called crystallization [10], but the production of rare earth carbonates in a similar manner is usually referred to as precipitation [11]. Table 1 shows the terminology of precipitation and crystallization based on the driving force of supersaturation and the form of the product.
Precipitation and crystallization processes both obey the same basic rules: supersaturation is generated, which is the driving force and the necessary prerequisite for nucleation and growth. Thermodynamically, the condition for when a solid phase forms in the solution at constant temperature and pressure can be written [2,20]: ∆µ = −RT · ln(a/a 0 ) < 0 (1) where ∆µ = the difference in solute chemical potential, corresponding to the change in molar Gibbs energy for the transformation, R = the ideal gas constant, T = the absolute temperature, a = the activity of the solute in the solution, and a 0 = the activity of the solute at equilibrium with the solid phase. The supersaturation ratio, S, is defined as the ratio a/a 0 . Provided activities can be approximated by concentrations (i.e., the concentration-dependence of activity coefficients can be neglected), the supersaturation ratio can be expressed as: where C = the solute concentration and C eq = the solubility. The solution is supersaturated when S > 1. Retaining the above-mentioned approximation of the activity ratio, the relative supersaturation may be defined as: Considering a common precipitation or crystallization reaction, the supersaturation ratio can be expressed in terms of the concentrations of reactants and the solubility product, K sp : In hydrometallurgical processes, precipitation by chemical reaction is common. Table 2 lists the main reaction-based methods. Precipitation by chemical reaction may yield both amorphous and crystalline products, e.g., precipitation of iron can form into crystalline spinel ferrite (Fe 3 O 4 ), or amorphous iron hydroxides. Table 2. Methods of precipitation based on chemical reactions [21,22].

Precipitation Methods Reaction
Hydrolysis precipitation Me n+ + nOH − → Me(OH) n(s) Sulfide precipitation As for aqueous crystallization methods, they can be classified based on the method of generating supersaturation. The main methods are listed in Table 3.

Li-Ion Battery Materials and Metal Salts Used in Their Synthesis
In Li-ion batteries, lithium ions move from the anode through an electrolyte to the cathode during discharge, and back during charge [32]. The cathode material is made of a powdered intercalated lithium compound. Layered oxides LiMO 2 or spinel type LiM 2 O 4 compounds (lithium-containing transition metal oxides, with M = Co, Ni, Mn of different composition) have mostly been used, but also LiMPO 4 compounds are increasingly popular. The anode material typically consists of powdered carbon, such as graphite. It is bonded with respective foils (Cu foil or Al foil) in the battery. The electrolyte consists of a salt of lithium and a non-coordinating anion (PF 6 − , BF 4 − , CF 3 SO 3 − , or N(SO 2 CF 3 ) 2 − ) dissolved in a polar organic solvent [32][33][34]. Table 4 lists some representative electrode materials that have been commercialized or have commercial prospects in rechargeable Li-ion batteries. Doping and coating have also been used to improve the performance of anode or cathode materials in Li-ion batteries [34]. Assuming Co as the transition metal in the cathode material, the overall discharge reaction can be written as [32]: ; (x is between 0 and 1) Antisolvent crystallization (NH 4 ) 3 ScF 6 crystallization by adding ethanol [18] Reaction crystallization/precipitation

Heterogeneous reaction
Crystallization of gypsum in neutralization with lime or limestone of sulfuric acid-based solutions [28] Homogeneous reaction Crystallization of PbCO 3 by fluidized-bed homogeneous crystallization from wastewater [29] Hydrothermal crystallization Crystallization of scorodite at temperatures above 150 • C [30] Membrane distillation crystallization Desalination of seawater [31]  In Li-ion batteries, the constrained specific capacity of the cathode materials is one of the major obstacles to improving the energy density. The synthesis cost of cathode materials can be high as some of the oxides require critical metals and the synthesis processes usually require elevated temperatures. Important synthesis methods of cathode materials include the solid state reaction method, the co-precipitation method, the sol-gel method, hydrothermal synthesis, and spray pyrolysis [39,40]. In most cases, metal salts serve as raw materials for the cathode materials. Using NCM as an example, Figures 1 and 2 show typical synthesis processes.  Common metal salts (or metal compounds) for cathode materials are listed in Table 5. The production of these raw materials is the upstream part of Li-ion battery manufacture. Hydrometallurgical processes are necessary for their production, and precipitation and crystallization are essential steps of the processes. Purity requirements of battery grade Li2CO3, CoSO4·7H2O, NiSO4·6H2O and MnSO4·H 2O, which are the most common commercial products, are reported to be (by mass) Li2CO3 ≥ 99.5%, Co ≥ 20.2%, Ni ≥ 22.0% and Mn ≥ 31.8% [41]. An increasingly important route for the production of metal salts is from secondary sources, such as spent Li-ion batteries. Recovering the metals from Li-ion batteries reduces their environmental impact and may increase the availability of critical elements.   Common metal salts (or metal compounds) for cathode materials are listed in Table 5. The production of these raw materials is the upstream part of Li-ion battery manufacture. Hydrometallurgical processes are necessary for their production, and precipitation and crystallization are essential steps of the processes. Purity requirements of battery grade Li2CO3, CoSO4·7H2O, NiSO4·6H2O and MnSO4·H 2O, which are the most common commercial products, are reported to be (by mass) Li2CO3 ≥ 99.5%, Co ≥ 20.2%, Ni ≥ 22.0% and Mn ≥ 31.8% [41]. Table 5. Metal salts (or metal compounds) used in the production of cathode materials [36,[39][40][41][42][43].

Precipitation and Crystallization of Metal Salts for Li-Ion Battery Materials
An increasingly important route for the production of metal salts is from secondary sources, such as spent Li-ion batteries. Recovering the metals from Li-ion batteries reduces their environmental impact and may increase the availability of critical elements.  Common metal salts (or metal compounds) for cathode materials are listed in Table 5. The production of these raw materials is the upstream part of Li-ion battery manufacture. Hydrometallurgical processes are necessary for their production, and precipitation and crystallization are essential steps of the processes. Purity requirements of battery grade Li 2 CO 3 , CoSO 4 ·7H 2 O, NiSO 4 ·6H 2 O and MnSO 4 ·H 2 O, which are the most common commercial products, are reported to be (by mass) Li 2 CO 3 ≥ 99.5%, Co ≥ 20.2%, Ni ≥ 22.0% and Mn ≥ 31.8% [41]. Table 5. Metal salts (or metal compounds) used in the production of cathode materials [36,[39][40][41][42][43].

Lithium
Cobalt Nickel Manganese Titanium Aluminum Iron An increasingly important route for the production of metal salts is from secondary sources, such as spent Li-ion batteries. Recovering the metals from Li-ion batteries reduces their environmental impact and may increase the availability of critical elements.

Precipitation and Crystallization of Metal Salts for Li-Ion Battery Materials
For the production of the metal salts mentioned in Table 5, the raw materials include some minerals and natural salt lake brine and some secondary resources, such as spent Li-ion batteries, waste residue, slag and tailings. The main minerals and natural brine are [1,3,8]: Lithium: lepidolite, spodumene, petalite, lithiophosphate and salt lake brine. Cobalt: cobalt sulfide minerals and cobalt oxide minerals. Nickel: laterite, nickel sulfide minerals and nickel arsenide mineral. Manganese: pyrolusite, psilomelane, braunite and rhodochrosite.
Hydrometallurgical processing is an important part in the process of utilizing these resources, and in general, the process can be summarized as pretreatment → leaching → separation/purification → product precipitation/crystallization [3]. Common techniques for the separation or purification unit include selective precipitation, solvent extraction, ion exchange and reverse osmosis. The concentration of metal ions in the streams from the separation or purification unit is relatively high. Precipitation or crystallization is then used to produce the final products of high purity. The sulfate system is most commonly used in the hydrometallurgical processes due to the low cost of sulfuric acid, leading to metal sulfate salts as the resulting products. Other salts can also be produced from these products. Figure 3 shows a phase diagram that summarizes the product crystallization routes followed by the methods discussed in this review for a generic binary salt-water system. Point A represents the starting composition and temperature of a batch solution. Evaporative crystallization (EC) is often employed to produce supersaturation via evaporation of the solvent at elevated temperature, which is represented by the process path A → B → C in Figure 3.

Precipitation and Crystallization of Metal Salts for Li-Ion Battery Materials
For the production of the metal salts mentioned in Table 5, the raw materials include some minerals and natural salt lake brine and some secondary resources, such as spent Li-ion batteries, waste residue, slag and tailings. The main minerals and natural brine are [1,3,8]: Lithium: lepidolite, spodumene, petalite, lithiophosphate and salt lake brine. Cobalt: cobalt sulfide minerals and cobalt oxide minerals. Nickel: laterite, nickel sulfide minerals and nickel arsenide mineral. Manganese: pyrolusite, psilomelane, braunite and rhodochrosite.
Hydrometallurgical processing is an important part in the process of utilizing these resources, and in general, the process can be summarized as pretreatment → leaching → separation/purification → product precipitation/crystallization [3]. Common techniques for the separation or purification unit include selective precipitation, solvent extraction, ion exchange and reverse osmosis. The concentration of metal ions in the streams from the separation or purification unit is relatively high. Precipitation or crystallization is then used to produce the final products of high purity. The sulfate system is most commonly used in the hydrometallurgical processes due to the low cost of sulfuric acid, leading to metal sulfate salts as the resulting products. Other salts can also be produced from these products. Figure 3 shows a phase diagram that summarizes the product crystallization routes followed by the methods discussed in this review for a generic binary salt-water system. Point A represents the starting composition and temperature of a batch solution. Evaporative crystallization (EC) is often employed to produce supersaturation via evaporation of the solvent at elevated temperature, which is represented by the process path A → B → C in Figure 3. EC is a conventional and widely used unit operation for industrial crystallization of many products, including metal salts for Li-ion battery materials as detailed above [44]. This process can attain relatively high crystal growth rates, as a result of the high temperatures employed, but it is not an energy-efficient process [45]. EC is a conventional and widely used unit operation for industrial crystallization of many products, including metal salts for Li-ion battery materials as detailed above [44]. This process can attain relatively high crystal growth rates, as a result of the high temperatures employed, but it is not an energy-efficient process [45].

Evaporative Crystallization
If the solubility of a solute is appreciably decreased by a reduction in temperature, evaporation is always followed by a cooling operation that can significantly increase the yield at low cost [44]. Then the process path is A → B → C → D as shown in Figure 3. Figure 4 shows the solubility curves of Li 2 SO 4 , CoSO 4 , MnSO 4 and NiSO 4 in H 2 O. As can be seen, for CoSO 4 and NiSO 4 , EC normally consists of solute concentration by evaporation and crystallization after cooling. The products are sulfate hydrates.  Multi-effect evaporative crystallization (MEEC) is the most frequently used EC technique. Figure 5 shows a triple-effect evaporative crystallization system. It consists of three evaporators in series and a final condenser. Compared to the single-effect evaporative system, MEEC has the advantages of lower energy consumption and higher crystallization rates, but it constitutes a complex operation [47]. An alternative to multiple-effect evaporation is vapor recompression. The principle of this method is to raise the temperature of the vapor as it exits the evaporator by raising its pressure, and passing it back into the evaporator heat exchanger. However, this method has not been widely applied, because it lacks reliability and is not applicable to systems with high boiling point components [48]. Multi-effect evaporative crystallization (MEEC) is the most frequently used EC technique. Figure 5 shows a triple-effect evaporative crystallization system. It consists of three evaporators in series and a final condenser. Compared to the single-effect evaporative system, MEEC has the advantages of lower energy consumption and higher crystallization rates, but it constitutes a complex operation [47].  Multi-effect evaporative crystallization (MEEC) is the most frequently used EC technique. Figure 5 shows a triple-effect evaporative crystallization system. It consists of three evaporators in series and a final condenser. Compared to the single-effect evaporative system, MEEC has the advantages of lower energy consumption and higher crystallization rates, but it constitutes a complex operation [47]. An alternative to multiple-effect evaporation is vapor recompression. The principle of this method is to raise the temperature of the vapor as it exits the evaporator by raising its pressure, and passing it back into the evaporator heat exchanger. However, this method has not been widely applied, because it lacks reliability and is not applicable to systems with high boiling point components [48].

Eutectic Freeze Crystallization
Eutectic freeze crystallization (EFC) produces supersaturation and attains the specific eutectic An alternative to multiple-effect evaporation is vapor recompression. The principle of this method is to raise the temperature of the vapor as it exits the evaporator by raising its pressure, and passing it back into the evaporator heat exchanger. However, this method has not been widely applied, because it lacks reliability and is not applicable to systems with high boiling point components [48].

Eutectic Freeze Crystallization
Eutectic freeze crystallization (EFC) produces supersaturation and attains the specific eutectic conditions through both freeze crystallization and crystallization by concentration [49]. This process is represented by the path A → E → F shown in Figure 3. EFC is a suitable technology for the simultaneous separation of an aqueous solution into ice and salt crystals near the eutectic point [50]. The composition of the feed solution can be either higher or lower than the concentration at the eutectic point F. Ideally, separation of the resulting ice and salt crystals can then be achieved through gravity, as ice has a lower density but salt crystals have a higher density than the solution. Particular advantages of EFC include the ability to recover pure water and treat corrosive streams due to the low temperatures employed. Also, EFC requires significantly less energy input than EC, since the heat of fusion of ice (6.01 kJ/mol) is six times lower than the heat of evaporation of water (40.65 kJ/mol) [51]. However, EFC requires higher capital costs than EC, since the technique is not currently mature.
EFC has been widely studied for the treatment of wastewater and the desalination of brine. Salt crystals that have been targeted include Na 2 SO 4 ·10H 2 O, MgSO 4 ·7H 2 O, CaSO 4 ·7H 2 O, Na 2 CO 3 , NaCl, NaNO 3 and KNO 3 , where the metals are commonly found in saline wastewater [52][53][54][55]. Research into the recovery and purification of other salts from industrial streams using EFC has so far been limited, but is attracting increased attention.
EFC could be a promising approach for the production of metal salts for Li-ion battery materials. However, the application of EFC for recovery of Li, Co, Mn has not yet been reported. Lu [50] studied the use of EFC for recovery of NiSO 4 ·7H 2 O from an industrial NiSO 4 solution containing Na 2 SO 4 , reporting that 70% NiSO 4 could be crystallized from solution before it became saturated with Na 2 SO 4 . Figure 6 shows the binary phase diagram of the NiSO 4 -H 2 O system at low temperatures. As can be seen, the eutectic temperature of the pure NiSO 4 solution is about −4 • C. All systems are unique, however, so more research is necessary to ensure efficacious production of Li, Ni, Co and Mn salts through EFC. The composition of the feed solution can be either higher or lower than the concentration at the eutectic point F. Ideally, separation of the resulting ice and salt crystals can then be achieved through gravity, as ice has a lower density but salt crystals have a higher density than the solution. Particular advantages of EFC include the ability to recover pure water and treat corrosive streams due to the low temperatures employed. Also, EFC requires significantly less energy input than EC, since the heat of fusion of ice (6.01 kJ/mol) is six times lower than the heat of evaporation of water (40.65 kJ/mol) [51]. However, EFC requires higher capital costs than EC, since the technique is not currently mature. EFC has been widely studied for the treatment of wastewater and the desalination of brine. Salt crystals that have been targeted include Na2SO4·10H2O, MgSO4·7H2O, CaSO4·7H2O, Na2CO3, NaCl, NaNO3 and KNO3, where the metals are commonly found in saline wastewater [52][53][54][55]. Research into the recovery and purification of other salts from industrial streams using EFC has so far been limited, but is attracting increased attention.
EFC could be a promising approach for the production of metal salts for Li-ion battery materials. However, the application of EFC for recovery of Li, Co, Mn has not yet been reported. Lu [50] studied the use of EFC for recovery of NiSO4·7H2O from an industrial NiSO4 solution containing Na2SO4, reporting that 70% NiSO4 could be crystallized from solution before it became saturated with Na2SO4. Figure 6 shows the binary phase diagram of the NiSO4-H2O system at low temperatures. As can be seen, the eutectic temperature of the pure NiSO4 solution is about −4 °C. All systems are unique, however, so more research is necessary to ensure efficacious production of Li, Ni, Co and Mn salts through EFC. NiSO4 (wt%) Figure 6. Binary phase diagram of the NiSO4-H2O system (solubility data based on literature reports [46,56]).
In order to make the EFC process more efficient, column crystallizers capable of simultaneously crystallizing and separating ice and salt crystals have been designed [51]. Figure 7 shows a typical cylindrical crystallization unit for this EFC process. The temperature of the system is controlled by indirect cooling. The scraping operation is very important during the EFC process, because the scrapers dislodge crystals of ice or inorganic salts from the chilled surface. This avoids salt or ice scaling on the chilled surfaces and promotes crystal growth in the bulk of the solution. This in turn In order to make the EFC process more efficient, column crystallizers capable of simultaneously crystallizing and separating ice and salt crystals have been designed [51]. Figure 7 shows a typical cylindrical crystallization unit for this EFC process. The temperature of the system is controlled by indirect cooling. The scraping operation is very important during the EFC process, because the scrapers dislodge crystals of ice or inorganic salts from the chilled surface. This avoids salt or ice scaling on the chilled surfaces and promotes crystal growth in the bulk of the solution. This in turn allows the ice and salt crystals to be separated on the basis of the difference in densities.

Antisolvent Crystallization
Antisolvent crystallization relies on the addition of a water-miscible organic solvent to reduce the solubility of the solute, generating supersaturation, and thereby forcing the targeted salts to crystallize [58,59]. The ability of a solvent to solubilize an ion can be captured by the dielectric constant (ε) of the solvent. The dielectric constant is a measure of the polarizability of a substance. If the dielectric constant of the additional solvent is lower than that of water, the solvation of the charged ions is decreased, and the solubility of the solute is reduced [60]. Antisolvent crystallization can produce high product yields over narrow temperature ranges [61]. It is also an energy saving and highly safe technique because the process can be executed at ambient temperature. In addition, comparatively simple equipment is required for antisolvent crystallization, which is thus attractive from both cost and safety perspectives. However, antisolvent crystallization requires large capital cost due to handling large volumes [49], and the recovery of antisolvent after crystallization introduces extra costs.
The literature on antisolvent crystallization contains numerous reports of potential organic solvents that are able to lower the solubility of some inorganic solutes. These include ethanol, acetic acid, 2-propanol, acetone, glycine, methanol, tert-butanol, methyl-ethyl-ketone (MEK), ethyl-acetate and isopropyl-acetate [17,18,59,61]. Recently, antisolvent crystallization has been examined for use in the selective recovery of critical metals from aqueous solutions. For example, Korkmaz et al. used antisolvent crystallization to selectively recover rare earth elements from NiMH battery leach liquors [17]. The antisolvents used in this case were ethanol and 2-propanol. Kaya et al. [18] reported on the recovery of scandium from an ammonium fluoride strip liquor using antisolvent crystallization. In this instance, the recovery yields were above 98% when acetone, ethanol, methanol or isopropanol were used. Antisolvent crystallization could also be used to recover Li, Ni, Co and Mn salts [62]. An antisolvent crystallization process using a LiOH-water-ethanol system has been reported to produce purified LiOH·H 2O [63]. Research by Cohen et al. first suggested that antisolvent crystallization could be used to recover NiSO4 from hydrometallurgical decopperized electrolytes [64]. Moldoveanu and

Antisolvent Crystallization
Antisolvent crystallization relies on the addition of a water-miscible organic solvent to reduce the solubility of the solute, generating supersaturation, and thereby forcing the targeted salts to crystallize [58,59]. The ability of a solvent to solubilize an ion can be captured by the dielectric constant (ε) of the solvent. The dielectric constant is a measure of the polarizability of a substance. If the dielectric constant of the additional solvent is lower than that of water, the solvation of the charged ions is decreased, and the solubility of the solute is reduced [60]. Antisolvent crystallization can produce high product yields over narrow temperature ranges [61]. It is also an energy saving and highly safe technique because the process can be executed at ambient temperature. In addition, comparatively simple equipment is required for antisolvent crystallization, which is thus attractive from both cost and safety perspectives. However, antisolvent crystallization requires large capital cost due to handling large volumes [49], and the recovery of antisolvent after crystallization introduces extra costs.
The literature on antisolvent crystallization contains numerous reports of potential organic solvents that are able to lower the solubility of some inorganic solutes. These include ethanol, acetic acid, 2-propanol, acetone, glycine, methanol, tert-butanol, methyl-ethyl-ketone (MEK), ethyl-acetate and isopropyl-acetate [17,18,59,61]. Recently, antisolvent crystallization has been examined for use in the selective recovery of critical metals from aqueous solutions. For example, Korkmaz et al. used antisolvent crystallization to selectively recover rare earth elements from NiMH battery leach liquors [17]. The antisolvents used in this case were ethanol and 2-propanol. Kaya et al. [18] reported on the recovery of scandium from an ammonium fluoride strip liquor using antisolvent crystallization. In this instance, the recovery yields were above 98% when acetone, ethanol, methanol or isopropanol were used. Antisolvent crystallization could also be used to recover Li, Ni, Co and Mn salts [62].
An antisolvent crystallization process using a LiOH-water-ethanol system has been reported to produce purified LiOH·H 2 O [63]. Research by Cohen et al. first suggested that antisolvent crystallization could be used to recover NiSO 4 from hydrometallurgical decopperized electrolytes [64]. Moldoveanu and Demopoulos [59] studied antisolvent crystallization for several metal sulfate and chloride systems of hydrometallurgical interest. It was concluded that 2-propanol is an efficient solvent for many metal sulfates and >90% crystallization yields under ambient conditions were reported, but the process was found to be ineffective for chloride systems [59]. The antisolvent could be recovered from the aqueous solution after crystallization and reused, e.g., by reduced pressure distillation [17,18].
In general, despite the many potential applications for antisolvent crystallization in inorganic processing, wastewater treatment and hydrometallurgy, further systematic study of the process is needed, as only limited data and a low number of studies have been reported and only for specific systems.

Precipitation (Reaction Crystallization)
As summarized in the Introduction, in the field of hydrometallurgy, reaction crystallization is usually referred to simply as 'precipitation'. Metal precipitation is a process in which the metal ion is allowed to react with other compounds to produce a product of low solubility. Metal precipitation is the most common method to recover metals as nearly insoluble compounds from solution [1]. Li 2 CO 3 is the common source of lithium when producing cathode materials for Li-ion batteries [10]. Precipitation is always employed to produce Li 2 CO 3 , since it is practically insoluble in neutral or basic aqueous solution. Anions for precipitation of Ni 2+ , Co 2+  , after which Ni(OH) 2 was prepared as a final product [66]. However, this process is generally used to produce crude products or precursors, because high product purity cannot be obtained using conventional precipitation methods.
A key issue with this method is that the precipitate formed by the addition of precipitating agents is typically not easily filtered, because the local supersaturation at the feed point where the reagent is added will generally be very high [67]. In order to improve the quality of the precipitate, the level of supersaturation should be kept low during the chemical reaction. Homogeneous precipitation can accomplish this. In homogeneous precipitation reaction, the precipitating agent is produced over a long period of time in a homogeneous solution [1]. The resulting precipitate is compact and crystalline. Methods employed for controlling supersaturation include pH control, metal complexation and dissociation, dissolution reaction, and redox reaction [2]. Homogeneous precipitation could also be used to produce metal salts (or metal compounds) for Li-ion battery materials. For example, highly pure Co(OH) 2 can be prepared by adding NaOH to a mixed solution of ammonia and cobalt sulfate, because Co 2+ forms complex compounds with NH 3 and then dissociates in a controlled manner [68].

Other Methods
Hydrothermal crystallization can sometimes be used to produce metal salts for Li-ion battery materials. As seen in Figure 4, the solubility of MnSO 4 at high temperature is very low, so that MnSO 4 hydrate crystals can be produced in an autoclave [3]. Another potential method is membrane distillation crystallization. This is a hybrid process of distillation and crystallization. Membrane distillation can separate the water from the concentrated solution, to produce a supersaturated solution for subsequent crystallization [31]. The current applications of membrane distillation crystallization are limited to protein crystallization and few wastewater treatments [31]. Further research into membrane distillation crystallization is needed to elucidate the best processes for the production of metal salts.

Precipitation and Crystallization in Hydrometallurgical Recycling of Li-Ion Batteries
With the ever-growing need for lithium-ion batteries, particularly from the electric mobility industry, a large amount of lithium-ion batteries are bound to be retired in the near future, thereby leading to serious disposal problems and detrimental impacts on environment and energy conservation [69]. A complete recycling process of Li-ion batteries includes physical treatment and pyrometallurgical/hydrometallurgical chemical process due to the complicated assembly of Li-ion batteries and the diversity of electrode materials [69,70]. In recycling of Li-ion batteries, the foils of Cu and Al and the anode material are easily physically separated. The composition of the cathode material is complex, and rich in valuable metals, and thus a key to a viable recycling is to "close the loop" with respect to the cathode material [10]. The recovery of cathode material using a hydrometallurgical process has been widely studied [6,10,65,66,69,70]. Figure 8 shows a general flowchart for the hydrometallurgical process for recycling Li-ion batteries. Some specific and detailed flowcharts have been investigated in literature [10,71,72]. Moreover, diversity in design and products is presented in the hydrometallurgical processes and some industrial-scale production have been achieved [73][74][75]. Relevant metal salts or precursors of cathode material are the main products of the recycling processes [71][72][73][74][75]. For example, in Recupyl (France) process [73] and AEA (UK) process [75], all the valuable metals have been recycled as metal salts.
Metals 2020, 10, x FOR PEER REVIEW 11 of 16 loop" with respect to the cathode material [10]. The recovery of cathode material using a hydrometallurgical process has been widely studied [6,10,65,66,69,70]. Figure 8 shows a general flowchart for the hydrometallurgical process for recycling Li-ion batteries. Some specific and detailed flowcharts have been investigated in literature [10,71,72]. Moreover, diversity in design and products is presented in the hydrometallurgical processes and some industrial-scale production have been achieved [73][74][75]. Relevant metal salts or precursors of cathode material are the main products of the recycling processes [71][72][73][74][75]. For example, in Recupyl (France) process [73] and AEA (UK) process [75], all the valuable metals have been recycled as metal salts.  As can be seen in Figure 8, after obtaining the cathode materials from spent Li-ion batteries, pretreatment, such as roasting, alkaline leaching and acid leaching are employed to solubilize the metals. Alkaline leaching is performed for materials with a high Al content, while acid leaching is always necessary for the recovery of Li, Ni, Co and Mn [72]. After obtaining the acidic leach solution, various processes, which have similar steps and methods, have been developed to achieve separation and purification [66,71]. It is possible to resynthesize cathode materials directly from strip or leach solutions in the lab after careful adjustment of the conditions (dosing of salts and evaporation) [76,77]. However, the need for careful control of the conditions in the presence of impurities makes it challenging to develop a robust and efficient industrial process for direct synthesis of new cathode materials. Figure 9 shows a general flowchart for the treatment of acidic leach solutions. Precipitation, crystallization, solvent extraction and electrolysis deposition techniques can be employed. The precipitation and crystallization processes involved are mainly for the removal of Al and Fe, and the separation and production of Li, Ni, Co and Mn salts. It is noteworthy that some mixed metal salts produced in hydrometallurgical processes with no purification stages can also be used for the production of new materials [78,79]. Table 6 summarizes some precipitation and crystallization techniques used in the recycling of Li-ion batteries. As can be seen in Figure 8, after obtaining the cathode materials from spent Li-ion batteries, pretreatment, such as roasting, alkaline leaching and acid leaching are employed to solubilize the metals. Alkaline leaching is performed for materials with a high Al content, while acid leaching is always necessary for the recovery of Li, Ni, Co and Mn [72]. After obtaining the acidic leach solution, various processes, which have similar steps and methods, have been developed to achieve separation and purification [66,71]. It is possible to resynthesize cathode materials directly from strip or leach solutions in the lab after careful adjustment of the conditions (dosing of salts and evaporation) [76,77]. However, the need for careful control of the conditions in the presence of impurities makes it challenging to develop a robust and efficient industrial process for direct synthesis of new cathode materials. Figure 9 shows a general flowchart for the treatment of acidic leach solutions. Precipitation, crystallization, solvent extraction and electrolysis deposition techniques can be employed. The precipitation and crystallization processes involved are mainly for the removal of Al and Fe, and the separation and production of Li, Ni, Co and Mn salts. It is noteworthy that some mixed metal salts produced in hydrometallurgical processes with no purification stages can also be used for the production of new materials [78,79]. Table 6 summarizes some precipitation and crystallization techniques used in the recycling of Li-ion batteries. As seen in Table 6, Cu precipitation (cementation) by adding iron is commonly used to recover Cu from acidic leach solution. Conventional precipitation techniques are suitable for the removal of impurities such as Fe(III) and Al(III) from acidic leach solutions, but the resulting precipitates are not pure. If selective precipitation of Ni(II), Co(II), Mn(II) is performed, further purification or transformation is required to obtain the final metal salts (or metal compounds) from the Li-ion battery materials, whereas evaporative crystallization can be used to produce pure metal salts. Evaporative crystallization is usually employed to produce Ni, Co and Mn sulfates after solvent extraction as this is the conventional and mature technique. The more novel precipitation and crystallization methods reviewed in this paper have as yet scarcely been reported in connection with recycling of Li-ion batteries.    [84] Note: 1 As there are many known hydrated oxides of Fe(III), for the sake of simplicity we represent all of them as Fe(OH) 3 in the above table. The varying degrees of hydration take the form of hydroxide disproportionation to oxide and water. The first precipitated Fe(III) hydrated oxide is known to be ferrihydrite, which has a variable hydration degree of 4-9 associated waters. 2 dmgH 2 (dimethylglyoxime) is a chemical compound described by the formula CH 3 C(NOH)C(NOH)CH 3 .
As seen in Table 6, Cu precipitation (cementation) by adding iron is commonly used to recover Cu from acidic leach solution. Conventional precipitation techniques are suitable for the removal of impurities such as Fe(III) and Al(III) from acidic leach solutions, but the resulting precipitates are not pure. If selective precipitation of Ni(II), Co(II), Mn(II) is performed, further purification or transformation is required to obtain the final metal salts (or metal compounds) from the Li-ion battery materials, whereas evaporative crystallization can be used to produce pure metal salts. Evaporative crystallization is usually employed to produce Ni, Co and Mn sulfates after solvent extraction as this is the conventional and mature technique. The more novel precipitation and crystallization methods reviewed in this paper have as yet scarcely been reported in connection with recycling of Li-ion batteries.

Conclusions
The unit operations precipitation and crystallization have been widely employed in hydrometallurgy. The reported methods, classified based on the induction of supersaturation include: evaporative crystallization, eutectic freeze crystallization, antisolvent crystallization, conventional reaction crystallization or precipitation, and homogeneous reaction crystallization or precipitation. Their mechanisms, advantages and some existing applications have been discussed. The characteristics of the hydrometallurgical streams and target products should dictate the selection of precipitation or crystallization methods used. Evaporative crystallization is a mature technique for producing Ni, Co, Mn sulfate salts. Eutectic freeze crystallization and antisolvent crystallization are energy saving and safe alternatives, but few case studies have been reported. Conventional precipitation is a poor choice for producing pure products, while homogeneous precipitation can offer some improvement. In addition, since the production of the metal salts from spent Li-ion batteries has become increasingly important, application of precipitation and crystallization processes in hydrometallurgical recycling processes for Li-ion batteries have also been reviewed. To date, the application of more novel and potentially superior precipitation and crystallization techniques for the recycling of Li-ion battery materials is lacking, but this situation is expected to change in the future.