ES3033513A1 - Method for recovering lithium from lithium-ion battery electrolytes - Google Patents

Abstract

The present invention relates to the recovery of lithium salts from spent batteries, and more specifically, to a method for recovering the degradation products of a liquid electrolyte composed of at least one lithium salt from the black mass of spent lithium batteries, comprising steps of hydrolysis, filtration, evaporation, and optionally heat treatment and subsequent selective dissolution, in order to obtain lithium salts from the electrolyte recovered from the spent lithium battery. This method is capable of recovering lithium salts in a simple manner, and without the use of high temperatures or chemical reagents. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

METHOD OF RECOVERING LITHIUM FROM ELECTROLYTES OF BATTERIES

LITHIUM ION

The present invention relates to the recovery of lithium salts from spent batteries, and more specifically, to a method for recovering the degradation products of a liquid electrolyte composed of at least one lithium salt from the black mass of spent lithium batteries, comprising steps of hydrolysis, filtration, evaporation, and optionally heat treatment and subsequent selective dissolution, in order to obtain lithium salts from the electrolyte recovered from the spent lithium battery. This method is capable of recovering lithium salts in a simple manner, and without the use of high temperatures or chemical reagents.

BACKGROUND OF THE INVENTION

Lithium-ion batteries that include a lithium electrolyte are widely known in the state of the art. In fact, most lithium-ion batteries used in computers, cameras, household appliances, scooters, bicycles, and electric vehicles include a lithium electrolyte. The electrolytes used in batteries consist of a mixture of organic solvents with lithium salts (with high solubility in the solvents), thus ensuring high ion mobility in the fluid (diffusion and migration). The electrolyte is commonly composed of a solvent, mainly organic carbonates (i.e., in carbonic acid esters H<2>CO<3>), and a conductive salt such as lithium hexafluorophosphate (LiPF6). Due to their stability against oxidation and their high conductivity, solutions of lithium salts, particularly LiPF6, in organic carbonates (i.e., in carbonic acid esters H<2>CO<3>), are the most commonly used electrolytes in the manufacture of lithium-ion batteries.

However, when the battery's useful life is reached and other reuse options are no longer possible, this lithium salt ends up becoming part of the rest of its components, that is, the battery's anode and cathode.

Normally, in order to recover either the metals from spent batteries, or the metal itself from the electrolyte (as is the case with lithium), what is known as a "black mass" is generated from the battery waste, which is a fraction that includes the components of interest from the spent battery. This "black mass" is normally obtained from the crushing of battery cells that have previously been separated from the modules. After crushing, there may be a component separation stage, mainly aluminum and copper, and sometimes a heat treatment to remove the organic compounds that form part of the electrolyte.

The fraction obtained after crushing and, sometimes, heat treatment, is known as "black mass" and includes a mixture of anode, cathode and impurities that may come from remains of the cell packaging. That is, it is a mixture of graphite and the metals that formed the cathode (manganese, cobalt, nickel, and lithium, mainly). Regarding lithium, the presence of this metal in the black mass is due to both the electrolyte (lithium mainly associated with LiF6P) and the lithium existing in the cathode. From this black mass, many techniques have been developed that allow the metals to be separated from each other, with greater or lesser difficulty and degree of purity. For this purpose, both hydrometallurgical and high-temperature pyrometallurgical techniques are used (Xie, F., Wang, C., Sun, Y., Fan, Y., Zhao, Z., Yao, Y., 2023. Sustainable and selective recovery of lithium from spent lithium-ion batteries based on hydrogen reduction: Theoretical analysis and phase transformation. Sep. Purif. Technol.

318, 123972). Lithium recovery can be carried out by different techniques, which allow obtaining lithium salts (carbonate, hydroxide and oxalate, mainly), but these techniques lead to products contaminated by fluorine salts from the degradation of the electrolyte, so it is necessary to first eliminate these fluorine salts to obtain high purity lithium salts.

Examples of metal recovery processes (such as lithium or its salts) from used lithium batteries and their black masses can be found in the literature, for example, in document US2007196725A1 which recovers lithium from crushed anodes of spent lithium batteries by means of a hydrometallurgical process that dissolves the metal oxides and lithium in water and finally by adding acid the lithium is precipitated as lithium carbonate (U<2>CO<3>). Another example of a similar process is that of document KR20210138922A which discloses a method for recovering lithium from positive electrodes and electrolytes of spent lithium batteries, comprising acid leaching, subsequent filtering, and the addition of an aqueous Na2CO3 solution to precipitate the dissolved lithium as lithium carbonate (U2CO3). These processes extract lithium from the depleted battery and then convert it back into another lithium salt, using high temperatures or strong reagents such as acids.

Therefore, it is necessary to develop new alternative methods for the recovery of lithium salts from electrolytes directly and avoiding the use of high temperatures and/or strong chemical reagents.

DESCRIPTION OF THE INVENTION

The present invention provides a method for recovering lithium from the degradation of electrolytes in spent lithium-ion batteries. The main advantage of lithium recovery is that lithium salts can be obtained, which can be used subsequently for any purpose, for example, to manufacture new electrolytes or to be integrated into the manufacture of new batteries. According to the method of the invention, this is achieved simply, using water as the sole reagent and a medium temperature in one of the stages of the invention.

To do this, we start from the black mass of these exhausted lithium batteries, which contains the liquid lithium electrolyte of interest.

In the present invention, "black mass" refers to the powdery material obtained from the mechanical crushing of battery cells, which comprise the cathode and anode of the battery, cells that have previously been separated from the battery modules. According to the method of the present invention, the starting black mass comprises at least the liquid lithium electrolyte to be recovered, although it may also comprise fluorine salts.

The black mass to be used as a starting material in the method of the present invention may be a black mass that has not undergone any pyrolysis process (heat treatment) or a black mass that has been pyrolyzed at low temperature.

"Non-pyrolyzed black mass" is understood to mean that which has not been treated by processes that include any pyrolysis process, since in this case pyrolysis destroys the electrolyte and transforms it into other compounds.

According to the invention, "low-temperature pyrolyzed black mass" is defined as that black mass that has been subjected to pyrolysis or heat treatment(s), provided that these are at a temperature below or equal to 600 °C. Therefore, "high-temperature pyrolyzed black mass" would be that which has undergone at least one pyrolysis or heat treatment process at a temperature above 600 °C.

"Medium-temperature heat treatment" refers to any heat treatment carried out at a temperature of 700 °C or less. This contrasts with other heat treatments carried out at high temperatures above 700 °C.

A first aspect of the invention, therefore, relates to a method for recovering lithium in the form of LiF and Li3PO4, from a non-pyrolyzed black mass or pyrolyzed at a temperature below or equal to 600 °C, comprising anode and cathode powders of lithium batteries comprising a lithium electrolyte, where the method comprises the steps of:

a) hydrolyze the black mass with a dust fraction with particle size less than or equal to 25 ^m, in water at a concentration between 100 and 400 g/L, at a temperature between 25°C and 1 °C,

b) filter the suspension resulting from step a), and

c) evaporate the liquid obtained from step b) at a temperature between 40 and 70°C, for at least 1 hour per 10 liters of solution, under vacuum conditions between 10 and 30 mbar of pressure.

This particle size, less than or equal to 25 microns, has been determined using laser diffraction and a suspension of the black mass in alcohol. Other alternative techniques for measuring particle size include dynamic image analysis (DIA), static laser light scattering (SLS, also called laser diffraction), dynamic light scattering (DLS), and sieve analysis.

Using a black mass with a powder fraction of less than or equal to 25 microns as the starting raw material to be hydrolyzed has the advantage that water can more easily access the electrolyte residues and promote their dissolution.

In another even more preferred embodiment, hydrolysis step a) is carried out with a black mass/water ratio of between 100 and 200 g/L. This provides the advantage that the amount of liquid to be evaporated is even smaller and the process will be even faster.

After evaporation step c), a product consisting of lithium salts with impurities is obtained, specifically LiF, U2CO3 and LiPF6 salts, with impurities of HF and various organic compounds (which they are), a brown solid. In the event that at least one lithium salt included in the initial black mass was LiPF6, said compound decomposes on contact with air, giving rise to impurities of HF, POF3 and other organic compounds, such as alkylfluorophosphates (OPF2OR or OPF(OR)2 where R = methyl or ethyl).

Optionally, a drying step d) can also be carried out after evaporation step c), which comprises drying the solid resulting from step c) at a temperature between 60 and 80 °C for at least 60 minutes.

This stage d) allows to extract the moisture that still remains in the product obtained after the evaporation stage c), which includes lithium salts, and thus obtain a product of greater purity.

However, in order to achieve greater efficiency in the extraction of lithium from the black mass, as well as to recover lithium salts of greater purity, in addition to the steps a-c) or a-d) above, in a preferred embodiment the method of the invention comprises additionally performing the following steps e-h):

e) heat treating the product obtained in step c) at a temperature between 500°C and 750°C.

f) hydrolyze the product resulting from step e) in water at a concentration of 50 and 80 g/L at a temperature between 25 °C and 1 °C,

g) filter the suspension resulting from step f), and

h) evaporating the liquid obtained in step g), preferably at a temperature between 40 and 70°C, preferably for at least 1 hour per 10 liters of solution, preferably under vacuum conditions between 10 and 30 mbar of pressure.

Stage e) of heat treatment, where the product obtained in stage c) or in stage d) is heated at medium temperature (500-750 °C), on the one hand, makes it possible to burn or carbonize any organic impurities that may be present in the product resulting from stage c) or d); and on the other hand, it favors the re-extraction of the lithium salts in the subsequent stage e) of hydrolysis. This medium temperature heating in stage e) also favors the selective separation of the LiF and U3PO4 salts resulting from the evaporation of stage h) in the subsequent stage i) of third subsequent hydrolysis, thus favoring the yield in the lithium extraction. Preferably, the product obtained in stage c) or d) is placed in a container resistant to at least said heat treatment temperature of 500-750 °C (for example, a crucible). The heat treatment is preferably carried out in a conventional furnace, without the need to use a protective atmosphere.

Stage f), which is the second stage of hydrolysis, is preferably carried out in a reactor with mechanical agitation where a motor provides a stirrer rotation speed of between 500 and 600 rpm, in order to promote the dissolution of the product components in water.

In a preferred embodiment, the product/water ratio in step f) of hydrolysis is between 50 and 80 g/L. This preferred range of 50-80 g/L of product (resulting from step e) per liter of water provides the advantage that in the subsequent evaporation process, the amount of water to be evaporated, and, in addition, the difference in solubilities of the lithium salts is optimal.

Filtration stage g) results in the recovery of the less water-soluble lithium salt in solid form (in this case, U3PO4), and the recovery of the liquid part where the more water-soluble lithium salt is dissolved (in this case, LiF).

The evaporation stage h) results in the recovery of the LiF salt and the Li3PO4 salt, both in solid state.

However, in order to achieve a separation of the LiF salt and the U3PO4 salt (after step h both salts are mixed in solid form), and thus be able to use them separately, another preferred embodiment of the method of the invention comprises carrying out, in addition to carrying out the previous steps a-h), that is, after carrying out the evaporation step h), also carrying out the following steps i-k):

i) hydrolyze the product obtained after step h) in water at a concentration of 5 and 15 g/L, at a temperature between 25 and 1°C,

j) filter the suspension resulting from step i), and

k) evaporate the liquid obtained in step j) at a temperature between 40 and 70°C, for at least 1 hour per 10 liters of solution, under vacuum conditions between 10 and 30 mbar of pressure.

Optionally, a final drying step l) can also be carried out after evaporation step k), drying step l) comprising drying the solid resulting from step k) at a temperature of between 60 and 80 °C for at least 60 minutes.

In a preferred embodiment, drying steps d) and l) provide the advantage that optimal drying is achieved (with humidity less than 10%) in a rapid time of less than 60 minutes.

Stage i) is a third hydrolysis stage, in which the product of stage h of evaporation (LiF salts and U<3>PO<4> salts) are dissolved in water at low temperature (below 25°C but above 0°C and therefore with the water still in a liquid state). This low temperature favors a greater difference in solubility between the LiF salt (more soluble in water) and the U<3>PO<4> salt (more insoluble in water), thus achieving the dissolution of most of the LiF salt without dissolving most of the U<3>PO<4> salt. Preferably, this third hydrolysis stage is carried out with stirring, which favors the dissolution of the most water-soluble salt (LiF). This allows the solid mixture of both salts obtained after step h) to be separated into a solid part that will be mainly composed of U<3>PO<4>, and a water solution that will mainly comprise the dissolved LiF salt in solution.

In a preferred embodiment, the hydrolysis of step i) is carried out at a temperature between 10°C and 1°C, more preferably at a temperature between 5-1°C, and even more preferably at 2-1°C. This has the effect of greater dissolution of the more soluble salt (LiF) and less dissolution of the less soluble salt (U<3>PO<4>), which optimizes the dissolution of a greater part of the LiF, and that a greater part of the U<3>PO<4> remains in solid form, which provides greater efficiency to the separation process of the LiF and U3PO4 salts after performing evaporation step k). In fact, the effect of the difference in solubility between both salts is even more noticeable at lower temperatures, hence temperatures close to the freezing of water (2-1 °C) are the most preferred.

Stage j) is a third filtration stage, in which the solution from stage i) comprising a liquid (LiF-rich water solution) and a solid (mostly U3PO4) is separated by filtration into a LiF solution and a solid U3PO4 salt, this stage being key in the separation of both salts.

Stage k) is a third stage of evaporation of the liquid obtained in stage j). The result of this stage is the evaporation of the solution containing the dissolved LiF salt, thus obtaining the solid LiF salt. The solid U3PO4 salt had already been separated in the previous stage j) of filtration as the solid fraction of the filtrate. Therefore, after stage k) 2 solid fractions are obtained, one being the solid LiF salt, and the other being the solid Li3PO4 salt.

The drying stage l) allows the extraction of the remaining moisture from the LiF and U3PO4 salts obtained after the evaporation stage of stage k), and thus obtain the salts with greater purity.

Since after evaporation step k) (or after drying step l) both LiF and U3PO4 salts are separated, this allows each salt to be used for a different purpose. These salts can in fact be reused directly in the manufacture of electrolytes for new batteries. For example, LiF salts can be used to form a new electrolyte for lithium-ion batteries, while the U3PO4 salt can be used to manufacture a new electrolyte for phosphate-ion batteries. Preferably, the potential new electrolyte to be manufactured comprises only one of the salts (i.e., without mixing both salts to form a single electrolyte). These LiF and U3PO4 salts obtained according to the method can also be used for the manufacture of LiPF6 salts, thus avoiding or minimizing the consumption of virgin raw materials (such as phosphorus (P) and fluorine (F). Note that LiPF6 is a particularly preferred salt for the manufacture of lithium electrolytes, among other reasons due to its lower price and better performance as an electrolyte.

After the third stage k) of evaporation (and the subsequent optional stage l) of drying) of said salts, it is possible to obtain lithium salts LiF and LÍ3PO4 separately and, in addition, of greater purity, with a higher yield (compared to the method whose last stage was the evaporation of stage c) or the drying of stage d).

In another preferred embodiment of the invention, at least one of the hydrolysis steps a) or f) or i) is preferably carried out in a reactor with mechanical stirring, even more preferably with mechanical stirring between 500 and 600 rpm, in order to promote the dissolution of components.

In another preferred embodiment, at least one of the filtering steps b) or g) or j) is preferably carried out by means of a conventional filter, whether a press type filter, a pressure filter, a vacuum filter or any other filtration system.

In another preferred embodiment, at least one of the evaporation steps c) or h) or k) is/are carried out at a temperature of between 40°C and 70°C, more preferably at a temperature of between 50°C and 60°C, and even more preferably at a temperature of 55°C. The use of a temperature between 40 and 70°C during evaporation has the advantage that the evaporation rate is higher and consequently the evaporation time is shorter while the use of a temperature between 50-60°C, preferably 55°C, has the advantage that the material obtained has a higher degree of crystallinity.

At least one of the evaporation steps c) or h) or k) can preferably be carried out in conventional evaporation and crystallization systems or in rotary evaporators, thus achieving evaporative crystallization of the dissolved solid.

In another preferred embodiment of the method of the present invention, at least one of the evaporation steps c) or h) or k) is/are carried out under vacuum conditions of between 10 and 30 mbar of pressure. The use of vacuum allows evaporation to be carried out at lower temperatures and/or in faster times, allowing the corresponding solid to be obtained from each evaporation step c) and/or h) and/.

As has been mentioned, the method of the invention can use as a starting material any black mass from lithium ion batteries comprising an electrolyte of a lithium salt, except those in which said lithium salt electrolyte has been exposed to a high temperature pyrolysis stage, since high temperature pyrolysis (above 600°C) destroys the electrolyte. What could be used is a black mass resulting from the mixture of pyrolyzed black mass and another black mass not pyrolyzed at high temperature (above 600°C), in the latter case recovering only the electrolyte not pyrolyzed at high temperature.

In the method of the present invention, the black mass is subjected to a hydrolysis step a). It has been observed that carrying out the first prior hydrolysis of step a) of the method of the invention, carried out before any subsequent step for the recovery of lithium according to the method of the invention, on any type of black mass, reduces or avoids the presence of lithium fluoride (LiF) impurities in said black mass, since the hydrolysis dissolves at least part of said LiF salts in water, this being one of the lithium salts to be recovered.

With this first hydrolysis, two objectives are achieved: on the one hand, fluorine impurities (e.g. LiF salts) are eliminated from the black mass, both those fluorine impurities initially present in the initial components of the black mass, as well as those fluorine compounds that are degradation compounds of the used electrolyte of the black mass, which helps that if said black mass wants to be used for another process, for example for the extraction of other metals from said black mass (for example from the used electrodes Co, Mn, Ni, Fe), said extraction process is simpler and allows to obtain said metals with greater efficiency and purity, given that the fluorine impurities that hinder said extraction process of these metals from the black mass have been previously eliminated.

Furthermore, since the LiF salt is soluble in water, during hydrolysis most of it dissolves in water, which allows said LiF salt to be precipitated in solid form after the corresponding evaporation step according to the method of the invention.

This represents a significant improvement over other technologies that do not perform a prior hydrolysis step to extract metals from the spent black mass, or to extract LiF from the spent electrolyte.

Another advantage of the method of the present invention that uses hydrolysis step/s is that it manages to separate and recover the LiF, either together with other impurities after the first evaporation c), or together with the U3PO4 salt after the second evaporation h), or as a separate and pure fraction of LiF after the third evaporation k). Especially advantageous is the execution of the method whose final step is the third evaporation k) that results in the separated fraction of LiF, since possible subsequent purification steps are avoided, especially taking into account that in other conventional routes or procedures the LiF salt is obtained mixed together with other lithium salts such as for example U2CO3 or Li(OH), which is a disadvantage. In summary, the addition of an initial hydrolysis step avoids the need for extra steps to eliminate said LiF that constitutes impurities in the lithium salts to be recovered.

The main advantage of this method is that it allows for the simple recovery of lithium from the black mass of spent batteries and its reuse in the manufacture of new electrolytes. Another significant advantage, as can be seen in steps a-c) or a-d) or a-h) or a-k) or a-l) of the method, is that it does not require high temperatures (over 750°C) or strong or aggressive chemical reagents. It only requires water, which is recovered during the evaporation process and can be reused cyclically.

In another preferred embodiment of the method of the present invention, the lithium batteries from which the black matter is obtained are batteries from electric vehicles. It has been observed that, in lithium batteries from electric vehicles, the lithium electrolyte is present in greater quantities, which means that when black matter from lithium batteries from electric vehicles is used as raw material, the method of recovering lithium from the electrolyte according to the invention is more efficient since the quantities of final salts will be greater.

In summary, the present invention provides a simple method for the direct recovery of lithium salts from lithium battery electrolytes, which does not require purification steps or additional reprocessing steps for the lithium salts to be recovered. Furthermore, LiF is recovered from the electrolyte, thereby avoiding certain problems such as the presence of fluorine impurities in the black mass, as well as the use of very high temperatures or strong chemical reagents.

Another aspect of the invention also comprises a method for manufacturing a lithium electrolyte from the LiF or U3PO4 salt obtained after evaporation step k) or drying step l), where the method comprises mixing the LiF or U3PO4 salt with one or more organic carbonates selected from the group comprising ethylene carbonate (ec), diethyl carbonate (dec), dimethyl carbonate (dmc), ethylmethyl carbonate (emc), propylene carbonate (pc), methylpropyl carbonate (mpc), etc.

Throughout the description and claims, the word "comprise" and its variants are not intended to exclude other technical features, additives, components, or steps. For those skilled in the art, other objects, advantages, and features of the invention will be apparent partly from the description and partly from the practice of the invention. The following examples and figures are provided for illustrative purposes only and are not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1.- Diagram of the steps comprising an embodiment of the method of the invention.

Figure 2.- X-ray diffraction diagrams of the Black Mass BM-1 (top) and BM-2 (bottom).

Figure 3. Appearance of the liquid L1 obtained after the hydrolysis stage.

Figure 4. 31P-NMR (a) and 19F-NMR (b) spectra of liquid L1 dissolved in DMSO.

Figure 5.(a) Solid obtained after hydrolysis and evaporation (S2) and b) image obtained by scanning electron microscopy of solid S2, at 1500 magnifications.

Figure 6. 31P-NMR (a) and 19F-NMR (b) spectra of solid S2 dissolved in DMSO. (inset figure in b): appearance of the S2 suspension in DMSO.

Figure 7. Infrared spectroscopy spectra of S2 obtained from the black masses BM-1 and BM-2.

Figure 8. X-ray diffraction diagram of solid S2 obtained from BM1.

Figure 9. DTA/TGA curves obtained from solid S2.

Figure 10. Macroscopic appearance of the product obtained after heating S2 at 700 °C for 2 hours.

Figure 11. X-ray diffraction diagrams and image obtained by scanning electron microscopy for the solid obtained from U3PO4 (a) and for the solid obtained from LiF (b).

EXAMPLES

The invention will now be illustrated by tests carried out by the inventors, which demonstrate the effectiveness of the lithium salt recovery method of the invention.

Two different black masses from spent NMC batteries obtained from electric and/or hybrid vehicles (called BM-1 and BM-2) were used as starting materials. They were obtained through successive stages of discharge, primary crushing, magnetic and eddy current separation, and secondary crushing of battery cells of the indicated type. This process produces a black mass formed by the mixture of anode, cathode, and electrolyte, along with varying amounts of aluminum and copper.

A diagram of the experimental procedure followed according to an embodiment of the method of the present invention is shown in Fig. 1. Both starting black masses BM-1 and BM-2 were treated with water at a rate of 200 g/L. This step will allow the solubilization of the electrolyte present in the black masses.

After mixing for 1 h at room temperature at 500 rpm with mechanical stirring, the suspensions were filtered through a Millipore Holder filter at a pressure of 7 bar. The filtrate (L1) was evaporated in a Bucchi rotary evaporator, mod. R-100, at 12 mbar pressure. The solid obtained (S2) was dried in an oven at 100 °C for 1 h.

After drying, the solid was treated at 700 °C for 3 h in a Thermolab muffle furnace using alumina crucibles.

The resulting solid (LiF U3PO4) was characterized using various techniques. The mixture of LiF and U3PO4 was dissolved in water (S/L ratio = 3.3 g/L) taking advantage of the difference in the water solubilities of each of these compounds. The aqueous suspension was filtered, obtaining a water-insoluble solid that was characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM); and an aqueous solution that was evaporated to give a white solid that was studied by X-Ray Diffraction (XRD) and SEM. The liquid L2 is considered a residual liquor of the process.

The solid resulting from filtration (S1) is used to carry out subsequent metal recovery steps, including the lithium associated with the cathode and the remaining metals. These steps are not part of this invention.

Characterization of the initial black mass

First, each black mass was dissolved in an aqua regia solution (HNO3/HCl = 1:3, v/v) to determine the major metal content. The metal contents in the corresponding solutions were measured by atomic absorption, and the calculated weight percentage values are summarized in Table 1.

In addition, the carbon content was determined by combustion in an induction furnace and infrared detection, and the results obtained are also shown in Table 1.

The main metals found can be associated with the main components of the batteries (i.e., lithium metal oxides in the cathode material and carbon graphite in the anode material). The other metals detected—aluminum, iron, and copper—can be attributed as follows: aluminum to the cathode current collector; iron to the casing; and copper to the anode current collector. Other elements could be present in both black masses. However, their composition is less than 0.1% by weight.

Table 1. Chemical composition of the black masses studied

The X-ray diffraction patterns of the starting black masses BM-1 and BM-2 are shown in Fig. 2. In both cases, the diffraction maxima can be attributed to the lithium nickel cobalt manganese oxide phases [01-070-4315] (LiNixMnyCo1-x-yO2) and graphite [03-065-6212]. Furthermore, in the case of the black mass BM-1, a reflection maximum corresponding to the copper phase [01-089-2838] was detected. No other species were detected within the sensitivity of the X-ray diffraction measurements.

Recovery of lithium from electrolyte decomposition

The starting black masses were treated in contact with water to eliminate any possible lithium compounds in the electrolyte. As mentioned above, LiPF6 is commonly used in lithium-ion battery electrolyte solutions. However, LiPF6 hydrolysis occurs at room temperature, although complete conversion requires days to weeks (Plakhotnyk et al., 2005).

After hydrolysis of the black mass with water and subsequent filtration, a light brown liquid (L1) is obtained (Fig. 3). This liquid is identical in both black masses. A representative sample of L1 from BM1 was studied by nuclear magnetic resonance (NMR) to identify the species (fluorides and phosphorus) present in the solutions obtained.

Fig. 4 shows the NMR spectra of L1. The spectra recorded for the different samples (BM1 and BM2) are very similar. The 19F spectra present the typical doublets with chemical shifts, 5<f>= -70.14 ppm [J(P, F) = 711.3 Hz] which is assigned to PF6-. In the case of the 31P spectra, it contains a septet with 5p = -143.1 ppm [J(P, F) = 711.9 Hz] also belonging to PF6- (Parimalam et al., 2017; Plakhotnyk et al., 2005; Xu et al., 2023).

Fig. 5a shows a macroscopic view of solid S2 obtained after hydrolysis of the black mass and its subsequent evaporation. Fig. 5b shows an appearance of the particles observed by scanning electron spectroscopy at 1500x magnification, with no defined morphology, and sizes ranging from 10-20 ^m.

To verify the presence of LiPF6 in solid S2, a suspension of S2 in DMSO was prepared (20 mg S2/mL DMSO). It was observed that only a portion of the solid dissolved in DMSO. The liquid containing the soluble portion was analyzed by NMR. Fig. 6 shows the NMR spectra of the DMSO-soluble portion of S2. It is observed that the spectra are identical to those obtained for L1, indicating that LiPF6 is part of the composition of S2, although there are other non-soluble compounds in DMSO that probably correspond to organic compounds such as carbonate-based solvents (Han et al., 2019), imide-based lithium salts (Kim et al., 2020), organic lithium salts (Feng et al., 2020), and cyclic sulfur compounds (Aurbach and Chusid, 2009).

The S2 solids were subjected to a digestion process to determine the composition of each element present in them. Table 2 shows the weight percentages calculated from atomic absorption measurements. It should be noted that the weight percentage is less than 100%. This could indicate the presence of organic compounds in the solid, such as the organic solvents commonly used in lithium battery electrolytes (Moller et al., 2001; Xu et al., 2023).

Table 2. Chemical composition of the S2 solids obtained from BM1 and BM2.

The amount of S2 obtained for each of the two treated black masses is 27.9 and 35.1 g/kg of black mass for BM1 and BM2, respectively. The values in Table 2 indicate the mineral phase composition: 22.81% by weight of LiPF6, 37.33% by weight of LiF, and 9.2% by weight of U2CO3 for BM1; and 45.2% by weight of LiPF6, 35.8% by weight of LiF, and 4.5% by weight of U2CO3 for BM2. The remainder, up to 100%, should correspond to organic compounds.

Fig. 7 shows the infrared spectra of the S2 solids obtained for each black mass. The broad absorption band in the range between 3600 and 3250 cm-1 indicates the -O-H stretching vibration of water molecules v(-O-H). The weak and broad band below 3000 cm-1 shows the stretching vibrations of the -C-H bonds (v(-CH2) and v(-CH3) of EC/DMC). Around 1650 cm-1, the -C=O stretching vibrations of EC/DMC v(-C=O) and the -OH bending vibration of water 5(-O-H) are observed. There are some peaks around 1500 cm-1 corresponding to the -C=O stretching vibration of carbonate-based material v(-C=O) (Tang et al., 2017), the -CH2 bending vibration of EC 5(sc)(-CH2), and the -CH3 bending vibrations of DMC 5(-CH3). Moreover, the bands in the range of 1200–1000 cm-1 are attributed to the asymmetric and symmetric -O-C-O- stretching vibration of EC/DMC (vs(-O-C-O) and vas(-O-C-O). Finally, the -P-F stretching band of (PF6)- anion (at 843 cm-1) splits into two bands in DMC and EC. These bands correspond to the ion pairing in the latter solvents v(-P-F) (Yang et al., 2006).

Fig. 8 shows the X-ray diffraction pattern of S2 obtained from BM1 (in the case of BM2 the pattern is identical and not represented). The diffraction signals corresponding to LiPF6 [01-082-0784], LiF [01-072-1538] and LiCO3 [01 0872-0729] are shown. The intensity of the signals suggests a higher relative proportion of LiPF6 and LiF than of the other mineral phases, in agreement with the values in Table 2.

Figure 9 shows the TGA and DTA curves for heating S2 in an N2 atmosphere (heating rate 20 °C/min). The DTA curve shows a broad endothermic effect between 25 °C and 175 °C, with a peak at 62.5 °C. This effect is associated with a mass loss of 6.3% (see TGA curve). Between 175 °C and 319 °C, small endothermic signals are observed, corresponding to a mass loss of 6.5%. Finally, a sharp endothermic effect is observed between 580 °C and 632 °C, with a peak at 604 °C. This effect is associated with a mass loss of 16.62%. The total mass loss in the range 25 °C - 700 °C was 29.75%.

The observed mass losses up to 319 °C could be due to the decomposition of organic compounds, according to the FTIR results. The endothermic peak at 604 °C could correspond to the decomposition of a carbonate. Shi et al. (2020) (Shi et al., 2020) studied the composition of pure lithium carbonate and found a decomposition temperature of 727 °C in an inert atmosphere.

However, Parsierb et al. (2001) (Pasierb et al., 2001) found that in a CO<2> atmosphere, the decomposition temperature of lithium carbonate drops below 727 °C. This means that the peak observed at 604 °C could correspond to the decomposition of lithium carbonate, since in a CO<2>-rich atmosphere it could exist due to the presence of organic compounds that decompose in the range of up to 319 °C, reducing the decomposition temperature.

The presence of lithium carbonate in electrolyte degradation products was previously investigated by An et al. (2016) (An et al., 2016). The solid-electrolyte interface (SEI) is a thin layer that forms on the electrode surfaces of lithium-ion batteries during their charge and discharge cycles. The layer is formed as a consequence of complex electrochemical reactions between the electrolyte (LiPF6 and organic carbonates such as EC, DMC, PC) and the electrodes, producing by-products such as lithium fluoride (LiF) and lithium carbonate (U<2>CO<3>). The SEI acts as a protective interface, preventing further reactions between the electrodes and the electrolyte, improving the performance and longevity of the batteries.

Subsequently, S2 was heated at 700 °C for 2 h in a muffle furnace to obtain a compound whose appearance is shown in Figure 10.

Figure 11 shows the X-ray diffraction diagram of the product obtained after heat treatment at 700 °C. It can be seen that the diffraction maxima correspond to U3PO4 [00-015-076] and LiF [01-072-1538]. The formation of lithium phosphate would follow reaction (1):

The above reaction was previously demonstrated by Du et al. (Du et al., 2023) when they studied the hydrolysis under hydrothermal conditions and decomposition above 500 °C of a LiPF6-based electrolyte.

The lithium fluoride-phosphate mixture is of little use as it is not suitable for reuse in the manufacture of new batteries. However, the two compounds can be separated by taking advantage of their different solubilities in water (L3PO4 solubility: 1.34 g/L and LiF solubility: 0.4 g/L). To test this, the product obtained at 700 °C (S2700) was dissolved in water at room temperature using an S/L ratio of 3.3 g/L. The mixture was stirred and filtered. The mixture was stirred and filtered after 20 min. The insoluble solid was analyzed by X-ray diffraction. The diffraction pattern corresponds to U3PO4 [01-084-00466] with small impurities of LiF [01-072-1538] (Fig. 11a). The scanning electron microscopy image of the pseudo-hexagonal phosphate sheet-like flat crystals (Michael, 2022) is shown in the inset diagram.

The liquid obtained after filtration was evaporated in vacuo. The resulting solid was analyzed by X-ray diffraction (Fig. 11b). The diffraction pattern corresponds to a highly crystalline LiF [01-072-1538] with a very low phosphate content. The inset shows a scanning electron microscopy image of the LiF crystals, which do not display a well-defined morphology.

Table 3 shows the chemical composition of the lithium phosphate and lithium fluoride obtained, as well as their percentage composition by component. It can be seen that LiF contains a small percentage of U3PO4 (3% wt.). U3PO4 contains 9% LiF. A second purification would likely reduce this percentage.

Table 3. Chemical composition of lithium fluoride and lithium phosphate and components present in each of them.

According to the method of the present invention, lithium recovery, expressed as a percentage of lithium carbonate equivalent recovery, obtained through a mass balance and analysis of the final products of the process, is 94.6% for BM1 and 90.3% for BM2.

In lithium-ion batteries, the presence of fluorinated species in the electrolyte, including LiF, is considered essential for their role in passivating the anode surface (Ripp et al., 2009; C. Wang et al., 2019). Furthermore, the present invention enables the relatively straightforward synthesis of U3PO4, which is one of the electrolyte materials for solid-state batteries and is an essential compound for the fabrication of lithium iron phosphate batteries (Spears et al., 2022).

Therefore, this invention allows the majority of the lithium to be recovered from the electrolyte in the form of LiF and U3PO4. Furthermore, the separation and recovery of LiF prevents fluorine contamination in the lithium carbonate.

Claims (10)

1. A method for recovering lithium in the form of LiF and Li3PO4, from a non-pyrolyzed black mass or pyrolyzed at a temperature below or equal to 600 °C, comprising anode and cathode powders of lithium batteries comprising a lithium electrolyte, where the method comprises the steps of: a) hydrolyze the black mass with a dust fraction with particle size less than or equal to 25 ^m, in water at a concentration between 100 and 400 g/L, at a temperature between 25°C and 1 °C, b) filter the suspension resulting from step a), and c) evaporate the liquid obtained from step b) at a temperature between 40 and 70°C, for at least 1 hour per 10 liters of solution, under vacuum conditions between 10 and 30 mbar of pressure. 2. Method according to claim 1, step a) of hydrolysis is carried out in water at a concentration of between 100 and 200 g/L. 3. Method according to claims 1 to 2, wherein after step c) a subsequent step d) is carried out which comprises drying the solid resulting from step c) at a temperature between 60 and 80 °C for at least 60 minutes. 4. Method according to any of claims 1 to 3, comprising, in addition to steps a-c) or a-d) above, carrying out the following steps eh): e) heat treating the product obtained in step c) or step d) at a temperature between 500°C and 750°C, f) hydrolyze the product resulting from step e) in water at a concentration of 50 and 80 g/L at a temperature between 25 °C and 1 °C, g) filter the suspension resulting from step f), and h) evaporate the liquid obtained in step g), at a temperature between 40 and 70°C, for at least 1 hour per 10 liters of solution, under vacuum conditions between 10 and 30 mbar of pressure. 5. Method according to claim 4, wherein after performing evaporation step h), the following steps i-k) are also performed: i) hydrolyze the product obtained after step h) in water at a concentration of 5 and 15 g/L, at a temperature between 25 and 1°C, j) filter the suspension resulting from step i), and k) evaporate the LiF solution obtained in step j) at a temperature between 40 and 70°C, for at least 1 hour per 10 liters of solution, under vacuum conditions between 10 and 30 mbar of pressure. 6. Method according to claim 5, wherein the hydrolysis is carried out at a temperature between 10°C and 1°C, more preferably at a temperature between 5-1°C, and even more preferably at 2-1°C, 7. Method according to claim 5, wherein after performing evaporation step k), step l) is also performed, which comprises drying the solid resulting from step k) at a temperature between 60 and 80 °C for at least 60 minutes. 8. Method according to any of claims 1 to 5, wherein at least one of the evaporation steps c) or h) or k) is carried out at a temperature between 50°C and 60°C, preferably 55°C. 9. The method according to any one of claims 1 to 8, wherein the lithium batteries from which the black mass is obtained are batteries from electric vehicles. 10. Method for manufacturing a lithium electrolyte from the LiF or U3PO4 salt obtained after evaporation step k) or drying step l), respectively, described in the method according to any of claims 5, 7 and 8, wherein the method comprises mixing the LiF or Li3PO4 salt with one or more organic carbonates selected from the group comprising ethylene carbonate (ec), diethyl carbonate (dec), dimethyl carbonate (dmc), ethylmethyl carbonate (emc), propylene carbonate (pc) and methylpropyl carbonate (mpc).