Patent Publication Number: US-2022231350-A1

Title: Method for recovering and recycling electrolyte salts from lithium batteries

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for recovering and recycling electrolyte salts from lithium batteries. 
     BACKGROUND OF THE INVENTION 
     Spent lithium battery waste is on the rise and is projected to increase to millions of metric tons in the next 20 years as electric vehicle production and stationary energy storage capability increases. Profitable recycling of spent batteries will be critical to the industry as lithium battery usage increases. One component of lithium batteries that is not generally recycled is the electrolyte, and particularly the electrolyte salt, which is typically the most valuable component of the electrolyte. Recovery of lithium electrolyte salts, such as LiPF 6 , can generate additional revenue for the battery recycling industry while at the same time eliminating another waste component. One issue with recovery of the lithium electrolyte salts is inclusion of electrolyte solvents within solid, solvated recovered salts. These solvents can be difficult to selectively remove from the solvated salts, since the salts are generally soluble in various extraction solvents, and the solvents are tenaciously held within the solvated salts, making removal difficult at atmospheric pressure as well as in vacuo. This is further exacerbated by the low decomposition temperature of LiPF 6  requiring the drying temperature to be kept low. For example, recovered from ethylene carbonate crystallizes as a mixed ethylene carbonate-LiPF 6  salt (i.e., X-ray diffraction analysis shows neither an ethylene carbonate structure, nor a LiPF 6  structure). Other carbonates, such as propylene carbonate, are even more problematic, since retained propylene carbonate will be released into the electrolyte solvent when the salt is reused in a lithium battery, and can intercalate into graphite-based anodes causing undesirable expansion. 
     There is an ongoing need for methods of recovering lithium electrolyte salts from spent lithium batteries with reduced solvent content. The methods described herein address this need. 
     SUMMARY OF THE INVENTION 
     A method for recovering a purified lithium battery electrolyte salt from battery waste comprises the steps of contacting supercritical carbon dioxide (CO 2 ) under pressure (e.g., at a pressure in the range of about 1,500 to about 30,000 pounds-per-square inch (psi), preferably at least about 2,000 psi) with a solvated, solid lithium electrolyte salt with, thereby extracting solvent from the electrolyte salt into the supercritical carbon dioxide to form a waste solution. The next step is separating the waste solution from the electrolyte salt. And the final step is recovering a purified, solid lithium electrolyte salt. The solvated salt in the first step is solvated with a solvent comprising an organic carbonate. 
     In some embodiments, the method includes steps to obtain the solvated solid lithium electrolyte salt from shredded battery waste before contacting with supercritical CO 2 . In these embodiments, the first step is mixing shredded lithium batteries with an organic carbonate solvent (e.g., a solvent is selected from the group consisting of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate; preferably diethyl carbonate), thereby extracting a lithium salt-containing electrolyte away from solid battery materials. The next step is separating the solid battery materials from a liquid phase containing the electrolyte dissolved in the organic carbonate solvent. And the third step is concentrating the liquid phase via drying at atmospheric or reduced pressure to remove a majority of solvents and recover a solvated, solid lithium electrolyte salt therefrom. This solvated, solid lithium electrolyte salt is solvated with a solvent comprising an organic carbonate solvent, such as the organic carbonate used for the extraction, and may include other organic carbonate solvents or other electrolytes solvents from the waste batteries. This solvated salt is then purified with the supercritical CO 2  as described above. 
     Optionally, solvent extracted for the lithium electrolyte salt is recovered from the waste solution, e.g., by reducing the pressure of the supercritical carbon dioxide to a level where the carbon dioxide becomes a gas, which is then vented away from the solvent. In some embodiments, the pressure is reduced in two or more stages. 
     The supercritical carbon dioxide can be contacted with the solvated, solid lithium electrolyte salt by pumping the supercritical carbon dioxide through a bed or column of the electrolyte salt. Any lithium electrolyte salt that is insoluble in supercritical carbon dioxide can be purified by the methods described herein. In preferred embodiments, the electrolyte salt is LiPF 6 . 
     The electrolyte salts recovered by the process described herein can be recycled by incorporating the recovered salts in new lithium batteries, either alone, or preferably as a mixture with a non-recycled lithium electrolytes salt. 
     Some advantages of the method described herein is that carbonate solvent can efficiently extract LiPF 6 -containing electrolytes from shredded batteries, since the LiPF 6  is soluble in carbonates, and is commonly used in lithium batteries. Removal of the solvents by drying is incomplete, however, because residual carbonate solvent is strongly held in the electrolyte salt once it solidifies. The resulting solvated salt may not be suitable for use in lithium batteries, however, supercritical carbon dioxide does not substantially dissolve lithium electrolyte salts, such as LiPF 6 , but does dissolve organic carbonate solvents from solvated lithium electrolyte salts. Using sequential extraction of the electrolyte from the shredded batteries, and then extraction of solvent from the solvated lithium electrolyte salt with supercritical carbon dioxide results in a much purer final electrolyte salt. At least about 90 percent of propylene carbonate (PC) can be removed from PC-solvated LiPF 6  with supercritical carbon dioxide at 2,000 psi. This is important because, as noted above, PC can intercalate into graphite-based anodes causing undesirable expansion. Thus, if too much PC is present in the recovered LiPF 6 , this could render the material unsuitable for use in lithium batteries that have a graphite-based anode. 
     The methods described herein comprise certain novel features hereinafter fully described, which are illustrated in the accompanying drawings, and particularly pointed out in the appended claims. It is to be understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the systems, electrochemical reactors, and methods described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  provides a schematic illustration of an apparatus for supercritical carbon dioxide extraction of solvents from solvated, solid lithium electrolyte salt, such as LiPF 6 . 
         FIG. 2  provides a bar graph of LiPF 6  and propylene carbonate content of PC/LiPF 6  before and after extraction with supercritical CO 2  according to the method described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     Methods for recovering and purifying lithium electrolyte salts from spent battery electrolytes are described herein. In some embodiments, the methods couple separate, sequential solvent extraction and supercritical carbon dioxide extraction processes to significantly improve the purity of recovered lithium electrolyte salts, such as LiPF 6  from spent lithium batteries. The recovered salts are of a purity level sufficient for recycling and reuse in lithium batteries. 
     In some embodiments the method comprises first extracting electrolyte containing a lithium electrolyte salt from shredded batteries (e.g., spent batteries at the end of their useful lifetime) with an organic carbonate solvent; concentrating the extracted electrolyte in vacuo to form a solid lithium electrolyte salt that is solvated with an organic carbonate. The solvated salt is then purified by extracting solvent from the solvated, solid lithium electrolyte salt with supercritical CO 2  to purify the lithium electrolyte salt sufficiently for reuse in lithium batteries. Of course, it is to be understood that the supercritical CO 2  purification can be performed on any lithium salt that is solvated with an organic carbonate, regardless of whether the salt was extracted from batteries. In the first extraction, the organic carbonate solvent is selected based on the solubility of the lithium electrolyte salt in the solvent, as well as the volatility of the solvent, to facilitate the concentration process. The supercritical CO 2  is preferably held at a pressure in the range of about 1,500 to about 30,000 psi and is passed through a bed or column of the solvated salt. Alternatively, the supercritical CO 2  can be contacted with the solvated salt with agitation and then drained or pumped away. Solvent extracted from the solvated, solid electrolyte salt can be recovered as well by evaporating the CO 2  after removal from the salt. 
     Suitable organic carbonate solvents for extracting the electrolyte from the shredded battery materials include, e.g., diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. A preferred organic carbonate is diethyl carbonate. 
     Inorganic salts generally have a fairly low solubility in supercritical CO 2  (see e.g., M Schoeder, R A Fifer, and J B Morris, U.S. Army Research Laboratory Report No. ARL-TR-745  “The Relationship of Chemical Structure to Supercritical Fluid Solubility and to Co - Solvent - Modifier Properties: A Literature Review ” June 1995, pages 1-57; particularly pg. 7), thus any lithium salt useful in lithium battery systems would be suitable for purification by the methods described herein. For example, electrolyte salts such as LiPF 6 , LiClO 4 , and Li(TFSI) would be suitable for purification and recovery by the methods described herein. A preferred electrolyte salt is LiPF 6 . 
       FIG. 1  provides a schematic illustration of an exemplary system for extracting solvent from a solvated, solid lithium electrolyte salt. System  100  comprises CO 2  source  102  (e.g., a CO 2  tank) connected to a high-pressure pump  106  by gas line  104 . The CO 2  is pressurized by pump  106  and transferred into heat exchanger  110  through gas line  108  to cool the pressurized gas and for supercritical CO 2 . The supercritical CO 2  is passed through line  112  into extraction vessel  112 . In use vessel  112  is filled or partially filled with a bed or column of a solvated, solid lithium electrolyte salt to be purified and desolvated. The supercritical CO 2  flows through the bed or column of the electrolyte salt and extracts solvents out of the solid salt and into the supercritical CO 2 . The resulting solution of solvent in supercritical CO 2  flows through line  116  which includes a back-pressure regulator  118 , and then into collection vessel  120 . Pressure in vessel  120  is reduced by second back pressure regulator  124  so that CO 2  evaporates away through line  122 , and solvent extracted from the salt collects in vessel  120 . 
     Example 1 
     Extraction of PC from LiPF 6  was performed on a simple mixture of LiPF 6  and PC with a concentration of about 25 wt. % PC. This material was then wrapped in filter paper before being placed in a supercritical CO 2  extractor. The extractor was then pressurized with CO 2  to 2000 psi at about 30° C. The pressure was held for 15 minutes (min) and then a valve was opened to start to allow CO 2  to flow into the first separation stage. The pressure was maintained at about 1,400 psi in the extractor and about 1,200 psi in the first separation stage for 5 min by continuously pumping the CO 2  into the extractor and allowing excess pressure from the first separation stage to flow into the second separation stage, which is held at near atmospheric pressure. After this dynamic phase the pressure was allowed to slowly drop to atmospheric pressure in both the extractor and first separation stage. The materials were then collected from the extractor and analyzed using nuclear magnetic resonance with trifluoroethanol as a reference. The results of the extraction are shown graphically in  FIG. 2 , which demonstrate removal of about 90% of the PC from the material. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing materials or methods (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “consisting of” and “consists of” are to be construed as closed terms, which limit any compositions or methods to the specified components or steps, respectively, that are listed in a given claim or portion of the specification. In addition, and because of its open nature, the term “comprising” broadly encompasses compositions and methods that “consist essentially of” or “consist of” specified components or steps, in addition to compositions and methods that include other components or steps beyond those listed in the given claim or portion of the specification. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All numerical values obtained by measurement (e.g., weight, concentration, physical dimensions, removal rates, flow rates, and the like) are not to be construed as absolutely precise numbers, and should be considered to encompass values within the known limits of the measurement techniques commonly used in the art, regardless of whether or not the term “about” is explicitly stated. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate certain aspects of the materials or methods described herein and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the claims. 
     Preferred embodiments are described herein, including the best mode known to the inventors for carrying out the claimed invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the claimed invention to be practiced otherwise than as specifically described herein. Accordingly, the claimed invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the claimed invention unless otherwise indicated herein or otherwise clearly contradicted by context.