Patent Application
20210261411
The present disclosure relates to improved methods of making anhydrous metal sulfide nanocrystals.
Metal chalcogenide (MYn, where Y=S, Se, and Te) nanocrystals have attracted a great deal of attention for a wide range of applications including next generation batteries, catalysts, semiconductors, and advanced lubricants. Table 1 provides several examples of metal chalcogenides and some of their various applications. However, while metal chalcogenides are promising materials, their practical deployment in these various applications is highly constrained by both costs and the ability to control their morphology. Conventional production techniques include chemical vapor deposition, carbothermal reduction, elemental conversion, and hydrothermal synthesis. These processes are not amenable to scale-up, are energy intensive, and suffer from poor yields.
In particular, anhydrous lithium sulfide (Li2S) has great potential for use in both lithium-sulfur (Li—S) and all solid-state batteries (ASSB). Li2S may be employed directly as a cathode material in Li—S batteries and is the key component and cost driver in the production of lithium supertonic conducting (LISICON) solid-state electrolytes (SSE) including as Li10GeP2S12, Li7P3S11, and argyrodites (Li6PS5X, where X=Cl, Br, and I). Li2S currently retails for $1000/kg in bulk and up to $15,000/kg for lab scale quantities, making potential battery technologies that rely on this material prohibitively expensive and uncompetitive. This cost reflects, in part, current synthetic techniques.
Synthetic techniques for producing Li2S at large scale include carbothermal reduction of lithium sulfate:
Li2SO4(s)+2C(s)→Li2S(s)+2CO2(g). (1)
This highly endothermic reaction requires temperatures >800° C. and releases significant quantities of greenhouse gases. This information is set forth in: N. Ikeda and K. Yamamoto, “Method of manufacturing lithium sulfide,” EP 0802159 B1, 2001; and M. Kohl, J. Bruckner, I. Bauer, H. Althues, and S. Kaskel, “Synthesis of highly electrochemically active Li2S nanoparticles for lithium-sulfur-batteries”, J. Mater. Chem. A 3, 16307 (2015) (each of which are incorporated herein in their entirety).
An alternative method is the hydrosulfurization of LiX salts (X=OH, Cl, Br, etc.):
2LiX(s)+excess H2S(g)→Li2S(s)+2HX(g)+H2S(g). (2)
This reaction is also endothermic and requires elevated temperatures, though somewhat lower temperatures than the carbothermal reduction method described above. Depending on the precursor, the temperature varies from 100-500° C., with LiOH being the most reactive. Lithium carbonate (Li2CO3) reacts similarly but requires much higher temperatures. This information is set forth in: EP 0802159 B1; and C. B. Dressel, H. Jha, A.-M. Eberle, H. A. Gasteiger, and T. F. Fässler, “Electrochemical performance of lithium-sulfur batteries based on a sulfur cathode obtained by H2S gas treatment of a lithium salt”, J. Power Sources 307, 844-848 (2016) (which is incorporated herein in its entirety). This process requires the handling of the hazardous H2S at elevated temperature as well as the generation of additional corrosive species (e.g., HCl, etc.)
For battery applications, the size and uniformity of the resulting Li2S powders is important to the success of subsequent processing. Material produced in the processes described above often results in particles that are too large for use in battery applications, mandating the use of energy-intensive and time-consuming ball milling to reduce the particle size. Ball milling is effective in reducing the average particle size but does little to improve uniformity of the particles and creates the possibility of contamination from the media employed.
As an alternative, Li2S may be synthesized in a solution-based approach. In this two-step process, metallic lithium is first dissolved in alcohol, forming an alkoxide precursor and releasing valuable hydrogen gas:
2Li(s)+2ROH(sol)→2ROLi(sol)+H2(g). (3)
Next H2S diluted in an inert gas is bubbled through the solution to produce Li2S and regenerate the alcohol:
2ROLi(sol)+H2S(g)→Li2S(sol)+2ROH(sol). (4)
H2S is supplied in a stoichiometric amount and is completely abated in the process. Both of these thermodynamically favorable reactions proceed spontaneously and rapidly go to completion at room temperature. The Li2S may either precipitate out of solution directly or may be recovered by evaporation, depending on the choice of co-solvent. The net reaction is:
2Li(s)+H2S(g)→Li2S(s)+H2(g). (5)
This process offers numerous advantages including abatement of a major industrial waste gas (H2S) and the co-generation of two high value products (H2 and Li2S) that are easily separated and recovered. It has a low energy budget and the alcohol reagent may be recovered and recycled. This information is set forth in: Y. Yang, C. A. Wolden, X. Li, and R. Morrish, “Production of alkali sulfide cathode material and methods for processing hydrogen sulfide”, U.S. Pat. No. 10,399,853 issued Sep. 3 2019; X. Li, Y. Zhao, A. Brennan, M. McCeig, C. A. Wolden, and Y. Yang, “Reactive precipitation of anhydrous alkali sulfide nanocrystals with concomitant abatement of hydrogen sulfide and cogeneration of hydrogen”, Chem Sus Chem 10, 2904 (2017); Y. Zhao, Y. Yang, and C. A. Wolden, “Scalable synthesis of size-controlled Li2S nanocrystals for next-generation battery technologies”, ACS Appl. Energy Mater. 2, 2246 (2019); K. Hietala, Y. Zhao, Y. Yang, and C. A. Wolden, “Scalable synthesis of alkali sulfide nanocrystals using a bubble column reactor”, Ind Eng. Chem. Res. 57, 8436 (2018); and Y. Zhao, W. Smith, and C. A. Wolden, “Scale-up of Li2S nanocrystal synthesis and application to solid state electrolytes”, J. Electrochem. Soc, 167, 070520 (2020) (each of which are incorporated herein in their entirety). However, while this process is thermodynamically favorable, it is still cost prohibitive as it requires the use of metallic lithium, which is 2-5 times more expensive on a lithium basis than the leading lithium battery precursors (e.g., Li2CO3, LiCl, and LiOH). Moreover, metallic lithium is pyrophoric and therefore requires careful handling in an expensive glove box environment, and the use of hazardous H2S remains a concern.
The present invention provides alternative methods to synthesis Li2S and other metal or metalloid sulfide (MSn) nanocrystals with significant advantages over the known processes described above.
One method disclosed herein is a solution-based process that employs low-cost lithium salts as a reagent and utilizes one or more metathesis reactions that are preferably spontaneous at ambient or near ambient conditions and that proceed rapidly to completion with near 100% efficiency. The products are readily phase-separated from the organic solution, which may then be recovered and recycled. The method advantageously produces Li2S nanocrystals having a morphology and particle size distribution suitable for use in battery applications, eliminating the need for further process steps.
Another method disclosed herein involves the same reagents but the metathesis reaction is conducted in the solid-state at elevated temperature without solvent. The resulting products are then phase separated in organic solution, retaining the ability to maintain control over Li2S crystal size and morphology.
The methods disclosed herein comprise mixing a first sulfide salt and a first lithium salt; allowing sufficient time to form Li2S and a product salt; and recovering the Li2S. In some embodiments, the first sulfide salt and the first lithium salt are mixed in a solution. The product salt is insoluble in or only sparingly soluble in the solution and the product salt is separated from the solution prior to recovering the Li2S. The solution may comprise a polar solvent having a boiling point of about 150° C. or less, and Li2S may be recovered via solvent evaporation in an inert atmosphere or in the presence of H2S. In some preferred embodiments, the first sulfide salt and the first lithium salt spontaneously react to form the Li2S at room temperature.
In other embodiments, the first sulfide salt and the first lithium salt are mixed, without the addition of a solvent, and the mixture is heated to a temperature of at least 400° C. The Li2S and the product salt are added to a solution in which the product salt is insoluble in or only sparingly soluble in and the product salt is separated from the solution prior to recovering the Li2S. The first solution may comprise a polar solvent having a boiling point of about 150° C. or less, and Li2S may be recovered via solvent evaporation in an inert atmosphere or in the presence of H2S.
In some embodiments, the method further comprises annealing the recovered Li2S at a temperature of about 150° C. to about 300° C. in an inert atmosphere or in the presence of H2S.
In some embodiments, the Li2S is in the form of nanocrystals have a volume-averaged mean particle size (D50) from 5 nm to 50 nm.
In some embodiments, the first sulfide salt is selected from the group consisting of Na2S, K2S, Rb2S, Cs2S, Fr2S, (NH4)2S, P2S5, NiS, and combinations thereof.
In some embodiments, the first lithium salt is selected from the group consisting of a lithium halide, lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium sulfate (Li2SO4), lithium sulfite (Li2SO3), lithium amide (LiNH2), lithium nitride (LiN3), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), and combinations thereof.
In an aspect of the invention, the Li2S produced in either of the two approaches described above can then be used as a reactant for the metathesis of additional sulfides. This reaction regenerates the lithium salt, which can then be used to produce additional Li2S. In other words, the lithium salt is not consumed but is simply a reactive intermediate that facilitates the synthesis of other desired metal or metalloid sulfides. This circular process is described as cascaded metathesis.
In some embodiments, the method further comprises mixing the recovered Li2S and a non-lithium containing salt; allowing sufficient time to form a metal or metalloid sulfide (MSn) and a second lithium salt; and recovering the MSn. In some embodiments, the recovered Li2S and non-lithium containing salt are mixed in a solution. The MSn is insoluble in or only sparingly soluble in the solution and MSn is separated from the solution. The solution may comprise a polar aprotic solvent having a boiling point of about 150° C. or less.
In other embodiments, the recovered Li2S and non-lithium containing salt are mixed, without the addition of a solvent, and the mixture is heated to a temperature of at least 400° C. MSn and the product salt are added to a solution in which MSn is insoluble in or only sparingly soluble in and it is separated from the solution. The solution may comprise a polar aprotic solvent having a boiling point of about 150° C. or less.
In some embodiments, the non-lithium containing salt comprises a metal or metalloid cation and an anion selected from the group consisting of a halide, hydroxide, carbonate, sulfate, sulfite, nitrate, nitrite, phosphate, acetate, citrate, and combinations thereof.
In some embodiments, MSn is selected from the group consisting of Cr2S3, MnS, ReS2, FeS2, RuS2, OsS2, CoS2, RhS2, IrS3, NiS2, PdS, PtS, HfS2, NbS2, TaS2, GeS2, SiS2, TiS2, SnS2, MoS2, ZrS2, CdS, ZnS, VS2, WS2, Al2S3, CaS, and MgS.
In some embodiments, the method further comprises recovering the second lithium salt and recycling the second lithium salt second to be used as the first lithium salt.
This Summary of the Invention is neither intended nor should it be construed as being representative of the full extent and scope of the present disclosure. The present disclosure is set forth in various levels of detail in the Summary of the Invention as well as in the attached drawings and the Detailed Description of the Invention, and no limitation as to the scope of the present disclosure is intended by either the inclusion or non-inclusion of elements, components, etc. in this Summary of the Invention. Additional aspects of the present disclosure will become more readily apparent from the Detailed Description, particularly when taken together with the drawings.
The phrases “at least one,” “one or more,” “or,” and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C,” “at least one of A, B, or C,” “one or more of A, B, and C,” “one or more of A, B, or C,” “A, B, and/or C,” and “A, B, or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together.
The term “a” or “an” entity refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more,” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising,” “including,” and “having” can be used interchangeably.
Unless otherwise indicated, all numbers expressing quantities, dimensions, conditions, ratios, ranges, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about” or “approximately”. Accordingly, unless otherwise indicated, all numbers expressing quantities, dimensions, conditions, ratios, ranges, and so forth used in the specification and claims may be increased or decreased by approximately 5% to achieve satisfactory results. In addition, all ranges described herein may be reduced to any sub-range or portion of the range.
The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Accordingly, the terms “including,” “comprising,” or “having” and variations thereof can be used interchangeably herein.
It shall be understood that the term “means” as used herein shall be given its broadest possible interpretation in accordance with 35 U.S.C., Section 112(f). Accordingly, a claim incorporating the term “means” shall cover all structures, materials, or acts set forth herein, and all of the equivalents thereof. Further, the structures, materials, or acts and the equivalents thereof shall include all those described in the Summary, Brief Description of the Drawings, Detailed Description, Abstract, and Claims themselves.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the disclosed system and together with the general description of the disclosure given above and the detailed description of the drawings given below.
Disclosed herein are methods of producing Li2S and other metal or metalloid sulfide (MSn) nanocrystals. In some embodiments, Li2S is a final reaction product. In other embodiments, Li2S is an intermediate and is reacted to form other metal or metalloid sulfide nanocrystals.
An aspect of the invention is a method of producing Li2S nanocrystals. The process involves mixing a sulfide salt and a lithium salt in one or more solvents. The sulfide salt may be an alkali metal sulfide salt (M2S, where M=Na, K, Rb, Cs, and Fr) or another sulfide salt, such as, by way of non-limiting example, (NH4)2S, P2S5, and NiS. The lithium salt may be a lithium halide such as LiF, LiCl, LiBr, and LiI, or another lithium salt, such as, by way of non-limiting example, LiOH, LiNH2, LiN3, and LiNO3. These salts are represented by the formula LiX, where X is an anion with a formal charge of −1. Other suitable lithium salts may be Li2CO3, Li2SO4, and Li2SO3. These salts are represented by the formula Li2X, where X is an anion with a formal charge of −2. Another suitable lithium salt may be Li3PO4. Li2S is formed via a metathesis or counter-ion exchange reaction in a suitable solvent. Two general reactions to produce Li2S are:
2LiX+M2→Li2S+2MX (6); and
Li2X+M2→Li2S+M2X. (7)
In an embodiment, the method comprises: preparing a solution comprising the sulfide salt; preparing a solution comprising the lithium salt; mixing the two solutions; and allowing sufficient time for the reaction to form Li2S and a product salt (MX or M2X). In another embodiment, the method comprises: preparing a solution of one of the sulfide salt or the lithium salt; adding the other of the sulfide salt or the lithium salt to the solution; and allowing sufficient time for the reaction to form Li2S and the product salt.
In embodiments, the formation of Li2S occurs spontaneously at low temperatures (i.e., temperatures of about 50° C. or less) and is ideally spontaneous under ambient or room temperatures (i.e., about 18° C. to about 28° C.). In some embodiments, the sulfide salt and the lithium salt spontaneously react to form Li2S at a temperature of about 50° C. or less, at a temperature of about 45° C. or less, at a temperature of about 40° C. or less, at a temperature of about 35° C. or less, at a temperature of about 30° C. or less, at a temperature of about 25° C. or less, at a temperature of about 20° C. or less, at a temperature of about 15° C. or less, at a temperature of about 10° C. or less, at a temperature of about 5° C. or less, at a temperature of about 0° C. or less, at a temperature of about −5° C. or less, at a temperature of about −10° C. or less, at a temperature of about −15° C. or less, at a temperature of about −20° C. or less, at a temperature of about −25° C. or less, at a temperature of about −30° C. or less, at a temperature of about −35° C. or less, at a temperature of about −40° C. or less, at a temperature of about −45° C. or less, at a temperature of about −50° C. or less, at a temperature of about −55° C. or less, at a temperature of about −60° C. or less, at a temperature of about −65° C. or less, or at a temperature of about −70° C. or less. In some embodiments, the reaction may be performed under ambient conditions (i.e., at room temperature). In some embodiments, the solution may be heated to a temperature of about 50° C., to a temperature of about 45° C., to a temperature of about 40° C., to a temperature of about 35° C., or to a temperature of about 30° C. In other embodiments, the solution may be cooled to a temperature of about 15° C., to a temperature of about 10° C., to a temperature of about 5° C., to a temperature of about 0° C., to a temperature of about −5° C., to a temperature of about −10° C., to a temperature of about −15° C., to a temperature of about −20° C., to a temperature of about −25° C., to a temperature of about −30° C., to a temperature of about −35° C., to a temperature of about −40° C., to a temperature of about −45° C., to a temperature of about −50° C., to a temperature of about −55° C., to a temperature of about −60° C., to a temperature of about −65° C., or to a temperature of about −70° C.
The reaction products, the Li2S and the product salt, may be separated from the solvent using standard chemical methods. For instance, depending upon the solvent or co-solvents, Li2S may either be precipitated from the solution upon formation or recovered via solvent evaporation. In some embodiments, the product salt is only sparingly soluble in the solvent and may be precipitated from the solution upon formation and then separated from the solvent by settling, centrifugation, filtration, decantation, or other suitable techniques. As used herein, “sparingly soluble” means that it requires about 1 to 2 L of the solvent to dissolve about 1 g the solute. The Li2S may then be recovered from the solvent via solvent evaporation once the product salt has been separated. The recovered Li2S may be in the form of nanocrystals. The evaporation process may be conducted at temperatures above the boiling point of the solvent and preferably less than about 150° C. The evaporation process may be conducted under an inert atmosphere, in the presence of H2S, or under reduced pressure. The solvent may be captured and reused in the process.
The sulfide salt is preferably in an anhydrous form. Examples of suitable sulfide salts include, but are not limited to Na2S, K2S, Rb2S, Cs2S, Fr2S, (NH4)2S, P2S5, NiS, and combinations thereof. In an embodiment, the sulfide salt is anhydrous Na2S, which can be made in a number of ways. Sodium sulfide is typically sold commercially as hydrate flakes which may contain approximately 40% water by weight. The commercial Na2S.xH2O may be dehydrated before use in reaction or perhaps in situ through the addition of hygroscopic compounds (e.g., CaSO4, molecular sieves, etc.). The Na2S may also be purified by reaction at elevated temperature, for example by reaction with H2, carbon, H2S, and/or S8. Therefore, anion impurities other than sulfide S2− (e.g., SO32−, S2O32−, SO42−, HS−, Sx2−, OH−) can be removed.
The lithium salt is also preferably in an anhydrous form. The lithium salt may have a general formula of LiX, Li2X, or Li3X where X is a singly, doubly, or triply charged anion, respectively. Examples of suitable lithium salts include, but are not limited to, LiF, LiCl, LiBr, LiI, LiOH, LiNH2, LiN3, and LiNO3 Li2CO3, Li2SO3, Li2SO4, Li3PO4, and combinations thereof. In an embodiment, the lithium salt is anhydrous LiCl.
The one or more solutions may contain any suitable solvent or mixture of solvents to dissolve the reactants, facilitate the reaction to form the Li2S, and also their recovery. The solvent may contain one or more co-solvents. In some embodiments, the solvent may be chosen such that the product salt (2MX or M2X) is not soluble or is only sparingly soluble therein, while Li2S is soluble therein. In other embodiments, the solvent may be chosen such that Li2S is not soluble or are only sparingly soluble therein, while the product salt is soluble therein. In embodiments, the solvent comprises one or more volatile organic compounds that preferably have a boiling point of less than about 150° C. The solvent is preferably substantially free of water. In some embodiments, the solvent is selected from the group consisting of alcohols, ethers, esters, ketones, amides, and combinations thereof. Suitable solvents and co-solvents of the present invention include, by way of non-limiting example, C2-C5 alcohols, tetrahydrofuran (THF), dimethylformamide (DMF), and acetonitrile. In preferred embodiments, the solvent is a polar solvent such as an alcohol, preferably one or more of ethanol, isopropanol, propanol, butanol, and combinations thereof; these solvents may aid in the precipitation of the product salt from the solution.
The sulfide salt and the lithium salt are mixed together in solution and allowed to react for a sufficient amount of time such that the reaction produces Li2S and a product salt. In certain embodiments, the mixture contains approximately stoichiometric amounts of the sulfide salt and the lithium salt. In some embodiments, the mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 48 hours, between about 0 minutes to about 36 hours, between about 0 minutes to about 30 hours, between about 0 minutes to about 24 hours, between about 0 minutes to about 18 hours, between about 0 minutes to about 12 hours, between about 0 minutes to about 10 hours, between about 0 minutes to about 8 hours, between about 0 minutes to about 6 hours, between about 0 minutes to about 5 hours, between about 0 minutes to about 4 hours, between about 0 minutes to about 3 hours, between about 0 minutes to about 2 hours, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 30 to about 60 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion. In some embodiments, the yield of Li2S nanocrystals is greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 91%, greater than about 92%, greater than about 93%, greater than about 94%, greater than about 95%, greater than about 96%, greater than about 97%, greater than about 98%, or greater than about 99%.
In some embodiments, once Li2S has been recovered or separated from the solution, it is then annealed. Annealing removes any residual solvent and may also improve the crystallinity and particle size distribution of the particles. Annealing is performed by subjecting the Li2S powders to temperatures ranging from about 100° C. to about 350° C., preferably from about 150° C. to about 300° C., or more preferably from about 200° C. to about 250° C., for a sufficient period of time, for example, ranging from about 0 to 3 hours in an inert atmosphere, preferably 1 to 2 hours in an inert atmosphere.
An alternative approach to Li2S synthesis is through a solid-state reaction at elevated temperature. The reactions are as described in (6) and (7) above, but no solvent is present. In this case stoichiometric amounts of a metal sulfide salt (MnS) and a lithium salt (LinX), which are disclosed above, are mixed together and heated in an inert environment to a sufficient temperature and for a sufficient amount of time. The reaction goes to completion, producing Li2S and the corresponding product salt. This solid-state mixture is separated by using a solvent that preferentially dissolves Li2S, such as for example a C2-C5 alcohol, while the product salt remains in solid form therein. After the product salt is separated from the solvent by centrifugation, filtration, settling, decantation, or other means, Li2S is recovered from solution by evaporating the solvent and it may also be annealed as described above.
The solution-free reaction between metal sulfide salt (MnS) and lithium salt (LinX) occurs at elevated temperatures, generally above 400° C. In some embodiments, the reactants are heated to a temperature of about 450° C. or more, to a temperature of about 475° C. or more, to a temperature of about 500° C. or more, to a temperature of about 525° C. or more, to a temperature of about 550° C. or more, to a temperature of about 575° C. or more, to a temperature of about 600° C. or more, to a temperature of about 625° C. or more, to a temperature of about 650° C. or more, to a temperature of about 675° C. or more, to a temperature of about 700° C. or more, to a temperature of about 725° C. or more, to a temperature of about 750° C. or more, to a temperature of about 775° C. or more, or to a temperature of about 800° C. In some embodiments, the reactants are heated to a temperature of no more than about 800° C.
In some embodiments, the solution-free mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 180 minutes, between about 0 minutes to about 120 minutes, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 60 to about 120 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion.
The methods disclosed herein can be used to produced Li2S nanocrystals that have a well-defined morphology and particle size distribution. As used herein, unless otherwise specified, the term “particle size” refers to a volume-averaged mean particle size as defined by X-ray scattering and diffraction, also be referred to as “D50” values. The polydispersity index (PDI) is a measure of the heterogeneity of a sample based on size and is the mean size based on volume divided by the mean size based on number. In some embodiments, the Li2S nanocrystals have a particle size of less than 100 nm. In some embodiments, the Li2S nanocrystals have a particle size from 1 nm to 50 nm, from 1 nm to 45 nm, from 1 nm to 35 nm, from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 15 nm, from 1 nm to 10 nm, from 10 nm to 50 nm, from 15 nm to 50 nm, from 20 nm to 50 nm, from 25 nm to 50 nm, from 30 nm to 50 nm, from 35 nm to 50 nm, from 40 nm to 50 nm, from 15 nm to 45 nm, or from 20 nm to 40 nm. In some embodiments, the Li2S nanocrystals have a PDI of 2 or less, of 1.8 or less, of 1.6 or less, of 1.4 or less, of 1.2 or less, of 1.0 or less, of 0.8 or less, of 0.6 or less, or of 0.4 or less. In some embodiments, the Li2S nanocrystals are in the form of nanoflakes. In some embodiments the nanoflakes are arranged in cauliflower-like agglomerates.
While Li2S is a valuable reaction product, it may also be used a precursor to form other metal or metalloid sulfide (MSn) nanocrystals. Another aspect of the invention is the use of Li2S as intermediate in a series of reactions to form other metal or metalloid sulfide nanocrystals. The synthesis of these other metal or metalloid sulfide nanocrystals is performed by employing at least two metathesis or counter-ion exchange reactions. A first metathesis reaction results in the formation of Li2S, which is then used to synthesize MSn nanocrystals (such as those shown in Table 1) through a second metathesis reaction with an appropriate salt (MX2n). The second metathesis reaction may be represented by:
nLi2S+MX2n→MSn+2nLiX. (8)
Here M is a metal or metalloid cation and n is an integer of 1 to 4, typically either 1 or 2. Non-limiting examples of M include, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Hf, Nb, Ta, Sn, Zr, V, Mo, W, Ge, Ti, Al, Ca, Mg, Cd, Zn, and Si. The MX2n salt does not comprise lithium. The non-lithium containing salt (MX2n) comprises an anion that may be selected from the group consisting of a halide, hydroxide, carbonate, sulfate, sulfite, nitrate, nitrite, phosphate, acetate, and citrate. The non-lithium containing salt preferably is chosen such that the reaction regenerates the same lithium salt that is used in the first metathesis reaction to form Li2S (e.g., reaction (6)).
Li2S may be prepared according to any of the methods described above. In some embodiments, Li2S and the non-lithium salt (MX2n) are mixed together in solution or suspension or a combination thereof and allowed to react for a sufficient amount of time such that the reaction produces MSn and a lithium salt. In an embodiment, the method comprises: preparing a solution comprising Li2S; preparing a solution comprising the non-lithium salt; mixing the two solutions; and allowing sufficient time for the reaction to form MSn nanocrystals and the product lithium salt. In another embodiment, the method comprises: preparing a solution of one of Li2S or the non-lithium salt; adding the other of the Li2S or the non-lithium salt to the solution; and allowing sufficient time for the reaction to form MSn and the product lithium salt.
In embodiments, the formation of MSn is spontaneous at low temperatures (i.e., temperatures of about 50° C. or less) and is ideally spontaneous under ambient or room temperature conditions. In some embodiments, Li2S and the non-lithium salt spontaneously react to form the MSn at a temperature of about 50° C. or less, at a temperature of about 45° C. or less, at a temperature of about 40° C. or less, at a temperature of about 35° C. or less, at a temperature of about 30° C. or less, at a temperature of about 25° C. or less, at a temperature of about 20° C. or less, at a temperature of about 15° C. or less, at a temperature of about 10° C. or less, at a temperature of about 5° C. or less, at a temperature of about 0° C. or less, at a temperature of about −5° C. or less, at a temperature of about −10° C. or less, at a temperature of about −15° C. or less, at a temperature of about −20° C. or less, at a temperature of about −25° C. or less, at a temperature of about −30° C. or less, at a temperature of about −35° C. or less, at a temperature of about −40° C. or less, at a temperature of about −45° C. or less, at a temperature of about −50° C. or less, at a temperature of about −55° C. or less, at a temperature of about −60° C. or less, at a temperature of about −65° C. or less, or at a temperature of about −70° C. or less. In some embodiments, the reaction may be performed under ambient conditions. In some embodiments, the solution may be heated to a temperature of about 50° C., to a temperature of about 45° C., to a temperature of about 40° C., to a temperature of about 35° C., or to a temperature of about 30° C. In other embodiments, the solution may be cooled to a temperature of about 15° C., to a temperature of about 10° C., to a temperature of about 5° C., to a temperature of about 0° C., to a temperature of about −5° C., to a temperature of about −10° C., to a temperature of about −15° C., to a temperature of about −20° C., to a temperature of about −25° C., to a temperature of about −30° C., to a temperature of about −35° C., to a temperature of about −40° C., to a temperature of about −45° C., to a temperature of about −50° C., to a temperature of about −55° C., to a temperature of about −60° C., to a temperature of about −65° C., or to a temperature of about −70° C.
In certain embodiments, the mixture contains approximately stoichiometric amounts of Li2S and the non-lithium salt. In certain embodiments, the mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 48 hours, between about 0 minutes to about 30 hours, between about 0 minutes to about 24 hours, between about 0 minutes to about 18 hours, between about 0 minutes to about 12 hours, between about 0 minutes to about 10 hours, between about 0 minutes to about 8 hours, between about 0 minutes to about 6 hours, between about 0 minutes to about 5 hours, between about 0 minutes to about 4 hours, between about 0 minutes to about 3 hours, between about 0 minutes to about 2 hours, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 30 to about 60 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion. In certain embodiments, the yield of MSn is greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 91%, greater than about 92%, greater than about 93%, greater than about 94%, greater than about 95%, greater than about 96%, greater than about 97%, greater than about 98%, or greater than about 99%.
The reaction products, MSn and a lithium salt, may be separated from the solvent using standard chemical methods. For instance, depending upon the solvent or co-solvents, MSn may either be precipitated from the solution or recovered via solvent evaporation. In some embodiments, MSn is only sparingly soluble in the solvent and may be precipitated from the solution upon formation and then separated from the solution by settling, centrifugation, filtration, or other suitable techniques. The lithium product salt may then be recovered from the solvent via solvent evaporation. The lithium product salt may be recycled back to the first metathesis reaction to be used a reactant to form Li2S (e.g., via equation (6) above). The solvent may be captured and reused in the process.
The one or more solutions for the second metathesis reaction may contain any suitable solvent or mixture of solvents to dissolve the reactants and facilitate the reaction to form and separate the MSn powders. The solvent may contain one or more co-solvents. In some embodiments, the solvent may be chosen such that the product lithium salt is not soluble in or is only sparingly soluble therein, while MSn is soluble therein. In other embodiments, the solvent may be chosen such that MSn is not soluble or is only sparingly soluble therein, while the lithium salt is soluble therein. In embodiments, the solvent comprises one or more volatile organic compounds that preferably have a boiling point of less than about 150° C. The solvent is preferably substantially free of water. In some embodiments, the solvent comprises alcohols, ethers, esters, ketones, amides, and combinations thereof. Suitable solvents and co-solvents of the present invention include, by way of non-limiting example, C2-C5 alcohols, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, and acetone. In preferred embodiments, the solvent is a polar aprotic solvent, preferably tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, and acetone, and combinations thereof such solvents may aid in the precipitation of MSn from the solution upon formation.
In some embodiments, once the MSn powders have been recovered or separated from the solution, they are annealed. Annealing removes any residual solvent and may also improve the crystallinity and particle size distribution of the powders. Annealing is performed by subjecting MSn to temperatures ranging from about 100° C. to about 350° C., preferably from about 150° C. to about 300° C., or more preferably from about 200° C. to about 250° C., for a period of time, for example, ranging from about 0 to 3 hours in an inert atmosphere, preferably 1 to 2 hours in an inert atmosphere.
As an alternative to the solution based approach, MSn may also be formed through a solid-state or solvent-free reaction at elevated temperature. In this case stoichiometric amounts of Li2S and a suitable non-lithium containing salt (MX2n), which are disclosed above, are mixed together and heated in an inert environment to sufficient temperature and for a sufficient amount of time. The reaction goes to completion, producing MSn and the corresponding lithium containing product salt. This solid-state mixture is separated by using a solvent that preferentially dissolves lithium containing product salt, such as for example polar aprotic solvent, while MSn remains in solid form therein. MSn may then be separated from the solvent by centrifugation, filtration, settling, decantation, or other means and the MSn powders may be annealed as described above. The lithium containing product salt can be recovered from solution by evaporating the solvent.
The solvent-free reaction between metal or metalloid salt (MX2n) and lithium sulfide (Li2S) occurs at elevated temperatures, generally above 400° C. In some embodiments, the reactants are heated to a temperature of about 450° C. or more, to a temperature of about 475° C. or more, to a temperature of about 500° C. or more, to a temperature of about 525° C. or more, to a temperature of about 550° C. or more, to a temperature of about 575° C. or more, to a temperature of about 600° C. or more, to a temperature of about 625° C. or more, to a temperature of about 650° C. or more, to a temperature of about 675° C. or more, to a temperature of about 700° C. or more, to a temperature of about 725° C. or more, to a temperature of about 750° C. or more, to a temperature of about 775° C. or more, or to a temperature of about 800° C. In some embodiments, the reactants are heated to a temperature of no more than about 800° C.
In some embodiments, the solvent-free mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 180 minutes, between about 0 minutes to about 120 minutes, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 60 to about 120 minutes. In some embodiments, the reaction time is sufficient such that the reaction proceeds to completion.
The methods disclosed herein may be used to produce MSn nanocrystals that have a well-defined morphology and particle size distribution. In some embodiments, the MSn nanocrystals have a particle size of less than 100 nm. In some embodiments, the MSn nanocrystals have a particle size from 1 nm to 50 nm, from 1 nm to 45 nm, from 1 nm to 35 nm, from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 15 nm, from 1 nm to 10 nm, from 10 nm to 50 nm, from 15 nm to 50 nm, from 20 nm to 50 nm, from 25 nm to 50 nm, from 30 nm to 50 nm, from 35 nm to 50 nm, from 40 nm to 50 nm, from 15 nm to 45 nm, or from 20 nm to 40 nm. In some embodiments, the MSn nanocrystals have a PDI of 2 or less, of 1.8 or less, of 1.6 or less, of 1.4 or less, of 1.2 or less, of 1.0 or less, of 0.8 or less, of 0.6 or less, or of 0.4 or less.
The cascaded metathesis approach, may be used to produce a variety of MSn powders. In some embodiments, Li2S may be formed from the reaction of Na2S with LiCl, shown in reaction (9), in alcohol or another suitable solvent. This reaction is spontaneous at ambient temperature.
2LiCl(sol)+Na2S(sol)→Li2S(sol)+2NaCl(s)ΔG°≈−90 kJ/mol (9)
A first solution may comprise less than about 12 g/100 g of Na2S in ethanol, preferably from about 9 g/100 g to about 11.5 g/100 g of Na2S in ethanol, and a second solution may comprise less than about 25 g/100 g of LiCl in ethanol, preferably from about 15.5 g/100 g to about 25 g/100 g of LiCl in ethanol. Stoichiometric amounts of Na2S and LiCl may then be mixed after both have been dissolved in ethanol in the first and second solutions. The sodium chloride precipitates out of solution as it is sparingly soluble in ethanol, while Li2S remains dissolved. The NaCl precipitate is removed from the solution by standard techniques and Li2S is then recovered by solvent evaporation.
In other embodiments, Li2S may be formed from the reaction of Na2S with LiCl by heating in an inert environment. Stoichiometric amounts of Na2S and LiCl powders may be mixed and heated to temperatures of about 600° C. under inert atmosphere. The reaction products are a mixture of NaCl and Li2S which can be separated using ethanol, which dissolves Li2S while NaCl is sparingly soluble. The NaCl precipitate is removed from the solution by standard techniques and the Li2S powders can then recovered by solvent evaporation as described previously.
The recovered Li2S may then be reacted through a second metathesis reaction with a chloride salt, as shown in reaction (10):
When the second metathesis reaction is conducted in a polar aprotic solvent (e.g., THF, DMF, acetonitrile, etc.) the MSn product precipitates from solution, while LiCl remains dissolved in the solution. Reaction (10) regenerates LiCl which can be recovered via solvent evaporation and used in reaction (9). Many examples of MSn synthesis via Reaction (10) have been demonstrated and can be found in R. R. Chianelli, M. B. Dines, “Low-Temperature Solution Preparation of Group 4B, 5B and 6B Transition-Metal Dichalcogenides”, Inorg. Chem. 17, 2758 (1978); R. R. Chianelli, E. B. Prestridge, T. A. Pecoraro, J. P. Deneufville, “Molybdenum Disulfide in the Poorly Crystalline “Rag” Structure”, Science 203, 1105 (1979); and T. Pecoraro, R. R. Chianelli, “Hydrodesulfurization catalysis by transition metal sulfides” J. of Catal. 67, 430 (1981) (each of which is incorporated herein in their entirety).
Alternatively, the second metathesis could be conducted thermally without solvent using the solid-state method described herein.
The central thesis of this approach is to couple reactions (9) and (10), or the analogous solid-state reactions, to enable the cascaded metathesis of innumerable metal sulfides using Li compounds as recycled intermediates. This reaction sequence is shown in
The direct implementation of reaction (11) is commonly utilized to synthesize a number of metal sulfide compounds. The key benefits of the cascaded metathesis approach, rather than the direct approach, are threefold: First, the cascaded approach allows for the removal of the lithium salt byproduct from the product using a nonprotic or weakly protic solvent. In the direct approach, the salt byproduct is typically NaCl, which can only be removed by dissolving in strongly protic solvents such as water, methanol, or ethylene glycol. Many metal sulfides and/or their metal chloride precursors cannot tolerate strongly protic solvents such as those listed above because of their tendency to undergo hydrolysis. Therefore, in some cases, it is beneficial to form LiCl as the byproduct rather than NaCl due to the high solubility of LiCl in nonprotic or weakly protic polar solvents. These solvents, such as tetrahydrofuran or dimethylformamide, can be tolerated by many metal sulfides/chlorides. The second benefit is that the use of nonprotic or weakly protic solvents can impart a unique morphology or particle size distribution to the MSn product, which may be beneficial for certain applications. These morphologies are often not accessible when using the direct approach. The third benefit is that the net reaction achieves the previous two advantages with no net consumption of lithium, which could be prohibitively expensive.
The reagents involved are low cost salts and the reactions are preferably spontaneously, proceeding to completion under ambient conditions or near ambient conditions in solution. The only significant energy requirements are those associated with the vaporization of volatile organic solvents for the recovery of the Li intermediates. The solvents may be readily condensed and recycled and the MSn product may be directly recovered in nanocrystal form.
By coupling reactions (9) and (10), Li2S can be formed and then be used to facilitate the synthesis other metal or metalloid sulfides of interest. By way of non-limiting examples, the following metal or metalloid sulfides can be produced from Li2S:
2Li2S(s)+SnCl4(sol)→SnS2(s)+4LiCl(sol)ΔG°=−382 kJ/mol; (12)
2Li2S(s)+SiCl4(sol)→SiS2(s)+4LiCl(sol)ΔG°=−248 kJ/mol; (13)
3Li2S(s)+2AlCl3(sol)→Al2S3(s)+6LiCl(sol)ΔG°=−449 kJ/mol; (14)
Li2S(s)+CaCl2(sol)→CaS(s)+2LiCl(sol)ΔG°=−56 kJ/mol (15); and
Li2S(s)+MgCl2(sol)→MgS(s)+2LiCl(sol)ΔG°=−77 kJ/mol. (16)
These reactions are all highly thermodynamically favorable, driven by the exothermicity of salt precipitation. Examples of metal or metalloid sulfides that may be formed using the cascaded metathesis approach are Cr2S3, MnS, ReS2, FeS2, RuS2, OsS2, CoS2, RhS2, IrS3, NiS2, PdS, PtS, HfS2, NbS2, TaS2, GeS2, SiS2, TiS2, SnS2, MoS2, ZrS2, CdS, ZnS, VS2, WS2, Al2S3, CaS, and MgS.
While the above reaction sequence has been disclosed with some degree of specificity, one skilled in the art will understand that other reactants may be employed. In addition, the reaction conditions (e.g., temperature and pressure) may be varied to optimize the production of the Li2S and/or MSn nanocrystals. The reactions disclosed herein may conducted in a batch process or in a continuous process.
Li2S nanocrystals were formed using the following solution-based method:
The resulting powders obtained from the above method were analyzed using X-ray diffraction (XRD) using a Philips X′Pert X-ray diffractometer with Cu Kα radiation (λ=0.15405 nm) between 10 and 70° at a scan rate of 5° min−1. Samples were prepared on a glass slide with a piece of Scotch Magic Tape covering the material to prevent undesired reactions with ambient moisture. The contribution from the glass slide was background subtracted with a polynomial fit.
Li2S nanocrystals were formed using the following solid-state method:
The resulting powders obtained from the above method after step (2) were analyzed using XRD using the previously described method.
Various modifications of the above-described invention will be evident to those skilled in the art. It is intended that such modifications are included within the scope of the following claims.
This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application 62/980,015 filed Feb. 21, 2020 and to U.S. Provisional Patent Application 63/023,550 filed May 12, 2020. The entirety of these applications is incorporated herein by reference.
This invention was made with government support under grant number 1825470 awarded by National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63023550 | May 2020 | US | |
| 62980015 | Feb 2020 | US |
