Patent Application
20250236982
The present invention generally relates to methods of producing purified rare earth metal deposits, and more particularly, to such methods in which electroplating is used to purify the rare earth metal.
Rare-earth elements (REEs), such as neodymium, samarium, and dysprosium, possess many unique magnetic and electrical properties. They may be used in such applications as lamp phosphors, permanent magnets, rechargeable nickel metal hydride batteries and catalysts. In permanent magnets, the REEs are used in their metallic form.
To secure the supply chain and maximize the utilization of REEs, efforts should not only be focused on the production of REEs from mineral ores but also on secondary sources such as end-of-life products, both of which require an efficient reduction process to produce REE metals for the synthesis of new products, such as magnets. Currently, the leading technology for the production of REE metals is molten fluoride salt electrolysis, a highly energy intensive and environmentally deleterious process. Therefore, there remains a keen interest in the development of alternative processes that can provide energy savings, without environmental impacts, such as room-temperature electrochemical deposition of REE metals.
Due to the high electropositivity of REE metals, a non-aqueous electrolyte with a wide electrochemical window is required for electrodeposition. Much effort has been focused on fluorine (F)-containing systems, and in particular, ionic liquids (ILs) and organic electrolytes containing bis(trifluoromethane)-sulfonimide (TFSI) or trifluoromethanesulfonate anions. While the electrodeposition of various REE metals has been reported in these systems, the application of F-containing ILs has been greatly hindered by various obstacles. Among them is the so-called passivation effect which can significantly impede stable deposition (e.g., P. Geysens et al., Phys. Chem. Chem. Phys. 2021, 23, 9070-9079). This effect likely originates from an electrically insulating layer formed by either coreduction of anions alongside the REE metal ions or decomposition of the organic solvent. This layer is likely a mixture of metal fluoride and metal organic compounds. The formation of this passivation layer is a general phenomenon that has been widely observed and studied in the electroplating processes of other highly electropositive metals, such as Li, Na, K, Ca, and Mg (e.g., C. Bao et al., Adv. Funct. Mater. 2020, 30, No. 2004891). Although this passivation layer has proven beneficial in battery applications for single-valent light elements (e.g., Li and Na), it is detrimental for metal deposition. Multivalent metals (e.g., Ca, Mg, and REE metals) with large radii are not able to diffuse through this insulating layer, which results in a significantly retarded reduction current or large overpotential for deposition and stripping. The passivation layer also introduces impurities into the product and lowers the current efficiency by introducing parasitic side reactions. This effect has been widely investigated in the electroplating of divalent metals (Mg and Ca) in F-based electrolytes, but little attention has been paid to this topic in relation to the electrodeposition of trivalent REE metals.
Thus, there would be a significant benefit in a more cost effective low-temperature method that could produce substantially pure deposits of rare earth metals. Such a method would be further advantageous if it does not rely on ionic liquids and if the resulting deposits do not contain passivated surface films.
The present disclosure is directed to an electrodeposition method for rare earth metals that overcomes the problems encountered in methods of the conventional art. In particular, the method described herein is generally lower in cost than known methods while producing substantially pure deposits of rare earth metals. The present method also advantageously does not rely on ionic liquids and produces rare earth deposits that do not contain passivated surface films, such as fluorinated passivated films. Moreover, the present method reduces the level of impurities normally incorporated in rare earth deposits by conventional electroplating methods. As further discussed below, the method achieves this by precipitating undesirable electrolyte ions before the electrodeposition process. Upon mixing, the undesirable ions precipitate by a double-salt metathesis reaction. Precipitation is achieved by selecting a solvent in which the undesirable ions are insoluble.
More particularly, the method can achieve a purified rare earth metal deposit from a rare earth metal halide salt by employing the following steps: (i) producing a rare earth metal borohydride salt (RE(BH4)3) from the rare earth metal halide salt ((RE)X3) by the following metathesis reaction:
n·ABH4+m·(RE)X3RE(BH4)3+n·AX
wherein the above reaction is conducted as a solid state reaction in the absence of a solvent, and wherein the variables are defined as follows: A is an alkali metal cation which counterbalances the borohydride (BH4−) anion; RE represents one or more rare earth metal cation(s) selected from scandium, yttrium, and lanthanide elements; X represents a halide selected from chloride, bromide, and iodide; n is the molar amount of ABH4; and m is the molar amount of (RE)X3; wherein the ratio of n:m is 3:1 to 5:1; (ii) mixing RE(BH4)3 and AX with a halogen-free organic solvent in which RE(BH4)3 is fully dissolved and in which AX is highly insoluble and precipitates, thereby forming a solution containing the RE(BH4)3 salt dissolved therein in the substantial absence of AX; and (iii) contacting the solution from step (ii) with an anode and cathode in electrical communication, and electrically charging the anode and cathode to electroplate the rare earth metal onto the cathode. In particular embodiments, A is lithium (Li). In further or separate embodiments, RE represents one or more lanthanide elements, such as Nd, Dy, and/or Sm. In further or separate embodiments, the ratio of n:m is about 4:1. In further or separate embodiments, the halogen-free organic solvent is selected from ethers, dialkyl sulfides, nitriles, and carbonates, or more particularly, ether solvents, wherein the ether solvent may be one or a combination selected from, for example, diethyl ether (EE), ethyl propyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, methyl t-butyl ether, dimethoxy ethane (monoglyme), diethylene glycol dimethyl ether (diglyme), di triethylene glycol dimethyl ether (triglyme), tetrahydrofuran, and dioxolane. In further or separate embodiments, the halogen-free organic solvent is selected from dialkyl sulfide solvents, such as one or a combination of dimethyl sulfide (MS), diethyl sulfide, dipropyl sulfide, dibutyl sulfide, and ethyl propyl sulfide.
In particular embodiments, RE borohydride salts, particularly those of Nd and Dy, are synthesized by a mechanochemical metathesis reaction between the rare earth chloride salt and LiBH4, followed by dissolution of the RE borohydride salt into a suitable organic solvent and electrodeposition of the RE from the solution. Ethyl ether (EE) and dimethyl sulfide (MS) may be particularly selected as solvents. As further discussed below, the RE borohydride-based electrolyte was characterized by cyclic voltammetry (CV), chronoamperometry, and Raman spectroscopy, and the obtained deposits were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS). Experimental results indicate that the RE borohydride-based electrolyte can effectively eliminate the passivation effect and produce substantially pure RE deposits with high rare earth content.
In a first step (i.e., step (i)) of the method, a rare earth metal borohydride salt (RE(BH4)3) is produced from a rare earth metal halide salt ((RE)X3) by the following metathesis reaction:
n·ABH4+m·(RE)X3RE(BH4)3+n·AX
The symbol “RE,” as used in the above reaction, represents one or more rare earth metal cations. Notably, the symbol “RE” may be used interchangeably with the acronym “REE”, which refers to “rare earth element”. The rare earth metal cations are generally trivalent (+3) charged. As well known, rare earth metals refer to the elements selected from scandium, yttrium, and lanthanides. The lanthanides are those elements having an atomic number of 57-71. The lanthanide elements are listed as follows: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In some embodiments, RE is selected from Nd, Dy, or Sm. The symbol “X,” as used in the above reaction, represents a halide selected from chloride, bromide, and iodide. Generally, X does not include fluoride, which is in a further effort to make the resulting electroplating solution fluorine-free to prevent formation of a fluorine-containing passivation layer on the RE deposit. Any of the halides (X) can be combined with any one or more of the RE cations to form a rare earth halide salt of the formula (RE)X3. Some examples of (RE)X3 salts include NdCl3, NdBr3, NdI3, DyCl3, DyBr3, DyI3, SmCl3, SmBr3, SmI3, LaCl3, CeCl3, PrCl3, EuCl3, GdCl3, and TbCl3. In some embodiments, RE in the (RE)X3 salt is a single RE metal, which would lead to RE in RE(BH4)3 being a single RE metal. In other embodiments, RE in the (RE)X3 salt contains at least two RE metals, which would lead to RE in RE(BH4)3 being at least two RE metals. The foregoing scenario is equivalent to two different salts, (RE)X3 and (RE)′X3, being present. In addition to the possibilities of one or more RE salts being present, one or more non-RE metal salts may be present. The non-RE metal salt may be, for example, a halide salt of aluminum or zinc.
By the metathesis reaction shown above, X groups in (RE)X3 become exchanged with BH4 groups from ABH4, thus yielding RE(BH4)3 and AX product salts. As the RE(BH4)3 product salt directly derives from the (RE)X3 reactant salt, the RE(BH4)3 product salt may be derived from any of the exemplary (RE)X3 salts provided above by simply replacing X groups with BH4 groups. Some examples of RE(BH4)3 product salts include Nd(BH4)3, Dy(BH4)3, Sm(BH4)3, La(BH4)3, Ce(BH4)3, Pr(BH4)3, Eu(BH4)3, Gd(BH4)3, and Tb(BH4)3. In some embodiments, RE in the RE(BH4)3 salt is a single RE metal and the purified rare earth metal deposited on the cathode is a single metal. In other embodiments, RE in the RE(BH4)3 salt contains at least two RE metals and the purified rare earth metal deposited on the cathode is an alloy. The foregoing scenario may equivalently be viewed as the presence of two different RE borohydride salts, RE(BH4)3 and RE′(BH4)3. In addition to the possibilities of one or more RE borohydride salts being present, one or more non-RE metal borohydride salts may be present. The non-RE borohydride metal salt may be, for example, a borohydride salt of aluminum or zinc. In the case where a non-RE metal is included, the resulting RE deposit would be an alloy composed of RE and non-RE metals. As indicated above, the non-RE metal can be included in the deposit by including a non-RE metal halide salt in the metathesis reaction. If the non-RE metal is denoted by the symbol “NRE”, the metathesis reaction may include a (NRE)X3 salt in combination with an (RE)X3 salt to form (NRE)(BH4)3 salt in combination with an (RE)(BH4)3 salt, wherein the latter two salts would be dissolved into the halogen-free organic solvent for subsequent electroplating of the NRE-RE alloy. In some embodiments, an (NRE)X3 salt, such as AlX3 salt or ZnCl2 salt, are excluded from the metathesis reaction. In some embodiments, the metathesis reaction contains solely the ABH4 and (RE)X3 reactants and the RE(BH4)3 and AX products. In some embodiments, the process yields a deposit of a substantially pure single metal rather than an alloy. In other embodiments, an alloy is formed.
The symbol “A,” as used in the above reaction, represents an alkali metal cation which counterbalances the borohydride (BH4−) anion. The alkali metal cation can be, for example, Li+, Na+, K+, Rb+, or Cs+. More typically, A is Li+ or Na+. The combination of A and BH4 results in an alkali metal borohydride salt of the formula ABH4. Some examples of alkali metal borohydride salts include LiBH4, NaBH4, KBH4, RbBH4, and CsBH4. The byproduct salt AX formed by the metathesis reaction combines the A species and X species used in the reaction. Thus, AX may be, for example, LiCl, LiBr, LiI, NaCl, NaBr, NaI, KCl, KBr, or KI depending on the A and X species used.
The subscript n is the molar amount of ABH4, while m is the molar amount of (RE)X3. For purposes of the present invention, the ratio of n:m is 3:1 to 5:1. In different embodiments, the ratio of n:m is precisely or about 3:1, 3.5:1, 4:1, 4.5:1, or 5:1, or a ratio within a range bounded by any two of the foregoing ranges (e.g., 3.5:1 to 4.5:1).
The metathesis reaction (step (i)) is conducted as a solid state reaction in the absence of a solvent. Thus, at least (or solely) ABH4 and (RE)X3 are combined and integrally mixed by a solid state mixing process. The solid state mixing process may employ, for example, ball milling, high-speed mixing, shear mixing, compounding, and/or extrusion, all of which are well known in the art. Preferably, step (i) is conducted under an inert atmosphere, which typically contains argon or nitrogen in a predominant amount relative to oxygen. In some embodiments, the inert atmosphere is substantially or completely oxygen free.
In a second step (i.e., step (ii)) of the method, the rare earth metal borohydride salt (RE(BH4)3) and AX, as produced in step (i), are mixed with a halogen-free organic solvent (i.e., “solvent”) in which RE(BH4)3 is fully dissolved and in which AX is highly insoluble and precipitates. The result is a solution containing the RE(BH4)3 salt dissolved therein in the substantial (or complete) absence of AX dissolved in the solvent. In one embodiment, the AX species is left as a precipitate in the solution in the subsequent electroplating step. The precipitate is typically settled at the bottom of the solution. In another embodiment, the precipitated AX species is removed from the solution (e.g., by filtering or centrifuging or both) to form a substantially (or completely) AX-free solution that is used in the subsequent electroplating step.
In some embodiments, the solution containing the RE(BH4)3 salt in step (ii) excludes an ionic liquid. As well known, an ionic liquid is typically a liquid at room temperature (e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. Some examples of types of ionic liquids that may be excluded include those containing imidazolium, ammonium, pyridinium, and pyrrolidinium groups as the cationic component, or alternatively, those ionic liquids which contain fluorine in the anionic component, such as bis(perfluoroalkylsulfonyl)imide (e.g., bistriflimide, NTf2 or TFSI), perfluoroalkylsulfonate, tetrafluoroborate, and hexafluorophosphate types of anions.
The solvent in step (ii) may be selected from, for example, ethers, dialkyl sulfides, nitriles, and carbonates. Some examples of ether solvents include diethyl ether, ethyl propyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, methyl t-butyl ether, dimethoxy ethane (monoglyme), diethylene glycol dimethyl ether (diglyme), di triethylene glycol dimethyl ether (triglyme), tetrahydrofuran, and dioxolane. Some examples of dialkyl sulfide solvents include dimethyl sulfide, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, and ethyl propyl sulfide. Some examples of nitrile solvents include acetonitrile, propionitrile, butyronitrile, benzonitrile, and m-tolunitrile. Some examples of carbonate solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Some other possible solvents include N-methylpyrrolidone (NMP), hexamethylphosphoramide (HMPA), and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU). In some embodiments, the solution is fluorine-free. In further or separate embodiments, the solvent is non-ionic. In some embodiments, any one or more of the above classes or specific types of solvents is/are excluded from the solution in step (ii).
In some embodiments, the solution containing the RE(BH4)3 salt in step (ii) has a concentration of at least 0.1 M of RE(BH4)3 salt. In different embodiments, the solution containing the RE(BH4)3 salt in step (ii) has a concentration of precisely, about, or at least 0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1 M, 1.2 M, 1.5 M, 1.8 M, or 2 M of RE(BH4)3 salt. In further or separate embodiments, ABH4 is in admixture with the RE(BH4)3 salt in the solution in step (ii). The ABH4 in the solution may be present in an amount of precisely, about, or at least, for example, 10%, 20%, 30%, 40%, or 50% of the amount of RE(BH4)3 salt. As further discussed in the Examples section, inclusion of ABH4 in the solution was found to provide a significantly enhanced ionic conductivity and resulting enhanced efficiency in the RE deposition. The enhanced ionic conductivity can be attributed to an unexpected synergistic effect between ABH4 and RE(BH4)3 during the electroplating step.
In a third step (i.e., step (iii)) of the method, the solution from step (ii) is contacted with an anode and cathode in electrical communication, and the anode and cathode are electrically charged (i.e., a potential is established between anode and cathode) to electroplate the rare earth metal onto the cathode. As well known in the art, the electroplating process involves the reduction of the cationic RE element in the RE metal salt (in this case, RE(BH4)3) at the cathode to form a deposit of elemental RE at the cathode. The anode and cathode are typically interconnected by conductive wiring to establish electrical communication. A suitable voltage potential is established between the anode and cathode sufficient to electrically reduce the RE cation and produce an RE deposit on the cathode. The voltage for the electrodeposition typically ranges from 2V to 5V. As the electroplating step is preferably conducted in the absence of fluorine, the RE metal deposit typically does not contain a fluorine-containing passivation layer. In some embodiments, the electroplating step is conducted directly on the solution from step (ii) in which AX salt has precipitated. In other embodiments, the electroplating step is conducted by first removing AX salt that precipitated from the solution from step (ii) to form a substantially (or completely) AX-free solution, followed by contacting the AX-free solution with the anode and cathode, and electroplating, as above, to result in RE metal deposition on the cathode. Any of the rare earth metals or alloys thereof, as described earlier above, may be deposited in substantially pure form by this method. In particular embodiments, the RE metal deposit is or includes Nd, Dy, or Sm.
The anode used in the electroplating process can be any of the anodes known in the art for electroplating of rare earth elements. The anode is typically an inert anode, such as a porous or non-porous graphite, titanium-containing, tantalum-containing, or platinum-containing anode. The cathode may be any suitable electronic conductive substrate, such as, for example, platinum, stainless steel, iron, copper, nickel, aluminum, carbon, titanium, molybdenum, tungsten, or alloys of any of these.
In some embodiments, the electroplating process is conducted in air. More typically, the electroplating process is conducted under a reduced oxygen atmosphere, which can be partially or completely composed of an inert gas. The inert gas may be, for example, nitrogen or argon. Oxygen, if present, is typically kept to a concentration of no more than (or less than) 10 ppm or 1 ppm. Moisture may also be similarly minimized. The use of an inert gas and low moisture is helpful in preventing or lessening formation of an oxide passivation layer on the RE deposit.
Typically, the electroplating process is conducted with the electroplating solution being at or below room temperature, e.g., a temperature of about, up to, or less than 15, 20, 25, or 30° C. However, the electroplating process may be conducted with the electroplating solution being at an elevated temperature, such as a temperature of about 40, 50, 60, 70, 80, 90, 100, 110, or 120° C. In other embodiments, the electroplating process is conducted with the electroplating solution being at temperature within a range bounded by any two of the foregoing exemplary temperatures.
The electroplating process may use direct or pulse current. Any suitable current density may also be used, such as a current density of at least 0.01, 0.05, 0.1, 0.5, or 1 A/dm2 and up to 2, 5, 10, 15, 20, 25, 30, 40, or 50 A/dm2. The electroplating time may be suitably varied and used in conjunction with a particular current density and temperature to achieve a desired thickness of the RE coating. The electroplating time may be, for example, 1, 5, 10, 20, 30, 40, 50, 60, 90, or 120 minutes depending on the current density and temperature to achieve a desired thickness. The thickness of the RE metal coating may be precisely, about, at least, above, up to, or less than, for example, 1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, or 300 microns, or a thickness within a range bounded by any two of the foregoing values.
Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.
Chemicals and Materials. Anhydrous NdCl3 (99.9%), anhydrous DyCl3 (99.9%), LiBH4 (95%), tris[N,N-bis(trimethylsilyl)amide]neodymium (98%), ethyl ether, dimethyl sulfide (99%), borane-dimethyl sulfide complex (94%), pentane, and tetrahydrofuran (THF) were all obtained from commercial sources and used without further purification.
Synthesis of Rare Earth Borohydride Electrolyte. The synthesis of rare earth borohydride was based on a mechanochemical driven metathesis reaction of REE chloride and LiBH4. The synthesis was performed in a high-speed ball miller under argon in an 80 mL stainless-steel jar and beads (diameter 5 mm). The mole ratio between REE chloride (NdCl3, DyCl3) and LiBH4 was 1:4. The ball milling was conducted for 15 min followed by 5-min break to prevent overheating. The total milling time was 2 hours. After ball milling, the mixture was mixed with ethyl ether or dimethyl sulfide. The rare earth borohydride solution and the insoluble LiCl precipitate were separated by centrifugation. The concentration of the prepared electrolyte was 0.6 M. The solvent-free rare earth borohydride was obtained after the reaction mixture was dried at room temperature under vacuum for 2 hours and then dried at 90° C. under vacuum for 2 hours.
Lithium-free rare earth borohydride was also synthesized. The synthesis of lithium-free rare earth borohydride was based on a previously reported method under an argon atmosphere (R. A. Andersen, Inorg. Chem. 1979, 18, 1507-1509). Borane dimethyl sulfide (0.4 g, 0.0053 mol) was dissolved in dry pentane, and tris((hexamethyldisilyl)amido)-neodymium (0.70 g, 0.001 mol) was added, which resulted in the immediate precipitation of neodymium borohydride. The mixture was stirred for 24 hours and evaporated to dryness under vacuum. The solids were recrystallized from THF to yield Nd(BH4)3·1.5 THF (36% yield).
Electrochemistry of Rare Earth Borohydride Electrolyte. The electrochemistry of the rare earth borohydride electrolyte was studied in a custom three-electrode cell inside an argon-filled glovebox with oxygen and moisture level less than 1 ppm. Cyclic voltammetry (CV) and chronoamperometry were performed with a potentiostat without convection and a potentiostat with a Pt wire (1 mm in diameter) or Cu foil as the working electrode, Pt wire as the quasi reference electrode, and carbon paper as the counter electrode. The potential of redox peaks versus ferrocene/ferrocenium (Fc/Fc+) was calibrated by Fc/Fc+ redox couple as the internal reference.
Material Characterizations. Powder X-ray diffraction (XRD) data were recorded with a diffractometer operated at 45 kV and 40 mA (scanning step: 0.020 per step). The diffraction patterns were recorded in the range of 10-80°. λ=0.1540598 nm. XPS experiments were performed with a spectrometer equipped with an Al anode source operated at 15 KV and an applied power of 350 W and a pass energy of 93.5 eV. Samples were mounted on a foil since the C is binding energy was used to calibrate the binding energy shifts of the sample (C 1s=284.8 eV). SEM images and EDX mapping images were collected on a scanning electron microscope. ICP-OES analysis was performed using an ICP-OES spectrometer. Raman spectroscopy was performed using a Raman spectrum instrument with an emission laser wavelength of 532 nm.
The mechanochemical synthesis of REE borohydrides, as described herein, is driven by an efficient solid-state metathesis reaction between an REE chloride and LiBH4. This method avoids the high-pressure synthesis of REE hydride precursors and exhibits a much faster reaction rate than solution-phase metathesis reactions.
This ethyl ether-based Nd(BH4)3 electrolyte [Nd(BH4)3-EE]was studied by CV before performing electrodeposition.
In order to gain a better understanding of this peak, control experiments were performed by studying the CV curve of a lithium-free Nd(BH4)3-EE solution (denoted as pure-Nd(BH4)3-EE) and a neodymium-free LiBH4-EE solution. Lithium-free Nd(BH4)3 was synthesized via the reaction between neodymium silylamide and borane-dimethyl sulfide.
The XRD pattern of the lithium-free Nd(BH4)3 was found to be in agreement with the literature and confirmed to be Nd(BH4)3·1.5 THF (B. Richter et al., Ibid.). ICP-OES analysis of this product indicated the absence of alkaline impurities below the detection limit of a 0.7% mole ratio. However, surprisingly, as shown in the CV curve of LiBH4 in ethyl ether (
Raman spectroscopy of ethyl ether-based electrolytes was studied to gain further understanding of the electrolyte speciation. Raman spectra were obtained from pure ethyl ether, and from LiBH4-EE, Nd(BH4)3-EE, and Dy(BH4)3-EE solution. The ethyl ether mode was observed around 840 cm−1, which can be ascribed to the C—O skeleton stretching and the methyl group rocking of the ethyl ether (J. Wieser et al., Acta, Part A: Mol. Spectrosc. 1968, 24, 1055-1089). The splitting of the peak into a doublet with a main peak at 846 cm−1 and a shoulder peak at 837 cm−1 is attributed to the presence of TT and TG conformers of ethyl ether (J. Wieser et al., Ibid.). The C—O stretching mode of ether is usually sensitive to the local environment and is a good indicator of coordination, and hence, it is utilized here to study the borohydride solutions. An increase in the relative intensity of the TG peak can be observed for all boron hydride containing ethyl ether solutions, which indicates an increased population of TG conformers. This is likely due to the coordination of ethyl ether to Li+, Nd3+, or Dy3+ ions since the TG conformer is more sterically conducive to coordination with cations through oxygen. The Raman peaks in the 2050-2550 cm−1 region can be ascribed to B—H vibrations, which reveal the anionic features of ethyl ether-based solutions. For the LiBH4-EE solution, a doublet can be found at 2250 and 2375 cm−1, which can be attributed to the presence of aggregates rather than fully dissociated free ions, as evidenced by its extremely low ionic conductivity (<0.001 mS cm−1) (A. E. Shirk et al., J. Am. Chem. Soc. 1973, 95, 5901-5904. For Nd(BH4)3-EE and Dy(BH4)3-EE solution, the spectra are composed of three peaks at 2137, 2214, and 2467 cm−1, which can be assigned to ionic (BH4)− (or free (BH4)−), bridging B—Hbridging, and terminal B—Hterminal vibrations. This suggests the coexistence of anion-coordinated ion pairs and free (BH4)− anions in REE borohydride-ethyl ether solution. The absence of peaks at 2250 and 2375 cm−1 suggests that REE(BH4)3 can promote the dissociation of LiBH4 and contribute to higher ionic conductivity of REE(BH4)3-EE solutions, likely via a synergistic mechanism. Based on these results, the possible active species in REE-EE solutions are assumed to be REE3+, Li+, [REE (EE)n (BH4)m](3−m)+, [LiREE (EE)n (BH4)m](4−m)+, and (BH4)−.
Based on these CV results on the mechanochemically synthesized Nd(BH4)3-EE solution, REE electrodeposition was performed by chronoamperometry at −3.2 V vs Fc/Fc+ for 10 h using a Pt wire as the cathode. The current-time curve of chronoamperometry of Nd(BH4)3-EE electrolyte at −3.2 V vs Fc/Fc+, with Pt wire as working electrode is shown in
The morphology and composition of the electrodeposited Nd and Dy samples were characterized by SEM, EDX, and ICPOES. As shown in the SEM and EDX mapping micrographs in
The weight percentages of Nd and Dy were determined to be 58.5 and 55.2%, accompanied by a significant amount of Cl (over 30%). The Cl impurity in these deposits was also observed in previously studied borohydride systems (P. Geysens et al., Ibid.). However, the formation mechanism may be different since a majority of Cl− was precipitated as insoluble LiCl in the present synthesis, whereas all of Cl− exists as soluble species in the Nd2Cl6(DME)4-LiBH4 system (P. Geysens et al., Ibid.). The high Cl content is unlikely to originate from solvated LiCl or NdCl3 as both of them have very low solubility in ethyl ether. Rather, it is likely due to the formation of a Cl-containing complex, such as the LiRE(BH4)3Cl complex, which is reported to form during ball milling or heating (M. B. Ley et al., J. Phys. Chem. C 2012, 116, 21267-21276). Interestingly, chlorine does not feature any passivation unlike its neighboring halogen fluorine, and it is reported that Cl can act as a depassivating agent in the electrodeposition of Mg (R. Attias et al., ACS Catal. 2020, 10, 7773-7784).
In order to more precisely quantify the REE content, the Nd sample was analyzed by ICP-OES, with the results shown in Table 2 below. Based on the whole weight of the sample, Nd concentration is about 61%, which is consistent with the EDX results. The lithium concentration is only 1 wt %, which is lower than the ratio (10 wt %) in the electrolyte. By combining CV and ICP results, the electrodeposition of Nd is presumed to be Li-mediated reduction of Nd.
While the REE(BH4)3-ethyl ether solution can produce deposits with reasonably high REE content via passivation-free electrodeposition, Cl contamination remains a major problem. Dimethyl sulfide was also attempted as a solvent to replace ethyl ether. The use of dimethyl sulfide as a solvent for the REE metathesis mixture yielded a pink Nd(BH4)3 solution with an insoluble white LiCl precipitate (see
The speciation of dimethyl sulfide-based electrolyte was investigated by Raman spectroscopy. Raman spectra were obtained of Nd(BH4)3-MS, pure-Nd(BH4)3-MS, and Dy(BH4)3-MS electrolytes at 720-760 cm−1 and 2050-2550 cm−1 regions, which correspond to the C—S asymmetric stretching of dimethyl sulfide and the B—H stretching of (BH4)−, respectively (T. H. Joon et al., J. Mol. Struct. 1987, 162, 191-200). A shift of the C—S stretching mode from 744.2 cm−1 (for pure dimethyl sulfide) to 742.6 cm−1 (for REE(BH4)3 in dimethyl sulfide) could be ascribed to the coordination of dimethyl sulfide to REE cations, which involves the change of C—S bond length induced by charge transfer between sulfur and REE3+. The B—H stretching mode of the REE-containing dimethyl sulfide solution exhibits a similar pattern as the ethyl ether-based solutions, which feature the coexistence of anion-coordinated ion pairs and free (BH4)− anions and indicates the partial dissociation of REE(BH4)3 in dimethyl sulfide. The additional peak located at 2361 cm−1 suggests the formation of a different complex.
It is worth noting that pure Nd(BH4)3-MS displays a low intensity at 2221 cm−1, which indicates that only a low ratio of ionic (BH4)− can be formed without LiBH4 present in solution, which consequently results in a low ionic conductivity. However, the significant enhancement of the 2221 cm−1 peak can be observed in the Li-containing REE(BH4)3-MS solution. This suggests that the presence of Li can facilitate the disassociation of REE(BH4)3, thereby improving the ionic conductivity and electrodeposition performance.
This observation is similar to that of ethyl ether-based solutions, which confirms the synergistic effect in dimethyl sulfide solutions. Based on the above results, it can be surmised that the electroactive species in the REE(BH4)3-MS solution may be REE3+, Li+, [REE (MS)n (BH4)m](3−m)+, [LiREE (MS)n (BH4)m](4−m)+, and (BH4)−.
The electrodeposition in Nd(BH4)3-MS solution was carried out potentiostatically at −3.5 V using copper film as the working electrode. The chronoamperogram, as shown in
The data in Table 3 indicates a nearly 17% increase of Nd from 58.5 to 75.2%, and more importantly, more than 10-fold decrease of Cl from 33.8 to 2.3%, compared with the ethyl ether solution. The slightly higher O level in the sample can be attributed to air exposure during EDX sample preparation. The presence of 3% sulfur indicates a small extent of solvent decomposition during the electrodeposition. Corresponding EDX mapping images were used to determine the distribution of Nd, Cl, O, and S in the Nd sample. Quantitative analysis results from ICP-OES are shown in Table 4 below.
The data in Table 4 indicate an elemental composition of Nd, B, and Li at 75, 7, and 1%, respectively. Similar compositional results were obtained for the Dy sample. It is worth mentioning that both deposits can generate sparks when directly contacted with water, which is an indicator of extremely high reactivity of metallic REE material. Notably, such a high reactivity has also been previously observed on nanosized rare earth metal particles that are chemically reduced by lithium naphthalenide in ether solution (D. Bartenbach et al., Angew. Chem., Int. Ed. 2021, 60, 17373-17377).
The in-depth XPS analysis was carried out to gain additional insight into the chemical state of deposited REE samples.
A survey of spectra indicates the presence of Li, O, and Cl for sample from Nd(BH4)3-EE and Li, O, Cl, and S for the sample from Nd(BH4)3-EE, which is consistent with the EDX results. In particular, the high-resolution Li is spectra was analyzed to study the chemical state. For the Nd(BH4)3-EE sample, there is a shift of Li is peak to lower binding energy after etching, which indicates a change of average chemical state from oxide to metallic and confirms the co-reduction of Li. The intensity decrease of Li is after etching reveals a lower concentration of Li inside the sample. This can be explained by the Li-mediated reduction mechanism since the co-deposited Li at the earlier stage may be continuously consumed through the chemical reduction reaction with Nd3+.
Here is described a passivation-free system for the electrodeposition of rare earth metals. Nd(BH4)3 and Dy(BH4)3 salts were first synthesized by a mechanochemically driven metathesis reaction between REE chloride and LiBH4. These borohydride salts were then dissolved in ethyl ether or dimethyl sulfide, and their electrochemical properties were studied by Raman spectroscopy, CV, and chronoamperometry. Evidence was found for a synergistic effect between REE borohydride and LiBH4 that promotes the dissociation of borohydride salts and facilitates the electrochemical properties. Combined with the in-depth XPS analysis of deposits, the reduction mechanism of these rare earth metals likely proceeds through a co-reduction or Li-mediated reduction pathway. Passivation-free electrodeposition can be performed in both ethyl ether and dimethyl sulfide. However, the product obtained from the dimethyl sulfide solvent exhibits superior purity and suppresses Cl contamination by preventing the formation of Cl-containing complexes. XPS, EDX, and ICP-OES analyses of the deposits indicate that the overall concentration of the REEs reach 75%, which contains 40-48% metallic phase.
While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.
The present application claims benefit of U.S. Provisional Application No. 63/624,010, filed on Jan. 23, 2024, all of the contents of which are incorporated herein by reference.
This invention was made with government support under Prime Contract No. DE-AC05-000R22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63624010 | Jan 2024 | US |
