High-purity lithium borohydride and a method for preparing the same

Abstract

This invention discloses a high-purity lithium borohydride and its preparation method, belonging to the field of inorganic synthesis technology. The method includes: surface activation of lithium hydride with an aminosilane activator, reaction with a borane-tetrahydrofuran complex at 30-80℃, followed by impurity removal through a combination of lithium bromide and alkaline alumina, negative pressure decomposition, crystallization, washing, and drying to obtain high-purity lithium borohydride. The obtained product is a white crystalline powder with a purity ≥99.90%, chloride ions ≤15ppm, free alkali ≤50ppm, and metallic impurity Na. + / K + / Al 3+ ≤5 / 5 / 1ppm, particle size D50=5-15μm, ionic conductivity at 25℃≥3.0×10 ‑5 S / cm, moisture absorption rate ≤0.1% / 24h. This invention has wide raw material adaptability, mild process, and high product purity, and is suitable for solid electrolytes, pharmaceutical synthesis and other fields.

Description

Technical Field

[0001] This invention relates to the field of inorganic synthesis technology, and in particular to a high-purity lithium borohydride and its preparation method. Background Technology

[0002] Lithium borohydride (LiBH4) is a high-performance complex hydride with strong reducing properties and high hydrogen storage density. It is widely used in organic synthesis reduction reactions, new energy hydrogen storage, lithium-ion battery solid electrolytes, and aerospace propellants. High-end applications (such as solid electrolytes, pharmaceutical intermediate synthesis, and precision electronic chemicals) require extremely high purity lithium borohydride, typically exceeding 99.9%, with strict limitations on chloride ions, free alkali, and metallic impurities (Na₂O₃). + K + Al 3+ (and moisture content).

[0003] Existing preparation processes mainly include: (1) Schlesinger method: prepared by reacting lithium hydride with boron trichloride at high temperature (about 300°C). Although it can achieve kilogram-level production, boron trichloride is highly corrosive and toxic, and the byproduct LiCl formed by the reaction forms a solid solution with the target product, which is difficult to completely separate by conventional washing, resulting in the residual chlorine content in the product usually exceeding 1000 ppm; at the same time, the high-temperature process consumes a lot of energy and is prone to local overheating, which leads to product decomposition.

[0004] (2) Metathesis method: Sodium / potassium borohydride and lithium chloride are reacted in solvents such as tetrahydrofuran. The byproduct sodium / potassium chloride is removed by filtration, and the product is obtained by concentration and crystallization. This method is simple, but the raw materials are prone to impurities. The chloride ion content is usually ≥50ppm, the purity is only 95%-98%, and the byproduct salt is difficult to treat. The solvent recovery rate is low, and the environmental pressure is high.

[0005] (3) Direct synthesis method: The reaction of lithium hydride and borane complex under high temperature and pressure is energy-intensive, the reaction is violent and easily out of control, the product is easily oxidized, and the purity is difficult to reach 99.9%.

[0006] (4) Solid-phase ball milling: Lithium hydride and boron source are ball milled at room temperature, but the reaction efficiency is low, the product particle size is uneven, impurities are easily encapsulated, and the purity is ≤98.5%.

[0007] Therefore, it is of great significance to develop a method for preparing high-purity lithium borohydride with a mild process, stable product purity, and low chloride ion content. Summary of the Invention

[0008] The purpose of this invention is to provide a high-purity lithium borohydride and its preparation method to solve the problems in the background art.

[0009] To achieve the above objectives, the present invention provides a method for preparing high-purity lithium borohydride, specifically comprising the following steps: (1) Surface activation of lithium hydride: Under an inert atmosphere, lithium hydride is mixed with an aminosilane activator to activate it, thereby obtaining highly active surface-activated lithium hydride; (2) Low-temperature synthesis: Under an inert atmosphere, surface-activated lithium hydride is mixed with an organic solvent, heated to 30-80℃, and then borane-tetrahydrofuran complex is added dropwise to react and obtain a crude lithium borohydride mixture. The organic solvent is either a pure ether solvent or an ether-ionic liquid composite solvent. (3) Synergistic impurity removal: Lithium bromide auxiliary and alkaline alumina adsorbent are added sequentially to the crude product mixture, and then the filtrate is obtained by precision filtration; (4) Crystallization and drying: The filtrate is subjected to negative pressure decomposition, crystallization, washing and vacuum drying to obtain high-purity lithium borohydride.

[0010] Preferably, in step (1), the aminosilane activator is one or more of hexamethyldisilazane, trimethylsilyldiethylamine, and N,N-dimethyltrimethylsilane, the molar ratio of the aminosilane activator to lithium hydride is 0.001-0.05:1, and the activation time is 30-60 min.

[0011] Preferably, in step (2), the mass ratio of lithium hydride to organic solvent is 1:5-1:20, the molar ratio of lithium hydride to borane-tetrahydrofuran complex is 1.05-1.15:1, and the reaction time is 4-8h; The borane-tetrahydrofuran complex was purified by molecular sieve adsorption and vacuum distillation before use, with a purity ≥99.9% and a moisture content ≤20ppm.

[0012] Preferably, in step (2), the pure ether solvent is one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether.

[0013] Preferably, in step (2), the ether-ionic liquid composite solvent is one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether; the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, and the volume ratio of the ionic liquid to the ether solvent is 1:5-1:20.

[0014] Preferably, in step (3), the molar ratio of lithium bromide auxiliaries to the initial lithium hydride in step (1) is 0.0001-0.001:1; the amount of alkaline alumina adsorbent is 0.05-0.5 wt% of the crude product mixture, and the adsorption time is 0.5-1.5 h; the pore size of the filter medium for precision filtration is 0.22-1 μm.

[0015] Preferably, in step (4), the vacuum degree of the negative pressure decomposition is -0.08MPa to -0.098MPa, the decomposition temperature is 40-80℃, and the decomposition time is 1-4h.

[0016] Preferably, in step (4), the concentrated solution after decomplexation is redispersed with anhydrous ether solvent, and then crystallized using any of the following methods: (a) Temperature-controlled crystallization: Transfer to a crystallization vessel, first cool to 10-15℃, hold for 1-2 hours, then cool to -5℃ to 5℃ at a rate of 0.5-2℃ / h, and hold for 4-8 hours; (b) Antisolvent crystallization: At 5-15°C, add 2-4 times the volume of anhydrous n-hexane or anhydrous diethyl ether to the concentrate at a dropping rate of 1-5 L / min.

[0017] Preferably, in step (4), the solvent used for washing is anhydrous n-hexane or anhydrous diethyl ether, and the washing is performed 2-3 times; the vacuum drying conditions are vacuum degree ≤ -0.095MPa, temperature 40-60℃, and time 8-16h.

[0018] This invention also provides a high-purity lithium borohydride prepared by the above-described method, wherein the lithium borohydride is a white crystalline powder with a purity ≥99.90%, chloride ion content ≤15ppm, free alkali content ≤50ppm, moisture content ≤35ppm, and metallic impurity Na. + Content ≤5ppm, K + Content ≤5ppm, Al 3+ Content ≤1ppm, particle size D50=5-15μm, ionic conductivity at 25℃ ≥3.0×10 -5 S / cm, moisture absorption rate ≤0.1% / 24h.

[0019] Therefore, the high-purity lithium borohydride and its preparation method provided by the present invention have the following beneficial effects: (1) This invention removes impurities by surface activation of lithium hydride, in-situ precipitation of lithium bromide additive and adsorption of alkaline alumina, and can obtain ultra-high purity lithium borohydride products. The contents of chloride ions, free alkali and metal impurities such as sodium, potassium and aluminum are all controlled at extremely low levels, which can meet the stringent purity requirements of high-end applications such as solid electrolytes and pharmaceutical synthesis.

[0020] (2) The present invention has broad requirements for the purity of lithium hydride raw materials. Even if industrial-grade low-purity lithium hydride (containing a variety of metal impurities and high free alkali) is used, qualified high-purity products can still be obtained after processing by the process of the present invention. This effectively reduces the dependence on high-purity, high-cost lithium hydride raw materials and reduces supply chain risks and production costs.

[0021] (3) The synthesis reaction of the present invention is carried out under normal pressure and low temperature conditions, without the need for high temperature and high pressure equipment, and the energy consumption is significantly reduced compared with traditional methods; moreover, some materials can be recycled, which meets the requirements of green chemistry and sustainable development.

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0023] Figure 1 This is a bar chart comparing the chloride ion and free alkali contents of various embodiments and comparative examples of the present invention. Detailed Implementation

[0024] This invention provides a method for preparing high-purity lithium borohydride, specifically including the following steps: (1) Surface activation of lithium hydride Under an inert atmosphere (argon, dew point ≤ -40℃), lithium hydride (particle size 5-20μm, purity down to 97.5%) is mixed with an aminosilane activator, selected from one or more of hexamethyldisilazane, trimethylsilyldiethylamine, and N,N-dimethyltrimethylsilane. The molar ratio of activator to lithium hydride is 0.001-0.05:1. The mixture is stirred for 30-60 min to allow the activator to coordinate with the oxide / hydroxyl layer on the surface of lithium hydride, exposing highly active Li-H bonds, thus obtaining surface-activated highly active lithium hydride powder.

[0025] (2) Low temperature synthesis In an anhydrous and oxygen-free reactor protected by an inert atmosphere (argon), an organic solvent and surface-activated highly active lithium hydride are added. The mass ratio of lithium hydride to organic solvent is 1:5-1:20. The mixture is stirred and heated to 30-80℃ (preferably 30-50℃). Borane-tetrahydrofuran complex (BH3·THF, concentration 1-2 mol / L, purified by molecular sieve adsorption and vacuum distillation before use, purity ≥99.9%, water ≤20ppm) is slowly added dropwise. The molar ratio of lithium hydride to borane-tetrahydrofuran complex is 1.05-1.15:1. The addition time is 2-3 h, and the reaction is maintained at this temperature for 4-8 h. The reaction equation is as follows: LiH + BH3·THF → LiBH4 + THF After the reaction was completed, a crude lithium borohydride mixture was obtained.

[0026] In this invention, the organic solvent is a pure ether solvent or an ether-ionic liquid composite solvent. The pure ether solvent is selected from one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether. In the composite solvent, the ionic liquid is selected from 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTFSI), and the ether solvent is selected from one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether; the volume ratio of the ionic liquid to the ether solvent is 1:5-1:20 (preferably 1:8-1:15).

[0027] In this invention, the borane-tetrahydrofuran complex is purified by molecular sieve adsorption and vacuum distillation before use, with a concentration of 1-2 mol / L. The purity is ≥99.9%, and the water content is ≤20 ppm. Borane is always present in dilute solution form. The maximum online quantity per batch is ≤1 kg (based on a 100L reactor), which meets the low-risk level of the "Specifications for Safety Risk Assessment of Fine Chemical Reactions" (GB / T42300-2022).

[0028] The purified BH3·THF solution should be stored in a sealed container protected from light at -10℃ to 0℃, with a shelf life of no more than 7 days; or it can be stored frozen at -20℃ after being filled with inert gas, with a shelf life of 3 months. Before use, the effective borane concentration must be re-determined by hydrolysis-sodium hydroxide titration.

[0029] (3) Collaborative impurity removal (a) In-situ precipitation-assisted impurity removal: Add lithium bromide to the crude product mixture. The molar ratio of lithium bromide to the initial lithium hydride in step (1) is 0.0001-0.001:1. Stir for 10-30 min to promote the in-situ complexation precipitation of trace amounts of lithium chloride that may be generated during the reaction.

[0030] (b) Adsorption depth purification: Add alkaline alumina adsorbent at a dosage of 0.05-0.5 wt% of the crude product mixture, stir and adsorb for 0.5-1.5 h to remove free alkali (LiOH) and metal impurities (Na). + K + Al 3+ ) and trace amounts of moisture.

[0031] (c) Precision filtration: Precision filtration is performed under an inert atmosphere (argon) (filtration medium pore size 0.22-1μm) to obtain filtrate.

[0032] The control mechanism of free alkali (LiOH): The aminosilane activator undergoes an irreversible reaction with LiOH (taking hexamethyldisilazane as an example: (CH3)3Si-NH-Si(CH3)3+2LiOH→2(CH3)3Si-OLi+NH3↑), converting solid LiOH into soluble lithium silicate salt, which is removed during filtration. Alkaline alumina further reduces the residual free alkali to ≤50ppm through ion exchange or physical adsorption.

[0033] (4) Crystallization and drying (a) Negative pressure decomposition: The filtrate is transferred to a decomposition reactor and heated to 40-80℃ under a vacuum of -0.08MPa to -0.098MPa for 1-4 hours. The solvent recovered by distillation is dried and then recycled.

[0034] (b) Crystallization: The concentrated solution after decomplexation is redispersed with anhydrous ether solvent, and then crystallized using any of the following methods: Temperature-controlled crystallization: Transfer to a crystallization vessel, first cool to 10-15℃, hold for 1-2 hours, then slowly cool to -5℃ to 5℃ at a rate of 0.5-2℃ / h, and hold for 4-8 hours; Antisolvent crystallization: At 5-15℃, slowly add anhydrous n-hexane or anhydrous diethyl ether at a rate of 2-4 times the volume of the concentrate to the concentrate, at a rate of 1-5 L / min.

[0035] After crystallization, the mixture is centrifuged under an inert atmosphere (argon) (3000-4000 rpm, 10-20 min) to obtain a wet cake of lithium borohydride crystals, and the mother liquor is recovered.

[0036] (c) Washing and drying: The wet cake is washed 2-3 times with anhydrous n-hexane or anhydrous diethyl ether (moisture content ≤10ppm), each time using 1.5-3 times the mass of the wet cake; after washing, it is vacuum dried for 8-16 hours at a vacuum degree ≤-0.095MPa and a temperature of 40-60℃ to obtain high-purity lithium borohydride product.

[0037] This invention can also recycle and regenerate materials: (a) Solvent recovery: The ether solvents distilled under negative pressure are dried and dehydrated by 4Å molecular sieve (moisture content ≤ 20ppm) and returned to step (2) for recycling.

[0038] (b) When using ionic liquids, ionic liquid recovery and deep regeneration: The crystallization mother liquor and washing liquid are combined and the ionic liquid is recovered by vacuum distillation. In actual production, when online monitoring or batch sampling shows that the chloride ion concentration in the product is ≥10 ppm, the ionic liquid is deeply regenerated. The regeneration process is as follows: Activated carbon adsorption and decolorization: Add 5-10% activated carbon by mass of ionic liquid, stir at 60℃ for 2 hours, and filter; Ion exchange resin treatment: Removes accumulated metal cation impurities using cation exchange resins (such as Amberlite IRC-86); Drying by vacuum distillation: Distill at a vacuum of -0.095 MPa and a temperature of 80-100℃ for 4-6 hours.

[0039] (c) Regeneration of activator and adsorbent: aminosilane activator is recycled after simple distillation; alkaline alumina adsorbent is recycled after high-temperature activation (300-400℃, 2-4h).

[0040] In this invention, the reaction system should be equipped with nitrogen / argon purging, an emergency pressure relief valve, a flame arrester, and a combustible gas alarm device; the reactor material should be 316L stainless steel or Hastelloy; operators must wear antistatic protective clothing and chemical-resistant gloves.

[0041] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0042] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0043] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0044] Unless otherwise specified, the reagents, instruments, and equipment used in this invention are all commonly used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0045] Example 1 This embodiment provides a method for preparing high-purity lithium borohydride, the specific steps of which are as follows: (1) Surface activation of lithium hydride: Under argon protection (dew point ≤ -40℃), take 75.4g of micron-sized LiH (particle size 10μm, purity 99.5%), add 2.6g of hexamethyldisilazane, stir for 45min to obtain highly active LiH powder with surface activation.

[0046] (2) Low-temperature synthesis: In a 10L reactor under argon protection, a composite solvent consisting of 80mL EMIMBF4 and 720mL anhydrous tetrahydrofuran, along with the activated LiH obtained above, was added, and the mixture was stirred and heated to 40℃. 4.37L of purified 2mol / L BH3·THF was slowly added dropwise over 2.5h, and the reaction was maintained at this temperature for 5h. After the reaction was completed, a crude lithium borohydride mixture was obtained.

[0047] (3) Synergistic removal of impurities: Add 0.38g of anhydrous lithium bromide to the crude product mixture and stir for 15min; then add 3.0g of alkaline alumina adsorbent and stir for 1h; finally, filter through a 0.45μm filter membrane under argon protection to obtain the filtrate.

[0048] (4) Crystallization and drying: The filtrate was decomplexed at -0.095 MPa and 60 °C for 2.5 h to distill off tetrahydrofuran. The concentrate was dispersed in 75 mL of anhydrous tetrahydrofuran and then crystallized using a temperature-controlled program. The temperature was first lowered to 12 °C and held for 1.5 h, then lowered to 0 °C at a rate of 1 °C / h and held for 5 h. After centrifugation, the wet cake was washed twice with 120 mL of anhydrous n-hexane and dried under vacuum at 45 °C and -0.095 MPa for 12 h to obtain 177.2 g of white crystalline powder product, with a yield of 93.1%.

[0049] Example 2 This embodiment is essentially the same as Example 1, except that: 150.8 g of LiH (twice that of Example 1) was used, the composite solvent was 160 mL of BMIMTFSI and 3040 mL of anhydrous tetrahydrofuran, and the activator was trimethylsilyldiethylamine (5.2 g). 8.74 L of 2 mol / L BH3·THF was added dropwise in a 20 L reactor. The final product yield was 354.4 g, with a yield of 93.1%.

[0050] Example 3 Referring to Example 1, the feed amounts were increased tenfold: 754g LiH (99.5% purity), 43.7L 2mol / L BH3·THF, and 0.8L of ionic liquid and 7.2L of THF forming a composite solvent, using a 200L reactor. The total reaction time was 8 hours, yielding 1698g of product, with a yield of 89.2%.

[0051] Example 4 The difference between this embodiment and Example 1 is that industrial-grade LiH (purity 97.5%, main impurities: Na 150ppm, K 80ppm, Cl 200ppm, LiOH 0.8wt%, Ca≈120ppm, Mg≈60ppm, Fe≈40ppm) was used instead of high-purity LiH. To maintain the same effective LiH molar number as in Example 1, the amount of industrial-grade LiH added was 76.9g. 4.37L of purified 2mol / L BH3·THF was added dropwise. Under the same conditions, 171.8g of product was obtained, with a yield of 90.2%.

[0052] Example 5 The difference between this embodiment and Example 1 is that the composite solvent was replaced with pure anhydrous tetrahydrofuran (800 mL, without adding ionic liquid). The final product yield was 173.4 g, with a yield of 91.1%.

[0053] Example 6 This embodiment is the same as the steps (1)-(3) of Example 1. After obtaining the filtrate and decomposing it, antisolvent crystallization is used. The decomposed concentrate is dispersed in 75 mL of anhydrous tetrahydrofuran, and the temperature is controlled at 10°C. 188 mL of anhydrous n-hexane (about 2.5 times the volume of the concentrate) is added dropwise at a rate of 1.5 L / min with stirring. After the addition is completed, stirring is continued for 30 min, and the mixture is allowed to stand for 1 h. Subsequent centrifugation, washing, and drying are the same as in Example 1. 176.5 g of product is obtained, with a yield of 92.7%.

[0054] Comparative Example 1 Take 1 mol of potassium borohydride and 1.1 mol of lithium chloride, add 2 L of tetrahydrofuran, react at 25 °C for 30 h, filter, concentrate and crystallize, and dry.

[0055] Comparative Example 2 Same as Example 5, but omitting the lithium hydride surface activation step.

[0056] Comparative Example 3 Same as Example 1, but omitting the step of synergistic impurity removal using lithium bromide and alkaline alumina.

[0057] Comparative Example 4 Take 100g of unactivated LiH, add 2LTHF, and react at 120℃ and 5bar borane for 8h.

[0058] Comparative Example 5 Same as in Example 1, except that BH3·THF was replaced with an equimolar amount of trimethyl borate, and the reaction conditions were increased to 150°C, 3 bar, and 12 h.

[0059] Comparative Example 6 Same as in Example 1, except that BH3·THF was replaced with an equimolar amount of BH3·SMe2 (2 mol / L toluene solution).

[0060] The products prepared in the above embodiments or comparative examples were subjected to performance testing, and the results are shown in Tables 1-4. Figure 1 As shown.

[0061] (1) Purity determination (acid-base titration): Accurately weigh 0.5 g of product (accurate to 0.0001 g), dissolve it in 50 mL of deionized water (resistivity ≥18.2 MΩ·cm) under argon protection, and titrate to the endpoint with 0.1 mol / L HCl standard solution using bromocresol green-methyl red as an indicator. Perform a blank test at the same time.

[0062] (2) Determination of chloride and bromide ions (ion chromatography): Refer to GB / T38575-2020. Sample pretreatment: Weigh 0.2g of the product in a glove box, dilute to 100mL with high-purity water, filter through a 0.22μm filter membrane, and then inject the sample. Instruments: Ion chromatograph, anion analyzer column (AS19), KOH gradient elution, conductivity detector. Detection limit Cl - 0.5ppm, Br - 0.5 ppm. Spiked recoveries range from 95% to 105%.

[0063] (3) Determination of free base (calculated as LiOH) (acid-base titration method): Refer to the relevant methods in the "Industrial Standard for Lithium Borohydride". In an argon glove box, accurately weigh approximately 1.0 g of the product (accurate to 0.0001 g), place it in a 250 mL Erlenmeyer flask, add 50 mL of anhydrous isopropanol, and magnetically stir to dissolve for 10 min (lithium borohydride dissolves in isopropanol, while LiOH is insoluble, forming a suspension). Add 2-3 drops of phenolphthalein indicator (1% ethanol solution), and titrate with 0.01 mol / L hydrochloric acid-isopropanol standard solution until colorless. Perform a blank test simultaneously. The free base content (calculated as LiOH mass fraction) is calculated using the following formula: w LiOH =[c×(V1-V0)×M] / [m×1000]×100% Where: c is the concentration of the hydrochloric acid-isopropanol standard solution, in mol / L; V1 is the volume of standard solution consumed in the sample titration, in mL; V0 is the volume of standard solution consumed in the blank titration, in mL; M is the molar mass of LiOH (23.95 g / mol); m is the sample mass, in g. Each batch of samples was measured in triplicate, and the arithmetic mean was taken.

[0064] Note: This method is based on the principle that LiBH4 is readily soluble in isopropanol while LiOH is insoluble. The free base content is determined by titrating the LiOH in the suspension.

[0065] (4) Determination of metal impurities (ICP-MS method): Refer to GB / T38575-2020. Sample pretreatment: Weigh 0.5g of product in a glove box, dissolve in 5mL of high-purity water, transfer to a 50mL centrifuge tube, and bring to a final volume. Instrument: ICP-MS (collision cell mode, internal standard method). Detection limit Na + 0.1ppm, K + 0.05ppm, Al 3+ 0.1 ppm; spiked recovery rate 90%-110%; repeatability RSD ≤ 5% (n=6).

[0066] (5) Moisture determination (Karl Fischer method): Refer to GB / T6283-2008. Weigh 0.1-0.3g of the product in a glove box and determine the moisture content using a coulometric Karl Fischer moisture analyzer. The detection limit is 1ppm.

[0067] (6) Ionic conductivity test: In an argon glove box, 0.5g of product was placed in a Φ10mm mold and cold-pressed into a sheet at 300MPa, with the sheet thickness controlled at 0.5-1.5mm. Electrochemical impedance spectroscopy (EIS) was used with a stainless steel blocking electrode as the electrode, a frequency range of 1Hz-1MHz, an AC amplitude of 10mV, and a temperature of 25±0.5℃. The conductivity was calculated by σ=L / (R·S), where L is the thickness, S is the electrode area, and R is the intersection of the semicircle and the real axis in the Nyquist plot.

[0068] (7) Particle size test (laser particle size distribution method): Refer to GB / T19077-2016. In an argon glove box, take an appropriate amount of product powder and disperse it in anhydrous n-hexane (solid content about 0.5-1wt%). After ultrasonic dispersion for 1 min, use a laser particle size analyzer (such as Malvern Mastersizer 3000) to determine the particle size distribution and calculate D50 (median diameter of volume distribution). The dispersion process must be strictly isolated from moisture and air.

[0069] (8) Determination of moisture absorption rate (dynamic moisture adsorption method): Refer to GB / T29505-2013. In an argon glove box, accurately weigh approximately 2g of product and spread it evenly on a weighing bottle (already at constant weight). Place the bottle in a constant temperature and humidity chamber (temperature 25℃±0.5℃, relative humidity 50%±2%), expose it openly for 24 hours, then remove it, seal it immediately, and weigh it. The moisture absorption rate is calculated using the following formula: Moisture absorption rate (%) = (weight gain / initial mass) × 100%. Each sample is measured in parallel three times, and the average value is taken.

[0070] Note: "—" indicates that no detection was performed or the detection limit was exceeded.

[0071] Table 1: Product Purity and Key Impurities Data

[0072] Note: Bromine ion (Br) - All of them were found to be below the detection limit (<0.5 ppm), so they are not listed in the table.

[0073] Table 2: Metal Impurity Content

[0074] Table 3: Process Yield, Particle Size, and Moisture Absorption Rate

[0075] Table 4: Comparison of Electrochemical Performance

[0076] From Tables 1-4 and Figure 1 It can be seen that the lithium borohydride products obtained in each embodiment of the present invention have a purity ≥99.90%, chloride ion content ≤12 ppm, free alkali ≤48 ppm, and moisture ≤35 ppm, which are far superior to the comparative example, indicating that surface activation and the selection of a specific boron source are crucial to product quality; in addition, Na in each embodiment + K + Al 3+ All metallic impurities were controlled to extremely low levels. In particular, Example 4, using industrial-grade low-purity LiH (containing impurities such as Ca, Mg, and Fe), still managed to reduce the Ca, Mg, and Fe content in the final product to ≤2 ppm through synergistic impurity removal, demonstrating the broad-spectrum and high-efficiency nature of the impurity removal system of this invention. The yields of each example were consistently between 89% and 94%, higher than the comparative example. Furthermore, the products of Examples 1 and 2 exhibited uniform particle size and low moisture absorption, indicating a mature crystallization process and good product stability. Moreover, the room-temperature ionic conductivity of Examples 1, 2, and 5 was ≥3.0 × 10⁻⁶. -5 The conductivity S / cm is much higher than that of Comparative Example 6, proving that the product obtained by the process of this invention is more suitable for solid electrolytes.

[0077] In summary, this invention successfully prepared a high-purity, low-impurity, high-conductivity lithium borohydride product with ideal yield through surface activation, low-temperature synthesis, synergistic impurity removal, and optimized crystallization, while also exhibiting good raw material adaptability.

[0078] (8) Long-term cycling experiment of ionic liquids: The process was repeated 20 times under the conditions of Example 2, with approximately 3-5% fresh ionic liquid added to each batch. When the product chloride ion concentration was ≥10 ppm or the purity was ≤99.93%, the ionic liquid was deeply regenerated (activated carbon decolorization + ion exchange resin + vacuum distillation).

[0079] The results showed that the first 15 batches of products consistently met the optimal selection criteria (purity ≥ 99.94%, Cl). -7-9 ppm); batches 16-18 have a purity of 99.92-99.93%, Cl - At 10-11 ppm, the basic requirements are still met; after regeneration, the purity of the ionic liquid is restored to 99.6%, and after 5 more batches of recycling, the purity of the product is restored to 99.94%.

[0080] In summary, this invention successfully produces high-purity lithium borohydride through a complete process including lithium hydride surface activation, low-temperature synthesis, synergistic impurity removal with lithium bromide and alkaline alumina, negative pressure decomposition, and optimized crystallization and drying. This method offers broad raw material adaptability, suitable for industrial-grade low-purity lithium hydride; mild process conditions, eliminating the need for high temperature and pressure; stable yield; and recyclable solvents and ionic liquids. It combines the advantages of high purity, low impurities, environmental friendliness, and controllable cost, making it suitable for high-end applications such as solid-state electrolytes and pharmaceutical synthesis.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing high-purity lithium borohydride, characterized in that, Specifically, the following steps are included: (1) Surface activation of lithium hydride: Under an inert atmosphere, lithium hydride is mixed with an aminosilane activator to activate it, thereby obtaining highly active surface-activated lithium hydride. (2) Low-temperature synthesis: Under an inert atmosphere, surface-activated lithium hydride is mixed with an organic solvent, heated to 30-80℃, and then borane-tetrahydrofuran complex is added dropwise to react and obtain a crude lithium borohydride mixture. The organic solvent is either a pure ether solvent or an ether-ionic liquid composite solvent. (3) Synergistic impurity removal: Lithium bromide auxiliary and alkaline alumina adsorbent are added sequentially to the crude product mixture, and then the filtrate is obtained by precision filtration; (4) Crystallization and drying: The filtrate is subjected to negative pressure decomposition, crystallization, washing and vacuum drying to obtain high-purity lithium borohydride.

2. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (1), the aminosilane activator is one or more of hexamethyldisilazane, trimethylsilyldiethylamine, and N,N-dimethyltrimethylsilane, the molar ratio of the aminosilane activator to lithium hydride is 0.001-0.05:1, and the activation time is 30-60 min.

3. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (2), the mass ratio of lithium hydride to organic solvent is 1:5-1:20, the molar ratio of lithium hydride to borane-tetrahydrofuran complex is 1.05-1.15:1, and the reaction time is 4-8h; The borane-tetrahydrofuran complex was purified by molecular sieve adsorption and vacuum distillation before use, with a purity ≥99.9% and a moisture content ≤20ppm.

4. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (2), the pure ether solvent is one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether.

5. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (2), the ether-ionic liquid composite solvent is one or more of tetrahydrofuran, diethyl ether, methyl tert-butyl ether, and diethylene glycol dimethyl ether; the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, and the volume ratio of the ionic liquid to the ether solvent is 1:5-1:

20.

6. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (3), the molar ratio of lithium bromide auxiliaries to the initial lithium hydride in step (1) is 0.0001-0.001:1; the amount of alkaline alumina adsorbent is 0.05-0.5 wt% of the crude product mixture, and the adsorption time is 0.5-1.5 h; the pore size of the filter medium for precision filtration is 0.22-1 μm.

7. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (4), the vacuum degree of the negative pressure decomposition is -0.08MPa to -0.098MPa, the decomposition temperature is 40-80℃, and the decomposition time is 1-4h.

8. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (4), the concentrated solution after decomposition is redispersed with anhydrous ether solvent, and then crystallized using any of the following methods: (a) Temperature-controlled crystallization: Transfer to a crystallization vessel, first cool to 10-15℃, hold for 1-2 hours, then cool to -5℃ to 5℃ at a rate of 0.5-2℃ / h, and hold for 4-8 hours; (b) Antisolvent crystallization: At 5-15°C, add 2-4 times the volume of anhydrous n-hexane or anhydrous diethyl ether to the concentrate at a dropping rate of 1-5 L / min.

9. The method for preparing high-purity lithium borohydride according to claim 1, characterized in that, In step (4), the solvent used for washing is anhydrous n-hexane or anhydrous diethyl ether, and the washing is performed 2-3 times; the vacuum drying conditions are vacuum degree ≤ -0.095MPa, temperature 40-60℃, and time 8-16h.

10. A high-purity lithium borohydride, characterized in that, The lithium borohydride is prepared by the method described in any one of claims 1-9, and is a white crystalline powder with a purity ≥99.90%, chloride ion content ≤15ppm, free alkali content ≤50ppm, moisture content ≤35ppm, and metallic impurity Na. + Content ≤5ppm, K + Content ≤5ppm, Al 3+ Content ≤1ppm, particle size D50=5-15μm, ionic conductivity at 25℃ ≥3.0×10 -5 S / cm, moisture absorption rate ≤0.1% / 24h.

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