Method for preparing lithium metal water-soluble sample

Abstract

The invention discloses a method for preparing a lithium metal water-soluble sample. The method comprises the following steps: dissolving ammonium acetate in glacial acetic acid to form a solution; and putting a solid lithium strip into the obtained solution, introducing water into a reaction system to dissolve the metal lithium, heating to remove redundant acetic acid and ammonium acetate, and digesting residual solid substances with aqua regia to obtain a metal lithium water-soluble sample. According to the method, the reaction for oxidizing the metal lithium into the lithium ions is highly controllable, no danger is generated, no extra metal ions are introduced into the metal lithium water-soluble sample, and the caused sample pollution is small.

Description

Technical Field

[0001] The invention relates to the technical field of atomic emission spectroscopy (ICP) determination, and more specifically to the determination of impurity elements in metallic lithium, and in particular to a method for preparing a metallic lithium water-soluble sample. Background Art

[0002] In the manufacture of lithium-ion batteries, metallic lithium can be directly added to the negative electrode material to improve the problem of lithium waste in the positive electrode material caused by low initial coulombic efficiency. This is called pre-lithiation of the negative electrode (Gao Dongli, Wu Liangce. Patent Status of Pre-lithiation of Lithium Battery Negative Electrode [J]. China Science and Technology Information, 2023(22):24-28.). The pre-lithiation process generally requires the addition of high-purity metallic lithium into the negative electrode material. In order to ensure battery performance, it is necessary to analyze the impurity elements contained in each batch of metallic lithium. GB / T20931 stipulates that the content of various impurities in metallic lithium should be determined by flame atomic absorption spectrometry, but battery material testing laboratories are generally not equipped with flame atomic absorption spectrometers. On the other hand, since the national standards generally require or recommend the use of inductively coupled plasma atomic emission spectroscopy (ICP) to determine the content of impurity elements in positive, negative or electrolyte materials, battery material testing laboratories are generally equipped with ICP instruments. In order to shorten the determination cycle of impurities in lithium metal and improve the utilization rate of the instrument, it is necessary to develop a method for determining the content of impurity elements in lithium metal by ICP. In order to prepare the ion water solution required by ICP, the lithium strip needs to be dissolved in a strong acid solution. However, the chemical properties of lithium metal are extremely active and its density is low. It is easy to cause an explosion if it is directly placed in water or acid. In practice, small pieces of solid lithium strips not exceeding 0.25 g are often placed in ice water precooled to 0°C to form a solution. This method still has certain risks. And only a small amount of lithium metal can be processed each time.

[0003] In the above technical background, we need to develop a method for preparing water-soluble samples of metallic lithium to address the problem that metallic lithium reacts too violently with water or acid and can easily cause explosion. Summary of the invention

[0004] In view of the problem that metallic lithium reacts too violently with water and is likely to cause explosion, the invention aims to provide a method for preparing a water-soluble sample of metallic lithium.

[0005] The object of the present invention is achieved by the following scheme: A method for preparing a water-soluble sample of metallic lithium comprises the following steps: (1) In a reactor, dissolving ammonium acetate in glacial acetic acid to form an acetic acid / ammonium acetate solution; (2) placing the solid lithium ribbon into the solution obtained in step (1), and dissolving the lithium oxide on the surface of the lithium ribbon; (3) The solution obtained in step (2) is extracted from the reactor and mixed with pure water to form a diluted solution, so that the water content in the reaction system is always less than 20%. The solution is then re-injected into the reactor to allow the metallic lithium in the lithium belt to react with acetic acid and gradually dissolve, while releasing hydrogen. Be careful not to add water directly into the reactor! (4) Repeat step (3) to completely dissolve the lithium ribbon.

[0006] (5) Place the reactor on a heating plate to heat and evaporate excess acetic acid and ammonium acetate.

[0007] (6) Inject aqua regia into the reactor and heat it to completely dissolve other metal ions. The volume is fixed to obtain a water sample suitable for the determination of impurity ions in lithium strips by ICP.

[0008] Preferably, high-grade pure acetic acid and ammonium acetate reagents are used in step (1).

[0009] Before the experiment begins, the cavities in the lithium ribbon are eliminated by mechanical force to ensure that the lithium ribbon added in step (2) is solid.

[0010] In step (3) and step (4), the water content in the reaction system is always below 20%.

[0011] The heating temperature in step (5) is 120°C.

[0012] The mechanism of the present invention is: ammonium acetate is introduced into the reaction system of acetic acid and metallic lithium to reduce hydrogen ions. If the lithium strip is directly put into acetic acid, a violent reaction will occur and may cause an explosion. This is because acetic acid can ionize hydrogen ions, and hydrogen ions and metallic lithium undergo a violent redox reaction: CH3COOH → CH3COO - + H + (1) Li + H + → Li + + 1 / 2 H2↑ (2) When ammonium acetate is dissolved in acetic acid, it also ionizes into acetate ions: CH3COONH4→ CH3COO - + NH4 + (3) Since reaction (1) is a reversible reaction, when there are additional acetate ions ionized by ammonium acetate in the system, the reaction will shift to the left, reducing the hydrogen ions in the system. Experiments show that the pH of pure acetic acid is ≈ 3, while the pH of acetic acid / ammonium acetate is ≈ 7. Therefore, when the lithium ribbon in equation (2) is put into acetic acid / ammonium acetate, the lithium in the lithium ribbon does not react; however, the lithium oxide on the surface of the lithium ribbon will dissolve: 1 / 2Li2O + CH3COOH → CH3COOLi + 1 / 2H2O (4) This reaction does not involve redox, and the heat released is very low, which will not cause the reaction system to heat up.

[0013] In step (3), after water is introduced into the reaction system, the lithium in the lithium ribbon reacts with the water: Li + H2O → LiOH + 1 / 2H2↑ (5) Lithium hydroxide then reacts with acetic acid: LiOH + CH3COOH → CH3COOLi + H2O (6) The sum of their responses is: Li + CH3COOH → CH3COOLi + 1 / 2H2↑ (7) It is not difficult to see that the overall reaction does not consume water, and water acts as a catalyst in the reaction system. Therefore, in step (3) and step (4), it is necessary to control the amount of water in the reaction system to control the reaction rate of lithium and acetic acid.

[0014] Experiments have shown that when operating at 25°C, if the water content in the reaction system exceeds 20%, the reaction may be too fast and out of control, which must be avoided. In addition, water must not be allowed to directly contact the lithium belt, which will cause a local reaction to be too fast and cause an explosion. Therefore, in both steps (3) and (4), acetic acid / ammonium acetate must be extracted from the reactor first, water must be dispersed in the extracted acetic acid / ammonium acetate reaction system, and then the acetic acid / ammonium acetate aqueous solution must be re-injected into the reaction system.

[0015] In the above process, the temperature of the reaction system can be always controlled below the flash point of acetic acid (33°C), so there is no risk of combustion of acetic acid. Hollow lithium ribbons tend to float above the liquid and accumulate hydrogen in the cavity, which may be dangerous. Therefore, before the experiment begins, the cavity in the lithium ribbon should be eliminated by mechanical pressing.

[0016] In step (5), too high a heating temperature may cause violent decomposition of ammonium acetate and explosion, so the heating temperature must be controlled.

[0017] Advantages of the present invention: (1) The oxidation reaction of metallic lithium into lithium ions in this method is highly controllable and will not cause any danger.

[0018] (2) No additional metal ions will be introduced into the aqueous lithium metal sample, resulting in less sample contamination. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with specific examples. The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial sources. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0020] Example 1 Prepare a 1 g lithium strip water sample at 25°C as follows: (1) Preparation of acetic acid / ammonium acetate solution: Add 5.0 g of high-grade pure ammonium acetate and 10.0 g of high-grade pure glacial acetic acid to a 100 mL polytetrafluoroethylene beaker placed in a fume hood, heat the beaker to 40°C on a hot plate to help dissolve, and obtain a transparent and viscous acetic acid / ammonium acetate liquid reaction solution, which is cooled to room temperature (25°C) for later use; (2) Add lithium ribbon to the reaction solution: Accurately weigh 1.0 g of lithium ribbon in a glove box and place it in a sealed soft plastic package. Use your hands to pinch the lithium ribbon into a solid block through the soft plastic package. Transfer the lithium ribbon out of the glove box and immediately place it in the acetic acid / ammonium acetate liquid obtained in step (1) until the black lithium oxide layer on the surface of the lithium ribbon gradually dissolves and the metallic lithium with a white metallic luster is exposed inside. (3) Start the reaction between lithium and acetic acid: Use a clean plastic dropper to extract 7 mL of acetic acid / ammonium acetate liquid from the reaction device obtained in step (2) into another clean 100 mL polytetrafluoroethylene beaker, paying attention to careful observation to avoid extracting lithium blocks; add 1.0 mL of pure water to the extracted 7 mL of acetic acid / ammonium acetate, and gently stir with a plastic dropper for 1 minute to evenly disperse the water in the acetic acid / ammonium acetate; then, use a plastic dropper to re-inject the water / acetic acid / ammonium acetate solution into the reaction device and immediately shake it gently. The reaction of metallic lithium with acetic acid under the catalysis of water immediately begins, and small hydrogen bubbles are generated on the surface of the lithium ribbon; gently stir the reaction solution every 15 minutes; in this step of the experiment, ensure smooth ventilation to avoid hydrogen accumulation, and it is absolutely not allowed to add water directly to the reaction system! (4) Accelerate the reaction between lithium and acetic acid: After the reaction was carried out for 1 h and the reaction rate gradually decreased, step (3) was repeated and 1 mL of water was added to the reaction system. After the reaction was carried out for 2 h, the lithium strip was completely dissolved to obtain a viscous transparent solution. (5) Removal of ammonium acetate and acetic acid: The reaction system was heated with a graphite heating plate under sufficient ventilation, and the heating temperature was set to 120 degrees Celsius. Water, acetic acid and ammonium acetate were gradually evaporated to obtain a solid residue.

[0021] 5. Water Sample Preparation Add 10 mL of 3:1 aqua regia to the reaction system, cover the beaker mouth with a clean plastic cloth and set the heating plate temperature to 80°C for 30 min to heat the reaction. After cooling, a clear solution is obtained. Transfer all the solution to a 100 mL volumetric flask and dilute to obtain a water sample suitable for ICP determination.

[0022] GB / T 20930-2015 stipulates that the potassium, sodium, calcium, iron, aluminum, nickel, copper, magnesium and lead impurities that need to be controlled in lithium strip samples can all be determined through this water sample.

[0023] Example 2 Prepare a 1 g lithium strip water sample at 15°C as follows: At 15°C, the reaction rate decreases. In step (4), 1 mL of pure water is added to the reaction system after an interval of 1 h to make the total water content in the reaction system reach 3 mL. Careful control is required to ensure that the water content in the reaction system is always less than 20%. The other steps are the same as in Example 1.

[0024] The above embodiments are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing a water-soluble sample of metallic lithium, characterized in that: The steps include: (1) In a reactor, dissolving ammonium acetate in glacial acetic acid to form an acetic acid / ammonium acetate solution; (2) placing the solid lithium ribbon into the solution obtained in step (1), maintaining the solution temperature below the flash point of acetic acid, and dissolving the lithium oxide on the surface of the lithium ribbon; (3) The solution obtained in step (2) is extracted from the reactor and mixed with pure water to form a solution so that the water content is less than 20%, and then the solution is re-injected into the reactor, and the metallic lithium in the lithium belt begins to react with acetic acid and gradually dissolves, while releasing hydrogen. It is necessary to strictly prevent water from being directly added to the reactor; (4) Repeat step (3) until the lithium strip is completely dissolved; (5) placing the reactor on a heating plate to heat and evaporate excess acetic acid and ammonium acetate; (6) Inject aqua regia into the reactor and heat it to completely dissolve other metal ions. After cooling, adjust the volume to obtain a water sample suitable for the determination of impurity ions in lithium strips by ICP.

2. A method for preparing a water-soluble sample of metallic lithium according to claim 1, characterized in that: Before the experiment begins, the cavities in the lithium ribbon are eliminated by mechanical force to ensure that the lithium ribbon added in step (2) is solid.

3. A method for preparing a water-soluble sample of metallic lithium according to claim 1, characterized in that: In step (3) and step (4), the water content in the reaction system is always below 20%.

4. A method for preparing a water-soluble sample of metallic lithium according to claim 1, characterized in that: The heating temperature in step (5) is 120°C.

5. A method for preparing a water-soluble sample of metallic lithium according to any one of claims 1 to 4, characterized in that: Prepare a 1 g lithium strip water sample at 25°C as follows: (1) Preparation of acetic acid / ammonium acetate solution: Add 5.0 g of high-grade pure ammonium acetate and 10.0 g of high-grade pure glacial acetic acid to a 100 mL polytetrafluoroethylene beaker placed in a fume hood, heat the beaker to 40°C on a hot plate to help dissolve, and obtain a transparent and viscous acetic acid / ammonium acetate liquid reaction solution, which is cooled to room temperature (25°C) for later use; (2) Add lithium ribbon to the reaction solution: Accurately weigh 1.0 g of lithium ribbon in a glove box and place it in a sealed soft plastic package. Use your hands to pinch the lithium ribbon into a solid block through the soft plastic package. Transfer the lithium ribbon out of the glove box and immediately place it in the acetic acid / ammonium acetate liquid obtained in step (1) until the black lithium oxide layer on the surface of the lithium ribbon gradually dissolves and the metallic lithium with a white metallic luster is exposed inside. (3) Start the reaction between lithium and acetic acid: Use a clean plastic dropper to extract 7 mL of acetic acid / ammonium acetate liquid from the reaction device obtained in step (2) into another clean 100 mL polytetrafluoroethylene beaker, paying attention to careful observation to avoid extracting lithium blocks; add 1.0 mL of pure water to the extracted 7 mL of acetic acid / ammonium acetate, and gently stir with a plastic dropper for 1 minute to evenly disperse the water in the acetic acid / ammonium acetate; then, use a plastic dropper to re-inject the water / acetic acid / ammonium acetate solution into the reaction device and immediately shake it gently. The reaction of metallic lithium with acetic acid under the catalysis of water immediately begins, and small hydrogen bubbles are generated on the surface of the lithium ribbon; gently stir the reaction solution every 15 minutes; in this step of the experiment, ensure smooth ventilation to avoid hydrogen accumulation, and it is absolutely not allowed to add water directly to the reaction system! (4) Accelerate the reaction between lithium and acetic acid: After the reaction was carried out for 1 h and the reaction rate gradually decreased, step (3) was repeated and 1 mL of water was added to the reaction system. After the reaction was carried out for 2 h, the lithium strip was completely dissolved to obtain a viscous transparent solution. (5) Removal of ammonium acetate and acetic acid: The reaction system was heated with a graphite heating plate under sufficient ventilation, and the heating temperature was set to 120 degrees Celsius. Water, acetic acid and ammonium acetate were gradually evaporated to obtain a solid residue. (6) Water sample preparation: Add 10 mL of 3:1 aqua regia to the reaction system, cover the beaker mouth with a clean plastic cloth and set the heating plate temperature to 80°C for 30 min to heat the reaction. After cooling, a clear solution is obtained. Transfer all the solution to a 100 mL volumetric flask and dilute to obtain a water sample suitable for ICP determination.

6. A method for preparing a water-soluble sample of metallic lithium according to any one of claim 5, characterized in that: A water sample of 1 g of lithium strip was prepared at 15°C. In step (1), the transparent and viscous acetic acid / ammonium acetate liquid reaction solution was cooled to 15°C for standby use. In step (4), 1 mL of pure water was added to the reaction system after an interval of 1 h, so that the total water content in the reaction system reached 3 mL, and the water content in the reaction system was always less than 20%.