Organic pretreatment method in lithium sulfide ICP-OES impurity test

Abstract

The invention relates to the technical field of battery materials, in particular to an organic pretreatment method in lithium sulfide ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometer) impurity test. The method mainly comprises the following steps: (1) weighing a lithium sulfide solid sample, and adding ultrapure water for dissolving to obtain a sample treatment solution; (2) adding chloroacetic acid into the sample treatment liquid, and converting the sulfur element in the lithium sulfide into the organic matter to obtain an intermediate liquid 1; (3) adding ammonia water into the intermediate solution 1 to adjust the pH value to 7-8, and converting excessive chloroacetic acid in the intermediate solution 1 and mercaptoacetic acid generated by the reaction into thiodiacetic acid to obtain an intermediate solution 2; and (4) adding hydrochloric acid into the intermediate solution 2 to adjust the pH value to 1-2, and then adding an organic solvent for extraction to obtain a water phase solution which is a solution to be detected. After extraction, the concentration of the S element in a water phase solution is extremely low, the experimental process is safe and controllable, and no chemical reagent influencing the test of the content of metal element impurities is added.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of battery materials, in particular to an organic pretreatment method in lithium sulfide ICP-OES impurity testing. BACKGROUND

[0002] Sulfur is abundant in the earth, has the characteristics of low price and environmental friendliness, and a lithium sulfide battery prepared by using lithium sulfide as a positive electrode material has high material theoretical specific capacity and battery theoretical specific energy, and has a wide application prospect.

[0003] Impurity content testing of battery-grade lithium sulfide is the "lifeline" of whether it can be applied to high-performance lithium-sulfur batteries. It directly determines the core performance and safety bottom line of the battery: impurities will irreversibly consume active lithium, leading to capacity attenuation and low coulombic efficiency; and will also cause a side reaction to produce gas, causing a bulge and even a safety risk of thermal runaway. In industrial production, strict impurity control is the cornerstone of ensuring material batch stability, improving the yield of battery cells, and ultimately achieving the commercial success of lithium-sulfur batteries. Therefore, accurate impurity testing is not ordinary quality inspection, but a prerequisite for ensuring high energy density, long cycle life and safety and reliability of the battery.

[0004] There is no national standard for lithium sulfide testing at present, and the industry usually uses the difference method to test the purity of lithium sulfide, so the accuracy of impurity testing is crucial. At present, the inductively coupled plasma optical emission spectrometry (ICP-OES) is commonly used for content testing of metal elements in lithium sulfide.

[0005] In the content testing of metal elements in lithium sulfide, since the content of sulfur is as high as 70%, if the dilution multiple of the solid sample is small, there will be very large matrix interference from S 2- , and in the dissolution process, the highly toxic gas H2S will be released, so generally S 2- in lithium sulfide needs to be pretreated first to reduce its matrix interference.

[0006] At present, the technical method for removing S 2- in lithium sulfide is mainly the method of water-soluble heating and then acidification or oxidation, which can convert part of S 2- into H2S, but the solubility of H2S is very large, and there is still a large amount of S 2- and HS - in the test solution. To solve this problem, the application proposes a new lithium sulfide pretreatment method based on lithium sulfide ICP-OES impurity testing. SUMMARY

[0007] This invention aims to perform sulfur removal pretreatment on lithium sulfide materials requiring impurity content testing, thereby preventing the matrix effect of sulfur from affecting the accuracy of the test. Through a mild and efficient chemical pretreatment method, the sulfur bulk in lithium sulfide is separated and removed in a controllable manner, while retaining the target impurity elements to the maximum extent. This enables accurate quantitative analysis of major metallic and non-metallic impurity elements such as K, Ca, Na, Mg, Fe, Pb, Zn, Al, Si, and Ti in lithium sulfide.

[0008] To address the aforementioned technical problems, this invention provides an organic pretreatment method for lithium sulfide ICP-OES impurity testing, comprising the following steps: (1) Weigh the lithium sulfide solid sample, add ultrapure water to dissolve it, and obtain the sample processing solution; (2) Add chloroacetic acid to the sample processing solution to convert the sulfur element in lithium sulfide into organic matter to obtain intermediate solution 1; (3) Add ammonia to intermediate liquid 1 to adjust the pH value to 7-8, so that the excess chloroacetic acid in intermediate liquid 1 and the mercaptoacetic acid generated in step (2) are converted into thiodiacetic acid, and intermediate liquid 2 is obtained. (4) Add hydrochloric acid to intermediate liquid 2 to adjust the pH value to 1~2, then add organic solvent for extraction. The resulting aqueous solution is the test solution, thus completing the organic pretreatment for lithium sulfide ICP-OES impurity testing.

[0009] Preferably, in step (1), the mass ratio of lithium sulfide solid sample to ultrapure water is 1:(10~20), and more preferably 1:20; Preferably, in step (2), the molar ratio of chloroacetic acid to lithium sulfide solid sample is (2~2.5):1, and the molar ratio of lithium sulfide solid sample is calculated based on 100% purity, preferably 2.5:1; Preferably, the mass concentration of ammonia in step (3) is 1% to 10%, more preferably 5% to 10%; Preferably, the organic solvent in step (4) is either ethyl acetate or dichloromethane; the number of extractions is 2 to 3, preferably 3; and the concentration of hydrochloric acid is 0.5 to 2.5 mol / L, preferably 1.5 to 2.5 mol / L.

[0010] The reaction principle is as follows: Starting material: solid lithium sulfide (Li2S) containing impurities.

[0011] Step 1: Dissolve solid lithium sulfide in a small amount of water to make S 2- Most of them were converted to HS - .

[0012] Reaction equation: Li₂S + H₂O → LiHS + LiOH A small portion of HS - Secondary hydrolysis occurs, HS - + H2O→H2S+OH - Kh1 = 9.09 × 10 -3 Kh2 = 1.0989 × 10 -7 Step Two: Adding chloroacetic acid (ClCH2COOH) converts sulfur (S) into organic matter.

[0013] Reaction equation: ClCH2COOH+LiOH→ClCH2COOLi+H2O ClCH2COOH+LiHS→HSCH2COOH+LiCl ClCH2COOH+Li2S→S(CH2COOH)2+LiCl Step 3: Add ammonia water to adjust the pH to 7-8. Excess chloroacetic acid reacts with mercaptoacetic acid to form thiodiacetate.

[0014] Reaction equation: ClCH2COOH+NH3·H2O→ClCH2COONH4+H2O ClCH2COOLi+NH3·H2O→ClCH2COONH4+LiOH HSCH2COOH+NH3·H2O→HSCH2COONH4+H2O ClCH2COONH4+ HSCH2COONH4→S(CH2COONH4)2+NH4Cl Step Four: Adjust the pH to 1-2 with HCl to convert thiodiacetate to thiodiacetic acid, and extract the thiodiacetic acid 2-3 times with ethyl acetate or dichloromethane. Acidification: This process converts thiodiacetate into thiodiacetic acid. The pKa values ​​of thiodiacetic acid are pK1: 3.32 and pK2: 4.29. Adjusting the pH to 1-2 ensures that thiodiacetic acid exists in an electrically neutral molecular form, making it easier to extract. Lowering the pH prevents the carboxyl groups of thiodiacetic acid from forming complexes with metal ions in the solution.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects: ① It has a high degree of sulfur removal, and the sulfur concentration in the aqueous phase after extraction is extremely low, with a removal rate of up to 99% or more; ② Compared to generating highly toxic soluble hydrogen sulfide gas, generating organic extractable thiodiacetic acid makes the experiment more controllable and safer; ③ The experiment can be carried out at room temperature, and the reaction is rapid but not violent; ④ No chemical reagents that affect the metal element impurities are added during the entire process, so there is no impact on impurity testing. Detailed Implementation

[0016] The present invention will be further described below with reference to embodiments. These embodiments are illustrative of the present invention, but the present invention is not limited to these embodiments: In the following embodiments, the operating conditions of the inductively coupled plasma atomic emission spectrometer were set as follows: the detector was Thermo Scientific iCAP PRO; the number of repetitions was 3; the pump speed was 45 rpm; the boost delay was 35 s; the readout time was 15 s; the RF power was 1150 W; the stabilization time was 10 s; the observation mode was axial; the atomizing gas flow rate was 0.5 L / min; the cooling gas flow rate was 12.5 L / min; the auxiliary gas flow rate was 0.5 L / min; and the test wavelengths are shown in Table 1 below.

[0017] Table 1 The partial COA data for the lithium sulfide solid samples are shown in Table 2 below: Table 2 Example 1 (1) Weigh 0.5004 g of lithium sulfide solid sample, add 10 mL of ultrapure water to dissolve it, and obtain the sample treatment solution; (2) Add 2.5998 g of chloroacetic acid to the sample processing solution to convert the sulfur element in lithium sulfide into organic matter, and obtain intermediate solution 1; (3) Add 5wt% ammonia water to intermediate solution 1 at a rate of 1-2 mL / min to adjust the pH value to 7.53, and convert the excess chloroacetic acid in intermediate solution 1 and the mercaptoacetic acid generated in step (2) into thiodiacetic acid to obtain intermediate solution 2; (4) Add 1.5 mol / L hydrochloric acid to intermediate solution 2 at a rate of 1-2 mL / min to adjust the pH value to 1.50, then add ethyl acetate for extraction 3 times, each time using 15 mL, to form two layers of substances. The upper layer is a mixture of organic solution and thiodiacetic acid, and the lower layer is an aqueous solution containing impurity elements. The aqueous solution is the solution to be tested. (5) Transfer the solution to be tested to a 50 mL volumetric flask, dilute to the mark with ultrapure water, and test the content of impurity elements using ICP-OES.

[0018] The test results are shown in Table 3 below: Table 3 Wherein: The test results have included the dilution factor in the calculation, which means that the test result = the result directly output by the ICP-OES instrument * the volume of the final volume / the sample mass, that is, the impurity content contained in 1g of sample.

[0019] Based on the test results, the COA (Certificate of Analysis) of a solid lithium sulfide sample was used for verification. It can be seen that the detected impurity content after processing using the method of this embodiment is basically consistent with its COA result. However, compared with the traditional detection of solid lithium sulfide impurities, the pretreatment method of this embodiment firstly generates organic extractable thiodiacetic acid, avoiding the generation of highly toxic soluble hydrogen sulfide gas, making the experiment more controllable and safer; secondly, the entire experiment can be carried out at room temperature, and the reaction is rapid and not violent; finally, no chemical reagents that affect the metal element impurities are added in the entire process, so there is no impact on the impurity test, which can ensure the accuracy of the detection.

[0020] Calculate the sulfur removal rate using purity data: S removal rate (%) = (original sulfur content - measured sulfur content) / original sulfur content * 100%.

[0021] Original sulfur content = (32.06 / 45.942) * 99.69% = 69.5673%. The final S removal rate was 99.28%.

[0022] Example 2 (1) Weigh 1.0029 g of lithium sulfide solid sample, add 10 mL of ultrapure water to dissolve it, and obtain the sample treatment solution; (2) Add 5.0058 g of chloroacetic acid to the sample processing solution to convert the sulfur element in lithium sulfide into organic matter, and obtain intermediate solution 1; (3) Add 10wt% ammonia water dropwise to intermediate solution 1 at a rate of 1-2 mL / min to adjust the pH value to 7.31, and convert the excess chloroacetic acid in intermediate solution 1 and the mercaptoacetic acid generated in step (2) into thiodiacetic acid to obtain intermediate solution 2; (4) Add 2.5 mol / L hydrochloric acid to intermediate liquid 2 at a rate of 1-2 mL / min to adjust the pH value to 2.00, then add ethyl acetate for extraction 3 times, each time using 15 mL, to form two layers of substances. The upper layer is a mixture of organic solution and thiodiacetic acid, and the lower layer is an aqueous solution containing impurity elements. The aqueous solution is the solution to be tested. (5) Transfer the solution to be tested to a 50 mL volumetric flask, dilute to the mark with ultrapure water, and test the content of impurity elements using ICP-OES.

[0023] The test results are shown in Table 4 below: Table 4 Based on the test results, the final S removal rate was 99.12%, calculated in the same way as in Example 1.

[0024] Example 3 (1) Weigh 0.7505 g of lithium sulfide solid sample, add 10 mL of ultrapure water to dissolve it, and obtain the sample treatment solution; (2) Add 4.0032 g of chloroacetic acid to the sample processing solution to convert the sulfur element in lithium sulfide into organic matter, and obtain intermediate solution 1; (3) Add 8wt% ammonia to intermediate solution 1 at a rate of 1-2 mL / min to adjust the pH value to 7.50, and convert the excess chloroacetic acid in intermediate solution 1 and the mercaptoacetic acid generated in step (2) into thiodiacetic acid to obtain intermediate solution 2; (4) Add 2 mol / L hydrochloric acid to intermediate solution 2 at a rate of 1-2 mL / min to adjust the pH value to 1.55, then add dichloromethane for extraction 3 times, each time using 15 mL, to form two layers of substances. The lower layer is a mixture of organic solution and thiodiacetic acid, and the upper layer is an aqueous solution containing impurity elements. The aqueous solution is the solution to be tested. (5) Transfer the solution to be tested to a 50 mL volumetric flask, dilute to the mark with ultrapure water, and test the content of impurity elements using ICP-OES.

[0025] The test results are shown in Table 5 below: Table 5 Based on the test results, the final S removal rate was 99.35%, calculated in the same way as in Example 1.

[0026] Example 4 Spiking recovery experiment for lithium sulfide samples: (1) Weigh four lithium sulfide solid samples, designated as groups A to D, with group D serving as the blank control group. Calculate the background value using the data from group D. The masses of the four lithium sulfide solid samples are as follows: A: 1.0013 g, B: 1.0120 g, C: 1.0008 g, D: 1.0109 g Dissolve the sample in 10 mL of ultrapure water, and add 0.5 mL, 1 mL, and 2.5 mL of 100 mg / L mixed standard solution to A, B, and C respectively to obtain the sample treatment solution. (The method for preparing the mixed standard solution is to take 10 mL of each of the following elements, K, Ca, Na, Mg, Fe, Pb, Zn, Al, Si, and Ti, at a concentration of 1000 mg / L (with H2O as the medium), and mix them thoroughly in a 100 mL volumetric flask.) (2) Chloroacetic acid was added to the sample treatment solution respectively, and the mass of each added was: 5.0032 g of A, 5.0109 g of B, 5.0102 g of C, and 5.0014 g of D were used to convert the sulfur element in lithium sulfide into organic matter, resulting in intermediate solutions A1, B1, C1, and D1. (3) Add 5wt% ammonia water dropwise to intermediate solutions A1, B1, C1, and D1 at a rate of 1-2 mL / min to adjust the pH value to 7.50, 7.44, 7.55, and 7.49, respectively. The excess chloroacetic acid in intermediate solutions A1, B1, C1, and D1 is converted into thiodiacetic acid by reacting with mercaptoacetic acid generated in step (2), and intermediate solutions A2, B2, C2, and D2 are obtained. (4) Add 2.5 mol / L hydrochloric acid to intermediate solutions A2, B2, C2, and D2 at a rate of 1-2 mL / min to adjust the pH value to 2.03, 2.01, 1.98, and 2.00, respectively. Then add dichloromethane for extraction three times, each time using 15 mL, to form two layers of substances. The lower layer is a mixture of organic solution and thiodiacetic acid, and the upper layer is an aqueous solution containing impurity elements. The aqueous solutions are the test solutions A3, B3, C3, and D3. (5) Transfer the solutions A3, B3, C3 and D3 to 50 mL volumetric flasks respectively, dilute to the mark with ultrapure water, and test the impurity element content using ICP-OES.

[0027] The test results for the blank control group D are shown in Table 6 below, and the test results for the spiked recovery groups A, B, and C are shown in Tables 7, 8, and 9 below: Table 6 The background levels of samples A, B, and C are calculated based on the test results of D. Taking the K element in sample A as an example: K (background level) = 0.95 μg / g * ​​1.0013 g / 50 mL = 0.02 μg / mL Table 7 Sample A - 1.0013 g Table 8 Sample B - 1.0120 g Table 9 Sample C - 1.0008 g According to the test results, the recoveries of spiked substances at different concentrations were all between 90% and 110%, showing good spiked recovery performance. The method is accurate and the operation technique is reliable, and it can accurately determine the content of target substances in samples.

[0028] As can be seen from the detection results in Examples 1-3 above, the elemental content values ​​in Examples 1-3 of this application are basically consistent with the elemental content values ​​in the COA of the sample; as can be seen from the spiked recovery results in Example 4, the spiked recovery rate is between 96% and 110%, and the spiked performance is good; indicating that the method of this application can stably and accurately obtain detection results consistent with the COA of the sample, and has good reliability and repeatability.

[0029] All other parts not described in detail are existing technologies. Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. An organic pretreatment method for lithium sulfide ICP-OES impurity testing, characterized in that, Includes the following steps: (1) Weigh the lithium sulfide solid sample, add ultrapure water to dissolve it, and obtain the sample processing solution; (2) Add chloroacetic acid to the sample processing solution to convert the sulfur element in lithium sulfide into organic matter to obtain intermediate solution 1; (3) Add ammonia to intermediate liquid 1 to adjust the pH value to 7-8, so that the excess chloroacetic acid in intermediate liquid 1 and the mercaptoacetic acid generated in step (2) are converted into thiodiacetic acid, and intermediate liquid 2 is obtained. (4) Add hydrochloric acid to intermediate liquid 2 to adjust the pH value to 1~2, then add organic solvent for extraction. The resulting aqueous solution is the test solution, thus completing the organic pretreatment for lithium sulfide ICP-OES impurity testing.

2. The organic pretreatment method according to claim 1, characterized in that, In step (1), the mass ratio of lithium sulfide solid sample to ultrapure water is 1:(10~20).

3. The organic pretreatment method according to claim 1, characterized in that, In step (2), the molar ratio of chloroacetic acid to lithium sulfide solid sample is (2~2.5):1, and the molar amount of lithium sulfide solid sample is calculated based on 100% purity.

4. The organic pretreatment method according to claim 1, characterized in that, The mass concentration of ammonia in step (3) is 1% to 10%.

5. The organic pretreatment method according to claim 1, characterized in that, In step (4), the organic solvent is either ethyl acetate or dichloromethane; the extraction is performed 2 to 3 times; and the concentration of hydrochloric acid is 0.5 to 2.5 mol / L.

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