Digestion method for lithium sulfide impurity element analysis and content determination method thereof

Abstract

The invention relates to a digestion method for lithium sulfide impurity element analysis and a content determination method thereof, the digestion method comprises the following steps: (A1) pre-digestion: weighing a lithium sulfide sample, and adding ultrapure water to obtain a sample aqueous solution; dropwise adding oxidizing acid into the sample aqueous solution, and heating to enable the lithium sulfide sample in the sample aqueous solution to fully react, so as to obtain a pre-digested sample; (A2) microwave digestion: carrying out microwave digestion on the pre-digested sample under a closed condition to obtain a microwave digested sample, and fixing the volume to obtain a to-be-detected sample solution. Compared with a conventional multi-time acid-adding digestion method, the digestion method has the advantages that oxidizing acid does not need to be added repeatedly, so that the operation steps are simple, the digestion time is short, and compared with the multi-time acid-adding digestion method, the method can meet quantitative analysis of most impurity elements in lithium sulfide.

Description

Technical Field

[0001] The present invention relates to the fields of analytical chemistry and lithium sulfide solid-state batteries, and in particular to a digestion method for analyzing impurity elements in lithium sulfide and a method for determining their content. Background Art

[0002] Lithium batteries, as both power and consumer batteries, are widely used in automobiles, mobile phones, and various electronic products. Most existing lithium batteries are liquid batteries, using liquid electrolytes. Liquid electrolytes are organic solvents, which are flammable and pose a safety risk. Using solid-state electrolytes can significantly improve battery safety and other performance, leading lithium-ion battery companies, R&D institutions, and universities to invest significant resources in solid-state battery research and development.

[0003] Currently, existing literature has disclosed many materials that may be suitable for solid-state electrolytes, with lithium sulfide being one of the main components of an ideal solid-state electrolyte. As the main material for sulfide solid-state batteries, Li2S materials have relatively harsh production conditions, and there are currently no relevant mature standards to evaluate the purity of lithium sulfide. When lithium sulfide is dissolved in water, due to the strong reducing properties of divalent sulfur ions, it undergoes an oxidation-reduction reaction with oxygen in the air at the liquid-gas interface to produce elemental sulfur. However, elemental sulfur is insoluble in water, so a solution cannot be obtained. If the elemental sulfur is filtered out, then due to elemental adsorption on the surface of the elemental sulfur, it will inevitably cause element loss, which will result in a small quantitative element test result. Therefore, the filtration method is not the optimal solution. Therefore, a method for testing the impurity element content of lithium sulfide using inductively coupled plasma atomic emission spectrometry (ICP-OES) has been developed. Patent CN 116754349A discloses a method for removing elemental sulfur by oxidation using multiple acid additions, but this method is cumbersome to operate, takes a long time to digest, and can only detect a few elements. In view of this, the present invention provides a digestion method for analyzing impurity elements in lithium sulfide and a method for determining its content. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a digestion method for analyzing impurity elements in lithium sulfide and a method for determining its content, so as to meet the quantitative analysis of most impurity elements in lithium sulfide.

[0005] The technical solution of the present invention to solve the above technical problems is as follows:

[0006] In a first aspect, a digestion method for analyzing impurity elements in lithium sulfide comprises the following steps:

[0007] (A1) Predigestion: Weigh a lithium sulfide sample and add ultrapure water to obtain a sample aqueous solution; dropwise add an oxidizing acid to the sample aqueous solution, and heat to allow the lithium sulfide sample in the sample aqueous solution to fully react, thereby obtaining a predigested sample;

[0008] (A2) Microwave digestion: The pre-digested sample is subjected to microwave digestion under sealed conditions to obtain a microwave-digested sample, which is then fixed to volume to obtain a sample solution to be tested.

[0009] Principle description: Step (A1) pre-digestion of the present invention: weigh a lithium sulfide sample, add ultrapure water to obtain a sample aqueous solution; wait for the sample to react preliminarily with water, and shake or stir, then drop an oxidizing acid into the sample aqueous solution, heat to allow the lithium sulfide sample in the sample aqueous solution to fully react, and obtain a pre-digested sample; in this step, water is first added for wetting to avoid a violent reaction that may occur when acid is directly added, which may cause sample sputtering, thereby avoiding possible danger to experimenters and possible loss of elements in the sample; then, an oxidizing acid is added dropwise and heated for reaction, so that the oxidizing acid reacts with divalent sulfur anions in the sample aqueous solution to generate hydrogen sulfide, which partially overflows first, to prevent the generation of a large amount of hydrogen sulfide gas in the sealed microwave digestion tank, which may exceed the upper pressure limit of the tank body and cause the tank to explode, causing damage to experimental equipment and personnel, and the experiment will also fail;

[0010] Step (A2) microwave digestion: This step mainly allows the sulfur element and divalent sulfur anions in the sample to continue to react with the oxidizing acid under a high temperature, high pressure, and microwave environment to generate oxidation products such as sulfite and sulfate. This prevents the sulfur element from existing as solid particles and affecting the sample injection in the ICP-OES instrument (clogging and atomization effect). At the same time, it prevents the divalent sulfur anions from forming precipitation with the positive ions of the element to be measured, which may affect the measurement results.

[0011] The beneficial effects of the present invention are as follows: the present invention performs pre-digestion by adding water and adding an oxidizing acid, and then performs microwave digestion under closed conditions. Compared with the conventional method of multiple acid digestion, the method does not need to repeatedly add an oxidizing acid, so the operation steps are simple and the digestion time is short. Moreover, the method can meet the quantitative analysis requirements of most impurity elements in lithium sulfide.

[0012] On the basis of the above technical solution, the present invention can also be improved as follows.

[0013] Furthermore, the oxidizing acid in step (A1) comprises at least one of nitric acid, aqua regia, and perchloric acid;

[0014] And / or in step (A1), the ratio of the lithium sulfide sample to the ultrapure water is (0.01-1) g: (1-10) mL;

[0015] and / or in step (A1), the ratio of the lithium sulfide sample to the oxidizing acid is (0.01-1) g: (5-20) mL;

[0016] And / or the heating parameters in step (A1): temperature is 80° C. to 180° C., and time is 10 min to 30 min.

[0017] The beneficial effect of adopting the above further scheme is that the S element generated by adding the oxidizing acid in step (A1) can be easily oxidized to sulfite and sulfate under the conditions of high temperature, high pressure, microwave and oxidizing acid in step (A2), thereby avoiding affecting the measurement results.

[0018] Furthermore, the specific process of step (A2) is as follows: the pre-digested sample is sealed and assembled into a microwave digestion instrument, temperature and pressure sensors are connected in sequence, the microwave digestion instrument is turned off, a microwave digestion program is set, and the digestion program is run to obtain a microwave digestion sample after completion.

[0019] The beneficial effect of adopting the above further scheme is: this step mainly allows the S element and divalent sulfur anions in the sample to react with the oxidizing acid under a high temperature, high pressure, and microwave environment to generate oxidation products sulfite and sulfate, thereby preventing the presence of solid particles of S element from affecting the injection (clogging and atomization effect) in the ICP-OES instrument, and at the same time preventing the divalent sulfur anions from forming precipitation with the positive ions of the element to be measured, thereby affecting the measurement results.

[0020] Furthermore, the microwave digestion program is specifically set as follows: the temperature of the first program is 130° C. to 150° C., and the time is 10 min to 20 min; the temperature of the second program is 170° C. to 220° C., and the time is 30 min to 50 min.

[0021] The beneficial effect of adopting the above further scheme is: since different tanks have different pressure limits, it is necessary to decide whether to stop heating based on the pressure limit. By observing the tank pressure in the lower temperature heating program of the first program, if a pressure surge occurs, heating can be stopped in advance to prevent the tank from exploding.

[0022] Furthermore, the method further includes preparing a spiked solution of the sample to be processed, comprising the following steps: weighing a lithium sulfide sample, adding each impurity element to be measured in the lithium sulfide sample for spiked treatment, and obtaining a spiked lithium sulfide sample; and processing the spiked lithium sulfide sample according to the descriptions of steps (A1) and (A2) to obtain a spiked solution of the sample to be processed.

[0023] The beneficial effect of adopting the above further scheme is that the recovery rate obtained by spiked is a key data for measuring data accuracy.

[0024] Furthermore, the amount of the impurity element to be detected added during the spike addition process is 1 to 10 times the content of the element to be detected. Generally, the amount of the impurity element to be detected added during the spike addition process is 1 to 5 times the content of the element to be detected. For elements that are not detected or have very low content, the amount of the impurity element to be detected added is 1 to 10 times the content of the element to be detected. If the impurity element content in the sample is less than 1 ppm, the spike addition amount can be set at 1 ppm to 5 ppm.

[0025] Furthermore, each impurity element to be measured in the lithium sulfide sample is at least one of Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Sr, Ti, Tl, V and Zn.

[0026] In a second aspect, a method for determining the content of lithium sulfide impurities comprises the following steps:

[0027] (B1) taking the sample solution to be treated obtained by the digestion method, heating the solution until the volume is less than 5 mL (the solution volume is very small but the solvent is not completely evaporated and no crystallization occurs), then constant the volume to obtain a sample solution, and testing on a computer to obtain the solution concentration of each impurity element to be measured in the sample solution; calculating the mass content of each impurity element to be measured in the lithium sulfide sample based on the solution concentration of each impurity element to be measured in the sample solution;

[0028] (B2) taking the sample spiked solution to be processed obtained by the digestion method, heating it to a solution volume of less than 5 mL (the solution volume is very small but the solvent is not completely evaporated and no crystallization occurs), then constant the volume to obtain a spiked sample solution, and performing element determination using a standard curve method (external standard method) on an ICP-OES instrument to obtain the solution concentration of each impurity element to be measured in the spiked sample solution; calculating the spiked recovery rate of each impurity element to be measured in the lithium sulfide sample based on the solution concentration of each impurity element to be measured in the sample solution and the solution concentration of each impurity element to be measured in the spiked sample solution.

[0029] The beneficial effect of adopting the above scheme is that the contents of various impurity elements in the sample can be obtained, and the reliability of the determination results can be evaluated by the spike recovery rate.

[0030] Furthermore, step (B1) includes the following specific steps:

[0031] (B1-1) taking the sample solution to be treated, heating it until the volume of the solution is less than 5 mL, cooling it, and then constant volume to obtain a sample solution;

[0032] (B1-2) measuring the spectral line intensity of the sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample;

[0033] (B1-3) Calculate the mass content of each impurity element to be measured in the lithium sulfide sample according to formula (1):

[0034]

[0035] In formula (1): i is the mass content of the impurity element to be measured in the lithium sulfide sample, in mg / kg; c i is the concentration of each impurity element to be measured in the sample solution, in mg / L; c is the content of the impurity element to be measured in the blank solution, in mg / L; V is the fixed volume of the sample solution, in mL; m is the weighed mass of the lithium sulfide sample, in g.

[0036] Further, step (B2) includes the following specific steps:

[0037] (B2-1) taking the spiked solution of the sample to be processed, heating it until the volume of the solution is less than 5 mL, cooling it, and then constant to volume to obtain a spiked sample solution;

[0038] (B2-2) measuring the spectral line intensity of the spiked sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the spiked sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample;

[0039] (B2-3) Calculate the spiked recovery of each impurity element to be measured in the lithium sulfide sample according to formula (2):

[0040] R=(c R -c i )×100% (2);

[0041] In formula (2): R is the recovery rate; c R is the concentration of each impurity element to be measured in the spiked sample solution, in mg / L; c i It is the concentration of each impurity element to be measured in the sample solution, in mg / L. DETAILED DESCRIPTION

[0042] The principles and features of the present invention are described below. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in this field or the product instructions are used. Where the manufacturers of the reagents or instruments used are not specified, they are all conventional products that can be purchased through regular channels.

[0043] This embodiment relates to a digestion method for analyzing impurity elements in lithium sulfide, comprising the following steps:

[0044] (A1) Predigestion: Weigh a lithium sulfide sample and add ultrapure water to obtain a sample aqueous solution; dropwise add an oxidizing acid to the sample aqueous solution, and heat to allow the lithium sulfide sample in the sample aqueous solution to fully react, thereby obtaining a predigested sample;

[0045] (A2) Microwave digestion: The pre-digested sample is subjected to microwave digestion under sealed conditions to obtain a microwave-digested sample, which is then fixed to volume to obtain a sample solution to be tested.

[0046] In this embodiment, the oxidizing acid in step (A1) preferably comprises at least one of nitric acid, aqua regia, and perchloric acid; wherein the nitric acid, aqua regia, and perchloric acid are all of high purity or above.

[0047] And / or the ratio of the lithium sulfide sample to the ultrapure water in step (A1) is (0.01-1) g: (1-10) mL; specifically, for example, 0.01 g: 1 mL, 0.1 g: 5 mL, 1 g: 10 mL, etc.;

[0048] And / or in step (A1), the ratio of the lithium sulfide sample to the oxidizing acid is (0.01-1) g: (5-20) mL; specifically, for example, 0.01 g: 5 mL, 0.1 g: 10 mL, 1 g: 20 mL, etc.;

[0049] And / or heating parameters in step (A1): temperature is 80°C to 180°C, specifically 80°C, 100°C, 120°C, 180°C, etc., time is 10min to 30min, specifically 10min, 20min, 30min, etc.

[0050] In this embodiment, preferably, the heating in step (A1) is performed in a graphite heater.

[0051] In this embodiment, the specific process of step (A2) is as follows: the pre-digested sample is sealed and assembled into a microwave digestion instrument, temperature and pressure sensors are connected in sequence, the microwave digestion instrument is turned off, a microwave digestion program is set, and the digestion program is run to obtain a microwave digestion sample after completion.

[0052] Preferably, the microwave digestion program is set as follows: the temperature of the first program is 130°C to 150°C, specifically 130°C, 140°C, 150°C, etc., and the time is 10min to 20min, specifically 10min, 20min, etc.; the temperature of the second program is 170°C to 220°C, specifically 170°C, 180°C, 200°C, 220°C, etc., and the time is 30min to 50min, specifically 30min, 40min, 50min, etc.

[0053] Preferably, this embodiment further includes preparing a spiked solution of the sample to be processed, comprising the following steps: weighing a lithium sulfide sample, adding each impurity element to be measured in the lithium sulfide sample for spike treatment, and obtaining a spiked lithium sulfide sample; and processing the spiked lithium sulfide sample according to the description of steps (A1) and (A2) to obtain a spiked solution of the sample to be processed.

[0054] In this embodiment, the spiked amount of the impurity element to be measured in the spiked process is preferably 1 to 10 times the content of the element to be measured. Generally, the spiked amount of the impurity element to be measured in the spiked process is 1 to 5 times the content of the element to be measured. For elements that are not detected or have very low content, the spiked amount of the impurity element to be measured is 1 to 10 times the content of the element to be measured. If the content of the impurity element in the sample is less than 1 ppm, the spiked amount can be set at 1 ppm to 5 ppm.

[0055] Preferably, in this embodiment, the impurity elements to be measured in the lithium sulfide sample are at least one of Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Sr, Ti, Tl, V and Zn.

[0056] This embodiment also relates to a method for determining the content of lithium sulfide impurities, comprising the following steps:

[0057] (B1) taking the sample solution to be treated obtained by the digestion method, heating the solution until the volume is less than 5 mL, the solution volume is very small but the solvent is not completely evaporated, and no crystallization occurs, and then constant volume to obtain a sample solution, and obtaining the solution concentration of each impurity element to be measured in the sample solution; according to the solution concentration of each impurity element to be measured in the sample solution, calculating the mass content of each impurity element to be measured in the lithium sulfide sample;

[0058] (B2) taking the sample spiked solution to be processed obtained by the digestion method, heating it until the solution volume is less than 5 mL, the solution volume is very small but the solvent is not completely evaporated, and no crystallization occurs, then constant volume to obtain a spiked sample solution, and obtaining the solution concentration of each impurity element to be measured in the spiked sample solution; according to the solution concentration of each impurity element to be measured in the sample solution and the solution concentration of each impurity element to be measured in the spiked sample solution, calculating the spiked recovery rate of each impurity element to be measured in the lithium sulfide sample.

[0059] In this embodiment, step (B1) preferably includes the following specific steps:

[0060] (B1-1) taking the sample solution to be processed, heating the solution until the volume is less than 5 mL, the solution volume is very small but the solvent is not completely evaporated, and no crystallization occurs, and then cooling and constant volume to obtain a sample solution;

[0061] (B1-2) measuring the spectral line intensity of the sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample;

[0062] (B1-3) Calculate the mass content of each impurity element to be measured in the lithium sulfide sample according to formula (1):

[0063]

[0064] In formula (1): i is the mass content of the impurity element to be measured in the lithium sulfide sample, in mg / kg; c i is the concentration of each impurity element to be measured in the sample solution, in mg / L; c is the content of the impurity element to be measured in the blank solution, in mg / L; V is the fixed volume of the sample solution, in mL; m is the weighed mass of the lithium sulfide sample, in g.

[0065] In this embodiment, step (B2) preferably includes the following specific steps:

[0066] (B2-1) taking the spiked solution of the sample to be processed, heating the solution until the volume is less than 5 mL, the solution volume is very small but the solvent is not completely evaporated and no crystallization occurs, and then cooling and constant volume to obtain a spiked sample solution;

[0067] (B2-2) measuring the spectral line intensity of the spiked sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the spiked sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample;

[0068] (B2-3) Calculate the spiked recovery of each impurity element to be measured in the lithium sulfide sample according to formula (2):

[0069] R=(c R -c i )×100% (2);

[0070] In formula (2): R is the recovery rate; c R is the concentration of each impurity element to be measured in the spiked sample solution, in mg / L; c i It is the concentration of each impurity element to be measured in the sample solution, in mg / L.

[0071] The standard curve method describes the relationship between the spectral line intensity y and the concentration x by a linear regression equation: y = a + bx, where a is the intercept of the line and b is the slope of the line. When making a standard curve, n experimental points (x1, y1), (x2, y2) ... (x n ,y n ) The error between each experimental point and the regression line can be quantitatively described by the following formula (3):

[0072]

[0073] The sum of squares of the errors between the regression line and all experimental points is given by the following formula (4):

[0074]

[0075] In order to make the determined regression equation and regression line closest to the actual distribution state of the experimental points, Q must take the minimum value. In the analysis and correction, different x i Value measurement y i , use the least squares method to estimate the values ​​of a and b, so that the Q value reaches the minimum value. Using the mathematical method of finding the extreme value, there is and The calculation formulas (5) and (6) for a and b can be derived:

[0076]

[0077] Where, are the average values ​​of x and y respectively. After the intercept a and slope b of the straight line are determined, the linear regression equation and the regression line are determined. The intercept a, slope b and correlation coefficient of each impurity element to be measured in the lithium sulfide sample, namely Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Sr, Ti, Tl, V and Zn, are shown in Table 1.

[0078] Table 1 Intercept a, slope b and correlation coefficient of each impurity element to be measured

[0079]

[0080]

[0081] Example 1

[0082] This embodiment relates to a digestion method for analyzing lithium sulfide impurity elements and a method for determining its content, comprising the following specific steps:

[0083] ① Weigh m = 0.1 (± 0.05) g of Li2S sample into a microwave digestion vessel (made of PTFE) and add 5 ml of ultrapure water to moisten the vessel (to avoid the violent reaction of adding acid directly, which may cause splashing and sample loss); the sample is basically dissolved in water;

[0084] Note: Both a blank group and a sample spike group need to be prepared. Since the test object is a trace element, the spike amount is recommended to be 1 mg / L. The spike amount can be adjusted according to the content of the element to be tested in the sample.

[0085] ② Slowly add 10ml of oxidizing acid such as nitric acid (high-grade pure reagent) to the microwave digestion tank.

[0086] ③ Use a microwave digestion tank to heat in a graphite heater at a temperature of 110°C for 20 minutes to ensure that the lithium sulfide is basically completely reacted.

[0087] ④ Seal the microwave digestion vessel, install the outer vessel, assemble it into the microwave digestion instrument, connect the temperature and pressure sensors, and turn off the microwave digestion instrument.

[0088] ⑤Set the microwave digestion program: digestion temperature: ①140℃, 150min; ②210℃, 45min.

[0089] After the program is set up, run the digestion program. After it ends, wait for the instrument to cool to room temperature and take out the microwave digestion tank.

[0090] ⑥ Clean the tank cover with ultrapure water and pour the cleaning solution into the microwave digestion tank.

[0091] ⑦ Heat the sample in a microwave digester in a graphite heater until it is nearly dry. After cooling to room temperature, remove the sample and dilute the solution to a plastic volumetric flask to obtain the sample solution.

[0092] ⑧ Establish a calibration curve: Accurately pipette 10 mL of a 1000 mg / L standard solution of the element to be measured into a 100 mL volumetric flask. Constant the volume to obtain a 100 mg / L calibration stock solution. Subsequently, pipette 0 mL, 0.5 mL, 1 mL, 2 mL, 3 mL, and 5 mL of the calibration stock solution into six 100 mL volumetric flasks, respectively. Pipet 7.4 mL of high-grade pure hydrochloric acid into each flask (the amount of pipette can be adjusted appropriately depending on the sample solution matrix). This yields a series of calibration solutions with a concentration gradient of 0 mg / L, 0.5 mg / L, 1 mg / L, 2 mg / L, 3 mg / L, and 5 mg / L in a 3% hydrochloric acid matrix. The elemental spectral lines are selected as shown in Table 1 (other non-interfering spectral lines can also be selected). The test conditions are shown in Table 3. After the instrument is properly started, measure the spectral line intensities of the series of standard solutions and plot the calibration curve based on the spectral line intensities and element concentrations.

[0093] ⑨ Test the spectral line intensity of the blank solution, sample solution and spiked sample solution, and substitute the spectral line intensity into the calibration curve to obtain the corresponding solution element concentration c, c i and c R . Sample element concentration ω i The spike recovery R was calculated according to the above formulas (1) and (2). The test data are shown in Table 2.

[0094] Compared to the plate digestion method of multiple acid additions, this method does not require repeated addition of oxidizing acid, resulting in simpler steps and shorter digestion times. Testing has shown that this method can meet the quantitative analysis requirements of most elements.

[0095] Table 2 Sample test results and recovery rates of each element

[0096]

[0097]

[0098] In Table 2, ND means that the element content in the sample solution is below the detection limit.

[0099] Table 3 Test conditions

[0100]

[0101] Example 2: Comparative Example

[0102] The experiment was conducted according to the patent publication number CN 116754349A. The details are as follows:

[0103] This embodiment relates to a method for digesting lithium sulfide impurity elements based on ICP-OES, comprising the following steps:

[0104] (1) Removing H2S: Weighing a lithium sulfide (Li2S) sample, adding ultrapure water to obtain a sample aqueous solution; heating the sample aqueous solution until 2 / 3-9 / 10 of the ultrapure water in the sample aqueous solution evaporates, preliminarily removing H2S generated in the sample aqueous solution, and obtaining a sample free of H2S;

[0105] The mass volume ratio of the lithium sulfide sample to the ultrapure water in step (1) is 1 / 5 g·mL -1 In step (1), the H2S-removed sample is subjected to the H2S-removed step again, and the step is repeated at least once; the H2S-removed sample is subjected to the H2S-removed step again, which includes the following specific steps: adding ultrapure water to the H2S-removed sample to obtain a new sample aqueous solution, wherein the mass volume ratio of the H2S-removed sample to the ultrapure water added again is 1 / 10-1 / 20 g·mL -1 , heating the new sample aqueous solution until the ultrapure water added again evaporates 2 / 3-9 / 10 of the added amount, driving away the H2S generated in the new sample aqueous solution, and obtaining the H2S-removed sample again; in step (1), the heating temperature is 100-200°C.

[0106] Specifically, 1) weigh m = 0.1 (± 0.05) g of Li2S sample into a beaker and add 10 ml of ultrapure water to moisten it (direct addition of acid will cause a violent reaction, resulting in splashing and sample loss); the sample is basically dissolved in the ultrapure water; heat at 180°C until nearly dry. This process can be repeated multiple times, the main purpose of which is to initially remove H2S;

[0107] (2) Adjusting to acidity: adding a non-oxidizing acid to the sample from which H2S has been removed to obtain an acidic turbid liquid; the pH value of the acidic turbid liquid in step (2) is below 3. The non-oxidizing acid in step (2) includes an aqueous solution of hydrochloric acid or hydrogen bromide; the purity of the non-oxidizing acid is above premium grade.

[0108] Specifically, 2) slowly add 10 ml of hydrochloric acid (high-grade pure reagent) to the beaker and cover with a watch glass; a large amount of bubbles and a large amount of white precipitate are generated, and the solution becomes a white turbid liquid;

[0109] (3) Oxidation and precipitation removal: an oxidizing agent is added dropwise to the acidic turbid liquid until a clear solution is obtained, wherein the clear solution is the sample solution to be treated. The purpose is to oxidize elemental sulfur into sulfate or sulfite. In step (3), the oxidizing agent includes hydrogen peroxide, ozone, or perchloric acid; the purity of the oxidizing agent is higher than high purity; and the temperature at which the oxidizing agent is oxidized is 60-90°C.

[0110] Specifically, 3) add 10 ml of H2O2, and place the beaker covered with a watch glass on a hot plate and heat slowly at 80°C; the white turbid liquid gradually becomes clear.

[0111] The rest is the same as that of Example 1.

[0112] Table 3 Recovery R of Examples 1 and 2

[0113]

[0114]

[0115] In summary, the present invention performs pre-digestion by adding water and adding oxidizing acid, and then performs microwave digestion under closed conditions. Compared with the conventional method of multiple acid digestion, this method does not require repeated addition of oxidizing acid, so the operation steps are simple and the digestion time is short. In addition, this method can meet the quantitative analysis of most impurity elements in lithium sulfide.

[0116] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A digestion method for lithium sulfide impurity element analysis, characterized in that: The steps include: (A1) Predigestion: Weigh a lithium sulfide sample and add ultrapure water to obtain a sample aqueous solution; dropwise add an oxidizing acid to the sample aqueous solution, and heat to allow the lithium sulfide sample in the sample aqueous solution to fully react, thereby obtaining a predigested sample; (A2) Microwave digestion: The pre-digested sample is subjected to microwave digestion under sealed conditions to obtain a microwave-digested sample, which is then fixed to volume to obtain a sample solution to be tested.

2. A digestion method for lithium sulfide impurity element analysis according to claim 1, characterized in that, The oxidizing acid in step (A1) comprises at least one of nitric acid, aqua regia, and perchloric acid; And / or in step (A1), the ratio of the lithium sulfide sample to the ultrapure water is (0.01-1) g: (1-10) mL; and / or in step (A1), the ratio of the lithium sulfide sample to the oxidizing acid is (0.01-1) g: (5-20) mL; And / or the heating parameters in step (A1): temperature is 80° C. to 180° C., and time is 10 min to 30 min.

3. A digestion method for lithium sulfide impurity element analysis according to claim 1, characterized in that: The specific process of step (A2) is as follows: the pre-digested sample is sealed and assembled into a microwave digestion instrument, temperature and pressure sensors are connected in sequence, the microwave digestion instrument is turned off, a microwave digestion program is set, and the digestion program is run to obtain a microwave digested sample after completion.

4. A digestion method for lithium sulfide impurity element analysis according to claim 3, characterized in that: The microwave digestion program was set as follows: the temperature of the first program was 130°C to 150°C, and the time was 10 min to 20 min; the temperature of the second program was 170°C to 220°C, and the time was 30 min to 50 min.

5. A digestion method for analyzing impurity elements in lithium sulfide according to any one of claims 1 to 4, characterized in that: The method also includes preparing a spiked solution of the sample to be processed, comprising the following steps: weighing a lithium sulfide sample, adding each impurity element to be measured in the lithium sulfide sample for spiked treatment, and obtaining a spiked lithium sulfide sample; and processing the spiked lithium sulfide sample according to the descriptions of steps (A1) and (A2) to obtain a spiked solution of the sample to be processed.

6. A digestion method for lithium sulfide impurity element analysis according to claim 5, characterized in that: The spiked amount of the impurity element to be measured added in the spiked treatment is 1 to 10 times the content of the element to be measured; if the content of the impurity element in the sample is lower than 1 ppm, the spiked amount can be set at 1 ppm to 5 ppm.

7. A digestion method for analyzing impurity elements in lithium sulfide according to claim 5, characterized in that: The impurity elements to be tested in the lithium sulfide sample are at least one of Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Si, Sn, Sr, Ti, Tl, V and Zn.

8. A method for determining the content of lithium sulfide impurities, characterized in that: The steps include: (B1) taking a sample solution to be treated obtained by the digestion method according to any one of claims 1 to 7, heating the solution until the volume is less than 5 mL, then constant to obtain a sample solution, and testing on a machine to obtain the solution concentration of each impurity element to be measured in the sample solution; and calculating the mass content of each impurity element to be measured in the lithium sulfide sample based on the solution concentration of each impurity element to be measured in the sample solution; (B2) taking the sample spiked solution to be processed obtained by the digestion method according to any one of claims 1 to 7, heating the solution until the volume is less than 5 mL, and then fixing the volume to obtain a spiked sample solution, and obtaining the solution concentration of each impurity element to be detected in the spiked sample solution; The spiked recovery rate of each impurity element to be measured in the lithium sulfide sample is calculated according to the solution concentration of each impurity element to be measured in the sample solution and the solution concentration of each impurity element to be measured in the spiked sample solution.

9. The method for determining the content of lithium sulfide impurities according to claim 8, characterized in that: Step (B1) includes the following specific steps: (B1-1) taking the sample solution to be treated, heating it until the volume of the solution is less than 5 mL, cooling it, and then fixing the volume to obtain a sample solution; (B1-2) measuring the spectral line intensity of the sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample; (B1-3) Calculate the mass content of each impurity element to be measured in the lithium sulfide sample according to formula (1): In formula (1): i is the mass content of the impurity element to be measured in the lithium sulfide sample, in mg / kg; c i is the concentration of each impurity element to be measured in the sample solution, in mg / L; c is the content of the impurity element to be measured in the blank solution, in mg / L; V is the fixed volume of the sample solution, in mL; m is the weighed mass of the lithium sulfide sample, in g.

10. The method for determining the content of lithium sulfide impurities according to claim 8, characterized in that: Step (B2) includes the following specific steps: (B2-1) taking the spiked solution of the sample to be processed, heating it until the volume of the solution is less than 5 mL, cooling it, and then constant to volume to obtain a spiked sample solution; (B2-2) measuring the spectral line intensity of the spiked sample solution by ICP-OES, and calculating the solution concentration of each impurity element to be measured in the spiked sample solution using a calibration curve of each impurity element to be measured in the lithium sulfide sample; (B2-3) Calculate the spiked recovery of each impurity element to be measured in the lithium sulfide sample according to formula (2): R=(c R -c i )×100% (2); In formula (2): R is the recovery rate; c R is the concentration of each impurity element to be measured in the spiked sample solution, in mg / L; c i It is the concentration of each impurity element to be measured in the sample solution, in mg / L.

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