Method for detecting residual lithium on surface of positive electrode lithium supplementing agent

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a method for detecting residual lithium on the surface of a positive electrode lithium supplement agent, which comprises the following steps: adding a positive electrode lithium supplement agent into a first solvent for dispersion to obtain a first mixed solution, then adding a second solvent for dispersion to obtain a second mixed solution, the first solvent comprising at least one of ketone and ester, and the second solvent comprising alcohol; performing vacuum filtration on the second mixed solution to obtain filtrate and residues, wherein the filtrate comprises lithium hydroxide and lithium carbonate; drying the residues to obtain mixed powder, wherein the mixed powder comprises lithium carbonate; calculating the contents of lithium hydroxide and lithium carbonate in the filtrate by using a chemical titration method, and detecting the lithium content and the non-lithium main element content in the mixed powder by using an inductively coupled plasma emission spectrum detection method; calculating the content of lithium carbonate in the positive electrode lithium supplementing agent according to the contents of lithium and non-lithium main elements and the content of lithium carbonate in the filtrate, and calculating the content of residual alkali and the content of residual lithium in the positive electrode lithium supplementing agent according to the contents of lithium hydroxide and lithium carbonate. The method can accurately characterize the residual lithium content.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a method for detecting residual lithium on the surface of a positive electrode lithium supplement. BACKGROUND

[0002] In the first charge-discharge process of a lithium ion battery, lithium ions released from the positive electrode material react with the electrolyte to form an SEI film, thereby irreversibly consuming lithium sources in the positive electrode material. To solve this problem, the existing scheme adds a lithium supplement to the battery system to supplement the irreversible lithium source loss during formation, thereby improving the energy density and cycle life of the lithium ion battery. When preparing the lithium supplement, an excess of lithium source is added to participate in the reaction, and the excess lithium becomes residual lithium. Excessive residual lithium can affect the performance of the lithium ion battery, and thus the residual lithium index is of important reference significance for evaluating the performance of the positive electrode material of the lithium ion battery and the battery preparation process.

[0003] However, the existing residual lithium test method can cause deviations in the test results due to the destruction of the crystal lattice structure inside the lithium supplement or the presence of partially insoluble residual lithium, making it difficult to accurately characterize the residual lithium content in the lithium supplement. SUMMARY

[0004] In view of this, the present application provides a method for detecting residual lithium on the surface of a positive electrode lithium supplement, which can solve at least one of the above technical problems.

[0005] The present application provides a method for detecting residual lithium on the surface of a positive electrode lithium supplement, comprising the following steps: adding a positive electrode lithium supplement to a first solvent for dispersion to obtain a first mixed solution, and then adding a second solvent for dispersion to obtain a second mixed solution, wherein the first solvent comprises at least one of a ketone and an ester, and the second solvent comprises an alcohol; the second mixed solution is subjected to vacuum filtration to obtain a filtrate and a residue, and the filtrate comprises lithium hydroxide and lithium carbonate; the residue is dried to obtain a mixed powder, and the mixed powder comprises lithium carbonate; the content of lithium hydroxide and the content of lithium carbonate in the filtrate are calculated by chemical titration, and the content of lithium element and the content of non-lithium main elements in the mixed powder are detected by inductively coupled plasma emission spectrometry; and the content of lithium carbonate in the positive electrode lithium supplement is calculated according to the content of lithium element, the content of non-lithium main elements, and the content of lithium carbonate in the filtrate, and the content of residual alkali and the content of residual lithium in the positive electrode lithium supplement are calculated according to the content of lithium hydroxide in the filtrate and the content of lithium carbonate in the positive electrode lithium supplement.

[0006] In the detection method of the present application, the first solvent and the second solvent are used to disperse the positive electrode lithium supplement agent. The first solvent added first can form an interface protection film, which protects the surface of the positive electrode lithium supplement agent from being eroded by the second solvent. The second solvent added later can dissolve the residual lithium hydroxide and a small amount of lithium carbonate on the surface of the positive electrode lithium supplement agent. After filtration, the content of lithium hydroxide and lithium carbonate in the filtrate can be obtained by chemical titration method, solving the problem that when the second solvent is used as the solvent, the lithium dissolved after the surface of the positive electrode lithium supplement agent is eroded will affect the detection result, improving the accuracy of the test result of the content of lithium hydroxide. At the same time, for the lithium carbonate in the mixed powder, the inductively coupled plasma emission spectrometry (ICP-AES) is used to test the content of lithium element and non-lithium main element M in the mixed powder. According to the content of lithium element, the content of non-lithium main element M and the content of lithium carbonate in the filtrate, the content of lithium carbonate on the surface of the positive electrode lithium supplement agent can be calculated, solving the problem that the content of lithium carbonate on the surface of the existing positive electrode lithium supplement agent is difficult to detect. According to the content of lithium hydroxide and lithium carbonate in the positive electrode lithium supplement agent, the content of residual lithium in the positive electrode lithium supplement agent can be accurately characterized.

[0007] In some embodiments, the ketone includes at least one of acetone and nitrogen-methyl pyrrolidone. The basic substances on the surface of the positive electrode lithium supplement agent are difficult to dissolve in the above-mentioned ketone, so the above-mentioned ketone can strip the basic substances (lithium hydroxide and lithium carbonate) on the surface of the positive electrode lithium supplement agent and form a protective film on the surface of the positive electrode lithium supplement agent, solving the problem that lithium is dissolved after the surface of the positive electrode lithium supplement agent is eroded in the residual lithium content detection process, improving the accuracy of the residual lithium content detection result, and accurately characterizing the content of residual lithium on the surface of the positive electrode lithium supplement agent.

[0008] In some embodiments, the ester includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate and methyl ethyl carbonate. The basic substances on the surface of the positive electrode lithium supplement agent are difficult to dissolve in the above-mentioned ester, so the above-mentioned ester can strip the basic substances (lithium hydroxide and lithium carbonate) on the surface of the positive electrode lithium supplement agent and form a protective film on the surface of the positive electrode lithium supplement agent, solving the problem that lithium is dissolved after the surface of the positive electrode lithium supplement agent is eroded in the residual lithium content detection process, improving the accuracy of the residual lithium content detection result.

[0009] In some embodiments, the alcohol includes at least one of anhydrous methanol, anhydrous ethanol, ethylene glycol, isopropyl alcohol and glycerol. The lithium hydroxide on the surface of the positive electrode lithium supplement agent can be dissolved in the alcohol solvent to form a filtrate, which is convenient for subsequent chemical titration method to detect the content of lithium hydroxide, and further accurately characterize the content of residual lithium on the surface of the positive electrode lithium supplement agent.

[0010] In some embodiments, the concentration of the positive electrode lithium supplement agent in the second mixed solution is 2 g / L-40 g / L. Controlling the concentration of the positive electrode lithium supplement agent within the above range can fully dissolve the positive electrode lithium supplement agent and lithium hydroxide on the surface thereof, reduce the saturation precipitation of lithium hydroxide during the dissolution process, improve the accuracy of the content test results of lithium hydroxide and lithium carbonate, and provide a reliable data basis for improving the performance of the positive electrode material of the lithium ion battery and the battery preparation process.

[0011] In some embodiments, the positive electrode lithium supplement agent comprises Li x MO y wherein 1≤x≤6, 2≤y≤4, and M comprises at least one of Fe, Co, Ni, Cu, and Si. The detection method of the present application has wide applicability and can be accurately and reliably applied to a variety of different positive electrode lithium supplement agents.

[0012] In some embodiments, the content of lithium hydroxide and the content of lithium carbonate in the filtrate are calculated by using a chemical titration method, which comprises titrating the filtrate using a first indicator and a second indicator, wherein the first indicator comprises phenolphthalein and the second indicator comprises a bromocresol green-methyl red mixed indicator. The change of phenolphthalein from pink to colorless can indicate that the lithium hydroxide in the filtrate is completely neutralized and a small amount of lithium carbonate in the filtrate is neutralized to lithium bicarbonate; and the change of the bromocresol green-methyl red mixed indicator from green to orange yellow can indicate that the lithium bicarbonate in the filtrate is completely neutralized to lithium carbonate. The clear and stepwise end point signals provided by the two indicators in the continuous titration can accurately determine the respective contents of lithium hydroxide and lithium carbonate in the filtrate, and thus accurately characterize the content of residual lithium on the surface of the positive electrode lithium supplement agent.

[0013] In some embodiments, the positive electrode lithium supplement agent is weighed before being added to the first solvent, and the weighing of the positive electrode lithium supplement agent is performed in an environment with a relative humidity of less than or equal to -30%. Weighing in an environment with a humidity within the above range can minimize the interference of moisture in the air on the weighing result, thereby ensuring the accuracy and reliability of the weighing result, improving the detection accuracy of the content of lithium carbonate, and thus accurately characterizing the content of residual lithium on the surface of the positive electrode lithium supplement agent.

[0014] In some embodiments, the first mixed solution and the second mixed solution are dispersed by magnetic stirring, wherein the rotation speed of the magnetic stirring is 700 rpm-800 rpm, the dispersion time of the first mixed solution is 5 min-10 min, and the dispersion time of the second mixed solution is 20 min-30 min. Controlling the rotation speed of the magnetic stirring dispersion and the dispersion time of the first mixed solution within the above range can form a uniform interface protection film on the surface of the positive electrode lithium supplementing agent, and the interface protection film can protect the surface of the positive electrode lithium supplementing agent from being eroded by the second solvent, thereby solving the problem of lithium dissolution caused by the erosion of the second solvent on the surface of the positive electrode lithium supplementing agent. Controlling the rotation speed of the magnetic stirring dispersion and the dispersion time of the second mixed solution within the above range can completely dissolve the lithium hydroxide on the surface of the positive electrode lithium supplementing agent, improve the accuracy of the test result of the lithium hydroxide content, and further accurately characterize the content of the residual lithium on the surface of the positive electrode lithium supplementing agent.

[0015] In some embodiments, the drying temperature of the residue is 55℃-65℃, and the drying time is 55 min-65 min. Controlling the drying temperature and time of the residue within the above range can efficiently and controllably remove the first solvent and the second solvent in the residue, improve the accuracy of the test result of the lithium carbonate content, and further accurately characterize the content of the residual lithium on the surface of the positive electrode lithium supplementing agent. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The flowchart of the method for detecting the residual lithium on the surface of the positive electrode lithium supplementing agent provided in the present application.

[0017] Figure 2 The XRD comparison chart of the positive electrode lithium supplementing agent before and after being dispersed by the first solvent and the second solvent provided in the present application.

[0018] Figure 3 The SEM comparison chart of the positive electrode lithium supplementing agent before being dispersed by the first solvent and the second solvent provided in the present application.

[0019] Figure 4 The SEM comparison chart of the positive electrode lithium supplementing agent after being dispersed by the first solvent and the second solvent provided in the present application. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments.

[0021] In view of the problem that the detection result accuracy of the existing residual lithium content detection method is not high, the present application provides a method for detecting the residual lithium on the surface of the positive electrode lithium supplementing agent, which can accurately characterize the content of the residual lithium on the surface of the positive electrode lithium supplementing agent by accurately measuring the content of lithium hydroxide and lithium carbonate.

[0022] The application provides a method for detecting residual lithium on the surface of a positive electrode lithium supplement, which comprises the following steps Figure 1 , and specifically comprises the following steps: Step 1: dispersing the positive electrode lithium supplement in a first solvent to obtain a first mixed solution, and then dispersing the first mixed solution in a second solvent to obtain a second mixed solution, wherein the first solvent comprises at least one of ketone and ester, and the second solvent comprises alcohol.

[0023] In some embodiments, the positive electrode lithium supplement is weighed before being added into the first solvent, and the weighing is performed in an environment with a relative humidity of less than or equal to -30%. Optionally, the humidity of the weighing environment can be -30% RH, -31% RH, -32% RH, -33% RH, -34% RH, -35% RH, or a range formed by any two of the above values. The humidity of the weighing environment in the above range can minimize the interference of moisture in the air on the weighing result, thereby ensuring the accuracy and reliability of the weighing result, improving the detection accuracy of the lithium carbonate content, and further accurately characterizing the content of residual lithium on the surface of the positive electrode lithium supplement.

[0024] In some embodiments, the ketone comprises at least one of acetone and N-methyl pyrrolidone. The basic substances on the surface of the positive electrode lithium supplement are difficult to dissolve in the above-mentioned ketones, and therefore the above-mentioned ketones can strip the basic substances (lithium hydroxide and lithium carbonate) on the surface of the positive electrode lithium supplement and form a protective film on the surface of the positive electrode lithium supplement, thereby solving the problem of lithium dissolution due to the erosion of the surface of the positive electrode lithium supplement by the solvent during the detection of the residual lithium content, improving the accuracy of the detection result of the residual lithium content, and further accurately characterizing the content of residual lithium on the surface of the positive electrode lithium supplement.

[0025] In some embodiments, the ester comprises at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate and methyl ethyl carbonate. The basic substances on the surface of the positive electrode lithium supplement are difficult to dissolve in the above-mentioned esters, and therefore the above-mentioned esters can strip the basic substances (lithium hydroxide and lithium carbonate) on the surface of the positive electrode lithium supplement and form a protective film on the surface of the positive electrode lithium supplement, thereby solving the problem of lithium dissolution due to the erosion of the surface of the positive electrode lithium supplement by the solvent during the detection of the residual lithium content, and improving the accuracy of the detection result of the residual lithium content.

[0026] In some embodiments, the alcohol comprises at least one of anhydrous methanol, anhydrous ethanol, ethylene glycol, isopropyl alcohol and glycerol. The lithium hydroxide on the surface of the positive electrode lithium supplement can be dissolved in the alcohol solvent to form a filtrate, which facilitates the subsequent chemical titration method for detecting the content of lithium hydroxide, and further accurately characterizing the content of residual lithium on the surface of the positive electrode lithium supplement.

[0027] In some embodiments, the concentration of the positive electrode lithium supplement agent in the second mixed solution is 2 g / L-40 g / L, and the concentration of the positive electrode lithium supplement agent can be 2 g / L, 10 g / L, 15 g / L, 20 g / L, 25 g / L, 30 g / L, 35 g / L, 40 g / L, or a range formed by any two of the above values. Controlling the concentration of the positive electrode lithium supplement agent in the above range can fully dissolve the positive electrode lithium supplement agent and lithium hydroxide on the surface of the positive electrode lithium supplement agent, reduce the saturation precipitation of lithium hydroxide during the dissolution process, improve the accuracy of the test results of the content of lithium hydroxide and lithium carbonate, and provide reliable data basis for improving the performance of the positive electrode material of the lithium ion battery and the battery preparation process. Optionally, the concentration of the positive electrode lithium supplement agent is 20 g / L.

[0028] In some embodiments, the positive electrode lithium supplement agent includes Li x MO y wherein 1≤x≤6, 2≤y≤4, and M includes at least one of Fe, Co, Ni, Cu, and Si. The detection method of the present application has wide applicability and can be accurately and reliably applied to a variety of different positive electrode lithium supplement agents.

[0029] In some embodiments, the dispersion method of the first mixed solution and the second mixed solution includes magnetic stirring, wherein the rotation speed of the magnetic stirring is 700 rpm-800 rpm, the dispersion time of the first mixed solution is 5 min-10 min, and the dispersion time of the second mixed solution is 20 min-30 min. The rotation speed of the magnetic stirring can be 700 rpm, 720 rpm, 740 rpm, 760 rpm, 780 rpm, or 800 rpm, the dispersion time of the first mixed solution can be 5 min, 6 min, 7 min, 8 min, 9 min, or 10 min, and the dispersion time of the second mixed solution can be 20 min, 22 min, 24 min, 26 min, 28 min, or 30 min, or a range formed by any two of the above values. Controlling the rotation speed of the magnetic stirring and the dispersion time of the first mixed solution in the above range can form a uniform interfacial protective film on the surface of the positive electrode lithium supplement agent, and the interfacial protective film can protect the surface of the positive electrode lithium supplement agent from being eroded by the second solvent, thereby solving the problem of lithium dissolution caused by the erosion of the second solvent on the surface of the positive electrode lithium supplement agent. Controlling the rotation speed of the magnetic stirring and the dispersion time of the second mixed solution in the above range can completely dissolve the lithium hydroxide on the surface of the positive electrode lithium supplement agent, improve the accuracy of the test results of the content of lithium hydroxide, and accurately characterize the content of the residual lithium on the surface of the positive electrode lithium supplement agent. Optionally, the rotation speed of the magnetic stirring is 720 rpm, the dispersion time of the first mixed solution is 5 min, and the dispersion time of the second mixed solution is 20 min.

[0030] Step two, the second mixed solution is vacuum filtered to obtain a filtrate and a residue, and the filtrate includes lithium hydroxide and lithium carbonate.

[0031] In some embodiments, the lithium carbonate is dissolved in the second solvent in a small amount, and the filtrate contains both lithium hydroxide and lithium carbonate.

[0032] Step three, the residue is dried to obtain a mixed powder, and the mixed powder includes lithium carbonate.

[0033] In some embodiments, the drying temperature of the residue is 55-65℃, and the drying time is 55-65min. The drying temperature of the residue can be 55℃, 56℃, 57℃, 58℃, 59℃, 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, or any range defined by any two of the above values. The drying temperature and time of the residue within the above range can efficiently and controllably remove the first solvent and the second solvent in the residue, and improve the accuracy of the lithium carbonate content detection result. Optionally, the drying temperature of the residue is 60℃, and the drying time is 60min.

[0034] After step three, a mixed powder containing the positive electrode lithium supplement agent and lithium carbonate can be obtained.

[0035] Step four, the content of lithium hydroxide and the content of lithium carbonate in the filtrate are calculated by a chemical titration method, and the content of lithium element and the content of non-lithium main element in the mixed powder are detected by an inductively coupled plasma emission spectrometry method; and the content of lithium carbonate in the positive electrode lithium supplement agent is calculated according to the content of lithium element, the content of non-lithium main element in the mixed powder, and the content of lithium carbonate in the filtrate, and the content of residual alkali and the content of residual lithium in the positive electrode lithium supplement agent are calculated according to the content of lithium hydroxide in the filtrate and the content of lithium carbonate in the positive electrode lithium supplement agent.

[0036] In some embodiments, the content of lithium hydroxide and the content of lithium carbonate in the filtrate are calculated by a chemical titration method: the filtrate is titrated using a first indicator and a second indicator, wherein the first indicator includes phenolphthalein, and the second indicator includes a bromocresol green-methyl red mixed indicator. The change of phenolphthalein from pink to colorless can indicate that the lithium hydroxide in the filtrate is completely neutralized and a small amount of lithium carbonate in the filtrate is neutralized to lithium bicarbonate; and the change of the bromocresol green-methyl red mixed indicator from green to orange yellow can indicate that the lithium bicarbonate in the filtrate is completely neutralized to lithium carbonate. Through the clear and step-by-step end point signals provided by the two indicators in the continuous titration, the respective contents of lithium hydroxide and lithium carbonate in the filtrate can be accurately obtained, and the content of residual lithium on the surface of the positive electrode lithium supplement agent can be accurately characterized.

[0037] In some embodiments, the content of lithium hydroxide W1% in the filtrate and the content of trace amounts of lithium carbonate W in the filtrate are calculated using chemical titration and conversion relationships. 4-1 %.

[0038] ; .

[0039] Where C is the concentration of the hydrochloric acid standard solution in mol / L. -1 V is the volume of the filtrate after dilution (L); V1 is the volume of hydrochloric acid standard solution consumed during the first color change (L); V2 is the volume of hydrochloric acid standard solution consumed during the second color change (L); V3 is the volume of the filtrate aliquoted (L); m is the mass of the filtrate sample (g).

[0040] In some embodiments, the lithium carbonate content in the positive electrode lithium supplement is the sum of the small amount of lithium carbonate in the filtrate and the lithium carbonate content in the mixed powder, and the lithium carbonate content in the positive electrode lithium supplement is calculated. x MO y The calculation process for the residual alkali and residual lithium content can be broken down into steps: First, calculate the lithium elemental content W2% (measured lithium elemental content) in the mixed powder, the content of the non-lithium main element M W3% and the content of a small amount of lithium carbonate in the filtrate W... 4-1 % Calculate the lithium carbonate content W4% in the positive electrode lithium replenishment agent, W4 = W 4-1 + (W2-W5) / 0.19, where W5% is the theoretical lithium-ion content, W5=xW3, that is, W4= W 4-1 + (W2-xW3) / 0.19; then, based on the lithium hydroxide content W1% in the filtrate and the lithium carbonate content W4 / % in the positive electrode lithium supplement, calculate the residual alkali content W6 / % in the positive electrode lithium supplement, W6=1.54W1+W4, that is, W6=1.54W1+W 4-1 + (W2-xW3) / 0.19; Finally, based on the residual alkali content W6 / % in the positive electrode lithium supplement, calculate the residual lithium content W% in the positive electrode lithium supplement, W=0.19W6, that is, W=0.2926W1+0.19W 4-1 +W2-xW3.

[0041] In the detection method of the present application, the first solvent and the second solvent are used to disperse the positive electrode lithium supplement agent. The first solvent added first can form an interface protection film, which can protect the surface of the positive electrode lithium supplement agent from being eroded by the second solvent. The second solvent added later can dissolve the residual lithium hydroxide and a small amount of lithium carbonate of the positive electrode lithium supplement agent. Then, the content of lithium hydroxide and lithium carbonate in the filtrate can be obtained by chemical titration method after filtration, so as to solve the problem that when the second solvent is used as the solvent, the lithium dissolved after the surface of the positive electrode lithium supplement agent is eroded will affect the detection result, improve the accuracy of the test result of the content of lithium hydroxide, and at the same time, the inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to test the content of lithium element and non-lithium main element M in the mixed powder. According to the content of lithium element, the content of non-lithium main element M and the content of lithium carbonate in the filtrate, the content of lithium carbonate on the surface of the positive electrode lithium supplement agent can be calculated, so as to solve the problem that the content of lithium carbonate on the surface of the existing positive electrode lithium supplement agent is difficult to detect. According to the content of lithium hydroxide and lithium carbonate, the content of residual lithium in the positive electrode lithium supplement agent can be accurately characterized.

[0042] The technical solutions of the present application are exemplified below in combination with specific embodiments.

[0043] Example 1 (1) In the drying room (environmental humidity ≤-30%RH), two kinds of iron-based positive electrode lithium supplement agents (Li5FeO4) samples prepared by different processes were weighed as 2.0g, and the accurate weighing value was recorded. The two samples were defined as LFO-1 and LFO-2, respectively. Dimethyl carbonate (first solvent) 50mL was added, and a magnetic stirrer was used to stir and disperse at a speed of 720rpm for 5min to obtain a first mixed solution. Anhydrous ethanol (second solvent) was continuously added, and a magnetic stirrer was used to stir and disperse at a speed of 720rpm for 20min to obtain a second mixed solution. (2) The second mixed solution was vacuum filtered using a polytetrafluoroethylene filter paper with a filter hole diameter of 0.22um to obtain a filtrate and a residue. The filtrate was collected for testing, and the residue was dried in a 60℃ vacuum drying oven for 60min to obtain a mixed powder for testing.

[0044] (3) 50mL of the filtrate was transferred to a 250mL conical flask, and 2 drops of phenolphthalein indicator were added. 0.05mol / L hydrochloric acid was titrated to the end point, and the standard hydrochloric acid consumption volume V1 was recorded. 2 drops of bromocresol green-methyl red indicator were added, and the titration was continued to the end point, and the hydrochloric acid consumption volume V2 was recorded. The content of lithium hydroxide was calculated. Then, according to the conventional ICP test process, the content of non-lithium main element M and lithium element in the mixed powder was tested, the content of lithium carbonate in the mixed powder was calculated, and then the residual alkali content and residual lithium content in the sample were obtained.

[0045] Comparative Example 1 Unlike Example 1, the first solvent is not used in step (1), ultrapure water is used as the second solvent, and the mixed powder is not detected in step (3). The iron content in the filtrate after filtration is tested using ICP. The specific steps are as follows: 5 mL of filtrate is transferred using a pipette, 5 mL of hydrochloric acid solution is added, and ultrapure water is added to a 100 mL volumetric flask. The iron content is then tested using ICP.

[0046] Comparative Example 2 Unlike Example 1, step (3) does not involve testing the mixed powder; instead, ICP is used to test the iron content in the filtrate after filtration. The specific steps are as follows: 5 mL of the filtrate is transferred to a 150 mL quartz beaker using a pipette, heated to dryness on a 60°C hot plate, then dissolved in 10 mL of hydrochloric acid. The solution is then diluted to 100 mL with ultrapure water in a volumetric flask. The iron content is then tested using ICP.

[0047] Comparative Example 3 The difference from Example 1 is that: in step (1), the first solvent is not used, the second solvent is replaced with anhydrous ethanol, and in step (3), the mixed powder is not detected, and the iron content in the filtrate after filtration is tested using ICP. The specific steps are as follows: 5 mL of filtrate is transferred to a 150 mL quartz beaker using a pipette, heated to dryness on a 60°C hot plate, then 10 mL of hydrochloric acid is added to dissolve it, and the volume is adjusted to 100 mL with ultrapure water. 5 mL of hydrochloric acid solution is added to the volumetric flask, and the volume is adjusted to 100 mL with ultrapure water. The iron content is then tested using ICP.

[0048] Comparative Example 4 The difference from Example 1 is that: in step (1), the first solvent is not used, the second solvent is replaced with ethylene glycol, and in step (3), the mixed powder is not detected, and the iron content in the filtrate after filtration is tested using ICP. The specific steps are as follows: 5 mL of filtrate is transferred to a 150 mL quartz beaker using a pipette, heated to dryness on a 60°C hot plate, then 10 mL of hydrochloric acid is added to dissolve it, and the volume is adjusted to 100 mL with ultrapure water. 5 mL of hydrochloric acid solution is added to the volumetric flask, and the volume is adjusted to 100 mL with ultrapure water. The iron content is then tested using ICP.

[0049] The test results are shown in Table 1. Table 1 Test Result Analysis: According to Table 1, the lithium carbonate value measured by titration + powder ICP (Example 1) of the positive electrode lithium supplement prepared by the first process is 8635 ppm higher than the lithium carbonate value measured by titration + filtrate ICP (Comparative Example 2); the lithium carbonate value measured by titration + filtrate ICP (Example 1) of the positive electrode lithium supplement prepared by the second process is 4149 ppm higher than the lithium carbonate value measured by titration + filtrate ICP (Comparative Example 2). It is fully illustrated that the lithium carbonate which is difficult to dissolve in the organic solvent cannot be characterized by the titration method to obtain the real level value of the sample, and the powder ICP and the filtrate titration method can better characterize the lithium carbonate content on the surface of the positive electrode lithium supplement.

[0050] According to Table 1, the lithium hydroxide content value measured after the positive electrode lithium supplement prepared by the first process is dispersed by anhydrous ethanol (Comparative Example 3) is 57204 ppm higher than the value measured after the positive electrode lithium supplement is dispersed by the mixed solvent (Example 1 uses the first solvent and the second solvent); the lithium hydroxide value measured after the positive electrode lithium supplement prepared by the second process is dispersed by anhydrous ethanol (Comparative Example 3) is 11177 ppm higher than the value measured after the positive electrode lithium supplement is dispersed by the mixed solvent (Example 1 uses the first solvent and the second solvent). It is illustrated that the mixed solvent (the first solvent + the second solvent) can effectively avoid the erosion of the single alcohol solvent to the surface of the positive electrode lithium supplement and avoid the influence of the dissolved lithium on the test results.

[0051] The XRD patterns of the iron-based positive electrode lithium supplement prepared by two different processes before and after being dispersed by the mixed solvent (the first solvent and the second solvent) are as shown in Figure 2 It can be seen that no obvious impurity phase is generated after the sample is dispersed by the mixed solvent; the SEM patterns of the iron-based positive electrode lithium supplement prepared by two different processes before and after being dispersed by the mixed solvent are as shown in Figure 3 and Figure 4 It can be seen that the micro-morphology of the sample after being dispersed by the mixed solvent does not change obviously, which further confirms that the mixed solvent of the present application will not damage the surface structure of the positive electrode lithium supplement and can avoid the problem of dissolution of the internal lattice lithium of the positive electrode lithium supplement. Because the alkaline substance on the surface of the positive electrode lithium supplement is difficult to dissolve in the first solvent (ester), the first solvent first peels off the alkaline substance on the surface of the sample, thereby forming a protective film on the surface of the sample; the second solvent (alcohol) is continuously added, and the alkaline substance is immediately dissolved therein without eroding the surface of the material. Therefore, the mixed solvent is more suitable for the test requirements of residual lithium than the single organic reagent.

[0052] From the comparative example 1, the residual lithium value of the positive electrode lithium supplementing agent prepared by different processes is sharply increased after stirring and dispersion in ultrapure water, and the test result is completely deviated from the normal value, which is due to the collapse of the internal lattice structure of the positive electrode lithium supplementing agent caused by the violent reaction of the positive electrode lithium supplementing agent with water. From the comparative example 2, the content of iron element in the filtrate of the positive electrode lithium supplementing agent prepared by the first process and dispersed in the mixed solvent is 32 ppm. From the comparative examples 1, 3 and 4, the contents of iron element in the filtrates dispersed in ultrapure water, anhydrous ethanol and ethylene glycol are 64688 ppm, 25689 ppm and 43769 ppm respectively, which are much higher than the content of iron element in the mixed solvent used in the comparative example 2. From the comparative example 2, the content of iron element in the filtrate of the positive electrode lithium supplementing agent prepared by the second process and dispersed in the mixed solvent is 15 ppm. From the comparative examples 1, 3 and 4, the contents of iron element in the filtrates dispersed in ultrapure water, anhydrous ethanol and ethylene glycol are 36028 ppm, 14628 ppm and 23561 ppm respectively, which are much higher than the content of iron element in the mixed solvent used in the comparative example 2. Further, from the ICP test results of the content of iron element in the filtrate, it can be seen that the content of iron element in the filtrate of a single solvent is much higher than the content of iron element in the mixed solvent, which further indicates that the use of the mixed solvent in the present application can slow down the erosion of the surface of the positive electrode lithium supplementing agent, which meets the requirements of the residual lithium test for the solvent.

[0053] In addition, those skilled in the art should understand that the above embodiments are only used to illustrate the present application, and are not used as a limitation to the present application. Any appropriate changes and modifications made to the above embodiments within the spirit and principle of the present application fall within the disclosure range of the present application.

Claims

1. A method for detecting residual lithium on the surface of a positive electrode lithium replenishing agent, characterized in that, Includes the following steps: The positive electrode lithium supplement is added to a first solvent for dispersion to obtain a first mixed solution, and then a second solvent is added for dispersion to obtain a second mixed solution. The first solvent includes at least one of ketones and esters, and the second solvent includes an alcohol. The second mixed solution was subjected to vacuum filtration to obtain filtrate and residue, wherein the filtrate included lithium hydroxide and lithium carbonate; The residue was dried to obtain a mixed powder, which included lithium carbonate. The contents of lithium hydroxide and lithium carbonate in the filtrate were calculated using chemical titration, and the contents of lithium and non-lithium main elements in the mixed powder were detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The content of lithium carbonate in the positive electrode lithium supplement is calculated based on the content of lithium element in the mixed powder, the content of non-lithium main element, and the content of lithium carbonate in the filtrate. The content of residual alkali and residual lithium in the positive electrode lithium supplement is calculated based on the content of lithium hydroxide in the filtrate and the content of lithium carbonate in the positive electrode lithium supplement.

2. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The ketone includes at least one of acetone and N-methylpyrrolidone.

3. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The ester includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, and methyl ethyl carbonate.

4. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The alcohol includes at least one of anhydrous methanol, anhydrous ethanol, ethylene glycol, isopropanol, and glycerol.

5. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The concentration of the positive electrode lithium replenishing agent in the second mixed solution is 2 g / L-40 g / L.

6. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The positive electrode lithium supplement includes Li x MO y Where 1≤x≤6, 2≤y≤4, and M includes at least one of Fe, Co, Ni, Cu and Si.

7. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The method of calculating the content of lithium hydroxide and lithium carbonate in the filtrate by chemical titration includes: titrating the filtrate with a first indicator and a second indicator, wherein the first indicator includes phenolphthalein and the second indicator includes a bromocresol green-methyl red mixed indicator.

8. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The positive electrode lithium replenishing agent is weighed before being added to the first solvent. The weighing of the positive electrode lithium replenishing agent is carried out in an environment with a relative humidity of less than or equal to -30%.

9. The method for detecting residual lithium on the surface of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The first mixed solution and the second mixed solution are dispersed by magnetic stirring, wherein the magnetic stirring speed is 700 rpm-800 rpm, the dispersion time of the first mixed solution is 5 min-10 min, and the dispersion time of the second mixed solution is 20 min-30 min.

10. The method for detecting residual lithium on the surface of the positive electrode lithium replenishing agent according to claim 1, characterized in that, The drying temperature of the residue is 55℃-65℃, and the drying time is 55min-65min.

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