Method for evaluating surface coating effect of positive electrode lithium supplementing agent

Abstract

The present application belongs to the technical field of lithium ion battery cathode material, and particularly relates to a method for evaluating surface coating effect of a cathode lithium supplement agent, which comprises dispersing a standard lithium supplement agent and a lithium supplement agent to be tested in an alcohol solvent under the same conditions, the standard lithium supplement agent being a sample with good surface coating effect, sampling at different dispersion times to determine residual lithium amount in the liquid, drawing a broken line graph with dispersion time as the x-axis and residual lithium amount as the y-axis, recording the stable section of the curve of the standard lithium supplement agent as platform time T0, and recording the stable section of the curve of the lithium supplement agent to be tested as platform time T1; comparing platform time T1 of the lithium supplement agent to be tested with platform time T0 of the standard lithium supplement agent to determine the coating effect. The method for evaluating surface coating effect of a cathode lithium supplement agent provided by the present application can reflect the overall coating state of the material, and can evaluate the coating uniformity of the coating layer of basically all coating types.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a method for evaluating the surface coating effect of cathode lithium replenishment agent. Background Technology

[0002] During the initial charge and discharge cycle of a lithium-ion battery, a solid electrolyte interphase (SEI) film forms on the negative electrode surface through a side reaction between the electrolyte and lithium ions. This permanently depletes the active lithium reserves within the system, leading to a decrease in battery energy density and a deterioration in cycle stability. To address this issue, positive electrode lithium replenishers have been developed. By adding lithium replenishers to the battery system, they compensate for the irreversible lithium source loss during formation and operation, thereby improving the energy density and cycle life of lithium-ion batteries.

[0003] Most lithium replenishers currently exhibit poor air stability, with alkaline surfaces and high hygroscopicity, making them prone to reacting with moisture in the air, leading to material deterioration, increased internal resistance, and ultimately affecting their lithium replenishment performance. Existing methods primarily employ carbon coating, oxide coating, and phosphide coating to inhibit the contact between the lithium replenisher and air. Better coating results in greater improvements to the lithium replenisher's air stability, kinetic properties, and electrical properties. A key aspect of coating modification is the uniformity and integrity of the coating, which reflects the material's corrosion resistance. Therefore, detection methods for characterizing the coating effect are particularly important.

[0004] CN120629269A discloses an evaluation method for the surface coating effect of electrode materials. The electrode materials, coating materials, and coated electrode materials to be tested are respectively made into electrode sheets under the same conditions, and then made into batteries under the same conditions. Electrochemical impedance spectroscopy tests are performed on each battery to obtain a relaxation time distribution curve. The relaxation time curves of the battery containing the electrode material and the battery containing the coating material are used as standard curves, and the relaxation curve of the coated electrode material to be tested is compared with the standard curves to evaluate the coating effect of the material. However, this evaluation method tests through the electrical performance of the battery and indirectly judges the coating effect. This method has a long evaluation cycle and cannot exclude the influence of battery preparation processes and other material factors on the battery performance, and it is impossible to accurately judge the coating effect of the material in practical applications. CN116593447A discloses an evaluation method for the surface alumina coating effect of lithium cobaltate. This method first removes the surface-coated aluminum in the standard sample and the sample to be tested through solid-base melting digestion, then collects the digested filter residue by filtration, adds a suitable acid reagent to dissolve the filter residue, and performs inductively coupled plasma testing to obtain the aluminum content in the standard product and the sample to be tested; if the aluminum content to be tested is greater than or equal to the aluminum content in the standard product, the coating effect is qualified, otherwise it is unqualified. However, this method involves solid-base melting digestion, with cumbersome steps and greater risks, and only evaluates one coating type, namely aluminum coating. Furthermore, another common detection method is to observe the coated material through a scanning electron microscope or a transmission electron microscope to directly judge the coating effect, but the microscopic characterization has a small field of view and high costs, and cannot reflect the overall coating state of the particles. In addition, the existing technology only targets a certain coating type and cannot uniformly test various coating types. Summary of the Invention

[0005] The object of the present invention is to provide a new evaluation method for the surface coating effect of a positive electrode lithium supplement agent. This evaluation method is accurate, effective, and has a short evaluation cycle, can reflect the overall coating state of the material, and can simultaneously evaluate the coating layer uniformity of basically all coating types.

[0006] Specifically, the evaluation method for the surface coating effect of the positive electrode lithium supplement agent provided by the present invention includes the following steps: S1: Disperse the standard lithium supplement agent and the lithium supplement agent to be tested in an alcohol solvent under the same conditions respectively. The standard lithium supplement agent is a sample with a good surface coating effect. Samples are taken at different dispersion times to measure the residual lithium amount in the liquid, and a broken line graph with the dispersion time as the x-axis and the residual lithium amount as the y-axis is drawn. The stable section of the curve of the standard lithium supplement agent is denoted as the platform time T0, and the stable section of the curve of the lithium supplement agent to be tested is denoted as the platform time T1; S2: Compare the plateau time T1 of the lithium replenishing agent to be tested with the plateau time T0 of the standard lithium replenishing agent to determine the coating effect; when the plateau time T1 of the lithium replenishing agent to be tested is greater than or equal to the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is good; when the plateau time T1 of the lithium replenishing agent to be tested is less than the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is poor.

[0007] like Figure 1 As shown, both the standard lithium replenisher and the test lithium replenisher are coated with a layer containing lithium hydroxide (LiOH) and lithium carbonate (Li2CO3). When evaluating the coating effect of the positive electrode lithium replenisher using the method provided by this invention, the coated standard lithium replenisher and the test lithium replenisher are first dispersed in an alcohol solvent. At this point, the alcohol solvent dissolves residual lithium (LiOH and Li2CO3) on the sample surface. As the dispersion time increases, the alcohol solvent begins to erode part of the coating layer. At this stage, the substrate remains intact, and the measured residual lithium amount remains unchanged. After further dispersion, the coating layer on the lithium replenisher surface is completely destroyed, and the solvent begins to erode the substrate sample, forming alkaline substances, which leads to an increase in the measured residual lithium value. In other words, the coating effect of the test lithium replenisher can be evaluated by the difference in the plateau time of the standard lithium replenisher and the test lithium replenisher dispersed in the alcohol solvent. The method for evaluating the surface coating effect of cathode lithium replenishment agents provided by this invention can reflect the overall coating state of the material and can assess the coating uniformity of the coating layer in almost all coating types. Furthermore, the evaluation method for evaluating the surface coating effect of cathode lithium replenishment agents provided by this invention has a short evaluation cycle, simple steps, and low cost. Attached Figure Description

[0008] Figure 1 A mechanism diagram of the method for evaluating the surface coating effect of the positive electrode lithium replenishment agent provided by the present invention; Figure 2 This is a line graph of the standard lithium supplement A in Example 1; Figure 3 This is a line graph of the lithium supplement A to be tested in Example 1; Figure 4 This is a line graph of the lithium supplement agent B to be tested in Example 1; Figure 5 This is a line graph of the standard lithium supplement B in Example 2; Figure 6 This is a line graph of the lithium supplement C to be tested in Example 2; Figure 7 Microscopic morphology images of standard lithium replenisher B and test lithium replenisher C (a: standard lithium replenisher B; b: test lithium replenisher C). Figure 8 This is a line graph of the standard lithium supplement C in Example 3; Figure 9This is a line graph of the lithium supplement agent D to be tested in Example 3; Figure 10 Microscopic morphology images of standard lithium replenisher C and lithium replenisher D to be tested (a: standard lithium replenisher C; b: lithium replenisher D to be tested). Detailed Implementation

[0009] The method for evaluating the surface coating effect of the positive electrode lithium replenishment agent provided by the present invention includes the following steps: S1: Disperse the standard lithium replenisher and the lithium replenisher to be tested in alcohol solvent under the same conditions. Take samples at different dispersion times to determine the residual lithium in the liquid. Plot a line graph with dispersion time as the x-axis and residual lithium as the y-axis. Record the stable segment of the curve of the standard lithium replenisher as the plateau time T0 and the stable segment of the curve of the lithium replenisher to be tested as the plateau time T1. S2: Compare the plateau time T1 of the lithium replenishing agent to be tested with the plateau time T0 of the standard lithium replenishing agent to determine the coating effect; when the plateau time T1 of the lithium replenishing agent to be tested is greater than or equal to the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is good; when the plateau time T1 of the lithium replenishing agent to be tested is less than the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is poor.

[0010] In the evaluation of the surface coating effect of the above-mentioned positive electrode lithium replenisher, the standard lithium replenisher is a sample with good surface coating effect, that is, a sample in which the substrate surface is coated with a coating layer and the coating layer thickness is uniform. The thickness of the coating layer in the standard lithium replenisher is preferably 10~30nm, such as 10nm, 12nm, 14nm, 16nm, 18nm, 20nm, 22nm, 24nm, 26nm, 28nm, 30nm, etc.

[0011] In the evaluation of the surface coating effect of the above-mentioned positive electrode lithium replenishing agent, the coating type of the standard lithium replenishing agent and the lithium replenishing agent to be tested is not particularly limited, and can be selected from at least one of carbon coating, oxide coating, and phosphide coating. Furthermore, to avoid the influence of the coating type on the test results and to make the evaluation results more accurate, preferably, the standard lithium replenishing agent and the lithium replenishing agent to be tested have the same coating type, and the coating layer thickness of the standard lithium replenishing agent and the lithium replenishing agent to be tested is similar. The difference in the average thickness of the two coating layers is preferably no more than 10 nm, such as 10 nm, 9 nm, 8 nm, 7 nm, 6 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1 nm, 0 nm, etc.

[0012] In the evaluation of the surface coating effect of the positive electrode lithium replenishment agent, the stable segment of the curve is located from the point when the alcohol solvent begins to erode the coating layer to the point when the coating layer is completely destroyed. During the stable segment, the y-axis of the line graph remains essentially unchanged, and the preferred range of its variation Δy is 0~0.005%, such as 0, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.

[0013] In the evaluation of the surface coating effect of the aforementioned positive electrode lithium supplement, alcohol solvent was chosen as the dispersion solvent primarily because it can react with the carbon coating layer, oxide coating, phosphide coating, and substrate. The reaction of alcohol solvent eroding the carbon coating layer is essentially the reaction of alcohol as a nucleophile with the functional groups on the carbon surface; the reaction of alcohol solvent eroding the oxide coating layer is essentially the interaction of alcohol as a weak acid and nucleophile with the metal sites and hydroxyl groups on the oxide surface, accompanied by chemisorption and proton transfer, ultimately leading to etching of the oxide surface; the reaction of alcohol solvent eroding the phosphide coating layer is essentially the interaction between the nucleophilicity of the phosphide and the weak acidity of the alcohol; the reaction of alcohol solvent eroding the substrate itself is essentially an acid-base neutralization reaction and a redox reaction, first involving surface passivation and proton exchange, followed by redox reactions to generate gases and alkaline substances. The type of alcohol solvent is not particularly limited and can be at least one of low-carbon alcohols, medium-carbon alcohols, and high-carbon alcohols. The lower alcohols preferably have C1 to C5 carbon atoms, and can be specifically listed as at least one of methanol, ethanol, ethylene glycol, n-propanol, isopropanol, glycerol, n-butanol, isobutanol, tert-butanol, n-pentanol, and isopentanol. The middle alcohols preferably have C6 to C10 carbon atoms, and can be specifically listed as at least one of hexanol, heptanol, n-octanol, isooctanol, nonanol, and decanol. The higher alcohols preferably have more than C11 carbon atoms, and can be specifically listed as at least one of undecyl alcohol, lauryl alcohol, tridecanol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecanol, stearyl alcohol, nonadecanol, and eicosyl alcohol. The alcohol solvent is particularly preferably a lower alcohol, as it has stronger corrosive power. This is because as the carbon chain length increases, the steric hindrance effect of the hydrocarbon group increases, physically protecting the hydroxyl group and reducing its nucleophilic reactivity. Simultaneously, the electron-donating effect of the long-chain alkyl group weakens the polarity of the OH bond, thus reducing the ability to corrode the coating layer, or even preventing it from reacting.

[0014] In the evaluation of the surface coating effect of the positive electrode lithium replenisher, the sampling time interval can be determined based on the solubility of the coating layers of the standard lithium replenisher and the lithium replenisher to be tested in the alcohol solvent. If the coating layer dissolves quickly in the alcohol solvent, the sampling time interval should be set shorter; if the coating layer dissolves slowly in the alcohol solvent, the sampling time interval should be set longer. Generally, the sampling time interval can be 5 min to 10 min, such as 5 min, 5.5 min, 6 min, 6.5 min, 7 min, 7.5 min, 8 min, 8.5 min, 9 min, 9.5 min, 10 min, etc.

[0015] In the evaluation of the surface coating effect of the positive electrode lithium replenisher, the dispersion can be carried out under static conditions or under stirring conditions. The stirring can be mechanical or magnetic, preferably magnetic, because magnetic stirring is easier to control under the same conditions. Furthermore, the stirring speed of the magnetic stirrer used is preferably 700 rpm to 800 rpm, such as 700 rpm, 720 rpm, 740 rpm, 760 rpm, 780 rpm, 800 rpm, etc. In addition, the dispersion time can be determined based on the solubility of the coating layers of the standard lithium replenisher and the lithium replenisher to be tested in the alcohol solvent. If the coating layer dissolves quickly in the alcohol solvent, the dispersion time can be set shorter; if the coating layer dissolves slowly in the alcohol solvent, the dispersion time should be set longer. Generally, the dispersion time is preferably 5 min to 300 min, such as 5 min, 10 min, 20 min, 50 min, 80 min, 100 min, 120 min, 150 min, 180 min, 200 min, 220 min, 250 min, 280 min, 300 min, etc.

[0016] In the evaluation of the surface coating effect of the positive electrode lithium supplement above, the sampling method is not particularly limited. Preferably, it includes taking samples of the dispersed solid-liquid mixture at different dispersion times for solid-liquid separation, and using the resulting filtrate as the test sample. The solid-liquid separation method is not particularly limited and can include pressure filtration, vacuum filtration, centrifugation, etc. When vacuum filtration is used for solid-liquid separation, the filter membrane can be a polytetrafluoroethylene (PTFE) membrane. Furthermore, the pore size of the filter membrane is preferably 0.22 μm to 0.8 μm, such as 0.22 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, etc.

[0017] In the evaluation of the surface coating effect of the above-mentioned positive electrode lithium replenishment agent, the sample volume is preferably 10mL~50mL when measuring the residual lithium content, such as 10mL, 15mL, 20mL, 25mL, 30mL, 35mL, 40mL, 45mL, 50mL, etc.

[0018] In the evaluation of the surface coating effect of the aforementioned positive electrode lithium replenisher, during the acquisition of the standard lithium replenisher line graph, one sample of the standard lithium replenisher can be taken, and a portion of the solid-liquid mixture formed by dispersion is extracted from the dispersion system at different dispersion times for filtration. The resulting clear filtrate is then used as the test sample. Alternatively, multiple samples of the standard lithium replenisher can be taken, each dissolved in an alcohol solvent under different conditions, and a sample of the standard lithium replenisher dispersion is extracted at different dispersion times for filtration. The resulting clear filtrate is then used as the test sample. The determination process for the lithium replenisher to be tested is the same.

[0019] In the evaluation of the surface coating effect of the above-mentioned positive electrode lithium replenishment agent, the method for determining the residual lithium content includes titration with a standard hydrochloric acid solution. The concentration of the standard hydrochloric acid solution is preferably 0.05 mol / L to 0.3 mol / L, such as 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc. Furthermore, the titration process first uses phenolphthalein indicator to determine the titration endpoint, and then uses bromocresol green-methyl red indicator to determine the titration endpoint again. When using phenolphthalein indicator to determine the titration endpoint, the system color changes from red to colorless, which is the titration endpoint. The volume of standard hydrochloric acid solution consumed at this point is recorded as V1, in mL. When using bromocresol green-methyl red indicator to determine the titration endpoint, the system color changes from green to orange-red, which is the titration endpoint. The volume of standard hydrochloric acid solution consumed at this point is recorded as V2, in mL. Residual lithium content (%) = (1.54) w LiOH + w Li2CO3 The specific calculation process is as follows: 0.19 × 0.19 LiOH mass fraction: w LiOH =(c×(2V1-V2)×10 -3 ×23.95) / (m×V3 / V)×100; Li2CO3 mass fraction: w Li2CO3 =(c×(V2-V1)×10 -3 ×73.89) / (m×V3 / V)×100; Residual alkali mass fraction: w ra =1.54× w LiOH + w Li2CO3 ; Residual lithium mass fraction: w rl =0.19× wra ; c represents the concentration of the standard hydrochloric acid solution, mol / L; V represents the volume of alcohol solvent added, mL; V1 represents the volume of standard hydrochloric acid solution consumed at the first titration endpoint, mL; V2 represents the volume of standard hydrochloric acid solution consumed at the second titration endpoint, mL; V3 represents the volume of sample transferred during the titration test, mL; m represents the accurate mass of the sample weighed, g. In the above formulas, 23.95 and 73.89 are the molar masses of LiOH and Li2CO3, respectively, both in g / mol.

[0020] The embodiments of the present invention will be described in detail below with reference to specific examples and comparative examples. However, those skilled in the art will understand that the embodiments described below are only some embodiments of the present invention, not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. All embodiments based on the present invention fall within the protection scope of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments used are not specified, they are all conventional samples that can be purchased commercially.

[0021] Example 1 In a drying room (humidity < -30%RH), 2.0 g of each of 13 standard lithium supplement A (base component is Li5FeO4, coating layer is carbon coating layer, coating layer thickness is 25nm±3nm) was added to a 150 mL conical flask. A stir bar was added, and then 100 mL of anhydrous ethanol solvent was added to each flask to bring the volume to a final volume. The flasks were sealed with sealing film and stirred at 700 rpm for 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, and 120 min, respectively. The filtrate was then collected by vacuum filtration through a 0.22 μm polytetrafluoroethylene filter membrane for analysis. Transfer 50 mL of the above filtrate and add phenolphthalein indicator; titrate with 0.05 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V1 (mL); when the color of the system changes from red to colorless, continue to add bromocresol green-methyl red indicator, titrate with 0.05 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V2; the titration endpoint is reached when the color of the system changes from green to orange-red. The residual lithium amount = w rl =0.19× w ra Record the residual lithium content data corresponding to different dispersion times, as detailed in Table 1. Plot a line graph of standard lithium replenishing agent A with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in the table below. Figure 2 .

[0022] S2`. In a drying room (humidity < -30%RH), 13 samples of lithium replenishing agent A (base component: Li5FeO4, coating layer: carbon coating layer, average coating layer thickness: 20nm, maximum thickness difference between different particles and different regions: 15nm) were tested under the same conditions as standard lithium replenishing agent A. The residual lithium content data corresponding to different dispersion times were obtained, as detailed in Table 1. A line graph of standard lithium replenishing agent A was plotted with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in Table 1. Figure 3 .

[0023] In a drying chamber (humidity < -30%RH), 13 samples of lithium replenishing agent B (base component: Li5FeO4, coating layer: carbon coating layer; observed by transmission electron microscopy, average coating layer thickness: 30 nm; maximum thickness difference between different particles and different regions: 10 nm) were tested under the same conditions as standard lithium replenishing agent A. The residual lithium content data corresponding to different dispersion times are shown in Table 1. A line graph of standard lithium replenishing agent A was plotted with dispersion time as the x-axis and residual lithium content as the y-axis. See Table 1 for details. Figure 4 .

[0024] Combining the time plateau curves of standard lithium replenisher A, test lithium replenisher A, and test lithium replenisher B, and the data in Table 1, it can be seen that standard lithium replenisher A is a standard with ideal coating effect. The difference between its residual lithium values ​​in the 20-60 min time is ≤0.005%, indicating that the residual lithium on the surface has been completely dissolved after 20 min of stirring and dispersion, and the ethanol solvent begins to erode the surface of the coating layer. As the stirring and dispersion time is extended, the residual lithium value does not change significantly. When the stirring time reaches 70 min, the test residual lithium value increases significantly, indicating that the ethanol solvent has completely eroded the surface coating layer and begins to erode the base of the lithium replenisher. The reaction generates alkaline substances, which cause the residual lithium value to rise rapidly. Therefore, the plateau time T0 of the standard is 40 min. The test results of lithium supplement A show that the difference in residual lithium value between 20 and 40 minutes is ≤0.005%, indicating that the residual lithium on the surface was completely dissolved after 20 minutes of stirring and dispersion, and the ethanol solvent began to erode the coating layer surface. As the stirring and dispersion time increased, the residual lithium value did not change significantly. When the stirring time reached 50 minutes, the residual lithium value increased significantly, indicating that the ethanol solvent had completely eroded the surface coating layer and began to erode the lithium supplement base. Therefore, the plateau time of lithium supplement A was 20 minutes, which was shorter than that of the standard sample, indicating poor coating effect. The test results of lithium supplement agent B show that the difference in residual lithium value between 20 and 70 minutes is ≤0.005%, indicating that the residual lithium on the surface was completely dissolved after 20 minutes of stirring and dispersion, and the ethanol solvent began to erode the coating layer surface. As the stirring and dispersion time was extended, the residual lithium value did not change significantly. When the stirring time reached 80 minutes, the residual lithium value increased significantly, indicating that the ethanol solvent had completely eroded the surface coating layer and began to erode the lithium supplement agent substrate. Therefore, the plateau time of lithium supplement agent B was 50 minutes, which is longer than that of the standard sample, indicating excellent coating effect.

[0025] The microstructures of standard lithium replenisher A, test lithium replenisher A, and test lithium replenisher B were observed using scanning electron microscopy. The results showed that the surface coating of standard lithium replenisher A was very smooth, dense, and uniform; the surface of test lithium replenisher A was relatively rough, with significant differences in coating thickness between different particles and different regions of the same particle, failing to achieve the ideal coating effect; the surface of test lithium replenisher B was very smooth and dense, achieving the ideal coating effect. This demonstrates that the evaluation process for the surface coating effect of the positive electrode lithium replenisher provided by this invention is accurate and effective.

[0026] Table 1

[0027] Example 2 In a drying room (humidity < -30%RH), 2.0 g of each of 13 standard lithium supplement B (base component: Li6CoO4, coating layer: phosphide layer, coating layer thickness: 20nm±2nm) was added to a 150 mL Erlenmeyer flask. A stir bar was added, and then 100 mL of isopropanol was added to each flask to bring the volume to a final volume. The flasks were sealed with sealing film and stirred at 800 rpm for 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, 95 min, 100 min, 105 min, 110 min, 115 min, and 120 min, respectively. The filtrate was then collected by vacuum filtration through a 0.22 μm polytetrafluoroethylene filter membrane and prepared for analysis. Transfer 50 mL of the above filtrate and add phenolphthalein indicator; titrate with 0.3 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V1 (mL); when the color of the system changes from red to colorless, continue to add bromocresol green-methyl red indicator, titrate with 0.3 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V2; the titration endpoint is reached when the color of the system changes from green to orange-red. The residual lithium content = w rl =0.19× w ra Record the residual lithium content data corresponding to different dispersion times, as detailed in Table 2. Plot a line graph of standard lithium replenishing agent B with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in [see table 2]. Figure 5 .

[0028] S2`. In a drying room (humidity < -30%RH), 13 samples of the lithium replenishing agent C to be tested (base component: Li6CoO4, coating layer: phosphide layer, average coating layer thickness: 20nm, maximum thickness difference between different particles and different regions: 10nm) were subjected to the same conditions as the standard lithium replenishing agent B. The residual lithium content data corresponding to different dispersion times were obtained, as detailed in Table 2. A line graph of the standard lithium replenishing agent B was plotted with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in Table 2. Figure 6 .

[0029] Combining the time plateau curves of standard lithium replenisher B and the lithium replenisher C to be tested, and the data in Table 2, it can be seen that standard lithium replenisher B is a standard with ideal coating effect. The difference between its residual lithium values ​​in the 25-60 min time is ≤0.005%, indicating that the residual lithium on the surface has been completely dissolved after 25 min of stirring and dispersion, and isopropanol begins to erode the surface of the coating layer. As the stirring and dispersion time is extended, the residual lithium value does not change significantly. When the stirring time reaches 65 min, the tested residual lithium value increases significantly, indicating that the isopropanol solvent has completely eroded the surface coating layer and begins to erode the base of the lithium replenisher. The reaction generates alkaline substances, which cause the residual lithium value to rise rapidly. Therefore, the plateau time T0 of the standard is 35 min. The test results of lithium supplement C show that the difference in residual lithium value between 25 and 40 minutes is ≤0.005%, indicating that the residual lithium on the surface was completely dissolved after 25 minutes of stirring and dispersion, and the isopropanol solvent began to erode the coating layer surface. As the stirring and dispersion time was extended, the residual lithium value did not change significantly. When the stirring time reached 45 minutes, the residual lithium value increased significantly, indicating that the isopropanol had completely eroded the surface coating layer and began to erode the lithium supplement base. Therefore, the plateau time of lithium supplement C was 15 minutes, which was shorter than that of the standard sample, indicating poor coating effect.

[0030] SEM images of standard lithium supplement B and lithium supplement C to be tested are shown below. Figure 7 ,from Figure 7 It can be seen that the standard lithium replenishing agent has a dense and uniform surface coating; the tested lithium replenishing agent C has a rough and unevenly distributed surface coating, with significant differences in coating thickness between different particles and different regions of the same particle. Therefore, the evaluation process for the surface coating effect of the positive electrode lithium replenishing agent provided by this invention is accurate and effective.

[0031] Table 2

[0032] Example 3 In a drying room (humidity < -30%RH), 2.0 g of each of 13 standard lithium supplement C (base component is Li5FeO4, coating layer is oxide layer, and the average thickness of the coating layer is 25nm±3nm) were added to 150 mL Erlenmeyer flasks. Stirring rods were added, and then 100 mL of n-butanol was added to each flask to bring the volume to a final volume. The flasks were sealed with sealing film and stirred at 750 rpm for 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, and 120 min, respectively. The filtrate was then collected by vacuum filtration through a 0.22 μm polytetrafluoroethylene filter membrane for analysis. Transfer 50 mL of the above filtrate and add phenolphthalein indicator; titrate with 0.05 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V1 (mL); when the color of the system changes from red to colorless, continue to add bromocresol green-methyl red indicator, titrate with 0.05 mol / L standard hydrochloric acid solution to the titration endpoint and record the volume of standard hydrochloric acid solution consumed, V2; the titration endpoint is reached when the color of the system changes from green to orange-red. The residual lithium amount = w rl =0.19× w ra Record the residual lithium content data corresponding to different dispersion times, as detailed in Table 3. Plot a line graph of standard lithium replenishing agent C with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in [see table 3]. Figure 8 .

[0033] S2`. In a drying room (humidity < -30%RH), 13 samples of the lithium replenishing agent D (base component: Li5FeO4, coating layer: oxide layer, average coating layer thickness: 26nm, maximum thickness difference between different particles and different regions: 13nm) were tested under the same conditions as the standard lithium replenishing agent C. The residual lithium content data corresponding to different dispersion times were obtained, as detailed in Table 3. A line graph of the standard lithium replenishing agent C was plotted with dispersion time as the x-axis and residual lithium content as the y-axis, as shown in Table 3. Figure 9 .

[0034] Combining the time plateau curves of standard lithium replenisher C and the lithium replenisher D to be tested, and the data in Table 3, it can be seen that standard lithium replenisher C is a standard with ideal coating effect. The difference between its residual lithium values ​​in the 30-70 min time is ≤0.005%, indicating that the residual lithium on the surface has been completely dissolved after 30 min of stirring and dispersion, and n-butanol begins to erode the surface of the coating layer. As the stirring and dispersion time is extended, the residual lithium value does not change significantly. When the stirring time reaches 80 min, the tested residual lithium value increases significantly, indicating that n-butanol has completely eroded the surface coating layer and begins to erode the base of the lithium replenisher. The reaction generates alkaline substances, which cause the residual lithium value to rise rapidly. Therefore, the plateau time T0 of the standard is 40 min. The test results of lithium supplement agent D show that the difference in residual lithium value between 30-70 min is ≤0.005%, indicating that the residual lithium on the surface was completely dissolved after 30 min of stirring and dispersion, and n-butanol began to erode the coating layer surface. As the stirring and dispersion time was extended, the residual lithium value did not change significantly. When the stirring time reached 80 min, the residual lithium value increased significantly, indicating that n-butanol had completely eroded the surface coating layer and began to erode the lithium supplement agent base. Therefore, the plateau time of lithium supplement agent D was 40 min, which is equal to the plateau time of the standard sample, indicating that the coating effect was ideal.

[0035] SEM images of standard lithium supplement C and lithium supplement D to be tested are shown below. Figure 10 ,from Figure 10 It can be seen that the surface coating of the standard lithium replenishing agent is uniform and dense; the surface coating of the lithium replenishing agent C to be tested is also uniform and dense. Therefore, the evaluation process for the surface coating effect of the positive electrode lithium replenishing agent provided by this invention is accurate and effective.

[0036] Table 3

[0037] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A method for evaluating the surface coating effect of a positive electrode lithium replenishment agent, characterized in that, Includes the following steps: S1: The standard lithium replenishing agent and the lithium replenishing agent to be tested are dispersed in alcohol solvent under the same conditions. The standard lithium replenishing agent is a sample with good surface coating effect. The residual lithium in the liquid is measured at different dispersion times. A line graph with dispersion time as the x-axis and residual lithium as the y-axis is plotted. The stable segment of the curve of the standard lithium replenishing agent is recorded as the plateau time T0, and the stable segment of the curve of the lithium replenishing agent to be tested is recorded as the plateau time T1. S2: Compare the plateau time T1 of the lithium replenishing agent to be tested with the plateau time T0 of the standard lithium replenishing agent to determine the coating effect; when the plateau time T1 of the lithium replenishing agent to be tested is greater than or equal to the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is good; when the plateau time T1 of the lithium replenishing agent to be tested is less than the plateau time T0 of the standard lithium replenishing agent, it indicates that the coating effect of the lithium replenishing agent to be tested is poor.

2. The method for evaluating the surface coating effect of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The standard lithium replenishing agent and the lithium replenishing agent to be tested have the same coating type; Preferably, the coating type of the standard lithium replenishing agent and the lithium replenishing agent to be tested is selected from at least one of carbon coating, oxide coating and phosphide coating; Preferably, the difference in coating thickness between the standard lithium replenishing agent and the lithium replenishing agent to be tested does not exceed 10 nm.

3. The method for evaluating the surface coating effect of the positive electrode lithium replenishing agent according to claim 1, characterized in that, The Δy range of the stable segment of the curve is 0~0.005%.

4. The method for evaluating the surface coating effect of the positive electrode lithium replenishing agent according to claim 1, characterized in that, The alcohol solvent is a C1-C5 low-carbon alcohol.

5. The method for evaluating the surface coating effect of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The dispersion is carried out under stirring conditions; Preferably, the stirring is magnetic stirring; Preferably, the magnetic stirrer used for magnetic stirring rotates at a speed of 700 rpm to 800 rpm.

6. The method for evaluating the surface coating effect of the positive electrode lithium replenishing agent according to claim 1, characterized in that, The sampling time interval is 5 min to 10 min.

7. The method for evaluating the surface coating effect of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The sampling method includes taking out the solid-liquid mixture formed by dispersion at different dispersion times, performing solid-liquid separation, and using the resulting filtrate as the test sample.

8. The method for evaluating the surface coating effect of the positive electrode lithium replenishing agent according to claim 7, characterized in that, The solid-liquid separation method is vacuum filtration, and the filter membrane used in the vacuum filtration is a polytetrafluoroethylene membrane with a pore size of 0.22μm~0.8μm.

9. The method for evaluating the surface coating effect of the positive electrode lithium replenishing agent according to claim 7, characterized in that, When determining the residual lithium content, the sample volume should be 10 mL to 50 mL.

10. The method for evaluating the surface coating effect of the positive electrode lithium replenishment agent according to claim 1, characterized in that, The method for determining the residual lithium content includes hydrochloric acid titration. Preferably, the concentration of the standard hydrochloric acid solution used in the hydrochloric acid titration method is 0.05 mol / L to 0.3 mol / L; Preferably, in the hydrochloric acid titration process, the titration endpoint is first determined using phenolphthalein indicator, and then using bromocresol green-methyl red indicator. When using phenolphthalein indicator to determine the titration endpoint, the endpoint is reached when the system color changes from red to colorless, and the volume of standard hydrochloric acid solution consumed at this point is recorded as V1. When using bromocresol green-methyl red indicator to determine the titration endpoint, the endpoint is reached when the system color changes from green to orange-red, and the volume of standard hydrochloric acid solution consumed at this point is recorded as V2. Residual lithium content = (1.54) w LiOH + w Li2CO3 )×0.19, w LiOH =(c×(2V1-V2)×10 -3 ×23.95) / (m×V3 / V)×100, w Li2CO3 =(c×(V2-V1)×10 -3 ×73.89) / (m×V3 / V)×100; c represents the concentration of the standard hydrochloric acid solution, in mol / L; V3 represents the volume of the sample transferred during the titration test, in mL; V represents the volume of the added alcohol solvent, in mL; m represents the mass of the sample, in g.

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