Negative electrode performance evaluation method

Abstract

The invention relates to the technical field of batteries, in particular to a negative electrode performance evaluation method which comprises the following steps: S1, preparing a button cell by taking hard carbon as a negative electrode active material; s2, performing a first charge and discharge test on the button cell in a voltage interval of 0-1.5 V to obtain a first discharge specific capacity A1; carrying out a second charge and discharge test in a voltage interval of 0V-0.8 V to obtain a second discharge specific capacity A2; carrying out a third charge and discharge test in a voltage interval of 0.05-0.8 V to obtain a third discharge specific capacity A3; and carrying out a fourth charge and discharge test in a voltage interval of-0.005 V to 1.5 V to obtain a fourth discharge specific capacity A4. According to the evaluation method provided by the invention, the dynamic performance of the hard carbon negative electrode can be quickly judged, necessary support data is provided for full battery application, and different types of hard carbon can be quickly screened.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly relates to a method for evaluating the performance of a negative electrode. Background Art

[0002] With the development and progress of society, the environmental problems brought about by the shortage of oil resources have attracted great attention. Therefore, it is urgent to develop new battery systems to meet the efficient utilization of renewable energy. Lithium-ion batteries have been widely used in various industries due to their high energy density. However, lithium resources are scarce and unevenly distributed in the earth's crust, making it difficult to meet the demand for large-scale energy storage. Sodium is widely present in the earth's crust and oceans. Sodium-ion batteries (SIBs) can complement lithium-ion batteries and be used for large-scale electrochemical energy storage, electric vehicles, etc. Among them, the negative electrode material is crucial for the performance of sodium-ion batteries. As an important component of the negative electrode material, the performance of hard carbon plays a decisive role in the sodium storage capacity of sodium-ion batteries. The mechanism of sodium storage in hard carbon is roughly divided into three pathways: adsorption on hard carbon defects, interlayer insertion, and pore filling. In the adsorption pathway on hard carbon defects, the irreversible capacity increases due to the excessive proportion of some capacity, and the initial Coulomb efficiency becomes low. In the interlayer insertion pathway, due to the relatively large proportion of some capacity, the rate performance is likely to deteriorate. In the pore filling pathway, the morphology of sodium ions during filling in micropores is similar to that in the 0-valent state. Due to the excessive proportion of capacity, metallic sodium is extremely likely to precipitate, which not only makes it difficult to guarantee the battery safety performance but also affects the electrochemical performance of the battery. It can be seen that sodium-ion batteries prepared with hard carbon of different performances vary greatly in terms of capacity, rate, and cycle performance. Therefore, how to evaluate the comprehensive performance of hard carbon is crucial.

[0003] In the prior art, when using a coin cell to evaluate the performance of a hard carbon negative electrode, only the specific capacity of hard carbon can be roughly tested, but the actual capacity utilization and rate performance in a full cell cannot be accurately evaluated. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the actual capacity utilization and rate performance in a full cell cannot be accurately evaluated when using a coin cell to evaluate the performance of a hard carbon negative electrode in the prior art, so as to provide a method for evaluating the performance of a negative electrode.

[0005] To this end, the present invention provides a method for evaluating the performance of a negative electrode. The evaluation method includes the following steps: S1. Prepare a button cell using hard carbon as the negative electrode active material; S2. Perform a first charge-discharge test on the button cell in the voltage range of 0V to 1.5V to obtain the first discharge specific capacity A1; perform a second charge-discharge test in the voltage range of 0V to 0.8V to obtain the second discharge specific capacity A2; perform a third charge-discharge test in the voltage range of 0.05V to 0.8V to obtain the third discharge specific capacity A3; perform a fourth charge-discharge test in the voltage range of -0.005V to 1.5V to obtain the fourth discharge specific capacity A4; evaluate the actual sodium storage and desodium capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the first discharge specific capacity A1; evaluate the actual sodium storage specific capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the second discharge specific capacity A2; evaluate the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the third discharge specific capacity A3; evaluate the maximum value of the sodium storage capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the fourth discharge specific capacity A4.

[0006] In some embodiments, in S2, the first charge-discharge test includes: (1) discharging to 0V at a first current; (2) discharging to 0V at a second current; (3) discharging to 0V at a third current; (4) charging to 1.5V at the first current; wherein the first current is greater than the second current, and the second current is greater than the third current.

[0007] In some embodiments, the second charge-discharge test includes: (1) discharging to 0V at a first current; (2) discharging to 0V at a second current; (3) discharging to 0V at a third current; (4) charging to 0.8V at the first current; wherein the first current is greater than the second current, and the second current is greater than the third current.

[0008] In some embodiments, the third charge-discharge test includes: (1) discharging to 0.05V at a first current; (2) discharging to 0.05V at a second current; (3) discharging to 0.05V at a third current; (4) charging to 0.8V at the first current; wherein the first current is greater than the second current, and the second current is greater than the third current.

[0009] In some embodiments, the third charge-discharge test includes: (1) discharging to -0.005V at a first current; (2) discharging to -0.005V at a second current; (3) discharging to -0.005V at a third current; (4) charging to 1.5V at the first current; wherein the first current is greater than the second current, and the second current is greater than the third current.

[0010] In some embodiments, the first current is 0.01C to 0.2C, the second current is 0.01C to 0.1C, and the third current is 0.01C to 0.05C.

[0011] In some embodiments, the first current is 0.1C, the second current is 0.05C, and the third current is 0.01C.

[0012] In some embodiments, the evaluation method further includes: calculating the ratio of the first discharge specific capacity A1 to the fourth discharge specific capacity A4 as A1 / A4, calculating the ratio of the third discharge specific capacity A3 to the second discharge specific capacity A2 as A3 / A2, and evaluating the capacity performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material according to A1 / A4; evaluating the rate performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material according to A3 / A2.

[0013] In some embodiments, when A1 / A4 is greater than or equal to 85%, it is determined that the capacity performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material meets the standard; when A3 / A2 is greater than or equal to 32%, it is determined that the rate performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material meets the requirements.

[0014] In some embodiments, the precursor of the hard carbon includes at least one of biomass, polymer, and fossil fuel. Preferably, the biomass includes at least one of coconut shell, wood, cotton, straw, and bamboo, the polymer includes at least one of phenolic resin, polyacrylonitrile, polyimide, and polyvinyl chloride, and the fossil fuel includes at least one of pitch or bituminous coal.

[0015] In some embodiments, in the active coating of the negative electrode sheet of the button battery, the mass fraction of the hard carbon is 94% to 97%.

[0016] The technical solution of the present invention has the following advantages:

[0017] A method for evaluating the performance of a negative electrode provided by the present invention, the evaluation method comprising the following steps: S1. Prepare a button cell using hard carbon as a negative electrode active material; S2. Perform a first charge-discharge test on the button cell within a voltage range of 0V to 1.5V to obtain a first discharge specific capacity A1; perform a second charge-discharge test within a voltage range of 0V to 0.8V to obtain a second discharge specific capacity A2; perform a third charge-discharge test within a voltage range of 0.05V to 0.8V to obtain a third discharge specific capacity A3; perform a fourth charge-discharge test within a voltage range of -0.005V to 1.5V to obtain a fourth discharge specific capacity A4; evaluate the actual sodium storage and desodium capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the A1; evaluate the actual sodium storage specific capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the A2; evaluate the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the A3; evaluate the maximum value of the sodium storage capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material according to the A4. The present invention evaluates the actual sodium storage and desodium capacity, sodium storage specific capacity, sodium storage specific capacity in the adsorption region, and the maximum value of the sodium storage capacity of the sodium-ion battery prepared using the hard carbon as the negative electrode active material by testing the discharge specific capacity of the button cell formed by the hard carbon in different voltage ranges, and can thus determine the fast charging performance of the hard carbon negative electrode material applied to the sodium-ion full battery. The evaluation method provided by the present invention can quickly measure the rate performance of the sodium-ion battery prepared using the hard carbon as the negative electrode active material, accurately obtain the first Coulomb efficiency of the sodium-ion battery prepared using the hard carbon as the negative electrode active material, quickly judge the kinetic performance of the hard carbon negative electrode, provide necessary support data for the application of the full battery, and quickly screen different types of hard carbon. Detailed implementation manners

[0018] The following embodiments are provided to better further understand the present invention, and are not limited to the best implementation manner, and do not constitute a limitation to the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts falls within the protection scope of the present invention.

[0019] For those not specifying specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in the art can be followed. For those reagents or instruments not specifying the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0020] Since the negative electrode has a negative polarity, that is, the potential of the negative electrode is negative relative to metallic sodium, the button cell prepared with the negative electrode material needs to be discharged first and then charged to facilitate sodium storage and desorption in the negative electrode. The charge and discharge of the full cell are opposite to those of the button cell. During charging, it corresponds to sodium storage, and during discharging, it corresponds to sodium desorption. Therefore, the discharge cut-off voltage of the full cell corresponds to the charge cut-off voltage of the button cell, and the charge cut-off voltage of the full cell corresponds to the discharge cut-off voltage of the button cell. When the full cell is charged to the upper limit cut-off voltage, during the R & D process, the inventor found that due to the polarization effect of the battery, the actual potential of the negative electrode is often lower than 0 V, and if the duration is too long, metallic sodium will precipitate. When the full cell is discharged to the lower limit cut-off voltage, due to the voltage hysteresis effect of hard carbon, the actual potential of the negative electrode cannot reach 1.5 V, and the actual utilization capacity of the negative electrode is lower than the test result of the button cell. Therefore, by testing the specific capacity of the button cell in different voltage ranges, the actual capacity performance and rate performance of the hard carbon negative electrode in the full cell can be determined.

[0021] Furthermore, through research, it is found that when the voltage range of charge and discharge of the button cell is 0 V to 1.5 V, the discharge specific capacity is A1, and based on the A1, the actual sodium storage and desorption capacity of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated; when the voltage range of charge and discharge of the button cell is 0 V to 0.8 V, the discharge specific capacity is A2, and based on the A2, the actual sodium storage specific capacity of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated; when the voltage range of charge and discharge of the button cell is 0.05 V to 0.8 V, the discharge specific capacity is A3, and based on the A3, the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated; when the voltage range of charge and discharge of the button cell is -0.005 V to 1.5 V, the discharge specific capacity is A4, and based on the A4, the maximum value of the sodium storage capacity of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated.

[0022] Furthermore, based on the A1 / A4, the capacity performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated. The larger the ratio of A1 / A4, the smaller the proportion of the micropore filling capacity, the higher the actual available capacity of the full cell, and the higher the initial Coulombic efficiency; the lower the ratio of A1 / A4, the larger the proportion of the micropore filling capacity, the excessive proportion of the capacity below 0 V, the lower the actual available capacity of the full cell, and the lower the initial Coulombic efficiency. Based on the A3 / A2, the rate performance of the sodium-ion battery prepared with the hard carbon as the negative electrode active material is evaluated. The larger the ratio of A3 / A2, the better the rate performance of the full cell, but the larger the initial irreversible capacity, which will lead to a lower initial Coulombic efficiency of the full cell.

[0023] Example 1

[0024] This example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0025] (1) Determination process for preparing coin cells using hard carbon as raw material:

[0026] Prepare hard carbon using biomass coconut shell as precursor. The specific steps are as follows:

[0027] Pretreatment: Crush the coconut shell into particles with a diameter of 1 - 3 cm, wash with water to remove surface impurities, and dry at 120 °C until the moisture content is less than 5%.

[0028] Carbonization under inert atmosphere treatment: Place the pretreated coconut shell particles in a tubular furnace, and under an argon atmosphere, heat to 1000 °C at a rate of 5 °C / min and hold for 3.5 hours to completely pyrolyze the organic components to form hard carbon precursors.

[0029] Post-treatment cooling and screening: After naturally cooling the hard carbon precursors to room temperature, screen out particles that are too small (<1 μm) or too large (>20 μm) to obtain hard carbon.

[0030] Mix hard carbon, conductive agent, dispersant, and binder according to a mass ratio of 94:2:1.5:2.5. Add hard carbon, conductive agent (carbon nanotubes), dispersant (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber), and add an appropriate amount of water and stir well to prepare a negative electrode slurry with a solid content of 45 wt%.

[0031] Coat the negative electrode slurry on copper foil, dry at 85 °C for 3 h, roll press to obtain a negative electrode sheet, and punch the rolled negative electrode sheet into a negative electrode circular sheet with a diameter of 12 mm.

[0032] Assemble the negative electrode circular sheet with a sodium sheet with a diameter of 14 mm into a coin cell. The structure of the coin cell is successively a negative electrode shell, sodium sheet, glass fiber separator, negative electrode circular sheet, gasket, spring sheet, and positive electrode shell. After assembly, inject the electrolyte. Among them, the electrolyte solvent is composed of ethylene carbonate (EC), diethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:2:2, and the solute is sodium hexafluorophosphate (NaPF 6 )), and the concentration of NaPF 6 in the electrolyte is 1.5 mol / L.

[0033] Measure the charge-discharge specific capacity in different voltage ranges for the assembled coin cells according to the following charge-discharge steps:

[0034] Charge-discharge steps for 0 V - 1.5 V: Discharge at 0.1 C to 0 V, discharge at 0.05 C to 0 V, discharge at 0.01 C to 0 V to obtain the discharge specific capacity A1; then charge at 0.1 C to 1.5 V.

[0035] Charge-discharge steps for 0 V - 0.8 V: Discharge at 0.1 C to 0 V, discharge at 0.05 C to 0 V, discharge at 0.01 C to 0 V to obtain the discharge specific capacity A2; then charge at 0.1 C to 0.8 V.

[0036] Charge and discharge process from 0.05V to 0.8V: Discharge at 0.1C to 0.05V, discharge at 0.05C to 0.05V, discharge at 0.01C to 0.05V to obtain the discharge specific capacity A3; then charge at 0.1C to 0.8V.

[0037] Charge and discharge process from -0.005V to 1.5V: Discharge at 0.1C to -0.005V, discharge at 0.05C to -0.005V, discharge at 0.01C to -0.005V for 72h to obtain the discharge specific capacity A4; then charge at 0.1C to 1.5V.

[0038] The test results are shown in Table 1.

[0039] (2) Determination process for preparing a full cell using hard carbon as raw material:

[0040] According to the mass ratio of O3-type layered oxide (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 ), conductive agent (carbon black), dispersant (carbon nanotube), binder (polyvinylidene fluoride) being 96:1:1:2, mix the layered oxide, carbon black, carbon nanotube and polyvinylidene fluoride in N-methylpyrrolidone, stir evenly to obtain the positive electrode slurry with a solid content of 65wt%; coat the positive electrode slurry on the surface of aluminum foil and dry at 85°C for 2h, then roll to obtain the positive electrode sheet.

[0041] According to the mass ratio of hard carbon, conductive agent (carbon nanotube), dispersant (sodium carboxymethyl cellulose), binder (polyacrylate) being 94:2:1.5:2.5, mix hard carbon, conductive agent, dispersant and binder, add appropriate amount of water, fully stir and configure to obtain a negative electrode slurry with a solid content of 48wt%, coat the negative electrode slurry on the surface of the foil, dry at 75°C for 2h, then roll to obtain the negative electrode sheet.

[0042] Assemble the negative electrode sheet, positive electrode sheet and separator in a soft package laminated manner to form a battery, the separator substrate is polyethylene, inject the sodium ion electrolyte and then carry out formation and grading to obtain the full cell. The solvent of the sodium ion electrolyte is composed of ethylene carbonate (EC), diethyl carbonate (DMC), ethyl methyl carbonate (EMC) with a volume ratio of 1:2:2, and the solute is sodium hexafluorophosphate (NaPF 6 ), and the concentration of NaPF 6 in the sodium ion electrolyte is 1.5mol / L.

[0043] Test the first Coulomb efficiency and 0.33C discharge capacity of the full cell, and the 3C rate charging constant current ratio.

[0044] The charge-discharge range of the full battery is 1.5 to 4.0 V;

[0045] The formation and formation steps are as follows: charge at 0.1C to 3.7V, hold at 45°C for 48h, then charge at constant current and constant voltage at 0.1C to 4.0V, and cut off at 0.05C; discharge at 0.33C to 1.5V.

[0046] The first Coulomb efficiency is E, and E = D / C, where C is the total charge capacity during the entire formation and formation process, and D is the discharge capacity during the entire formation and formation process.

[0047] The calculation method of the constant current ratio for 3C rate charging: charge at constant current and constant voltage at 1C to 4.0V, cut off at 0.05C; discharge at 0.33C to 1.5V; charge at constant current and constant voltage at 3C to 4.0V, cut off at 0.05C; discharge at 0.33C to 1.5V. Derive the constant current section capacities of 1C and 3C charging from the charge-discharge data, and the constant current ratio for 3C rate charging is the constant current section capacity of 3C charging / the constant current section capacity of 1C charging.

[0048] The measurement results are shown in Table 3.

[0049] Example 2

[0050] This example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are the same as those in Example 1, except that hard carbon prepared from fossil fuel asphalt as the precursor is used to replace the hard carbon prepared from biomass coconut shell in Example 1, and the mass of the hard carbon in this example is the same as that of the hard carbon in Example 1.

[0051] Among them, the specific steps for preparing hard carbon from fossil fuel asphalt as the precursor are as follows:

[0052] Pretreatment: Heat the asphalt to 300°C to melt it, and filter to remove impurities;

[0053] Pre-oxidation: Pre-oxidize the asphalt after pretreatment in air at 350°C for 3 hours to form a cross-linked structure to inhibit graphitization;

[0054] Carbonization: Place the asphalt with the cross-linked structure in a tubular furnace, and heat it to 1200°C at a rate of 5°C / min in an argon atmosphere, and hold for 5 hours to promote further cross-linking of asphalt molecules to form a hard carbon precursor;

[0055] Crushing and cleaning: After the hard carbon precursor is naturally cooled to room temperature, mechanically crush it, and then wash it with hydrochloric acid to remove ash to obtain hard carbon.

[0056] Example 3

[0057] This embodiment provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are the same as those in Embodiment 1, except that the hard carbon prepared using the high molecular polymer phenolic resin as the precursor replaces the hard carbon prepared using biomass coconut shell as the precursor in Embodiment 1, and the mass of the hard carbon in this embodiment is the same as that of the hard carbon in Embodiment 1.

[0058] Among them, the specific steps for preparing hard carbon using the high molecular polymer phenolic resin as the precursor are as follows:

[0059] Pretreatment and curing molding: The phenolic resin is pre-cured at 120 °C for 3 hours to form a cross-linked network structure, and then broken into particles with a particle size of 0.5 - 2 mm.

[0060] Carbonization: The broken phenolic resin particles are placed in a tubular furnace, and under an argon atmosphere, they are heated to 1300 °C at a rate of 5 °C / min and held for 3 hours to pyrolyze and generate a hard carbon precursor.

[0061] Pickling and purification: After the hard carbon precursor is naturally cooled to room temperature, it is washed with dilute hydrochloric acid to remove residual metal impurities, and hard carbon is obtained.

[0062] Embodiment 4

[0063] This embodiment provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0064] (1) The measurement process for preparing a button cell using hard carbon as the raw material:

[0065] Using the hard carbon prepared in Embodiment 1 as the hard carbon in this embodiment, a button cell is assembled according to the method provided in Embodiment 1.

[0066] The assembled button cell is measured for the charge-discharge specific capacity in different voltage ranges according to the following charge-discharge steps:

[0067] Charge-discharge step of 0 V - 1.5 V: Discharge at 0.2 C to 0 V, discharge at 0.1 C to 0 V, discharge at 0.05 C to 0 V to obtain the discharge specific capacity A1; then charge at 0.2 C to 1.5 V.

[0068] Charge-discharge step of 0 V - 0.8 V: Discharge at 0.2 C to 0 V, discharge at 0.1 C to 0 V, discharge at 0.05 C to 0 V to obtain the discharge specific capacity A2; then charge at 0.2 C to 0.8 V.

[0069] Charge-discharge step of 0.05 V - 0.8 V: Discharge at 0.2 C to 0.05 V, discharge at 0.1 C to 0.05 V, discharge at 0.05 C to 0.05 V to obtain the discharge specific capacity A3; then charge at 0.2 C to 0.8 V.

[0070] Charge and discharge process from -0.005V to 1.5V: Discharge at 0.2C to -0.005V, discharge at 0.1C to -0.005V, discharge at 0.05C to -0.005V, with a time limit of 72h, to obtain the discharge specific capacity A4; then charge at 0.2C to 1.5V.

[0071] (2) The measurement process for preparing the full cell with hard carbon as the raw material is the same as that in step (2) of Example 1.

[0072] Example 5

[0073] This example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0074] (1) Measurement process for preparing the button cell with hard carbon as the raw material:

[0075] Use the hard carbon prepared in Example 1 as the hard carbon in this example, and assemble the button cell according to the method provided in Example 1;

[0076] Measure the charge and discharge specific capacities in different voltage ranges for the assembled button cell according to the following charge and discharge process:

[0077] Charge and discharge process from 0V to 1.5V: Discharge at 0.05C to 0V, discharge at 0.02C to 0V, discharge at 0.01C to 0V, to obtain the discharge specific capacity A1; then charge at 0.05C to 1.5V.

[0078] Charge and discharge process from 0V to 0.8V: Discharge at 0.05C to 0V, discharge at 0.02C to 0V, discharge at 0.01C to 0V, to obtain the discharge specific capacity A2; then charge at 0.05C to 0.8V.

[0079] Charge and discharge process from 0.05V to 0.8V: Discharge at 0.05C to 0.05V, discharge at 0.02C to 0.05V, discharge at 0.01C to 0.05V, to obtain the discharge specific capacity A3; then charge at 0.05C to 0.8V.

[0080] Charge and discharge process from -0.005V to 1.5V: Discharge at 0.05C to -0.005V, discharge at 0.02C to -0.005V, discharge at 0.01C to -0.005V, with a time limit of 72h, to obtain the discharge specific capacity A4; then charge at 0.05C to 1.5V.

[0081] (2) The measurement process for preparing the full cell with hard carbon as the raw material is the same as that in step (2) of Example 1.

[0082] Comparative Example 1

[0083] This comparative example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0084] A button cell was prepared according to the steps and parameters in Step (1) of Example 1.

[0085] The assembled button cell was measured for charge-discharge specific capacity in different voltage ranges according to the following charge-discharge process steps:

[0086] Discharge at 0.1C to 0V, then charge at 0.1C to 2V to obtain a discharge capacity C1 and a charge capacity C2. The first Coulombic efficiency of the button cell is C2 / C1;

[0087] The test results are shown in Table 2.

[0088] Comparative Example 2

[0089] This comparative example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0090] Button cells were prepared respectively according to the steps and parameters in Step (1) of Example 2.

[0091] The assembled button cell was measured for charge-discharge specific capacity in different voltage ranges according to the following charge-discharge process steps:

[0092] Discharge at 0.1C to 0V, then charge at 0.1C to 2V to obtain a discharge capacity C1 and a charge capacity C2. The first Coulombic efficiency of the button cell is C2 / C1;

[0093] The test results are shown in Table 2.

[0094] Comparative Example 3

[0095] This comparative example provides a method for evaluating the performance of the negative electrode. The specific steps and parameters are as follows:

[0096] Button cells were prepared respectively according to the steps and parameters in Step (1) of Example 3.

[0097] The assembled button cell was measured for charge-discharge specific capacity in different voltage ranges according to the following charge-discharge process steps:

[0098] Discharge at 0.1C to 0V, then charge at 0.1C to 2V to obtain a discharge capacity C1 and a charge capacity C2. The first Coulombic efficiency of the button cell is C2 / C1;

[0099] The test results are shown in Table 2.

[0100] Table 1 Test Results of Button Cells in Examples

[0101]

[0102] Table 2 Test Results of Button Cells in Comparative Examples

[0103] Comparative Example C1 (mAh / g) C2 (mAh / g) C2 / C1 Comparative Example 1 300.0 271.0 90.3％ Comparative Example 2 265.1 242.3 91.4％ Comparative Example 3 345.8 321.5 93.0％

[0104] Table 3 Full cell test results

[0105] Example Initial Coulombic Efficiency Constant Current Ratio of 3C Rate Charging Example 1 85.0％ 90.8％ Example 2 81.0％ 86.7％ Example 3 83.0％ 81.1％

[0106] It should be noted that in each embodiment and each comparative example of the present invention, the content of the active material loaded on the negative electrode sheet in the button cell and the content of the active material loaded on the negative electrode sheet in the full cell are exactly the same. The test results of the present invention are only related to the performance of the hard carbon and have nothing to do with other factors. The hard carbon used in Examples 4-5 and Comparative Example 1 is the same as that in Example 1. Therefore, the full cell test results of Examples 4-5 and Comparative Example 1 are the same as those of Example 1; the hard carbon used in Comparative Example 2 is the same as that in Example 2. Therefore, the full cell test results of Comparative Example 2 are the same as those of Example 2; the hard carbon used in Comparative Example 3 is the same as that in Example 3. Therefore, the full cell test results of Comparative Example 3 are the same as those of Example 3.

[0107] According to the data in Table 1, the first discharge specific capacity A1 of the coin cell prepared with the hard carbon provided in Example 1 as the negative electrode active material was 326.0 mAh / g in the first charge-discharge test in the voltage range of 0V to 1.5V, which characterized the actual sodium storage and desorption capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The second discharge specific capacity A2 of the coin cell prepared with the hard carbon provided in Example 1 as the negative electrode active material was 299.8 mAh / g in the second charge-discharge test in the voltage range of 0V to 0.8V, which characterized the actual sodium storage specific capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The third discharge specific capacity A3 of the coin cell prepared with the hard carbon provided in Example 1 as the negative electrode active material was 123.0 mAh / g in the third charge-discharge test in the voltage range of 0.05V to 0.8V, which characterized the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The fourth discharge specific capacity A4 of the coin cell prepared with the hard carbon provided in Example 1 as the negative electrode active material was 365.1 mAh / g in the fourth charge-discharge test in the voltage range of -0.005V to 1.5V, which characterized the maximum sodium storage capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. And A1 / A4 was 89.3%, which characterized the capacity utilization of the sodium-ion battery prepared with this hard carbon as the negative electrode active material, indicating that the proportion of the micropore filling capacity in this hard carbon was relatively small, the actual available capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material was relatively high, and the initial Coulomb efficiency was relatively high. It was determined that the initial Coulomb efficiency of the sodium-ion battery prepared with this hard carbon as the negative electrode active material met the standard. A3 / A2 was 41.0%, which characterized the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. It was determined that the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material met the requirements. Further, according to Table 3, the initial Coulomb efficiency of the sodium-ion battery prepared with the hard carbon in Example 1 as the negative electrode active material was 85.0%, and the constant current ratio during 3C rate charging was 90.8%. The test results were consistent with the test results of the coin cell prepared with the hard carbon in Example 1 as the negative electrode active material.

[0108] According to the data in Table 1, the first discharge specific capacity A1 of the coin cell prepared with the hard carbon provided in Example 2 as the negative electrode active material is 280.0 mAh / g in the first charge-discharge test in the voltage range of 0V to 1.5V, which characterizes the actual sodium storage and desorption capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The second discharge specific capacity A2 of the coin cell prepared with the hard carbon provided in Example 2 as the negative electrode active material is 271.7 mAh / g in the second charge-discharge test in the voltage range of 0V to 0.8V, which characterizes the actual sodium storage specific capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The third discharge specific capacity A3 of the coin cell prepared with the hard carbon provided in Example 2 as the negative electrode active material is 81.4 mAh / g in the third charge-discharge test in the voltage range of 0.05V to 0.8V, which characterizes the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The fourth discharge specific capacity A4 of the coin cell prepared with the hard carbon provided in Example 2 as the negative electrode active material is 319.5 mAh / g in the fourth charge-discharge test in the voltage range of -0.005V to 1.5V, which characterizes the maximum sodium storage capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. And A1 / A4 is 87.6%, which characterizes the capacity utilization of the sodium-ion battery prepared with this hard carbon as the negative electrode active material, indicating that the proportion of the micropore filling capacity in this hard carbon is relatively small, the actual available capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material is relatively high, and the initial Coulomb efficiency is relatively high. It is determined that the initial Coulomb efficiency of the sodium-ion battery prepared with this hard carbon as the negative electrode active material meets the standard. A3 / A2 is 30.0%, which characterizes the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. It is determined that the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material does not meet the standard. Further, according to Table 3, the initial Coulomb efficiency of the sodium-ion battery prepared with the hard carbon of Example 2 as the negative electrode active material is 81.0%, and the constant current ratio of 3C rate charging is 86.7%. This test result is consistent with the test result of the coin cell prepared with the hard carbon of Example 2 as the negative electrode active material.

[0109] According to the data in Table 1, for the coin cell prepared with the hard carbon provided in Example 3 as the negative electrode active material, the first discharge specific capacity A1 obtained from the first charge-discharge test in the voltage range of 0V to 1.5V is 348.8 mAh / g, which characterizes the actual sodium storage and desodium capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The second discharge specific capacity A2 obtained from the second charge-discharge test in the voltage range of 0V to 0.8V for the coin cell prepared with the hard carbon provided in Example 3 as the negative electrode active material is 331.3 mAh / g, which characterizes the actual sodium storage specific capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The third discharge specific capacity A3 obtained from the third charge-discharge test in the voltage range of 0.05V to 0.8V for the coin cell prepared with the hard carbon provided in Example 3 as the negative electrode active material is 95.5 mAh / g, which characterizes the sodium storage specific capacity in the adsorption region of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. The fourth discharge specific capacity A4 obtained from the fourth charge-discharge test in the voltage range of -0.005V to 1.5V for the coin cell prepared with the hard carbon provided in Example 3 as the negative electrode active material is 395.7 mAh / g, which characterizes the maximum sodium storage capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material. And A1 / A4 is 88.1%, which characterizes the capacity utilization of the sodium-ion battery prepared with hard carbon as the negative electrode active material, indicating that the proportion of micropore filling capacity in this hard carbon is relatively small, the actual available capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material is relatively high, and the initial Coulomb efficiency is relatively high. It is determined that the actual available capacity of the sodium-ion battery prepared with this hard carbon as the negative electrode active material meets the standard. A3 / A2 is 28.8%, which characterizes the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material, and it is determined that the rate performance of the sodium-ion battery prepared with this hard carbon as the negative electrode active material does not meet the standard. Further, according to Table 3, the initial Coulomb efficiency of the sodium-ion battery prepared with the hard carbon of Example 3 as the negative electrode active material is 83.0%, and the constant current ratio during 3C rate charging is 81.1%. The test results are consistent with those of the coin cell prepared with the hard carbon of Example 3 as the negative electrode active material.

[0110] According to the data in Table 1, during the determination of the coin cells prepared with the hard carbons provided in Examples 4 and 5 as raw materials, the discharge current in different voltage ranges was changed, and the calculated A1 / A4 and A3 / A2 changed slightly, but still met the determination criteria provided by the present invention.

[0111] Comparing Comparative Examples 1 to 3 with Examples 1 to 3, the same hard carbon was tested using the conventional coin cell evaluation method. For example, according to the test results of Comparative Example 1 and Comparative Example 3, the first Coulombic efficiency C2 / C1 of the coin cell prepared with the hard carbon of Comparative Example 3 as the negative electrode active material was better than that of Comparative Example 1. However, the first Coulombic efficiency of the sodium ion battery prepared with the hard carbon of Comparative Example 3 as the negative electrode active material was inferior to that of Comparative Example 1. Therefore, it is proved that the traditional evaluation method cannot accurately judge the capacity performance of hard carbon and there are certain errors, and the effective data provided for the full cell design is limited. The evaluation method provided by the present invention uses the capacity parameters of the coin cell prepared with hard carbon as the negative electrode active material in different voltage ranges to reflect the sodium storage performance of the sodium ion battery prepared with hard carbon as the negative electrode active material. Since the sodium storage rate of hard carbon varies greatly in different voltage ranges, by testing the capacity in different voltage ranges, the rate performance of the sodium ion battery prepared with hard carbon as the negative electrode active material can be accurately judged, providing necessary support data for the full cell application and quickly screening different types of hard carbon.

[0112] Obviously, the above examples are merely illustrations for clear explanation and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. The obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A method for evaluating negative electrode performance, characterized in that: The evaluation method comprises the following steps: S1. Preparing a button cell using hard carbon as the negative electrode active material; S2, performing a first charge and discharge test on the button battery in a voltage range of 0V to 1.5V to obtain a first discharge specific capacity A1; performing a second charge and discharge test in a voltage range of 0V to 0.8V to obtain a second discharge specific capacity A2; performing a third charge and discharge test in a voltage range of 0.05V to 0.8V to obtain a third discharge specific capacity A3; performing a fourth charge and discharge test in a voltage range of -0.005V to 1.5V to obtain a fourth discharge specific capacity A4; According to the first discharge specific capacity A1, the actual sodium storage and sodium removal capacity of the sodium ion battery prepared by using the hard carbon as the negative electrode active material is evaluated; According to the second discharge specific capacity A2, an actual sodium storage specific capacity of a sodium ion battery prepared by using the hard carbon as a negative electrode active material is evaluated; According to the third discharge specific capacity A3, the adsorption zone sodium storage specific capacity of the sodium ion battery prepared by using the hard carbon as the negative electrode active material is evaluated; The maximum value of the sodium storage capacity of the sodium ion battery prepared with the hard carbon as the negative electrode active material is evaluated according to the fourth discharge specific capacity A4.

2. The negative electrode performance evaluation method according to claim 1, characterized in that: In S2, the first charge and discharge test includes: (1) discharging to 0 V with a first current; (2) discharging to 0 V with a second current; (3) discharging to 0 V with a third current; (4) charging to 1.5 V with a first current; The first current is greater than the second current, and the second current is greater than the third current.

3. The negative electrode performance evaluation method according to claim 1, characterized in that: The second charge and discharge test includes: (1) discharging to 0 V with a first current; (2) discharging to 0 V with a second current; (3) discharging to 0 V with a third current; (4) charging to 0.8 V with a first current; The first current is greater than the second current, and the second current is greater than the third current.

4. The negative electrode performance evaluation method according to claim 1, characterized in that: The third charge and discharge test includes: (1) discharging to 0.05 V with a first current; (2) discharging with a second current to 0.05 V; (3) discharging with a third current to 0.05 V; (4) charging to 0.8 V with a first current; The first current is greater than the second current, and the second current is greater than the third current.

5. The negative electrode performance evaluation method according to claim 1, characterized in that: The third charge and discharge test includes: (1) discharging to -0.005 V with a first current; (2) discharging with a second current to -0.005 V; (3) discharging at a third current to -0.005 V; (4) charging to 1.5 V with a first current; The first current is greater than the second current, and the second current is greater than the third current.

6. The negative electrode performance evaluation method according to any one of claims 2 to 5, characterized in that: The first current is 0.01C to 0.2C, the second current is 0.01C to 0.1C, and the third current is 0.01C to 0.05C.

7. The negative electrode performance evaluation method according to claim 6, characterized in that: The first current is 0.1C, the second current is 0.05C, and the third current is 0.01C.

8. The negative electrode performance evaluation method according to claim 1, characterized in that: The evaluation method further includes: calculating the ratio of the first discharge specific capacity A1 to the fourth discharge specific capacity A4 as A1 / A4, calculating the ratio of the third discharge specific capacity A3 to the second discharge specific capacity A2 as A3 / A2, According to the A1 / A4, the capacity of the sodium ion battery prepared by using the hard carbon as the negative electrode active material is evaluated; The rate performance of the sodium ion battery prepared with the hard carbon as the negative electrode active material was evaluated based on the A3 / A2.

9. The negative electrode performance evaluation method according to claim 1, characterized in that: The precursor of the hard carbon includes at least one of biomass, high molecular polymer and fossil fuel.

10. The negative electrode performance evaluation method according to claim 1, characterized in that: In the active coating of the negative electrode sheet of the button cell, the mass fraction of the hard carbon is 94% to 97%.

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