Method for measuring sodium intercalation capacity of sodium-ion battery hard carbon negative electrode

Abstract

The application provides a method for measuring the sodium intercalation capacity of a sodium-ion battery hard carbon negative electrode, comprising the following steps: providing a plurality of test batteries with hard carbon electrode plates and sodium plates as positive and negative electrodes; using an electrochemical workstation to perform constant-current discharge on the plurality of test batteries, setting the cut-off gram capacity as the cut-off condition of the constant-current discharge process, so that each test battery corresponds to a cut-off gram capacity value, each cut-off gram capacity value is a known number, and the cut-off gram capacity values corresponding to the plurality of test batteries are not completely the same; performing electrochemical impedance spectroscopy testing on the plurality of discharged test batteries to obtain electrochemical impedance spectrograms corresponding to the plurality of test batteries one by one; comparing the radii of the second arc segments of each electrochemical impedance spectrogram corresponding to the plurality of test batteries and determining the maximum one; and taking the cut-off gram capacity of the test battery corresponding to the maximum one as the reference value of the actual gram capacity of the test battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery. Background Technology

[0002] With the advent of the era of electric vehicles and smart grids, the severe shortage of lithium resources has seriously constrained the development of lithium-ion batteries. Therefore, there is an urgent need to develop a next-generation energy storage battery system with excellent comprehensive performance. Sodium-ion batteries do not face the resource shortage problem faced by lithium-ion batteries, and their comprehensive performance has considerable room for improvement, making research on sodium-ion batteries increasingly important.

[0003] For sodium-ion batteries, hard carbon anodes have a disordered internal crystal arrangement and numerous pores; moreover, the interlayer structure, closed micropores, and surface defect sites can all contribute to sodium storage capacity, making them considered the most promising anode material for sodium-ion batteries. However, when evaluating the sodium intercalation capacity of hard carbon anodes in coin cell fabrication, the presence of polarization can lead to test results that are lower than the actual capacity.

[0004] How to solve the above problems and achieve effective measurement of the sodium intercalation capacity of hard carbon anodes in sodium-ion batteries is a question that needs to be considered by those skilled in the art. Summary of the Invention

[0005] To address the problems in the prior art, this application provides a method for determining the sodium intercalation capacity of a hard carbon anode in a sodium-ion battery, which can effectively measure the actual sodium intercalation capacity.

[0006] This application provides a method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery, comprising the following steps:

[0007] Multiple test batteries with hard carbon and sodium electrodes are provided.

[0008] An electrochemical workstation is used to perform constant current discharge on multiple test batteries. The cutoff specific capacity is set as the cutoff condition corresponding to the constant current discharge, so that each test battery has a corresponding cutoff specific capacity value. Each cutoff specific capacity value is a known number, and the cutoff specific capacity values ​​corresponding to multiple test batteries are not completely the same.

[0009] Electrochemical impedance spectroscopy was performed on multiple test batteries after discharge to obtain electrochemical impedance spectra corresponding to each of the multiple test batteries.

[0010] The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

[0011] In one embodiment, the step of comparing the radii corresponding to the second arc segment of each of the electrochemical impedance spectra corresponding to the plurality of test batteries and determining the largest one, and using the cutoff specific capacity of the test battery corresponding to the largest one as a reference value for the actual specific capacity of the test battery includes:

[0012] The test batteries are sorted from smallest to largest according to their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries and n is a natural number greater than 1, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively;

[0013] If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and X m If no sodium deposition was observed in the corresponding test battery, then the actual specific capacity of the test battery is determined to be within X. m The corresponding cutoff capacity and X m+1 The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

[0014] In one embodiment, the method further includes the following steps:

[0015] Multiple test batteries with hard carbon and sodium electrodes as positive and negative electrodes are provided again;

[0016] The cutoff gram capacity value corresponding to the multiple test batteries provided again is between X. m The corresponding cutoff capacity and X m+1 Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra.

[0017] The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

[0018] In one embodiment, the step of comparing the radii corresponding to the second arc segment of each of the electrochemical impedance spectra corresponding to the plurality of test batteries and determining the largest one, and using the cutoff specific capacity of the test battery corresponding to the largest one as a reference value for the actual specific capacity of the test battery includes:

[0019] The test batteries are sorted from smallest to largest according to their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries and n is a natural number greater than 2, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively;

[0020] If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and the occurrence of X m The corresponding radius is greater than X m-1 The corresponding radius, and X m If sodium deposition occurs in the corresponding test battery, then the actual specific capacity of the test battery is determined to be between X. m-1 The corresponding cutoff capacity and X m The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

[0021] In one embodiment, the method further includes the following steps:

[0022] Multiple test batteries with hard carbon and sodium electrodes as positive and negative electrodes are provided again;

[0023] The cutoff gram capacity value corresponding to the multiple test batteries provided again is between X. m-1 The corresponding cutoff capacity and X m Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra.

[0024] The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

[0025] In one embodiment, the cutoff capacity values ​​corresponding to the plurality of test batteries are all different.

[0026] In one embodiment, multiple test batteries are prepared using the same execution standard.

[0027] In one embodiment, when the multiple test batteries are subjected to constant current discharge using an electrochemical workstation, the constant current discharge current corresponding to the multiple test batteries is the same, the value of the constant current discharge current ranges from 50 μA to 200 μA, and the value of the cutoff specific capacity corresponding to the multiple test batteries ranges from 200 mAh / g to 350 mAh / g.

[0028] In one embodiment, the frequency range of the electrochemical impedance spectroscopy test is 10 mHz to 100 kHz, the amplitude is 10 mV, and the test voltage is the open-circuit voltage.

[0029] In one embodiment, the method further includes the following steps:

[0030] A constant current charge-discharge test was performed on multiple test batteries after electrochemical impedance spectroscopy testing. The voltage was set as the corresponding cutoff condition, and the charge-discharge efficiency of the multiple test batteries after the constant current charge-discharge test was calculated.

[0031] The method for determining the sodium intercalation capacity of the hard carbon anode in a sodium-ion battery disclosed in this application replaces the cutoff condition of constant current discharge from "zero voltage" to "reaching a predetermined specific capacity," thus avoiding premature test termination due to polarization of the test battery and obtaining a larger specific capacity value. Simultaneously, after the constant current discharge test, the electrochemical impedance spectroscopy of the battery is obtained, and combined with the analysis of sodium deposition within the battery, the method selects the one that is theoretically closest to the actual specific capacity of the test battery from among multiple cutoff specific capacities corresponding to various existing test batteries, and uses this measured specific capacity as a reference value for the actual specific capacity. The method for determining the sodium intercalation capacity of the hard carbon anode in a sodium-ion battery disclosed in this application effectively avoids the negative impact of battery polarization on the test, obtaining a more reliable reference value that is closer to the actual specific capacity. Attached Figure Description

[0032] Figure 1 Electrochemical impedance spectroscopy images of multiple test batteries corresponding to the sodium intercalation capacity determination method of the hard carbon negative electrode of sodium-ion battery provided in the embodiments of this application.

[0033] Figure 2 Comparison images of charge-discharge results between embodiments and comparative examples of the method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery provided in this application.

[0034] Figure 3 This is a schematic diagram illustrating the results of disassembling the test batteries corresponding to cutoff specific capacities of X1 to X7 in an embodiment of the method for determining the sodium intercalation capacity of the hard carbon negative electrode of a sodium-ion battery provided in this application, to observe whether sodium deposition occurs on the hard carbon electrode.

[0035] The following detailed description, in conjunction with the accompanying drawings, will further illustrate this application. Detailed Implementation

[0036] The following description will be given with reference to the accompanying drawings for a more complete description of the present application. The drawings illustrate exemplary embodiments of the present application. However, the present application may be implemented in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. These exemplary embodiments are provided to make the present application thorough and complete, and to fully convey the scope of the present application to those skilled in the art. Similar reference numerals denote the same or similar components. The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to limit the present application. As used herein, the singular forms “a,” “an,” and “the” are intended to also include the plural forms unless the context clearly indicates otherwise. Furthermore, when used herein, “comprising” and / or “including” and / or “having,” integers, steps, operations, components, and / or components, but without excluding the presence or addition of one or more other features, regions, integers, steps, operations, components, and / or groups thereof. Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. Furthermore, unless explicitly defined herein, terms such as those defined in general dictionaries should be interpreted as having the same meaning as they have in the relevant art and the content of this application, and will not be interpreted as having an idealized or overly formal meaning. Exemplary embodiments will be described below in conjunction with the accompanying drawings. It should be noted that components depicted with reference to the drawings are not necessarily shown to scale; and identical or similar components will be given the same or similar reference numerals or similar technical terms.

[0037] When evaluating the sodium intercalation capacity of the hard carbon negative electrode in coin cells, the presence of polarization can cause the measured capacity to be lower than the actual capacity. The sodium intercalation potential of the hard carbon negative electrode in sodium-ion batteries is around 0.05V, very close to the discharge cutoff voltage of 0V during battery testing. In the negative electrode half-cell, the sodium sheet serves as both the counter electrode and the reference electrode, leading to significant polarization in the coin cell. During discharge testing, due to this polarization, the instantaneous voltage drops below 0V at a certain moment, causing the test to be prematurely interrupted, ultimately resulting in inaccurate measurements of the hard carbon sodium intercalation capacity.

[0038] This application provides a method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery, comprising the following steps:

[0039] Multiple test batteries with hard carbon and sodium electrodes are provided.

[0040] An electrochemical workstation is used to perform constant current discharge on multiple test batteries. The cutoff specific capacity is set as the cutoff condition corresponding to the constant current discharge, so that each test battery has a corresponding cutoff specific capacity value. Each cutoff specific capacity value is a known number, and the cutoff specific capacity values ​​corresponding to multiple test batteries are not completely the same.

[0041] Electrochemical impedance spectroscopy was performed on multiple test batteries after discharge to obtain electrochemical impedance spectra corresponding to each of the multiple test batteries.

[0042] The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

[0043] The method for determining the sodium intercalation capacity of the hard carbon anode in a sodium-ion battery disclosed in this application replaces the cutoff condition of constant current discharge from "zero voltage" to "reaching a predetermined specific capacity," thus avoiding premature test termination due to polarization of the test battery and obtaining a larger specific capacity value. Simultaneously, after the constant current discharge test, the electrochemical impedance spectroscopy of the battery is obtained, and combined with the analysis of sodium deposition within the battery, the method selects the one that is theoretically closest to the actual specific capacity of the test battery from among multiple cutoff specific capacities corresponding to various existing test batteries, and uses this measured specific capacity as a reference value for the actual specific capacity. The method for determining the sodium intercalation capacity of the hard carbon anode in a sodium-ion battery disclosed in this application effectively avoids the negative impact of battery polarization on the test, obtaining a more reliable reference value that is closer to the actual specific capacity.

[0044] As will be understood by those skilled in the art, "specific capacity" refers to the ratio of the electrical capacity that the active material inside the battery can release to the mass of the active material. Specific capacity is usually expressed in milliampere-hours per gram (mA·h / g).

[0045] Those skilled in the art will understand that “electrochemical impedance spectroscopy” is also known as electrochemical impedance spectroscopy (EIS).

[0046] Those skilled in the art will understand that a "negative electrode half-cell," also known as a negative electrode material half-cell, refers to a battery in which only one type of negative electrode material is used, while the positive electrode is the electrolyte. The principle of a negative electrode half-cell is to form an electrolyte interface film on the surface of the negative electrode material, which can prevent reactions between the electrode and the electrolyte, thereby extending the battery life.

[0047] Those skilled in the art will understand that a "reference electrode (RE)" refers to an electrode used as a reference for comparison when measuring the potential of various electrodes.

[0048] Those skilled in the art will understand that the "counter electrode (CE)" is also known as the auxiliary electrode. This electrode and the working electrode form a circuit to ensure that the current in the working electrode is unimpeded, so as to ensure that the reaction under study occurs on the working electrode.

[0049] The specific embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0050] This application provides a method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery, comprising the following steps:

[0051] Step S1: Provide multiple test cells with hard carbon plates and sodium plates as positive and negative electrodes.

[0052] In one embodiment, hard carbon, SP, CMC adhesive, SBR, and deionized water are mixed and stirred, and then uniformly coated onto aluminum foil. The dried electrode sheets are then punched. A button cell is assembled in a glove box using hard carbon and sodium electrodes as the positive and negative electrodes. The proportions of hard carbon, SP, CMC adhesive, and SBR are 90% to 96%, 1% to 3%, 1% to 3%, and 1% to 2%, respectively. The electrode punching die is 10mm to 14mm. The button cell assembly sequence is: negative electrode shell, gasket, negative electrode sheet, separator, sodium electrode sheet, gasket, spring sheet, and positive electrode shell.

[0053] In other embodiments, the test battery may be a coin cell, which includes a negative electrode shell, a positive electrode shell, a separator, nickel foam, a sodium sheet, and a hard carbon negative electrode.

[0054] In one embodiment, multiple test batteries are prepared using the same execution standard, for example:

[0055] The method for preparing the test battery may include the following steps:

[0056] Hard carbon, conductive agent, binder and solvent are mixed to obtain slurry;

[0057] The slurry is coated onto the substrate surface to obtain the hard carbon negative electrode sheet.

[0058] Hard carbon, a conductive agent, a binder, and a solvent are mixed to obtain a slurry. The hard carbon can be hard carbon powder, and the average particle size of the hard carbon powder can be from 0.5 μm to 50 μm, or more specifically from 2 μm to 20 μm.

[0059] In this embodiment, the conductive agent may include conductive carbon black (SUPER P), Ketjen black, acetylene black, carbon nanotubes, or conductive agent KS 6, and may even be conductive carbon black.

[0060] In this embodiment, the adhesive may include one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and adhesive LA133 BP 7, or more specifically, one or two of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyvinylidene fluoride, or even a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber or polyvinylidene fluoride. In this embodiment, when the adhesive comprises two or more of the above-mentioned specific substances, this application does not have a special limitation on the ratio of the specific substances; any ratio can be used. In this embodiment, when the adhesive is a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber, the mass ratio of sodium carboxymethyl cellulose and styrene-butadiene rubber is 1:1. In this embodiment, the polyvinylidene fluoride may be PVDF 5130 or PVDF HSV900.

[0061] In this embodiment, the solvent may include ultrapure water or methylpyrrolidone. In this embodiment, when the solvent is ultrapure water, the slurry is an aqueous slurry; when the solvent is methylpyrrolidone, the slurry is an oil-based slurry.

[0062] In this embodiment, the mass ratio of the hard carbon fiber, conductive agent, and binder can be (80 to 97):(1 to 10):(2 to 10), or more preferably (88 to 94):(2 to 4):(4 to 8). Specifically, in this embodiment, the mass ratio of the hard carbon fiber, conductive agent, and binder is 94:2:4, 80:10:10, or 88:4:8.

[0063] In this embodiment, the solid content of the slurry can be 28% to 50%, or even 30% to 45%.

[0064] In this embodiment, the mixing can be carried out under stirring conditions. This application has no special requirements for the stirring, as long as the mixture can be mixed evenly.

[0065] After obtaining the slurry, this application coats the slurry onto the substrate surface to obtain the hard carbon negative electrode sheet. In this embodiment, the thickness of the coated slurry can be from 50 μm to 350 μm, and more specifically from 100 μm to 250 μm. This application does not have any particular limitation on the coating method; any coating method conventional in the art can be used.

[0066] In this embodiment, the process after coating may further include drying the substrate coated with the slurry. In this embodiment, the drying temperature may be 100°C to 150°C, or more preferably 105°C to 120°C; the drying time may be 1 hour to 4 hours, or more preferably 2 hours to 3 hours.

[0067] In this embodiment, the diaphragm may include a polypropylene film, a polyethylene film, or a PVDF ceramic-modified diaphragm, and may be a PVDF ceramic-modified diaphragm.

[0068] In this embodiment, the solvent may include carbonate solvents or carboxylic acid ester solvents, and more preferably carbonate solvents. In this embodiment, the carbonate solvent may include one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), and methyl propyl carbonate (MPC), and more preferably one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. In this embodiment, when the carbonate solvent includes two or more of the above-mentioned specific substances, this application does not have a special limitation on the ratio of the specific substances, and any ratio can be used. In the embodiments of this application, the carbonate solvent is a mixed solution of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, wherein the volume ratio of ethylene carbonate to propylene carbonate is 1:1, and the volume ratio of fluoroethylene carbonate to the total volume of ethylene carbonate and propylene carbonate is 5:100.

[0069] In this embodiment, the carboxylic acid ester solvent may include one or more of methyl formate (MF), methyl acetate (MA), methyl butyrate (MB), and ethyl propionate (EP), and may more specifically be methyl formate or methyl acetate. In this embodiment, when the carboxylic acid ester solvent includes two or more of the above-mentioned specific substances, this application does not have a special limitation on the ratio of the specific substances, and any ratio can be used.

[0070] Step S2: Use an electrochemical workstation to perform constant current discharge on multiple test batteries, set the cutoff specific capacity as the cutoff condition corresponding to the constant current discharge, so that each test battery has a corresponding cutoff specific capacity value, and each cutoff specific capacity value is a known number. The cutoff specific capacity values ​​corresponding to multiple test batteries are not completely the same.

[0071] In one embodiment, the cutoff capacity values ​​corresponding to the plurality of test batteries are all different.

[0072] In this embodiment, the cutoff gram capacity values ​​corresponding to the plurality of test batteries are arranged in an arithmetic sequence. The difference between the cutoff gram capacity values ​​corresponding to two adjacent test batteries in this sequence ranges from 10 mAh / g to 60 mAh / g, and can specifically be 15 mAh / g, 20 mAh / g, 25 mAh / g, 30 mAh / g, 35 mAh / g, 40 mAh / g, 45 mAh / g, 50 mAh / g, and 55 mAh / g.

[0073] In one embodiment, a predicted gram capacity is obtained by measuring the actual gram capacity of the test battery, such that at least one of the plurality of cutoff gram capacities falls within the range of the predicted gram capacity.

[0074] In one embodiment, the cutoff gram capacity values ​​corresponding to the plurality of test batteries range from 200 mAh / g to 350 mAh / g.

[0075] Step S3: Perform electrochemical impedance spectroscopy on the multiple test batteries after discharge to obtain electrochemical impedance spectra corresponding to each of the multiple test batteries.

[0076] In one embodiment, the frequency range of the electrochemical impedance spectroscopy test is 10 mHz to 100 kHz, the amplitude is 10 mV, and the test voltage is the open-circuit voltage.

[0077] In one embodiment, when the electrochemical workstation is used to perform constant current discharge on multiple test batteries, the constant current discharge current corresponding to each of the multiple test batteries is the same, and the value of the constant current discharge current ranges from 50 μA to 200 μA.

[0078] Step S4: Compare the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries and determine the largest one. Use the cutoff specific capacity of the test battery corresponding to the largest one as a reference value of the actual specific capacity of the test battery.

[0079] Those skilled in the art will understand that the electrochemical impedance spectroscopy (EIS) corresponding to each of the test cells generally includes two connected arc segments. The first arc segment is the one closer to the origin of the coordinate system, and its radius is highly correlated with the SEI film. The second arc segment is the one closest to the origin of the coordinate system, excluding the first arc segment, and its radius is generally directly related to the interfacial charge transfer resistance (Rct). In EIS testing, the interfacial charge transfer resistance Rct initially shows a gradual increasing trend. When the hard carbon electrode cannot accommodate more sodium ions for insertion, causing sodium ions to begin to precipitate in the form of metallic sodium, the interfacial charge transfer resistance decreases sharply, and correspondingly, the radius of the second arc segment begins to decrease significantly.

[0080] In one embodiment, step S4 includes:

[0081] Step S411: Sort the multiple test batteries in ascending order of their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries and n is a natural number greater than 1, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively.

[0082] Step S412: If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and X m If no sodium deposition was observed in the corresponding test battery, then the actual specific capacity of the test battery is determined to be within X. m The corresponding cutoff capacity and X m+1 The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

[0083] Step S413: Provide multiple test cells again, with hard carbon and sodium plates as positive and negative electrodes.

[0084] Step S414: Set the cutoff gram capacity value of the plurality of test batteries provided again to be between X. m The corresponding cutoff capacity and X m+1 Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra.

[0085] Step S415: Compare the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the plurality of test batteries and determine the largest one, and use the cutoff specific capacity of the test battery corresponding to the largest one as a reference value of the actual specific capacity of the test battery.

[0086] In one embodiment, step S4 includes:

[0087] Step S421: Sort the multiple test batteries in ascending order of their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries and n is a natural number greater than 2, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively.

[0088] Step S422: If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and the occurrence of X m The corresponding radius is greater than X m-1 The corresponding radius, and X m If sodium deposition occurs in the corresponding test battery, then the actual specific capacity of the test battery is determined to be between X. m-1 The corresponding cutoff capacity and X m The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

[0089] Step S423: Provide multiple test cells again, with hard carbon and sodium plates as positive and negative electrodes.

[0090] Step S424: Set the cutoff gram capacity value corresponding to the plurality of test batteries provided again to be between X. m-1 The corresponding cutoff capacity and X m Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra.

[0091] Step S425: Compare the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the plurality of test batteries and determine the largest one, and use the cutoff specific capacity of the test battery corresponding to the largest one as a reference value of the actual specific capacity of the test battery.

[0092] Those skilled in the art will understand that after step S415 or step S425, steps S411 to S415 or steps S421 to S425 may be repeated until a more accurate reference value for the required volume is obtained.

[0093] In one embodiment, the following steps may also be included:

[0094] Step S5: Perform constant current charge-discharge tests on multiple test batteries after electrochemical impedance spectroscopy testing, set the voltage as the corresponding cutoff condition, and calculate the charge-discharge efficiency of multiple test batteries after the constant current charge-discharge test.

[0095] As those skilled in the art will understand, the cutoff condition set in the method for determining the sodium intercalation capacity of the hard carbon anode of sodium-ion batteries in this application is "cutoff specific capacity." This means that there may be cases where the test battery deposits sodium and continues to discharge. In such cases, the measured value will be greater than the maximum sodium intercalation capacity of the hard carbon anode. Therefore, after performing electrochemical impedance spectroscopy, a constant current charge-discharge test should also be performed on the test battery, and the charge-discharge efficiency of the test battery should be calculated based on the test results. If the charge-discharge efficiency is within a reasonable range (e.g., greater than 80%, which can be estimated based on the type of material), then the reference value of specific capacity determined in step S4 is valid. If the charge-discharge efficiency is significantly lower than a reasonable range (e.g., only 50%), then a portion of the specific capacity is contributed by the test battery under sodium deposition conditions, and the corresponding reference value of specific capacity determined in step S4 may deviate significantly from the actual specific capacity.

[0096] Example

[0097] Step S1: Prepare multiple test batteries: Mix and stir hard carbon, SP, CMC adhesive, SBR, and deionized water, and coat evenly onto aluminum foil; punch holes in the dried electrode sheets; assemble button batteries in a glove box using hard carbon and sodium electrodes as positive and negative electrodes. The proportions of hard carbon, SP, CMC adhesive, and SBR are 90%–96%, 1%–3%, 1%–3%, and 1%–2%, respectively. The electrode punching mold is 10mm–14mm. The button battery assembly sequence is: negative electrode shell, gasket, negative electrode sheet, separator, sodium electrode sheet, gasket, spring sheet, positive electrode shell.

[0098] Step S2: Use an electrochemical workstation to perform constant current discharge on the 7 test batteries, and set the cutoff specific capacity as the cutoff condition corresponding to the constant current discharge, so that each test battery has a cutoff specific capacity value X1 to X7.

[0099] Among them, X1 corresponds to 200mAh / g, X2 corresponds to 250mAh / g, X3 corresponds to 270mAh / g, X4 corresponds to 290mAh / g, X5 corresponds to 300mAh / g, X6 corresponds to 350mAh / g, and X7 corresponds to 400mAh / g.

[0100] Step S3: Perform electrochemical impedance spectroscopy (EIS) on the seven test batteries after discharge to obtain EIS spectra corresponding to each of the seven test batteries, and then organize the data to obtain... Figure 1 .

[0101] The electrochemical impedance spectroscopy test has a frequency range of 10 mHz to 100 kHz and an amplitude of 10 mV, and the test voltage is the open-circuit voltage. When multiple test cells are subjected to constant current discharge using an electrochemical workstation, the constant current discharge current corresponding to each of the multiple test cells is the same, and the value of the constant current discharge current ranges from 50 μA to 200 μA.

[0102] Step S4: Compare the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the seven test cells and determine the largest one. The cutoff specific capacity of the test cell corresponding to X4 is obtained as a reference value for the actual specific capacity of the test cell.

[0103] Step S5: Perform a constant current charge-discharge test on the test battery corresponding to X4 after electrochemical impedance spectroscopy testing. Set the voltage as the corresponding cutoff condition, and calculate the charge-discharge efficiency of the test battery corresponding to X4 and the comparative ratio obtained from the experiment with a cutoff voltage of 0V. Summarize the results to obtain... Figure 2 .

[0104] Step S6: Disassemble the test batteries with the same cutoff capacity in a glove box and observe whether sodium deposition occurs on the hard carbon electrodes. The corresponding results are as follows: Figure 3 As shown.

[0105] Combination Figure 1 , Figure 2 and Figure 3 It can be seen that the actual specific capacity measured by the method for determining the sodium intercalation capacity of the hard carbon anode of sodium-ion batteries in this application is 290 mAh / g, which is much higher than the approximately 80 mAh / g of the comparative example, and closer to the predicted specific capacity of 290 mAh / g to 295 mAh / g. When the cutoff specific capacity is set to 290 mAh / g, no white sodium metal precipitation appears on the surface of the electrode after disassembly in the glove box. When the cutoff specific capacity is slightly higher, obvious sodium metal precipitation appears on the surface of the electrode. This proves that the method for determining the sodium intercalation capacity of the hard carbon anode of sodium-ion batteries in this application is indeed effective.

[0106] The specific embodiments of this application have been described above with reference to the accompanying drawings. However, those skilled in the art will understand that various changes and substitutions can be made to the specific embodiments of this application without departing from the spirit and scope of this application. All such changes and substitutions fall within the scope defined by this application.

Claims

1. A method for determining the sodium intercalation capacity of a hard carbon negative electrode in a sodium-ion battery, characterized in that, Includes the following steps: Multiple test batteries with hard carbon and sodium electrodes are provided. An electrochemical workstation is used to perform constant current discharge on multiple test batteries. The cutoff specific capacity is set as the cutoff condition corresponding to the constant current discharge, so that each test battery has a corresponding cutoff specific capacity value. Each cutoff specific capacity value is a known number, and the cutoff specific capacity values ​​corresponding to multiple test batteries are not completely the same. Electrochemical impedance spectroscopy was performed on multiple test batteries after discharge to obtain electrochemical impedance spectra corresponding to each of the multiple test batteries. The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

2. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, The step of comparing the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the plurality of test batteries and determining the largest one, and using the cutoff specific capacity of the test battery corresponding to the largest one as a reference value for the actual specific capacity of the test battery includes: The test batteries are sorted from smallest to largest according to their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries, and n is a natural number greater than 1, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively; If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and X m If no sodium deposition was observed in the corresponding test battery, then the actual specific capacity of the test battery is determined to be within X. m The corresponding cutoff capacity and X m+1 The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

3. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 2, characterized in that, It also includes the following steps: Multiple test batteries with hard carbon and sodium electrodes as positive and negative electrodes are provided again. The cutoff gram capacity value corresponding to the multiple test batteries provided again is between X. m The corresponding cutoff capacity and X m+1 Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra. The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

4. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, The step of comparing the radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the plurality of test batteries and determining the largest one, and using the cutoff specific capacity of the test battery corresponding to the largest one as a reference value for the actual specific capacity of the test battery includes: The test batteries are sorted from smallest to largest according to their cutoff capacity values, corresponding to X1 to X... n Where n is the total number of the various test batteries and n is a natural number greater than 2, for X1 to X... n The radii corresponding to the second arc segment of the electrochemical impedance spectrum are compared respectively; If X appears m The corresponding radius is greater than X m+1 The corresponding radius, and the occurrence of X m The corresponding radius is greater than X m-1 The corresponding radius, and X m If sodium deposition occurs in the corresponding test battery, then the actual specific capacity of the test battery is determined to be between X. m-1 The corresponding cutoff capacity and X m The corresponding cutoff capacity is between, where m is a natural number less than n and greater than 0.

5. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 4, characterized in that, It also includes the following steps: Multiple test batteries with hard carbon and sodium electrodes as positive and negative electrodes are provided again. The cutoff gram capacity value corresponding to the multiple test batteries provided again is between X. m-1 The corresponding cutoff capacity and X m Between the corresponding cutoff specific capacities, constant current charge-discharge and electrochemical impedance spectroscopy tests were performed again to obtain the corresponding electrochemical impedance spectra. The radii of the second arc segment of the electrochemical impedance spectrum corresponding to each of the multiple test batteries are compared and the largest one is determined. The cutoff specific capacity of the test battery corresponding to the largest one is used as a reference value of the actual specific capacity of the test battery.

6. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, The cutoff capacity values ​​corresponding to the various test batteries are all different.

7. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, All of the test batteries were prepared using the same execution standards.

8. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, When the multiple test batteries are subjected to constant current discharge using an electrochemical workstation, the constant current discharge current corresponding to the multiple test batteries is the same, the value of the constant current discharge current ranges from 50μA to 200μA, and the value of the cutoff specific capacity corresponding to the multiple test batteries ranges from 200mAh / g to 350mAh / g.

9. The method for determining the sodium intercalation capacity of the hard carbon negative electrode in a sodium-ion battery as described in claim 1, characterized in that, The frequency range of the electrochemical impedance spectroscopy test is 10 mHz to 100 kHz, the amplitude is 10 mV, and the test voltage is the open circuit voltage.

10. The method for determining the sodium intercalation capacity of the hard carbon negative electrode of a sodium-ion battery as described in any one of claims 1 to 9, characterized in that, It also includes the following steps: A constant current charge-discharge test was performed on multiple test batteries after electrochemical impedance spectroscopy testing. The voltage was set as the corresponding cutoff condition, and the charge-discharge efficiency of the multiple test batteries after the constant current charge-discharge test was calculated.

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