Method for nondestructively detecting sodium precipitation of negative electrode of sodium-ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a method for nondestructively detecting sodium precipitation of a negative electrode of a sodium-ion battery. The method comprises the following steps: S1, preparing a sodium ion three-electrode battery cell by adopting a pole piece of a sodium ion battery cell to be detected; s2, adjusting the SOC of the sodium ion three-electrode battery cell, performing primary sodium plating on the negative electrode, and testing the electrochemical impedance spectrum of the negative electrode of the battery cell to obtain the charge transfer impedance of the negative electrode of the first battery cell; s3, carrying out charging and discharging tests on the battery cell subjected to primary sodium plating under different multiplying powers; s4, adjusting the SOC of the battery cell after the charging and discharging test, performing secondary sodium plating on the negative electrode, and testing the electrochemical impedance spectrum of the negative electrode of the battery cell to obtain the charge transfer impedance of the negative electrode of the second battery cell; and S5, according to the negative charge transfer impedance of the first battery cell and the second battery cell, obtaining the sodium precipitation condition of the battery cell during charging under different multiplying powers. The method can be used for non-destructively analyzing whether the sodium ion battery cell is subjected to sodium precipitation or not and the severity of sodium precipitation.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery. Background Art

[0002] Although lithium-ion batteries have a series of advantages such as high energy density and long cycle life, the global reserves of lithium resources are scarce (the relative abundance in the earth's crust is only 20 ppm). Moreover, with the increase in the application volume, lithium resources are becoming more and more expensive, and it is becoming increasingly difficult for lithium-ion batteries to meet people's needs for obtaining reliable, convenient, and stable energy at low cost. As an alkali metal element, sodium ion has similar chemical properties to lithium ion. Sodium element is inexpensive and widely distributed globally, and its development and utilization will not be restricted by factors such as cost and resource shortage. Therefore, sodium-ion batteries can be used as a supplement to lithium-ion batteries and are widely used in various application scenarios with low energy density requirements.

[0003] Graphite is mainly used as the negative electrode material for lithium-ion batteries. However, due to the small d002 layer spacing of graphite, it cannot be used in sodium-ion batteries. Therefore, the negative electrode material of sodium-ion batteries is mainly hard carbon at present. Compared with graphite, hard carbon materials have more irregularly shaped micropores / heteroatoms and a lower degree of graphitization. In addition, at high sodium intercalation state of charge (SOC), the diffusion coefficient of Na + in hard carbon is also lower than that of Li + in graphite. These reasons result in the much lower intrinsic electron and ion conductivity of hard carbon materials than that of graphite. When hard carbon is used as the main material in high-energy-density sodium-ion batteries, when the charging rate is high, sodium deposition is likely to occur in sodium-ion batteries, thus bringing performance and safety risks to the batteries. Therefore, for sodium-ion battery cells, how to quickly determine whether sodium deposition occurs in the cells during charging at different rates is of great significance for guiding the safe use of the cells.

[0004] At present, the method for judging whether sodium deposition occurs in sodium-ion batteries after charging at different rates is mainly to select a certain number of battery cells, number the cells, and then perform charge and discharge at different rates for a certain number of times (such as 10 - 20 times) at 25°C (or other temperatures). After the charge and discharge are completed, the fully charged cells are taken out of the cabinet, and the cells are disassembled in a drying room to check the negative electrode interface conditions to determine whether sodium deposition occurs during charging at different charging rates.

[0005] Although the above method can relatively intuitively judge whether sodium deposition occurs in sodium-ion batteries and the severity of sodium deposition, since it involves destructive disassembly of the battery cells, other useful information cannot be obtained, such as the degree of change in the storage characteristics and safety characteristics of the battery cells after sodium deposition compared with normal battery cells; regular change information such as the K value, gas generation, and calendar life of battery cells with different degrees of sodium deposition cannot be obtained, thus limiting the practical value of the battery cells.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The object of the present invention is to provide a method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery, which can non-destructively analyze whether sodium deposition occurs on the sodium-ion battery cell and the severity of sodium deposition by measuring the charge transfer impedance of the negative electrode using electrochemical impedance spectroscopy.

[0008] In order to achieve the above object of the present invention, the following technical solutions are specifically adopted:

[0009] The present invention provides a method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery, comprising the following steps:

[0010] S1. Prepare a sodium-ion three-electrode battery cell using the electrode sheet of the sodium-ion battery cell to be tested;

[0011] S2. Adjust the SOC of the sodium-ion three-electrode battery cell, perform primary sodium plating on the negative electrode, and obtain a first battery cell; measure the electrochemical impedance spectroscopy of the negative electrode of the first battery cell to obtain the charge transfer impedance of the negative electrode of the first battery cell;

[0012] S3. Perform charge and discharge tests on the first battery cell at different rates;

[0013] S4. Adjust the SOC of the first battery cell after the charge and discharge test, perform secondary sodium plating on the negative electrode, and obtain a second battery cell; measure the electrochemical impedance spectroscopy of the negative electrode of the second battery cell to obtain the charge transfer impedance of the negative electrode of the second battery cell;

[0014] S5. Obtain the sodium deposition situation of the negative electrode of the sodium-ion battery cell to be tested during charging at different rates according to the charge transfer impedance of the negative electrode of the first battery cell and the charge transfer impedance of the negative electrode of the second battery cell.

[0015] Further, in step S1, the sodium-ion three-electrode battery cell is prepared using the positive electrode sheet of the sodium-ion battery cell to be tested, the negative electrode sheet of the sodium-ion battery cell to be tested, and the treated copper wire.

[0016] Further, in step S1, the copper wire is immersed in concentrated sulfuric acid for 0.5 - 5.0 h, and after being washed with water and dried in sequence, the treated copper wire is obtained.

[0017] Further, in step S1, the diameter of the copper wire is 30 - 200 μm, and the length is 10 - 300 mm.

[0018] Further, in step S2, the SOC of the sodium-ion three-electrode battery cell is adjusted to 5% - 100% SOC, wherein the characteristic value is 50% SOC.

[0019] Further, in step S2, the test temperature of the electrochemical impedance spectrum of the negative electrode of the first battery cell is 25°C.

[0020] Further, in step S3, the charge-discharge temperature of the charge-discharge test is 25°C, and the number of charge-discharge cycles is 50 weeks.

[0021] Further, in step S4, the SOC of the first battery cell after the charge-discharge test is adjusted to 5% - 100% SOC, where the characteristic value is 50% SOC.

[0022] Further, in step S4, the test temperature of the electrochemical impedance spectrum of the negative electrode of the second battery cell is 25°C.

[0023] Further, in step S5, when the charge transfer impedance of the negative electrode of the second battery cell is greater than that of the negative electrode of the first battery cell, no sodium deposition occurs on the negative electrode of the sodium-ion battery cell to be tested; when the charge transfer impedance of the negative electrode of the second battery cell is less than that of the negative electrode of the first battery cell, sodium deposition occurs on the negative electrode of the sodium-ion battery cell to be tested.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] The method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery according to the present invention can, by comparing and analyzing the change rules of the charge transfer impedance of the negative electrode of the battery cell after charge and discharge at different rates, determine whether sodium deposition occurs on the negative electrode of the battery cell and the severity of sodium deposition without disassembling the battery cell; this method not only avoids disassembling the battery cell but also does not interfere with the acquisition of other useful information; at the same time, it is also possible to explore the charging capacity boundary of the battery cell based on this method to obtain the maximum available charging rate without sodium deposition on the negative electrode, thereby guiding the safe use of sodium-ion battery cells and showing good practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0027] Figure 1 It is the electrochemical impedance spectrum of the negative electrodes of a fresh three-electrode battery cell, Group A battery cells, and Group B battery cells in Embodiment 1 of the present invention.

[0028] Figure 2 It is the surface states of the negative electrodes of Group A battery cells (A) and Group B battery cells (B) after the charge-discharge test in Embodiment 1 of the present invention.

[0029] Figure 3The negative electrode electrochemical impedance spectra of the fresh three-electrode cell, Group A cells, Group B cells, Group C cells, and Group D cells in Example 2 of the present invention.

[0030] Figure 4 The surface states of the negative electrodes of Group A cells (A), Group B cells (B), Group C cells (C), and Group D cells (D) after charge-discharge testing in Example 2 of the present invention. Detailed implementation manners

[0031] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific implementation manners. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention. Conditions not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments not specified by the manufacturer can be obtained as conventional products through commercial purchase.

[0032] A method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery in an embodiment of the present invention will be specifically described below.

[0033] In some embodiments of the present invention, a method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery is provided, including the following steps:

[0034] S1. Prepare a sodium-ion three-electrode cell using the electrode sheet of the sodium-ion cell to be tested;

[0035] S2. Adjust the SOC of the sodium-ion three-electrode cell, perform primary sodium plating on the negative electrode, and obtain a first cell; test the negative electrode electrochemical impedance spectrum (EIS) of the first cell to obtain the negative electrode charge transfer impedance of the first cell;

[0036] S3. Perform charge-discharge testing on the first cell at different rates;

[0037] S4. Adjust the SOC of the first cell after charge-discharge testing, perform secondary sodium plating on the negative electrode, and obtain a second cell; test the negative electrode electrochemical impedance spectrum (EIS) of the second cell to obtain the negative electrode charge transfer impedance of the second cell;

[0038] S5. Obtain the sodium deposition situation on the negative electrode of the sodium-ion cell to be tested during charging at different rates according to the negative electrode charge transfer impedance of the first cell and the negative electrode charge transfer impedance of the second cell.

[0039] When hard carbon is used as the anode material of a sodium-ion battery, different sodium intercalation mechanisms will be exhibited at different electrode potentials of the anode. When the anode electrode potential > 0.1 V, at this time, hard carbon stores sodium by surface adsorption and shows a certain capacity externally. In this stage, due to the relatively high electrode potential, the problem of sodium precipitation is not likely to occur; as the amount of sodium intercalated in the anode increases, the anode electrode potential gradually decreases. When its electrode potential < 0.1 V, sodium ions gradually intercalate and microporous deposit in the graphite domain of hard carbon to form a NaC x compound. In this stage, due to the relatively low anode electrode potential and the charging polarization at high rates resulting in a relatively low actual anode electrode potential, when the anode electrode potential reaches the sodium precipitation potential, sodium precipitation occurs at the anode.

[0040] When charging a sodium-ion battery at a low rate (low current density), the migration amount of Na + in the bulk of hard carbon is higher than the amount of Na transferred to the surface of hard carbon + quantity. At this time, because the Na + on the surface of hard carbon is "consumed" in real time and will not accumulate on the surface of hard carbon, so the problem of sodium precipitation will not occur; as the charging rate gradually increases, the content of Na + transferred to the surface of hard carbon gradually increases, while the transfer coefficient of Na + from the surface of hard carbon to the bulk remains unchanged. When the charging rate is higher than a certain specific value, the amount of Na + transferred to the surface of hard carbon is higher than the amount of Na + transferred to the bulk of hard carbon, Na + will accumulate on the surface of hard carbon. And as the charging rate increases, the charging polarization received by the anode also gradually increases. When the polarization potential reaches the sodium precipitation potential, the Na + on the surface of hard carbon will precipitate into Na metal and deposit on the anode surface, manifested as sodium precipitation at the anode.

[0041] When sodium precipitation occurs at the anode, the charging of the battery cell does not stop. The Na + migrating to the anode surface through the electrolyte, part of it continues to intercalate into hard carbon, manifested as the capacity of the battery cell; another part of Na + will deposit and precipitate on the surface of hard carbon. This part does not show the capacity of the battery cell due to the loss of active sodium; during this process, the ohmic impedance of the electrolyte / separator, etc. and the transfer impedance of SEI basically remain unchanged, while the charge transfer impedance decreases because a part of Na + is transformed into Na metal, and the higher the proportion of metallic Na deposited on the anode surface in the active Na, the more obvious the decrease in the charge transfer impedance; therefore, by using electrochemical impedance spectroscopy (EIS) to test the charge transfer impedance of the battery cell after charge and discharge at different rates, it is possible to non-destructively analyze whether sodium precipitation occurs in the battery cell and the severity of sodium precipitation.

[0042] The present invention provides a method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery. A sodium-ion three-electrode battery cell is prepared using the electrode sheet of the sodium-ion battery cell to be tested. After the battery cell is fabricated, the state of charge (SOC) of the battery cell is adjusted. After sodium plating on the negative electrode, the charge transfer impedance of the negative electrode is measured using electrochemical impedance spectroscopy (EIS) (at 50% SOC). After the measurement, the battery cell is charged and discharged at different rates. After the charge and discharge test, the battery cell is adjusted to the SOC and sodium plating is performed on the negative electrode again. The charge transfer impedance of the negative electrode of the sodium-plated battery cell is measured again using EIS. By comparing the differences in the charge transfer impedance between the two measurements, it is analyzed whether sodium is deposited inside the battery cell.

[0043] Through the above method, a qualitative judgment can be obtained on whether sodium is deposited and the severity of sodium deposition in the battery cell after charge and discharge at different rates without performing destructive disassembly on the sodium-ion battery cell. Moreover, by measuring the charge transfer impedance of the battery cell using EIS, it will not affect the internal state of the battery cell and can maintain the internal state of the battery cell without interfering with the acquisition of other information. This method is a simple, convenient, and non-destructive non-destructive analysis method that avoids destructive disassembly of the battery cell and does not affect the acquisition of other useful information. In addition, through the analysis of the change in the charge transfer impedance, this method can also be used quantitatively to explore the charging capacity boundary of the battery cell, that is, to study at what rate the battery cell with a given design can be charged and discharged without sodium deposition problems, thereby guiding the safe use of the battery cell.

[0044] In some embodiments of the present invention, in step S1, a sodium-ion three-electrode battery cell is prepared using the positive electrode sheet of the sodium-ion battery cell to be tested, the negative electrode sheet of the sodium-ion battery cell to be tested, and the treated copper wire.

[0045] In some embodiments of the present invention, in step S1, the copper wire is immersed in concentrated sulfuric acid for 0.5 - 5.0 h, and after washing with water and drying in sequence, the treated copper wire is obtained; preferably, the copper wire is immersed in concentrated sulfuric acid for 2 h.

[0046] In some embodiments of the present invention, in step S1, the diameter of the copper wire is 30 - 200 μm and the length is 10 - 300 mm; if the diameter of the copper wire is too thin, the specific surface area is small, and it is not easy for metallic sodium to adhere to the copper wire during sodium plating, resulting in the failure of the reference electrode and the loss of reference value of the test result; at the same time, the processability is poor, and the copper wire is easy to break and not easy to process; if the diameter of the copper wire is too thick, it will cause poor encapsulation, and the copper wire needs to be led out from the inside of the battery cell to the outside; preferably, the diameter of the copper wire is 90 - 110 μm and the length is 140 - 160 mm.

[0047] In some embodiments of the present invention, in step S1, the homogenized positive electrode slurry (for example, the positive electrode material in the positive electrode slurry is a sodium-ion layered oxide positive electrode material) is coated on the aluminum foil, and after drying, rolling, and die-cutting in sequence, the positive electrode sheet is obtained;

[0048] The homogenized anode slurry (the anode material in the anode slurry is hard carbon material) is coated on the aluminum foil, and then dried, roll-pressed and die-cut in sequence to obtain the anode sheet.

[0049] A number of copper wires (for example, with a diameter of 100 μm and a length of 150 mm) are immersed in concentrated sulfuric acid for 0.5 - 5.0 h (for example, 2 h). After the immersion, they are washed with deionized water and dried to obtain the treated copper wires.

[0050] The above-mentioned positive electrode sheet, negative electrode sheet and treated copper wires are used to prepare a sodium-ion three-electrode battery cell. After preparation, the sodium-ion three-electrode battery cell is transferred normally, and then baked, injected with electrolyte, and pre-charged and formed.

[0051] In some embodiments of the present invention, in step S2, the SOC of the sodium-ion three-electrode battery cell is adjusted to 5% - 100% SOC, where the characteristic value is 50% SOC.

[0052] In some embodiments of the present invention, in step S2, the test temperature of the electrochemical impedance spectrum of the negative electrode of the first battery cell is 25°C.

[0053] In some embodiments of the present invention, in step S2, sodium-ion three-electrode battery cells with similar capacity, liquid retention, internal resistance and thickness are selected. After adjusting to 50% SOC, the negative electrode is plated with sodium once, and then the charge transfer resistance of the negative electrode at 50% SOC is tested by EIS at 25°C, and the EIS is fitted with Zview software to obtain the charge transfer impedance value of the negative electrode of the first battery cell.

[0054] In some embodiments of the present invention, in step S3, the charge-discharge test temperature is 25°C and the number of charge-discharge cycles is 50 cycles (cls). The first battery cell is grouped, and the first battery cell is tested for charge and discharge 50 times at 25°C with different rates.

[0055] In some embodiments of the present invention, in step S4, the SOC of the first battery cell after the charge-discharge test is adjusted to 5% - 100% SOC, where the characteristic value is 50% SOC.

[0056] In some embodiments of the present invention, in step S4, the test temperature of the electrochemical impedance spectrum of the negative electrode of the second battery cell is 25°C.

[0057] In some embodiments of the present invention, in step S4, after the charge-discharge test is completed, the first battery cell is adjusted to 50% SOC and its negative electrode is re-plated with sodium, and the charge transfer impedance of the negative electrode of the battery cell after plating with sodium is tested twice by EIS.

[0058] In some embodiments of the present invention, in step S5, when the negative electrode charge transfer impedance of the second battery cell is greater than that of the first battery cell, no sodium deposition occurs at the negative electrode of the sodium ion battery cell to be tested; when the negative electrode charge transfer impedance of the second battery cell is less than that of the first battery cell, sodium deposition occurs at the negative electrode of the sodium ion battery cell to be tested.

[0059] In some embodiments of the present invention, in step S5, when the negative electrode charge transfer impedance of the second battery cell is less than that of the first battery cell, the smaller the negative electrode charge transfer impedance of the second battery cell, the higher the degree of sodium deposition at the negative electrode.

[0060] In some embodiments of the present invention, a method for non-destructively detecting sodium deposition at the negative electrode of a sodium ion battery specifically includes the following steps:

[0061] S1. Prepare a sodium ion three-electrode battery cell using the positive electrode and negative electrode of the sodium ion battery cell to be tested (a certain given design battery cell) and the treated copper wire.

[0062] S2. Select sodium ion three-electrode battery cells with similar capacity, liquid retention, internal resistance, and thickness; adjust the SOC of the sodium ion three-electrode battery cells to 50%, and after performing primary sodium plating on the negative electrode, obtain the first battery cell (fresh three-electrode battery cell); test the EIS of the first battery cell (fresh three-electrode battery cell, all battery cells are tested, adding parallel samples and data persuasiveness) at 25°C to obtain the negative electrode charge transfer impedance of the first battery cell (fresh three-electrode battery cell).

[0063] S3. Divide the first battery cell (fresh three-electrode battery cell) into groups, and each group of battery cells (4 - 5 battery cells in each group, through the test of parallel samples, adding persuasiveness) are respectively subjected to charge and discharge tests at 25°C for 50 weeks at different rates.

[0064] S4. After the charge and discharge cycle test is completed, adjust the SOC of each group of battery cells to 50%, perform secondary sodium plating on the negative electrode, and then perform EIS tests on each group of battery cells after secondary sodium plating at 25°C to obtain the negative electrode charge transfer impedance of each group of battery cells after charge and discharge tests at different rates.

[0065] S5. For a certain given design battery cell, compared with the fresh three-electrode battery cell, for each group of battery cells that have undergone charge and discharge at different rates for 50 weeks, when the negative electrode charge transfer impedance increases, it indicates that no sodium deposition occurs at the negative electrode; when the negative electrode charge transfer impedance decreases, it indicates that sodium deposition occurs at the negative electrode, and the greater the degree of decrease in the negative electrode charge transfer impedance, the greater the degree of sodium deposition.

[0066] Existing methods for detecting sodium deposition at the negative electrode of sodium-ion batteries include: identifying whether sodium deposition occurs in the battery cell through the peak heat generation, determining whether sodium deposition occurs in the battery based on the expansion force of the battery and the differential curve of the capacity, detecting the Coulomb efficiency during each charge and discharge process of the battery cell by using a high-precision test instrument, and identifying whether sodium deposition occurs in the battery cell through a sudden decrease in the Coulomb efficiency. For sodium-ion batteries, the above methods are not only cumbersome and unintuitive, but also require the use of expensive equipment to analyze the relevant parts of the battery cell. They not only have a high cost and are difficult to adopt on a large scale, but also cannot be used as a quantifiable method to explore and quantitatively analyze the charging capacity boundary of the battery cell of a given design system. However, for the method of the present invention, only by making the positive and negative electrode sheets of the sodium-ion battery cell into a three-electrode battery cell and testing the change of the charge transfer impedance of the negative electrode of the three-electrode battery cell through EIS, not only can a qualitative determination be made on whether sodium deposition occurs in a large number of battery cells after charge and discharge at different rates, but also the degree of sodium deposition in different battery cells can be identified based on the change amount of the charge transfer impedance after charge and discharge. In addition, through the test of the charge transfer impedance of the negative electrode of the battery cell after charge and discharge at different rates, the charging boundary of the sodium-ion battery of a given chemical system and the maximum charging rate without sodium deposition can also be quantified, thereby providing a good basis for guiding the safe use of the battery cell.

[0067] Example 1

[0068] The method for non-destructively detecting sodium deposition at the negative electrode of a sodium-ion battery provided in this example includes the following steps:

[0069] S1. Coating the homogenized positive electrode slurry (the positive electrode material in the positive electrode slurry is a sodium-based layered oxide positive electrode material) on the aluminum foil, and sequentially drying, rolling, and die-cutting to obtain a positive electrode sheet;

[0070] Coating the homogenized negative electrode slurry (the negative electrode material in the negative electrode slurry is a hard carbon material) on the aluminum foil, and sequentially drying, rolling, and die-cutting to obtain a negative electrode sheet;

[0071] Soaking several copper wires with a diameter of 100 μm and a length of 150 mm in concentrated sulfuric acid for 2 h. After the soaking is completed, washing with deionized water and drying to obtain the treated copper wires;

[0072] Preparing a three-electrode battery cell using the above positive electrode sheet, negative electrode sheet, and treated copper wires; after the preparation is completed, the three-electrode battery cell is normally transferred, and the battery cell is baked, injected with electrolyte, and pre-charged and formed;

[0073] S2. Selecting three-electrode battery cells with similar capacity, liquid retention, internal resistance, and thickness, adjusting to 50% SOC, and then performing a single sodium plating on the negative electrode to obtain fresh three-electrode battery cells; testing the EIS of the negative electrode of the fresh three-electrode battery cells at 25 °C, and fitting the EIS with Zview software to obtain the charge transfer impedance value of the negative electrode of the fresh three-electrode battery cells;

[0074] S3. Randomly divide the fresh three - electrode cells into two groups, namely Group A and Group B. Among them, the cells in Group A are charged and discharged for 50 weeks at 25°C with a charge - discharge rate of 0.8C / 1C and a voltage range of 1.5 - 3.95V, and the cells in Group B are charged and discharged for 50 weeks at 25°C with a charge - discharge rate of 1.5C / 1C and a voltage range of 1.5 - 3.95V.

[0075] S4. After the charge - discharge of the cells in Group A and Group B is completed, adjust the cells to 50% SOC, then perform secondary sodium plating on the negative electrode. At 25°C, perform EIS tests on the negative electrodes of the two groups of cells after sodium plating, and use Zview software to fit the EIS to obtain the negative - electrode charge - transfer impedance value of the cells in Group A and the negative - electrode charge - transfer impedance value of the cells in Group B respectively.

[0076] S5. The electrochemical impedance spectra of the negative electrodes of the fresh three - electrode cells, the cells in Group A, and the cells in Group B are as Figure 1 shown. Analyze the results of the negative - electrode charge - transfer impedance values before and after 50 - week charge - discharge. The results are shown in Table 1. Compared with the fresh three - electrode cells, for the cells in Group A and Group B that have undergone charge - discharge at different rates for 50 weeks, their negative - electrode ohmic impedance and the transfer impedance of SEI basically remain unchanged, while the charge - transfer impedance shows obvious changes. Compared with the fresh three - electrode cells, among them, the negative - electrode charge - transfer impedance of the cells in Group A slightly increases after 50 - week charge - discharge, indicating that charging the cells at 0.8C will not cause sodium deposition on the negative - electrode surface, and the increase in impedance is caused by the increase in side reactions on the negative electrode during the charge - discharge process; while the negative - electrode charge - transfer impedance of the cells in Group B shows a significant decrease after 50 - week charge - discharge, indicating that after charging at 1.5C, sodium + precipitation occurs on the negative - electrode surface of the sodium - ion cells. Since a part of the Na + conducted to the negative - electrode surface is deposited as sodium metal, the negative - electrode charge - transfer impedance decreases.

[0077] Table 1

[0078] Negative charge transfer impedance (mohm) Fresh three-electrode cell 28.74 Group A cells were charged and discharged at 0.8C / 1C for 50 cycles 29.49 Group B cells were charged and discharged at 1.5C / 1C for 50 cycles 23.71

[0079] To verify the reliability of the above - mentioned method, recharge the cells in Group A and Group B that have completed the EIS test to 100% SOC, disassemble the two groups of cells in a drying room, and check the sodium precipitation situation at the negative - electrode interface; the surface morphology of the negative electrode is as Figure 2 shown.

[0080] The disassembly results of the two groups of cells show that there is no sodium precipitation on the negative - electrode surface of the cells in Group A after charge - discharge at 25°C, while the sodium precipitation on the negative - electrode surface of the cells in Group B is relatively serious. The measured results are consistent with the analysis of the change in the negative - electrode charge - transfer impedance of the sodium - ion three - electrode cells, indicating that through the test and analysis of the negative - electrode charge - transfer impedance of the cells after charge - discharge tests at different rates using EIS, it can meet the requirements of non - destructive detection of whether sodium precipitation occurs in the cells after high - rate charge - discharge.

[0081] Example 2

[0082] The method for non-destructively detecting sodium deposition on the negative electrode of a sodium-ion battery provided in this embodiment includes the following steps:

[0083] S1. Coating the homogenized positive electrode slurry (the positive electrode material in the positive electrode slurry is a sodium-ion layered oxide positive electrode material) on aluminum foil, and successively drying, rolling, and die-cutting to obtain a positive electrode sheet;

[0084] Coating the homogenized negative electrode slurry (the negative electrode material in the negative electrode slurry is a hard carbon material) on aluminum foil, and successively drying, rolling, and die-cutting to obtain a negative electrode sheet;

[0085] Soaking several copper wires with a diameter of 100 μm and a length of 150 mm in concentrated sulfuric acid for 2 h. After soaking, washing with deionized water and drying to obtain the treated copper wires;

[0086] Preparing a three-electrode battery cell using the above positive electrode sheet, negative electrode sheet, and treated copper wires; after preparation, the three-electrode battery cell is transferred normally, and the battery cell is baked, injected with electrolyte, and pre-charged and formed;

[0087] S2. Selecting three-electrode battery cells with similar capacity, liquid retention, internal resistance, and thickness, adjusting them to 50% SOC, and then performing a first sodium plating on the negative electrode to obtain fresh three-electrode battery cells; testing the EIS of the negative electrode of the fresh three-electrode battery cells at 25 °C, and fitting the EIS with Zview software to obtain the negative electrode charge transfer impedance value of the fresh three-electrode battery cells;

[0088] S3. Randomly dividing the fresh three-electrode battery cells into four groups: A, B, C, and D; among them, the battery cells in group A are charged and discharged for 50 cycles at 25 °C, 0.8C / 1C, 1.5 - 4.0 V, the battery cells in group B are charged and discharged for 50 cycles at 25 °C, 1.0C / 1C, 1.5 - 4.0 V, the battery cells in group C are charged and discharged for 50 cycles at 25 °C, 1.2C / 1C, 1.5 - 4.0 V, and the battery cells in group D are charged and discharged for 50 cycles at 25 °C, 1.5C / 1C, 1.5 - 4.0 V;

[0089] S4. After the battery cells in groups A, B, C, and D are charged and discharged, adjusting the battery cells to 50% SOC, and then performing a second sodium plating on the negative electrode. At 25 °C, performing an EIS test on the negative electrodes of the four groups of battery cells after sodium plating, and using Zview software to fit the EIS to obtain the negative electrode charge transfer impedance values of the battery cells in group A, group B, group C, and group D respectively;

[0090] S5. The negative electrode electrochemical impedance spectra of the fresh three-electrode battery cells, the battery cells in group A, group B, group C, and group D are as follows Figure 3As shown; the results of analyzing the negative electrode charge transfer impedance values before and after 50 - cycle charge - discharge are shown in Table 2. Compared with the fresh three - electrode battery cells, after 50 - cycle charge - discharge, the negative electrode charge transfer impedances of both Group A and Group B battery cells increase, indicating that charging with these two charging rates will not cause sodium deposition problems in the battery cells. After 50 - cycle charge - discharge, the negative electrode charge transfer impedances of both Group C and Group D battery cells decrease, and with the increase of the charging rate, the decrease of the negative electrode charge transfer impedance is more obvious. The results show that both Group C and Group D battery cells have sodium deposition after 50 - cycle charge - discharge, and the sodium deposition in Group C battery cells is less serious than that in Group D battery cells.

[0091] Table 2

[0092]

[0093] To verify the above conclusion, the four groups of battery cells A, B, C, and D that have completed the EIS test are charged to 100% SOC and disassembled in a drying room to check the sodium deposition situation at the negative electrode interface. The detailed results are shown in Figure 4 as follows:

[0094] The disassembly results of the four groups of battery cells show that it can be seen that there is no sodium deposition in both Group A and Group B battery cells after 50 - cycle charge - discharge tests, while sodium deposition appears in both Group C and Group D battery cells, and the sodium deposition in Group D is more serious. Judging from the negative electrode charge transfer impedance results tested by EIS, the decrease in the charge transfer impedance of Group D battery cells is the largest, so the sodium deposition is also the most serious. From the perspective of the negative electrode charge transfer impedance, for this designed battery cell, the maximum charging rate should be < 1.2C. At this time, slight sodium deposition will occur in the battery cell. If the rate is further increased, the sodium deposition in the battery cell will be more serious, which may pose a safety risk in use. This result is also confirmed by the actual disassembly results of the battery cells.

[0095] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for non-destructive detection of sodium precipitation in the negative electrode of a sodium ion battery, characterized in that: The following steps are involved: S1. Prepare a sodium ion three-electrode cell using the electrode piece of the sodium ion cell to be tested; S2, adjusting the SOC of the sodium ion three-electrode battery cell, performing sodium plating on the negative electrode once, and obtaining a first battery cell; testing the electrochemical impedance spectrum of the negative electrode of the first battery cell, and obtaining the charge transfer impedance of the negative electrode of the first battery cell; S3, performing charge and discharge tests on the first battery cell at different rates; S4, adjusting the SOC of the first battery cell after the charge and discharge test, performing a secondary sodium plating on the negative electrode to obtain a second battery cell; testing the electrochemical impedance spectrum of the negative electrode of the second battery cell to obtain the charge transfer impedance of the negative electrode of the second battery cell; S5. According to the charge transfer impedance of the negative electrode of the first battery cell and the charge transfer impedance of the negative electrode of the second battery cell, the sodium precipitation condition of the negative electrode of the sodium ion battery cell to be tested when charging at different rates is obtained.

2. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S1, the sodium ion three-electrode battery cell is prepared by using the positive electrode sheet of the sodium ion battery cell to be tested, the negative electrode sheet of the sodium ion battery cell to be tested and the treated copper wire.

3. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 2, characterized in that, In step S1, the copper wire is immersed in concentrated sulfuric acid for 0.5 to 5.0 hours, and then washed and dried in sequence to obtain the treated copper wire.

4. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 3, characterized in that, In step S1, the copper wire has a diameter of 30 to 200 μm and a length of 10 to 300 mm.

5. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that, In step S2, the SOC of the sodium ion three-electrode battery cell is adjusted to 5% to 100% SOC, wherein the characteristic value is 50% SOC.

6. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S2, the test temperature of the electrochemical impedance spectroscopy of the negative electrode of the first battery cell is 25°C.

7. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S3, the charge and discharge temperature of the charge and discharge test is 25° C., and the number of charge and discharge cycles is 50 cycles.

8. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S4, the SOC of the first battery cell after the charge and discharge test is adjusted to 5% to 100% SOC, wherein the characteristic value is 50% SOC.

9. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S4, the test temperature of the electrochemical impedance spectroscopy of the negative electrode of the second battery cell is 25°C.

10. The method for nondestructive testing of sodium ion battery negative electrode sodium precipitation according to claim 1, characterized in that: In step S5, when the charge transfer impedance of the negative electrode of the second battery cell is greater than the charge transfer impedance of the negative electrode of the first battery cell, no sodium precipitation occurs at the negative electrode of the sodium ion battery cell to be tested; when the charge transfer impedance of the negative electrode of the second battery cell is less than the charge transfer impedance of the negative electrode of the first battery cell, sodium precipitation occurs at the negative electrode of the sodium ion battery cell to be tested.

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