Method for testing cycle performance of hard carbon negative electrode material and application

Abstract

The invention discloses a method for testing the cycle performance of a hard carbon negative electrode material and application, and belongs to the technical field of sodium-ion batteries. The method for testing the cycle performance of the hard carbon negative electrode material comprises the following steps: S1, preparing a to-be-tested hard carbon material into a negative electrode plate, laminating a positive electrode plate, a diaphragm, the negative electrode plate and a reference electrode, and injecting an electrolyte to prepare a three-electrode battery; s2, discharging the three-electrode battery to a cut-off voltage at a current of 0.5-1 C, and then discharging the three-electrode battery to the cut-off voltage at a current of 0.01-0.1 C; and S3, charging the three-electrode battery, and recording the minimum potential value of the negative electrode relative to the reference electrode and the SOC of the corresponding battery when the potential of the negative electrode is zero. According to the test method provided by the invention, the three-electrode battery is used for testing, the reference electrode is introduced to monitor the real potential of the negative electrode in the charging process of the battery, and the cycle performance of the hard carbon negative electrode material is judged according to the lowest potential of the negative electrode, so that the test period is shortened, and the research and development time of the sodium ion battery is saved.

Description

Technical Field

[0001] The invention relates to a sodium ion battery, and in particular to a test method and application of the cycle performance of a hard carbon negative electrode material. Background Art

[0002] The biggest advantage of sodium-ion batteries over lithium-ion batteries is the widespread and low cost of sodium resources. Sodium is the sixth most abundant element in the earth's crust and is almost ubiquitous, while lithium resources are relatively limited and unevenly distributed. Therefore, sodium-ion batteries are not affected by raw material supply and price fluctuations. Due to the abundance of sodium, as well as the low-cost materials and technologies that may be used in the manufacturing process of sodium-ion batteries, this type of battery is expected to reduce the overall cost of energy storage systems, making large-scale energy storage more economically feasible. Sodium-ion batteries may have less impact on the environment than lithium-ion batteries during production and recycling because they may use fewer harmful chemicals. In addition, the widespread availability of sodium resources reduces environmental problems caused by mining rare metals.

[0003] In summary, the application of sodium-ion batteries can not only meet the energy storage needs in different scenarios, but also play an important role in promoting green and low-carbon economy and ensuring energy security. With the advancement of technology, sodium-ion batteries are expected to play an increasingly important role in the future energy structure. At present, the fastest commercialization progress is the sodium-based layered oxide system battery. During the cycle of the battery, the hard carbon negative electrode with poor power performance is often accompanied by sodium precipitation, which will lead to the accelerated decay of the battery capacity. At the same time, the precipitated sodium will react with the electrolyte to generate gas, causing safety hazards; especially in the late cycle, sodium precipitation will cause the battery capacity to plummet and be accompanied by severe bloating, which seriously affects the performance and safety of sodium-ion batteries. Therefore, it is very important to screen out hard carbon negative electrode materials with excellent power performance for the industrialization and safe use of sodium-ion batteries. Different manufacturers have different technical routes, and the physicochemical characteristics of the hard carbon negative electrode materials they produce are quite different. They often need to undergo physical property characterization, buckle electrochemical performance characterization, full battery preparation, and short-term and long-term electrochemical performance characterization to analyze their performance, which takes a long time.

[0004] Different hard carbon negative electrode materials have quite different performances. The traditional method of screening battery cycle performance requires preparing the hard carbon negative electrode material into a full battery and then testing the number of cycles. The number of cycles tested is as high as 600 or more, and the test period ranges from 3 to 12 months. The time cost of the cycle performance test of traditional hard carbon negative electrode materials is high, resulting in a long development cycle for the corresponding sodium-ion battery. Summary of the invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a test method for quickly evaluating the cycle performance of hard carbon negative electrode materials; another purpose of the present invention is to provide an application of the test method for the cycle performance of hard carbon negative electrode materials.

[0006] The present invention discloses a method for testing the cycle performance of a hard carbon negative electrode material, comprising the following steps: S1: Prepare the hard carbon material to be tested into a negative electrode sheet, stack the positive electrode sheet, separator, negative electrode sheet and reference electrode, inject electrolyte, and make a three-electrode battery; S2: discharging the three-electrode battery at a current of 0.5-1C to a cut-off voltage, and then discharging at a current of 0.01-0.1C to a cut-off voltage; S3: Charge the three-electrode battery and record the minimum potential value of the negative electrode relative to the reference electrode and the SOC (state of charge) of the battery when the negative electrode potential is zero.

[0007] A reference electrode is introduced to monitor the actual potential of the negative electrode of the battery during charging. If the minimum potential of the negative electrode reaches 0V or below, it can be determined that there is a risk of sodium precipitation. The smaller the potential value of the minimum potential of the negative electrode (vs the reference electrode), the higher the risk of sodium precipitation and the worse the cycle performance. The higher the SOC of the battery corresponding to a negative electrode potential of 0V, or the higher the minimum potential of the negative electrode, the better the performance of the hard carbon material.

[0008] The cut-off voltage refers to the voltage limit set in a specific application. When the device or battery reaches this voltage value during the charging or discharging process, the circuit will stop charging or discharging, usually to protect the normal operation of the battery or device, or to prevent the battery from over-discharging and overcharging. For batteries, the cut-off voltage usually refers to the voltage boundary set during the charging or discharging process. The cut-off voltage of the test can be selected according to the specific situation of the battery.

[0009] Furthermore, in step S1, the negative electrode plate includes a negative electrode current collector, a hard carbon material, a binder, and a conductive agent; measured by mass percentage, the hard carbon material is 90-95%, the binder is 1-5%, and the conductive agent is 1-5%.

[0010] The material of the negative electrode current collector can be selected from that used in conventional sodium ion batteries, preferably aluminum foil with a thickness of 8-16µm.

[0011] The binder and the conductive agent may be commercially available products used in conventional sodium ion batteries.

[0012] Furthermore, in step S1, the reference electrode in the three-electrode battery includes a conductive metal and sodium plated on the surface of the conductive metal.

[0013] Advantages of preparing reference electrodes by electroplating: Strong controllability: The electroplating method can accurately control the thickness and uniformity of the metal layer on the reference electrode surface. By adjusting parameters such as electroplating time, solution concentration, and current density, a high degree of control over the electrode surface morphology and composition can be achieved, thereby ensuring the stability and consistency of the reference electrode.

[0014] High purity and high uniformity: Electroplating can be used to uniformly deposit high-purity metal layers (such as silver, silver / silver chloride, copper, etc.) on the surface of metal substrates. This ensures that the chemical composition of the reference electrode surface is consistent, thereby improving the electrochemical performance of the electrode and reducing errors caused by impurities or uneven deposition.

[0015] Lower cost: Compared with other preparation methods (such as evaporation, sputtering, etc.), electroplating usually requires simpler equipment and operating conditions and lower costs, which is particularly suitable for large-scale production or laboratory-scale electrode preparation.

[0016] Easy to operate and implement: The electroplating method is easy to operate and suitable for laboratory environments. For experiments that require batch preparation of reference electrodes, the electroplating method provides a simple and efficient solution. In addition, different metal ion solutions can be selected during the electroplating process to achieve the deposition of different metals.

[0017] Adjustable electrode surface structure: Through the electroplating process, the surface roughness, crystal structure and other characteristics of the metal layer can be adjusted, thereby affecting the electrochemical stability of the reference electrode. For example, using different electroplating conditions, the current response characteristics of the reference electrode can be optimized to improve its reliability and accuracy in electrochemical measurements.

[0018] Good mechanical bonding: The electroplating method can firmly deposit the metal layer on the base material (such as gold, copper or stainless steel). This close bonding helps to improve the mechanical stability of the reference electrode, reduce the risk of the metal layer peeling or falling off, and ensure its long-term stable operation.

[0019] Furthermore, in step S1, the reference electrode is prepared by laminating a conductive metal as a reference electrode with a positive electrode sheet, a separator, and a negative electrode sheet to form a three-electrode battery, and then plating sodium on the surface of the conductive metal.

[0020] Since sodium metal is very active, a three-electrode battery is assembled using a conductive metal as a reference electrode, and then the reference electrode is plated with sodium, which not only makes the performance of the reference electrode better, but also makes it easier to operate and less expensive.

[0021] Furthermore, the sodium plating operation is: connecting the positive electrode plate of the three-electrode battery with a SOC of 40-60% and the reference electrode, and forwardly plating sodium for 1-2 hours at a current of 0.01-0.1 mA; connecting the negative electrode plate of the three-electrode battery and the reference battery, and reversely plating sodium for 1-2 hours at a current of 0.01-0.1 mA.

[0022] Furthermore, the diaphragm includes one or more of glass fiber, polypropylene, polyethylene, and polyethylene terephthalate.

[0023] Furthermore, it includes a positive electrode current collector, a layered oxide, a binder, and a conductive agent; measured by mass percentage, the layered oxide is 90-95%, the binder is 1-5%, and the conductive agent is 1-5%.

[0024] Layered oxides are usually composed of transition metals (such as nickel, manganese, cobalt, iron, etc.) and oxygen, and are doped with sodium ions. For example, NFM111 (NaFeMn) and NFM424 are two specific layered oxide materials.

[0025] The material of the positive electrode current collector can be selected from that used in conventional sodium ion batteries, preferably aluminum foil with a thickness of 8-16µm.

[0026] Furthermore, the electrolyte includes sodium hexafluorophosphate, vinyl acetate, ethyl methyl carbonate and fluoroethylene carbonate.

[0027] Furthermore, in step S3, a constant current is used when charging the three-electrode battery, and the constant current corresponds to a current value of 0.01C-2C.

[0028] According to the future usage scenarios of the battery, the appropriate charging current needs to be selected for testing, so that the test results are more in line with the performance requirements of the assembled sodium-ion battery, making the test results more accurate.

[0029] The present invention also discloses an application of a test method for the cycle performance of a hard carbon negative electrode material. For example, the test method for the electrochemical performance of a hard carbon negative electrode material described above is applied to screening hard carbon negative electrode materials.

[0030] The present invention provides a method for testing the electrochemical properties of a hard carbon negative electrode material. The method uses a three-electrode battery for testing, introduces a reference electrode to monitor the actual potential of the negative electrode of the battery during charging, and judges the overall electrical performance and cycle performance of the hard carbon negative electrode material based on the lowest potential of the negative electrode, thereby shortening the test cycle and saving battery research and development time. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 1 is a schematic diagram of the structure of a soft-pack three-electrode battery assembled in an embodiment of the present invention, wherein 1 is a positive electrode, 2 is a negative electrode, 3 is a reference electrode, and 4 is a reference electrode tab; Figure 2 1 is a graph showing the test results of samples No. 1 to No. 6 of Examples of the present invention; Figure 3 is a graph showing the full battery cycle test results of samples 1 to 6 in Comparative Example 2 of the present invention; Figure 4 This is a disassembled picture of the battery cells No. 2 and No. 4 after the full battery cycle test in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0032] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] The positive electrode sheet, negative electrode sheet and electrolyte of the present invention are specifically as follows: Method for preparing positive electrode sheet: Nickel iron manganese (molar ratio of 1:1:1) ternary layered oxide, conductive agent (conductive carbon black), and polyvinylidene fluoride are added to a stirring tank at a mass ratio of 95:2:3, and the stirring solvent is N-methylpyrrolidone (NMP). The mixture is stirred evenly to obtain a composite coated layered oxide positive electrode slurry. The positive electrode slurry is coated on the current collector aluminum foil, and is uniformly placed in a 90°C drying oven to dry and remove NMP. The rolled positive electrode is cut and rolled to obtain a layered oxide positive electrode sheet.

[0034] Method for preparing negative electrode sheet: 1kg of sample 1-6 hard carbon negative electrode material was added to a stirring tank with a conductive agent, polyvinylidene fluoride, and sodium carboxymethyl cellulose in a mass ratio of 95:2:2.5:0.5, and the stirring solvent was deionized water. The mixture was stirred evenly to obtain hard carbon negative electrode slurry. The negative electrode slurry was coated on the current collector aluminum foil, and uniformly entered the 100°C drying oven to dry and remove NMP. The rolled negative electrode was cut and rolled to obtain hard carbon negative electrode sheets 1-6.

[0035] Electrolyte: The sodium salt is 1 mol / L NaPF6, the solvent is ethylene carbonate and ethyl methyl carbonate (volume ratio 1:1), and the additive is 1% of the total mass of the electrolyte fluoroethylene carbonate.

[0036] Examples 1-6

[0037] The negative electrode sheets prepared from the hard carbon negative electrode materials No. 1-6 were subjected to the following operations: S1: The copper wire is soaked in 0.1 mol / L dilute hydrochloric acid to remove the surface copper oxide, rinsed with ethanol, and vacuum dried at 80°C for 30 min. The positive electrode sheet and the negative electrode sheet prepared according to the above method are stacked in the order of diaphragm-positive electrode sheet-diaphragm-negative electrode sheet-diaphragm-copper wire-diaphragm-positive electrode sheet-diaphragm (such as Figure 1 As shown), the positive and negative electrode sheets are the positive and negative electrode sheets of Examples 1 and 2, the separator is a 16μm polypropylene base film, the copper wire is soldered to the copper nickel-plated ear (reference electrode), and then a soft-pack three-electrode battery is obtained through packaging, electrolyte injection, formation, capacity division and aging processes; a soft-pack three-electrode battery with a SOC of 50% is taken, the positive electrode of the power supply is connected to the positive electrode of the battery cell, the negative electrode of the power supply is connected to the reference electrode, and sodium is plated in the forward direction for 2h at a current of 48μA; then the positive electrode of the power supply is connected to the negative electrode of the battery cell, the negative electrode of the power supply is connected to the reference electrode, and sodium is plated in the reverse direction for 2h at a current of 48μA to complete the sodium plating of the reference electrode.

[0038] S2: Discharge the three-electrode battery to 1.5V at a rate of 0.5C, and then discharge the battery to 1.5V at 0.1C to complete the initial discharge of the battery cell.

[0039] S3: Connect the positive electrode of the power supply to the positive electrode of the cell of the three-pole battery, the negative electrode of the power supply to the negative electrode of the cell of the three-pole battery, and the reference electrode to the reference electrode of the cell. Charge at a current of 0.5C, and record the minimum potential value of the negative electrode relative to the reference electrode (negative electrode sodium precipitation potential) and the SOC (state of charge) of the battery when the negative electrode potential is zero. The results are shown in Table 1 and Figure 2 shown.

[0040] Comparative Examples 1-6

[0041] The negative electrode sheets, positive electrode sheets and 16μm polypropylene base film separators made of hard carbon No. 1-6 were stacked in a Z-shaped stacking method, and then packaged, injected with electrolyte, formed, divided and aged to obtain soft-pack batteries, which were then tested for battery cycle performance. The test conditions were room temperature test environment, test voltage of 1.5-3.9V, charge and discharge current density of 0.5C, and the test results are shown in Table 1 and Figure 3 shown.

[0042] Table 1 Test results sample The battery SOC corresponding to the negative electrode when 0V Negative electrode sodium potential Full battery cycle number Full battery cycle capacity retention rate 1 43.42% -0.0207 506 81.2% 2 23.22% -0.0928 609 75.3% 3 45.91% -0.0154 607 91% 4 72.53% -0.0126 580 97.8% 5 43.97% -0.0193 506 85.1% 6 25.30% -0.0490 610 78.5% As shown in Table 1 and Figure 2 As shown, the three-electrode test results show that the lowest sodium precipitation potential of the negative electrode is sample No. 2, and the highest is sample No. 4. Therefore, the highest risk of sodium precipitation is sample No. 2, and the lowest is sample No. 4. At the same time, the electrical properties of hard carbon negative electrode materials from high to low are No. 4> No. 3> No. 5> No. 1> No. 6> No. 2.

[0043] As shown in Table 1 and Figure 3As shown, the full battery cycle performance test of samples 1-6 shows that the cycle performance from high to low is No. 4>No. 3>No. 5>No. 1>No. 6>No. 2.

[0044] After disassembling the soft pack battery after the full battery cycle test, Figure 4 As shown, the battery cell assembled with the No. 2 hard carbon negative electrode material exhibited sodium precipitation, while the battery cell assembled with the No. 4 hard carbon negative electrode material did not exhibit sodium precipitation. The results after the battery was fully cycled were consistent with the test results of the three-electrode battery test.

[0045] It can be seen that the test results obtained by the three-electrode battery are highly consistent with the results obtained by the full-battery cycle test, but the full-battery cycle test requires more than 600 cycles, which takes 3-12 months, while the test time of the three-electrode battery only takes 1-2 weeks. It can be seen that the test method provided by the present invention is more accurate than the full-battery cycle test, but can greatly save test time.

[0046] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for testing the cycle performance of a hard carbon negative electrode material, characterized in that: The following steps are involved: S1: Prepare the hard carbon material to be tested into a negative electrode sheet, stack the positive electrode sheet, separator, negative electrode sheet and reference electrode, inject electrolyte, and make a three-electrode battery; S2: discharging the three-electrode battery at a current of 0.5-1C to a cut-off voltage, and then discharging at a current of 0.01-0.1C to a cut-off voltage; S3: Charge the three-electrode battery and record the minimum potential value of the negative electrode relative to the reference electrode and the SOC of the battery corresponding to when the negative electrode potential is zero.

2. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: In the step S1, the negative electrode plate includes a negative electrode current collector, a hard carbon material, a binder, and a conductive agent; measured by mass percentage, the hard carbon material is 90-95%, the binder is 1-5%, and the conductive agent is 1-5%.

3. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: In step S1, the reference electrode in the three-electrode battery includes a conductive metal and sodium plated on the surface of the conductive metal.

4. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 3, characterized in that: In the step S1, the reference electrode is prepared by laminating a conductive metal as a reference electrode with a positive electrode sheet, a separator, and a negative electrode sheet to form a three-electrode battery, and then plating sodium on the surface of the conductive metal.

5. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 4, characterized in that: The sodium plating operation is: connecting the positive electrode plate of the three-electrode battery with a SOC of 40-60% and the reference electrode, and plating sodium in the forward direction for 1-2 hours at a current of 0.01-0.1 mA; connecting the negative electrode plate of the three-electrode battery and the reference battery, and plating sodium in the reverse direction for 1-2 hours at a current of 0.01-0.1 mA.

6. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: The separator includes one or more of glass fiber, polypropylene, polyethylene, and polyethylene terephthalate.

7. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: It includes a positive electrode current collector, a layered oxide, a binder, and a conductive agent; measured by mass percentage, the layered oxide is 90-95%, the binder is 1-5%, and the conductive agent is 1-5%.

8. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: The electrolyte includes sodium hexafluorophosphate, vinyl acetate, ethyl methyl carbonate and fluoroethylene carbonate.

9. The method for testing the cycle performance of a hard carbon negative electrode material according to claim 1, characterized in that: In step S3, a constant current is used when charging the three-electrode battery, and the constant current corresponds to a current value of 0.01C-2C.

10. Application of a method for testing the cycle performance of a hard carbon negative electrode material, characterized in that: The method for testing the electrochemical properties of the hard carbon negative electrode material as described in any one of claims 1 to 9 is used to screen hard carbon negative electrode materials.

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