Method for determining the content of electrolyte additives

Abstract

The application discloses a method for determining the content of an electrolyte additive, which comprises the following steps: S100, performing formation treatment on a plurality of battery cells containing the same additive with different additive contents to obtain a battery; S200, performing a floating test on the battery obtained in the step S100 to obtain the real-time thickness of the battery in a set number of days, and obtaining the thickness change rate of the battery in the set number of days; S300, obtaining the slope value of the thickness change rate according to adjacent n thickness change rates; S400, obtaining the thickness inflection point value of the battery according to the slope value of the thickness change rate; and S500, determining the content of the electrolyte additive according to the thickness inflection point value to improve the floating performance. The application establishes the relationship between the floating performance of the battery and the content of the additive by using the thickness inflection point value, and the optimal content of the electrolyte additive for improving the floating performance of the battery can be quickly and accurately obtained.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a method for determining the content of electrolyte additives. Background Technology

[0002] Float charging is a continuous, long-term process that uses a small current to slowly increase the depth of charge of a battery at the end of its charging phase, or to compensate for self-discharge losses caused by prolonged inactivity, bringing the battery to a fully charged state. Currently, there is no scientific and effective method to evaluate the effectiveness of float charging in improving battery performance. The only visual assessment is that a later appearance of the float thickness inflection point indicates better float charging performance. However, this visual identification method has the following drawbacks: First, inconsistent standards for manually identifying the inflection point can interfere with the identification of thickness inflection point samples. Second, in existing experimental techniques, testing continues even after the float thickness inflection point appears until the battery thickness change rate reaches a predetermined failure threshold, resulting in high experimental costs for lithium-ion battery float charging and hindering batch processing, thus affecting sample detection efficiency. Finally, current technology cannot establish a relationship between float charging performance and additive content, therefore it is impossible to determine the optimal electrolyte additive content for improving float charging performance. Summary of the Invention

[0003] In view of this, this application provides a method for determining the content of electrolyte additives, which can obtain the optimal content of electrolyte additives that is beneficial to improving float charging performance.

[0004] Firstly, this application provides a method for determining the content of electrolyte additives. The method includes the following steps: S100: performing formation treatment on multiple battery cells containing the same additive but with different additive contents to obtain batteries with different additive contents; S200: performing a float charge test on the batteries obtained in step S100 to obtain the real-time thickness of the batteries during the float charge test and to obtain the thickness change rate of the batteries; S300: obtaining the slope value of the battery thickness change rate based on n adjacent thickness change rates; S400: obtaining the thickness inflection point value of the batteries based on the slope value of the battery thickness change rate; S500: determining the electrolyte additive content based on the battery thickness inflection point value to improve the battery float charge performance. This application analyzes the thickness change rate curve during the float charge process of lithium-ion batteries, confirms that the slope values ​​of a suitable number of adjacent thickness change rates can be used as parameters to identify the thickness inflection point, and then uses the thickness inflection point value to establish the relationship between battery float charge performance and electrolyte additives, which can quickly and accurately obtain the optimal content of electrolyte additives to improve battery float charge performance.

[0005] In some embodiments, the mass percentage of the additive is 0.01 to 10 wt% based on the mass of the electrolyte. Exemplarily, the mass percentage of the additive is 0.01 wt%, 0.1 wt%, 1 wt%, 2 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, or a range of any two of the above values.

[0006] In some embodiments, the method further includes obtaining the initial state thickness of the battery before step 200. The initial state thickness is the thickness of the finished battery, which is prepared by winding positive and negative electrode sheets after slitting, with the positive and negative electrode sheets separated by a separator, thereby obtaining a wound bare cell. The bare cell is then subjected to top-side sealing, inkjet printing, vacuum drying, electrolyte injection, high-temperature settling, and formation and capacity determination to obtain the finished battery.

[0007] In some embodiments, step S200 includes: before the start of the float charge test process: placing the battery in an environment of 30-60°C for 20-30 minutes, charging it at a constant current to the rated voltage of the battery with the battery calibration current, switching the battery to constant voltage charging, stopping charging when the current reaches 0.03-00.05C, letting it stand for 8-10 minutes, and then placing the battery in an environment of 22-28°C for 1-2 hours to obtain the thickness of the battery on day 0 of float charge.

[0008] Preferably, obtaining the thickness of the battery on day 0 of float charging includes: placing the battery in a 45°C environment for 30 minutes, charging it at a constant current to the battery's rated voltage, switching the battery to constant voltage charging, stopping charging when the current reaches 0.05C, placing it in a 25°C environment for 1 hour, and obtaining the thickness of the battery on day 0 of float charging.

[0009] In some embodiments, in step S200, the float charge test process includes: placing the battery in an environment of 30–60°C for 20–30 minutes, discharging the battery to 2.5–3.0V with a current of 0.1–1.5C, placing it in the environment for 8–10 minutes, charging it to the battery's rated voltage using the battery's calibrated current, and then switching the battery to constant voltage charging to perform the float charge test. The purpose of charging and discharging the battery before the float charge test is to ensure that the battery is in a fully charged state.

[0010] Preferably, the battery is left to stand in an environment of 45°C for 30 minutes, discharged to 2.5-3.0V with a current of 0.5C, left to stand for 10 minutes, charged to the rated voltage of the battery with constant current at the battery's calibrated current, and then the battery is switched to constant voltage charging for float charging test.

[0011] In some embodiments, step S200 includes: pre-setting a battery thickness testing frequency based on different cathode systems, with the start day of the float charge test recorded as day 1, and obtaining the real-time battery thickness based on the battery thickness testing frequency, specifically including:

[0012] For lithium-ion batteries with a lithium cobalt oxide cathode system, the real-time thickness of the battery is acquired every 5 to 8 days. For example, in some examples, the real-time thickness of the battery is acquired on day 0, day 5, day 10, ... during float charging; in other examples, the real-time thickness is acquired on day 0, day 7, day 14, ... during float charging. It should be noted that the above are merely illustrative examples, and the specific interval is not limited by this application, as long as it meets the above range.

[0013] For lithium-ion batteries with a lithium iron phosphate cathode system, the battery thickness is measured every 30 days from day 1 to day 60 of the float charge test, and then every 8 to 15 days thereafter. For example, in some examples, the real-time battery thickness is measured on float charge days 0, 30, 60, 70, ...; in other examples, it is measured on float charge days 0, 30, 60, 75, ... . It should be noted that the above are merely examples, and the interval after day 60 of the float charge test can be chosen freely, as long as it meets the above range; this application does not impose any restrictions.

[0014] For lithium-ion batteries with a ternary cathode system, the battery thickness is measured every 30 days from day 1 to day 90 of the float charge test, and then every 8 to 15 days thereafter. For example, for lithium-ion batteries with a ternary cathode system, in some examples, the real-time battery thickness is measured on float charge days 0, 30, 90, 98, ...; in other examples, it is measured on float charge days 0, 30, 90, 115, ... . It should be noted that the above are merely examples, and the interval after day 90 of the float charge can be chosen freely, as long as it meets the above range; this application does not impose any restrictions.

[0015] In some embodiments, in step S200, the thickness change rate is obtained according to formula I; thickness change rate % = [(real-time thickness of the battery / initial state thickness) - 1] * 100% Formula I.

[0016] In some embodiments, in step S300, obtaining the slope value of the battery thickness change rate based on n adjacent thickness change rates includes: using a preset number of days as the abscissa and the thickness change rate as the ordinate, creating a linear standard curve based on n adjacent thickness change rates to obtain the slope value of the thickness change rate; recording the first slope value obtained as the first slope value of the battery; and recording the slope value as k, where k is obtained according to Equation II.

[0017]

[0018] Where x is the preset number of days, and y is the battery thickness change rate corresponding to the preset number of days. The average of n preset days. It is the average of n adjacent thickness change rates.

[0019] In some embodiments, in step 400, obtaining the battery thickness inflection point value based on the slope value of the battery thickness change rate includes: dividing all real-time obtained slope values ​​other than the first slope value by the first slope value; when the ratio is higher than a predetermined value M, the corresponding preset number of days is determined as the battery thickness inflection point value. The value of M ranges from 2 to 8. For lithium-ion batteries with a lithium cobalt oxide cathode, the predetermined value M is 3; for lithium-ion batteries with a lithium iron phosphate cathode, the predetermined value M is 5; and for lithium-ion batteries with a ternary cathode, the predetermined value M is 6.

[0020] In some embodiments, in step S500, determining the electrolyte additive content based on the battery thickness inflection point value includes: plotting a linear standard relationship curve between the electrolyte additive content and the corresponding thickness inflection point value, with the electrolyte additive content as the abscissa and the thickness inflection point value as the ordinate. The content on the abscissa corresponding to the highest point on the ordinate of the linear standard relationship curve is the optimal electrolyte additive content that can improve the battery's float charging performance. That is, the electrolyte content at this position can achieve the appearance of the thickness inflection point at the latest possible thickness days; the later the thickness inflection point appears, the better the battery's float charging performance. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the thickness change rate curves of lithium cobalt oxide batteries with different additive contents in Example 1 of this application;

[0023] Figure 2 This is a schematic diagram of the thickness change rate curve of the lithium cobalt oxide battery group B4 in Example 1 of this application;

[0024] Figure 3 for Figure 2 A schematic diagram of the slope value of the corresponding thickness change rate of lithium cobalt oxide battery;

[0025] Figure 4 This is a schematic diagram of the curves showing the relationship between different additive contents and thickness inflection point values ​​in Example 1 of this application;

[0026] Figure 5 This is a schematic diagram of the thickness change rate curves of lithium cobalt oxide batteries with different additive contents in Example 2 of this application;

[0027] Figure 6 This is a schematic diagram of the curves showing the relationship between different additive contents and thickness inflection point values ​​in Example 2 of this application.

[0028] Figure 7 This is a schematic diagram of the thickness change rate curves of lithium cobalt oxide batteries with different additive contents in Example 3 of this application;

[0029] Figure 8 This is a schematic diagram showing the curves of different additive contents and thickness inflection point values ​​in Example 3 of this application;

[0030] Figure 9 This is a schematic diagram of the thickness change rate curves of lithium cobalt oxide batteries with different additive contents in Example 4 of this application;

[0031] Figure 10 This is a schematic diagram of the curves showing the relationship between different additive contents and thickness inflection points in Example 4 of this application. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0033] Implementation Example Design:

[0034] Example 1

[0035] (I) Lithium-ion batteries

[0036] Preparation of positive electrode sheet

[0037] Aluminum foil is used as the positive electrode current collector. A layer of lithium cobalt oxide slurry is uniformly coated on the surface of the aluminum foil. The lithium cobalt oxide slurry includes 97.8 wt% LiCoO2 (LCO), 0.8 wt% polyvinylidene fluoride (PVDF) and 1.4 wt% conductive carbon black. The positive electrode sheet is then cold-pressed to prepare the positive electrode sheet.

[0038] Preparation of negative electrode sheet

[0039] A copper foil is used as the negative electrode current collector. A layer of graphite slurry is uniformly coated on the surface of the copper foil. The graphite slurry includes 97.7 wt% artificial graphite, 1.3 wt% carboxymethyl cellulose (CMC) and 1.0 wt% styrene-butadiene rubber (SBR). The negative electrode sheet is then prepared by cold pressing.

[0040] Preparation of the separating membrane

[0041] A 9μm thick porous polyethylene (PE) membrane was selected as the separator.

[0042] Preparation of electrolyte

[0043] In a dry argon-atmospheric glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 6:34:20:40. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvents to dissolve and mix thoroughly. Lithium difluorooxalate borate (LiDFOB, electrolyte additive) was then added to obtain the electrolyte. The electrolyte contained 12% LiPF6, 2% LiDFOB, and the remainder was the mass percentage of the organic solvents. Specifically, EC comprised 5.2% of the electrolyte, PC 29.2%, EMC 17.2%, and DEC 34.4%.

[0044] Preparation of lithium-ion batteries

[0045] The positive and negative electrode sheets are slit and wound together, with the separator separating them to prepare multiple wound bare cells. These cells are then subjected to top-side sealing, inkjet printing, vacuum drying, injection of different concentrations of LiDFOB (electrolyte additive), high-temperature settling, and formation and capacity testing to obtain multiple finished batteries. The different concentrations of LiDFOB are 0%, 1%, 2%, 3%, and 4%, and the resulting batteries are numbered B0, B1, B2, B3, and B4 respectively. Float charge testing is then performed.

[0046] (II) Battery Float Charge Test

[0047] Test method: (1) Use a thickness tester to test the thickness and record the initial thickness of the battery; (2) Place the battery in a high-temperature chamber and let it stand in a 45°C environment for 30 minutes. Charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.45V (rated voltage). Then switch the battery to constant voltage charging. Stop charging when the current reaches 0.05C. Let the battery stand for 10 minutes. Remove the battery and use a thickness tester to test the thickness. Record this as the thickness of the battery on day 0 of float charging; (3) Place the battery back in the high-temperature chamber and charge it in a 45°C environment. After standing for 30 minutes, discharge the battery to 3.0V with a current of 0.5C, stand for 10 minutes, and then charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.45V (rated voltage). Then switch the battery to constant voltage charging to start float charging; (4) Test the thickness every 7 days until the battery thickness change rate is >40%. Obtain the thickness value corresponding to each 7 days of float charging. Take the thickness of full charge as the reference thickness. Thickness change rate % = [(thickness at each point / initial state thickness) - 1] * 100%. By analogy, multiple battery thickness change rate curves are obtained. See Figure 1 .from Figure 1 As can be seen, the float charge thickness growth rate varies with the LiDFOB concentration. Using the slope of the float charge thickness growth rate, the inflection point value of the float charge thickness for LiDFOB batteries with different concentrations can be calculated. Figure 1 .

[0048] Specifically, the calculation process uses B4 as an example (the calculation process for other battery thickness inflection point values ​​is the same as for B4), see [link to relevant documentation]. Figure 2 and Figure 3 The slope value of the thickness change rate is calculated using the first three points of the thickness change rate. For example, the slope value is calculated using the thickness change rates at points 0, 7, and 14 as the first slope value. The slope value is calculated using the thickness change rates at points 7, 14, and 21 as the second slope value (i.e., other slope values ​​obtained in real time). This process is repeated to obtain the slope value change curve. The calculation process of the slope value k is shown in Table 1. The calculation methods of the following embodiments can all refer to the calculation process of Embodiment 1, and will not be repeated here.

[0049] Table 1

[0050]

[0051]

[0052] Among them, 1#, 2#, and 3# are three sets of parallel samples, x is the preset number of days, and y is the average thickness change rate of the three sets of parallel samples corresponding to the preset number of days. The average of n preset days. It is the average of the average thickness change rates of n adjacent values.

[0053] For example, when x = 14, y = (3.50% + 3.31% + 3.48%) / 3 = 3.43%. Substitute into the formula: That is, k = 0.00058.

[0054] from Figure 3 As can be seen, with the increase of float charging days, the thickness change rate initially increases linearly and slowly. Using the least squares formula, the slope value at the first point can be calculated to be 0.00058. As the float charging days continue, the thickness change rate increases significantly and rapidly. When float charging reaches 70 days, the slope value is 0.00196. For lithium-ion batteries with a lithium cobalt oxide cathode system, with a limit value M of 3, 0.00196 / 0.00058 = 3.38 > 3. Therefore, it can be determined that the float charging thickness reaches an inflection point. Figure 2 It can also be clearly seen that the battery thickness inflection point occurs after 70 days of float charging.

[0055] See Figure 4 Plotting different concentrations of LiDFOB on the x-axis and the inflection point value of float thickness on the y-axis, we obtain... Figure 4 A linear standard curve showing the relationship between different concentrations of LiDFOB and their corresponding thickness inflection points. From... Figure 4 As the LiDFOB concentration increases, the number of days corresponding to the inflection point of float charging thickness first increases and then decreases; it is evident that the B2 group of batteries shows the thickness inflection point after 112 days of float charging, making it the battery with the latest appearance of the float charging thickness inflection point. Figure 4 The optimal concentration of electrolyte additive (at the highest point in the linear standard curve) indicates the best float charging performance of the battery, meaning the electrolyte additive content is 2.0%. It should be noted that the optimal concentration of electrolyte additive remains consistent for different lithium salts (all other conditions being equal), and this applies hereinafter.

[0056] Example 2

[0057] (I) Lithium-ion batteries

[0058] Preparation of positive electrode sheet

[0059] Aluminum foil is used as the positive electrode current collector. A layer of lithium cobalt oxide slurry is uniformly coated on the surface of the aluminum foil. The lithium cobalt oxide slurry includes 97.8 wt% LiCoO2 (LCO), 0.8 wt% polyvinylidene fluoride (PVDF) and 1.4 wt% conductive carbon black. The positive electrode sheet is then cold-pressed to prepare the positive electrode sheet.

[0060] Preparation of negative electrode sheet

[0061] A copper foil is used as the negative electrode current collector. A layer of graphite slurry is uniformly coated on the surface of the copper foil. The graphite slurry includes 97.7 wt% artificial graphite, 1.3 wt% carboxymethyl cellulose (CMC) and 1.0 wt% styrene-butadiene rubber (SBR). The negative electrode sheet is then prepared by cold pressing.

[0062] Preparation of the separating membrane

[0063] A 9μm thick porous polyethylene (PE) membrane was selected as the separator.

[0064] Preparation of electrolyte

[0065] In a dry argon-atmospheric glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 6:34:20:40. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvents and dissolved and mixed thoroughly. Lithium difluorophosphate (LiPO2F2, an electrolyte additive) was then added to obtain the electrolyte. The electrolyte contained 13.0% LiPF6, 1.0% LiPO2F2, and the remainder was the mass percentage of the organic solvents. Specifically, EC comprised 5.2% of the electrolyte, PC 29.2%, EMC 17.2%, and DEC 34.4%.

[0066] Preparation of lithium-ion batteries

[0067] The positive and negative electrode sheets are slit and wound together, with the separator separating them to prepare multiple wound bare cells. These cells are then top-side sealed, inkjet-coded, vacuum-dried, injected with different concentrations of LiPO2F2 (electrolyte additive), and subjected to high-temperature settling before formation and capacity testing to obtain multiple finished batteries. The different concentrations of LiPO2F2 are 0%, 0.1%, 0.3%, 0.6%, 0.9%, and 1.0%, respectively. The resulting batteries are numbered B-0, B-1, B-2, B-3, B-4, and B-5. Float charge testing is then performed.

[0068] (II) Battery Float Charge Test

[0069] Test method: (1) Use a thickness tester to test the thickness and record the initial thickness of the battery; (2) Place the battery in a high-temperature chamber and let it stand in a 40°C environment for 30 minutes. Charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.25V (rated voltage). Then switch the battery to constant voltage charging. Stop charging when the current reaches 0.05C. Let the battery stand for 10 minutes. Remove the battery and use a thickness tester to test the thickness. Record this as the thickness of the battery on day 0 of float charging; (3) Place the battery back in the high-temperature chamber and let it stand in a 40°C environment. After standing for 30 minutes, discharge the battery to 2.8V with a current of 0.5C, stand for 10 minutes, and then charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.25V (rated voltage). Then switch the battery to constant voltage charging to start float charging; (4) Test the thickness every 8 days until the battery thickness change rate is >40%. Obtain the thickness value corresponding to each 8 days of float charging. Take the thickness of full charge as the base thickness. Thickness change rate % = [(thickness at each point / initial state thickness) - 1] * 100%. By analogy, multiple battery thickness change rate curves are obtained. See Figure 5 .from Figure 5 As can be seen, the float charge thickness growth rate varies with the change of LiPO2F2 concentration. Using the slope value of the float charge thickness growth rate, the thickness inflection point value of float charge for LiPO2F2 batteries with different concentrations can be calculated. The calculation process can refer to the calculation process of B4 in Example 1, which will not be repeated here.

[0070] See Figure 6 Plotting different concentrations of LiPO2F2 on the x-axis and the inflection point value of the float thickness on the y-axis, we obtain... Figure 6 A linear standard curve showing the relationship between different concentrations of LiPO2F2 and the corresponding inflection point values. From... Figure 6 As the concentration of LiPO2F2 increases, the number of days corresponding to the inflection point of float charge thickness first increases and then decreases. It can be clearly seen that the thickness inflection point of the B-3 group of batteries occurs after 288 days of float charging, which is the battery with the latest occurrence of the float charge thickness inflection point. That is, when the electrolyte additive content is 0.6%, the battery float charging performance is the best.

[0071] Example 3

[0072] (I) Lithium-ion batteries

[0073] Preparation of positive electrode sheet

[0074] Aluminum foil is used as the positive electrode current collector. A layer of lithium cobalt oxide slurry is uniformly coated on the surface of the aluminum foil. The lithium cobalt oxide slurry includes 97.8 wt% LiCoO2 (LCO), 0.8 wt% polyvinylidene fluoride (PVDF) and 1.4 wt% conductive carbon black. The positive electrode sheet is then cold-pressed to prepare the positive electrode sheet.

[0075] Preparation of negative electrode sheet

[0076] A copper foil is used as the negative electrode current collector. A layer of graphite slurry is uniformly coated on the surface of the copper foil. The graphite slurry includes 97.7 wt% artificial graphite, 1.3 wt% carboxymethyl cellulose (CMC) and 1.0 wt% styrene-butadiene rubber (SBR). The negative electrode sheet is then prepared by cold pressing.

[0077] Preparation of the separating membrane

[0078] A 9μm thick porous polyethylene (PE) membrane was selected as the separator.

[0079] Preparation of electrolyte

[0080] In a dry argon-atmospheric glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 6:34:20:40. Lithium hexafluorophosphate (LiPF6) was then added to the organic solvents, dissolved, and mixed thoroughly. Ethyl sulfate (an electrolyte additive) was then added to obtain the electrolyte. The electrolyte contained 11.5% LiPF6, 2.5% ethylene sulfate, and the remainder was the mass percentage of the organic solvents. Specifically, EC comprised 5.2% of the electrolyte, PC 29.2%, EMC 17.2%, and DEC 34.4%.

[0081] Preparation of lithium-ion batteries

[0082] The positive and negative electrode sheets are slit and wound together, with the separator separating them to prepare multiple wound bare cells. These cells are then subjected to top-side sealing, inkjet printing, vacuum drying, injection of different concentrations of ethylene sulfate (electrolyte additive), high-temperature settling, and formation and capacity testing to obtain multiple finished batteries. The different concentrations of ethylene sulfate are 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%, respectively. The resulting batteries are numbered A-0, A-1, A-2, A-3, A-4, and A-5. Float charging tests are then performed.

[0083] (II) Battery Float Charge Test

[0084] Test method: (1) Use a thickness tester to test the thickness and record the initial thickness of the battery; (2) Place the battery in a high-temperature chamber and let it stand in a 40°C environment for 30 minutes. Charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.35V (rated voltage). Then switch the battery to constant voltage charging. Stop charging when the current reaches 0.05C. Let the battery stand for 10 minutes. Remove the battery and use a thickness tester to test the thickness. Record this as the thickness of the battery on day 0 of float charging; (3) Place the battery back in the high-temperature chamber and let it stand in a 40°C environment for 30 minutes. After 30 minutes, discharge the battery to 2.75V with a 0.5C current, let it stand for 10 minutes, and then charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.35V (rated voltage). Then switch the battery to constant voltage charging to start float charging; (4) Test the thickness every 5 days until the battery thickness change rate is >40%, and obtain the thickness value corresponding to each 5 days of float charging. Take the thickness of full charge as the base thickness. Thickness change rate % = [(thickness at each point / initial state thickness) - 1] * 100%. By analogy, multiple battery thickness change rate curves are obtained, see Figure 7 .from Figure 7 As can be seen, the float charge thickness growth rate varies with the change of ethylene sulfate concentration. Using the slope value of the float charge thickness growth rate, the thickness inflection point value of float charge batteries with different concentrations of ethylene sulfate can be calculated. The calculation process can refer to the calculation process of B4 in Example 1, and will not be repeated here.

[0085] See Figure 8 Plotting different concentrations of vinyl sulfate on the x-axis and the inflection point values ​​of float thickness on the y-axis, we obtain... Figure 8 A linear standard curve showing the relationship between different concentrations of vinyl sulfate and the corresponding thickness inflection point values. From... Figure 8 As the concentration of ethylene sulfate increases, the number of days corresponding to the inflection point of float charge thickness first increases and then decreases. It can be clearly seen that the A-3 group of batteries experienced the thickness inflection point after 296 days of float charging, making it the battery with the latest appearance of the float charge thickness inflection point. That is, when the electrolyte additive content is 1.5%, the battery float charging performance is the best.

[0086] Example 4

[0087] (I) Lithium-ion batteries

[0088] Preparation of positive electrode sheet

[0089] Aluminum foil is used as the positive electrode current collector. A layer of lithium cobalt oxide slurry is uniformly coated on the surface of the aluminum foil. The lithium cobalt oxide slurry includes 97.8 wt% LiCoO2 (LCO), 0.8 wt% polyvinylidene fluoride (PVDF) and 1.4 wt% conductive carbon black. The positive electrode sheet is then cold-pressed to prepare the positive electrode sheet.

[0090] Preparation of negative electrode sheet

[0091] A copper foil is used as the negative electrode current collector. A layer of graphite slurry is uniformly coated on the surface of the copper foil. The graphite slurry includes 97.7 wt% artificial graphite, 1.3 wt% carboxymethyl cellulose (CMC) and 1.0 wt% styrene-butadiene rubber (SBR). The negative electrode sheet is then prepared by cold pressing.

[0092] Preparation of the separating membrane

[0093] A 9μm thick porous polyethylene (PE) membrane was selected as the separator.

[0094] Preparation of electrolyte

[0095] In a dry argon-atmospheric glove box, organic solvents ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 6:34:20:40. Lithium bis(oxalato)borate (LiBOB) was then added to the organic solvents and dissolved and mixed thoroughly. Finally, 1,4-butene sulfonate lactone (an electrolyte additive) was added to obtain the electrolyte. The mass percentages of LiBOB and 1,4-butene sulfonate lactone in the electrolyte were 13.5% and 0.5%, respectively, with the remainder being the mass percentage of the organic solvents. Specifically, the mass percentages of EC, PC, EMC, and DEC in the electrolyte were 5.2%, 29.2%, 17.2%, and 34.4%, respectively.

[0096] Preparation of lithium-ion batteries

[0097] The positive and negative electrode sheets are slit and wound together, with the separator separating them to prepare multiple wound bare cells. These cells are then top-side sealed, inkjet-coded, vacuum-dried, injected with different concentrations of 1,4-butene sulfonate lactone (electrolyte additive), and subjected to high-temperature settling before formation and capacity testing to obtain multiple finished batteries. The different concentrations of 1,4-butene sulfonate lactone are 0%, 0.2%, 0.5%, 1.0%, and 1.5%, respectively. The resulting batteries are numbered B-0, B-1, B-2, B-3, and B-4, respectively. Float charge testing is then performed.

[0098] (II) Battery Float Charge Test

[0099] Test method: (1) Use a thickness tester to test the thickness and record the initial thickness of the battery; (2) Place the battery in a high-temperature chamber and let it stand in a 45°C environment for 30 minutes. Charge it with a constant current of 1.0C (1C is the rated capacity of the battery) until the battery voltage reaches 4.45V (rated voltage). Then switch the battery to constant voltage charging. Stop charging when the current reaches 0.05C. Let the battery stand for 10 minutes. Remove the battery and use a thickness tester to test the thickness. Record this as the thickness of the battery on day 0 of float charging; (3) Place the battery back in the high-temperature chamber and let it stand in a 45°C environment for 30 minutes. Discharge the battery to 3.0V with a current of 0.5C. Let it stand for 10 minutes. Then charge it with a current of 1.0C (1C is the rated capacity of the battery). (3) Charge the battery with constant current until the battery voltage reaches 4.45V (rated voltage), then switch to constant voltage charging to start float charging; (4) Test the thickness every 8 days until the battery thickness change rate is >40%, and obtain the thickness value corresponding to each 8 days of float charging. Take the full charge thickness as the base thickness, and the thickness change rate % = [(thickness at each point / initial state thickness) - 1] * 100%. By analogy, multiple battery thickness change rate curves can be obtained. The thickness inflection point value of float charging batteries with different concentrations of 1,4-butene sulfonate can be calculated. With different concentrations of 1,4-butene sulfonate as the horizontal axis and the float charging thickness inflection point value as the vertical axis, the linear standard relationship curve of different concentrations of 1,4-butene sulfonate and the corresponding thickness inflection point value can be obtained. Figures 9-10 It is evident that the B2 group of batteries exhibited a thickness inflection point after 192 days of float charging, making it the battery with the latest appearance of the thickness inflection point during float charging. Therefore, it was determined that the battery float charging performance was best when the electrolyte additive content was 0.5%. It should be noted that the above embodiments are merely illustrative examples, and the determination method described in this application is also applicable to other battery systems to determine the optimal concentration of electrolyte additives. For details, please refer to the calculation process of the above embodiments; this application will not elaborate further.

[0100] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for determining the content of electrolyte additives, characterized in that, The determination method includes the following steps: S100: Formation treatment is performed on multiple cells containing the same additive but with different additive contents to obtain batteries with different additive contents. S200: Perform a float charge test on the battery obtained in step S100, obtain the real-time thickness of the battery during the float charge test, and obtain the thickness change rate of the battery. S300: Based on n adjacent thickness change rates, obtain the slope value of the battery thickness change rate; S400: Obtain the inflection point value of the battery thickness based on the slope value of the battery thickness change rate; S500: Determine the electrolyte additive content based on the battery thickness inflection point value to improve battery float charging performance; In step S300, obtaining the slope value of the battery thickness change rate based on n adjacent thickness change rates includes: Using the preset number of days as the horizontal axis and the thickness change rate as the vertical axis, a linear standard curve is created based on n adjacent thickness change rates to obtain the slope value of the thickness change rate. The first slope value obtained is recorded as the first slope value of the battery; The slope value is denoted as k, which is obtained according to Equation II; Formula II Where x is the preset number of days, and y is the battery thickness change rate corresponding to the preset number of days. The average of n preset days. It is the average of n consecutive thickness change rates; In step 400, obtaining the inflection point value of the battery thickness based on the slope value of the battery thickness change rate includes: Divide the slope values ​​obtained in real time, other than the first slope value, by the first slope value. When the ratio is higher than the limit value M, the corresponding preset number of days is determined as the thickness inflection point value of the battery. The value of M ranges from 2 to 8.

2. The determination method according to claim 1, characterized in that, The additive has a mass percentage content of 0.01~10wt% based on the mass of the electrolyte.

3. The determination method according to claim 1, characterized in that, Before step S200, the method further includes: obtaining the initial state thickness of the battery.

4. The determination method according to claim 1, characterized in that, Step S200 includes: before the start of the float charge test: placing the battery in an environment of 30~60℃ for 20~30 minutes, charging it at a constant current to the rated voltage of the battery with the battery calibration current, switching the battery to constant voltage charging, stopping charging when the current reaches 0.03~00.05C, letting it stand for 8~10 minutes, and then placing the battery in an environment of 22~28℃ for 1~2 hours to obtain the thickness of the battery on day 0 of float charge.

5. The determination method according to claim 1, characterized in that, In step S200, the float charge test process includes: The battery is left to stand in an environment of 30~60℃ for 20~30 minutes, discharged to 2.5~3.0V with a current of 0.1~1.5C, left to stand for 8~10 minutes, charged to the rated voltage of the battery with constant current at the battery's calibrated current, and then the battery is switched to constant voltage charging for float charging test.

6. The determination method according to claim 1, characterized in that, Step S200 includes: Based on different cathode systems, a preset battery thickness testing frequency is established. The day the float charge test begins is designated as day 1. The real-time battery thickness is obtained according to this testing frequency, specifically including: For lithium-ion batteries with lithium cobalt oxide cathode system, the real-time thickness of the battery is obtained every 5 to 8 days. For lithium-ion batteries with lithium iron phosphate cathode system, the battery thickness was acquired every 30 days from day 1 to day 60 of the float charge test, and the real-time battery thickness was acquired every 8 to 15 days after day 60. For lithium-ion batteries with ternary cathode system, the battery thickness was acquired every 30 days from day 1 to day 90 of the float charge test, and then every 8 to 15 days thereafter.

7. The determination method according to claim 3, characterized in that, In step S200, the thickness change rate is obtained according to formula I; Thickness change rate % = [(real-time thickness of battery / initial state thickness) - 1] * 100% Equation I.

8. The determination method according to claim 1, characterized in that, In step S500, determining the electrolyte additive content based on the battery thickness inflection point value includes: Plot a linear standard relationship curve between electrolyte additive content and thickness inflection point value, with the content of electrolyte additive as the x-axis and the thickness inflection point value as the y-axis. The content corresponding to the highest point in the linear standard relationship curve is the optimal content of electrolyte additive that can improve the battery float charging performance.

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