Rapid screening method for self-discharge of lithium ion battery

Abstract

The invention discloses a rapid screening method for self-discharge of lithium ion batteries, and belongs to the technical field of batteries. The method specifically comprises the following steps: grading to-be-detected battery cells according to weight, preparing the battery cells into a lithium ion battery, then performing formation, and recording cut-off voltage V0 and formation time T; then standing, recording the voltage V1 at the end, and calculating the voltage difference deltaV of each lithium ion battery according to the formula deltaV = V0-V1; and in each weight grade, the average pressure difference delta Vave and the standard deviation sigma are calculated according to the pressure difference delta V, the average formation time Tave is calculated according to the T, and self-discharge screening is achieved through the relation between the delta V and the delta Vave and the standard deviation sigma and the relation between the T and the Tave. According to the method, the battery cell with large self-discharge can be identified in advance by utilizing the formation time and the formation voltage drop in the formation section, so that on one hand, the test period is greatly shortened, on the other hand, the identified risk battery cell does not need to continue to circulate, a part of energy consumption can be saved, and the production efficiency can be greatly improved without long-time placement.

Description

Technical Field

[0001] The invention relates to a rapid screening method for lithium ion battery self-discharge, belonging to the technical field of batteries. Background Art

[0002] In theory, batteries don't react with each other when not in use; redox reactions only occur when an electrical appliance is connected to the battery. However, in reality, ordinary batteries undergo internal chemical reactions. This means that even without any connection between the electrodes, trace amounts of chemicals within the battery react. These internal reactions reduce the battery's stored charge, gradually reducing its capacity. This phenomenon is known as self-discharge. Self-discharge occurs in virtually all types of batteries, including lithium-ion, sodium-ion, lead-acid, and nickel-metal hydride batteries. Self-discharge reduces the battery's usable capacity, affecting its battery life. Long-term self-discharge also accelerates internal chemical reactions and material degradation, shortening the battery's lifespan. Therefore, self-discharge testing is required during the production process.

[0003] Currently, battery self-discharge detection is performed after battery capacity sorting is completed. However, battery capacity sorting requires the battery to be placed for a long time after being fully charged and screened repeatedly to identify all cells with self-discharge risks. This makes the entire battery test cycle longer and affects battery production efficiency. Summary of the Invention

[0004] To solve the above problems, the present invention provides a method for self-discharge screening of lithium-ion batteries by combining a specific grading method and a specific voltage drop detection step. This method does not require time-consuming capacity division or a long-term static step to eliminate polarization. Foreign matter or micro-short-circuited cells inside the battery cell can be identified in advance during the formation stage, greatly saving detection time.

[0005] A first object of the present invention is to provide a method for screening self-discharge of a lithium ion battery, comprising the following steps:

[0006] S1. Sort several cells to be tested by weight;

[0007] S2. Prepare the graded cells into lithium-ion batteries, perform formation on each lithium-ion battery, and record the charge cut-off voltage V0 and formation time T;

[0008] S3. Let the formed lithium-ion battery rest for no more than 30 minutes, record the voltage V1 at the end of the rest period, and calculate the voltage difference ΔV of each lithium-ion battery, ΔV=V0-V1;

[0009] S4. In each weight range, calculate the average pressure difference ΔV based on the pressure difference ΔV of all lithium-ion batteries aveThe average formation time T is calculated based on the formation time T of all lithium-ion batteries and the standard deviation σ. ave , through the voltage difference ΔV of each lithium-ion battery and the average voltage difference ΔV ave The relationship between the standard deviation σ and the formation time T of each lithium-ion battery and the average formation time T ave The relationship between the two determines whether the self-discharge screening is passed.

[0010] Furthermore, in step S1, the difference between the maximum weight and the minimum weight of all the cores to be tested after hot pressing does not exceed 9g, and the cells are divided into different grades according to the maximum difference in weight of all the cells in each grade not exceeding 3g.

[0011] Furthermore, the difference between the maximum weight and the minimum weight of all the roll cores to be tested after hot pressing does not exceed 9g, for example, 9g, 8.8g, 8.5g, 8.2g, 8g, 7.5g, 7g, 6.5g, 6g, 5.5g, 5g, 4.5g, 4g, 3.5g, 3g, 2g, 1g, etc., including but not limited to the differences listed above.

[0012] Furthermore, the difference between the maximum weight and the minimum weight of the battery cells in each level does not exceed 3g, such as 3g, 2.8g, 2.5g, 2.2g, 2g, 1.7g, 1.5g, 1.3g, 1g, 0.8g, 0.5g, etc., including but not limited to the differences listed above.

[0013] Furthermore, before weight classification, the method further includes the steps of winding and / or pressing the battery cells.

[0014] Furthermore, the lithium-ion battery cell includes a positive electrode sheet, which includes a current collector and a positive electrode active layer disposed on at least one side of the current collector along the thickness direction, wherein the positive electrode active layer includes a positive electrode active material. The positive electrode active material is a nickel-cobalt-manganese ternary material.

[0015] Furthermore, in the nickel-cobalt-manganese ternary material, the molar ratio of Ni, Co, and Mn is (2-6): (0.1-1.5): (0.1-1.5). Most preferably, the nickel-cobalt-manganese ternary material is NCM311.

[0016] Furthermore, the lithium-ion battery cell also contains a negative electrode plate, which includes a current collector and a negative electrode active layer arranged on at least one side of the current collector along the thickness direction. The negative electrode active layer contains a negative electrode active material, wherein the negative electrode active material includes one or more of graphite, silicon, silicon oxide, titanium phosphorus oxide (including sodium titanium phosphate, sodium manganese titanium phosphate, and titanium phosphate).

[0017] Furthermore, the step of forming comprises:

[0018] (1) Charge to 3V with a preset constant current;

[0019] (2) charging with a current greater than the preset current to a charging cut-off voltage V0;

[0020] The preset current is greater than or equal to 0.04C and less than or equal to 0.06C.

[0021] Furthermore, the battery is charged to 3V at a preset constant current, wherein the preset current is greater than or equal to 0.04C and less than or equal to 0.06C. For example, 0.04C, 0.042C, 0.045C, 0.05C, 0.055C, 0.06C, etc., including but not limited to the currents listed above. Among them, 0.05C is most preferred.

[0022] Furthermore, in step (2), when charging to the cut-off voltage V0 with a current greater than the preset current, charging is performed with at least one constant current. When the charging is multi-stage constant current charging, the current of the latter stage is greater than the current of the former stage. Wherein, the "current greater than the preset current" is greater than or equal to 0.1C and less than or equal to 0.5C.

[0023] Furthermore, after being charged to 3V with a preset constant current, the battery is charged to a cut-off voltage V0 with a current greater than the preset current, wherein the charging current is greater than or equal to 0.1C and less than or equal to 0.5C. For example, 0.1C, 0.12C, 0.15C, 0.2C, 0.25C, 0.3C, 0.35C, 0.4C, 0.45C, 0.5C, etc., including but not limited to the currents listed above.

[0024] Furthermore, the cut-off voltage V0 may be in the range of 3.5-4V.

[0025] Furthermore, after formation, the battery is allowed to rest for no longer than 30 minutes. For example, 1 minute, 1.5 minutes, 2 minutes, 2.5 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 29 minutes, etc., including but not limited to the above-mentioned durations. Preferably, the resting time is 60-120 seconds.

[0026] Furthermore, the self-discharge detection method includes:

[0027] When the voltage difference of a single lithium-ion battery ΔV>ΔV ave When +σ*a, it fails the self-discharge screening, otherwise it passes, where a is any value from 3 to 3.5;

[0028] When the formation time of a single lithium-ion battery T<T aveWhen the self-discharge screening is not successful, the battery fails the self-discharge screening. Wherein, the formation time is the time it takes to charge to the cut-off voltage V0.

[0029] Furthermore, a is any value from 3 to 3.5, such as 3.01, 3.05, 3.1, 3.15, 3.2, 3.25, 3.3, 3.35, 3.4, 3.45, 3.49, etc., including but not limited to the values listed above.

[0030] Beneficial effects of the present invention:

[0031] (1) The present invention associates formation time with voltage drop, and can identify risky cells in advance during the formation stage, greatly shortening the detection cycle;

[0032] (2) The present invention identifies risky cells through formation, which greatly saves energy consumption and improves production efficiency without requiring additional standing time.

[0033] (3) The cost of identifying risky cells in the present invention is low, and does not require a large amount of equipment, space, and temperature control, which greatly reduces the cost of battery production. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to specific examples so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention.

[0035] The scheme that the present invention relates to is as follows:

[0036] In existing technologies, self-discharge detection typically requires first performing capacity division and then detecting the voltage drop generated by prolonged static standing. However, this capacity division requires a certain period of time, and the voltage drop after capacity division typically requires a long period of time (approximately 24 hours) after polarization is eliminated before it can be used as a basis for self-discharge detection. This undoubtedly hinders efficient battery production. Therefore, finding a method to quickly implement self-discharge detection is urgent.

[0037] The present invention provides a new method for rapid self-discharge detection, which does not involve steps such as capacity division and polarization elimination, but instead uses weight classification and polarization for rapid identification. Specifically:

[0038] Step 1: Binning

[0039] The hot-pressed cores are weighed and graded, and the difference between the maximum weight and the minimum weight of all the hot-pressed cores to be tested does not exceed 9g (for example, the present invention controls it within 105g-114g), and the difference between the maximum weight and the minimum weight of the battery cells in each grade is controlled to be no more than 3g. In the present invention, grading the weight of the battery cells is crucial to the accuracy and stability of the test results. This is mainly because: there will be fluctuations in the coating process of the electrode, which will affect the capacity of the battery cells. If the capacity is affected, it will affect the formation time, and the formation time is one of the key conditions for screening batteries with large self-discharge in this solution. In addition, the present invention uses the voltage drop parameter to determine whether the battery is abnormal during the self-discharge screening process. The weight difference may cause the battery to have different voltage drop performance under the same conditions, thereby affecting the screening results. In addition, batteries with large weight differences may have quality problems during the production process, such as electrolyte leakage, electrode defects, etc., which will affect the self-discharge performance of the battery and also affect the accuracy of batch detection. In summary, weight differences will affect the uniformity of the test environment, so core grading is also one of the essential steps of this solution.

[0040] Step 2: Formation

[0041] After injecting the electrolyte, the lithium battery is formed and allowed to rest. This involves charging the battery to a cutoff voltage V0 and then allowing it to rest. At the end of the rest period, the voltage V1 is recorded, and the formation time T, which is the time it takes to reach the cutoff voltage V0, is also recorded. In the present invention, this resting step is crucial and indispensable. It is a key step in determining self-discharge and represents the present invention's greatest innovation compared to the prior art. Specifically, battery self-discharge refers to the phenomenon in which a battery's charge naturally decreases due to internal chemical reactions or other factors without external charge or discharge. Self-discharge can lead to charge loss, voltage drop, and accelerated battery aging, increasing safety risks. In the prior art, lithium batteries are typically allowed to rest to eliminate polarization before undergoing formation. Self-discharge detection is performed based on the formation time and voltage drop. It is generally believed that if polarization is not eliminated during self-discharge screening, the actual battery voltage will be affected by the polarization voltage, resulting in inaccurate voltage measurement and affecting self-discharge determination. Eliminating polarization can shorten the battery's rest period, quickly reaching a stable open-circuit voltage, and thus more accurately screening batteries with abnormal self-discharge. However, the present invention has discovered that this is not the case. Our self-discharge screening does not involve the step of "eliminating polarization". Instead, we use this polarization process to achieve self-discharge screening and significantly shorten the detection cycle. It has been verified that it can achieve "rapid screening" with accurate results: when there is metal foreign matter during the formation process, the polarization of the battery cell is relatively large. On the one hand, it will reach the charge cut-off voltage earlier, which is manifested as a short formation time. On the other hand, the battery cell with metal foreign matter will have a relatively large voltage drop during the shelf time after the formation is completed, which is manifested as a voltage difference.

[0042] Step 3: Data Collection

[0043] 1) Calculate the voltage difference of a single cell in the winding core after binning: ΔV = V0 - V1;

[0044] 2) Extract the formation time T of the single cell,

[0045] 3) Calculate the average ΔV and T of each battery pack.

[0046] Step 4: Self-discharge determination

[0047] Cells with a value greater than ΔV mean + standard deviation * a (a can be between 3 and 3.5) and a formation time less than T mean are considered to be self-discharging cells.

[0048] In the present invention:

[0049] Preferably, the battery cells are wound and / or pressed (e.g., hot pressed) before weight classification. Hot pressed cores are chosen because they are relatively loose after winding and difficult to manipulate. Hot pressed cores are already shaped and will not undergo other movements, thus reducing measurement errors.

[0050] Preferably, the present invention optimizes the self-discharge screening of lithium-ion batteries: the lithium-ion battery cell contains a positive electrode plate, the positive electrode plate contains a current collector and a positive electrode active layer arranged on at least one side of the current collector along the thickness direction, and the positive electrode active layer contains a positive electrode active material. Among them, the positive electrode active material is a nickel-cobalt-manganese ternary material. The self-discharge screening of the present invention is mainly optimized based on lithium-ion batteries containing such positive electrode materials. After preliminary screening and testing, the weight distribution range of all battery cells is limited to no more than 9g (such as 105g-114g), and the weight range of all battery cells in each range is no more than 3g. Under such conditions, the accuracy and consistency of the self-discharge screening results are high. This is because: there are many kinds of positive electrode materials, such as lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, nickel cobalt manganese oxide, etc., each material has different characteristics, and these characteristics may affect the self-discharge of the battery. Specifically: (1) The chemical stability of the positive electrode material directly affects the self-discharge behavior of the battery. For example, the positive electrode material is prone to side reactions in the electrolyte, or its structure is unstable during the charge and discharge process, which will cause changes in the self-discharge situation. (2) The crystal structure of the positive electrode material will also affect the self-discharge. Some materials have a loose crystal structure, which makes it easy for ions in the electrolyte to penetrate, resulting in an increase in side reactions. On the contrary, materials with a dense structure can better prevent this penetration, resulting in different self-discharge behaviors of lithium batteries. (3) Impurities in the positive electrode material (such as metal ions) may dissolve and migrate to the surface of the negative electrode during the charge and discharge process, causing internal short circuits and thus affecting self-discharge. Therefore, when conducting self-discharge screening, it is necessary to fully consider the characteristics of the positive electrode material to improve the accuracy and reliability of the screening.

[0051] Preferably, the difference between the maximum weight and the minimum weight of all the roll cores to be tested after hot pressing does not exceed 9g, for example, 9g, 8.8g, 8.5g, 8.2g, 8g, 7.5g, 7g, 6.5g, 6g, 5.5g, 5g, 4.5g, 4g, 3.5g, 3g, 2g, 1g, etc., including but not limited to the differences listed above.

[0052] Preferably, the difference between the maximum weight and the minimum weight of the battery cells in each gear does not exceed 3g, for example 3g, 2.8g, 2.5g, 2.2g, 2g, 1.7g, 1.5g, 1.3g, 1g, 0.8g, 0.5g, etc., including but not limited to the differences listed above.

[0053] Preferably, the lithium-ion battery cell also contains a negative electrode plate, which includes a current collector and a negative electrode active layer arranged on at least one side of the current collector along the thickness direction, and the negative electrode active layer contains a negative electrode active material, wherein the negative electrode active material includes one or more of graphite, silicon element, silicon oxide, titanium phosphorus oxide (including sodium titanium phosphate, sodium manganese titanium phosphate, and titanium phosphate).

[0054] Preferably, in the nickel-cobalt-manganese ternary material, the molar ratio of Ni, Co, and Mn is (2-6): (0.1-1.5): (0.1-1.5). Most preferably, the nickel-cobalt-manganese ternary material is NCM311.

[0055] Preferably, the molar ratio of Ni, Co, and Mn is (2-6):(0.1-1.5):(0.1-1.5), such as 2:1:1, 3:1:1.5, 3:1.5:1, 4:1:1, etc., including but not limited to the ratios listed above. Among them, 3:1:1 is most preferred.

[0056] Preferably, when preparing the graded battery cells into lithium-ion batteries, the hot-pressed cores are brought into contact with the electrolyte. As long as the electrolyte completely soaks the electrode sheets, there is no impact on the electrical performance.

[0057] Preferably, in the new self-discharge detection method of the present invention, when the system is a lithium-ion battery, there are no requirements for the current and process of the formation process, and the formation can be performed according to the system or process requirements of the battery cell itself. There is no requirement for the charging cut-off voltage, which can be determined according to the battery system. More preferably, when the positive electrode material of the lithium-ion battery is NCM, the formation steps include: (1) charging to 3V with a preset current constant current; (2) charging to a charging cut-off voltage V0 with a current greater than the preset current; the preset current is greater than or equal to 0.04C and less than or equal to 0.06C. Different formation processes (including current, voltage, etc.) will affect the formation quality and consistency of the SEI film inside the battery, affect the self-discharge rate and thus affect the self-discharge detection. More importantly, different formation process parameters may lead to differences in the internal structure of the battery. Excessively high charging voltages may damage the positive and negative electrode materials and increase the self-discharge rate. Different parameters also affect the uniformity and defects of the SEI film, affecting the diffusion of lithium ions. Therefore, self-discharge detection is usually performed under specific conditions. Whether the battery state after formation is stable and whether it has been fully activated will affect the accuracy of the test results. The present invention optimizes the formation process for specific electrode materials to form a stable SEI film. At the same time, a small current is required to form a dense and thin SEI. A small current is used for charging before 3V (when the SEI is basically formed), and a large current is used afterward to shorten the formation time and increase production capacity.

[0058] Preferably, the high current (current greater than the preset current) of the formation process is charged to the cut-off voltage V0, and charging is performed with at least one constant current; when the charging is multi-stage constant current charging, the current of the latter stage is greater than the current of the previous stage. More preferably, the charging current is greater than or equal to 0.1C and less than or equal to 0.5C. In the case of multi-stage constant current charging, the cut-off voltage of each stage is calculated as follows:

[0059] ΔV n =(V0-3) / n, where V0 is the charging cut-off voltage, n represents the number of multiple constant current charging stages from charging to 3V to charging to the cut-off voltage V0, n is an integer and 1≤n≤5, ΔV n is the change in the charging cut-off voltage of each section relative to the previous section;

[0060] V m =ΔV n +V m-1 , V m is the charging cut-off voltage base value of the current segment, V m-1 The charging cut-off voltage base value of the previous section (initial value is 3V, i.e. V m-1 =3V);

[0061] In the multi-stage constant current charging process of the present invention, the charging cut-off voltage of each stage is Vm ±0.1.

[0062] The design of the formation process of the present invention is related to the reaction process inside the battery. At the beginning, the materials inside the battery may not be fully activated. If charging is not appropriate, it may cause local overheating or material decomposition. The SEI film is formed during the charging process. If the current is too small, it may not be able to effectively form a sufficient SEI film. If the current is too large, it may destroy the film being formed. The method of the present invention can ensure the stable formation of the SEI film without causing excessive pressure. Moreover, in the early stage of formation, the internal resistance of the battery is relatively large. As charging proceeds, the internal structure is gradually optimized. More importantly, gradually increasing the current intensity not only avoids the stress that may be caused to the battery structure by excessive changes in current intensity during the formation process, but also provides the battery with an "adaptation" process, making the internal reaction more uniform and stable. This stability can reduce the self-discharge detection error caused by internal non-uniform reactions.

[0063] Preferably, the battery is charged to 3V at a preset constant current, wherein the preset current is greater than or equal to 0.04C and less than or equal to 0.06C. For example, 0.04C, 0.042C, 0.045C, 0.05C, 0.055C, 0.06C, etc., including but not limited to the currents listed above. Among them, 0.05C is most preferred.

[0064] Preferably, after constant current charging to 3V with a preset current, the battery is charged to the cut-off voltage V0 with a current greater than the preset current. The charging current is greater than or equal to 0.1C and less than or equal to 0.5C. For example, 0.1C, 0.12C, 0.15C, 0.2C, 0.25C, 0.3C, 0.35C, 0.4C, 0.45C, 0.5C, etc., including but not limited to the currents listed above. Among them, it is more preferred to divide the constant current charging into at least two stages and the current of the latter stage is stronger than the former stage. It is most preferred that the current of the former stage is 0.1C and the current of the latter stage is 0.5C.

[0065] Preferably, the cut-off voltage V0 in the present invention may be in the range of 3.5-4V.

[0066] Preferably, after formation is completed, the battery is left to stand for no more than 30 minutes, and the pressure difference ΔV before and after the standing is calculated. Self-discharge can be determined based on this pressure difference. Of course, one of the innovations of the present invention is that, because this solution uses the voltage after standing to calculate ΔV, the standing time is between 60-120s (of course, those skilled in the art will understand that if the standing time is appropriately extended, the pressure difference will increase and the detection effect will be better). By redesigning the entire detection route, we can achieve rapid detection of self-discharge within a 2-minute standing time.

[0067] Preferably, the battery is left to stand for no longer than 30 minutes after formation, for example, 1 minute, 1.5 minutes, 2 minutes, 2.5 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 29 minutes, etc., including but not limited to the time periods listed above.

[0068] Preferably, the self-discharge detection determination method includes:

[0069] In each weight range, the average pressure difference ΔV is calculated based on the pressure difference ΔV of all lithium-ion batteries. ave The average formation time T is calculated based on the formation time T of all lithium-ion batteries and the standard deviation σ. ave , through the voltage difference ΔV of each lithium-ion battery and the average voltage difference ΔV ave The relationship between the standard deviation σ and the formation time T of each lithium-ion battery and the average formation time T ave The relationship between the two determines whether the self-discharge screening is passed;

[0070] When the voltage difference of a single lithium-ion battery ΔV>ΔV ave +σ*a, it fails the self-discharge screening, otherwise it passes, where a is any value from 3 to 3.5; when the formation time of a single lithium-ion battery T<T ave When the self-discharge screening is not successful, the battery fails the self-discharge screening. Wherein, the formation time is the time it takes to charge to the cut-off voltage V0.

[0071] Preferably, a is any value between 3 and 3.5, such as 3.01, 3.05, 3.1, 3.15, 3.2, 3.25, 3.3, 3.35, 3.4, 3.45, 3.49, etc., including but not limited to the values listed above.

[0072] In the lithium-ion battery of the present invention, except for the positive and negative active materials, the rest are common choices.

[0073] Preferably, the (positive or negative) electrode sheet includes an active layer; the active layer includes an active material, a conductive agent and a binder.

[0074] Preferably, the active layer comprises the following components by mass percentage: 75-95% active material, 1-15% conductive agent and 1-15% binder.

[0075] Preferably, the conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes and graphene.

[0076] Preferably, the binder is a negative electrode binder well known to those skilled in the art without particular limitation, such as one or more of chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacryloyl, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymer and guar gum copolymer.

[0077] Preferably, the type of the diaphragm is any diaphragm well known to those skilled in the art, without any particular limitation, such as one or more of polyethylene, polypropylene, polyacrylonitrile, polyacrylonitrile fiber, polyamic acid, polyimide, polyamide, polydopamine, polyarylethersulfone, polyvinylidene fluoride, polyethylene oxide, polyethylene terephthalate, polyester, non-woven fabric membrane and cellulose paper-based isolation membrane.

[0078] Preferably, the electrolyte is an electrolyte well known to those skilled in the art without any special restrictions, the solvent is at least one of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate and diethylene glycol dimethanol ether; the electrolyte salt is at least one of lithium perchlorate, lithium bis(oxalate)borate, and lithium hexafluorophosphate.

[0079] The cell preparation methods involved in the following embodiments are as follows:

[0080] 1. Preparation of negative electrode sheet:

[0081] Graphite (as the negative electrode active material), conductive agent (conductive carbon black), and binder (polyvinylidene fluoride) were mixed in a mass ratio of 85:10:5, and deionized water was added and stirred thoroughly to obtain a uniform active material slurry, which was then coated on the copper foil current collector with a coating surface density of 89.3 g / mm 2 (single-sided) and dried at 85°C. The thickness of the copper foil current collector is 8um, and then it is rolled, and the compaction density is 1.35g / cm 3 , and then die-cut into 93*3185mm,

[0082] 2. Preparation of positive electrode sheet

[0083] NCM (as the positive electrode active material), conductive agent (conductive carbon black), and binder (PVDF) were mixed in a mass ratio of 93:1.5:5.5, and NMP was added and stirred thoroughly to obtain a uniform active material slurry. The slurry was then coated on an aluminum foil current collector and dried at 90°C. The coating surface density was 177.1g / mm 2 (single-sided), the thickness of the aluminum foil current collector is 13 μm, and then it is rolled, with a compaction density of 3.1 g / cm 3, and then die-cut into 88*3053mm,

[0084] 3. Core preparation

[0085] The positive electrode sheet separator and the negative electrode sheet separator were rolled into a rectangle and hot pressed at 90°C for 60 seconds.

[0086] 4: Assembly

[0087] Then weld the tabs on the winding core to the adapter, and weld the top cover.

[0088] 5: Baking: Bake at 110℃ for 16h.

[0089] 6: Liquid injection: Inject about 40g of electrode liquid into the liquid injection hole.

[0090] Example 1

[0091] This solution selects 5.0Ah battery cells, 300EA battery cells (100EA battery cells were selected from each group of a batch of batteries for experiments), ternary (NCM311) positive electrode material, graphite negative electrode material, conventional electrolyte in the industry, electrolyte formula (LiFP6:EC:PC:EMC:FEC:VC:PS:DTD=13:20:7:50:6:1:2:1), and PP diaphragm.

[0092] Step 1: Grading: Weigh and grade the cores after hot pressing. The weight of the cells is distributed between 105-114g. Then the cores are divided into A, B, and C grades. Each grade is 100EA, 105<A≤108g, 108<B≤111g, 111<C≤114g. The weight difference is controlled between 0-3g as one grade.

[0093] Step 2: Formation: After the lithium battery is injected with the electrolyte, it is left to stand for 12 hours (for full infiltration) and then formed. The formation process is: 0.05C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and then standing for 60s to obtain the voltage V1 after standing.

[0094] Step 3: Obtain the cut-off voltage V0 = 3.75V for each cell (V0 is the cut-off voltage set for formation). Combined with the voltage V1 after standing, calculate the ΔV of each cell, and then calculate the average ΔV of each cell:

[0095] When 105<A≤108g, ΔV is 35-45mv, formation time is 2600-2800s, ΔV average = 38mv, T average = 2650s;

[0096] 108<B≤111g, ΔV is 31-45mv, formation time is 2700-2900s, ΔV average = 35mv, T average = 2800s;

[0097] 111<C≤114g, ΔV is 37-45mv, formation time is 2930-3150s, ΔV average = 36mv, T average = 3000s;

[0098] Step 4: For Grade A, the screening criteria are: ΔV mean + standard deviation * 3.5 = 40. Cells with a value greater than 40mV and a formation time less than 2650s are considered unqualified. For Grade B, the criteria are: ΔV mean + standard deviation * 3.5 = 38. Cells with a value greater than 38mV and a formation time less than 2800s are considered unqualified. For Grade C, the criteria are: ΔV mean + standard deviation * 3.5 = 40. Cells with a value greater than 40mV and a formation time less than 3000s are considered unqualified.

[0099] Step 5: Disassemble the self-discharging battery cell to find the short-circuit point, and perform EDS analysis on the short-circuit point to find that it contains a large amount of Fe element.

[0100] Example 2

[0101] The weight is divided into grades and the difference of each grade is controlled to be 0-2g, and the rest is the same as in Example 1.

[0102] Example 3

[0103] The formation process was replaced with: 0.05C constant current charging to 3V, 0.1C constant current charging to 3.25V, 0.3C constant current charging to 3.5V, 0.5C constant current charging to 3.75V, and standing for 60s. The rest was the same as in Example 1.

[0104] Example 4

[0105] The formation process was replaced with: 0.04C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and standing for 60s. The rest was the same as in Example 1.

[0106] Example 5

[0107] The formation process was replaced with: 0.06C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and resting for 60s. The rest was the same as in Example 1.

[0108] Example 6

[0109] The formation process was replaced with: 0.06C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and resting for 60s. The rest was the same as in Example 1.

[0110] Example 7

[0111] The formation process was replaced with: 0.05C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and resting for 120s. The rest was the same as in Example 1.

[0112] Example 8

[0113] The formation process was replaced with: 0.05C constant current charging to 3V, 0.1C constant current charging to 3.4V, 0.5C constant current charging to 3.75V, and standing for 30 minutes. The rest was the same as in Example 1.

[0114] Comparative Example 1

[0115] No weight classification is performed, and the rest is the same as in Example 1.

[0116] Comparative Example 2

[0117] After the cell capacity test, the cell was left for 24 hours to measure OCV1 and then left for 96 hours or 120 hours to measure OCV2. The K value was then calculated, K = (OCV1 - OCV2) / t, where t is 96 or 120. The rest was the same as in Example 1.

[0118] Comparative Example 3

[0119] The formation process was replaced with: 0.1C constant current charging to 3.75V, and then standing for 60s. The rest was the same as in Example 1.

[0120] Comparative Example 4

[0121] The positive electrode active material was replaced with lithium iron phosphate, and the rest was the same as in Example 1.

[0122] The test results are as follows:

[0123] (1) The test results are shown in detail using Example 1 as an example, wherein the K value of Example 1 is calculated based on (V1-V0) / t, where t is the standing time after formation:

[0124]

[0125]

[0126]

[0127]

[0128] (2) The detection rate results of the self-discharge cells of the examples and comparative examples are shown in the table below.

[0129]

[0130]

[0131] The above results show:

[0132] Comparing Example 1 with Comparative Example 1, it can be seen that if the cells are not graded, there will be a risk of missed screening. This may be related to the fact that the ΔV standard deviation becomes larger after the cells are not graded, resulting in a wider screening range, which in turn may miss some cells with self-discharge risks.

[0133] By comparing Example 1 and Comparative Example 2, it can be concluded that the present solution can identify risky cells in a short time, while the conventional test can only identify 33% of the risky cells in 96 hours and can only identify all the risky cells in 120 hours.

[0134] Generally, the longer the standing time, the K value will experience a process of first increasing, then decreasing, and then tending to be stable. In Example 1 and Comparative Example 1, the self-discharge screening cycle is relatively short, or the standing time is relatively short, so the K value is relatively small. In Example 2, the self-discharge screening cycle is relatively long, 96h and 120h respectively, so the K value is relatively large. Compared with 96h, the K value of 120h is in a decreasing stage, so the K value of 120h is smaller than the K value of 96h. The K value is an indicator used to measure the self-discharge rate of lithium batteries. We generally believe that the K value of a good battery is generally less than 0.08mV / h, and the present invention basically meets this requirement.

[0135] From the perspective of the detection rate of self-discharge cells in each group, all embodiments can achieve 100% detection, while the detection efficiency of comparative examples 1-3 is low, especially comparative example 4 after replacing the positive electrode material, which cannot even be detected. This shows that the method of the present invention has certain limitations and is not suitable for self-discharge screening of all lithium-ion batteries.

[0136] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for screening self-discharge of lithium-ion batteries, characterized in that: The following steps are involved: S1. Sort several cells to be tested by weight; S2. Prepare the graded cells into lithium-ion batteries, perform formation on each lithium-ion battery, and record the charge cut-off voltage V0 and formation time T; S3. Allow the formed lithium-ion battery to rest for no more than 30 minutes, record the voltage V1 at the end of the rest period, and calculate the voltage difference ΔV of each lithium-ion battery, where ΔV = V0 - V1; S4. In each weight range, calculate the average pressure difference ΔV based on the pressure difference ΔV of all lithium-ion batteries ave The average formation time T is calculated based on the formation time T of all lithium-ion batteries and the standard deviation σ. ave , through the voltage difference ΔV of each lithium-ion battery and the average voltage difference ΔV ave The relationship between the standard deviation σ and the formation time T of each lithium-ion battery and the average formation time T ave The relationship between the two determines whether the self-discharge screening is passed.

2. The self-discharge screening method according to claim 1, characterized in that: In step S1, the difference between the maximum weight and the minimum weight of all battery cells does not exceed 9g; and the battery cells are divided into grades according to the extreme difference in weight of all battery cells in each grade not exceeding 3g.

3. The self-discharge screening method according to claim 1, wherein: In step S2, the forming step includes: (1) Charge to 3V with a preset constant current; (2) charging with a current greater than the preset current to a charging cut-off voltage V0; The preset current is greater than or equal to 0.04C and less than or equal to 0.06C.

4. The self-discharge screening method according to claim 3, wherein: In step (2), charging is performed with at least one constant current; when the charging is multi-stage constant current charging, the current of the latter stage is greater than the current of the former stage.

5. The self-discharge screening method according to claim 3 or 4, characterized in that: In step (2), the charging current is greater than or equal to 0.1C and less than or equal to 0.5C.

6. The self-discharge screening method according to claim 1, characterized in that: In step S3, the standing time is 60-120s.

7. The self-discharge screening method according to claim 1, characterized in that: In step S4, the determination includes: When the voltage difference of a single lithium-ion battery ΔV>ΔV ave +σ*a, it fails the self-discharge screening, otherwise it passes, where a is any value from 3 to 3.5; and, When the formation time of a single lithium-ion battery T<T ave If , it fails the self-discharge screening, otherwise it passes.

8. The self-discharge screening method according to claim 1, wherein: In step S1, before the cells are sorted by weight, the steps of winding and / or pressing the cells are also included.

9. The self-discharge screening method according to claim 1, wherein: The lithium-ion battery cell contains a positive electrode sheet and a negative electrode sheet. The positive electrode sheet contains a current collector and a positive electrode active layer provided on at least one side of the current collector along the thickness direction. The negative electrode sheet contains a current collector and a negative electrode active layer provided on at least one side of the current collector along the thickness direction. The positive electrode active layer or the negative electrode active layer comprises at least one of the following features: (I) The positive electrode active layer comprises a positive electrode active material, and the positive electrode active material is a nickel-cobalt-manganese ternary material; (II) The negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises one or more of graphite, silicon, silicon oxide, and titanium phosphorus oxide.

10. The self-discharge screening method according to claim 9, characterized in that: In the nickel-cobalt-manganese ternary material, the molar ratio of Ni, Co and Mn is (2-6): (0.1-1.5): (0.1-1.5).

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