Method for calculating lithium supplement amount based on electric core cycle test

Abstract

The application provides a method for calculating lithium supplement amount based on battery cycle test, comprising judging whether the battery is normal by obtaining the battery cycle curve, and performing multiple lithium supplement experiments on the abnormal battery; performing charge-discharge cycle test on the battery after lithium supplement until the battery after lithium supplement is judged as a normal battery. When the battery after lithium supplement is a normal battery, the lithium supplement amount y of the normal battery is obtained, the difference diff between the offset of the EFC point of the abnormal battery without lithium supplement and the offset of the EFC point of the normal battery is calculated, and the discrete point (diff, y) is obtained; the discrete points of the batteries with different formulations are determined, the relationship equation y=f(diff) (a*diff+b) between the lithium supplement amount and the deviation is established; the charge-discharge cycle test is performed on the battery with unknown lithium supplement amount, the offset of the EFC point of the battery with unknown lithium supplement amount is obtained, and the lithium supplement amount of the battery with unknown lithium supplement amount is determined according to the relationship equation. The method for calculating lithium supplement amount based on battery cycle test can realize accurate lithium supplement of the positive electrode of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery replenishment technology, specifically to a method for calculating the amount of lithium replenishment based on cell cycle testing. Background Technology

[0002] Lithium-ion batteries are widely used in portable power supply fields due to their advantages such as high energy density and long cycle life. With the continuous development of consumer electronics, electric vehicles, energy storage and other fields, further improving the energy density of lithium-ion batteries has become an urgent problem to be solved.

[0003] During the first charge of a lithium-ion battery, the solid electrolyte interphase (SEI) layer formed on the negative electrode surface consumes a large amount of lithium ions, resulting in a low initial coulombic efficiency, irreversible reduction in overall capacity, and further impacting the battery's cycle life. In graphite-based anode battery systems, approximately 10% of the lithium source is consumed during the first charge, while in silicon-based anode battery systems, the consumption is even more pronounced, reaching around 30%.

[0004] Currently, lithium replenishment is an important means to improve the energy density and cycle life of lithium-ion batteries. There are many existing methods for lithium replenishment, the most common being positive electrode lithium replenishment and negative electrode lithium replenishment. The most common method for negative electrode lithium replenishment is to directly pre-lithiate the lithium metal powder / lithium metal foil. However, lithium metal has extremely high activity and inherent safety risks, which can bring uncontrollable risks to the battery production and use process. The most common method for positive electrode lithium replenishment is to directly add a lithium replenishing agent to the positive electrode. This agent releases active lithium during the first charge of the battery, compensating for the irreversible loss of active lithium caused by the formation of the SEI film on the negative electrode.

[0005] Existing cathode lithium replenishment methods cannot achieve precise lithium replenishment of the battery cell. Previous literature has analyzed the phase transition behavior of high-nickel cathode materials during electrochemical cycling in lithium-ion batteries, examining the intrinsic phase behavior of high-nickel materials and quantitatively analyzing the H2-H3 phase transition process using dQ / dV curves. The results indicate that the capacity decay of high-nickel ternary materials during cycling mainly originates from the H2-H3 phase transition of the cathode active material during charge and discharge. To compensate for the loss of some active lithium caused by the H2-H3 phase transition, and to improve battery capacity and cycle performance, it is necessary to quantitatively calculate the required lithium replenishment amount based on the capacity decay during cycling, thus enabling precise lithium replenishment of the battery's cathode. Summary of the Invention

[0006] The method for calculating lithium replenishment based on cell cycle testing provided by this invention solves the problem of how to accurately replenish lithium to the positive electrode of a battery.

[0007] To address the aforementioned technical problems, this application provides a method for calculating lithium replenishment capacity based on cell cycle testing, comprising:

[0008] Perform a preset number of charge-discharge cycle tests on the battery cells and then evaluate the battery cells.

[0009] The method of judgment is as follows:

[0010] Obtain the dQ / dV-V cycle curve for each cycle of the battery cell;

[0011] Obtain the maximum peak point (V1, dQ1 / dV1) of the dQ / dV-V cycle curve in the first cycle and the maximum peak point (Vn, dQn / dVn) in the Nth cycle, where N is an integer greater than 1;

[0012] Calculate the offset of the maximum peak point up to the Nth cycle, delta_N = |dQ1 / dV1 - dQn / dVn|, to obtain the initial capacity and the capacity up to the Nth cycle in the charge-discharge cycle test. The number of cycles when the capacity up to the Nth cycle reaches a preset proportion of the initial capacity is the EFC point. If the EFC point is lower than the preset number of cycles, the cell is judged to be an abnormal cell; otherwise, it is a normal cell.

[0013] When a battery cell is determined to be abnormal, multiple lithium replenishment experiments are performed on the abnormal battery cell; the battery cell after lithium replenishment is subjected to a preset number of charge-discharge cycle tests until the battery cell after lithium replenishment is determined to be a normal battery cell, at which point the lithium replenishment experiment is stopped.

[0014] When the lithium-replenished cell is a normal cell, obtain the lithium replenishment amount y of the normal cell, calculate the difference diff between the offset of the EFC point of the non-replenished abnormal cell and the offset of the EFC point of the normal cell, and obtain the discrete point (diff, y). The offset of the EFC point of the abnormal cell is delta_EFC = |

[0015] dQ1 / dV1-dQefc / dVefc|;The normal cell obtains a one-to-one correspondence between capacity retention rate and offset in a preset number of charge-discharge cycles, and determines the offset of the EFC point of the normal cell based on the one-to-one correspondence.

[0016] Charge-discharge cycle tests were conducted on cells with different formulations to determine the discrete points of cells with different formulations, and the relationship equation between lithium replenishment and deviation was established as y=f(diff).

[0017] Charge-discharge cycle tests are performed on cells with unknown lithium replenishment to obtain the offset of the EFC point of the cells with unknown lithium replenishment. The lithium replenishment of the cells with unknown lithium replenishment is determined according to the relational equation.

[0018] In one embodiment, the determination method for obtaining the dQ / dV-V cycle curve for each cycle of the battery cell includes: obtaining the charge / discharge time, voltage, and capacity of the battery cell, and calculating the dQ / dV-V cycle curve for each cycle of the battery cell using the charge / discharge time, voltage, and capacity of the battery cell.

[0019] In one implementation, after obtaining the charging and discharging time, voltage, and capacity of the battery cell, the obtained voltage and capacity values ​​are smoothed, and the dQ / dV-V cycle curve of the battery cell for each cycle is calculated based on the smoothed values.

[0020] In one implementation, the smoothing process is as follows:

[0021] The original values ​​are numbered according to the order of the obtained original values ​​to obtain a value group from the first original value to the Mth original value, where M is an integer greater than 41.

[0022] Based on M original values, M smoothed values ​​are obtained. The first smoothed value is the first original value in the value group. The second smoothed value is the average of the first and second original values ​​in the value group. The values ​​of the third to the fortieth smoothed values ​​are the average of the original value of the corresponding smoothed value number in the value group and the original values ​​of the first two numbers of the corresponding smoothed value number. The values ​​of the forty-first to the Mth smoothed values ​​are the average of the original value of the corresponding smoothed value number in the value group and the original values ​​of the first 39 numbers of the corresponding smoothed value number.

[0023] In one embodiment, the steps of performing charge-discharge cycle testing on the lithium-replenished battery cell include:

[0024] The lithium-replenished battery cell is first charged at the upper limit voltage V1. After the first charge, the lithium-replenished battery cell is charged and discharged in the voltage range V-V0.

[0025] When the capacity of the lithium-replenished cell reaches 90% of the initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V2, and then the lithium-replenished cell is charged and discharged in the voltage range V-V0.

[0026] When the capacity of the lithium-replenished cell reaches 80% of the initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V3, and then the lithium-replenished cell is charged and discharged in the voltage range V-V0.

[0027] When the capacity of the lithium-replenished cell reaches 70% of its initial capacity, the charge-discharge cycle test of the lithium-replenished cell is completed.

[0028] Where V is the lower limit voltage during the charge-discharge cycle, and V0 is the upper limit voltage during the charge-discharge cycle of the lithium-ion battery, V < V0 < V1 < V2 < V3.

[0029] In one embodiment, before the lithium-replenished cell is first charged at the upper limit charging voltage V1, a coin-holding experiment is performed on the lithium replenishing agent. The voltage V3 corresponding to the complete release of the lithium replenishing agent is obtained from the coin-holding experiment, and voltages V1 and V2 are calculated based on voltages V3 and V0. Here, V1 and V2 are two trisection points of the V0-V3 interval, and V and V0 are determined based on the voltage platform of the cell system of the lithium-replenished cell.

[0030] In one embodiment, the dQ / dV-V cycle curve is either a charging dQ / dV-V cycle curve or a discharging dQ / dV-V cycle curve.

[0031] In one implementation, the relationship between the amount of lithium replenished and the deviation is established as y = a * diff + b.

[0032] In one implementation, the EFC point is the number of revolutions when the capacity reaches 80% of the initial capacity.

[0033] In one implementation, the preset number of revolutions in the determination method is 1600 revolutions.

[0034] The method for calculating lithium replenishment amount based on cell cycle testing provided in this application involves obtaining the dQ / dV-V cycle curve for each cycle of a preset number of cycles, calculating the offset delta_N of the maximum peak point at the Nth cycle, and performing multiple lithium replenishment experiments on the abnormal cell when the cycle test results indicate that the cell is abnormal. After lithium replenishment, the cell undergoes charge-discharge cycle testing until it is determined to be a normal cell, at which point the lithium replenishment experiment is stopped. When the cell is determined to be a normal cell, the lithium replenishment amount y of the normal cell is obtained and calculated. The difference *diff* between the offset of the EFC point of an abnormal cell without lithium replenishment and that of a normal cell is used to obtain a discrete point (diff, y). Charge-discharge cycle tests are performed on cells with different formulations to determine the discrete points for each formulation, establishing the relationship equation between lithium replenishment amount and deviation: y = f(diff)(a*diff + b). Charge-discharge cycle tests are then performed on cells with unknown lithium replenishment amounts to obtain the offset of the EFC point. The lithium replenishment amount for cells with unknown lithium replenishment amounts is determined based on this relationship equation. This application's method for calculating lithium replenishment amount based on cell cycle tests establishes a quantitative relationship equation between offset and actual lithium replenishment amount using the dQ / dV-V cycle curve for each cell cycle. For a cell with unknown lithium replenishment amount and a new offset *diff* value, the required lithium replenishment amount can be calculated using this relationship equation. In other words, this application's method can quantitatively calculate the required lithium replenishment amount for the battery based on the capacity decay during cycling, achieving precise lithium replenishment of the battery's positive electrode. Attached Figure Description

[0035] Figure 1 The partial dQ / dV-V cycle curves are for charge-discharge cycle tests of normal and abnormal battery cells;

[0036] Figure 2 A diagram illustrating the experimental formula for lithium replenishment at the positive electrode;

[0037] Figure 3 The capacity retention rate versus cycle number graphs are obtained from the cyclic testing of Scheme 1, Scheme 2, and Scheme 3. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of this application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, certain operations related to this application are not shown or described in the specification. This is to avoid obscuring the core parts of this application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0039] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification and drawings are only for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0040] The serial numbers assigned to components in this document, such as "first" and "second," are used only to distinguish the described objects and have no sequential or technical meaning. The terms "connection" and "linkage" used in this application, unless otherwise specified, include both direct and indirect connections (linkages).

[0041] Abnormal cycle data in lithium-ion batteries during use is mainly caused by the loss of positive electrode capacity, which is primarily due to the H2-H3 phase transition during charge and discharge. After reaching a specified number of cycles, there is a certain capacity difference between normal and abnormal cells. Since the H2-H3 phase point corresponds to the maximum peak point of the dQ / dV-V curve of the charge-discharge cycle, this application uses cycle performance as a metric for precise lithium replenishment of the positive electrode to compensate for the capacity difference between normal and abnormal cells.

[0042] This application provides a method for calculating lithium replenishment capacity based on cell cycle testing, the method comprising:

[0043] Step S1: Perform a preset number of charge-discharge cycle tests on the battery cell and evaluate the battery cell.

[0044] For reference Figure 1 , Figure 1 The partial dQ / dV-V cycle curves for charge-discharge cycle tests of normal and abnormal battery cells are shown. Figure 1The four graphs above are partial dQ / dV-V cycle curves of a normal battery cell under charge-discharge cycle testing. Figure 1 The four graphs below are partial dQ / dV-V cycle curves of the charge-discharge cycle test of abnormal battery cells.

[0045] In step S1, judging the battery cell means judging whether the battery cell is a normal battery cell or a abnormal battery cell.

[0046] Specifically, the method of judgment is as follows:

[0047] like Figure 1 As shown, the dQ / dV-V cycle curve of the cell is obtained for each cycle.

[0048] Obtaining the dQ / dV-V cycle curve for each cycle of the battery cell may include: obtaining the charge / discharge time, voltage, and capacity of the battery cell, and calculating the dQ / dV-V cycle curve for each cycle of the battery cell using the charge / discharge time, voltage, and capacity of the battery cell.

[0049] Obtain the maximum peak point (V1, dQ1 / dV1) of the dQ / dV-V cycle curve in the first cycle and the maximum peak point (Vn, dQn / dVn) in the Nth cycle, where N is an integer greater than 1;

[0050] Calculate the offset of the maximum peak point to the Nth cycle, delta_N = |dQ1 / dV1 - dQn / dVn|, and obtain the initial capacity and the capacity to the Nth cycle in the charge-discharge cycle test based on the original data. The number of cycles when the capacity to the Nth cycle reaches the preset ratio of the initial capacity is the EFC point. If the EFC point is lower than the preset number of cycles, the cell is judged to be an abnormal cell; otherwise, it is a normal cell.

[0051] The offset can be the offset of the charging dQ / dV-V cycle curve or the offset of the discharging dQ / dV-V cycle curve. The EFC point can be the number of cycles when the capacity reaches 80% of the initial capacity. The preset number of cycles can be 1600 cycles.

[0052] from Figure 1 As can be seen, the offset of an abnormal cell after multiple charge-discharge cycles is greater than that of a normal cell.

[0053] Step S2: When the cell is determined to be abnormal, perform multiple lithium replenishment experiments on the abnormal cell; perform a preset number of charge-discharge cycle tests on the replenished cell until the replenished cell is determined to be a normal cell, then stop the lithium replenishment experiment.

[0054] Among them, conducting multiple lithium replenishment experiments on abnormal cells refers to adding different doses of lithium replenishing agent to abnormal cells to create lithium replenished cells, and then conducting charge-discharge cycle tests on the lithium replenished cells until they are judged to be normal cells according to the above judgment method, at which point the lithium replenishment experiment is stopped.

[0055] Step S3: When the lithium-replenished cell is a normal cell, obtain the lithium replenishment amount y of the normal cell, that is, the amount of lithium replenishment required to turn the abnormal cell of this formula into a normal cell is y; calculate the difference diff between the offset of the EFC point of the abnormal cell without lithium replenishment and the offset of the EFC point of the normal cell to obtain the discrete point (diff, y).

[0056] Because abnormal cells degrade capacity faster and have a larger offset when reaching the set number of turns than normal cells, the diff does not need to be taken as an absolute value.

[0057] The offset of the EFC point of the abnormal cell is delta_EFC = |dQ1 / dV1 - dQefc / dVefc|. Since the EFC point of the abnormal cell can be directly measured in the preset number of charge-discharge cycle tests, it can be directly obtained using the offset calculation formula. However, the normal cell has not reached the EFC point after the preset number of charge-discharge cycle tests. Therefore, it is necessary to obtain a one-to-one correspondence between the capacity retention rate and the offset of the normal cell in the preset number of charge-discharge cycles: capacity retention rate C% = f(delta_N). The offset of the EFC point of the normal cell is determined according to the one-to-one correspondence (the EFC point is the point where the capacity retention rate is a preset percentage).

[0058] Step S4: Perform charge-discharge cycle tests on cells with different formulations to determine the discrete points of cells with different formulations and establish the relationship equation between lithium replenishment and deviation y=f(diff); for example, it can be y=a*diff+b.

[0059] Here, "cells with different formulations" typically refers to cells with different formulations from different experimental settings within the same system. The following provides examples to illustrate steps S3 and S4:

[0060] Assuming the offset of the EFC point of a normal cell is 0.429 and the offset of the EFC point of an abnormal cell is 1.10; diff = 0.429 – 1.10 = -0.671, in the lithium replenishment experiment, the required amount of lithium replenishment is 1.5g, and a discrete point (-0.671, 1.50) can be obtained; through experiments on cells with different formulations, multiple discrete points can be obtained, such as: (-0.671, 1.50), (-0.81, 1.82), (-0.421, 1.39), ... (-0.681, 1.55), thus establishing the relationship between the amount of lithium replenishment and the offset through multiple discrete points.

[0061] S5: Perform charge-discharge cycle tests on cells with unknown lithium replenishment to obtain the offset of the EFC point of the cell with unknown lithium replenishment, and determine the lithium replenishment of the cell with unknown lithium replenishment according to the relational equation.

[0062] The method for calculating lithium replenishment based on cell cycle testing in this application establishes a quantitative relationship equation between the offset and the actual lithium replenishment amount through the dQ / dV-V cycle curve of each cell cycle. For a cell with an unknown lithium replenishment amount, the new offset diff value can be calculated using the relationship equation to determine the required lithium replenishment amount. In other words, the method in this application can quantitatively calculate the required lithium replenishment amount of the battery based on the capacity decay during the cycle process, thus achieving precise lithium replenishment of the positive electrode of the battery.

[0063] In order to highlight the maximum peak point of the dQ / dV-V cycle curve, in one embodiment, after obtaining the charge and discharge time, voltage and capacity of the battery cell, the obtained multiple voltage and capacity values ​​are smoothed, and the dQ / dV-V cycle curve of the battery cell for each cycle is calculated based on the smoothed values.

[0064] In one implementation, the smoothing process can be carried out in the following manner:

[0065] The original values ​​are numbered according to the order of the obtained original values ​​to obtain a value group from the first original value to the Mth original value, where M is an integer greater than 41.

[0066] Based on M original values, M smoothed values ​​are obtained. The first smoothed value is the first original value in the value group. The second smoothed value is the average of the first and second original values ​​in the value group. The values ​​of the third to the fortieth smoothed values ​​are the average of the original value of the corresponding smoothed value number in the value group and the original values ​​of the first two numbers of the corresponding smoothed value number. The values ​​of the forty-first to the Mth smoothed values ​​are the average of the original value of the corresponding smoothed value number in the value group and the original values ​​of the first 39 numbers of the corresponding smoothed value number.

[0067] In one embodiment, this application proposes a step-by-step continuous lithium replenishment technique, wherein the step of performing charge-discharge cycle testing on the replenished battery cell includes:

[0068] The lithium-replenished battery cell is first charged at the upper limit voltage V1. After the first charge, the lithium-replenished battery cell is charged and discharged in the voltage range V-V0.

[0069] When the capacity of the lithium-replenished cell reaches 90% of the initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V2, and then the lithium-replenished cell is charged and discharged in the voltage range V-V0.

[0070] When the capacity of the lithium-replenished cell reaches 80% of the initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V3, and then the lithium-replenished cell is charged and discharged in the voltage range V-V0.

[0071] When the capacity of the lithium-replenished cell reaches 70% of its initial capacity, the charge-discharge cycle test of the lithium-replenished cell is completed.

[0072] Where V is the lower limit voltage during the charge-discharge cycle, and V0 is the upper limit voltage during the charge-discharge cycle of the lithium-ion battery, V < V0 < V1 < V2 < V3.

[0073] In one embodiment, before the lithium-replenished cell is first charged at the upper limit charging voltage V1, a coin-holding experiment is performed on the lithium replenishing agent. The voltage V3 corresponding to the complete release of the lithium replenishing agent is obtained from the coin-holding experiment, and voltages V1 and V2 are calculated based on voltages V3 and V0. Here, V1 and V2 are two trisection points of the V0-V3 interval, and V and V0 are determined based on the voltage platform of the cell system of the lithium-replenished cell.

[0074] Currently, existing positive electrode lithium replenishment technologies typically involve adding lithium replenishing agents. To prevent lithium plating at the negative electrode during the initial charge, the amount of agent added is usually small, resulting in limited actual lithium replenishment effectiveness. Furthermore, current positive electrode lithium replenishment technologies can only provide a one-time lithium replenishment to the negative electrode during the initial charge. After replenishment, residual materials remain in the battery, negatively impacting battery performance. Moreover, existing positive electrode lithium replenishment technologies cannot achieve dynamic and continuous lithium replenishment during subsequent cycles of lithium-ion batteries.

[0075] This application proposes the aforementioned stepwise continuous lithium replenishment technology. On the one hand, this stepwise continuous lithium replenishment technology can achieve lithium replenishment at a lower voltage during the first charge, avoiding the risk of lithium plating during the first charge of the negative electrode. On the other hand, this stepwise continuous lithium replenishment technology can, in subsequent cycles, repeatedly activate the active lithium in the lithium replenishment material by increasing the battery charging voltage, thereby achieving continuous lithium replenishment to the negative electrode.

[0076] This application employs the aforementioned distributed continuous lithium replenishment technology to conduct cathode lithium replenishment experiments. This cathode lithium replenishment experiment can be applied not only to lithium iron phosphate systems but also to ternary systems. The following experiment uses a lithium iron phosphate system as an example. Figure 2 As shown, Figure 2 The experimental formula diagram for the lithium replenishment experiment of the positive electrode is shown.

[0077] Option 1:

[0078] 1.1 Preparation of the positive electrode

[0079] The positive electrode active material lithium iron phosphate SF-17, conductive carbon black SP, carbon nanotubes CN-1N, binder HSV900, and solvent NMP are mixed in a mass ratio of 100:0.8:0.8:2.0:40 and mixed evenly under the action of a vacuum stirrer to obtain a positive electrode slurry. Then, the positive electrode slurry is uniformly coated on the front and back sides of a positive electrode current collector aluminum foil with a certain areal density. The electrode sheets are dried in ovens at different temperatures in sequence. After drying, the electrode sheets are rolled and cut to obtain positive electrode sheets.

[0080] 1.2 Preparation of negative electrode

[0081] The negative electrode formulation, graphite ZTH72, conductive agent SP, dispersant (Jiangmen CMC), binder B9, and solvent H2O were mixed in a mass ratio of 100:0.5:1.5:3.2:79. The mixture was stirred evenly under vacuum to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated on both sides of a copper foil current collector of a certain thickness with a certain surface density. The electrode sheets were dried sequentially in ovens at different temperatures. After drying, the electrode sheets were rolled and slit to obtain the negative electrode sheets.

[0082] 1.3 Preparation of Electrolyte

[0083] The electrolyte selected is Xinyashanshan XYSS230512-01 electrolyte.

[0084] 1.4 Selection of Diaphragm

[0085] The diaphragm was selected from Xingyuan material SH312E13(9+3)um*60.5mm*1000m.

[0086] 1.5 Preparation of Lithium-ion Batteries

[0087] The positive electrode, separator, and negative electrode are wound into a core in sequence. The separator is located between the positive and negative electrodes in the core to prevent the positive and negative electrodes from directly contacting and short-circuiting. The core is then assembled into a cell through relevant steps. After drying, a certain amount of electrolyte is injected to make a lithium-ion battery.

[0088] 1.6 Lithium-ion battery formation

[0089] The formation process for lithium-ion batteries is as follows:

[0090] ① Charge at a constant current of 0.2C to 4.27V, then charge at a constant voltage of 4.27V until the cutoff current is 0.01C, and let stand for 10 minutes;

[0091] ② Discharge at a constant current of 0.2C to 2.5V, and let stand for 10 minutes;

[0092] ③ Charge at a constant current of 0.2C to 4.2V, then charge at a constant voltage of 4.2V until the cutoff current is 0.01C, and let stand for 10 minutes;

[0093] 1.7 Lithium-ion battery capacity assessment

[0094] The capacity grading process for lithium-ion batteries is as follows:

[0095] ① Discharge at a constant current of 0.2C to 2.5V and let stand for 10 minutes;

[0096] ② Charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V until the cutoff current is 0.01C, and let stand for 10 minutes.

[0097] ③ Discharge at a constant current of 0.5C to 2.5V and let stand for 10 minutes. The discharge capacity measured at this time is the actual initial capacity of the battery with a designed capacity of 5Ah.

[0098] 1.8 Cyclic performance testing and step-by-step continuous lithium replenishment process

[0099] The cycle performance of lithium-ion batteries is tested, and the specific testing process is as follows:

[0100] During the initial charge, the battery is charged at the upper limit voltage V1 = 4.27V, allowing some of the lithium ions in the lithium replenishment additive to be released for the formation of the SEI film. The following steps ① and ② are then repeated:

[0101] ① Charge at a constant current of 0.5C within the voltage range to 4.2V, then charge at a constant voltage of 4.2V until the cutoff current is 0.01C, and let stand for 10 minutes.

[0102] ② Discharge at a constant current of 0.5C to 2.5V and let stand for 10 minutes.

[0103] Step ② measures the discharge capacity as the actual cycle capacity of the battery after this cycle is completed.

[0104] When the battery capacity retention rate is 90%, which is equivalent to a battery cycle capacity of 4.5Ah, step ① is adjusted as follows:

[0105] Charge the capacitor at a constant current of 0.5C until V2 = 4.33V, then charge it at a constant voltage of 4.33V until the cutoff current is 0.01C, and let it stand for 10 minutes. Then proceed to step ②, and finally repeat steps ① and ②.

[0106] When the battery capacity retention rate is 80%, that is, when the battery cycle capacity is 4Ah, adjust step ①. The specific adjustment process is as follows:

[0107] Charge the battery with a constant current of 0.5C to 4.4V, then charge it with a constant voltage at 4.4V until the cutoff current is 0.01C, and let it stand for 10 minutes. Then proceed to step ②, and finally repeat steps ① and ②.

[0108] When the battery capacity retention rate is 70%, which is the battery cycle capacity of 3.5Ah, record the total number of battery cycles at this time.

[0109] Option 2:

[0110] Adjust the positive electrode formulation in step 1.1 of Scheme 1 to: Lithium iron phosphate (SF-17): Lithium-rich nickel oxide (LNO): Conductive carbon black (sp): Carbon nanotubes (CN-1N): Binder (HSV900): Solvent (NMP) = 98.5: 1.5: 0.8: 0.8: 2.0: 40. Then complete steps 1.1-1.8 in the same way as Scheme 1, and record the total number of battery cycles at this time.

[0111] Option 3:

[0112] Adjust the positive electrode formulation in step 1.1 of Scheme 1 to: Lithium iron phosphate (SF-17): Lithium-rich nickel oxide (LNO): Conductive carbon black (sp): Carbon nanotubes (CN-1N): Binder (HSV900): Solvent (NMP) = 98.0: 2.0: 0.8: 0.8: 2.0: 40. Then complete steps 1.1-1.8 in the same way as Scheme 1, and record the total number of battery cycles at this time.

[0113] Experimental results: Please refer to Figure 3 Among them, BL4-1-15 and BL4-1-16 are the capacity retention rate-cycle cycle curves of two identical experiments in Scheme 1, BL4-2-66 and BL4-2-67 are the capacity retention rate-cycle cycle curves of two identical experiments in Scheme 2, and BL4-3-16 and BL4-3-34 are the capacity retention rate-cycle cycle curves of two identical experiments in Scheme 3. It can be seen that after 146 cycles at 25℃ and 0.5C / 1C, the capacity retention rate of Scheme 1 is 97.1%, the capacity retention rate of Scheme 2 is 97.7%, and the capacity retention rate of Scheme 3 is 97.6%, indicating that the use of distributed continuous lithium replenishment has a considerable improvement on the cycle performance of the battery.

[0114] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. A method for calculating lithium replenishment amount based on cell cycle testing, characterized in that, include: Perform a preset number of charge-discharge cycle tests on the battery cell and then evaluate the battery cell. The method of judgment is as follows: Obtain the dQ / dV-V cycle curve for each cycle of the battery cell; Obtain the maximum peak point (V1, dQ1 / dV1) of the dQ / dV-V cycle curve in the first cycle and the maximum peak point (Vn, dQn / dVn) in the Nth cycle, where N is an integer greater than 1; Calculate the offset of the maximum peak point up to the Nth cycle, delta_N = |dQ1 / dV1 - dQn / dVn|, to obtain the initial capacity and the capacity up to the Nth cycle in the charge-discharge cycle test. The number of cycles when the capacity up to the Nth cycle reaches a preset proportion of the initial capacity is the EFC point. If the EFC point is lower than the preset number of cycles, the cell is determined to be an abnormal cell; otherwise, it is a normal cell. When the battery cell is determined to be a faulty battery cell, multiple lithium replenishment experiments are performed on the faulty battery cell; the battery cell after lithium replenishment is subjected to the preset number of charge-discharge cycle tests until the battery cell after lithium replenishment is determined to be a normal battery cell, at which point the lithium replenishment experiment is stopped; When the lithium-replenished cell is a normal cell, the lithium replenishment amount y of the normal cell is obtained, and the difference diff between the offset of the EFC point of the abnormal cell without lithium replenishment and the offset of the EFC point of the normal cell is calculated to obtain the discrete point (diff, y). The offset of the EFC point of the abnormal cell is delta_EFC=|dQ1 / dV1-dQefc / dVefc|. The normal cell obtains a one-to-one correspondence between capacity retention rate and offset in the preset number of charge-discharge cycles, and the offset of the EFC point of the normal cell is determined according to the one-to-one correspondence. Charge-discharge cycle tests were conducted on cells with different formulations to determine the discrete points of the cells with different formulations, and the relationship equation between lithium replenishment and deviation was established as y = f(diff). A charge-discharge cycle test is performed on a battery cell with an unknown lithium replenishment amount to obtain the offset of the EFC point of the battery cell with the unknown lithium replenishment amount, and the lithium replenishment amount of the battery cell with the unknown lithium replenishment amount is determined according to the relationship equation.

2. The method for calculating lithium replenishment based on cell cycle testing according to claim 1, characterized in that, In the aforementioned determination method, obtaining the dQ / dV-V cycle curve for each cycle of the battery cell includes: obtaining the charge / discharge time, voltage, and capacity of the battery cell, and calculating the dQ / dV-V cycle curve for each cycle of the battery cell using the charge / discharge time, voltage, and capacity of the battery cell.

3. The method for calculating lithium replenishment based on cell cycle testing according to claim 2, characterized in that, After obtaining the charging and discharging time, voltage, and capacity of the battery cell, the obtained voltage and capacity values ​​are smoothed, and the dQ / dV-V cycle curve of the battery cell for each cycle is calculated based on the smoothed values.

4. The method for calculating lithium replenishment based on cell cycle testing according to claim 3, characterized in that, The smoothing process is as follows: The original values ​​are numbered according to the order of the acquired original values ​​to obtain a value group from the first original value to the Mth original value, where M is an integer greater than 41. Based on the M original values, M smoothed values ​​are obtained. The first smoothed value is the first original value in the value group. The second smoothed value is the average of the first and second original values ​​in the value group. The values ​​of the third to the fortieth smoothed values ​​are the average of the original value corresponding to the smoothed value number in the value group and the original values ​​of the first two numbers corresponding to the smoothed value number. The values ​​of the forty-first to the Mth smoothed values ​​are the average of the original value corresponding to the smoothed value number in the value group and the original values ​​of the first 39 numbers corresponding to the smoothed value number.

5. The method for calculating lithium replenishment based on cell cycle testing according to claim 1, characterized in that, The steps for performing charge-discharge cycle tests on the lithium-replenished battery cells include: The lithium-replenished battery cell is first charged at the upper limit charging voltage V1, and then the lithium-replenished battery cell is charged and discharged in the voltage range V-V0 after the first charge. When the capacity of the lithium-replenished cell reaches 90% of its initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V2, and then the lithium-replenished cell is charged and discharged in the voltage range V-V0. When the capacity of the lithium-replenished cell reaches 80% of its initial capacity, the lithium-replenished cell is charged with the upper limit charging voltage V3, and then the lithium-replenished cell undergoes charge-discharge cycles within the voltage range V-V0. When the capacity of the lithium-replenished cell reaches 70% of its initial capacity, the charge-discharge cycle test of the lithium-replenished cell is completed. Where V is the lower limit voltage during the charge-discharge cycle, and V0 is the upper limit voltage during the charge-discharge cycle of the lithium-ion battery, V < V0 < V1 < V2 < V3.

6. The method for calculating lithium replenishment based on cell cycle testing according to claim 5, characterized in that, Before the lithium-replenished cell is first charged at the upper limit charging voltage V1, a coin-stop test is performed on the lithium replenishing agent. The voltage V3 corresponding to the complete release of the lithium replenishing agent is obtained based on the coin-stop test, and voltages V1 and V2 are calculated based on voltages V3 and V0. V1 and V2 are two trisection points of the V0-V3 interval, and V and V0 are determined based on the voltage platform of the cell system of the lithium-replenished cell.

7. The method for calculating lithium replenishment amount based on cell cycle testing according to claim 1, characterized in that, The dQ / dV-V cycle curve is either a charging dQ / dV-V cycle curve or a discharging dQ / dV-V cycle curve.

8. The method for calculating lithium replenishment amount based on cell cycle testing according to claim 1, characterized in that, The established equation relating lithium replenishment amount to deviation is y = a * diff + b.

9. The method for calculating lithium replenishment based on cell cycle testing according to claim 1, characterized in that, In the aforementioned determination method, the EFC point is the number of revolutions when the capacity reaches 80% of the initial capacity.

10. The method for calculating lithium replenishment based on cell cycle testing according to claim 1, characterized in that, In the aforementioned determination method, the preset number of revolutions is 1600 revolutions.

Images