Activation method for improving cyclic climbing of battery cell and lithium ion battery

Abstract

The invention provides an activation method for improving cyclic climbing of a battery cell and a lithium ion battery. The activation method comprises the following steps: S1, performing first charging formation on a battery cell before formation to obtain a first battery cell; s2, sequentially carrying out first shelving and second charging formation on the first battery cell to obtain a second battery cell; s3, sequentially carrying out second shelving and discharge formation on the second battery cell to obtain a third battery cell; s4, the third battery cell is repeatedly and circularly subjected to the step S2 and the step S3 in sequence, and a fourth battery cell is obtained; wherein the number of repeated cycles is larger than or equal to 3; the cut-off SOC at the end of formation is the cut-off SOC for discharge formation; and S5, sequentially carrying out first standing, secondary liquid injection, seal welding and capacity grading on the fourth battery cell. According to the activation method disclosed by the invention, the off-line capacity grading capacity of the battery cell can be obviously close to the maximum capacity, the condition of low capacity is reduced, and the problem of capacity climbing at the initial stage of circulation is improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to an activation method for improving cell cycle ramping and a lithium-ion battery. Background Technology

[0002] Lithium iron phosphate (LFP) batteries are widely used in new energy fields such as electric vehicles, energy storage, and consumer electronics due to their mature technology and relatively good safety. However, the initial capacity of LFP cells is relatively low after capacity testing and production, and the capacity increases during the subsequent 20-200 cycles. The low initial capacity affects the cell capacity testing and selection of good products, while the cells with increased capacity may lead to poorer capacity distribution consistency, thus affecting cell grouping.

[0003] Currently, methods to improve the capacity ramp-up of lithium iron phosphate (LFP) battery cells mainly include adjusting the N / P ratio, secondary discharge (capacity grading), and simultaneous capacity grading during high-temperature aging. Chinese patent application CN116845369A discloses a method for limiting the N / P ratio during cell manufacturing to 1.09~1.13, effectively improving the capacity ramp-up phenomenon in the early stages of battery cycling. Chinese patent application CN117136455A discloses a method for secondary capacity grading. This method involves cycling and discharging a finished lithium iron phosphate battery at room temperature with a small current and then allowing it to stand. After standing, the lithium iron phosphate battery undergoes secondary capacity grading, which can improve cycle ramp-up issues while reducing costs. Chinese patent application CN117117361A discloses charging a lithium iron phosphate battery after electrolyte injection at high temperature, increasing the charging voltage to 4.2~4.3V, and then performing multiple charge-discharge cycles at high temperature, reducing the lower voltage limit to 1.6~1.8V. This can promote the formation and capacity utilization of the lithium iron phosphate battery and improve the cycle capacity ramp-up problem after the lithium iron phosphate battery is off the production line.

[0004] However, the above-mentioned formation and capacity testing processes often fail to address the issue of cell capacity ramp-up during the initial cycling phase. Furthermore, the capacity gained from secondary capacity testing is often generated after eliminating polarization caused by interfacial impedance, and the material itself has not fully reached its optimal delithiation and lithium insertion state. Therefore, it is urgent to optimize the formation process to improve the low capacity dropout rate of lithium iron phosphate cells during capacity testing, enabling the cells to reach a higher material activation state before capacity testing. Summary of the Invention

[0005] The main objective of this invention is to provide an activation method for improving the cycle ramp of battery cells and a lithium-ion battery, so as to solve the problems of low production capacity and cycle ramp of lithium iron phosphate battery cells in the prior art.

[0006] To achieve the above objectives, according to one aspect of the present invention, an activation method for improving the cycle ramping of a battery cell is provided. The activation method includes: step S1, performing a first charging formation on a battery cell before formation to obtain a first battery cell; setting the charging current of the first charging formation to I1 and the cutoff SOC of the first charging formation to Q1; step S2, performing a first resting and a second charging formation on the first battery cell sequentially to obtain a second battery cell; setting the charging current of the second charging formation to I2 and the cutoff SOC of the second charging formation to Q2; step S3, performing a second resting and a discharge formation on the second battery cell sequentially to obtain a third battery cell; setting the discharge current of the discharge formation to I3 and the cutoff SOC of the discharge formation to Q3; step S4, repeating steps S2 and S3 sequentially on the third battery cell to obtain a fourth battery cell; wherein the number of repeated cycles is R, R≥3; and the cutoff SOC at the end of formation is Q3; step S5, performing a first standing, a second electrolyte injection, a sealing weld, and a capacity test on the fourth battery cell sequentially.

[0007] Furthermore, in step S1 above, I1 is 0.02~0.1C, and Q1 is 2~20%.

[0008] Furthermore, in step S2 above, the first resting time is 10~30 min, the first resting temperature is 35~50℃; and / or, I2 is 0.1~0.5℃, and Q2 is 30~50%.

[0009] Furthermore, in step S3 above, the second resting time is 10~30 min, the second resting temperature is 35~50℃; and / or, I3 is 0.1~0.5℃, and Q3 is 5~15%.

[0010] Furthermore, in step S4 above, R is 3~10.

[0011] Furthermore, step S1 above also includes: sequentially performing a liquid injection and a second settling on the bare cell to obtain a pre-formation cell; placing the pre-formation cell in a formation cabinet for a third settling followed by a first charging formation to obtain a first cell; wherein, the temperature of the second settling is 43~47℃, and the time of the second settling is 36~60h; the time of the third settling is 10~30min, and the temperature of the third settling is 35~55℃; and / or, the pre-formation cell is selected from any one or more of prismatic cells, pouch cells, and cylindrical cells.

[0012] Furthermore, step S5 above also includes: step S51, after the fourth battery cell is sequentially placed in the fourth resting position, the first static resting position, the second liquid injection position, and the sealing welding position, it is placed in the capacity testing cabinet for sequential fifth resting and constant current and constant voltage charging to obtain the fifth battery cell. The charging current of the constant current and constant voltage charging is set to I4, the charging cutoff current of the constant current and constant voltage charging is set to I5, and the cutoff voltage of the constant current and constant voltage charging is set to V1, where V1 is 3.65V; step S52, the fifth battery cell is sequentially placed in the sixth resting position and capacity testing cabinet. The discharge current for the cascading discharge is set to I6, and the discharge cutoff voltage for the cascading discharge is V2, where V2 is 2.0V. The fourth resting temperature is 35~55℃, and the fourth resting time is 10~30min. The fifth resting temperature is 20~30℃, and the fifth resting time is 10~30min. I4 is 0.2~1C, and I5 is 0.03~0.07C. And / or, the first settling temperature is 43~47℃, and the first settling time is 18~30h.

[0013] Furthermore, the sixth resting temperature is 20~30℃, and the sixth resting time is 10~30min; I6 is 0.2~1℃.

[0014] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, the positive electrode comprising a positive electrode active material, the lithium-ion battery being prepared by the activation method described above.

[0015] Furthermore, the positive electrode active material includes lithium iron phosphate.

[0016] By applying the technical solution of this invention, the battery cell undergoes multiple charge-discharge cycles within a fixed SOC range during the formation stage, followed by capacity testing to obtain a battery cell with a high degree of activation. Compared with a single (single charging direction) formation process, this application, through step S4, allows the negative electrode material to undergo sufficient volume expansion, exposing more active lithium intercalation sites. This significantly improves the initial capacity ramp-up phenomenon and has a clear advantage in initial capacity testing and fewer activation cycles. Consequently, the battery cell capacity at the end of capacity testing is closer to the maximum capacity, effectively avoiding an excessively long activation process, and thus facilitating a more accurate assessment of the battery cell capacity yield. Controlling the cutoff SOC at the end of formation to Q3 ensures the initial formation of the SEI film, thereby fully opening the graphite interlayer spacing and exposing more lithium intercalation sites. In summary, the activation method of this application can significantly improve the battery cell's capacity at the end of capacity testing, making it closer to the maximum capacity, reducing instances of low capacity, and improving the capacity ramp-up problem in the early stages of cycling. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0018] As analyzed in the background section of this application, existing lithium iron phosphate cells suffer from low production capacity and low cycle capacity. To address these issues, this application provides an activation method for improving cell cycle ramping and a lithium-ion battery.

[0019] In a typical embodiment of this application, an activation method for improving the cycle ramping of battery cells is provided. The activation method includes: step S1, performing a first charging formation on the battery cell before formation to obtain a first battery cell; setting the charging current of the first charging formation to I1 and the cutoff SOC of the first charging formation to Q1; step S2, performing a first resting and a second charging formation on the first battery cell to obtain a second battery cell; setting the charging current of the second charging formation to I2 and the cutoff SOC of the second charging formation to Q2; step S3, performing a second resting and a discharge formation on the second battery cell to obtain a third battery cell; setting the discharge current of the discharge formation to I3 and the cutoff SOC of the discharge formation to Q3; step S4, repeating steps S2 and S3 on the third battery cell to obtain a fourth battery cell; wherein the number of repeated cycles is R, R≥3; and the cutoff SOC at the end of formation is Q3; step S5, performing a first standing, a second electrolyte injection, a sealing weld, and a capacity test on the fourth battery cell.

[0020] This application involves performing multiple charge-discharge cycles within a fixed SOC range during the formation stage of the battery cell, followed by capacity grading to obtain a battery cell with a high degree of activation. Compared to a single-cycle (single charging direction) formation process, this application, through step S4, allows for sufficient volume expansion of the negative electrode material, exposing more active lithium intercalation sites. This significantly improves the initial capacity ramp-up phenomenon and offers a clear advantage in initial capacity grading, along with fewer activation cycles. Consequently, the battery cell capacity at the end of the capacity grading process is closer to the maximum capacity, effectively avoiding an excessively long activation process and thus facilitating a more accurate assessment of the battery cell capacity yield. Controlling the cutoff SOC at the end of formation to Q3 ensures the initial formation of the SEI film, thereby fully opening the graphite interlayer spacing and exposing more lithium intercalation sites. In summary, the activation method of this application can significantly improve the battery cell's capacity at the end of the capacity grading process, ensuring it is close to the maximum capacity and reducing instances of underperformance, thus improving the capacity ramp-up problem in the early stages of cycling.

[0021] State of Charge (SOC) refers to the state of charge of a battery during charging or discharging, representing the ratio of remaining battery charge to total battery capacity.

[0022] In one embodiment of this application, in step S1 above, I1 is 0.02~0.1C and Q1 is 2~20%.

[0023] Preferably controlling the charging current (I1) and the cutoff SOC (Q1) of the first charging formation within the aforementioned range helps the electrode material to be initialized under mild conditions, thereby allowing the electrode material to gradually adapt to lithium ion insertion and extraction, reducing the underutilization of some materials or irreversible structural damage caused by excessive current or excessive SOC. Simultaneously, it also helps to form a stable and thin solid electrolyte interphase (SEI) film.

[0024] Furthermore, I1 can be 0.02C, 0.04C, 0.06C, 0.08C, or 0.1C; of course, I1 can be any value within the above range. Q1 can be 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, or 20%; of course, Q1 can be any value within the above range.

[0025] In one embodiment of this application, in step S2 above, the first resting time is 10~30 min, the first resting temperature is 35~50℃; and / or, I2 is 0.1~0.5℃, and Q2 is 30~50℃.

[0026] Preferably controlling the first resting time and temperature within the above range not only helps to promote the maturation and optimization of the SEI film inside the cell, but also helps to stabilize the electrochemical system inside the cell, so that the charge distribution inside the battery can be balanced, reducing internal resistance, thereby improving the electrochemical stability of the cell.

[0027] Preferably controlling the charging current (I2) and the cutoff SOC (Q2) of the second charging formation within the aforementioned range helps to deeply activate the electrode material, allowing more active sites to participate in lithium-ion insertion and extraction, thereby achieving a higher material activation state before capacity testing. Furthermore, I2 can be 0.1C, 0.2C, 0.3C, 0.4C, or 0.5C; of course, I2 can be any value within the aforementioned range. Q2 can be 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, or 50%; of course, Q2 can be any value within the aforementioned range.

[0028] In one embodiment of this application, in step S3 above, the second resting time is 10~30 min, the second resting temperature is 35~50℃; and / or, I3 is 0.1~0.5℃, and Q3 is 5~15%.

[0029] Preferably controlling the second resting time and temperature within the above range not only helps to further stabilize the SEI film, making it less susceptible to damage during subsequent discharge formation, but also helps to evenly distribute the charge inside the cell, reducing charge concentration, thereby improving the cell's cycle performance and service life.

[0030] Preferably controlling the discharge current (I3) and the state of charge (SOC) cutoff (Q3) during discharge formation within the aforementioned range helps to thoroughly activate lithium ions in the graphite anode material, enabling them to reach the optimal lithium intercalation state and improving the overall capacity of the cell. Furthermore, it helps to reduce capacity ramp-up in subsequent cycles, improves cycle performance, and reduces the activation cycle required to reach stable capacity. I3 can be 0.1C, 0.2C, 0.3C, 0.4C, or 0.5C; of course, I3 can be any value within the aforementioned range. Q3 can be 5%, 7%, 9%, 10%, 12%, 14%, or 15%; of course, Q3 can be any value within the aforementioned range.

[0031] To further improve the activation effect, in one embodiment of this application, R is 3 to 10 in step S4 above.

[0032] In one embodiment of this application, step S1 further includes: sequentially injecting electrolyte into the bare cell and then allowing it to stand for a second time to obtain a pre-formation cell; placing the pre-formation cell in a formation cabinet for a third resting and then performing a first charging formation to obtain a first cell; wherein the temperature of the second resting is 43~47℃ and the time of the second resting is 36~60h; the time of the third resting is 10~30min and the temperature of the third resting is 35~55℃; and / or, the pre-formation cell is selected from any one or more of prismatic cells, pouch cells, and cylindrical cells.

[0033] The first electrolyte injection refers to the injection of a specified amount of electrolyte into the battery cell after assembly and baking. This allows the electrolyte to make ion contact with the electrode materials and form the SEI film. After the electrolyte injection, it needs to be left to stand for a sufficient time to allow the electrolyte to fully penetrate the electrode plates and separator, so as to avoid violent local reactions during the formation process.

[0034] Preferably controlling the temperature and time of the second settling period within the aforementioned range helps promote the formation and stability of the SEI film and the uniform distribution of the electrolyte, thereby further improving the internal structural stability of the cell. Placing the cell in a formation cabinet for a third settling period before formation, and controlling the temperature and time of the third settling period within the aforementioned range, helps to further stabilize the internal active materials of the cell, thereby facilitating the lithium-ion insertion and deintercalation processes during subsequent formation.

[0035] In one embodiment of this application, step S5 further includes: step S51, after the fourth battery cell is sequentially subjected to fourth placement, first static placement, secondary liquid injection, and sealing welding, it is placed in a capacity testing cabinet for sequential fifth placement and constant current and constant voltage charging to obtain the fifth battery cell; the charging current of constant current and constant voltage charging is set to I4, the charging cutoff current of constant current and constant voltage charging is set to I5, and the cutoff voltage of constant current and constant voltage charging is set to V1, where V1 is 3.65V; step S52, the fifth battery cell is sequentially subjected to sixth placement and... The capacity discharge is set as follows: the discharge current is I6, the discharge cutoff voltage is V2, and V2 is 2.0V; the fourth resting temperature is 35~55℃, and the fourth resting time is 10~30min; the fifth resting temperature is 20~30℃, and the fifth resting time is 10~30min; I4 is 0.2~1C, I5 is 0.03~0.07C; and / or, the first settling temperature is 43~47℃, and the first settling time is 18~30h.

[0036] Secondary electrolyte injection refers to a second replenishment of electrolyte after the initial electrolyte injection (primary electrolyte injection) during the battery cell production process. This ensures that the electrolyte inside the battery cell is fully saturated, thereby improving the battery's performance and consistency.

[0037] Sealing welding refers to the crucial process of sealing the battery's electrolyte inlet or casing opening through welding after electrolyte injection, thus isolating the battery cell from air and moisture and preventing electrolyte leakage, thereby improving the battery's sealing performance and safety.

[0038] The above capacity assessment process helps to obtain the actual capacity of the battery cells more accurately, which is beneficial for selecting battery cells that meet specific capacity standards for use in battery module assembly.

[0039] Among them, the fourth cell (i.e. the cell after formation activation) is further placed in the capacity testing cabinet for the fifth placement after the fourth placement and the first static placement. Controlling the temperature and time of the fourth and fifth placements within the above range helps to stabilize the internal state of the cell before charging and reduces the side reactions and SEI film damage during charging caused by internal instability.

[0040] After the fifth resting stage, the battery cell undergoes constant current and constant voltage charging. Controlling the charging current (I4), charging cut-off current (I5), and charging cut-off voltage V1 within the above range helps the battery cell to safely and efficiently reach its maximum capacity.

[0041] In one embodiment of this application, the temperature of the sixth resting is 20~30°C, the resting time is 10~30 min, and I6 is 0.2~1°C.

[0042] Preferably controlling the temperature and time of the sixth resting period within the above range helps the cell reach a stable internal state before capacity discharge, reducing instability during the discharge process, such as polarization.

[0043] Preferably controlling the discharge current (I6) and discharge cutoff voltage (V2) of the grading discharge within the above range helps the cell to discharge for a sufficient time, thereby obtaining accurate discharge capacity data.

[0044] In addition, I4 can be 0.2C, 0.4C, 0.6C, 0.8C or 1C, I5 can be 0.03C, 0.04C, 0.05C, 0.06C or 0.07C, and I6 can be 0.2C, 0.4C, 0.6C, 0.8C or 1C. Of course, I4, I5 and I6 can be any value within the above range.

[0045] In another typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode comprising a positive electrode active material, the lithium-ion battery being prepared by the activation method described above.

[0046] The lithium-ion batteries prepared by the above activation method have excellent initial capacity performance and high capacity retention, thereby improving the overall performance of the batteries.

[0047] In one embodiment of this application, the positive electrode active material includes lithium iron phosphate.

[0048] The activation method of this application is particularly suitable for lithium-ion batteries with lithium iron phosphate as the main active cathode material, thereby improving the initial capacity and cycle performance of the battery.

[0049] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0050] Example 1

[0051] Step S1: The bare cylindrical lithium iron phosphate cells are sequentially injected with electrolyte once and then left to stand at 45°C for 48 hours to obtain pre-formation cells. The pre-formation cells are then placed in a formation cabinet and left to stand at 45°C for 20 minutes before undergoing the first charge formation to obtain the first cell. The charging current for the first charge formation is set to I1, where I1 is 0.05C, and the cutoff SOC for the first charge formation is Q1, where Q1 is 10%.

[0052] Step S2: After the first cell is placed at 45°C for 20 minutes, a second charging formation is performed to obtain the second cell; the charging current of the second charging formation is set to I2, the cutoff SOC of the second charging formation is Q2, and Q2 is 40%.

[0053] Step S3: After the second cell is placed at 45°C for 20 minutes, it is discharged and formed to obtain the third cell; the discharge current for discharge formation is set to I3, the cutoff SOC for discharge formation is Q3, and Q3 is 10%.

[0054] Step S4: Repeat steps S2 and S3 sequentially on the third cell to obtain the fourth cell; the number of repetitions is R, where R is 5. The cutoff SOC at the end of formation is Q3 (10%). The charging current I2 for the second charging formation during the repeated cycles is 0.1C (first time), 0.2C (second time), 0.33C (third time), 0.5C (fourth time), and 0.1C (fifth time); the discharging current I3 for the discharge formation is 0.1C (first time), 0.2C (second time), 0.33C (third time), 0.5C (fourth time), and 0.1C (fifth time).

[0055] Step S5: The fourth battery cell is sequentially placed at 45℃ for 20 minutes, then at 45℃ for 24 hours, followed by secondary electrolyte injection and sealing welding. It is then placed in a capacity-balancing cabinet and placed at 25℃ for 20 minutes before constant current and constant voltage charging to obtain the fifth battery cell. The charging current for constant current and constant voltage charging is set to I4, where I4 is 0.33C; the charging cutoff current is set to I5, where I5 ​​is 0.05C; and the cutoff voltage is set to V1, where V1 is 3.65V. The fifth battery cell is then placed at 25℃ for 20 minutes before capacity-balancing discharge. The discharge current for capacity-balancing discharge is set to I6, where I6 is 0.33C; and the discharge cutoff voltage for capacity-balancing discharge is V2, where V2 is 2.0V.

[0056] Example 2

[0057] Step S1: The bare cylindrical lithium iron phosphate cells are sequentially injected with electrolyte once and then left to stand at 45°C for 48 hours to obtain pre-formation cells. The pre-formation cells are then placed in a formation cabinet and left to stand at 45°C for 20 minutes before undergoing the first charge formation to obtain the first cell. The charging current for the first charge formation is set to I1, where I1 is 0.05C, and the cutoff SOC for the first charge formation is Q1, where Q1 is 10%.

[0058] Step S2: After the first cell is placed at 45°C for 20 minutes, a second charging formation is performed to obtain the second cell; the charging current of the second charging formation is set to I2, the cutoff SOC of the second charging formation is Q2, and Q2 is 40%.

[0059] Step S3: After the second cell is placed at 45°C for 20 minutes, it is discharged and formed to obtain the third cell; the discharge current for discharge formation is set to I3, the cutoff SOC for discharge formation is Q3, and Q3 is 10%.

[0060] Step S4: Repeat steps S2 and S3 sequentially for the third cell to obtain the fourth cell; the number of cycles is R, where R is 5; and the cutoff SOC at the end of formation is Q3 (10%). The charging current I2 for the second charging formation during the repeated cycles is 0.1C (first time), 0.5C (second time), 0.33C (third time), 0.2C (fourth time), and 0.1C (fifth time); the discharging current I3 for the discharge formation is 0.1C (first time), 0.5C (second time), 0.33C (third time), 0.2C (fourth time), and 0.1C (fifth time).

[0061] Step S5: The fourth cell is sequentially placed at 45℃ for 20 minutes and then at 45℃ for 24 hours. Following this, it undergoes secondary electrolyte injection and sealing welding. It is then placed in a capacity-balancing cabinet and placed at 25℃ for 20 minutes before being charged with constant current and constant voltage to obtain the fifth cell. The charging current for constant current and constant voltage charging is set to I4, where I4 is 0.33C; the charging cutoff current is set to I5, where I5 ​​is 0.05C; and the cutoff voltage is set to V1, where V1 is 3.65V. The fifth cell is then placed at 25℃ for 20 minutes before being discharged. The discharge current for the capacity-balancing discharge is set to I6, where I6 is 0.33C; and the discharge cutoff voltage for the capacity-balancing discharge is V2, where V2 is 2.0V.

[0062] Example 3

[0063] The difference from Example 1 is that I1 is 0.02C and Q1 is 2%.

[0064] Example 4

[0065] The difference from Example 1 is that I1 is 0.1C and Q1 is 20%.

[0066] Example 5

[0067] The difference from Example 1 is that Q2 is 25%.

[0068] Example 6

[0069] The difference from Example 1 is that Q2 is 30%.

[0070] Example 7

[0071] The difference from Example 1 is that Q2 is 50%.

[0072] Example 8

[0073] The difference from Example 1 is that Q2 is 60%.

[0074] Example 9

[0075] The difference from Example 1 is that Q3 is 5%.

[0076] Example 10

[0077] The difference from Example 1 is that Q3 is 15%.

[0078] Example 11

[0079] The difference from Example 1 is that Q3 is 20%.

[0080] Example 12

[0081] The difference from Example 1 is that the number of cycles R is 3, that is, the charging current I2 of the second charging is 0.1C (first time), 0.2C (second time), and 0.33C (third time) in sequence; the discharge current I3 of the discharge is 0.1C (first time), 0.2C (second time), and 0.33C (third time) in sequence.

[0082] Example 13

[0083] The difference from Example 1 is that the number of cycles R is 10, that is, the S4 process of Example 1 is repeated twice.

[0084] Example 14

[0085] The difference from Example 1 is that the number of cycles R is 20, that is, the S4 process of Example 1 is repeated four times.

[0086] Comparative Example 1

[0087] The difference from Example 1 is that in step S2, the first cell is placed at 45°C for 20 minutes and then subjected to a second charging formation to obtain a second cell; the charging current of the second charging formation is set to I2, where I2 is 0.1C, and the cutoff SOC of the second charging formation is Q2, where Q2 is 40%; the second cell is then directly subjected to step S5.

[0088] Comparative Example 2

[0089] The difference from Example 1 is that after the fifth cell is placed at 25°C for 20 minutes for a sixth time for capacity discharge, the discharged cell is placed in the capacity discharge cabinet for 30 minutes, and then a second capacity discharge is performed. The discharge current of the second capacity discharge is 0.1C, and the discharge cutoff voltage of the capacity discharge is 2.0V.

[0090] Comparative Example 3

[0091] Step S1: The bare cylindrical lithium iron phosphate cells are sequentially injected with electrolyte and then left to stand at 45°C for 48 hours to obtain the pre-formation cells. The pre-formation cells are placed in a formation cabinet and left to stand at 45°C for 20 minutes before undergoing the first charge formation. After standing for 30 minutes, the cells are placed in a high-temperature environment at 45°C for 24 hours to obtain the first cells. The charging current for the first charge formation is set to I1, where I1 is 0.05C, and the cutoff SOC for the first charge formation is Q1, where Q1 is 10%.

[0092] Step S2: After the first cell is placed at 45°C for 20 minutes, it undergoes a second charging formation. After being placed at 45°C for 30 minutes, the cell is placed in a high-temperature environment at 45°C for 24 hours to obtain the second cell. The charging current for the second charging formation is set to I2, where I2 is 0.1C and the cutoff voltage is 4.2V.

[0093] Step S3: Place the second battery cell in the capacity testing cabinet and let it rest at 35°C for 30 minutes before charging. The charging current is set to 0.5C, the charging cut-off current is 0.05C, and the charging cut-off voltage is set to 4.2V. After resting for 1 minute, begin the discharge process. The discharge current is set to 0.5C, and the discharge cut-off voltage is 1.8V. After resting for 30 minutes, charge again at a constant current and constant voltage of 0.33C to 3.65V, with a cut-off current of 0.05C. After resting for 30 minutes, discharge to 2.0V.

[0094] Test method:

[0095] Capacity determination: The cells were placed in a constant temperature capacity determination chamber and charged with a constant current and constant voltage using a current of 0.33C. The cutoff voltage was 3.65V and the cutoff current was 0.05C. After resting for 30 minutes, a constant current discharge of 1C was performed, with a discharge cutoff voltage of 2.0V. After the discharge was completed, the cells were rested for 30 minutes. This charge-discharge cycle was performed 3 times. The capacity of the third cycle of 1C constant current discharge was taken as C0. Five parallel samples were tested and recorded for each example and comparative example.

[0096] Cycling: After the cell is capacitated, it is cycled at room temperature (25℃). In each cycle, charging is performed using 1C constant current and constant voltage charging, with a cutoff voltage of 3.65V and a cutoff current of 0.05C. After resting for 30 minutes, 1C constant current discharge is performed, with a discharge cutoff voltage of 2.0V. After discharge, it is rested for 30 minutes. The discharge capacity of each cycle is recorded and denoted as C1, C2, and so on. n (n≤200)

[0097] Cyclic capacity ramp-up rate calculation: Statistics from C1, C2 to C n The maximum value C in max The corresponding number of cycles and the maximum value C max Compared to the climb rate at C0. The climb rate is calculated as follows (C...max -C0 / C0)×100%.

[0098] The test results are shown in Tables 1 and 2.

[0099] Table 1

[0100]

[0101] Table 2

[0102]

[0103] As can be seen from the above, in Embodiments 1 and 2 of this application, the material (graphite) is more fully activated during the charging and discharging process in a multi-turn fixed SOC range during the formation stage, thereby exposing more lithium intercalation sites, resulting in a higher capacity of the cell after capacity division, with the number of activation turns basically within 20 turns, and a smaller capacity ramp-up.

[0104] The setting of Q1 in the first formation stage mainly affects the initial SEI film formation process of the negative electrode. The initial formation of the SEI film requires a lower current to obtain a more stable structure and needs to reach a certain voltage threshold. In Example 5, the SEI film formation current is slightly larger, and the SEI film formation exhibits non-uniformity. Similarly, its initial capacity decreases and its cycle capacity retention rate decreases.

[0105] The SOC range (i.e., the range between Q2 and Q3) and the number of cycles during the second formation stage of repeated charge-discharge processes have a significant impact on the cell's capacity utilization and capacity retention. An excessively wide SOC range (i.e., smaller Q3, larger Q2, and a greater number of cycles) allows for more complete volume expansion of the negative electrode, requiring fewer activation cycles and maintaining a higher initial cell capacity. However, a wider SOC range means more electrolyte consumption, accelerating cell degradation during cycling, as in Example 8. Conversely, a narrower SOC range (i.e., larger Q3, smaller Q2, and fewer cycles) limits the volume expansion of the negative electrode, reducing the number of active lithium intercalation sites and requiring more activation cycles. This results in a lower initial cell capacity utilization, but less electrolyte consumption, maintaining a higher capacity retention during cycling, as in Example 11. In Example 14, a higher number of cycles means more electrolyte consumption, accelerating cell degradation during cycling.

[0106] Comparative Examples 1 and 2 were in the activation process during the first 200 cycles, resulting in increased capacity. However, Comparative Example 1 only had one cycle in the formation stage, leading to low material activation, the lowest capacity during capacity testing, and a larger number of activation cycles, resulting in a higher capacity ramp-up. Comparative Example 2, due to its two-stage capacity testing process, eliminated most of the polarization at the battery interface, allowing the cell capacity to be further utilized, and its capacity testing capacity was slightly higher than Comparative Example 1. However, the material activation level was still low, and the number of activation cycles and the later capacity ramp-up were still relatively large. Comparative Example 3 widened the upper and lower voltage limits before capacity testing, which to some extent eliminated some polarization and completed some activation treatment of the material. However, compared with the examples, Comparative Example 3 had a shorter activation duration for the material, a lower degree of optimization, and increased electrolyte side reactions due to the higher charging cut-off voltage. The capacity retention rate of the battery after 200 cycles was lower than that of the examples, which to some extent shortened the normal service life of the cell.

[0107] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0108] This application involves performing multiple charge-discharge cycles within a fixed SOC range during the formation stage of the battery cell, followed by capacity grading to obtain a battery cell with a high degree of activation. Compared to a single-cycle (single charging direction) formation process, this application, through step S4, allows for sufficient volume expansion of the negative electrode material, exposing more active lithium intercalation sites. This significantly improves the initial capacity ramp-up phenomenon and offers a clear advantage in initial capacity grading, along with fewer activation cycles. Consequently, the battery cell capacity at the end of the capacity grading process is closer to the maximum capacity, effectively avoiding an excessively long activation process and thus facilitating a more accurate assessment of the battery cell capacity yield. Controlling the cutoff SOC at the end of formation to Q3 ensures the initial formation of the SEI film, thereby fully opening the graphite interlayer spacing and exposing more lithium intercalation sites. In summary, the activation method of this application can significantly improve the battery cell's capacity at the end of the capacity grading process, ensuring it is close to the maximum capacity and reducing instances of underperformance, thus improving the capacity ramp-up problem in the early stages of cycling.

[0109] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An activation method for improving cell cycle ramping, characterized in that, The activation method includes: Step S1: Perform a first charging formation on the pre-formed battery cell to obtain a first battery cell; set the charging current of the first charging formation to I1 and the cutoff SOC of the first charging formation to Q1. Step S2: The first cell is sequentially subjected to a first resting and a second charging formation to obtain a second cell; the charging current of the second charging formation is set to I2, and the cutoff SOC of the second charging formation is Q2. Step S3: The second cell is sequentially subjected to a second rest and discharge formation to obtain a third cell; the discharge current of the discharge formation is set to I3, and the cutoff SOC of the discharge formation is Q3. Step S4: Repeat steps S2 and S3 sequentially on the third cell to obtain the fourth cell; wherein the number of repetitions is R, R≥3; and the cutoff SOC at the end of the formation is Q3. Step S5 involves sequentially performing the first settling, second electrolyte injection, sealing welding, and capacity testing on the fourth battery cell.

2. The activation method according to claim 1, characterized in that, In step S1, I1 is 0.02~0.1C and Q1 is 2~20%.

3. The activation method according to claim 1 or 2, characterized in that, In step S2, the first resting time is 10~30 min, the first resting temperature is 35~50℃; and / or, I2 is 0.1~0.5℃, and Q2 is 30~50%.

4. The activation method according to any one of claims 1 to 3, characterized in that, In step S3, the second resting time is 10-30 minutes, the second resting temperature is 35-50°C; and / or, I3 is 0.1-0.5°C, and Q3 is 5-15%.

5. The activation method according to any one of claims 1 to 4, characterized in that, In step S4, R is 3 to 10.

6. The activation method according to any one of claims 1 to 5, characterized in that, Step S1 further includes: sequentially injecting electrolyte into the bare cell and then allowing it to stand for a second time to obtain the pre-formation cell; placing the pre-formation cell in a formation cabinet for a third placement and then performing the first charging formation to obtain the first cell. The second standing temperature is 43~47℃, and the second standing time is 36~60h; the third resting time is 10~30min, and the third resting temperature is 35~55℃. And / or, the pre-formation cell is selected from any one or more of prismatic cells, pouch cells, and cylindrical cells.

7. The activation method according to any one of claims 1 to 6, characterized in that, Step S5 further includes: Step S51: After the fourth battery cell is placed in the capacity testing cabinet, it undergoes the fourth placement, the first static placement, the second liquid injection, and the sealing welding in sequence. Then, it undergoes the fifth placement and constant current and constant voltage charging in sequence to obtain the fifth battery cell. The charging current of the constant current and constant voltage charging is set to I4, the charging cut-off current of the constant current and constant voltage charging is set to I5, and the cut-off voltage of the constant current and constant voltage charging is set to V1, where V1 is 3.65V. Step S52: The fifth cell is sequentially subjected to a sixth rest and capacity-divided discharge. The discharge current of the capacity-divided discharge is set to I6, and the discharge cutoff voltage of the capacity-divided discharge is V2, where V2 is 2.0V. The temperature of the fourth resting is 35~55℃, and the time of the fourth resting is 10~30min; The fifth resting temperature is 20~30℃, and the fifth resting time is 10~30min; The I4 is 0.2~1C, and the I5 is 0.03~0.07C; And / or, the temperature of the first settling period is 43~47℃, and the time of the first settling period is 18~30h.

8. The activation method according to claim 7, characterized in that, The sixth resting temperature is 20~30℃, and the sixth resting time is 10~30min; I6 is 0.2~1℃.

9. A lithium-ion battery, comprising a positive electrode, said positive electrode comprising a positive electrode active material, characterized in that, The lithium-ion battery is prepared by the activation method according to any one of claims 1 to 8.

10. The lithium-ion battery according to claim 9, characterized in that, The positive electrode active material includes lithium iron phosphate.

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