Method of chemically converting and grading lithium ion battery

Abstract

To provide a method of chemically converting and grading a lithium ion battery for significantly reducing a cycle expansion rate of the lithium ion battery, reducing an internal resistance increase rate, and significantly improving a cycle capacity retention rate.SOLUTION: A method includes the following step of: a chemical conversion step of changing a lithium ion battery up to an SOC power of 100% or more, the charging process including at least first to fifth steps, applying in each step a pressure to two opposing surfaces of the lithium ion battery, the pressure applied being 0.1 to 0.3 Mpa in the first step, 0.3 to 0.5 Mpa in the second step, 0.8 to 1.2 Mpa in the third step, 0.3 to 0.5 Mpa in the fourth step, and 0.8 to 1.2 Mpa in the fifth step; and a grading step of fixing the lithium ion battery after the chemical conversion, and performing a grading cycle after applying a pressure of 50 to 100 kg to two opposing surfaces of the lithium ion battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority from a Chinese patent application filed with the China Patent Office on April 1, 2024, bearing application number 202410390690X, the entire contents of which are incorporated herein by reference.

[0002] The present application belongs to the technical field of lithium ion batteries, and specifically relates to the formation and grading methods of lithium ion batteries. [Background technology]

[0003] With the rapid development of the new energy industry, the application scenarios of lithium-ion batteries are becoming more and more, and important application areas include digital products such as smartphones and laptops. All of these products use aluminum-plastic film flexible packaging cases, which have advantages over metal-cased batteries, such as light weight, strong plasticity, and high conversion efficiency. However, the high cycle expansion rate of lithium-ion batteries and the low hardness of aluminum-plastic film flexible packaging cases make the safety of lithium-ion batteries poor. Summary of the Invention [Problem to be solved by the invention]

[0004] In the related art, there are many effective means for reducing the cycle expansion rate, for example, by using a current collector with a special mesh-like porous structure, the volume change of the silicon-based negative electrode material during the lithium ion embedding and de-embedding process can be effectively alleviated, so that the negative electrode active layer is always in contact with the current collector, and therefore, by combining the silicon-based negative electrode material with the current collector, the silicon-based negative electrode material repeatedly expands and contracts during the cycle process, which improves the situation of voids being formed in the electrode piece structure, reduces the expansion rate of the negative electrode piece, and alleviates capacity fade. However, the manufacturing process of the special mesh-like porous structure current collector is complicated and special, which increases the cost of raw materials.

[0005] In the related art, there is a design method for reducing the cycle expansion rate of lithium ion batteries by further reducing the surface density and compaction of the pole pieces, but the improvement achieved by this method reduces the electrical properties of the lithium ion battery, such as the energy density. [Means for solving the problem]

[0006] The present application provides a method for forming and grading a lithium ion battery, which includes the following steps: Step S1, formation: charging a lithium ion battery to an SOC of 100% or more, the charging process including at least a first stage, a second stage, a third stage, a fourth stage, and a fifth stage, applying pressure to two opposing surfaces of the lithium ion battery in each stage, the applied pressure in the first stage is 0.1-0.3 MPa, the applied pressure in the second stage is 0.3-0.5 MPa, the applied pressure in the third stage is 0.8-1.2 MPa, the applied pressure in the fourth stage is 0.3-0.5 MPa, and the applied pressure in the fifth stage is 0.8-1.2 MPa; Step S2, grading: The lithium ion battery after the formation is fixed, and a pressure of 50 to 100 kg is applied to the two opposing surfaces of the lithium ion battery, and then a grading cycle is performed. [Effects of the Invention]

[0007] In the lithium-ion battery formation and grading method of the present application, a stepwise variable pressure is used during the formation of the lithium-ion battery to charge a high amount of power, up to an SOC of 100% or more. To adapt to the change in the internal stress of the electrode piece during charging, stepwise variable pressure is applied to two opposing surfaces of the lithium-ion battery being formed, so that the internal stress of the electrode piece changes up and down and is effectively released. The use of a constant pressure prevents the electrode piece and lithium-ion battery from expanding uncontrollably. The pressures of the first to fifth stages are set within the above ranges, thereby ensuring the bonding of the electrode piece active material particles, fully demonstrating the battery's performance, and ensuring that the internal stress of the electrode piece is fully released. The lithium-ion battery can be charged to a high amount of power (fully charged or above fully charged), allowing the electrode piece to fully expand. In addition, the lithium-ion battery undergoes grading after chemical formation. During grading, the lithium-ion battery is clamped flat using fasteners and pressure is applied. The lithium-ion battery is then cycled through charge and discharge cycles. When the battery is clamped and a pressure of 50-100 kg is applied, some of the cycle expansion is accelerated. Applying a pressure of 50-100 kg ensures the bonding of the active material particles in the pole pieces, allowing the battery to fully demonstrate its performance and ensuring that internal stress in the pole pieces is fully released. The combined effect of the chemical formation and grading processes significantly reduces the cycle expansion rate of the lithium-ion battery, lowers the rate of increase in internal resistance, and significantly improves the cycle capacity retention rate of the lithium-ion battery. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a graph showing the cycle thickness expansion rates of the lithium ion batteries of Examples 1 to 3 and Comparative Examples 1 to 5. [Figure 2] 1 is a graph showing the cycle capacity retention rates of the lithium ion batteries of Examples 1 to 3 and Comparative Examples 1 to 5. DETAILED DESCRIPTION OF THE INVENTION

[0009] In some embodiments, in step S1, the formation includes charging the lithium ion battery to 103% to 125% SOC, for example, 103% SOC, 105% SOC, 108% SOC, 110% SOC, 113% SOC, 115% SOC, 118% SOC, 120% SOC, 123% SOC, or 125% SOC, but is not limited to the listed values, and other unlisted values ​​within the range are equally applicable.

[0010] In this solution, by charging the lithium-ion battery to a fully charged state or higher, the expansion of the pole pieces approaches its maximum value, and a larger portion of the cycle expansion is completed earlier than planned. Furthermore, the cycle expansion rate is significantly reduced, the internal resistance increase rate is reduced, and the cycle capacity retention rate of the lithium-ion battery is greatly improved.

[0011] In some embodiments, the ratio of the pressure application times in the first stage, the second stage, the third stage, the fourth stage, and the fifth stage may be (2-3):(3-5):(45-60):(5-15):(5-20), for example, 2:3:45:5:5, 2:4:50:8:10, 2:5:55:12:15, 2:5:60:15:20, but is not limited to the recited values, and other unrecited values ​​within the numerical range are equally applicable.

[0012] In some embodiments, in step S1, the formation method specifically includes charging at a constant current of 0.02 C to 0.05 C for 2 to 3 minutes in the first stage, charging at a constant current of 0.1 C to 0.2 C for 3 to 5 minutes in the second stage, charging at a constant current of 1 C to 1.5 C for 45 to 60 minutes in the third stage, charging at a constant current of 0.1 C to 0.3 C for 10 to 20 minutes in the fourth stage, and charging at a constant current of 0 C for 5 to 20 minutes in the fifth stage. The constant current in the first stage may be, for example, 0.02 C, 0.03 C, 0.04 C, or 0.05 C, but is not limited to the listed values ​​and other unlisted values ​​within the range are equally applicable. The constant current charging time in the first stage may be, for example, 2 min, 2.5 min, or 3 min (not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable); the constant current in the second stage may be, for example, 0.1 C, 0.15 C, or 0.2 C (not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable); the constant current in the third stage may be, for example, 1 C, 1.2 C, or 1.5 C (not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable); the constant current charging time in the third stage may be, for example, 45 min, 50 min, 55 min, or 60 min (not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable). The constant current in the fourth step may be, for example, 0.1 C, 0.2 C, or 0.3 C, but is not limited to the listed values ​​and other unlisted values ​​within the range are equally applicable. The constant current charging time in the fourth step may be, for example, 10 min, 15 min, or 20 min, but is not limited to the listed values ​​and other unlisted values ​​within the range are equally applicable.The constant current charging time in the fifth step may be, for example, 5 min, 10 min, 15 min, or 20 min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are equally applicable.

[0013] In step S1 of this solution, the formation method specifically involves charging at a constant current of 0.02C to 0.05C for 2 to 3 minutes in the first stage, charging at a constant current of 0.1C to 0.2C for 3 to 5 minutes in the second stage, charging at a constant current of 1C to 1.5C for 45 to 60 minutes in the third stage, charging at a constant current of 0.1C to 0.3C for 10 to 20 minutes in the fourth stage, and charging at a constant current of 0C for 5 to 20 minutes in the fifth stage. By maintaining the charging current and charging time within these ranges in the five stages, the state of charge (SOC) is improved, the battery is fully charged or more fully charged. Furthermore, the electrode expansion approaches a higher value, and a larger portion of the cycle expansion is completed earlier, significantly reducing the cycle expansion rate and the rate of increase in internal resistance, thereby significantly improving the cycle capacity retention of the lithium-ion battery.

[0014] In some embodiments, in step S1, the chemical conversion temperatures in the first, second, third, and fourth steps may all be 60 to 90°C, for example, 60°C, 70°C, 80°C, or 90°C. However, the temperatures are not limited to the listed values, and other unlisted values ​​within the range are equally applicable.

[0015] In step S1 of this solution, the formation temperatures in the first, second, third, and fourth steps are all 60 to 90° C. By setting the formation temperatures in the first, second, third, and fourth steps to 60 to 90° C., a sufficient temperature is ensured to ensure good bonding and strong adhesion of the electrode groups, ensuring that the lithium ion battery has appropriate hardness, and delaying the deterioration and decomposition of the electrode groups and electrolyte materials, preventing premature softening of the lithium ion battery, ensuring that the lithium ion battery has appropriate hardness, and improving the electrical characteristics of the lithium ion battery.

[0016] In some embodiments, in step S1, the chemical conversion temperature in the fifth step may be 20 to 30°C, for example, 20°C, 22°C, 25°C, 28°C, or 30°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are equally applicable.

[0017] In step S1 of this solution, the formation temperature in the fifth step is set to 20-30°C, thereby improving the activity and dynamics of the internal raw materials of the lithium ion battery and further enhancing the electrical properties of the lithium ion battery. In addition, the lithium ion battery is subjected to a cooling and shaping process, which effectively releases internal stress, further reducing the thickness rebound of the lithium ion battery and improving its electrical properties.

[0018] In some embodiments, in step S2, the grading cycle includes: hold, constant current charge at a first charge ratio to a first cutoff voltage, hold, constant current discharge at a second discharge ratio to a second cutoff voltage, and cycle through charge and discharge procedures.

[0019] In some embodiments, the first charge ratio is 0.5 to 5C, and the first cutoff voltage is 4.2V to 4.5V. The first charge ratio may be, for example, 0.5C, 1C, 1.5C, 2C, 2.5C, 3C, 3.5C, 4C, 4.5C, or 5C, but is not limited to the listed values ​​and other unlisted values ​​within the numerical range are equally applicable. The first cutoff voltage may be, for example, 4.2V, 4.3V, 4.4V, or 4.5V, but is not limited to the listed values ​​and other unlisted values ​​within the numerical range are equally applicable.

[0020] In some embodiments, the second discharge ratio is 0.5 to 3C, and the second cutoff voltage is 2.5V to 3.0V. The second discharge ratio may be, for example, 0.5C, 1C, 1.5C, 2C, 2.5C, or 3C, but is not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable. The second cutoff voltage may be, for example, 2.5V, 2.8V, or 3V, but is not limited to the recited values ​​and other unrecited values ​​within the range are equally applicable.

[0021] In some embodiments, the retention time may be 5 to 10 minutes, for example, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, or 10 minutes, but is not limited to the listed values, and other unlisted values ​​within the range are equally applicable.

[0022] In some embodiments, the number of cycles of the charge / discharge procedure may be 5 to 50, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50, but is not limited to the listed numbers, and other unlisted numbers within the numerical range are equally applicable.

[0023] In this solution, the number of cycles of the charge and discharge procedure is set to 5 to 50 times, which ensures that the internal stress of the pole pieces is effectively released, improves the completeness of the cycle expansion of the lithium ion battery, and reduces the reaction of the internal raw materials of the lithium ion battery, reducing the consumption of active materials, and improving the cycle capacity retention rate of the lithium ion battery.

[0024] The following examples and comparative examples are directed to the manufacture of lithium ion batteries after injection and standing.

[0025] (1) Manufacturing of positive electrode pieces The positive electrode active material LCO (lithium cobalt oxide), binder PVDF (polyvinylidene fluoride), dispersant PVP (polyvinylpyrrolidone), and conductive agent SP (conductive carbon black Super-P) are uniformly mixed and stirred in a mass ratio of 97.4%:1.6%:0.2%:0.8% to obtain positive electrode slurry. The positive electrode slurry is then applied to aluminum foil in the coating process, and the aluminum foil is dried and cold-rolled to obtain positive electrode pieces.

[0026] (2) Manufacturing of negative electrode pieces The negative electrode active material graphite, binder SBR (styrene butadiene latex), dispersant CMC (sodium carboxymethyl cellulose), and conductive agent SP (conductive carbon black Super-P) are uniformly mixed and stirred in a mass ratio of 96.7%:1.5%:1.2%:0.6% to obtain a negative electrode slurry, which is then applied to copper foil in a coating process, dried, and cold-rolled to obtain negative electrode pieces.

[0027] (3) Selection of electrolyte Ethylene carbonate electrolyte is selected to manufacture lithium batteries.

[0028] (4) Selection of isolation membrane PE (polyethylene) membrane is used as the separator.

[0029] (5) Lithium-ion battery manufacturing The positive electrode piece, separator, and negative electrode piece are stacked in this order, and the separator is placed between the positive electrode piece and the negative electrode piece to perform isolation. The stack is then rolled up to obtain a bare battery cell. The bare battery cell is placed in an outer packaging case, dried, and then injected with electrolyte. The cell is vacuum-packed and left to stand, resulting in a lithium-ion battery after injection and standing.

[0030] Example 1 Step S1: After the injection and standing, the lithium-ion battery is placed on the clamping plate of the pressure forming cabinet. Step S2: Set up the chemical conversion process, apply 0.1 MPa in the first stage, apply 0.3 MPa in the second stage, apply 0.8 MPa in the third stage, apply 0.3 MPa in the fourth stage, and apply 0.8 MPa in the fifth stage; in the first stage, charge at 0.02 C constant current for 2 minutes, in the second stage, charge at 0.1 C constant current for 3 minutes, in the third stage, charge at 1 C constant current for 60 minutes, in the fourth stage, charge at 0.1 C constant current for 15 minutes, in the fifth stage, charge at 0 C constant current for 10 minutes; set the temperatures in the first, second, third, and fourth stages to 75°C, and in the fifth stage to 25°C; start the chemical conversion process, and charge to a total SOC of 103%; Step S3: Take out the lithium-ion battery after chemical formation and put it into the clamping plate of the grading cabinet. Step S4: Install the grading system, fasten the formed lithium-ion battery using fasteners, and apply a pressure of 50 kg. 1) Hold for 10 minutes, 2) Charge at 0.5 C to a cutoff voltage of 4.5 V, 3) Hold for 5 minutes, 4) Discharge at 0.5 C to a cutoff voltage of 3.0 V, and repeat steps 2) to 4) 10 times.

[0031] Example 2 Step S1: After the injection and standing, the lithium-ion battery is placed in the clamping plate of the pressure forming cabinet. Step S2: Set up the chemical conversion process, apply 0.2 MPa in the first stage, apply 0.4 MPa in the second stage, apply 1 MPa in the third stage, apply 0.4 MPa in the fourth stage, and apply 1 MPa in the fifth stage; in the first stage, charge at 0.03 C constant current for 3 minutes, in the second stage, charge at 0.2 C constant current for 4 minutes, in the third stage, charge at 1.3 C constant current for 50 minutes, in the fourth stage, charge at 0.2 C constant current for 10 minutes, and in the fifth stage, charge at 0 C constant current for 15 minutes; set the temperatures in the first, second, third, and fourth stages to 80°C, and set the temperature in the fifth stage to 25°C; start the chemical conversion process; charge to a total SOC of 113.2%; Step S3: Take out the lithium-ion battery after chemical formation and put it into the clamping plate of the grading cabinet. Step S4: Install the grading system, fasten the formed lithium-ion battery using fasteners, and apply a pressure of 75 kg. 1) Hold for 10 minutes, 2) Charge at 3C to a cutoff voltage of 4.5V, 3) Hold for 5 minutes, 4) Discharge at 1C to a cutoff voltage of 3.0V, and repeat steps 2) to 4) 15 times.

[0032] Example 3 Step S1: After the injection and standing, the lithium-ion battery is placed in the clamping plate of the pressure forming cabinet. Step S2: Set up the chemical conversion process, apply 0.3 MPa in the first stage, apply 0.5 MPa in the second stage, apply 1.2 MPa in the third stage, apply 0.5 MPa in the fourth stage, and apply 1.2 MPa in the fifth stage; in the first stage, charge at 0.02 C constant current for 3 minutes, in the second stage, charge at 0.2 C constant current for 5 minutes, in the third stage, charge at 1.5 C constant current for 45 minutes, in the fourth stage, charge at 0.3 C constant current for 20 minutes, and in the fifth stage, charge at 0 C constant current for 20 minutes; set the temperatures in the first, second, third, and fourth stages to 85°C, and the temperature in the fifth stage to 30°C; start the chemical conversion process; charge to a total SOC of 124.3%; Step S3: Take out the lithium-ion battery after chemical formation and put it into the clamping plate of the grading cabinet. Step S4: Set up the grading process; The lithium-ion battery after chemical conversion was fastened and fixed using a clamping tool, and a pressure of 100 kg was applied. 1) Hold for 10 minutes, 2) Charge at 5C to the cutoff voltage of 4.5V, 3) Hold for 5 minutes, 4) Discharge at 1C to the cutoff voltage of 3.0V, and repeat steps 2) to 4) 30 times in a row.

[0033] Comparative Example 1 This comparative example is similar to Example 3, except that the conversion process does not include the fifth step.

[0034] Comparative Example 2 In step S4 of this comparative example, the lithium ion battery after chemical formation was subjected to the following cycles: 1) holding for 10 minutes, 2) charging at 5 C to a cutoff voltage of 4.5 V, 3) holding for 5 minutes, and 4) discharging at 1 C to a cutoff voltage of 3.0 V. The cycles were repeated 30 times in succession, following the steps 2) to 4). The rest of the cycle was the same as in Example 3.

[0035] Comparative Example 3 This comparative example is similar to Example 3, except that the total SOC was charged to 80% during the formation process.

[0036] Comparative Example 4 This comparative example is similar to Example 3, except that the pressure value applied during the grading process is 120 kg.

[0037] Comparative Example 5 This comparative example is similar to Example 3, except that the pressure value applied during the grading process is 30 kg.

[0038] Performance Test (1) Capacity retention and thickness expansion rate test The open circuit voltage was detected for the batteries manufactured in the above examples and comparative examples as cycle test subjects. After the open circuit voltage detection was completed, the batteries were taken out and placed in a charge / discharge test cabinet. The charge / discharge procedure was set as follows: 1) hold for 10 minutes, 2) charge at 3.0C to a cut-off voltage of 4.5V, 3) hold for 5 minutes, 4) discharge at 1.0C to a cut-off voltage of 3.0V, and a full charge / full discharge cycle test was performed. The procedure from 2) to 4) was repeated until 1 to 4 were completed. After 500 consecutive cycles, the cycle test procedure was initiated, the capacity retention rate was measured, and the lithium-ion batteries were removed to measure the thickness expansion rate (thickness expansion rate = (100% SOC thickness value after 380 or 500 cycles - 100% SOC thickness value before cycling) / 100% SOC thickness value before cycling). The results are shown in Tables 1 and 2 and in Figures 1 and 2. The curves in Figure 1 are, from bottom to top, Example 3, Example 2, Example 1, Comparative Example 5, Comparative Example 4, Comparative Example 3, Comparative Example 2, and Comparative Example 1. The curves in Figure 2 are, from top to bottom, Example 3, Example 2, Example 1, Comparative Example 5, Comparative Example 4, Comparative Example 3, Comparative Example 2, and Comparative Example 1.

[0039] Table 1 JPEG2025155485000002.jpg18170

[0040] Table 2 JPEG2025155485000003.jpg18170

[0041] (2) Internal resistance increase rate At 25°C, the batteries of the examples and comparative examples were adjusted to 50% SOC and discharged at a current of 1C for 18 seconds. The battery voltage U2 and current I before the discharge was stopped, and the battery voltage U1 after the battery voltage stabilized were recorded. The DC internal resistance R was obtained by calculating according to the formula R=(U2-U1) / I. The DC internal resistances of the batteries before and after the cycle were recorded as R0 and R1, respectively. The DC resistance change rate=(R1-R0) / R0. The detection results are shown in Table 3 below.

[0042] Table 3 JPEG2025155485000004.jpg18170

[0043] Results analysis As can be seen from Figures 1 and 2 and Tables 1 and 3, The cycle thickness expansion rates of the lithium ion batteries are, from smallest to largest, Example 3 < Example 2 < Example 1 < Comparative Example 5 < Comparative Example 4 < Comparative Example 3 < Comparative Example 2 < Comparative Example 1. The internal resistance increase rates of the lithium ion batteries are, from smallest to largest, Example 3 < Example 2 < Example 1 < Comparative Example 5 < Comparative Example 4 < Comparative Example 3 < Comparative Example 2 < Comparative Example 1. The cycle capacity retention rates of the lithium ion batteries are, from largest to smallest, Example 3 > Example 2 > Example 1 > Comparative Example 5 > Comparative Example 4 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1. In Examples 1 to 3, after 380 cycles, the cycle thickness expansion rates of the lithium ion batteries were all still 7.8% or less, the internal resistance increase rates of the lithium ion batteries were all still 16.1% or less, and the cycle capacity retention rates of the lithium ion batteries were all still 83.5% or more. In Examples 1 to 3, after 500 cycles, the cycle thickness expansion rate of the lithium ion batteries was still 8.5% or less, the internal resistance increase rate of the lithium ion batteries was still 17.5% or less, and the cycle capacity retention rate of the lithium ion batteries was still 75% or more. As can be seen, the formation and grading method for lithium ion batteries of the present invention significantly reduced the cycle expansion rate of the lithium ion batteries, reduced the internal resistance increase rate, and significantly improved the cycle capacity retention rate of the lithium ion batteries.

[0044] Compared to Example 3, Comparative Example 1 does not include the fifth step of the chemical formation process, and therefore the lithium ion battery does not undergo the cooling and shaping process. As a result, the pole pieces cannot be effectively restrained, and the expansion continues to increase, resulting in a large thickness rebound of the lithium ion battery and poor electrical properties. After 380 cycles, the cycle thickness expansion rate of the lithium ion battery of Comparative Example 1 reaches a maximum of 8.6%, the internal resistance increase rate of the lithium ion battery of Comparative Example 1 reaches a maximum of 18.9%, and the cycle capacity retention rate of the lithium ion battery of Comparative Example 1 drops to 76.2%.

[0045] Compared to Example 3, in Comparative Example 2, the pressure of 100 kg was not applied in step S4, so the grading process of the lithium ion battery could not be effectively restrained without pressure, and the expansion of the pole pieces continued to increase, which increased the reaction of the internal raw materials of the lithium ion battery, increased the consumption of active materials, and reduced the cycle capacity retention of the lithium ion battery. After 380 cycles, the cycle thickness expansion rate of the lithium ion battery of Comparative Example 2 reached a maximum of 8.5%, the internal resistance increase rate of the lithium ion battery of Comparative Example 2 reached a maximum of 18.6%, and the cycle capacity retention rate of the lithium ion battery of Comparative Example 2 decreased to 76.9%.

[0046] Compared to Example 3, the formation process in Comparative Example 3 was charged to a total SOC of 80%, but the pole pieces were unable to expand sufficiently. After 380 cycles, the cycle thickness expansion rate of the lithium ion battery of Comparative Example 3 reached a maximum of 8.4%, the internal resistance increase rate of the lithium ion battery of Comparative Example 3 reached a maximum of 18.4%, and the cycle capacity retention rate of the lithium ion battery of Comparative Example 3 decreased to 77.4%.

[0047] Compared to Example 3, the pressure applied during the grading process in Comparative Example 4 was 120 kg, which caused the positive electrode active material to fall off and the structure of the active material to be destroyed, resulting in a high thickness expansion rate, a high internal resistance increase rate, and a low capacity retention rate. After 380 cycles, the cycle thickness expansion rate of the lithium ion battery of Comparative Example 4 reached a maximum of 8.3%, the internal resistance increase rate of the lithium ion battery of Comparative Example 4 reached a maximum of 18.2%, and the cycle capacity retention rate of the lithium ion battery of Comparative Example 4 decreased to 79.9%.

[0048] Compared to Example 3, in Comparative Example 5, the pressure applied during the grading process was 30 kg, and the cycle expansion of some of the pole pieces completed earlier was significantly reduced. After 380 cycles, the cycle thickness expansion rate of the lithium ion battery of Comparative Example 5 reached a maximum of 8.2%, the internal resistance increase rate of the lithium ion battery of Comparative Example 5 reached a maximum of 18.1%, and the cycle capacity retention rate of the lithium ion battery of Comparative Example 5 decreased to 80.5%.

Claims

1. 1. A lithium ion battery formation and grading method, comprising: Formation: Step S1, charging a lithium ion battery to an SOC of 100% or more, the charging process including at least a first stage, a second stage, a third stage, a fourth stage, and a fifth stage, applying pressure to two opposing surfaces of the lithium ion battery in each stage, the applied pressure in the first stage being 0.1-0.3 MPa, the applied pressure in the second stage being 0.3-0.5 MPa, the applied pressure in the third stage being 0.8-1.2 MPa, the applied pressure in the fourth stage being 0.3-0.5 MPa, and the applied pressure in the fifth stage being 0.8-1.2 MPa; Grading: A method for forming and grading a lithium ion battery, comprising: step S2 of fixing the formed lithium ion battery and applying a pressure of 50-100 kg to two opposing surfaces of the lithium ion battery, followed by a grading cycle.

2. 2. The lithium ion battery formation and grading method according to claim 1, wherein in step S1, the formation includes charging the lithium ion battery to 103% to 125% SOC power.

3. 2. The lithium-ion battery formation and grading method of claim 1, wherein the formation method in step S1 is specifically performed by: charging at a constant current of 0.02C to 0.05C for 2 to 3 minutes in the first stage; charging at a constant current of 0.1C to 0.2C for 3 to 5 minutes in the second stage; charging at a constant current of 1C to 1.5C for 45 to 60 minutes in the third stage; charging at a constant current of 0.1C to 0.3C for 10 to 20 minutes in the fourth stage; and charging at a constant current of 0C for 5 to 20 minutes in the fifth stage.

4. 2. The method for forming and grading a lithium ion battery according to claim 1, wherein in step S1, the formation temperatures in the first, second, third and fourth steps are all 60 to 90°C.

5. 2. The method for forming and grading a lithium ion battery according to claim 1, wherein in step S1, the formation temperature in the fifth step is 20 to 30°C.

6. 2. The method of claim 1, wherein in step S2, the grading cycle includes: holding, constant current charging at a first charge rate to a first cut-off voltage, holding, constant current discharging at a second discharge rate to a second cut-off voltage, and cycling through charge and discharge procedures.

7. 7. The method of claim 6, wherein the first charge ratio is 0.5 to 5C, and the first cutoff voltage is 4.2V to 4.5V.

8. 7. The method of claim 6, wherein the second discharge rate is 0.5 to 3C, and the second cutoff voltage is 2.5V to 3.0V.

9. 7. The method for forming and grading a lithium ion battery according to claim 6, wherein the holding time is 5 to 10 minutes.

10. The method for chemically forming and grading a lithium ion battery according to any one of claims 1 to 9, wherein in step S2, the number of grading cycles is 5 to 50.

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