Lithium ion battery and preparation method thereof

Abstract

The invention discloses a lithium ion battery and a preparation method thereof, and belongs to the technical field of lithium ion batteries. The preparation method of the lithium ion battery comprises the following steps: S1, processing a regenerated graphite material into a negative plate, laminating a positive plate, a diaphragm and the negative plate, putting into a shell to obtain a dry battery cell, injecting an electrolyte into the dry battery cell, and sealing; s2, carrying out first infiltration on the sealed battery cell; s3, charging the battery cell infiltrated for the first time to a voltage of 300-1000mV by adopting a current of less than or equal to 0.05 C; s4, carrying out second infiltration on the charged battery cell; and S5, charging the battery cell after the second infiltration, performing formation, capacity grading and aging, and then discharging the battery cell from the production line to obtain the lithium ion battery. According to the preparation method of the lithium ion battery provided by the invention, the dry cell made of the regenerated graphite material is initially charged after liquid injection, and the voltage before formation is increased, so that the adverse reaction of an electrode and an electrolyte in the electrolyte infiltration process is blocked, and the adverse effect on the lithium ion battery is eliminated.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a lithium-ion battery and a preparation method thereof. Background Art

[0002] Due to its advantages such as high energy density, long cycle life, and low self-discharge rate, lithium-ion batteries have become the preferred power source in many application fields and are widely used in electric vehicles, mobile phones, laptops, energy storage, and other fields. With the continuous increase in the application of lithium-ion batteries, the installed capacity of lithium-ion batteries is getting larger and larger, and the scrapping of lithium-ion batteries has become an environmental problem and a resource problem that the industry must face. In this situation, the recycling and reuse of lithium-ion batteries have gradually been put on the agenda.

[0003] At present, when using recycled graphite materials as the negative electrode active material in the manufacturing process of lithium-ion batteries, the same process route as that of conventional materials is still adopted, that is, the dry battery cells are put into the shell, injected with electrolyte, infiltrated at a temperature of 45°C for 24 hours, and then the formation process is carried out. However, when this scheme is applied to lithium-ion batteries obtained by processing recycled graphite, compared with lithium-ion batteries obtained by processing conventional materials, a certain proportion of lithium-ion batteries have the problem of low initial voltage (initial voltage ≤ 20 mV) before formation, and after the production process using this scheme, a certain proportion of the battery cells have poor negative electrode interface conditions, which affect the life of lithium-ion batteries during subsequent cycling. And the above-mentioned adverse effects are currently difficult to be screened out through the evaluation of relevant electrical performance parameters of the battery cells, resulting in poor consistency during subsequent battery cell matching and affecting the performance of the subsequent lithium-ion battery pack. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing a lithium-ion battery using recycled graphite materials; another purpose of the present invention is to provide a lithium-ion battery prepared using recycled graphite materials with excellent cycle performance.

[0005] The present invention discloses a preparation method of a lithium-ion battery, including the following steps:

[0006] S1: Process the recycled graphite material into a negative electrode sheet, stack the positive electrode sheet, separator, and negative electrode sheet, put them into the shell to obtain a dry battery cell, and inject electrolyte into the dry battery cell and then seal it;

[0007] S2: Perform the first infiltration on the sealed battery cell;

[0008] S3: Charge the battery cell after the first infiltration with a current of ≤ 0.05C to a voltage of 300 - 1000 mV;

[0009] S4: Perform the second infiltration on the charged battery cell;

[0010] S5: After charging, forming, grading, and aging the battery cells after the second infiltration, take them offline to obtain lithium-ion batteries.

[0011] The regenerated graphite material is obtained by processes such as crushing, pickling, purification, and coating of the negative electrode sheets of waste lithium-ion batteries.

[0012] The active material of the positive electrode sheet is one or more mixtures of lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron manganese phosphate, and lithium iron phosphate.

[0013] The preparation method of the lithium-ion battery disclosed in the present invention does not require other controls and restrictions from the material end. The new process only adds one infiltration and charging operation, and the previous old equipment can be used without additional investment; it greatly improves the problem of interface abnormality of lithium-ion batteries obtained using regenerated graphite, and improves the cycle performance of lithium-ion batteries; it improves the consistency of products after mass production, which is beneficial to reducing losses and improving efficiency in later stage battery assembly.

[0014] Further, in the step S2, the first infiltration is carried out in an environment of 39 - 45 °C.

[0015] The infiltration temperature is 39 - 45 °C to ensure the infiltration degree.

[0016] Further, in the step S2, the time of the first infiltration is 3 - 7 h.

[0017] The time of the first infiltration is 3 - 7 h to meet the conditions for pre-charging after the first infiltration.

[0018] Further, in the step S3, the charging current is 0.005 - 0.015 C.

[0019] After the first infiltration of the battery cells, pre-charge the battery cells with a small current of 0.005 - 0.015 C to meet the charging current limit allowed after the first infiltration.

[0020] Further, in the step S3, charge the battery cells to a voltage of 300 - 500 mV.

[0021] Pre-charge to a voltage of 300 - 500 mV with a small current to meet the adverse reaction of blocking the electrolyte and the electrode in the later long-term infiltration stage.

[0022] Further, in the step S4, the second infiltration is carried out in an environment of 39 - 45 °C.

[0023] Further, in the step S4, the time of the second infiltration is 14 - 18 h.

[0024] The time of the second infiltration of the battery cells is 14 - 18 h to meet the conditions for forming after infiltration.

[0025] Further, in step S5, the formation method adopted is as follows: standing still for 5 min; charging with a current of 0.008C for 20 min; charging with a current of 0.03C for 75 min; charging with a current of 0.05C for 50 min; standing still for 5 min; charging with a current of 0.3C for 45 min; standing still for 5 min.

[0026] Adopting the above formation method can shorten the formation time without affecting the performance of the lithium-ion battery, which is beneficial to improving the production efficiency and thus increasing the production capacity.

[0027] The present invention also discloses a lithium-ion battery obtained by the preparation method described above.

[0028] Further, the capacity retention rate of the lithium-ion battery provided by the present invention is higher than 85% after 1200 cycles.

[0029] For the dry battery cell made of recycled graphite material, a preliminary charge is carried out immediately after injecting the electrolyte in the preparation method of a lithium-ion battery provided by the present invention, so as to increase the voltage before formation and block the adverse reaction between the electrode and the electrolyte during the electrolyte infiltration process, thereby eliminating the adverse effect on the lithium-ion battery. Description of the Drawings

[0030] Figure 1 is the initial voltage distribution diagram before formation of the examples and comparative examples in the present invention;

[0031] Figure 2 is the full-charge disassembled negative electrode interface diagram of the battery cells with normal initial voltage (a) and low initial voltage (b) in the examples of the present invention;

[0032] Figure 3 is the relationship diagram between the number of cycles and the capacity retention rate of the examples and comparative examples in the present invention. Detailed Embodiments

[0033] To make the technical solutions of the present invention clearer, the following further describes the present invention in detail with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Prepare a lithium-ion battery:

[0036] Using lithium iron phosphate as the main cathode material, it is mixed evenly with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in an NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. The cathode slurry is evenly coated on both sides of a 12+1+1μm carbon-coated aluminum foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted cathode electrode roll. After slicing with a hardware die cutter, a 148*215mm cathode electrode sheet is obtained; using recycled graphite as the main anode material, it is mixed evenly with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in a deionized water solvent to obtain an anode slurry with a solid content of 54%. The anode slurry is evenly coated on both sides of a 4.5μm copper foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted anode electrode roll. After slicing with a hardware die cutter, a 151*219mm anode electrode sheet is obtained; the above-mentioned anode and cathode electrode sheets are separated by a PP separator and laminated 20 / 21 layers to obtain a bare battery cell with a capacity of 30Ah. After being put into a shell, a semi-finished dry battery cell is obtained, and electrolyte injection is carried out. The composition of the electrolyte: LiPF 6 The concentration is 1.2mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and it is sealed.

[0037] It is placed in an environment of 42°C for the first infiltration for 5 hours.

[0038] The infiltrated battery cell is charged to 500mV at a current of 0.01C.

[0039] It is placed in an environment of 42°C for the second infiltration for 17 hours, and after formation, grading, and aging, it is taken off the production line.

[0040] Formation: Stand still for 5 minutes; charge at a current of 0.008C for 20 minutes; charge at a current of 0.03C for 75 minutes; charge at a current of 0.05C for 50 minutes; stand still for 5 minutes; charge at a current of 0.3C for 45 minutes; stand still for 5 minutes.

[0041] Grading: Charge at a constant current and constant voltage of 0.5C to 3.65V 0.05C, stand still for 10 minutes, discharge at a current of 1C to 2.5V, stand still for 5 minutes, discharge at a current of 0.5C to 2.0V, stand still for 5 minutes, discharge at a current of 0.1C to 2.0V, stand still for 5 minutes, charge at a current of 0.5C for 5 minutes, stand still for 5 minutes, and then transfer to aging.

[0042] Aging: Stand still at 42°C for 24 hours and at 25°C for 72 hours.

[0043] Example 2

[0044] Preparation of Lithium-Ion Batteries:

[0045] Using lithium iron phosphate as the main cathode material, it is mixed evenly with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. The cathode slurry is evenly coated on both sides of a 12+1+1μm carbon-coated aluminum foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted cathode electrode roll. After slicing with a hardware die cutter, a 148*215mm cathode electrode sheet is obtained; using recycled graphite as the main anode material, it is mixed evenly with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in deionized water solvent to obtain an anode slurry with a solid content of 54%. The anode slurry is evenly coated on both sides of a 4.5μm copper foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted anode electrode roll. After slicing with a hardware die cutter, a 151*219mm anode electrode sheet is obtained; the above positive and negative electrode sheets are separated by a PP separator and stacked 20 / 21 layers to obtain a bare battery cell with a capacity of 30Ah. After putting it into the shell, a semi-finished dry battery cell is obtained, and then liquid injection is carried out. The composition of the electrolyte: LiPF 6 The concentration is 1.2mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and then it is sealed.

[0046] It is placed in an environment of 42°C for the first soaking for 5 hours.

[0047] The soaked battery cell is charged to 500mV with a current of 0.01C.

[0048] It is placed in an environment of 42°C for the second soaking for 17 hours, and then undergoes formation, grading, and aging before coming off the production line.

[0049] Formation: Stand still for 5 minutes; charge with a current of 0.02C for 3 hours; stand still for 30 minutes; charge with a current of 0.1C for 3 hours; stand still for 5 minutes.

[0050] Grading: Constant current and constant voltage charge at 0.5C to 3.65V 0.05C, stand still for 10 minutes, discharge at a current of 1C to 2.5V, stand still for 5 minutes, discharge at a current of 0.5C to 2.0V, stand still for 5 minutes, discharge at a current of 0.1C to 2.0V, stand still for 5 minutes, charge at a current of 0.5C for 5 minutes, stand still for 5 minutes, and then transfer to aging.

[0051] Aging: Stand still at 42°C for 24 hours and at 25°C for 72 hours.

[0052] Example 3

[0053] Preparation of Lithium-ion Batteries:

[0054] Using lithium iron phosphate as the main cathode material, it is mixed evenly with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. The cathode slurry is evenly coated on both sides of a 12+1+1μm carbon-coated aluminum foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted cathode electrode roll. After slicing with a hardware die cutter, a 148*215mm cathode electrode sheet is obtained; using recycled graphite as the main anode material, it is mixed evenly with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in deionized water solvent to obtain an anode slurry with a solid content of 54%. The anode slurry is evenly coated on both sides of a 4.5μm copper foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted anode electrode roll. After slicing with a hardware die cutter, a 151*219mm anode electrode sheet is obtained; the above-mentioned anode and cathode sheets are separated by a PP separator and laminated 20 / 21 layers to obtain a bare battery cell with a capacity of 30Ah. After being put into the shell, a semi-finished dry battery cell is obtained, and then electrolyte injection is carried out. The composition of the electrolyte: LiPF 6 has a concentration of 1.2mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and then it is sealed.

[0055] It is placed in an environment of 42°C for the first soaking for 2 hours.

[0056] The soaked battery cell is charged to 500mV with a current of 0.01C.

[0057] It is placed in an environment of 42°C for the second soaking for 17 hours, and then undergoes formation, grading, and aging before being taken off the production line.

[0058] Formation: Stand still for 5 minutes; charge with a current of 0.008C for 20 minutes; charge with a current of 0.03C for 75 minutes; charge with a current of 0.05C for 50 minutes; stand still for 5 minutes; charge with a current of 0.3C for 45 minutes; stand still for 5 minutes.

[0059] Grading: Constant current and constant voltage charge to 3.65V at 0.5C, stand still for 10 minutes, discharge with a current of 1C to 2.5V, stand still for 5 minutes, discharge with a current of 0.5C to 2.0V, stand still for 5 minutes, discharge with a current of 0.1C to 2.0V, stand still for 5 minutes, charge with a current of 0.5C for 5 minutes, stand still for 5 minutes, and then transfer to aging.

[0060] Aging: Stand still at 42°C for 24 hours and at 25°C for 72 hours.

[0061] Example 4

[0062] Prepare a lithium-ion battery:

[0063] Using lithium iron phosphate as the main cathode material, mix it evenly with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. After passing through an extrusion coater, the cathode slurry is evenly coated on both sides of a 12+1+1μm carbon-coated aluminum foil, baked, and then pressed by a slit roller press to obtain a compacted cathode pole roll. After slicing with a hardware die cutter, a 148*215mm cathode pole piece is obtained; using recycled graphite as the main anode material, mix it evenly with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in deionized water solvent to obtain an anode slurry with a solid content of 54%. After passing through an extrusion coater, the anode slurry is evenly coated on both sides of a 4.5μm copper foil, baked, and then pressed by a slit roller press to obtain a compacted anode pole roll. After slicing with a hardware die cutter, a 151*219mm anode pole piece is obtained; separate the above positive and negative pole pieces with a PP separator and stack them 20 / 21 layers to obtain a bare battery cell with a capacity of 30Ah. After putting it into a shell, a semi-finished dry battery cell is obtained, and then it is injected with electrolyte. The composition of the electrolyte: LiPF 6 The concentration is 1.2mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and then it is sealed.

[0064] Place it in an environment of 42°C for the first soaking for 5 hours.

[0065] Charge the soaked battery cell with a current of 0.05C to 1000mV.

[0066] Place it in an environment of 42°C for the second soaking for 17 hours, and then take it off the production line after formation, grading, and aging.

[0067] Formation: Stand still for 5 minutes; charge with a current of 0.008C for 20 minutes; charge with a current of 0.03C for 75 minutes; charge with a current of 0.05C for 50 minutes; stand still for 5 minutes; charge with a current of 0.3C for 45 minutes; stand still for 5 minutes.

[0068] Grading: Charge at a constant current and constant voltage of 0.5C to 3.65V 0.05C, stand still for 10 minutes, discharge with a current of 1C to 2.5V, stand still for 5 minutes, discharge with a current of 0.5C to 2.0V, stand still for 5 minutes, discharge with a current of 0.1C to 2.0V, stand still for 5 minutes, charge with a current of 0.5C for 5 minutes, stand still for 5 minutes, and then transfer to aging.

[0069] Aging: Stand still at 42°C for 24 h and at 25°C for 72 h.

[0070] Comparative Example 1

[0071] Prepare a lithium-ion battery:

[0072] Use lithium iron phosphate as the main cathode material, mix it with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in an NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. The cathode slurry is evenly coated on both sides of a 12 + 1 + 1 μm carbon-coated aluminum foil through an extrusion coater, baked, and then pressed through a slit roll press to obtain a compacted cathode pole roll. After slicing with a hardware die cutter, a 148*215 mm cathode pole piece is obtained; use recycled graphite as the main anode material, mix it with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in a deionized water solvent to obtain an anode slurry with a solid content of 54%. The anode slurry is evenly coated on both sides of a 4.5 μm copper foil through an extrusion coater, baked, and then pressed through a slit roll press to obtain a compacted anode pole roll. After slicing with a hardware die cutter, a 151*219 mm anode pole piece is obtained; separate the above positive and negative pole pieces with a PP separator and stack them in 20 / 21 layers to obtain a bare battery cell with a capacity of 30 Ah. After putting it into the shell, a semi-finished dry battery cell is obtained, and then electrolyte injection is carried out. The composition of the electrolyte: The concentration of LiPF 6 is 1.2 mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and then it is sealed.

[0073] Place it in an environment at 42°C for infiltration for 22 h, carry out formation, and then offline after grading and aging.

[0074] Formation: Stand still for 5 min; charge at a current of 0.02C for 3 h; stand still for 30 min; charge at a current of 0.1C for 3 h; stand still for 5 min.

[0075] Grading: Charge at a constant current and constant voltage of 0.5C to 3.65V 0.05C, stand still for 10 min, discharge at a current of 1C to 2.5V, stand still for 5 min, discharge at a current of 0.5C to 2.0V, stand still for 5 min, discharge at a current of 0.1C to 2.0V, stand still for 5 min, charge at a current of 0.5C for 5 min, stand still for 5 min, and then transfer to aging.

[0076] Aging: Stand still at 42°C for 24 h and at 25°C for 72 h.

[0077] Comparative Example 2

[0078] Prepare a lithium-ion battery:

[0079] Using lithium iron phosphate as the main cathode material, it is mixed evenly with Super P (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 96%:1%:3% in an NMP (N-methylpyrrolidone) solvent to obtain a cathode slurry with a solid content of 65%. The cathode slurry is evenly coated on both sides of a 12+1+1μm carbon-coated aluminum foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted cathode pole roll. After slicing with a hardware die cutter, a 148*215mm cathode pole piece is obtained; using recycled graphite as the main anode material, it is mixed evenly with Super P, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber) in a mass ratio of 95%:1%:1.5%:2.5% in a deionized water solvent to obtain an anode slurry with a solid content of 54%. The anode slurry is evenly coated on both sides of a 4.5μm copper foil through an extrusion coater, and after baking, it is pressed through a slit roller press to obtain a compacted anode pole roll. After slicing with a hardware die cutter, a 151*219mm anode pole piece is obtained; the above anode and cathode pole pieces are separated by a PP separator and stacked 20 / 21 layers to obtain a bare battery cell with a capacity of 30Ah. After being put into the shell, a semi-finished dry battery cell is obtained, and electrolyte injection is carried out. The composition of the electrolyte: LiPF 6 The concentration is 1.2mol / L, the solvent is EC and EMC, and the volume ratio is 3:7, and it is sealed.

[0080] It is placed in an environment of 42°C for infiltration for 22h, then formed, and after grading and aging, it is taken off the production line.

[0081] Formation: Stand still for 5min; Charge at a current of 0.008C for 20min; Charge at a current of 0.03C for 75min; Charge at a current of 0.05C for 50min; Stand still for 5min; Charge at a current of 0.3C for 45min; Stand still for 5min.

[0082] Grading: Constant current and constant voltage charge at 0.5C to 3.65V 0.05C, stand still for 10min, discharge at a current of 1C to 2.5V, stand still for 5min, discharge at a current of 0.5C to 2.0V, stand still for 5min, discharge at a current of 0.1C to 2.0V, stand still for 5min, charge at a current of 0.5C for 5min, stand still for 5min, and then transfer to aging.

[0083] Aging: Stand still at 42°C for 24h and at 25°C for 72h.

[0084] Performance testing and performance analysis:

[0085] 1. Test the initial voltage before formation

[0086] After the semi-finished battery cells are electrically connected to the formation equipment, the first step of the formation process is to stand still for 5 minutes. The formation equipment can record the full-process voltage of the first step of the semi-finished battery cells, and the initial voltage is the voltage at the start time point of the first step. Record the initial voltages of a certain number of lithium-ion batteries in Examples 1-4 and a certain number of lithium-ion batteries in Comparative Examples 1-2. The results are as Figure 1 shown.

[0087] As Figure 1 shown, the initial voltages of the battery cells in Comparative Examples 1-2 that did not undergo small-current charging during the initial wetting are lower than those of the battery cells in Examples 1-4 that have been charged. Taking the initial voltage greater than 20 mV as the good product standard, the defective rate of Comparative Examples 1-2 reaches 11.2%, while the overall defective rate of the battery cells in Examples 1-4 that have been charged during the initial wetting is only 0.1%, greatly reducing the defective rate. Moreover, the initial voltages of the battery cells in Comparative Examples 1-2 are distributed to a certain extent in the range of 0-120 mV, and more than 95% of the battery cells in Examples 1-4 are concentrated between 150-220 mV, which is beneficial to improving the consistency of the lithium-ion batteries off the production line.

[0088] 2. Disassemble the fully charged battery cells with low initial voltage to check the interface conditions

[0089] During the production process, the battery cell barcodes will record the manufacturing parameters of each process one by one. After the battery cells are off the production line, find the battery cells with normal initial voltage and low initial voltage according to the barcodes, and charge them to full charge at a constant current and constant voltage of 0.5C (charge to 3.65V±0.05C), and then disassemble them to check the negative electrode interface.

[0090] Figure 2 Part (a) in Figure 2 is the full-charge disassembly negative electrode interface diagram of the battery cells with normal initial voltage, Figure 2 and part (b) in

[0091] is the full-charge disassembly negative electrode interface diagram of the battery cells with low initial voltage. As

[0092] shown, the interface of the normal battery cells is golden yellow and bright, with uniform color everywhere, no damage or defect, while there are obvious black fog-like substances in some areas of the negative electrode interface of the abnormal battery cells, and the interface is uneven.

[0093] Normal temperature charge storage: Charge the battery cells off the production line to full charge at a constant current and constant voltage of 0.5C, leave them at room temperature of 25°C for 28 days, discharge them to 2.5V at 1C (record the discharge retention capacity at this step as Qinitial), stand still for 10 minutes, charge them to full charge at a constant current and constant voltage of 0.5C, stand still for 10 minutes, and discharge them to 2.5V at 1C (record the discharge recovery capacity at this step as Q2);

[0094] High-temperature charged storage: The off-line battery cells are charged to full capacity at a constant current of 0.5C and a constant voltage, then left standing at 60°C for 7 days, discharged at 1C to 2.5V (record the discharge retention capacity at this step as Q1), left standing for 10 minutes, charged to full capacity at a constant current of 0.5C and a constant voltage, left standing for 10 minutes, and then discharged at 1C to 2.5V (record the discharge recovery capacity at this step as Q2);

[0095] Capacity retention rate = Q1 / Qinitial; Capacity recovery rate = Q2 / Qinitial.

[0096] The charged storage performance of Test Examples 1 to 4 and Comparative Examples 1 to 2 was tested, and the results are shown in Table 1.

[0097] Table 1 Comparison of charged data of battery cells in Examples 1 to 4 and Comparative Examples 1 to 2

[0098] Retention rate at 25°C for 28 days Recovery rate at 25°C for 28 days Retention rate at 60°C for 7 days Recovery rate at 60°C for 7 days Example 1 97.2% 97.8% 96.5% 97.0% Example 2 97.0% 97.7% 96.3% 97.1% Example 3 93.3% 93.5% 91.7% 92.0% Example 4 93.4% 94.1% 91.8% 92.5% Comparative Example 1 94.3% 94.7% 92.7% 93.2% Comparative Example 2 94.4% 94.9% 92.8% 93.5%

[0099] As shown in Table 1, after 28 days of storage at room temperature, the capacity attenuation of the battery cells in Examples 3 to 4 and Comparative Examples 1 to 2 is about 3% more than that in Examples 1 to 2, and about 4% more after high temperature. The decline in the charged storage performance of Example 3 may be due to energization and pressure increase without reaching a certain degree of infiltration. The decline in the charged storage performance of Example 4 may be that the charging boost current does not meet the charging current limit allowed after one infiltration, resulting in mild lithium plating and affecting the performance. The charged storage performance data of Examples 1 and 2 are similar. Thus, it can be seen that using a time-saving formation method does not affect the storage performance of lithium-ion batteries.

[0100] 4. Cycle performance test

[0101] The off-line lithium-ion batteries are charged to full capacity at a constant current of 1C and a constant voltage, left standing for 30 minutes, discharged at 1C to 2.5V, left standing for 30 minutes. This is one week of cycling. Repeat the above steps and record the capacity attenuation under different cycle numbers. Capacity (retention) ratio = discharge capacity after each cycle / initial capacity. The test results are as Figure 3 shown.

[0102] From Figure 3 it can be seen that the capacity retention rate of the battery cells in Examples 1 to 4 still exceeds 85% after 1200 cycles. Comparative Examples 1 to 2 show a significant acceleration of cycle decay compared to Examples 1 to 4, and obvious failure phenomena occur after 900 cycles. Due to the adverse effects caused by mild lithium plating in Examples 3 to 4, the cycle performance decreases, and the cycle is less than 1600 weeks when the capacity retention rate is lower than 80%. The capacity retention rate of the battery cells in Examples 1 and 2 still exceeds 85% after 2000 cycles, and the cycle performance data of Examples 1 and 2 are highly consistent. Thus, it can be seen that using a time-saving formation method does not affect the storage performance of lithium-ion batteries.

[0103] The above-described embodiments merely represent several implementation manners of the present invention. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. A method for preparing a lithium ion battery, characterized in that: The following steps are involved: S1: Processing the recycled graphite material into the negative electrode sheet, stacking the positive electrode sheet, the separator and the negative electrode sheet, putting them into the shell to obtain the dry battery cell, injecting the electrolyte into the dry battery cell and sealing it; S2: Wetting the sealed battery cell for the first time; S3: Charge the battery cell after the first soaking to a voltage of 300~1000mV with a current of ≤0.05C; S4: soak the charged battery cell for the second time; S5: After the second soaking, the battery cell is charged, formed, divided in capacity, aged, and then taken offline to obtain a lithium-ion battery.

2. The method for preparing a lithium ion battery according to claim 1, characterized in that: In step S2, the first infiltration is infiltration in an environment of 39-45°C.

3. The method for preparing a lithium ion battery according to claim 2, characterized in that: In step S2, the first infiltration time is 3 to 7 hours.

4. The method for preparing a lithium ion battery according to claim 1, characterized in that: In step S3, the charging current is 0.005-0.015C.

5. The method for preparing a lithium ion battery according to claim 4, characterized in that: In step S3, the battery cell is charged to a voltage of 300-500 mV.

6. The method for preparing a lithium ion battery according to claim 1, characterized in that: In step S4, the second infiltration is infiltration in an environment of 39-45°C.

7. A method for preparing a lithium ion battery according to claim 6, characterized in that: In step S4, the second infiltration time is 14 to 18 hours.

8. The method for preparing a lithium ion battery according to claim 1, characterized in that: In step S5, the formation method adopted is: standing for 5 minutes; charging at a current of 0.008C for 20 minutes; charging at a current of 0.03C for 75 minutes; charging at a current of 0.05C for 50 minutes; standing for 5 minutes; charging at a current of 0.3C for 45 minutes; standing for 5 minutes.

9. A lithium ion battery, characterized in that: Obtained by the preparation method described in any one of claims 1 to 8.

10. A lithium ion battery according to claim 9, characterized in that: The capacity retention rate of the lithium-ion battery after 1200 cycles is higher than 85%.

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