Preparation method of square lithium ion battery for energy storage

Abstract

The present invention relates to the technical field of lithium-ion batteries and discloses a method for preparing a square lithium-ion battery for energy storage, comprising the following steps: step 1, obtaining a positive electrode slurry; step 2, obtaining a positive electrode sheet; step 3, obtaining a negative electrode slurry; step 4, obtaining a negative electrode sheet; step 5, providing a lithium-ion battery separator; step 6, obtaining a bare battery cell by lamination; step 7, providing a square lithium-ion battery aluminum shell, a positive electrode cover plate, a negative electrode cover plate, a positive electrode isolation ring, a Mylar film, and a side support plate; step 8, welding the bare battery cell to the negative electrode cover plate; step 9, obtaining a semi-finished battery cell before liquid injection; and step 10, obtaining a square lithium-ion battery for energy storage. The present invention uses the method for preparing a square lithium-ion battery for energy storage to prepare a diversified lithium-ion battery for energy storage, which has both a long cycle life and a high-power charge and discharge capability, will greatly improve the service life of the energy storage system, and can easily cope with a variety of large-scale energy storage scenarios.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing a square lithium ion battery for energy storage. Background Art

[0002] New energy storage technologies, represented by lithium-ion batteries, have short construction cycles, strong regulation capabilities, and broad development potential, representing the direction of future development. Power battery energy storage has bidirectional millisecond-level rapid regulation capabilities, and can play a huge role in power system flexibility regulation, emergency power support, and accident backup. It is an effective means to address the challenges of power balance and safety and stability.

[0003] With the application of electrochemical energy storage, the cost will be further reduced, and its advantages such as flexible layout, fast response speed and high energy efficiency will continue to stand out.

[0004] However, there is still a certain degree of uncertainty in the battery life of new energy storage technologies. If the battery life is too short, the service life of the energy storage system will be greatly reduced. Lithium-ion battery energy storage products have a single function. Even if they have a long cycle life, they often cannot have high power and cannot cope with large-scale energy storage scenarios. In summary, it is crucial to provide a long-life and high-rate energy storage battery. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing a square lithium-ion battery for energy storage, so as to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a method for preparing a square lithium-ion battery for energy storage, comprising the following steps:

[0007] Step 1: Mix the lithium iron phosphate active material, graphene composite conductive slurry and polyvinylidene fluoride according to a mass ratio, then add N-methylpyrrolidone, and stir under a vacuum mixer to obtain a positive electrode slurry;

[0008] Step 2: evenly coating the positive electrode slurry on the positive electrode current collector aluminum foil, drying it in a coating oven, and then cold pressing, slitting, and sheeting to obtain a positive electrode sheet;

[0009] Step 3: Mix the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a certain mass ratio, then add deionized water, and stir under a vacuum mixer to obtain a negative electrode slurry;

[0010] Step 4: evenly coating the negative electrode slurry on the negative electrode current collector copper foil, drying it in a coating oven, and then cold pressing, slitting, and sheeting to obtain a negative electrode sheet;

[0011] Step 5: providing a lithium-ion battery separator;

[0012] Step 6: stack the positive electrode sheet, separator, and negative electrode sheet in order to obtain a bare cell;

[0013] Step 7: Provide a square lithium-ion battery aluminum shell, a positive electrode cover, a negative electrode cover, a positive electrode isolation ring, a Mylar film, and a side support sheet;

[0014] Step 8: First weld the bare cell to the negative electrode cover, then wrap and fix it with Mylar film, side support sheet, and positive electrode isolation ring, place it in a square lithium-ion battery aluminum shell, and then weld the positive electrode tabs to the positive electrode cover respectively;

[0015] Step 9: seal and weld the negative electrode cover plate, the positive electrode cover plate and the square lithium-ion battery aluminum shell to obtain a semi-finished battery cell before liquid injection;

[0016] Step 10: The semi-finished battery cell before liquid injection is subjected to baking, liquid injection, formation, secondary liquid injection, sealing, and capacity separation to obtain a square lithium-ion battery for energy storage.

[0017] Wherein, in step 1, the average particle size of the particles with lithium iron phosphate active material is 30 to 200 nm, and the specific surface area is 5 to 30 m 2 / g;

[0018] The mass ratio of the lithium iron phosphate active material, graphene composite conductive paste, polyvinylidene fluoride and N-methyl pyrrolidone is: 100: 0.8-1: 2-2.5: 30-60;

[0019] The vacuum degree of the stirrer is -60 to -80 kPa.

[0020] Wherein, in step 2, the density of the positive electrode slurry coated on the positive electrode current collector aluminum foil is 200-400 mg / m 2 , coating speed is 1~10m / min;

[0021] The oven temperature is 65-100°C;

[0022] The cold pressing roller pressure is 20-40T, the sheet length is 200mm-300mm, the sheet width is 80-150mm, the positive electrode sheet length is 10-40mm, and the positive electrode sheet width is 20-50mm.

[0023] Wherein, in step 3, the conductive agent is one or a mixture of conductive carbon black, carbon nanotubes and conductive graphite, the active material with graphite is one of primary particle artificial graphite and secondary particle artificial graphite, the D50 particle size of the active material with graphite is 8 to 15 μm, and the specific surface area is 1 to 10 m 2 / g.

[0024] Wherein, in step 3, the mass ratio of the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber is 90-96:0.5-1:1.5-2.5:1-3;

[0025] The vacuum degree of the stirrer is -60 to -80 kPa.

[0026] Wherein, in step 4, the density of the negative electrode slurry coated on the negative electrode current collector copper foil is 100-200 mg / m 2 , coating speed is 1~10m / min;

[0027] The oven temperature is 65-100°C;

[0028] The cold pressing roller pressure is 20-40T, the sheet length is 200mm-300mm, the sheet width is 80-150mm, the negative electrode sheet length is 10-40mm, and the negative electrode sheet width is 20-50mm.

[0029] Wherein, in step 5, the thickness of the diaphragm is 7 to 25 μm and the width is 210 to 310 mm;

[0030] In step 6, the numerical relationship between the thickness H of the laminate of the bare cell and the thickness H3 of the aluminum shell of the square lithium-ion battery satisfies 90%≤H / H3≤97%.

[0031] Among them, in step 7, the square lithium-ion battery aluminum shell has a special length and width ratio, length L1, width W1, large wall thickness H1, side wall thickness H2, the numerical relationship between W1 and L1 satisfies 90mm≤W1≤130mm and 2.7≤L1 / W1≤2.9, the numerical relationship between H1 and H2 satisfies H1 / H2=2 / 3, the square lithium-ion battery aluminum shell is hollow, and the positive electrode cover and the negative electrode cover are respectively applied to the two ends of the square lithium-ion battery aluminum shell.

[0032] In step 8, the positive electrode tab and the positive electrode cover are welded by one of laser, ultrasonic and friction welding; the Mylar film is fixed to the negative electrode cover and the positive electrode isolation ring by hot melting points, with 3 to 6 hot melting points on the large surface and 1 to 3 on the side surfaces;

[0033] In step 9, the negative electrode cover plate, the positive electrode cover plate and the aluminum shell of the lithium-ion battery are welded by one of ultrasonic, friction and laser welding.

[0034] Wherein, in step 10, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain an organic solvent; lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide are dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.05 mol / L;

[0035] The content of lithium hexafluorophosphate in the electrolyte is 10% to 15%, and the content of lithium bis(fluorosulfonyl)imide is 1% to 5%.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention uses a method for preparing square lithium-ion batteries for energy storage to prepare diversified lithium-ion batteries for energy storage. These batteries have both long cycle life and high-power charge and discharge capabilities, which will greatly increase the service life of the energy storage system and can easily cope with a variety of large-scale energy storage scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a three-dimensional diagram of the aluminum shell of a square lithium-ion battery of the present invention;

[0039] Figure 2 This is a schematic diagram of the explosion structure of a square lithium-ion battery of the present invention;

[0040] Figure 3 This is a graph showing how the conventional performance of the battery of the present invention changes with the number of cycles;

[0041] Figure 4 The graph shows the change of high power performance of the battery of the present invention with the number of cycles.

[0042] In the figure, 1. Aluminum shell of square lithium-ion battery; 2. Bare battery cell; 3. Mylar film; 4. Side support; 5. Negative electrode cover; 6. Positive electrode cover; 7. Positive electrode isolation ring. DETAILED DESCRIPTION

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0044] Example 1:

[0045] A method for preparing a square lithium-ion battery for energy storage comprises the following steps:

[0046] Step 1: Mix the lithium iron phosphate active material, graphene composite conductive slurry and polyvinylidene fluoride in a mass ratio of 96:2:2, then add N-methylpyrrolidone, and stir under a vacuum mixer at a vacuum degree of -60kPa to -80kPa to obtain a positive electrode slurry.

[0047] Step 2: evenly coat the positive electrode slurry on the positive electrode current collector aluminum foil with a coating density of 200-400 mg / m 2The coating speed is 1-10 m / min, and the coating is dried in an oven at a temperature of 65-100°C. The coating is then cold-rolled at a pressure of 20-40T, slit, and sliced. The slice length is 200-300 mm, the slice width is 80-150 mm, the positive electrode sheet length is 10-40 mm, and the positive electrode sheet width is 20-50 mm to obtain the positive electrode sheet.

[0048] Step 3: Mix the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a mass ratio of 96:0.5:1.5:2, then add deionized water, and stir under a vacuum mixer at a vacuum degree of -60 kPa to -80 kPa to obtain a negative electrode slurry.

[0049] Step 4: evenly coat the negative electrode slurry on the negative electrode current collector copper foil with a single-side coating density of 100-200 mg / m 2 The coating speed is 1 to 10 m / min, and the coating is dried in a coating oven at a temperature of 65 to 100°C. The coating is then cold pressed at a roller pressure of 20 to 40T, cut and sliced, and the slice length is 200 mm to 300 mm, the slice width is 80 to 150 mm, the negative electrode sheet length is 10 to 40 mm, and the negative electrode sheet width is 20 to 50 mm to obtain a negative electrode sheet.

[0050] Step 5: Providing a lithium-ion battery separator with a thickness of 7 to 25 μm and a width of 210 to 310 mm;

[0051] Step 6: stack the positive electrode sheet, separator, and negative electrode sheet in order to obtain a bare cell 2; the numerical relationship between the stacking thickness H of the bare cell 2 and the aluminum shell thickness H3 satisfies 90%≤H / H3≤97%.

[0052] Step 7: Provide a prismatic lithium-ion battery aluminum case 1, a positive electrode cover plate 6, a negative electrode cover plate 5, a positive electrode separator 7, a Mylar film 3, and side supports 4. The prismatic lithium-ion battery aluminum case 1 has a unique length-to-width ratio. The length is L1, the width is W1, the main wall thickness is H1, and the side wall thickness is H2. The numerical relationship between W1 and L1 satisfies 90 mm ≤ W1 ≤ 130 mm and 2.7 ≤ L1 / W1 ≤ 2.9. The numerical relationship between H1 and H2 satisfies H1 / H2 = 2 / 3. The prismatic lithium-ion battery aluminum case 1 is hollow, with the positive electrode cover plate 6 and the negative electrode cover plate 5 acting on both ends of the prismatic lithium-ion battery aluminum case 1.

[0053] Step 8. The bare cell 2 is first welded to the negative electrode cover 5, and then wrapped and fixed with the Mylar film 3, the side support sheet 4, and the positive electrode isolation ring 7, and then placed in the square lithium-ion battery aluminum shell 1, and then the tabs of the positive electrode sheet are welded to the positive electrode cover 6 respectively; the welding method of the tabs of the positive electrode sheet and the positive electrode cover 6 is one of laser, ultrasonic and friction welding; the Mylar film 3 and the negative electrode cover 5 and the positive electrode isolation ring 7 are fixed by hot melting points, with 3 to 6 hot melting points on the large surface and 1 to 3 on the side.

[0054] Step 9: seal-weld the negative electrode cover plate 5, the positive electrode cover plate 6 and the square lithium-ion battery aluminum shell 1 to obtain a semi-finished battery cell before liquid injection; the welding method of the negative electrode cover plate 5, the positive electrode cover plate 6 and the square lithium-ion battery aluminum shell 1 is one of ultrasonic, friction and laser welding.

[0055] Step 10: The semi-finished battery cell before liquid injection is baked, liquid injected (the volume ratio of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in the electrolyte is 1:1:1, the content of lithium hexafluorophosphate is 12.5%, and the content of lithium bis(fluorosulfonyl)imide is 3%), chemically formed, liquid injected a second time, sealed, and volume divided to obtain a square lithium-ion battery for energy storage.

[0056] Example 2:

[0057] A method for preparing a square lithium-ion battery for energy storage comprises the following steps:

[0058] Step 1: Mix the lithium iron phosphate active material, graphene composite conductive slurry and polyvinylidene fluoride in a mass ratio of 96.5:1.5:2, then add N-methylpyrrolidone, and stir under a vacuum mixer at a vacuum degree of -60kPa to -80kPa to obtain a positive electrode slurry.

[0059] Step 2: evenly coat the positive electrode slurry on the positive electrode current collector aluminum foil with a coating density of 200-400 mg / m 2 The coating speed is 1-10 m / min, and the coating is dried in an oven at a temperature of 65-100°C. The coating is then cold-rolled at a pressure of 20-40T, slit, and sliced. The slice length is 200-300 mm, the slice width is 80-150 mm, the positive electrode sheet length is 10-40 mm, and the positive electrode sheet width is 20-50 mm to obtain the positive electrode sheet.

[0060] Step 3: Mix the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a mass ratio of 96:0.5:1.5:2, then add deionized water, and stir under a vacuum mixer at a vacuum degree of -60 kPa to -80 kPa to obtain a negative electrode slurry.

[0061] Step 4: evenly coat the negative electrode slurry on the negative electrode current collector copper foil with a single-side coating density of 100-200 mg / m 2 The coating speed is 1 to 10 m / min, and the coating is dried in a coating oven at a temperature of 65 to 100°C. The coating is then cold pressed at a roller pressure of 20 to 40T, cut and sliced, and the slice length is 200 mm to 300 mm, the slice width is 80 to 150 mm, the negative electrode sheet length is 10 to 40 mm, and the negative electrode sheet width is 20 to 50 mm to obtain a negative electrode sheet.

[0062] Step 5: Providing a lithium-ion battery separator with a thickness of 7 to 25 μm and a width of 210 to 310 mm;

[0063] Step 6: stack the positive electrode sheet, separator, and negative electrode sheet in order to obtain a bare cell 2; the numerical relationship between the stacking thickness H of the bare cell 2 and the aluminum shell thickness H3 satisfies 90%≤H / H3≤97%.

[0064] Step 7: Provide a prismatic lithium-ion battery aluminum case 1, a positive electrode cover plate 6, a negative electrode cover plate 5, a positive electrode separator 7, a Mylar film 3, and side supports 4. The prismatic lithium-ion battery aluminum case 1 has a unique length-to-width ratio. The length is L1, the width is W1, the main wall thickness is H1, and the side wall thickness is H2. The numerical relationship between W1 and L1 satisfies 90mm≤W1≤130mm and 2.7≤L1 / W1≤2.9. The numerical relationship between H1 and H2 satisfies H1 / H2=2 / 3. The prismatic lithium-ion battery aluminum case 1 is hollow, with the positive electrode cover plate 6 and the negative electrode cover plate 5 acting on either end of the prismatic lithium-ion battery aluminum case.

[0065] Step 8. The bare cell 2 is first welded to the negative electrode cover 5, and then wrapped and fixed with the Mylar film 3, the side support sheet 4, and the positive electrode isolation ring 7, and then placed in the square lithium-ion battery aluminum shell 1, and then the tabs of the positive electrode sheet are welded to the positive electrode cover 6 respectively; the welding method of the tabs of the positive electrode sheet and the positive electrode cover 6 is one of laser, ultrasonic and friction welding; the Mylar film 3 and the negative electrode cover 5 and the positive electrode isolation ring 7 are fixed by hot melting points, with 3 to 6 hot melting points on the large surface and 1 to 3 on the side.

[0066] Step 9: seal-weld the negative electrode cover plate 5, the positive electrode cover plate 6 and the square lithium-ion battery aluminum shell 1 to obtain a semi-finished battery cell before liquid injection; the welding method of the negative electrode cover plate 5, the positive electrode cover plate 6 and the square lithium-ion battery aluminum shell 1 is one of ultrasonic, friction and laser welding.

[0067] Step 10: The semi-finished battery cell before liquid injection is baked, liquid injected (the volume ratio of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in the electrolyte is 1:1:1, the content of lithium hexafluorophosphate is 11.5%, and the content of lithium bis(fluorosulfonyl)imide is 3.5%), chemically formed, liquid injected a second time, sealed, and volume divided to obtain a square lithium-ion battery for energy storage.

[0068] Example 3:

[0069] A method for preparing a square lithium-ion battery for energy storage comprises the following steps:

[0070] Step 1: Mix the lithium iron phosphate active material, graphene composite conductive slurry and polyvinylidene fluoride in a mass ratio of 95.5:2.5:2, then add N-methylpyrrolidone, and stir under a vacuum mixer at a vacuum degree of -60kPa to -80kPa to obtain a positive electrode slurry.

[0071] Step 2: evenly coat the positive electrode slurry on the positive electrode current collector aluminum foil with a coating density of 200-400 mg / m 2 The coating speed is 1-10 m / min, and the coating is dried in an oven at a temperature of 65-100°C. The coating is then cold-rolled at a pressure of 20-40T, slit, and sliced. The slice length is 200-300 mm, the slice width is 80-150 mm, the positive electrode sheet length is 10-40 mm, and the positive electrode sheet width is 20-50 mm to obtain the positive electrode sheet.

[0072] Step 3: Mix the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a mass ratio of 96:0.5:1.5:2, then add deionized water, and stir under a vacuum mixer at a vacuum degree of -60 kPa to -80 kPa to obtain a negative electrode slurry.

[0073] Step 4: The negative electrode slurry is evenly coated on the negative electrode current collector copper foil, with a single-sided coating density of 100-200 mg / m2 and a coating speed of 1-10 m / min. The coating is dried in a coating oven at a temperature of 65-100°C, and then cold pressed with a roller pressure of 20-40T, cut and sliced, with a slice length of 200-300 mm and a slice width of 80-150 mm. The negative electrode sheet length is 10-40 mm and the negative electrode sheet width is 20-50 mm to obtain a negative electrode sheet.

[0074] Step 5: Providing a lithium-ion battery separator with a thickness of 7 to 25 μm and a width of 210 to 310 mm;

[0075] Step 6, the positive electrode sheet, the separator, the negative electrode sheet are stacked in sequence, and a bare battery cell 2 is obtained through the stacking; the numerical relationship between the stacking thickness H of the bare battery cell 2 and the thickness H3 of the aluminum shell satisfies 90%≤H / H3≤97%.

[0076] Step 7, provide a square lithium ion battery aluminum shell 1, a positive electrode cover plate 6, a negative electrode cover plate 5, a positive electrode isolation ring 7, a Mylar film 3, and a side support piece 4; the square lithium ion battery aluminum shell 1 has a special length-width ratio. The length L1, the width W1, the large surface wall thickness H1, and the side wall thickness H2 satisfy the numerical relationship 90mm≤W1≤130mm and 2.7≤L1 / W1≤2.9, and the numerical relationship between H1 and H2 satisfies H1 / H2=2 / 3. The square lithium ion battery aluminum shell 1 is a hollow shape, and the positive electrode cover plate 6 and the negative electrode cover plate 5 act on the two ends of the square lithium ion battery aluminum shell 1 respectively.

[0077] Step 8, the bare battery cell 2 is first welded with the negative electrode cover plate 5, then wrapped and fixed by the Mylar film 3, the side support piece 4, and the positive electrode isolation ring 7, and then placed in the square lithium ion battery aluminum shell 1, and then the positive electrode tab of the positive electrode sheet is welded with the positive electrode cover plate 6; the welding mode of the positive electrode tab of the positive electrode sheet and the positive electrode cover plate 6 is one of laser welding, ultrasonic welding, and friction welding; the Mylar film 3 is fixed with the negative electrode cover plate 5 and the positive electrode isolation ring 7 through hot melting points, and the hot melting points are 3-6 on the large surface and 1-3 on the side surface.

[0078] Step 9, the negative electrode cover plate 5 and the positive electrode cover plate 6 are sealed and welded with the square lithium ion battery aluminum shell 1 to obtain a pre-liquid injection semi-finished product battery cell; the welding mode of the negative electrode cover plate 5 and the positive electrode cover plate 6 with the square lithium ion battery aluminum shell 1 is one of ultrasonic welding, friction welding, and laser welding.

[0079] Step 10, the pre-liquid injection semi-finished product battery cell is subjected to baking, liquid injection (the volume ratio of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in the electrolyte is 1:1:1, the content of lithium hexafluorophosphate is 11.5%, and the content of lithium bisfluorosulfonylimide is 4%), formation, secondary liquid injection, sealing, and capacity distribution to obtain a square lithium ion battery for energy storage.

[0080] Embodiment

[0081] Battery samples are taken from each of Examples 1-3 for detection, and the detection type, results, and detection methods are shown in the following table:

[0082] Table 1 Battery property detection data

[0083]

[0084] The specific battery cycle number is shown in Table 1 and Figure 3

[0085] From Table 1 and Figure 3 ​,4 It can be seen that through the preparation method of square lithium-ion batteries for energy storage, diversified lithium-ion batteries for energy storage are prepared. The mass energy density of the diversified lithium-ion battery for energy storage can reach 182Wh / kg. At the same time, it has a long life of about 4000 cycles at room temperature 1C / 1C (100A / 100A) and a high power capacity of about 2C / 3C (200A / 300A). It has both a long cycle life and high power charging and discharging capabilities, which will greatly improve the service life of the energy storage system and can easily cope with a variety of large-scale energy storage scenarios.

[0086] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a square lithium-ion battery for energy storage, characterized in that: The following steps are involved: Step 1: Mix the lithium iron phosphate active material, graphene composite conductive slurry and polyvinylidene fluoride according to a mass ratio, then add N-methylpyrrolidone, and stir under a vacuum mixer to obtain a positive electrode slurry; Step 2: The positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried in a coating oven, and then cold pressed, cut, and sliced ​​to obtain a positive electrode sheet. The density of the positive electrode slurry coated on the positive electrode current collector aluminum foil is 200-400 mg / m 2 , the coating speed is 1-10 m / min; the oven temperature is 65-100°C; the roller pressing pressure of the cold pressing is 20-40T, the sheet length is 200mm-300mm, the sheet width is 80-150mm, the positive electrode sheet length is 10-40mm, and the positive electrode sheet width is 20-50mm; Step 3: Mix the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a certain mass ratio, then add deionized water, and stir under a vacuum mixer to obtain a negative electrode slurry; Step 4: evenly coat the negative electrode slurry on the negative electrode current collector copper foil, dry it in a coating oven, and then cold press, cut and slice it to obtain a negative electrode sheet. The density of the negative electrode slurry coated on the negative electrode current collector copper foil is 100-200 mg / m 2 , the coating speed is 1-10 m / min; the oven temperature is 65-100°C; the roller pressure of the cold press is 20-40T, the sheet length is 200mm-300mm, the sheet width is 80-150mm, the negative electrode sheet length is 10-40mm, and the negative electrode sheet width is 20-50mm; Step 5: providing a lithium-ion battery separator having a thickness of 7 to 25 μm and a width of 210 to 310 mm; Step 6: stack the positive electrode sheet, the separator, and the negative electrode sheet in order to obtain a bare cell. The relationship between the thickness H of the stack of the bare cell and the thickness H3 of the aluminum shell of the square lithium-ion battery satisfies 90%≤H / H3≤97%; Step 7: Provide a square lithium-ion battery aluminum shell, a positive electrode cover, a negative electrode cover, a positive electrode isolation ring, a Mylar film, and a side support. The square lithium-ion battery aluminum shell has a special length-to-width ratio, with a length of L1, a width of W1, a large wall thickness of H1, and a side wall thickness of H2. The numerical relationship between W1 and L1 satisfies 90 mm ≤ W1 ≤ 130 mm and 2.7 ≤ L1 / W1 ≤ 2.

9. The numerical relationship between H1 and H2 satisfies H1 / H2 = 2 / 3. The square lithium-ion battery aluminum shell is hollow, and the positive electrode cover and the negative electrode cover are respectively applied to the two ends of the square lithium-ion battery aluminum shell. Step 8: The bare cell is first welded to the negative electrode cover, and then wrapped and fixed with Mylar film, side support sheet, and positive electrode isolation ring, and then placed in a square lithium-ion battery aluminum shell. The tabs of the positive electrode sheet are then welded to the positive electrode cover respectively. The welding method of the tabs of the positive electrode sheet and the positive electrode cover is one of laser, ultrasonic and friction welding; the Mylar film is fixed to the negative electrode cover and the positive electrode isolation ring by hot melting points, with 3 to 6 hot melting points on the large surface and 1 to 3 on the side. Step 9: seal-weld the negative electrode cover plate, the positive electrode cover plate, and the square lithium-ion battery aluminum shell to obtain a semi-finished battery cell before liquid injection, wherein the welding method of the negative electrode cover plate, the positive electrode cover plate, and the square lithium-ion battery aluminum shell is one of ultrasonic, friction, and laser welding; Step 10: The semi-finished battery cell before liquid injection is subjected to baking, liquid injection, formation, secondary liquid injection, sealing, and capacity separation to obtain a square lithium-ion battery for energy storage.

2. The method for preparing a square lithium-ion battery for energy storage according to claim 1, wherein: In step 1, the particles with lithium iron phosphate active material have an average particle size of 30 to 200 nm and a specific surface area of ​​5 to 30 m 2 / g; The mass ratio of the lithium iron phosphate active material, graphene composite conductive paste, polyvinylidene fluoride and N-methyl pyrrolidone is: 100: 0.8-1: 2-2.5: 30-60; The vacuum degree of the stirrer is -60 to -80 kPa.

3. The method for preparing a square lithium-ion battery for energy storage according to claim 1, wherein: In step 3, the conductive agent is one or a mixture of conductive carbon black, carbon nanotubes and conductive graphite, the active material with graphite is one of primary particle artificial graphite and secondary particle artificial graphite, and the D50 particle size of the active material with graphite is 8 to 15 μm and the specific surface area is 1 to 10 m 2 / g.

4. The method for preparing a square lithium-ion battery for energy storage according to claim 1, wherein: In step 3, the mass ratio of the active material with graphite, the conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber is 90-96: 0.5～1：1.5～2.5：1～3； The vacuum degree of the stirrer is -60 to -80 kPa.

5. The method for preparing a square lithium-ion battery for energy storage according to claim 1, wherein: In step 10, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain an organic solvent; lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide are dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.05 mol / L; The electrolyte contains 10% to 15% lithium hexafluorophosphate and 1% to 5% lithium bis(fluorosulfonyl)imide.

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