Preparation process of lithium iron phosphate positive electrode sheet

Abstract

The application discloses a preparation process of a lithium iron phosphate positive electrode sheet, and relates to the following steps: adding lithium iron phosphate and conductive agent carbon black into a polyvinylidene fluoride N-methyl pyrrolidone solution, then adding a foaming agent, uniformly grinding to obtain a slurry; coating the slurry on an aluminum foil, drying to obtain an electrode sheet; and performing pressure roller pressing and punching on the dried electrode sheet to obtain the lithium iron phosphate positive electrode sheet. The pore structure of the electrode sheet is synergistically controlled through physical foaming and chemical foaming, the obtained electrode sheet has excellent ion conductivity and mechanical strength, the 0.5C rate efficiency is increased by more than 4.27%, and the capacity retention rate at 1C cycle for 1000 times is more than 98%.

Description

Technical Field

[0001] This invention relates to a lithium iron phosphate cathode sheet and its preparation process that synergistically regulates "physical foaming-chemical foaming". Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) has become the mainstream cathode material for power batteries and energy storage batteries due to its high safety, long cycle life, and low cost. However, LFP itself has low electronic conductivity (~10). -9 (S / cm) and low lithium-ion diffusion coefficient (~10-14 cm⁻¹) 2 The rate capability is poor due to the electrode's speed ( / s). To improve electrochemical performance, it is usually necessary to optimize the electrode structure, including: constructing conductive networks (such as conductive agents like carbon black and carbon nanotubes), optimizing ion transport channels (such as porosity control), and enhancing interfacial contact (such as ensuring uniform binder distribution).

[0003] Currently, the preparation of commercial LFP electrodes mainly faces the following key issues:

[0004] (1) The contradiction between high density and porosity of electrodes

[0005] Increase compaction density (>2.0 g / cm³) 3 It can increase volumetric energy density, but it will reduce electrode porosity, making electrolyte wetting difficult and hindering ion transport.

[0006] Traditional physical methods of creating pores (such as template method and mechanical pressing) are difficult to control the pore size distribution precisely, which can easily lead to local pore blockage or poor connectivity.

[0007] (2) Issues related to the uniformity of slurry dispersion and the consistency of electrode sheets

[0008] LFP particles are prone to agglomeration, making it difficult to uniformly coat conductive agents (such as SP carbon black), which affects electron conduction.

[0009] Traditional homogenization processes (single high-speed shearing) may cause PVDF molecular chains to break or conductive agents to be over-dispersed, reducing slurry stability. Summary of the Invention

[0010] To address the above problems, this invention proposes the following synergistic regulation strategy, with the core innovations as follows:

[0011] This invention provides a lithium iron phosphate (LFP) cathode electrode based on chemical foaming control and its preparation method, aiming to solve the technical problem that it is difficult to synergistically optimize the high solid density and pore structure of the electrode in traditional processes.

[0012] Synergistic Control of Physical and Chemical Blowing Agents: PEG 400 and W-101 are used as blowing agents. Their controlled volatilization during the drying process at 100-120℃ increases the porosity within the electrode. By adjusting the addition amounts of blowing agents PEG400 and W-101 to (4-6%) and (0.1-0.3%), respectively, electrolyte wettability and lithium-ion transport efficiency are significantly improved.

[0013] Gradient homogenization process: Innovative two-stage stirring (low speed stirring → high speed) is adopted. The low speed stage ensures that the PVDF molecular chains are fully swollen and avoids breakage caused by high speed shearing. The high speed stage forcibly disperses LFP agglomerates, so that conductive carbon black (SP) is uniformly coated, and the viscosity stability of the slurry is significantly improved.

[0014] Different methods are used to prepare battery cathode materials. The specific methods are as follows:

[0015] (1) Lithium iron phosphate (LFP) and conductive carbon black (SP) weighed in different proportions were added to N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) in a homogenizing tank, and then an appropriate amount of foaming agent was added to prepare battery electrode sheets.

[0016] (2) Place the homogenizing tank in the homogenizer, stir at low speed, and then stir at high speed.

[0017] (3) The slurry is coated on aluminum foil with a thickness of 10-15 µm and a doctor blade thickness of 150-200 nm.

[0018] (4) Place the coated aluminum foil and glass plate into the dryer.

[0019] (5) Press the dried electrode sheet to the corresponding compaction density using a pressure roller.

[0020] (6) The rolled electrode sheet is punched, and the diameter of the electrode sheet is 10-14mm.

[0021] (7) Select any 4 electrodes, with the range of the electrodes being ≤0.2mg.

[0022] The mass ratio of lithium iron phosphate, conductive agent carbon black, and polyvinylidene fluoride is 80-90:5-10:5-10.

[0023] The foaming agent includes polyethylene glycol, foaming agent (W-101), or a combination thereof.

[0024] The amount of PEG 400 foaming agent added is 4-6% of the total mass of the slurry.

[0025] The W-101 foaming agent was added at a rate of 0.1-0.3% of the total mass of the slurry and was purchased from Dongying Shengdu Petroleum Technology Co., Ltd. It underwent purification treatment before use to ensure that the total amount of metal impurities was below 100 ppb.

[0026] During the homogenization process, the mixture is first stirred at 800-1200 r / min for 30 seconds to 1 minute, and then stirred at 2000-3000 r / min for 20-35 minutes.

[0027] During the homogenization process, the mixture is first stirred at 800 r / min for 30 seconds to 1 minute, and then stirred at 2500 r / min for 20 to 35 minutes.

[0028] In step (3), the gas volume is 14-16% and the pressure is 7-9 t for roller pressing.

[0029] This invention also provides a lithium iron phosphate cathode sheet, in which the selected foaming agent PEG and W-101 are added in combination, which is beneficial to the density uniformity on a large-size 9×20cm electrode sheet, and the uniformity error is controlled within ±0.2%. The process of this invention is simple and low-cost, suitable for mass production, and can be extended to other cathode material systems (such as NCM, LMFP).

[0030] Compatibility: Polyethylene glycol (PEG 400), acting as a foaming agent, evaporates along with the solvent NMP during coating, forming cavities between molecular chains and leaving micron-sized pores. This also enhances the flowability of the slurry and prevents pore structure collapse. The foaming agent (W-101), as an anionic polyether surfactant, stabilizes the cell size structure and reduces pore coalescence and collapse. Both are highly compatible with the preparation process of lithium iron phosphate electrodes.

[0031] Pore ​​structure optimization: Adjusting the addition amount of PEG 400 (4-6%) and W-101 (0.1-0.3%) can increase porosity by 23.89% (SEM verification).

[0032] In the low-speed stage of the gradient homogenization process: ensure that PVDF swells sufficiently and avoid molecular chain breakage caused by high-speed shearing.

[0033] High-speed stage: Forced convection disperses LFP agglomerates, ensuring uniform coating of SP carbon black. Attached Figure Description

[0034] Figure 1 SEM images of the samples from Example 1(a), Example 2(b) and Comparative Example 1(c) at 10K magnification, and corresponding simulated images of the surface pore distribution of the samples from Example 1(d), Example 2(e) and Comparative Example 1(f). Detailed Implementation

[0035] To better understand the present invention, the following description, in conjunction with embodiments and related drawings, further illustrates the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0036] Different methods are used to prepare battery cathode materials. The specific methods are as follows:

[0037] 1) Lithium iron phosphate (LFP) and conductive carbon black (SP) weighed in different proportions are added to N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) in a homogenizing tank, and then appropriate amounts of foaming agent PEG 400 (4-6%) and W-101 (0.1-0.3%) are added to prepare battery electrodes.

[0038] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0039] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0040] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0041] 5) Press the dried electrode sheets with pressure rollers to the corresponding electrode sheet pressing density.

[0042] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0043] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0044] In step 1), lithium iron phosphate (LFP), conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are weighed in a ratio of 90:5:5.

[0045] In step 1), the foaming agent is added to the slurry at 4%wt, 5%wt, and 6%wt of PEG 400, and W-101 at 0.1%wt, 0.2%wt, and 0.3%wt of W-101, and mixed evenly.

[0046] In step 5), the gas volume of the pneumatic roller press is adjusted to 14-16%, and the electrode roller pressing pressure is set to 7-9t. Different roller pressing pressures are used to adjust the compaction density of the electrode to 1.9-2.0 g / cm³. 3 .

[0047] Example 1

[0048] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 4%wt and foaming agent W-101 at 0.1%wt to prepare battery electrode sheets.

[0049] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0050] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0051] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0052] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0053] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0054] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0055] Example 2

[0056] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 5%wt and foaming agent W-101 at 0.1%wt to prepare battery electrode sheets.

[0057] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0058] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0059] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0060] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0061] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0062] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0063] Example 3

[0064] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 5%wt and foaming agent W-101 at 0.2%wt to prepare battery electrode sheets.

[0065] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0066] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0067] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0068] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0069] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0070] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0071] Example 4

[0072] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 4%wt and foaming agent W-101 at 0.2%wt to prepare battery electrode sheets.

[0073] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0074] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0075] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0076] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0077] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0078] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0079] Example 5

[0080] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 5%wt and foaming agent W-101 at 0.3%wt to prepare battery electrode sheets.

[0081] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0082] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0083] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0084] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0085] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0086] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0087] Example 6

[0088] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 6%wt and foaming agent W-101 at 0.3%wt to prepare battery electrode sheets.

[0089] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0090] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0091] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0092] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0093] 6) The rolled electrode sheets are stamped to a diameter of 14 mm.

[0094] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0095] Comparative Example 1

[0096] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. No foaming agent is used to prepare battery electrodes.

[0097] 2) Place the homogenizing tank in the homogenizer, stir at 850 r / min for 1 minute, and then stir at 2000 r / min for 29 minutes.

[0098] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0099] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0100] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0101] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0102] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0103] Comparative Example 2

[0104] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent PEG 400 at 5%wt to prepare battery electrode sheets.

[0105] 2) Place the homogenizing tank in the homogenizer and homogenize at 2000 r / min for 30 min (canceling gradient homogenization).

[0106] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0107] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0108] 5) Press the dried electrode sheets with a pressure roller until the compaction density is 1.9 g / cm³. 3 .

[0109] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0110] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0111] Comparative Example 3

[0112] 1) Weigh lithium iron phosphate (LFP) and conductive carbon black (SP) in a mass ratio of 90:5:5 and add them to a polyvinylidene fluoride (PVDF) N-methylpyrrolidone (NMP) solution in a homogenizing tank. Add foaming agent W-101 at 0.2%wt to prepare battery electrode sheets.

[0113] 2) Place the homogenizing tank in the homogenizer and homogenize at 2000 r / min for 30 min (canceling gradient homogenization).

[0114] 3) The slurry is coated onto aluminum foil with a thickness of 13 µm and a doctor blade thickness of 150 nm.

[0115] 4) Place the coated aluminum foil and glass plate in an oven at 120℃ and dry for 2 hours.

[0116] 5) The dried electrode sheets are subjected to a pressure of 11 t until the roller pressing density reaches 1.9 g / cm³. 3 .

[0117] 6) The rolled electrode sheets are stamped to a diameter of 14mm.

[0118] 7) Select any 4 electrodes, with a range of ≤0.2mg.

[0119] Comparative Example 4

[0120] The steps are the same as in Example 1, except that the foaming agent W-101 is replaced with sodium fatty alcohol polyoxyethylene ether sulfate (AES).

[0121] The discharge capacity of the samples prepared by the above steps and the samples obtained by the comparative examples were tested, and the test results are shown in Table 1.

[0122] Table 1 shows the porosity, wetting time, specific capacity at 0.5C plateau, and capacity retention after 1000 cycles at 1C obtained from the positive electrode tests of the examples and comparative examples.

[0123]

[0124] Data analysis was performed on the positive electrode sheets prepared in Examples 1-6 and Comparative Examples 1-3, as shown in Table 1. PEG 400 was added to the slurry at proportions of 4%wt, 5%wt, and 6%wt, and W-101 at proportions of 0.1%wt, 0.2%wt, and 0.3%wt, respectively, and mixed uniformly. The porosity of Examples 1-3 increased sequentially, and the time required for the same electrolyte to completely wet the positive electrode sheet decreased sequentially. The 0.5 C discharge specific capacity also increased accordingly. The addition of foaming agents PEG 400 and W-101 resulted in a uniform foam structure and shortened the Li... + The diffusion pathway, with the combination of PEG 400 and W-101, can synergistically improve porosity and pore stability, contributing to enhanced electrochemical performance. Examples 4-6 show a slight increase in porosity compared to Examples 1-3; the increased W-101 content leads to this increase. The 0.5 C discharge specific capacity observed in Examples 3 and 4 is generally higher, and efficiency is also improved. A W-101 content of 0.2%wt yields better results. Comparative Example 1, without foaming agent, has significantly lower electrode porosity, hindering electrolyte wetting and resulting in uneven electrolyte penetration and fewer dense pores in the electrode. Figure 1 As shown in (c). Comparative Example 2 used only PEG 400 as the foaming agent, without W-101, and its electrode porosity was significantly lower than that of Examples 2 and 5. Comparative Example 3 used only W-101 as the foaming agent, without PEG 400, and its electrode porosity was significantly lower than that of Examples 2 and 4, indicating that compounding can synergistically improve porosity and cell stability. Compounding with W-101 can alleviate the volume change during the charging and discharging process of lithium iron phosphate, and the 1000C capacity can reach more than 96.5%, with a maximum retention rate of more than 98.9%.

Claims

1. A process for preparing a lithium iron phosphate positive electrode sheet, characterized in that, The steps include the following: (1) Lithium iron phosphate and conductive carbon black are added to N-methylpyrrolidone solution of polyvinylidene fluoride, and then a foaming agent is added. After homogenization, a slurry is obtained. The foaming agent includes polyethylene glycol and foaming agent W-101. During the homogenization process, the mixture is first stirred at 800-1200 r / min for 30s-1 minute, and then stirred at 2000-3000 r / min for 20-35 minutes. (2) The slurry is coated onto aluminum foil and dried to obtain an electrode sheet; (3) The dried electrode is pressed and punched by pressure rollers to obtain lithium iron phosphate positive electrode.

2. The preparation process of the lithium iron phosphate positive electrode sheet according to claim 1, characterized in that, The mass ratio of lithium iron phosphate, conductive agent carbon black, and polyvinylidene fluoride is 80-90:5-10:5-10.

3. The preparation process of the lithium iron phosphate positive electrode sheet according to claim 1, characterized in that, The amount of foaming agent added is 1-10% of the mass of lithium iron phosphate.

4. The preparation process of the lithium iron phosphate positive electrode sheet according to claim 3, characterized in that, The amount of foaming agent added is 3-8% of the mass of lithium iron phosphate.

5. The preparation process of the lithium iron phosphate positive electrode sheet according to claim 1, characterized in that, During the homogenization process, the mixture is first stirred at 800 r / min for 30 seconds to 1 minute, and then stirred at 2500 r / min for 20 to 35 minutes.

6. The preparation process of the lithium iron phosphate positive electrode sheet according to claim 1, characterized in that, In step (3), the gas volume is 14-16% and the pressure is 7-9t for roller pressing.

7. A lithium iron phosphate positive electrode sheet prepared by the process according to any one of claims 1-6, characterized in that, The compaction density of lithium iron phosphate cathode sheets is greater than 2.4 g / cm³. 3 .

8. A battery material, characterized in that, Including the lithium iron phosphate positive electrode sheet as described in claim 7.

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