Positive electrode sheet and lithium-ion battery

Abstract

A positive electrode sheet and a lithium-ion battery having the positive electrode sheet are provided. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material includes a first lithium iron phosphate material and a second lithium iron phosphate material. When the cumulative volume distribution percentage of the first lithium iron phosphate material is 50%, the particle size is D 1 v When the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the particle size is D 2 v 50μm. D 1 v 50 is in the range of 0.3 to 0.95, D 2 v When the volume density of the positive electrode active material reaches a maximum value, the particle size is D mo μm, and the compressed density of the positive electrode sheet is PDg / cm under a pressure of 1.5Mpa. 3 0.45≦PD×D mo ≦12.38.

Description

[Technical Field]

[0001] Cross-reference of related applications This disclosure claims priority to Chinese Patent Application No. 202210139512.0, entitled “Positive electrode sheet and lithium ion battery,” filed on 15 February 2022, which is incorporated herein by reference in its entirety.

[0002] This disclosure relates to the field of lithium-ion batteries, and more particularly to cathode sheets and lithium-ion batteries. [Background technology]

[0003] Lithium iron phosphate is widely used due to its advantages such as high safety, low cost, and environmental friendliness. However, lithium iron phosphate also has clear drawbacks. Specifically, its low compressive density results in low energy density in batteries made with it, making it unsuitable to meet the demand for long-life batteries. To improve the energy density of batteries, lithium iron phosphate with a higher compressive density is required. However, increasing the compressive density of lithium iron phosphate often degrades the electrochemical performance of the battery. [Overview of the project] [Means for solving the problem]

[0004] In a first embodiment, the disclosure provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material comprising a first lithium iron phosphate material and a second lithium iron phosphate material. When the cumulative volume distribution percentage of the first lithium iron phosphate material is 50%, the corresponding particle size is D 1 v It is 50 μm. When the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the corresponding particle size is D2 v It is 50 μm. D 1 v 50 is in the range of 0.3 to 0.95, D 2 v 50 is in the range of 1.0 to 3.5. When the volume density of the positive electrode active material reaches the maximum value, the corresponding particle size is D mo μm. The compression density of the positive electrode sheet is PD g / cm under a pressure of 1.5 Mpa 3 and PD satisfies 0.45 ≦ PD × D mo ≦ 12.38.

[0005] In some embodiments of the present disclosure, PD is in the range of 2.25 to 2.85.

[0006] In some embodiments of the present disclosure, PD is in the range of 2.3 to 2.7.

[0007] In some embodiments of the present disclosure, PD is in the range of 2.35 to 2.7.

[0008] In some embodiments of the present disclosure, D of the first lithium iron phosphate material 1 v 50 is in the range of 0.35 to 0.65, and D of the second lithium iron phosphate material 2 v 50 is in the range of 1.25 to 2.85.

[0009] [[ID=ID=40]]In some embodiments of the present disclosure, PD × D mo is in the range of 0.6 to 10.5.

[0010] In some embodiments of the present disclosure, D mo is in the range of 0.2 to 4.5.

[0011] In some embodiments of the present disclosure, D mo is in the range of 0.3 to 4.2.

[0012] In some embodiments of the present disclosure, when the cumulative volume distribution percentage of the positive electrode active material is 50%, the corresponding particle size is D vIt is 50 μm (median particle size), D v 50 falls within the range of 0.2 to 3.1.

[0013] In some embodiments of this disclosure, the mixed weight ratio of the first lithium iron phosphate material to the second lithium iron phosphate material is in the range of 1:(1 to 9).

[0014] In some embodiments of this disclosure, the first lithium iron phosphate material has a carbon coating layer on its surface, and / or the second lithium iron phosphate material has a carbon coating layer on its surface.

[0015] In some embodiments of this disclosure, the positive electrode active material layer further comprises a binder and a conductive agent. The compressed density of the positive electrode sheet is 2.25 to 2.75 g / cm³ under a pressure of 1.5 MPa. 3 It is within the range.

[0016] In some embodiments of this disclosure, the positive electrode active material layer is formed by coating a positive electrode paste, comprising a positive electrode active material, a conductive agent, a binder, and a solvent, onto a positive electrode current collector.

[0017] In a second embodiment, the Disclosure provides a lithium-ion battery, which includes a positive electrode sheet as described in the first embodiment of the Disclosure.

[0018] In some embodiments of this disclosure, the lithium-ion battery further includes a negative electrode sheet, an electrolyte and a separator located between the positive electrode sheet and the negative electrode sheet. [Brief explanation of the drawing]

[0019] [Figure 1] This figure shows the cycle performance curves for each pouch battery of Examples 1 to 5 and Comparative Example 1 of this disclosure. [Modes for carrying out the invention]

[0020] The foregoing description represents some embodiments of the present disclosure. It should be noted that several improvements and modifications may be made by those skilled in the art without departing from the principles of the present disclosure, and these will fall within the scope of the present disclosure.

[0021] This disclosure provides a positive electrode sheet and a method for manufacturing the same. This disclosure guarantees that the positive electrode sheet has a high compressive density, and that a battery manufactured using it has good electrochemical performance.

[0022] In a first embodiment, the disclosure provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material comprising a first lithium iron phosphate material and a second lithium iron phosphate material. When the cumulative volume distribution percentage of the first lithium iron phosphate material is 50%, the corresponding particle size is D 1 v It is 50 μm. When the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the corresponding particle size is D 2 v It is 50 μm. D 1 v 50 is in the range of 0.3 to 0.95, D 2 v 50 is in the range of 1.0 to 3.5. When the volume density of the positive electrode active material reaches its maximum value, the corresponding particle size is D mo The thickness is μm. The compressed density of the positive electrode sheet is PDg / cm² under a pressure of 1.5 MPa. 3 Therefore, PD is 0.45 ≤ PD × D mo It satisfies ≤ 12.38.

[0023] In this disclosure, two lithium iron phosphate materials having different particle sizes are used, and the particle size D of the positive electrode active material obtained after mixing is mo The mixing ratio of the two lithium iron phosphate materials is adjusted so that the particle size D of the first lithium iron phosphate material satisfies a specific relationship. This ensures that the positive electrode sheet has a sufficiently high compressive density and that the electrochemical performance (especially the cycle performance) of batteries made using this positive electrode sheet is excellent.1 v 50. Particle size D of the second lithium iron phosphate material 2 v 50, and particle size D of the positive electrode active material mo These may be obtained from their respective laser particle size distribution diagrams. The test method can be found in the particle size analysis laser diffraction method of GB / T19077-2016 / ISO13320:2009. The test apparatus is a laser particle size analyzer (such as Malvern3000). When the cumulative volume distribution percentage of the material is 50%, the corresponding particle size is called the median particle size of the material. PD may also be obtained by testing the positive electrode sheet with a compressor.

[0024] In some embodiments of this disclosure, the PD is in the range of 2.25 to 2.85. In this case, the positive electrode sheet is PD × D mo While satisfying the condition that the coefficient of gravity is in the range of 0.45 to 12.38, the positive electrode sheet has a high compressive density and appropriate porosity, and the battery has even better cycle performance and rate performance.

[0025] In some embodiments of this disclosure, PD is in the range of 2.3 to 2.7.

[0026] In some embodiments of this disclosure, PD is in the range of 2.35 to 2.7.

[0027] In this disclosure, D 1 v 50 <D 2 v It is 50. D of lithium iron phosphate material 1 v When 50 is small, the lithium-ion diffusion pathway is relatively short, and the fabricated battery has good electrical performance. (D of lithium iron phosphate material) 2 v If 50 is large, the compressive density of the positive electrode active material can be increased. The particle size D of the two materials. vWhen 50 is within the above range, adjusting the mixing ratio of the two materials ensures that the two lithium iron phosphate materials form a dense mass, and thus the compressed density of the resulting positive electrode sheet can be improved without impairing the battery's cycle performance.

[0028] In some embodiments of this disclosure, the first lithium iron phosphate material D 1 v 50 is in the range of 0.35 to 0.65, and is the second lithium iron phosphate material D 2 v 50 falls within the range of 1.25 to 2.85.

[0029] This disclosure provides information on the compressed density PD of the positive electrode sheet and the particle size D of the positive electrode active material. mo Rather than pursuing the highest possible PD value, a combination of factors is considered. The particle size of the positive electrode active material affects the electronic and ionic conductivity of the electrode sheet. mo This can indirectly reflect the content of medium to small particle sizes (or fine powder) in the positive electrode active material particles. In this disclosure, 0.45 ≤ PD × D mo By controlling the coefficient to ≤12.38, it is ensured that the positive electrode sheet has a sufficiently high compression density, while both the overall electronic and ionic conductivity of the positive electrode sheet are excellent, resulting in a battery with good cycle performance and rate performance.

[0030] In some embodiments of this disclosure, PD×D mo This value is in the range of 0.6 to 10.5. In this case, it is guaranteed that the positive electrode sheet has a high compression density, and that the battery has good electrochemical performance.

[0031] In some embodiments of this disclosure, D mo The value is in the range of 0.2 to 4.5. In this case, the positive electrode active material has an appropriate number of small to medium particle sizes, which ensures that the resistance to ion diffusion / movement within and between the particles of the positive electrode active material is low, contributing to the good rate performance and cycle performance of the battery.

[0032] In some embodiments of this disclosure, D mo It falls within the range of 0.3 to 4.2.

[0033] In some embodiments of this disclosure, when the cumulative volume distribution percentage of the positive electrode active material is 50%, the corresponding particle size is D v It is 50 μm (median particle size), D v 50 is in the range of 0.2 to 3.1. When the median particle size of the positive electrode active material after mixing is within this range, the positive electrode active material particles are properly stacked, the polarization intensity of the electrode sheet is low, the lithium ion transfer rate during the battery cycle process is high, and the energy density of the secondary battery is high.

[0034] In this disclosure, the mixed weight ratio of the first lithium iron phosphate material and the second lithium iron phosphate material is not particularly limited, as long as it is guaranteed that the particle size of the positive electrode active material formed by mixing the two satisfies the above relation.

[0035] In some embodiments of this disclosure, the mixed weight ratio of the first lithium iron phosphate material to the second lithium iron phosphate material is in the range of 1:(1 to 9), and further in the range of 1:(1 to 4). In this case, the positive electrode active material formed by mixing the two lithium iron phosphate materials can adequately ensure a high compressive density of the electrode sheet and good cycle performance of the battery.

[0036] In some embodiments of this disclosure, the first lithium iron phosphate material has a carbon coating layer on its surface, and / or the second lithium iron phosphate material has a carbon coating layer on its surface. The carbon coating layer may be obtained by sequentially polishing, spray-drying, and sintering a mixed slurry of a phosphorus source, an iron source, a lithium source, and a carbon source. This disclosure does not limit the specific methods for producing the two lithium iron phosphate materials. The presence of the carbon coating layer allows the first and second lithium iron phosphate materials to have good conductivity and fewer side reactions with the electrolyte. The positive electrode active material obtained by mixing the two has good conductivity and good battery cycle performance.

[0037] According to the positive electrode sheet provided in the first aspect of this disclosure, two lithium iron phosphate materials that satisfy the requirements of a specific particle size parameter are mixed, and the particle size D of the positive electrode active material obtained by mixing these two lithium iron phosphate materials is mo The compression density PD of the positive electrode sheet is given by 0.45 ≤ PD × D mo By controlling the coefficient to satisfy ≤12.38, it can be ensured that the resulting positive electrode sheet has high compressive density and good electrochemical performance, particularly cycle performance and rate performance.

[0038] In some embodiments of this disclosure, the positive electrode active material layer further comprises a binder and a conductive agent. The compressed density of the positive electrode sheet is 2.25 to 2.75 g / cm³ under a pressure of 1.5 MPa. 3 It is within the range.

[0039] In some embodiments of this disclosure, the positive electrode active material layer is formed by coating a positive electrode paste, comprising a positive electrode active material, a conductive agent, a binder, and a solvent, onto a positive electrode current collector. The solvent may be one or more of N-methylpyrrolidone (NMP), acetone, and dimethylacetamide (DMAC). The positive electrode current collector comprises one of aluminum foil, carbon-coated aluminum foil, and perforated aluminum foil. The conductive agent comprises, but is not limited to, one or more of carbon nanotubes, graphene carbon black, carbon fibers, etc. The binder comprises, but is not limited to, one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylate, polyacrylonitrile (PAN), sodium carboxymethylcellulose (CMC), and sodium alginate.

[0040] In a second embodiment, the Disclosure provides a lithium-ion battery, which includes a positive electrode sheet as described in the first embodiment of the Disclosure.

[0041] In some embodiments of this disclosure, the lithium-ion battery further includes a negative electrode sheet, an electrolyte and a separator located between the positive electrode sheet and the negative electrode sheet.

[0042] Lithium-ion batteries using this positive electrode sheet have high energy density and excellent cycle performance.

[0043] The technical solutions of this disclosure will be described below in combination with specific embodiments. [Examples]

[0044] The method for manufacturing the positive electrode sheet includes the following steps:

[0045] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.55 μm, i.e., D 1 v 50 was 0.55.

[0046] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 1.35 μm, i.e., D 2 v 50 was 1.35.

[0047] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 2:8. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 1.15 μm, i.e., D mo The value was 1.15.

[0048] The positive electrode active material LFP-3 was prepared into a positive electrode sheet by mixing the LFP-3 material with a binder (specifically, polyvinylidene fluoride, PVDF) and conductive carbon black in a weight ratio of 85:5:10, and then adding an appropriate amount of N-methylpyrrolidone (NMP) and mixing uniformly. To obtain a double-sided positive electrode sheet, the positive electrode paste was coated on both sides of carbon-coated aluminum foil and dried. The compressed density of the positive electrode sheet was 2.45 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, the PD was 2.45. PD × D of the positive electrode sheet mo The value was 2.82.

[0049] Fabrication of Pouch Lithium-Ion Battery: A positive electrode sheet made using the positive electrode active material LFP-3 from Example 1 was used as the positive electrode. A graphite electrode sheet was used as the negative electrode. A polypropylene film was used as the separator. A solution containing 1.0 mol / L of LiPF6 in an ethylene carbonate (EC):dimethyl carbonate (DMC) = 1:1 (volume ratio) was used as the electrolyte. After assembly, a pouch battery was obtained. [Examples]

[0050] Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0051] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.66 μm, i.e., D 1 v 50 was 0.66.

[0052] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 1.75 μm, i.e., D 2 v 50 was 1.75.

[0053] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 1:9. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 1.68 μm, i.e., D mo The value was 1.68.

[0054] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 2 was fabricated into a positive electrode sheet. The compressed density of the positive electrode sheet was 2.59 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, the PD was 2.59. PD × D of the positive electrode sheet mo The value was 4.35. [Examples]

[0055] Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0056] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.43 μm, i.e., D 1 v 50 was 0.43.

[0057] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 1.05 μm, i.e., D 2 v 50 was 1.05.

[0058] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 3:7. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 0.78 μm, i.e., D mo The value was 0.78.

[0059] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 3 was fabricated into a positive electrode sheet. The compressed density of the positive electrode sheet was 2.37 g / cm³ under a pressure of 1.5 MPa. 3That is, PD was 2.37. PD×D of the positive electrode sheet mo was 1.85.

Example

[0060] Different from the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0061] The first lithium iron phosphate material LFP-1 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.35 μm, that is, D 1 v 50 was 0.35.

[0062] The second lithium iron phosphate material LFP-2 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.79 μm, that is, D 2 v 50 was 1.79. <00,00347> To obtain the positive electrode active material LFP-3, the LFP-1 material and the LFP-2 material were mixed at a weight ratio of 4:6. When the volume density of LFP-3 reached the maximum value, the corresponding particle size was 1.22 μm, that is, D mo was 1.22.

[0064] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 4 was made into a positive electrode sheet. The compression density of the positive electrode sheet was 2.63 g / cm under a pressure of 3 1.5 Mpa, that is, PD was 2.63. PD×D of the positive electrode sheet mo was 3.21.

Example

[0065] Different from the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0066] The first lithium iron phosphate material LFP-1 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.47 μm, that is, D1 v 50 was 0.47.

[0067] The second lithium iron phosphate material LFP-2 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.48 μm, that is, D 2 v 50 was 1.48.

[0068] To obtain the positive electrode active material LFP-3, the LFP-1 material and the LFP-2 material were mixed at a weight ratio of 1:1. When the bulk density of LFP-3 reached the maximum value, the corresponding particle size was 1.14 μm, that is, D mo was 1.14.

[0069] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 5 was fabricated into a positive electrode sheet. The compression density of the positive electrode sheet was 2.67 g / cm under a pressure of 1.5 Mpa 3 That is, PD was 2.67. PD×D of the positive electrode sheet mo was 3.04.

Example

[0070] Different from the method of Example 1, the method for fabricating the positive electrode sheet includes the following steps.

[0071] The first lithium iron phosphate material LFP-1 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.91 μm, that is, D 1 v 50 was 0.91 μm.

[0072] The second lithium iron phosphate material LFP-2 was used. When the cumulative volume distribution percentage was 50%, the corresponding particle size was 2.32 μm, that is, D 2 v 50 was 2.32 μm.

[0073] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 2:8. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 3.78 μm, i.e., D mo The value was 3.78.

[0074] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 6 was fabricated into a positive electrode sheet. The compressed density of the positive electrode sheet was 2.56 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, PD is 2.56 g / cm³. 3 That was it. PD×D of the positive electrode sheet mo The result was 9.68. [Examples]

[0075] Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0076] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.91 μm, i.e., D 1 v 50 was 0.91 μm.

[0077] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 3.24 μm, i.e., D 2 v 50 was 3.24 μm.

[0078] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 3:7. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 3.89 μm, i.e., D mo It was 3.89 μm.

[0079] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 7 was prepared into a positive electrode sheet. The compressed density of the positive electrode sheet was 2.53 g / cm³ under a pressure of 1.5 MPa. 3Therefore, PD is 2.53 g / cm³. 3 That was it. PD×D of the positive electrode sheet mo The result was 9.84. [Examples]

[0080] Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0081] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.89 μm, i.e., D 1 v 50 was 0.89 μm.

[0082] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 3.21 μm, i.e., D 2 v 50 was 3.21 μm.

[0083] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 1:9. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 4.41 μm, i.e., D mo It was 4.41 μm.

[0084] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 8 was prepared into a positive electrode sheet. The compressed density of the positive electrode sheet was 2.61 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, PD is 2.61 g / cm³. 3 That was it. PD×D of the positive electrode sheet mo The value was 11.51.

[0085] To highlight the beneficial effects of this disclosure, the following comparative examples are provided.

[0086] Comparative Example 1 Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0087] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 0.45 μm, i.e., D 1 v 50 was 0.45.

[0088] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 3.45 μm, i.e., D 2 v 50 was 3.45.

[0089] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 3:7. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 4.98 μm, i.e., D mo The score was 4.98.

[0090] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Comparative Example 1 was prepared as a positive electrode sheet. The compressed density of the positive electrode sheet was 2.56 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, the PD was 2.56. PD × D of the positive electrode sheet mo The value was 12.75, which was outside the range of 0.45 to 12.38 of this disclosure.

[0091] Comparative Example 2 Unlike the method of Example 1, the method for producing the positive electrode sheet includes the following steps.

[0092] When the first lithium iron phosphate material LFP-1 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 1.04 μm, i.e., D 1 v 50 was 1.04, which was outside the range of 0.3 to 0.95 as defined in this disclosure.

[0093] When the second lithium iron phosphate material LFP-2 is used and the cumulative volume distribution percentage is 50%, the corresponding particle size is 3.74 μm, i.e., D 2 v 50 was 3.74, which was outside the range of 1.0 to 3.5 as defined in this disclosure.

[0094] To obtain the positive electrode active material LFP-3, LFP-1 material and LFP-2 material were mixed in a weight ratio of 8:2. When the volume density of LFP-3 reached its maximum value, the corresponding particle size was 2.96 μm, i.e., D mo It was 2.96 μm.

[0095] According to the method described in Example 1, the positive electrode active material LFP-3 obtained in Comparative Example 2 was prepared as a positive electrode sheet. The compressed density of the positive electrode sheet was 2.53 g / cm³ under a pressure of 1.5 MPa. 3 Therefore, PD is 2.53 g / cm³. 3 That was it. PD×D of the positive electrode sheet mo The value was 7.49.

[0096] To provide strong support for the beneficial effects of this disclosure, the pouch batteries of each example and comparative example were tested for the following electrochemical performance.

[0097] 1) Cycle Performance: Each pouch battery was subjected to a 0.5C / 0.5C charge-discharge cycle test at 25°C. The voltage range was 2.0~3.8V. During charging, the battery was charged to 3.8V with a constant current of 0.5C, and then charged to a cutoff current of 0.05C with a constant voltage. The Coulomb efficiency of the first cycle and the capacity retention rate after 1000 charge-discharge cycles were tested.

[0098] 2) Discharge capacity per gram of positive electrode active material: In a voltage range of 2.0 to 3.8 V, the positive electrode active material of each example and comparative example was charged to 3.8 V with a constant current of 0.1 C, and then charged to a cutoff current of 0.05 C with a constant voltage. Subsequently, the positive electrode active material was discharged to 2.0 V with a constant current of 0.1 C. The charging and discharging process was repeated three times. The discharge capacity of the third discharge was recorded as C0, and C0 divided by the mass of LFP-3 of each example and comparative example was the capacity per gram of LFP-3.

[0099] Figure 1 shows the cycle performance curves for each pouch battery in Examples 1-5 and Comparative Example 1. The Coulomb efficiency for the first cycle, the capacity retention rate after 1000 cycles, and other data for each example and comparative example battery are summarized in Table 1 below. [Table 1]

[0100] As can be seen from Figure 1 and Table 1, the cathode sheet produced by the method provided in this disclosure has a higher compressive density, and the electrochemical performance of batteries assembled using this cathode sheet is excellent. For example, the Coulomb efficiency in the first cycle can exceed 96%, the cathode has a high capacity per gram, and the capacity retention rate after 1000 cycles still exceeds 90%. However, the Coulomb efficiency and cycle performance of the batteries of Comparative Examples 1 and 2 in the first cycle are poor and far inferior to the batteries of this disclosure.

[0101] The embodiments described above illustrate only a few embodiments of the Disclosure and are described in detail, but they should not be construed as limiting the scope of the Disclosure. Those skilled in the art will understand that several modifications and improvements can be made without departing from the spirit of the Disclosure, all of which are intended to be within the scope of the Disclosure. Accordingly, the scope of the Disclosure shall be defined by the appended claims.

Claims

1. Positive electrode current collector and A positive electrode sheet comprising a positive electrode active material layer disposed on at least one side surface of the positive electrode current collector, The positive electrode active material layer comprises a positive electrode active material, and the positive electrode active material is First lithium iron phosphate material, and The second lithium iron phosphate material is provided, When the cumulative volume distribution percentage of the first lithium iron phosphate material is 50%, the corresponding particle size is D １ ｖ When the particle size is 50 μm and the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the corresponding particle size is D ２ ｖ It is 50 μm, D １ ｖ 50 is in the range of 0.3 to 0.95, D ２ ｖ 50 is in the range of 1.0 to 3.

5. When the volume density of the positive electrode active material reaches its maximum value, the corresponding particle size is D ｍｏ μm, and The compression density of the positive electrode sheet is PDg / cm² under a pressure of 1.5 MPa. ３ Therefore, PD is in the range of 2.25 to 2.85, and 0.45 ≤ PD × D ｍｏ A positive electrode sheet that satisfies ≤ 12.

38.

2. The positive electrode sheet according to claim 1, wherein PD is in the range of 2.3 to 2.

7.

3. The positive electrode sheet according to claim 1, wherein PD is in the range of 2.35 to 2.

7.

4. The first lithium iron phosphate material D １ ｖ 50 is in the range of 0.35 to 0.65, and the D of the second lithium iron phosphate material ２ ｖ The positive electrode sheet according to claim 1, wherein 50 is in the range of 1.25 to 2.

85.

5. PD×D ｍｏ The positive electrode sheet according to claim 1, wherein the value is in the range of 0.6 to 10.

5.

6. D ｍｏ The positive electrode sheet according to claim 1, wherein the value is in the range of 0.2 to 4.

5.

7. D ｍｏ The positive electrode sheet according to claim 1, wherein the value is in the range of 0.3 to 4.

2.

8. When the cumulative volume distribution percentage of the positive electrode active material is 50%, the corresponding particle size is D ｖ It is 50 μm, D ｖ The positive electrode sheet according to claim 1, wherein 50 is in the range of 0.2 to 3.

1.

9. The positive electrode sheet according to claim 1, wherein the weight ratio of the first lithium iron phosphate material to the second lithium iron phosphate material is in the range of 1:(1 to 9).

10. The positive electrode sheet according to claim 1, wherein the first lithium iron phosphate material has a carbon coating layer on its surface, and / or the second lithium iron phosphate material has a carbon coating layer on its surface.

11. The positive electrode active material layer further comprises a binder and a conductive agent, and the compressed density of the positive electrode sheet is 2.25 to 2.75 g / cm³ under a pressure of 1.5 MPa. ３ A positive electrode sheet according to claim 1, which is within the range of the specified range.

12. The positive electrode sheet according to claim 11, wherein the positive electrode active material layer is formed by coating the positive electrode current collector with a positive electrode paste comprising the positive electrode active material, a conductive agent, the binder, and a solvent.

13. A lithium-ion battery comprising a positive electrode sheet according to any one of claims 1 to 12.

14. The lithium-ion battery according to claim 13, further comprising a negative electrode sheet, an electrolyte and a separator located between the positive electrode sheet and the negative electrode sheet.

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