Patent Application
20240405209
The present disclosure relates to the field of lithium ion batteries, and specifically to a positive electrode sheet and a lithium ion battery.
Lithium iron phosphate material has wide use due to the high safety, low cost, environmental friendliness and other advantages. However, the lithium iron phosphate material also has obvious defects. That is, the compaction density of the lithium iron phosphate material is low, which leads to the low energy density of batteries prepared with the lithium iron phosphate material, so the demand for batteries with long battery life cannot be met. To improve the energy density of the battery, a lithium iron phosphate material having high compaction density is needed. However, when the compaction density of the lithium iron phosphate material is increased, the electrochemical performance of the battery is often reduced.
In a first aspect, the present disclosure provides a positive electrode sheet, which includes: a positive electrode current collector and a positive electrode active material layer arranged on at least a side surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and 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 corresponding particle size is D1v50 μm. When the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the corresponding particle size is D2v50 μm. D1v50 is in the range of 0.3-0.95, and D2v50 is in the range of 1.0-3.5.
When the volume density of the positive electrode active material reaches a maximum value, the corresponding particle size is Dmo μm. The compaction density of the positive electrode sheet is PD g/cm3 under a pressure of 1.5 Mpa, and PD meets: 0.45≤PD×Dmo≤12.38.
In a second aspect, the present disclosure provides a lithium ion battery. The lithium ion battery includes a positive electrode sheet as described in the first aspect of the present disclosure.
The foregoing descriptions are some implementations of the present disclosure. It should be noted that several improvements and modifications can be made by those of ordinary skill in the art without departing from the principle of the present disclosure, which shall fall within the protection scope of the present disclosure.
The present disclosure provides a positive electrode sheet and a preparation method thereof. By the present disclosure, the positive electrode sheet is ensured to have a high compaction density, and a battery prepared therewith has good electrochemical performance.
In a first aspect, the present disclosure provides a positive electrode sheet, which includes: a positive electrode current collector and a positive electrode active material layer arranged on at least a side surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and 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 corresponding particle size is D1v50 μm. When the cumulative volume distribution percentage of the second lithium iron phosphate material is 50%, the corresponding particle size is D2v50 μm. D1v50 is in the range of 0.3-0.95, and D2v50 is in the range of 1.0-3.5. When the volume density of the positive electrode active material reaches a maximum value, the corresponding particle size is Dmo μm. The compaction density of the positive electrode sheet is PD g/cm3 under a pressure of 1.5 Mpa, and PD meets: 0.45≤PD×Dmo≤12.38.
In the present disclosure, two lithium iron phosphate materials having different particle sizes are used, and the mixing ratio of the two lithium iron phosphate materials is adjusted such that the particle size Dmo of the positive electrode active material obtained after mixing and the compaction density PD of the positive electrode sheet meet a certain relation formula. This ensures that the positive electrode sheet has an appropriately high compaction density and the electrochemical performance (especially cycle performance) of a battery prepared with the positive electrode sheet is excellent. The particle size D1v50 of the first lithium iron phosphate material, the particle size D2v50 of the second lithium iron phosphate material, and the particle size Dmo of the positive electrode active material can each be obtained from their respective laser particle size distribution diagrams. The test method can be found in GB/T 19077-2016/ISO 13320:2009 Particle size analysis-Laser diffraction methods. The test instrument is a laser particle size analyzer (such as Malvern 3000). When the cumulative volume distribution percentage of a material is 50%, the corresponding particle size is referred to as median particle size of the material. PD can be obtained by testing the positive electrode sheet by a compaction machine.
In some embodiments of the present disclosure, PD is in the range of 2.25-2.85. In this case, while the positive electrode sheet meets the condition that PD×Dmo is in the range of 0.45-12.38, the positive electrode sheet has large compaction density and appropriate porosity, and the battery has even better cycle performance and rate performance.
In some embodiments of the present disclosure, PD is in the range of 2.3-2.7.
In some embodiments of the present disclosure, PD is in the range of 2.35-2.7.
In the present disclosure, D1v50<D2v50. When D1v50 of the lithium iron phosphate material is small, the lithium ion diffusion path is relatively short, and the prepared battery has good electrical performance. When D2v50 of the lithium iron phosphate material is large, the compaction density of the positive electrode active material can be increased. When the particle sizes Dv50 of the two materials are in the above ranges, by adjusting the mixing ratio of the two, the two lithium iron phosphate materials can be ensured to form a dense mass, thus improving the compaction density of the obtained positive electrode sheet without compromising the cycle performance of the battery.
In some embodiments of the present disclosure, D1v50 of the first lithium iron phosphate material is in the range of 0.35-0.65, and D2v50 of the second lithium iron phosphate material is in the range of 1.25-2.85.
In the present disclosure, the compaction density PD of the positive electrode sheet and the particle size Dmo of the positive electrode active material are considered in combination, rather than pursing a PD value as high as possible. The particle size of the positive electrode active material affects the electronic conductivity and ionic conductivity of the electrode sheet. Dmo can indirectly reflect the content of medium-to small-sized particles (or fine powder) in the positive electrode active material particles. In the present disclosure, by controlling 0.45≤PD×Dmo≤12.38, while the positive electrode sheet is ensured to have an appropriately high compaction density, the overall electronic conductivity and ion conductivity of the positive electrode sheet are both excellent, and the battery has good cycle performance and rate performance.
In some embodiments of the present disclosure, PD×Dmo is in the range of 0.6-10.5. In this case, the positive electrode sheet is ensured to have high compaction density and the battery is ensured to have good electrochemical performance.
In some embodiments of the present disclosure, Dmo is in the range of 0.2-4.5. In this case, the positive electrode active material has a proper number of small-to medium-sized particles, which can ensure that the resistance of diffusion/migration of ions inside and between particles of the positive electrode active material is small, and contributes to the good rate and cycle performance of the battery.
In some embodiments of the present disclosure, Dmo is in the range of 0.3-4.2.
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 Dv50 μm (median particle size), and Dv50 is in the range of 0.2-3.1. When the median particle size of the positive electrode active material after mixing is in the above range, the positive electrode active material particles are properly piled up, the polarization intensity of the electrode sheet is low, the lithium ion transmission speed is high during the cycle process of the battery, and the energy density of the secondary battery is high.
In the present disclosure, the mixing weight ratio of the first lithium iron phosphate material and the second lithium iron phosphate material is not particularly limited, as long as the particle size of the positive electrode active material formed by mixing the two is ensured to meet the above relation formula.
In some embodiments of the present disclosure, the mixing weight ratio of the first lithium iron phosphate material and the second lithium iron phosphate material is in the range of 1:(1-9), and further in the range of 1:(1-4). In this case, the positive electrode active material formed by mixing the two lithium iron phosphate materials can well ensure the high compaction density of the electrode sheet and the good cycle performance of the battery.
In some embodiments of the present disclosure, the first lithium iron phosphate material has a carbon coating layer on the surface and/or the second lithium iron phosphate material has a carbon coating layer on the surface. The carbon coating layer can be obtained by sequentially sanding, spray drying and sintering a mixed slurry of a phosphorus source, an iron source, a lithium source and a carbon source. In the present disclosure, the specific preparation method of the two lithium iron phosphate materials is not limited. The presence of the carbon coating layer allows the first and the second lithium iron phosphate material to have good electrical conductivity, and less side reactions with the electrolyte solution. The positive electrode active material obtained by mixing the two has good electrical conductivity, and the cycle performance of the battery is good.
According to the positive electrode sheet provided in the first aspect of the present disclosure, by mixing two lithium iron phosphate materials meeting the requirements of specific particle size parameters, and controlling the particle size Dmo of the positive electrode active material obtained by mixing the two lithium iron phosphate materials and the compaction density PD of the positive electrode sheet to meet 0.45≤PD×Dmo≤12.38, the obtained positive electrode sheet can be ensured to have high compaction density and good electrochemical performances, particularly cycle performance and rate performance.
In some embodiments of the present disclosure, the positive electrode active material layer further includes a binder and a conductive agent. The compaction density of the positive electrode sheet is in the range of 2.25-2.75 g/cm3 under a pressure of 1.5 Mpa.
In some embodiments of the present disclosure, the positive electrode active material layer is formed by coating a positive electrode paste including the positive electrode active material, a conductive agent, the binder and a solvent on the positive electrode current collector. The solvent can be one or more of N-methylpyrrolidone (NMP), acetone and dimethyl acetamide (DMAC). The positive electrode current collector includes any one of an aluminum foil, a carbon-coated aluminum foil and a perforated aluminum foil. The conductive agent includes, but is not limited to, one or more of carbon nanotubes, graphene carbon black, carbon fiber and the like. The binder includes, 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 carboxymethyl cellulose (CMC) and sodium alginate.
In a second aspect, the present disclosure provides a lithium ion battery. The lithium ion battery includes a positive electrode sheet as described in the first aspect of the present disclosure.
In some embodiments of the present disclosure, the lithium ion battery further includes a negative electrode sheet, and an electrolyte solution and a separator located between the positive electrode sheet and the negative electrode sheet.
The lithium ion battery using the positive electrode sheet has high energy density and excellent cycle performance.
In the following, the technical solution of the present disclosure will be described in combination with specific embodiments.
A method for preparing a positive electrode sheet includes the following steps.
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.55 μm, that is, D1v50 was 0.55.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.35 μm, that is, D2v50 was 1.35.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 2:8, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 1.15 μm, that is, Dmo was 1.15.
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 at a weight ratio of 85:5:10, adding an appropriate amount of N-methyl pyrrolidone (NMP), and mixing uniformly, to obtain a positive electrode paste. The positive electrode paste was coated on two sides of a carbon-coated aluminum foil, and dried, to obtain a two-sided positive electrode sheet. The compaction density of the positive electrode sheet was 2.45 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.45. PD×Dmo of the positive electrode sheet was 2.82.
Preparation of pouch lithium ion battery: The positive electrode sheet prepared with the positive electrode active material LFP-3 in Example 1 was used as a positive electrode. A graphite electrode sheet was used as a negative electrode. A polypropylene film was used as a separator. A solution containing 1.0 mol/L LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC)=1:1 (volume ratio) was used as an electrolyte solution. After assembly, a pouch battery was obtained.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.66 μm, that is, D1v50 was 0.66.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.75 μm, that is, D2v50 was 1.75.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 1:9, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 1.68 μm, that is, Dmo was 1.68.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 2 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.59 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.59. PD×Dmo of the positive electrode sheet was 4.35.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.43 μm, that is, D1v50 was 0.43.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.05 μm, that is, D2v50 was 1.05.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 3:7, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 0.78 μm, that is, Dmo was 0.78.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 3 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.37 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.37. PD×Dmo of the positive electrode sheet was 1.85.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.35 μm, that is, D1v50 was 0.35.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.79 μm, that is, D2v50 was 1.79.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 4:6, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 1.22 μm, that is, Dmo was 1.22.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 4 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.63 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.63. PD×Dmo of the positive electrode sheet was 3.21.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.47 μm, that is, D1v50 was 0.47.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.48 μm, that is, D2v50 was 1.48.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 1:1, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 1.14 μm, that is, Dmo was 1.14.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 5 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.67 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.67. PD×Dmo of the positive electrode sheet was 3.04.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.91 μm, that is, D1v50 was 0.91 μm.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 2.32 μm, that is, D2v50 was 2.32μ m.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 2:8, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 3.78 μm, that is, Dmo was 3.78.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Example 6 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.56 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.56 g/cm3. PD×Dmo of the positive electrode sheet was 9.68.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.91 μm, that is, D1v50 was 0.91 μm.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 3.24 μm, that is, D2v50 was 3.24 μm.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 3:7, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 3.89 μm, that is, Dmo was 3.89 μm.
Following 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 compaction density of the positive electrode sheet was 2.53 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.53 g/cm3. PD×Dmo of the positive electrode sheet was 9.84.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate materialLFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.89 μm, that is, D1v50 was 0.89 μm.
A second lithium iron phosphate materialLFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 3.21 μm, that is, D2v50 was 3.21 μm.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 1:9, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 4.41 μm, that is, Dmo was 4.41 μm.
Following 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 compaction density of the positive electrode sheet was 2.61 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.61 g/cm3. PD×Dmo of the positive electrode sheet was 11.51.
To highlight the beneficial effects of the present disclosure, the following comparative examples are provided.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 0.45 μm, that is, D1v50 was 0.45.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 3.45 μm, that is, D2v50 was 3.45.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 3:7, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 4.98 μm, that is, Dmo was 4.98.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Comparative Example 1 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.56 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.56. PD×Dmo of the positive electrode sheet was 12.75, which fell outside the range of 0.45-12.38 in the present disclosure.
Different from the method in Example 1, a method for preparing a positive electrode sheet includes the following steps:
A first lithium iron phosphate material LFP-1 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 1.04 μm, that is, D1v50 was 1.04, which fell outside the range of 0.3-0.95 defined in the present disclosure.
A second lithium iron phosphate material LFP-2 was used, where when the cumulative volume distribution percentage was 50%, the corresponding particle size was 3.74 μm, that is, D2v50 was 3.74, which fell outside the range of 1.0-3.5 defined in the present disclosure.
The LFP-1 material and the LFP-2 material were mixed at a weight ratio of 8:2, to obtain a positive electrode active material LFP-3. When the volume density of LFP-3 reached a maximum value, the corresponding particle size was 2.96 μm, that is, Dmo was 2.96 μm.
Following the method described in Example 1, the positive electrode active material LFP-3 obtained in Comparative Example 2 was prepared into a positive electrode sheet. The compaction density of the positive electrode sheet was 2.53 g/cm3 under a press of 1.5 Mpa, that is, PD was 2.53 g/cm3. PD×Dmo of the positive electrode sheet was 7.49.
To provide powerful support for the beneficial effects of the present disclosure, The pouch battery of each example and comparative example were tested for the following electrochemical performance.
As can be seen from
The foregoing embodiments show only several implementations of the present disclosure and are described in detail, which, however, are not to be construed as a limitation to the scope of the present disclosure. It is to be understood that for a person of ordinary skill in the art, several variations and improvements can be made without departing from the idea of the present disclosure, which are all contemplated in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202210139512.0 | Feb 2022 | CN | national |
The present application is a continuation application of PCT application No. PCT/CN2023/074704, filed on Feb. 7, 2023, which claims priority to Chinese Patent Application No. 202210139512.0 filed on Feb. 15, 2022 and entitled “Positive electrode sheet and lithium ion battery”, content of all of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2023/074704 | Feb 2023 | WO |
| Child | 18804688 | US |
