Positive pole piece, lithium battery and device

Abstract

The invention provides a positive pole piece, a lithium battery and a device. The positive pole piece comprises a positive current collector, a first positive material layer and a second positive material layer, the first positive material layer and the second positive material layer are sequentially stacked on at least one side of the positive current collector, the first positive material layer comprises a first lithium manganese iron phosphate material, and the molar ratio of Mn / (Mn + Fe) is x; the second positive electrode material layer comprises a second lithium manganese iron phosphate material, wherein the molar ratio of Mn / (Mn + Fe) is y; x < y, 0.4 < = x < = 0.65, and 0.6 < = y < = 0.8; the D50 particle size of the first lithium manganese iron phosphate material is in a range of 0.8 mu m-2 mu m, and the D50 particle size of the second lithium manganese iron phosphate material is in a range of 0.2 mu m-0. 6 mu m. The two lithium manganese iron phosphate materials are introduced into the positive pole piece at the same time and are arranged in a layered manner, so that the lithium battery adopting the positive pole piece has good fast charging performance and high energy density.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly relates to a positive electrode sheet, a lithium battery, and a device. Background Art

[0002] Compared with lithium iron phosphate (LFP) materials, lithium manganese phosphate (LMP) materials have a higher voltage plateau (about 4.0V), so their theoretical energy density is about 20% higher than that of LFP. However, LMP materials are almost insulators, and the actual specific capacity of the materials is extremely low, and there is almost no commercial possibility. Therefore, lithium manganese iron phosphate (LMFP) materials formed by partially replacing Fe with Mn are considered to be the next-generation phosphate-based positive electrode materials that can improve the energy density of batteries.

[0003] However, due to the introduction of Mn ions, the intrinsic impedance of LMFP materials is relatively large, and the polarization voltage of the positive electrode is relatively large during high-rate charging, resulting in a longer charging time for the battery. If the fast charging performance of LMFP materials is improved, the Mn content can be reduced, but the reduction of the Mn content will reduce the average discharge voltage, resulting in a decrease in the mass energy density of the materials. Therefore, it is necessary to provide a technical solution in which the battery positive electrode includes LMFP materials and can enable the battery to have both good fast charging performance and high energy density. Summary of the Invention

[0004] In view of this, the present application provides a double-coated positive electrode sheet to solve the problem that the existing lithium batteries using LMFP materials as the positive electrode cannot have both good fast charging performance and high energy density.

[0005] Specifically, in the first aspect of the present application, a positive electrode sheet is provided. The positive electrode sheet includes a positive electrode current collector and a first positive electrode material layer and a second positive electrode material layer laminated on at least one side of the positive electrode current collector. Among them, on the same side of the positive electrode current collector, the second positive electrode material layer is disposed on the side of the first positive electrode material layer away from the positive electrode current collector; the first positive electrode material layer includes a first lithium manganese iron phosphate material, and the second positive electrode material layer includes a second lithium manganese iron phosphate material; the molar ratio of Mn / (Mn + Fe) in the first lithium manganese iron phosphate material is x, and the molar ratio of Mn / (Mn + Fe) in the second lithium manganese iron phosphate material is y, where x < y, 0.4 ≤ x ≤ 0.65, and 0.6 ≤ y ≤ 0.8; the D50 particle size of the first lithium manganese iron phosphate material is in the range of 0.8 μm - 2 μm, and the D50 particle size of the second lithium manganese iron phosphate material is in the range of 0.2 μm - 0.6 μm.

[0006] In the above positive electrode sheet of the embodiment of the present application, two lithium manganese iron phosphate materials that meet the above requirements are introduced at the same time and are layered as described above, so that the lithium battery using the positive electrode sheet can have both good fast charging performance and high energy density characteristics, and the comprehensive performance is relatively excellent.

[0007] In a second aspect, the present application provides a lithium battery, which includes a positive electrode tab as described in the first aspect of the present application.

[0008] Due to the adoption of the above positive electrode tab, the lithium battery can better balance good fast charging performance and high energy density.

[0009] In a third aspect, the present application provides a device including the lithium battery described in the second aspect of the present application, and the device includes an electrical equipment or an energy storage system. Description of the Drawings

[0010] Figure 1 It is a schematic structural diagram of a positive electrode tab provided by the present application.

[0011] Figure 2 It is another schematic structural diagram of a positive electrode tab provided by the present application.

[0012] Figure 3 It shows the EIS test curves of the button cells prepared by using the positive electrode tabs of Examples 1-3 and Comparative Examples 6 and 7. Detailed Embodiments

[0013] Next, the technical solutions of the embodiments of the present application will be described in conjunction with the drawings.

[0014] Please refer to Figure 1 and Figure 2 , the embodiments of the present application provide a positive electrode tab 100 for a lithium battery. The positive electrode tab 100 includes a positive electrode current collector 10 and a first positive electrode material layer 21 and a second positive electrode material layer 22 stacked on at least one side of the positive electrode current collector 10. Among them, on the same side of the positive electrode current collector 10, the second positive electrode material layer 22 is disposed on the side away from the positive electrode current collector 10 of the first positive electrode material layer 21; the first positive electrode material layer 21 includes a first lithium iron manganese phosphate material, and the second positive electrode material layer 22 includes a second lithium iron manganese phosphate material; the molar ratio of Mn / (Mn + Fe) in the first lithium iron manganese phosphate material is x, and the molar ratio of Mn / (Mn + Fe) in the second lithium iron manganese phosphate material is y, where x < y, 0.4 ≤ x ≤ 0.65, 0.6 ≤ y ≤ 0.8; the D50 particle size of the first lithium iron manganese phosphate material is in the range of 0.8 μm - 2 μm, and the D50 particle size of the second lithium iron manganese phosphate material is in the range of 0.2 μm - 0.6 μm.

[0015] In the above-mentioned positive electrode sheet 100, two kinds of lithium iron manganese phosphate materials are introduced simultaneously and arranged in layers. The first lithium iron manganese phosphate material with a large D50 particle size and a small Mn / (Mn+Fe) molar ratio (hereinafter simply referred to as "LMFP-1") is arranged in the first positive electrode material layer 21 close to the positive electrode current collector 10, and the second lithium iron manganese phosphate material with a small D50 particle size and a large Mn / (Mn+Fe) molar ratio (hereinafter simply referred to as "LMFP-2") is arranged in the second positive electrode material layer 22 far from the positive electrode current collector 10. This can make the two LMFP materials "make the best use of their advantages and avoid their disadvantages", ensuring that the overall positive electrode sheet 100 takes into account good fast charging performance and high energy density characteristics.

[0016] Specifically, the Mn / (Mn+Fe) molar ratio of the LMFP-1 material is lower than that of the LMFP-2 material, and its intrinsic resistivity is lower. Setting it close to the positive electrode current collector 10 is more conducive to reducing the impedance of the electrode sheet, improving the electron transfer efficiency on the side of the positive electrode current collector, and ensuring better fast charging performance of the materials in the overall electrode sheet; in this application, the D50 particle size of the LMFP-1 material with a low Mn / (Mn+Fe) molar ratio is also controlled to be larger, which is conducive to ensuring a higher tap density and surface density of the first positive electrode material layer 21 where it is located, conducive to improving the energy density of the battery, and reducing the influence of the low Mn / (Mn+Fe) molar ratio of the LMFP-1 material on the reduction of energy density.

[0017] The LMFP-2 material has a small D50 particle size, a large specific surface area, and many lithium deintercalation and insertion active sites. And setting it far from the positive electrode current collector 10 (closer to the electrolyte) can make the porosity of the positive electrode material layer where it is located rich, with a strong liquid retention ability for the electrolyte, and a faster liquid phase transfer speed of active ions in this layer. Thus, during high-rate charging, the charging polarization is lower, the charging power is higher, the battery has strong fast charging ability and short fast charging time, and it also conforms to reducing the adverse effect of the high Mn / (Mn+Fe) molar ratio of the LMFP-2 on the fast charging performance of the battery. In addition, the high Mn / (Mn+Fe) molar ratio of the LMFP-2 material is helpful for increasing the average discharge voltage and mass energy density, and also reduces the influence of the small D50 particle size of the LMFP-2 material on the reduction of the battery volume energy density.

[0018] Therefore, by controlling the D50 particle size and Mn / (Mn+Fe) molar ratio of the two LMFP materials within the above ranges, their performances can be differentiated to a certain extent. And by arranging the two LMFP materials that meet the above requirements in layers as described above, the lithium battery using the above positive electrode sheet 100 can take into account good fast charging performance and high energy density characteristics, and has better comprehensive performance.

[0019] In addition, the active materials in the two layers of the positive electrode active material layer of the above-mentioned positive electrode sheet 100 are both homogeneous lithium iron manganese phosphate materials, and there is no problem of mismatched working voltage windows between heterogeneous materials, which has a relatively low deteriorating effect on the cycle life of the battery cell; the two LMFP materials are olivine-type positive electrode materials, and the intrinsic thermodynamic properties of the materials are stable, ensuring the safety performance of the battery.

[0020] In the present application, the molar ratio of Mn / (Mn + Fe) refers to the proportion of the amount of substance of the Mn element based on the sum of the amounts of substance of the Mn element and the Fe element in lithium iron manganese phosphate. Specifically, x refers to the proportion of the amount of substance of the Mn element based on the sum of the amounts of substance of the Mn element and the Fe element in the first lithium iron manganese phosphate material; y refers to the proportion of the amount of substance of the Mn element based on the sum of the amounts of substance of the Mn element and the Fe element in the second lithium iron manganese phosphate material.

[0021] In the present application, the above-mentioned LMFP-1 material may include LiMn x Fe 1-x PO4, and the above-mentioned LMFP-2 material may include LiMn y Fe 1-y PO4, where x < y, 0.4 ≤ x ≤ 0.65, and 0.6 ≤ y ≤ 0.8. Specifically, x may be 0.4, 0.42, 0.45, 0.48, 0.50, 0.52, 0.55, 0.56, 0.58, 0.60, 0.62, 0.63, 0.64, etc. Specifically, y may be 0.61, 0.62, 0.64, 0.65, 0.66, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.78, 0.79, etc. In some embodiments of the present application, 0.45 ≤ x ≤ 0.60 and 0.65 ≤ y ≤ 0.75. In this case, the performance of the LMFP-1 material and the LMFP-2 material can be more significantly distinguished. The impedance rate of the LMFP-1 material is lower, and the average discharge voltage and energy density of the LMFP-2 material are higher. Therefore, when the two are arranged in layers as described above, the battery can better balance the fast charging performance and high energy density.

[0022] In the present application, the D50 particle size of the LMFP-1 material may be specifically 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, or 1.9 μm, etc. The D50 particle size of the LMFP-2 material may be specifically 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, or 0.58 μm, etc. In some embodiments, the D50 particle size of the LMFP-1 material may be in the range of 1.0 μm-1.5 μm, and the D50 particle size of the LMFP-2 material may be in the range of 0.4 μm-0.6 μm.

[0023] In the present application, the specific surface area of ​​the LMFP-2 material is greater than the specific surface area of ​​the LMFP-1 material. In the embodiment of the present application, the specific surface area of ​​the LMFP-2 material is 10-25m 2 / g range. The LMFP-2 material with a small D50 particle size has a suitable specific surface area, which helps to reduce the problem of large interface side reactions with the electrolyte and high manganese dissolution during the battery charge and discharge cycle caused by the excessive specific surface area of ​​the material. In the embodiment of the present application, the specific surface area of ​​the above-mentioned LMFP-1 material is 2-12m 2 / g range. The specific surface area of ​​the LMFP-1 material with a large D50 particle size is within an appropriate range, which can help ensure that its specific capacity is as high as possible, and avoid its specific surface being too small and not conducive to the specific capacity. Specifically, the specific surface area of ​​the LMFP-2 material can be specifically 12m 2 / g, 15m 2 / g, 18m 2 / g, 20m 2 / g, 22m 2 / g, 23m 2 / g, 24m 2 / g, etc. The specific surface area of ​​LMFP-1 material can be specifically 3m 2 / g, 5m 2 / g, 6m 2 / g, 8m 2 / g, 10m 2 / g, 11m 2 / g, 11.5m 2 / g, etc.

[0024] In this application, the first positive electrode material layer 21 can be in direct contact with the positive electrode current collector 10, that is, the first positive electrode material layer 21 can be directly disposed on at least one surface of the positive electrode current collector 10; alternatively, other bottom coatings are further provided between the first positive electrode material layer 21 and the positive electrode current collector 10. The second positive electrode material layer 22 can be in direct contact with the first positive electrode material layer 21, that is, the second positive electrode material layer 22 can be directly disposed on the surface of the first positive electrode material layer 21; alternatively, other intermediate layers are further provided between the second positive electrode material layer 22 and the first positive electrode material layer 21.

[0025] In some embodiments of this application, the single-sided surface density of the first positive electrode material layer 21 is greater than or equal to the single-sided surface density of the second positive electrode material layer 22. Since the D50 particle size of the LMFP-1 material is larger than the D50 particle size of the LMFP-2 material, its tap density is greater than that of LMFP-2. As the LMFP-2 is a low-tap-density material, an excessively high mass ratio thereof will cause a decrease in the tap density of the positive electrode sheet 100. Therefore, controlling the surface density of the second positive electrode material layer 22 where LMFP-2 is located to be not higher than that of the first positive electrode material layer 21 where LMFP-1 is located is conducive to ensuring a relatively high energy density of the battery.

[0026] In some embodiments of this application, the ratio of the single-sided surface density of the first positive electrode material layer 21 to the single-sided surface density of the second positive electrode material layer 22 is 1-10. Controlling the surface density of the first positive electrode material layer 21 where the LMFP-1 material with a large D50 particle size is located at an appropriately high level can ensure that it can effectively improve the volume energy density of the battery. This is more conducive to reducing the impact of the low tap density of the small-particle-size LMFP-2 material on the reduction of the battery volume energy density, and avoiding the surface density of the second positive electrode material layer 22 being too low to effectively improve the fast charging performance of the battery. Specifically, the ratio of the single-sided surface density can be 1.1, 1.2, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 9.5, etc. In some embodiments, the ratio of the single-sided surface density is 1-9 times, and further can be 1-8 times or 1.5-9 times, etc.

[0027] In some embodiments of this application, the single-sided surface density of the second positive electrode material layer 22 is 20 g / m 2 -100 g / m 2 . When the single-sided surface density of the positive electrode material layer where the small-particle-size LMFP-2 material is located is within an appropriate range, it can not only ensure that its surface density will not be too high to reduce the tap density of the overall positive electrode sheet 100 and deteriorate the battery volume energy density, and will not increase the tortuosity of the electrode due to its too high surface density and too thick thickness, ensuring good fast charging performance of the battery, but also avoid not being able to effectively improve the fast charging performance of the battery due to its too low surface density. In some embodiments of this application, the single-sided surface density of the first positive electrode material layer 21 is 100 g / m 2 -200 g / m 2。The single-sided areal density of the cathode material layer where the large particle size LMFP-1 material is located is within an appropriate range, which can not only ensure a relatively high compaction density of the overall cathode electrode 100 and a large volumetric energy density of the battery, but also ensure good fast charging performance of the battery. Exemplarily, the single-sided areal density of the second cathode material layer 22 can specifically be 20 g / m 2 、30 g / m 2 、40 g / m 2 、50 g / m 2 、60 g / m 2 、70 g / m 2 、80 g / m 2 、90 g / m 2 、100 g / m 2 etc. The single-sided areal density of the first cathode material layer 21 can specifically be 100 g / m 2 、110 g / m 2 、120 g / m 2 、130 g / m 2 、140 g / m 2 、150 g / m 2 、160 g / m 2 、170 g / m 2 、180 g / m 2 、190 g / m 2 、195 g / m 2 etc.

[0028] In some embodiments of the present application, in the cathode electrode 100, the mass ratio of the LMFP-1 material to the LMFP-2 material is (1-10):1, preferably (1-9):1. This can enable the battery using the cathode electrode 100 to better improve the fast charging performance on the basis of ensuring a high energy density. Among them, the mass fraction of the large particle size LMFP-1 material is relatively high, and the compaction density of the first cathode material layer 21 where it is located is relatively high, mainly playing the role of ensuring a relatively high energy density of the battery. The small particle size LMFP-2 material is introduced into the second cathode material layer 22 on the surface of the electrode, mainly playing the role of improving the fast charging performance. Specifically, the mass ratio of the LMFP-1 material to the LMFP-2 material can be, for example, 1:1, 1.5:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, or 9:1, etc.

[0029] In some embodiments of the present application, the tap density of the first positive electrode material layer 21 is greater than that of the second positive electrode material layer 22. This is mainly because the D50 particle size of the LMFP-1 material is larger than that of the LMFP-2 material. The relatively high tap density of the first positive electrode material layer 21 helps to reduce the impact of the small Mn / (Mn + Fe) molar ratio of the LMFP-1 material on the mass energy density. In some embodiments of the present application, the tap density of the first positive electrode material layer 21 is 2.3 - 2.7 g / cm 3 . The tap density of the second positive electrode material layer 22 is 2.0 - 2.4 g / cm 3 . The positive electrode material layer where the LMFP-1 material with a large D50 particle size is located has an appropriate tap density, which is beneficial for the overall positive electrode sheet 100 to have a relatively high tap density and effectively improve the volumetric energy density of the battery. At the same time, it also ensures that the tap density of the first positive electrode material layer 21 is not too high, its porosity is appropriate, and the liquid retention ability is enhanced. The positive electrode material layer where the LMFP-2 material with a small D50 particle size is located has an appropriately high tap density, which can reduce the impact of the small particle size material on the volumetric energy density of the battery and is beneficial for improving the energy density by virtue of its high Mn content. Specifically, the tap density of the first positive electrode material layer 21 can be 2.35, 2.40, 2.45, 2.50, 2.55, 2.60, 2.65 or 2.68 g / cm 3 etc. The tap density of the second positive electrode material layer 22 can specifically be 2.05, 2.10, 2.15, 2.20, 2.25, 2.30, 2.35 or 2.38 g / cm 3 etc.

[0030] In some embodiments of the present application, the surface of the above-mentioned first lithium manganese iron phosphate material further has a conductive coating layer; and / or the surface of the above-mentioned second lithium manganese iron phosphate material further has a conductive coating layer. The presence of the conductive coating layer can improve the conductivity of the LMFP material and is beneficial for the performance of its rate capability. Among them, the conductive coating layer can specifically include a conductive carbon coating layer, etc. In addition, the above-mentioned first lithium manganese iron phosphate material and second lithium manganese iron phosphate material may also contain doping elements to improve their electronic conductivity and ionic conductivity.

[0031] In some embodiments of the present application, the mass percentage of the above-mentioned first lithium manganese iron phosphate material in the first positive electrode material layer 21 can be more than 85%; the mass percentage of the above-mentioned second lithium manganese iron phosphate material in the second positive electrode material layer 22 can be more than 80%. The relatively high mass percentage of the lithium manganese iron phosphate material as the positive electrode active material in each positive electrode material layer is beneficial for each positive electrode material layer to provide a relatively high reversible capacity, thereby ensuring a relatively high reversible capacity of the overall positive electrode sheet 100. Specifically, the above-mentioned mass percentage can independently be more than 85%, for example, 85% - 95%.

[0032] In this application, in addition to the above-mentioned first lithium iron manganese phosphate material in the first positive electrode material layer 21 and the second lithium iron manganese phosphate material in the second positive electrode material layer 22, the first positive electrode material layer 21 and the second positive electrode material layer 22 may independently include a binder and a conductive agent. Among them, each binder and each conductive agent can be a conventional choice in the battery field. For example, each conductive agent can independently be selected from one or more of conductive carbon black (such as acetylene black, Ketjen black, Super-P, 350G carbon black, etc.), carbon nanotubes (single-walled carbon nanotubes or multi-walled carbon nanotubes), graphene, carbon fiber, ordered mesoporous carbon, etc. Each binder can independently be selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyolefins (such as polyethylene (PE), polypropylene (PP)), polyacrylonitrile (PAN), polyacrylic acid (PAA), polyacrylate, polyimide (PI), styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), sodium alginate (SA), gelatin, etc., but not limited thereto.

[0033] Among them, the first positive electrode material layer 21 and the second positive electrode material layer 22 can be formed by coating and drying the corresponding slurries used for forming them respectively. The solvents contained in each slurry can be the same or different, and can independently be selected from one or more of N-methylpyrrolidone (NMP), N-ethylpyrrolidone, dimethylformamide (DMF), diethylformamide (DEF), dimethyl sulfoxide, tetrahydrofuran, alcohol solvents, etc., but not limited thereto. There is no special limitation on the solid content of each slurry, as long as it can meet the fluidity and uniformity of slurry coating.

[0034] The positive electrode current collector 10 can include but is not limited to aluminum foil, aluminum alloy foil, a polymer film material plated with metal aluminum, or the aforementioned materials with a carbon coating on the surface, etc. In some embodiments of this application, the positive electrode current collector 10 is aluminum foil or carbon-coated aluminum foil. In this application, a laminated structure of the first positive electrode material layer 21 and the second positive electrode material layer 22 can be formed on both opposite surfaces of the positive electrode current collector 10 (as shown in Figure 1 ), or a laminated structure of the first positive electrode material layer 21 and the second positive electrode material layer 22 can be formed on one surface of the positive electrode current collector 10 (as shown in Figure 2 ).

[0035] The embodiment of this application also provides a lithium battery, which includes the positive electrode plate 100 in the above-mentioned embodiment of this application.

[0036] In some embodiments of this application, the lithium battery further includes a negative electrode plate, an electrolyte, and a separator disposed between the negative electrode plate and the positive electrode plate.

[0037] Among them, the negative electrode plate generally includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The negative electrode material layer generally contains a negative electrode active material, a conductive agent, and a binder. In the embodiments of the present application, the negative electrode active material for the lithium battery can be selected from one or more of hard carbon, soft carbon, graphite, mesophase carbon microspheres, silicon-carbon composite materials, etc.

[0038] Among them, the separator is used to separate the positive electrode plate and the negative electrode plate, maintaining the insulation and liquid retention characteristics between the two; the separator, the positive electrode plate, and the negative electrode plate together constitute the battery core, and the core is accommodated in the battery housing and is wetted by the electrolyte contained in the housing. In some embodiments of the present application, the lithium battery can be assembled by the following method: the positive electrode plate, the separator, and the negative electrode plate are sequentially stacked to form a battery core; the battery core is accommodated in the battery housing, and the electrolyte is injected, and then the battery housing is sealed to obtain the battery. Among them, the battery core can be a wound type or a stacked type, etc.

[0039] Among them, any separator material in the battery can be used for the separator. Exemplarily, the separator can include, but is not limited to, polymer separators such as single-layer PP (polypropylene) film, single-layer PE (polyethylene) film, double-layer film PP / PE, double-layer film PP / PP, and three-layer PP / PE / PP, or non-woven fabric, etc. The electrolyte includes an electrolyte salt and an organic solvent, and the specific types and compositions of the electrolyte salt and the organic solvent are all conventional selections in the battery field and can be selected according to actual needs.

[0040] The embodiments of the present application also provide a device, and the device includes the above lithium battery of the embodiments of the present application. Among them, the device can be an electric vehicle (such as a car, a motorcycle, a bicycle, etc.), an electric toy, a 3C product (such as a mobile phone, a laptop computer, a tablet computer, a pen-input computer, an e-book player, a wearable device, etc.) and other electrical equipment; it can also be an energy storage system. The energy storage system can include a plurality of the above lithium batteries and a battery management system. The energy storage system can also supply power to the electrical equipment. Among them, the electrical equipment powered by the above lithium battery has a fast charging speed and a long running time.

[0041] Before introducing the specific embodiments of the present application, the test methods for the various parameters mentioned above in the present application are introduced first.

[0042] In this application, the test method for the Mn / (Mn+Fe) molar ratio of the above-mentioned LMFP materials is as follows: After the lithium-ion battery is fully discharged, it is disassembled to obtain the positive electrode sheet. The positive electrode sheet is immersed in the solvent dimethyl carbonate (abbreviated as DMC in English) for 10 min - 20 min to wash away the residual electrolyte. After the positive electrode sheet is dried, it is immersed in water to inactivate the binder so that the dressing layer can be peeled off from the positive electrode current collector. After drying the dressing layers on the positive electrode current collector side and the separator side, the upper and lower layers of the dressing layer are respectively scraped with a ceramic spatula (or polished with sandpaper) to obtain the first positive electrode material layer powder and the second positive electrode material layer powder. Then, the two positive electrode material layer powders are respectively tested with an inductively coupled plasma spectrometer (ICP) to obtain the Mn content and Fe content in the corresponding positive electrode material layer, and further obtain the Mn / (Mn+Fe) molar ratio of the LMFP material in each positive electrode material layer.

[0043] The test method for the D50 of the above-mentioned LMFP materials is as follows:

[0044] (1) After the lithium-ion battery is fully discharged, it is disassembled to obtain the positive electrode sheet. The positive electrode sheet is immersed in the solvent dimethyl carbonate (abbreviated as DMC in English) for 10 min - 20 min to wash away the residual electrolyte. After the positive electrode sheet is dried, it is immersed in water to inactivate the binder so that the dressing layer can be peeled off from the positive electrode current collector. After drying the dressing layers on the positive electrode current collector side and the separator side, the upper and lower layers of the dressing layer are respectively scraped with a ceramic spatula (or polished with sandpaper) to obtain the first positive electrode material layer powder and the second positive electrode material layer powder.

[0045] (2) Then, the first positive electrode material layer powder and the second positive electrode material layer powder are respectively dissolved in the solvent N-methylpyrrolidone (NMP) and heated at 80 °C to accelerate dissolution. The dissolved material is filtered by suction to separate the solid material, and the solid material is washed with NMP. Then, the obtained mixture of the solid material and NMP is centrifuged at a speed of 5000 rpm, and the upper material containing the conductive agent is discarded, and the lower material containing LMFP is collected. After the material containing LMFP is washed and centrifuged 6 times with NMP, a purified LMFP solid is obtained and vacuum dried at 105 °C for 2 hours to obtain a dried LMFP material.

[0046] (3) Finally, according to GB / T19077-2016 "Laser Diffraction Method for Particle Size Analysis", the dried LMFP materials are tested by laser particle size analysis to obtain the particle size distribution curve of the material. The particle size corresponding to the volume cumulative distribution percentage of 50% of the material can be read from the curve, that is, the D50 of each LMFP material is obtained.

[0047] Among them, the method for obtaining the single-sided surface density of each of the above positive electrode material layers is as follows:

[0048] (1) After fully discharging the lithium-ion battery, disassemble it to obtain the positive electrode plate. Immerse the positive electrode plate in the solvent dimethyl carbonate (abbreviated as DMC in English) for 10 min - 20 min to wash the residual electrolyte. After drying the positive electrode plate, strip the dressing layer on one side of the positive electrode plate (i.e., the laminated structure of the first positive electrode material layer and the second positive electrode material layer) with water, and after drying, use a sampler with a diameter of 0.015 m to sample and weigh the dressing layer to obtain the mass m. Through the formula M = m / (0.0075×0.0075×π), the sum M of the single-sided surface densities of the first positive electrode material layer 21 and the second positive electrode material layer 22 can be calculated;

[0049] (2) Perform ICP testing on the dried dressing layer to obtain the Mn content c in the dressing layer. Assume that the ratio of the surface density M1 of the first positive electrode material layer 21 to the surface density M2 of the second positive electrode material layer 22 is X (where M1 + M2 = M). According to the measured Mn contents of the LMFP materials in the upper and lower positive electrode material layers, which are a and b respectively, according to the following formula:

[0050]

[0051] The above X = (b - c) / (c - a) can be calculated, and thus the surface densities M1 and M2 of the upper and lower positive electrode material layers can be obtained according to the above M value.

[0052] Among them, the method for obtaining the tap density of each of the above positive electrode material layers includes the following steps: Cut the positive electrode plate through CP testing (argon ion thinning), specifically, use an argon ion beam to cut the positive electrode plate at the microscale to obtain a complete electrode interface; Take a scanning electron microscope (SEM) photo of the electrode interface. Due to the large difference in particle sizes of the upper and lower LMFP materials, the thicknesses of the upper and lower positive electrode material layers can be measured as h1 and h2 respectively. And according to the measured surface densities M1 and M2 of the upper and lower positive electrode material layers, the tap density of the first positive electrode material layer can be calculated according to M1 / h1, and the tap density of the second positive electrode material layer can be calculated according to M2 / h2.

[0053] Next, in combination with multiple specific embodiments, the technical solution of the present application will be further described.

[0054] Example 1

[0055] A positive electrode plate, the preparation method of which includes:

[0056] The chemical formula is LiMn 0.6 Fe 0.4The LMFP-1 material of PO4 (as shown in Table 1, its Mn content is 60%, and the D50 particle size is 1 μm) is mixed with the binder PVDF and the conductive agent carbon black in a mass ratio of 92:3:2 in the NMP solvent, and stirred evenly to obtain the first slurry. The LMFP-2 material of LiMn 0.75 Fe 0.25 PO4 (its Mn content and D50 particle size are as shown in Table 1) is mixed with the binder PVDF and the conductive agent carbon black in a mass ratio of 93:3:4 in the NMP solvent, and stirred evenly to obtain the second slurry.

[0057] The first slurry is first coated on the positive current collector - aluminum foil, and after drying, a first positive electrode material layer with a surface density of 150 g / m 2 is formed (abbreviated as "coating 1" in Table 1). Then, the second slurry is coated on the first positive electrode material layer, and after drying, a second positive electrode material layer with a surface density of 50 g / m 2 is formed (abbreviated as "coating 2" in Table 1). Then, the preparation of the first positive electrode material layer and the second positive electrode material layer is carried out on the reverse side of the aluminum foil. After rolling, a double-sided positive electrode plate is obtained.

[0058] According to the parameters summarized in Table 1, the positive electrode plates of other examples and comparative examples are prepared.

[0059] Among them, the difference between Example 2 and Example 1 is only that: the ratio of the surface density of coating 1 to coating 2 is 9. That is, in Example 2, the surface density of coating 1 is changed to 180 g / m 2 , and the single-sided surface density of coating 2 is changed to 20 g / m 2 .

[0060] The difference between Example 3 and Example 1 is only that: the ratio of the surface density of coating 1 to coating 2 is 1.5. That is, the surface density of coating 1 is changed to 120 g / m 2 , and the single-sided surface density of coating 2 is changed to 80 g / m 2 .

[0061] The difference between Example 4 and Example 1 is only that: the D50 particle size of the LMFP-2 material is changed to 0.6 μm.

[0062] The difference between Example 5 and Example 1 is only that: the D50 particle size of the LMFP-1 material is changed to 1.5 μm.

[0063] The difference between Example 6 and Example 1 is only that: the Mn content in the LMFP-1 material (i.e., Mn / (Mn + Fe)) is 50%. That is, the chemical formula of the LMFP-1 material in Example 6 includes LiMn 0.5 Fe 0.5 PO4.

[0064] Example 7 differs from Example 1 only in that the Mn content in the LMFP-2 material is 65%, that is, the chemical formula of the LMFP-2 material in Example 7 includes LiMn 0.65 Fe 0.35 PO4.

[0065] Example 8 differs from Example 1 only in that the Mn content in the LMFP-1 material is changed to 65% and the Mn content in the LMFP-2 material is changed to 80%. That is, the chemical formula of the LMFP-1 material in Example 8 includes LiMn 0.65 Fe 0.35 PO4, and the chemical formula of the LMFP-2 material includes LiMn 0.8 Fe 0.2 PO4.

[0066] Example 9 differs from Example 1 only in that the Mn content in the LMFP-1 material is changed to 40% and the Mn content in the LMFP-2 material is changed to 60%. That is, the chemical formula of the LMFP-1 material in Example 9 includes LiMn 0.4 Fe 0.6 PO4, and the chemical formula of the LMFP-2 material includes LiMn 0.6 Fe 0.4 PO4.

[0067] Example 10 differs from Example 1 only in that the D50 particle size of the LMFP-1 material is changed to 2 μm and the D50 particle size of the LMFP-2 material is changed to 0.6 μm.

[0068] Example 11 differs from Example 1 only in that the D50 particle size of the LMFP-1 material is changed to 0.8 μm and the D50 particle size of the LMFP-2 material is changed to 0.2 μm.

[0069] Example 12 differs from Example 1 only in that the surface density ratio of Coating 1 to Coating 2 is changed to 10. That is, in Example 12, the surface density of Coating 1 is 181.8 g / m 2 and the single-sided surface density of Coating 2 is 18.2 g / m 2 .

[0070] Example 13 differs from Example 1 only in that the surface density ratio of Coating 1 to Coating 2 is changed to 1. That is, in Example 13, the surface densities of both Coating 1 and Coating 2 are 100 g / m 2 .

[0071] Example 14 differs from Example 1 only in that the surface density ratio of Coating 1 and Coating 2 is changed to 0.5. That is, in Example 14, the surface density of Coating 1 is 66.67 g / m 2, the single-sided areal density of Coating 2 is 133.33 g / m 2 .

[0072] The difference between Comparative Example 1 and Example 1 is only that: the Mn contents of the LMFP-1 and LMFP-2 materials are 75% and 85% respectively.

[0073] The difference between Comparative Example 2 and Example 1 is only that: the Mn contents of the LMFP-1 and LMFP-2 materials are 30% and 50% respectively.

[0074] The difference between Comparative Example 3 and Example 1 is only that: the D50 particle sizes of the LMFP-1 and LMFP-2 materials are 3 μm and 1 μm respectively.

[0075] The difference between Comparative Example 4 and Example 1 is only that: the D50 particle sizes of the LMFP-1 and LMFP-2 materials are 0.5 μm and 0.1 μm respectively.

[0076] The difference between Comparative Example 5 and Example 1 is only that: the Mn contents of the LMFP-1 and LMFP-2 materials are both 70%.

[0077] In Comparative Example 6, the positive electrode coating on the aluminum foil is one layer, and the single-sided areal density is 200 g / m 2 , specifically prepared by coating and drying a slurry obtained by mixing a LiMn 0.6 Fe 0.4 PO4 material with a D50 particle size of 1 μm, a binder PVDF, and a conductive agent.

[0078] In Comparative Example 7, the positive electrode coating on the aluminum foil is one layer, and the single-sided areal density is 200 g / m 2 , specifically prepared by coating and drying a slurry obtained by mixing a LiMn 0.5 Fe 0.5 PO4 material with a D50 particle size of 1.5 μm, a binder PVDF, and a conductive agent.

[0079] Table 1 Design parameters of the positive electrode sheets of each example and comparative example

[0080]

[0081] Note: In Table 1, the Mn content specifically refers to the ratio of the molar amount of the Mn element in the LMFP material to the sum of the molar amounts of the Mn and Fe elements.

[0082] The single-sided thickness h1 of the first positive electrode material layer, the single-sided thickness h2 of the second positive electrode material layer, and the total single-sided thickness of the positive electrode coating composed of Coating 1 + Coating 2 in the positive electrode sheets of each embodiment of the present application and Comparative Examples 1-4 are summarized in Table 2 below. Based on Coating thickness = Coating surface density / Coating compaction density, the compaction density of Coating 1, the compaction density of Coating 2, and the mixed compaction density of the positive electrode coating can be calculated, and the relevant results are summarized in Table 2. In addition, the thickness and compaction density values of the positive electrode coatings in the positive electrode sheets of Comparative Examples 5-6 with single-layer coating are also summarized in Table 2.

[0083] Table 2 Summary of relevant test parameters of positive electrode sheets of each embodiment and comparative example

[0084]

[0085]

[0086] The lithium metal sheets were used as counter electrodes for the positive electrode sheets provided in the above embodiments or comparative examples, and button lithium batteries of model CR2016 were assembled. Electrochemical Impedance Spectroscopy (EIS) tests were performed on each button lithium battery to evaluate the positive electrode impedance.

[0087] Figure 3 The EIS test curve graphs of the button batteries prepared using the positive electrode sheets of Examples 1-3 and Comparative Examples 6 and 7 are shown. Figure 3 The larger the arc radius of the EIS curve in, the greater the positive electrode impedance. From Figure 3 It can be known that the positive electrodes of Comparative Examples 6 and 7 are single-layer coated with large-particle-size LMFP-1 material, and the positive electrode impedance is relatively large. After the positive electrode sheets of the embodiments of the present application adopt double-layer coating of different types of LMFP materials, the positive electrode impedance can be significantly reduced, which is beneficial to improving the kinetic performance of the battery positive electrode. Among them, when the sum of the single-sided surface densities of the two positive electrode material layers is equal (such as in Examples 1-3), the higher the surface density of the coating 2 where the LMFP-2 material is located, the lower the positive electrode impedance.

[0088] The positive electrode sheets provided in the above embodiments and comparative examples were respectively assembled with a separator and a negative electrode sheet into a soft-pack lithium battery. The specific preparation process of the soft-pack lithium battery is as follows: (1) Prepare a double-sided negative electrode sheet: Mix graphite negative electrode material with SBR binder, CMC thickener, conductive agent carbon black powder and water in a mass ratio of 100:1:0.5:1:80 to obtain a negative electrode slurry; coat the negative electrode slurry on the opposite two surfaces of the negative electrode current collector respectively, dry to form a negative electrode material layer, and finally roll to obtain a double-sided negative electrode sheet, where the sum of the double-sided surface densities of the negative electrode material layer is 180 g / m 2;(2) Stack the above-mentioned negative electrode sheet, PP separator, and positive electrode sheet in sequence to obtain a bare battery cell (including 7 positive electrode sheets and 8 negative electrode sheets), and place the bare battery cell into a battery case for welding; use a mixed organic solvent containing 1.0 mol / L LiPF6 and 2 wt% vinylene carbonate (VC) (specifically, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a volume ratio of 3:7) as the electrolyte, and inject the electrolyte into the aforementioned battery case to assemble a soft-pack lithium battery.

[0089] Perform formation on the soft-pack lithium battery. Specifically, the formation process is as follows: After the battery is filled with electrolyte, age it at 45°C for 24 h to fully infiltrate the electrolyte, then charge it at 0.05C to 15% SOC, then charge it at a constant current of 0.1C until the cut-off voltage is 4.3V, and then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C; then place it in an oven at 45°C for aging for 24 h.

[0090] Perform room-temperature capacity calibration on each formed soft-pack battery. Specifically, it includes: At room temperature (25°C), charge each soft-pack full battery at a constant current and constant voltage of 0.33C until the upper limit voltage is 4.3V, then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C, and let it stand for 30 min; then discharge it at a constant current of 0.33C until the lower limit voltage is 2V, and let it stand for 30 min; repeat the above charge-discharge steps 3 times, and record the capacity discharged in the third time as C0, with the unit of Ah; and record the average discharge voltage during the third discharge. Among them, this C0 is the room-temperature calibrated capacity of the full battery. In addition, calculate the specific capacity per gram of the positive electrode, the mass energy density of the positive electrode, and the volume energy density of the positive electrode for each soft-pack battery. Among them, specific capacity per gram of the positive electrode = C0 / total mass of the positive active material in a single battery; mass energy density of the positive electrode = specific capacity per gram of the positive electrode × average discharge voltage; volume energy density of the positive electrode = mass energy density of the positive electrode × compaction density of the positive electrode coating in the positive electrode sheet (see Table 2 above). The relevant results are summarized in Table 3 below.

[0091] Assemble each positive electrode sheet and negative electrode sheet into a soft-pack three-electrode battery. The difference from the above-mentioned assembly of the two-electrode soft-pack battery is that: Place a copper wire with a diameter of 2 μm and a separator between a separator and a negative electrode sheet in the middle of the bare battery cell, so that the copper wire is separated from the negative electrode sheet by the separator. After performing formation on the soft-pack battery following the aforementioned process, lithium plating is performed on the copper wire (charge at 50 μA for 4 h), and the potential of the copper wire after lithium plating is 0 mV vs. Li. Therefore, the copper wire after lithium plating can be used as a reference electrode to calibrate the potential of the negative electrode during charging.

[0092] Fast charging tests were conducted on the above-mentioned soft-pack three-electrode batteries: the test current was gradually changed as 3C - 2.5C - 2C - 1.5C - 1C (specifically, first, constant current charging was performed on the test current at 3C. When the battery reached the lithium plating boundary, the current was reduced to 2.5C for constant current charging of the test current. When the battery reached the lithium plating boundary, the current was reduced to 2C for constant current charging of the test current. When the battery reached the lithium plating boundary, the current was reduced to 1.5C for constant current charging of the test current. When the battery reached the lithium plating boundary, the current was reduced to 1C for constant current charging of the test current). The total fast charging time was the total charging time for the state of charge (SOC) of the battery to change from 10% SOC to 80% SOC. In the above fast charging test, whether the battery reached the lithium plating boundary was determined by the change in the negative electrode potential during the charging process. When the negative electrode potential decreased to 0 mV, it was considered that the lithium plating boundary was reached. The fast charging test results are also summarized in Table 3 below.

[0093] Table 3 Electrochemical performance test results of each soft-pack battery-

[0094]

[0095]

[0096] It can be learned from Table 3 that:

[0097] The positive electrodes of Comparative Examples 6 and 7 are single-layer coated large particle size LMFP-1 materials. Although the volume energy density of the battery positive electrode is relatively high, the fast charging performance of the batteries is not good. The positive electrode plates of Examples 1-3, 4, 7, 12-14 of the present application are layered coated with large particle size low Mn content LMFP-1 materials and small particle size high Mn content LMFP-2 materials compared with Comparative Example 6. The fast charging performance of these example batteries has been improved to a certain extent compared with Comparative Example 6.

[0098] Among them, the difference in the positive electrode sheets of Examples 1-3 and Examples 12-14 lies in the difference in the surface density ratio of Coating 1 to Coating 2. From the comparison between them, it can be learned that when the surface density ratio of Coating 1 to Coating 2 is less than 1 (such as Example 14), the mass proportion of the small particle LFMP-2 in Coating 2 is too high, and the mixed compaction density of the positive electrode coating is significantly reduced, thereby significantly reducing the volume energy density of the battery. When the surface density ratio of Coating 1 to Coating 2 is within the range of 1-10, the battery can better balance the volume energy density and a shorter fast charging time. And when the surface density ratio of Coating 1 to Coating 2 is relatively high (for example, this ratio is 10 in Example 12), it reflects that the mass proportion of the small particle in Coating 2 is relatively low. The improvement amplitude of the fast charging performance of the battery compared to Comparative Example 5 with single-layer coating is not as high as that of other Examples 1-3 and 13. It also reflects that the mass proportion of the large particle in Coating 1 is relatively high. The specific capacity of the battery is closer to that of Comparative Example 5 where all the positive electrodes use large particle size LFP materials, lower than the specific capacity of the positive electrodes in Examples 1-3 and 13, and the mass energy density of the battery is also lower. Therefore, the surface density ratio of Coating 1 to Coating 2 is preferably within the range of 1-9, and more preferably within the range of 1-3.

[0099] In other Examples 4-11 of this application, compared with Example 1, the D50 particle size of the upper / lower layer LMFP materials is changed, or the Mn content of the upper / lower layer LMFP materials is changed. The fast charging time and energy density of the fully packaged battery have changed. However, overall, the battery can still better balance these two performances, and the comprehensive performance is relatively good. In addition, from the comparison between Examples 1, 6-9, it can be learned that it is preferable that the Mn content of the LMFP-1 material is within the range of 0.5-0.6 and the Mn content of the LMFP-2 material is within the range of 0.65-0.75.

[0100] In addition, the Mn content of the two LMFP materials in Comparative Examples 1-2 is not within our required range. Among them, too high Mn content of the LMFP material will lead to a relatively large intrinsic impedance of the material, significantly affect the capacity performance, energy density and fast charging performance of the material; too low Mn content will lead to a significant decrease in the average discharge voltage, and further lead to a decrease in the volume energy density of the battery.

[0101] In Comparative Examples 3-4, the D50 particle sizes of the two LMFP materials are not within our required range. Among them, if the D50 particle size of the material is too small, although it is beneficial to the fast charging performance, it will lead to a significant decrease in the compaction of the positive electrode sheet, thereby affecting the volume energy density of the positive electrode; while too large D50 particle size of the material will deteriorate the fast charging performance of the battery.

[0102] In Comparative Example 5, the Mn content x of the lower-layer LMFP material is the same as the Mn content y of the upper-layer LMFP material, not satisfying x < y. The excessively high Mn content of the lower-layer LMFP material will result in a relatively large intrinsic impedance of the material. When two such LMFP materials are double-coated, the specific capacity of the positive electrode of the battery is poorly utilized and the energy density is low, and the improvement of the fast charging performance of the battery is not significant.

[0103] The above-described embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but should not be construed as a limitation on the scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive electrode current collector and a first positive electrode material layer and a second positive electrode material layer stacked on at least one side of the positive electrode current collector. Among them, on the same side of the positive electrode current collector, the second positive electrode material layer is arranged on the side away from the positive electrode current collector of the first positive electrode material layer; the first positive electrode material layer includes a first lithium iron manganese phosphate material, and the second positive electrode material layer includes a second lithium iron manganese phosphate material; the molar ratio of Mn / (Mn + Fe) in the first lithium iron manganese phosphate material is x, and the molar ratio of Mn / (Mn + Fe) in the second lithium iron manganese phosphate material is y, where x < y, 0.4 ≤ x ≤ 0.65, 0.6 ≤ y ≤ 0.8; the D50 particle size of the first lithium iron manganese phosphate material is in the range of 0.8 μm - 2 μm, and the D50 particle size of the second lithium iron manganese phosphate material is in the range of 0.2 μm - 0.6 μm.

2. The positive electrode sheet according to claim 1, wherein, The single-sided areal density of the first positive electrode material layer is greater than or equal to the single-sided areal density of the second positive electrode material layer.

3. The positive electrode sheet according to claim 2, characterized in that, The ratio of the single-sided areal density of the first positive electrode material layer to the single-sided areal density of the second positive electrode material layer is 1 - 10.

4. The positive electrode sheet according to claim 3, characterized in that, The ratio of the single-sided areal density of the first positive electrode material layer to the single-sided areal density of the second positive electrode material layer is 1 - 9.

5. The positive electrode sheet according to any one of claims 1-4, characterized in that, The single-sided areal density of the first positive electrode material layer is 100 g / m 2 - 200 g / m 2 .

6. The positive electrode sheet according to any one of claims 1-5, characterized in that, The single-sided areal density of the second positive electrode material layer is 20 g / m 2 -100 g / m 2 .

7. The positive electrode sheet according to any one of claims 1-6, characterized in that, The tap density of the first positive electrode material layer is greater than the tap density of the second positive electrode material layer.

8. The positive electrode sheet according to any one of claims 1-7, characterized in that, The tap density of the first positive electrode material layer is 2.3-2.7 g / cm 3 ; and / or the tap density of the second positive electrode material layer is 2.0-2.4 g / cm 3 .

9. The positive electrode sheet according to any one of claims 1-8, characterized in that, 0.45 ≤ x ≤ 0.60, 0.65 ≤ y ≤ 0.

75.

10. The positive electrode sheet according to any one of claims 1-9, characterized in that, The mass proportion of the first lithium iron manganese phosphate material in the first positive electrode material layer is more than 85%; the mass proportion of the second lithium iron manganese phosphate material in the second positive electrode material layer is more than 80%.

11. The positive electrode sheet according to any one of claims 1 to 10, characterized in that, The mass ratio of the first lithium iron manganese phosphate material to the second lithium iron manganese phosphate material is 1 - 10; optionally, the mass ratio of the first lithium iron manganese phosphate material to the second lithium iron manganese phosphate material is 1 - 9.

12. The positive electrode sheet according to any one of claims 1 - 11, wherein the first positive electrode material layer is in direct contact with the positive electrode current collector, and / or the second positive electrode material layer is in direct contact with the first positive electrode material layer.

13. A lithium battery, characterized in that, Comprising the positive electrode sheet according to any one of claims 1 - 12.

14. An apparatus comprising the lithium battery according to claim 13, the apparatus comprising an electrical device or an energy storage system.

Images