Positive electrode active material, positive electrode slurry, positive electrode sheet, and battery

Abstract

The present application belongs to the technical field of batteries, and provides a positive electrode active material, a positive electrode slurry, a positive electrode sheet, and a battery. The positive electrode active material comprises lithium manganese iron phosphate and lithium-rich lithium ferrite; the mass ratio of the lithium-rich lithium ferrite added to the positive electrode active material is 0.5%-5%; the particle size D50 of the lithium manganese iron phosphate is d1, and the particle size D50 of the lithium-rich lithium ferrite is d2, 0.05≤d1 / d2≤0.32.

Description

A positive electrode active material, a positive electrode slurry, a positive electrode sheet and a battery

[0001] The present application claims priority to the Chinese patent application No. 202411281588.2 filed on September 12, 2024 with the Chinese Patent Office, the whole content of the above application being incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of batteries, in particular to a positive electrode active material, a positive electrode slurry, a positive electrode sheet and a battery. BACKGROUND

[0003] As a new generation of lithium ion battery positive electrode material, lithium manganese iron phosphate (LMFP) is an upgraded version of lithium iron phosphate (LFP) material. Due to the presence of transition metal manganese (Mn) in lithium manganese iron phosphate material, the voltage platform of lithium manganese iron phosphate is higher than that of lithium iron phosphate, and the energy density of lithium manganese iron phosphate is also more advantageous than that of lithium iron phosphate. SUMMARY

[0004] Due to the presence of transition metal manganese in lithium manganese iron phosphate material, the lithium ion diffusion rate and electronic conductivity of lithium manganese iron phosphate are also lower than those of lithium iron phosphate material (the lithium ion diffusion rate of lithium manganese iron phosphate is 10-15 cm2 / S, the lithium ion diffusion rate of lithium iron phosphate is 10-14 cm2 / S; the electronic conductivity of lithium manganese iron phosphate is 10-13 S / cm, and the electronic conductivity of lithium iron phosphate is 10-9 S / cm), thereby the specific capacity of lithium manganese iron phosphate is lower than that of lithium iron phosphate. In addition, lithium hexafluorophosphate in the electrolyte will react with trace water in the battery to generate hydrofluoric acid, which will destroy the structure of lithium manganese iron phosphate. In addition, the Jahn-Teller effect of manganese ions exists, and manganese will be dissolved in the cycle process, reducing the stability of the positive electrode material. And the dissolved manganese ions will be reduced and precipitated at the negative electrode, destroying the SEI film, so that the cycle performance of lithium manganese iron phosphate is greatly inferior to that of lithium iron phosphate.

[0005] The present application provides a positive electrode active material, which comprises lithium manganese iron phosphate and lithium-rich lithium iron phosphate. The mass ratio of lithium-rich lithium iron phosphate added in the positive electrode active material is 0.5%-5%; the particle size D50 of lithium manganese iron phosphate is d1, the particle size D50 of lithium-rich lithium iron phosphate is d2, and 0.05≤d1 / d2≤0.32.

[0006] The present application also provides a positive electrode slurry, which comprises the positive electrode active material described above, a conductive agent, a binder and a solvent.

[0007] The present application also provides a positive electrode sheet, which is prepared by using the positive electrode active material described above or the positive electrode slurry described above.

[0008] The application also provides a battery comprising the positive electrode sheet. Advantages

[0009] The positive electrode active material provided by the embodiments of the application comprises lithium iron manganese phosphate and lithium-rich lithium iron oxide. When the positive electrode active material is applied to a battery, the lithium-rich lithium iron oxide can supplement the active lithium lost irreversibly in the first charging of the lithium iron manganese phosphate, thereby improving the gram capacity. After the addition of the lithium-rich lithium iron oxide, the active oxygen released during the first charging process can help generate an SEI film rich in Li2O components, thereby enhancing the kinetic performance, reducing the DCR, and improving the constant current ratio. The additional active lithium released by the lithium-rich lithium iron oxide during the first delithiation can continuously provide a lithium source during the cycling process, thereby improving the cycling performance. Moreover, the lithium-rich lithium iron oxide is alkaline, and can neutralize the hydrogen fluoride generated by the reaction between the electrolyte and moisture in the battery, thereby stabilizing the structure of the lithium iron manganese phosphate during the cycling process, reducing the dissolution of manganese, and improving the cycling performance. By controlling the mass ratio of the lithium iron manganese phosphate and the lithium-rich lithium iron oxide to be (19-199): 1, the lithium-rich lithium iron oxide can achieve good effects, while the problems such as serious gas generation caused by excessive addition of the lithium-rich lithium iron oxide can be avoided, and good rate performance can be maintained. By controlling the particle size D50 of the lithium iron manganese phosphate and the lithium-rich lithium iron oxide to satisfy the condition of 0.05≤d1 / d2≤0.32, a better stacking effect can be achieved between the two, thereby establishing a more effective conductive network and improving the gram capacity.

[0010] The positive electrode slurry provided by the embodiments of the application can achieve good gram capacity and cycling performance by comprising the positive electrode active material, the conductive agent, the binder, and the solvent.

[0011] The positive electrode sheet provided by the embodiments of the application can achieve good gram capacity and cycling performance by being prepared from the positive electrode slurry.

[0012] The battery provided by the embodiments of the application can achieve good gram capacity and cycling performance by comprising the positive electrode sheet, and has good comprehensive performance. Embodiments of the application

[0013] In the description of the application, unless otherwise explicitly specified and limited, the terms "connected", "connected", "fixed" should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal connection of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0014] In the present application, unless specifically defined and limited otherwise, the first feature is "on" or "under" the second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, the first feature "on", "above" and "over" the second feature includes that the first feature is directly above and obliquely above the second feature, and the horizontal height of the first feature is higher than that of the second feature. The first feature "under", "below" and "under" the second feature includes that the first feature is directly below and obliquely below the second feature, and the horizontal height of the first feature is lower than that of the second feature.

[0015] In the description of the present embodiment, the terms "upper", "lower", "left", "right", "front", "back" and the like orientation or position relationship are based on the orientation or position relationship shown in the drawings, which is for the convenience of description and simplification of operation, and is not intended to indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first" and "second" are used to distinguish in the description and have no special meaning.

[0016] In a first aspect, the embodiments of the present application provide a positive electrode active material, comprising lithium manganese iron phosphate and lithium-rich lithium iron oxide (Li5FeO4). That is, in the embodiments of the present application, lithium manganese iron phosphate and lithium-rich lithium iron oxide are compounded to obtain a positive electrode active material. When the positive electrode active material provided by the embodiments of the present application is applied to a battery, first, the lithium-rich lithium iron oxide can supplement the active lithium lost irreversibly in the first charging of the lithium manganese iron phosphate, thereby improving the gram capacity of the battery; second, after adding the lithium-rich lithium iron oxide, the active oxygen released during the first charging process can help to generate an SEI film rich in Li2O components, enhance the kinetic performance, reduce the DCR, and improve the constant current ratio; third, the additional active lithium released by the lithium-rich lithium iron oxide during the first delithiation can continuously provide lithium source during the cycle process, thereby improving the cycle performance; fourth, the lithium-rich lithium iron oxide is alkaline, which can neutralize the hydrogen fluoride generated by the reaction of the electrolyte and moisture in the battery, thereby stabilizing the structure of the lithium manganese iron phosphate during the cycle process, reducing the dissolution of manganese, and improving the cycle performance.

[0017] Therefore, by adding lithium-rich lithium iron oxide to the lithium manganese iron phosphate material, the problems of low gram capacity and poor cycle performance of the lithium manganese iron phosphate as a positive electrode active material alone are solved. The lithium manganese iron phosphate material can also retain the advantage of high energy density, and has obvious advantages over the related art of using lithium manganese iron phosphate or lithium iron phosphate alone as a positive electrode active material. That is, the lithium manganese iron phosphate has a higher voltage platform and energy density than the lithium iron phosphate, and by compounding with the lithium-rich lithium iron oxide, the defects of low gram capacity and poor cycle performance are compensated for, thereby having better comprehensive performance.

[0018] The mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is 0.5%-5%, the particle size D50 of the lithium manganese iron phosphate is d1, the particle size D50 of the lithium-rich lithium iron oxide is d2, and 0.05≤d1 / d2≤0.32. When the amount of the lithium-rich lithium iron oxide is too low, the relevant effect is difficult to achieve, and when the amount of the lithium-rich lithium iron oxide is too high, it is easy to cause serious gas production, thereby affecting the specific capacity and cycle performance. Therefore, in the embodiments of the present application, the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is set to 0.5%-5%. When the small particles of the lithium manganese iron phosphate and the large particles of the lithium-rich lithium iron oxide are compounded, a better stacking effect can be achieved, so that the lithium manganese iron phosphate and the lithium-rich lithium iron oxide can establish a more effective conductive network, thereby achieving the effect of improving the specific capacity and cycle performance and maintaining good rate performance.

[0019] Specifically, when the lithium-rich lithium iron oxide with a large particle size is compounded with the lithium manganese iron phosphate with a small particle size, the lithium-rich lithium iron oxide with a large particle size can be stacked, and there will be gaps between adjacent lithium-rich lithium iron oxide particles, and the lithium manganese iron phosphate particles with a small particle size will fill in these gaps, so that the lithium manganese iron phosphate and the lithium-rich lithium iron oxide can be uniformly distributed and in contact with each other, reducing or avoiding the problems such as stratification of the lithium manganese iron phosphate and the lithium-rich lithium iron oxide, so that they can fully interact with each other to better achieve the effect.

[0020] Exemplarily, d1 / d2 can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.2, 0.25, 0.3 or 0.32.

[0021] It can be understood that the positive electrode active material can only include the lithium manganese iron phosphate and the lithium-rich lithium iron oxide, or can include the lithium manganese iron phosphate and the lithium-rich lithium iron oxide and other positive electrode active materials.

[0022] As preferred in the embodiments of the present application, the positive electrode active material is composed of the lithium manganese iron phosphate and the lithium-rich lithium iron oxide. That is, the positive electrode active material only includes the lithium manganese iron phosphate and the lithium-rich lithium iron oxide, so as to better balance the advantages of the lithium manganese iron phosphate and reduce or avoid the defects of the lithium manganese iron phosphate through the lithium-rich lithium iron oxide.

[0023] In some embodiments, the particle size D50 of the lithium manganese iron phosphate satisfies: 0.8 μm≤d1≤1.6 μm. The particle size D50 of the lithium manganese iron phosphate is small, which is beneficial to improve the discharge performance and cycle life of the battery when applied to the battery. This is because the material with a small particle size has a larger specific surface area and a shorter ion diffusion path, which is beneficial to the intercalation and deintercalation reaction of lithium ions, thereby improving the charge-discharge rate performance of the battery and reducing the internal resistance of the battery. However, if the particle size D50 of the lithium manganese iron phosphate is too small, agglomerates are easy to form, which increases the difficulty of preparation of the battery material, and the particle size that is too small may also cause the mechanical strength of the material to decrease, thereby reducing the cycle life and stability of the material.

[0024] Exemplarily, the particle size D50 of the lithium manganese iron phosphate is 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm or 1.6 μm.

[0025] In some embodiments, the particle size D50 of the lithium-rich lithium iron oxide satisfies: 5 μm≤d2≤15 μm. The particle size D50 of the lithium-rich lithium iron oxide is large, which can form a better packing effect with the lithium manganese iron phosphate with small particle size, so as to establish a more effective conductive network, thereby better playing the effect of improving the gram capacity and the cycle performance. The lithium manganese iron phosphate with small particle size is surrounded around the lithium-rich lithium iron oxide with large particle size, so that the interaction between the lithium-rich lithium iron oxide and the lithium manganese iron phosphate material is better.

[0026] Exemplarily, the particle size D50 of the lithium-rich lithium iron oxide is 5 μm, 7 μm, 10 μm, 12 μm, 14 μm or 15 μm.

[0027] It should be noted that D50 refers to the particle size corresponding to the cumulative particle size distribution percentage of 50% of a sample, also known as the median particle size or median particle size.

[0028] In some embodiments, the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is greater than or equal to 0.5% and less than 2%. When the addition ratio of the lithium-rich lithium iron oxide is within this range, the gram capacity can be improved more obviously, the cycle performance can be improved, and the good rate performance can be maintained, so as to compensate for the defects of the lithium manganese iron phosphate relative to the lithium iron phosphate to some extent and improve the comprehensive performance.

[0029] Exemplarily, the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is 0.5%, 0.8%, 1%, 1.2%, 1.5% or 1.8%.

[0030] Preferably, the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is 1.5%-1.8%. When the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is within this range, the gram capacity and the cycle performance of the lithium manganese iron phosphate can be further improved, and the good rate performance can be maintained.

[0031] In some embodiments, the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is 2%-5%. Within a certain range, the higher the addition ratio of the lithium-rich lithium iron oxide, the better the improvement effect on the gram capacity and the cycle performance, but too high addition amount of the lithium-rich lithium iron oxide can also easily lead to too high gas production, which in turn affects the gram capacity and the cycle performance. When the mass ratio of the lithium-rich lithium iron oxide added in the positive electrode active material is 2%-5%, the improvement of the gram capacity and the cycle performance is more obvious, and the rate performance can be maintained within a good range.

[0032] Exemplarily, the mass ratio of the lithium-rich lithium iron phosphate added in the positive electrode active material is 2%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, 4.7% or 5.0%.

[0033] In some embodiments, the mass ratio of the lithium-rich lithium iron phosphate added in the positive electrode active material is 3.1%-5%. That is, the mass ratio of the lithium-rich lithium iron phosphate added in the positive electrode active material can be preferably set to 3.1%-5% to make the lithium-rich lithium iron phosphate fully play a role and avoid the serious problem of gas production caused by excessive lithium-rich lithium iron phosphate.

[0034] In some embodiments, the specific surface area of the lithium manganese iron phosphate is S1, the specific surface area of the lithium-rich lithium iron phosphate is S2, and 7≤S1 / S2≤40. The particle size of the lithium manganese iron phosphate is small, and its specific surface area is large, while the particle size of the lithium-rich lithium iron phosphate is large, and its specific surface area is small. By setting the specific surface area of the two to meet the condition of 7≤S1 / S2≤40, it is beneficial to form a better stacking effect of the two, so as to establish a more effective conductive network and play a better effect.

[0035] Exemplarily, S1 / S2 can be 7, 10, 12, 15, 17, 20, 22, 25, 28, 30, 32, 35, 38 or 40.

[0036] It can be understood that the specific surface area of the material is inversely proportional to the particle size, that is, the larger the particle size, the smaller the specific surface area, and the smaller the particle size, the larger the specific surface area. When the specific surface is large, more reaction sites can be provided, and when the specific surface area is small, the material has relatively stable properties. The lithium manganese iron phosphate as the main material of the positive electrode active material has a small particle size and a large specific surface area, which can provide more reaction sites and is beneficial to improve the energy density. The addition ratio of the lithium-rich lithium iron phosphate is small, and it is mainly used to provide lithium supplement and the like, and has a large particle size and a small specific surface area, which has a certain stability and plays a better role.

[0037] In some embodiments, the specific surface area of the lithium manganese iron phosphate satisfies: 8m 2 / g≤S1≤18m 2 / g. If the specific surface area of the lithium manganese iron phosphate is too small, it can affect the gram capacity, so that the gram capacity cannot be fully played. When the specific surface area of the lithium manganese iron phosphate is large, agglomeration and other problems can easily occur during the preparation of the positive electrode slurry by using the positive electrode active material.

[0038] Exemplarily, the specific surface area of the lithium manganese iron phosphate can be 8m 2 / g, 10m 2 / g, 12m 2 / g, 14m 2 / g, 16m 2 / g or 18m2 / g.

[0039] In some embodiments, the specific surface area of the lithium-rich lithium-iron- phosphate satisfies: 0.2m 2 / g≤S2≤2.4m 2 / g. If the specific surface area of the lithium-rich lithium-iron-phosphate is too small, the gram capacity will also be affected, and the gram capacity will not be as expected. If the specific surface area is too large, the battery will easily produce a large amount of gas, which is difficult to completely remove during production and manufacturing, thereby affecting the performance of the battery.

[0040] For example, the specific surface area of the lithium-rich lithium-iron-phosphate can be 0.2m 2 / g, 0.5m 2 / g, 0.7m 2 / g, 1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, or 2.4m 2 / g.

[0041] In some embodiments, in the positive electrode active material, the molar ratio of lithium element to manganese element is (1.5-2.9): 1, and the molar ratio of lithium element to iron element is (1.7-3.5): 1. That is, the positive electrode active material provided by the embodiments of the present application as a whole, the molar ratio of lithium element to manganese element is (1.5-2.9): 1, and the molar ratio of lithium element to iron element is (1.7-3.5): 1. By controlling the molar ratio of lithium element to manganese element and the molar ratio of lithium element to iron element in the positive electrode active material within a suitable range, the advantages of the mixed manganese-iron-lithium-phosphate and lithium-rich lithium-iron-phosphate can be fully utilized, the cycle performance is better, the gram capacity is improved while the voltage platform is guaranteed, thereby having better comprehensive performance.

[0042] In some embodiments, the molecular formula of the manganese-iron-lithium-phosphate is Li a M b (PO4) c .

[0043] wherein, M is Fe 1-x-z Mn x D z , D is one or more of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr and B; 0.95≤a / c≤1.1, 0.90≤b / c≤1.15, 1.01≤a / b≤1.1. By controlling the ratio of lithium to metal, the ratio of lithium to phosphorus, and the ratio of metal to phosphorus in the manganese-iron-lithium-phosphate material within a suitable range, the material can have a better crystal structure, thereby having better comprehensive performance.

[0044] In a second aspect, embodiments of the present application provide a positive electrode slurry, which comprises the positive electrode active material, the conductive agent, the binder and the solvent as described above.

[0045] The positive electrode slurry provided by embodiments of the present application has the same beneficial effects as the positive electrode active material, which will not be repeated here.

[0046] In some embodiments, the positive electrode slurry further comprises a dispersant. The addition of the dispersant in the positive electrode slurry can improve the flowability, make the dispersion performance of the positive electrode active material better, and maintain the stability of the positive electrode slurry.

[0047] It can be understood that the conductive agent, the binder, the dispersant and the solvent can be conventional materials in the art, and embodiments of the present application will not be specifically limited here.

[0048] For example, the conductive agent can be selected from one or more of carbon black, conductive graphite, carbon nanotubes, carbon fibers and graphene. The binder can be selected from an oily binder such as polyvinylidene fluoride or an aqueous binder such as carboxymethyl cellulose, polyacrylic acid, polyamide, etc. The dispersant can be selected from an inorganic dispersant such as metal oxide or hydroxide or an organic dispersant such as a polymer and a surfactant. The solvent can be selected from solvents such as N-methyl-2-pyrrolidone, dimethyl carbonate, ethylene carbonate, diethylene carbonate and dimethyl sulfoxide.

[0049] In addition, the positive electrode active material, the conductive agent, the binder, the dispersant and the solvent can also be conventional settings in the art, and embodiments of the present application will not be specifically limited here.

[0050] In a third aspect, embodiments of the present application provide a positive electrode sheet, which is prepared by using the positive electrode active material as described above or the positive electrode slurry as described above.

[0051] The positive electrode slurry described above is coated on the current collector, and after processes such as drying, punching and rolling, the positive electrode sheet can be obtained.

[0052] The positive electrode sheet provided by embodiments of the present application has the same beneficial effects as the positive electrode active material, which will not be repeated here.

[0053] In some embodiments, the compaction density of the positive electrode sheet is 2.2 g / cm 3 -2.6 g / cm 3The compaction density = the surface density / the thickness of the material. The compaction density has a great influence on the performance of the battery. On the one hand, by increasing the compaction density of the positive plate, more active substances can be accommodated, thereby increasing the energy storage capacity of the battery and optimizing the utilization rate of the material. On the other hand, with the increase of the compaction density, the conductive channels and bridges inside the positive plate can also be increased, so that the transmission of electrons and ions is more smooth, so as to improve the conductivity of the battery. In addition, the positive plate with high compaction density also has better structural stability. However, too high or too low compaction density of the positive plate is easy to cause the increase of the internal resistance and polarization of the battery. The compaction density of the positive plate in the embodiments of the present application is set to 2.2 g / cm 3 -2.6 g / cm 3 , which can reduce or avoid the increase of the internal resistance and polarization of the battery while improving the effects of improving the gram capacity and cycle performance, thereby improving the comprehensive performance of the battery.

[0054] In a fourth aspect, the embodiments of the present application provide a battery comprising the positive plate as described above.

[0055] The beneficial effects of the battery provided by the embodiments of the present application are basically the same as those of the positive active material, which will not be repeated here.

[0056] The embodiments of the present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods in the following examples are not specified, which are generally in accordance with the conditions recommended by the manufacturer.

[0057] It should be noted that the conditions in the embodiments and comparative examples of the present application are consistent except for the differences mentioned.

[0058] The manufacturing process of the battery in the embodiments and comparative examples of the present application includes: mixing and stirring lithium manganese iron phosphate and lithium-rich lithium iron oxide to form a positive active material according to a certain proportion; preparing a positive electrode slurry by mixing and stirring the positive active material, a conductive agent, a binder, a dispersing agent and a solvent; the proportion of the positive active material is 97.5%, the conductive agent is carbon black, the proportion is 0.7%, the binder is polyvinylidene fluoride, the proportion is 1.5%, and the dispersing agent is a polyester substance, the proportion is 0.3%. Artificial graphite is used as a negative active material, and is mixed and stirred with a conductive agent and a binder to prepare a negative electrode slurry; the proportion of the negative active material is 96.9%, the conductive agent is carbon black, the proportion is 0.5%, and the binder is carboxymethyl cellulose sodium and styrene butadiene rubber, wherein the proportion of carboxymethyl cellulose sodium is 1.1% and the proportion of styrene butadiene rubber is 1.5%. The positive electrode slurry and the negative electrode slurry are respectively obtained by coating and sheet making process to obtain positive plate and negative plate; the positive plate, the negative plate, the separator, the electrolyte, the cover plate and the aluminum shell are assembled together to form a square aluminum shell battery, and then the finished battery is obtained after formation and capacity distribution.

[0059] Example 1

[0060] In this example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 1.6%. The particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery is prepared according to the above battery production process.

[0061] Example 2

[0062] In this example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 2.9%. The particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery is prepared according to the above battery production process.

[0063] Example 3

[0064] In this example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 3.1%. The particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery is prepared according to the above battery production process.

[0065] Example 4

[0066] In this example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 0.5%. The particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery is prepared according to the above battery production process.

[0067] Example 5

[0068] In this example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 5.0%. The particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery is prepared according to the above battery production process.

[0069] Example 6

[0070] In this example, the positive electrode active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 2.9%. The particle size D50 of the lithium manganese iron phosphate is 0.8 μm, and the specific surface area is 18 m 2 / g, and the D50 of the lithium-rich lithium iron phosphate is 5 μm, and the specific surface area is 2.4 m 2 / g. The battery was prepared according to the above-described battery production process.

[0071] Example 7

[0072] In this example, the positive electrode active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 2.9%. The particle size D50 of the lithium manganese iron phosphate is 1.6 μm, and the specific surface area is 8 m 2 / g, and the D50 of the lithium-rich lithium iron phosphate is 15 μm, and the specific surface area is 0.2 m 2 / g. The battery was prepared according to the above-described battery production process.

[0073] Comparative Example 1

[0074] In this comparative example, the positive electrode active material consists only of lithium manganese iron phosphate, with the particle size D50 of the lithium manganese iron phosphate being 1.2 μm, and the specific surface area being 13 m 2 / g. The battery was prepared according to the above-described battery production process.

[0075] Comparative Example 2

[0076] In this comparative example, the positive electrode active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 0.3%. The particle size D50 of the lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of the lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery was prepared according to the above-described battery production process.

[0077] Comparative Example 3

[0078] In this comparative example, the positive electrode active material consists of lithium manganese iron phosphate and lithium-rich lithium iron phosphate, with the addition ratio of lithium-rich lithium iron phosphate being 6%. The particle size D50 of the lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, and the D50 of the lithium-rich lithium iron phosphate is 10 μm, and the specific surface area is 1.3 m 2 / g. The battery was prepared according to the above-described battery production process.

[0079] Comparative Example 4

[0080] In the present comparative example, the positive active material consists of lithium manganese iron phosphate and lithium-rich lithium-iron phosphate, wherein the addition ratio of lithium-rich lithium-iron phosphate is 2.9%. The particle size D50 of lithium manganese iron phosphate is 1.8 μm, the specific surface area is 6 m 2 / g, the D50 of lithium-rich lithium-iron phosphate is 3 μm, and the specific surface area is 2.6 m 2 / g. The battery is prepared according to the above battery production process.

[0081] The batteries in Examples 1-7 and Comparative Examples 1-4 are subjected to gram capacity test, DCR test, cycle test, EOL Mn deposition test and rate performance test (3C capacity retention rate), respectively. Among them, the gram capacity test method includes: placing the battery in a 25±2℃ constant temperature box, charging at 0.33C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; resting for 30 min; discharging at 0.33C constant current to 2.5V; the capacity / dry weight of active material during discharging is the gram capacity. The DCR test method includes: placing the battery in a 25±2℃ constant temperature box, charging at 0.33C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; resting for 30 min; discharging at 0.33C constant current to 80% SOC; resting for 1h, and the voltage is recorded as U1; then discharging at 1C (current expressed in A) constant current for 10s, and the voltage is recorded as U2; (U1-U2) / A is the DCR data. The cycle test method includes: placing the battery in a 25±2℃ constant temperature box, charging at 1C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; resting for 30 min; discharging at 1C constant current to 2.5V; resting for 30 min; repeating the above steps until the discharging capacity / initial discharging capacity = 80%, and the cycle number is recorded. The EOL Mn deposition test method includes: emptying the battery cycled to 80% SOH, disassembling and taking out the negative electrode sheet, scraping the negative electrode powder on the current collector, and then performing ICP test to test the manganese content. The rate performance (3C capacity retention rate) test method includes: placing the battery in a 25±2℃ constant temperature box, charging at 0.1C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C, resting for 30 min, discharging at 0.1C constant current to 2.5V, and the discharging capacity is recorded as Q0; charging at 0.1C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C, resting for 30 min, discharging at 3C constant current to 2.5V, and the discharging capacity is recorded as Q; Q / Q0 is the 3C capacity retention rate. The results are shown in Table 1:

[0082] Table 1 Performance test results of different examples and comparative examples

[0083]

[0084] As can be seen from Table 1, compared with the comparative example 1 without adding lithium-rich lithium iron phosphate in the positive active material, the gram capacity and cycle performance are greatly improved after adding 1.6% of lithium-rich lithium iron phosphate in the example 1, and the rate performance (3C capacity retention rate) of the example 1 is also maintained at a high level compared with the comparative example 1, the manganese deposition in the negative electrode has no obvious change, and the DCR is reduced. Compared with the comparative example 1, the gram capacity and cycle times of the example 2 are greatly improved after adding 2.9% of lithium-rich lithium iron phosphate in the positive active material, and the manganese deposition in the negative electrode is reduced, and the DCR is also reduced, but the rate performance is greatly reduced due to the high proportion of lithium-rich lithium iron phosphate. Compared with the comparative example 1, the gram capacity and cycle times of the examples 3-7 are improved to different degrees after adding 3.1%, 0.5%, 5.0%, 2.9% and 2.9% of lithium-rich lithium iron phosphate in the positive active material respectively, and the manganese deposition in the negative electrode is reduced, and the DCR is also reduced. Although lithium-rich lithium iron phosphate is added in the positive active material in the comparative example 2, the gram capacity, cycle times, manganese deposition in the negative electrode and DCR have no obvious change compared with the comparative example 1 without adding lithium-rich lithium iron phosphate, but the rate performance is reduced. The comparative example 3 adds excessive lithium-rich lithium iron phosphate, which can easily lead to excessive gas production, so that the gram capacity of the comparative example 3 is basically the same as that of the comparative example 1 without adding lithium-rich lithium iron phosphate, the cycle times are reduced, the manganese deposition in the negative electrode is reduced, and the DCR is increased. That is, although lithium-rich lithium iron phosphate is added in the comparative example 3, the manganese deposition in the negative electrode is reduced, which helps to improve the cycle performance, but the final cycle performance is reduced and the rate performance is obviously reduced due to the excessive amount of lithium-rich lithium iron phosphate. In the comparative example 4, the particle size of the lithium manganese iron phosphate is too large and the specific surface area is too small, and the particle size of the lithium-rich lithium iron phosphate is too small and the specific surface area is too large, so that the two cannot form a good conductive network after compounding, and the specific surface area of the lithium-rich lithium iron phosphate is too large, which increases the gas production of the battery and increases the side reaction, resulting in the decrease of the cycle performance and the great decrease of the rate performance.

[0085] In summary, the examples of the present application can improve the problems of low gram capacity and poor cycle performance of the lithium manganese iron phosphate alone by adding lithium-rich lithium iron phosphate in the positive active material, and maintain good rate performance. By controlling the addition proportion of lithium-rich lithium iron phosphate in a suitable range, the gram capacity and cycle performance are improved to different degrees, and the high rate performance is maintained. By compounding the small particle size lithium manganese iron phosphate and the large particle size lithium-rich lithium iron phosphate, it is ensured that the two can form a stacking effect when compounded, so as to establish a more effective conductive network, and better play the effect of lithium-rich lithium iron phosphate on the improvement of gram capacity and cycle performance.

Claims

1. A positive electrode active material, comprising lithium manganese iron phosphate and lithium-rich lithium-iron phosphate; wherein the lithium-rich lithium-iron phosphate is added in the positive electrode active material in a mass ratio of 0.5%-5%; a particle size D50 of the lithium manganese iron phosphate is d1, and a particle size D50 of the lithium-rich lithium-iron phosphate is d2, and 0.05≤d1 / d2≤0.

32.

2. The positive electrode active material according to claim 1, wherein the particle size D50 of the lithium manganese iron phosphate satisfies: 0.8μm≤d1≤1.6μm.

3. The positive electrode active material according to claim 1 or 2, wherein the particle size D50 of the lithium-rich lithium-iron phosphate satisfies: 5μm≤d2≤15μm.

4. The positive electrode active material according to any one of claims 1 to 3, wherein the lithium-rich lithium-iron phosphate is added in the positive electrode active material in a mass ratio of 0.5%-2%.

5. The positive electrode active material according to claim 4, wherein the lithium-rich lithium-iron phosphate is added in the positive electrode active material in a mass ratio of 1.5%-1.8%.

6. The positive electrode active material according to any one of claims 1 to 3, wherein the lithium-rich lithium-iron phosphate is added in the positive electrode active material in a mass ratio of 2%-5%.

7. The positive electrode active material according to claim 6, wherein the lithium-rich lithium-iron phosphate is added in the positive electrode active material in a mass ratio of 3.1%-5%.

8. The positive electrode active material according to any one of claims 1 to 7, wherein a specific surface area of the lithium manganese iron phosphate is S1, and a specific surface area of the lithium-rich lithium-iron phosphate is S2, and 7≤S1 / S2≤40.

9. The positive electrode active material according to claim 8, wherein The specific surface area of the lithium manganese iron phosphate satisfies: 8m 2 / g≤S1≤18m 2 / g.

10. The positive electrode active material according to claim 8 or 9, wherein The specific surface area of the lithium-rich lithium-iron-phosphate satisfies: 0.2 m 2 / g ≤ S2 ≤ 2.4 m 2 / g.

11. The positive electrode active material according to any one of claims 1 to 10, wherein in the positive electrode active material, a molar ratio of lithium element to manganese element is (1.5-2.9):1, and a molar ratio of lithium element to iron element is (1.7-3.5):

1.

12. The positive electrode active material according to any one of claims 1 to 11, wherein The molecular formula of the lithium iron manganese phosphate is Li a M b (PO4) c ; wherein M is Fe 1-x-z Mn x D z D is one or more of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr and B; 0.95≤a / c≤1.1, 0.90≤b / c≤1.15, 1.01≤a / b≤1.

1. 13.A positive electrode slurry, comprising the positive electrode active material according to any one of claims 1-12, a conductive agent, a binder, and a solvent. 14.The positive electrode slurry according to claim 13, further comprising a dispersant.

15. The positive electrode slurry according to claim 13 or 14, wherein, the conductive agent comprises one or more of carbon black, conductive graphite, carbon nanotube, carbon fiber, and graphene.

16. The positive electrode paste according to any one of claims 13 to 15, wherein, the binder comprises one of polyvinylidene fluoride, carboxymethyl cellulose, polyacrylic acid, and polyamide.

17. The positive electrode paste of any one of claims 13-16, wherein, the solvent comprises at least one of N-methyl-2-pyrrolidone, dimethyl carbonate, ethylene carbonate, diethylene carbonate, and dimethyl sulfoxide. 18.A positive electrode sheet prepared using the positive electrode active material according to any one of claims 1-2 or the positive electrode slurry according to any one of claims 13-17.

19. The positive electrode sheet according to claim 18, wherein The compacted density of the positive electrode sheet is 2.2 g / cm 3 - 2.6 g / cm 3 . 20.A battery comprising the positive electrode sheet according to claim 18 or 19.

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