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

Abstract

The invention provides a positive electrode active material, positive electrode slurry, a positive plate and a battery, and belongs to the technical field of batteries, the positive electrode active material comprises lithium manganese iron phosphate and lithium-rich lithium ferrite; wherein the mass ratio of the lithium-rich lithium ferrite 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 ferrite is d2, and d1 / d2 is larger than or equal to 0.05 and smaller than or equal to 0.32. In the embodiment of the invention, the positive electrode active material comprises lithium manganese iron phosphate and lithium-rich lithium ferrite. When the positive electrode active material is applied to a battery, lithium-rich lithium ferrite and lithium iron manganese phosphate are compounded, a proper adding proportion of the lithium-rich lithium ferrite is set, and the particle sizes of the lithium iron manganese phosphate and the lithium-rich lithium ferrite are in a proper proportion, so that the gram volume and the cycle performance can be improved when the positive electrode active material is applied to the battery; and the rate capability is kept in a good range.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode active material, a positive electrode slurry, a positive electrode sheet and a battery. Background Art

[0002] As a new generation of lithium-ion battery cathode 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 LFP material, the voltage platform of LFP is higher than that of LFP, and the energy density of LFP is also more advantageous than that of LFP.

[0003] However, due to the presence of transition metal manganese in lithium iron phosphate materials, the lithium ion diffusion rate and electronic conductivity of lithium iron phosphate are also lower than those of lithium iron phosphate materials (the lithium ion diffusion rate of lithium iron phosphate is 10-15cm 2 / S, the lithium ion diffusion rate of lithium iron phosphate is 10-14cm 2 / S; the electronic conductivity of lithium manganese iron phosphate is 10-13S / cm, and the electronic conductivity of lithium iron phosphate is 10-9S / cm), so that the gram capacity of lithium manganese iron phosphate is lower than that of lithium iron phosphate. In addition, the lithium hexafluorophosphate in the electrolyte will react with the trace water in the battery, and the generated hydrofluoric acid will destroy the structure of lithium manganese iron phosphate. In addition, the Jahn-Teller effect of manganese ions exists, and manganese will be dissolved during the cycle, reducing the stability of the positive electrode material. In addition, the dissolved manganese ions will be reduced and precipitated at the negative electrode, destroying the SEI film, making the cycle performance of lithium manganese iron phosphate more insufficient than that of lithium iron phosphate. Summary of the invention

[0004] The embodiments of the present application provide a positive electrode active material, a positive electrode slurry, a positive electrode sheet and a battery, which can simultaneously improve the gram capacity, cycle performance and rate performance of lithium manganese iron phosphate materials.

[0005] In a first aspect, an embodiment of the present application provides a positive electrode active material, including lithium manganese iron phosphate and lithium-rich lithium iron oxide;

[0006] Wherein, the mass ratio of lithium-rich lithium iron oxide added to the positive electrode active material is 0.5%-5%;

[0007] The particle size D50 of lithium manganese iron phosphate is d1, the particle size D50 of lithium-rich lithium iron oxide is d2, and 0.05≤d1 / d2≤0.32.

[0008] In some embodiments, the particle size D50 of lithium manganese iron phosphate satisfies: 0.8 μm≤d1≤1.6 μm;

[0009] And / or, the particle size D50 of the lithium-rich lithium ferrite satisfies: 5μm≤d2≤15μm.

[0010] In some embodiments, the mass ratio of lithium-rich lithium ferrite added to the positive electrode active material is 0.5%-2%.

[0011] In some embodiments, the mass ratio of lithium-rich lithium ferrite added to the positive electrode active material is 1.5%-1.8%.

[0012] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate is S1, the specific surface area of ​​lithium-rich lithium iron oxide is S2, and 7≤S1 / S2≤40.

[0013] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate satisfies: 8m 2 / g≤S1≤18m 2 / g;

[0014] And / or, the specific surface area of ​​lithium-rich lithium iron oxide satisfies: 0.2m 2 / g≤S2≤2.4m 2 / g.

[0015] 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.

[0016] In some embodiments, the molecular formula of lithium manganese iron phosphate is Li a M b (PO4) c ;

[0017] Where 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.

[0018] In a second aspect, an embodiment of the present application provides a positive electrode slurry, comprising the positive electrode active material, a conductive agent, a binder and a solvent as described above.

[0019] In a third aspect, an embodiment of the present application provides a positive electrode sheet, which is prepared using the positive electrode active material as described above or the positive electrode slurry as described above.

[0020] In some embodiments, the compaction density of the positive electrode sheet is 2.2 g / cm 3 -2.6g / cm 3 .

[0021] In a fourth aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet as described above.

[0022] Beneficial effects of the embodiments of the present application:

[0023] In an embodiment of the present application, the positive electrode active material includes lithium iron manganese phosphate and lithium-rich lithium iron oxide. When the positive electrode active material is applied to the battery, by compounding lithium-rich lithium iron oxide and lithium iron manganese phosphate, the lithium-rich lithium iron oxide can supplement the active lithium that is irreversibly lost in the first charge of the battery, thereby increasing the gram capacity. After adding lithium-rich lithium iron oxide, active oxygen can also be released during the first charge process to help generate a SEI film rich in Li2O components, enhance the kinetic performance, reduce DCR, and improve the constant current ratio. The additional active lithium released by the first lithium de-lithiation of lithium-rich lithium iron oxide can continuously provide a lithium source during the cycle process, thereby improving the cycle performance. And lithium-rich lithium iron oxide is alkaline, which can neutralize the hydrofluoric acid generated by the reaction of electrolyte and water in the battery, stabilize the structure of lithium iron manganese phosphate during the cycle process, reduce the dissolution of manganese, and improve the cycle performance. By controlling the mass ratio of lithium iron manganese phosphate and lithium-rich iron oxide to (19-199): 1, it is possible to ensure that the lithium-rich iron oxide can play a better role, and avoid the serious gas production caused by excessive addition of lithium-rich iron oxide, while maintaining good rate performance. By controlling the particle size D50 of lithium iron manganese phosphate and lithium-rich iron oxide to meet the condition of 0.05≤d1 / d2≤0.32, a better stacking effect can be formed between the two, thereby establishing a more effective conductive network and improving the gram capacity. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0025] In the first aspect, the embodiments of the present application provide a positive electrode active material, including lithium iron manganese phosphate and lithium iron-rich lithium (Li5FeO4). That is, in the embodiments of the present application, lithium iron manganese phosphate and lithium iron-rich lithium are compounded to obtain a positive electrode active material. When the positive electrode active material provided in the embodiments of the present application is applied to a battery, in the first aspect, lithium iron-rich lithium can supplement the active lithium that is irreversibly lost in the first charge of the lithium iron phosphate, thereby increasing the gram capacity of the battery; in the second aspect, after adding lithium iron-rich lithium, it can also release active oxygen during the first charge process to help generate a SEI film rich in Li2O components, enhance the kinetic performance, reduce DCR, and improve the constant current ratio; in the third aspect, the additional active lithium released by the first lithium de-lithiation of lithium iron-rich lithium can continuously provide a lithium source during the cycle process, thereby improving the cycle performance; in the fourth aspect, lithium iron-rich lithium is alkaline, which can neutralize the hydrofluoric acid generated by the reaction of electrolyte and water in the battery, stabilize the structure of lithium iron manganese phosphate during the cycle process, reduce the dissolution of manganese, and improve the cycle performance.

[0026] Therefore, by adding lithium-rich lithium iron oxide to the lithium iron phosphate material, the problem of low gram capacity and poor cycle performance when lithium iron phosphate is used alone as the positive electrode active material is solved. It can also retain the advantage of high energy density of lithium iron phosphate material, which has obvious advantages over the related art of using lithium iron phosphate or lithium iron phosphate alone as the positive electrode active material. That is, lithium iron phosphate has a higher voltage platform and energy density than lithium iron phosphate, and by compounding with lithium-rich lithium iron oxide, it makes up for its low gram capacity and poor cycle performance, thus having better comprehensive performance.

[0027] Among them, the mass ratio of lithium-rich iron phosphate added to the positive electrode active material is 0.5%-5%, the particle size D50 of lithium iron phosphate is d1, and the particle size D50 of lithium-rich iron phosphate is d2, 0.05≤d1 / d2≤0.32. When the amount of lithium-rich iron phosphate added is too low, it is difficult to exert the relevant effect, and when the amount of lithium-rich iron phosphate added is too high, it is easy to cause serious gas production, thereby affecting the gram capacity and cycle performance. Therefore, in the embodiment of the present application, the mass ratio of lithium-rich iron phosphate added to the positive electrode active material is set to 0.5%-5%. When small particles of lithium iron phosphate and large particles of lithium-rich iron phosphate are compounded, a better stacking effect can be formed, so that lithium iron phosphate and lithium-rich iron phosphate establish a more effective conductive network, thereby exerting the effect of improving gram capacity and cycle performance, and maintaining good rate performance.

[0028] Specifically, when lithium iron phosphate with larger particle size is compounded with lithium iron phosphate with smaller particle size, the lithium iron phosphate with larger particle size can be stacked, and there will be gaps between adjacent lithium iron phosphate particles, while the lithium iron phosphate particles with smaller particle size will fill in these gaps, so that lithium iron phosphate and lithium iron phosphate can be evenly distributed and contact each other, reducing or avoiding problems such as stratification of lithium iron phosphate and lithium iron phosphate, so that the two can fully interact with each other to better exert their effects.

[0029] Illustratively, d1 / d2 may be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.2, 0.25, 0.3 or 0.32.

[0030] It is understandable that the positive electrode active material may include only lithium iron manganese phosphate and lithium-rich lithium ferrate, or may include both lithium iron manganese phosphate and lithium-rich lithium ferrate and other positive electrode active materials.

[0031] As a preferred embodiment of the present application, the positive electrode active material is composed of lithium iron manganese phosphate and lithium-rich ferric acid lithium. That is, the positive electrode active material only includes lithium iron manganese phosphate and lithium-rich ferric acid lithium, so as to better take into account the advantages of lithium iron manganese phosphate and reduce or avoid the defects of lithium iron manganese phosphate through lithium-rich ferric acid lithium.

[0032] In some embodiments, the particle size D50 of lithium iron manganese phosphate satisfies: 0.8μm≤d1≤1.6μm. The particle size D50 of lithium iron manganese phosphate is relatively small, which is beneficial to improving the discharge performance and cycle life of the battery when applied to the battery. This is because materials with small particle sizes have a larger specific surface area and a shorter ion diffusion path, which is beneficial to the insertion and deinsertion reaction of lithium ions, thereby improving the charge and discharge rate performance of the battery and reducing the internal resistance of the battery. However, if the particle size D50 of lithium iron manganese phosphate is too small, it is easy to form agglomerates, which increases the difficulty of preparing battery materials, and too small a particle size may also cause the mechanical strength of the material to decrease, thereby reducing the cycle life and stability of the material.

[0033] Illustratively, the particle size D50 of lithium manganese iron phosphate is 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm or 1.6 μm.

[0034] In some embodiments, the particle size D50 of the lithium-rich iron oxide satisfies: 5 μm ≤ d2 ≤ 15 μm. The particle size D50 of the lithium-rich iron oxide is larger, and can form a better stacking effect with the small-particle lithium iron phosphate, thereby establishing a more effective conductive network, thereby better playing the role of improving the gram capacity and cycle performance. The small-particle lithium iron phosphate surrounds the large-particle lithium-rich iron oxide, so that the lithium-rich iron oxide and the lithium iron phosphate material interact better.

[0035] Exemplarily, the particle size D50 of the lithium-rich lithium ferrite is 5 μm, 7 μm, 10 μm, 12 μm, 14 μm or 15 μm.

[0036] It should be noted that D50 refers to the particle size corresponding to when the cumulative particle size distribution percentage of a sample reaches 50%, also known as the median particle size or median particle size.

[0037] In some embodiments, the mass ratio of lithium-rich lithium iron oxide added to the positive electrode active material is greater than or equal to 0.5% and less than 2%. When the addition ratio of lithium-rich lithium iron oxide is within this range, it can have a more obvious effect on improving the gram capacity, while improving the cycle performance and maintaining good rate performance, thereby compensating for the defects of lithium manganese iron phosphate relative to lithium iron phosphate to a certain extent and improving its comprehensive performance.

[0038] Exemplarily, the mass ratio of lithium-rich lithium ferrite added to the positive electrode active material is 0.5%, 0.8%, 1%, 1.2%, 1.5% or 1.8%.

[0039] The mass ratio of lithium-rich iron oxide added to the positive electrode active material is preferably 1.5%-1.8%. When the mass ratio of lithium-rich iron oxide added to the positive electrode active material is within this range, it can further ensure that the gram capacity and cycle performance of lithium manganese iron phosphate are improved, and maintain good rate performance.

[0040] In some embodiments, the mass ratio of lithium-rich iron oxide added to the positive electrode active material is 2%-5%. Within a certain range, as the addition ratio of lithium-rich iron oxide increases, the effect of improving gram capacity and cycle performance is better, but if the addition amount of lithium-rich iron oxide is too high, it is easy to cause excessive gas production, which in turn affects the gram capacity and cycle performance. When the mass ratio of lithium-rich iron oxide added to the positive electrode active material is 2%-5%, the improvement of gram capacity and cycle performance is more obvious, and the rate performance can be kept within a good range.

[0041] Exemplarily, the mass ratio of lithium-rich lithium ferrite added to 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%.

[0042] In some embodiments, the mass ratio of lithium-rich iron oxide added to the positive electrode active material is 3.1%-5%. That is, the mass ratio of lithium-rich iron oxide added to the positive electrode active material can be preferably set to 3.1%-5% to fully exert the effect of lithium-rich iron oxide and avoid the serious gas production problem caused by excessive lithium-rich iron oxide.

[0043] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate is S1, and the specific surface area of ​​lithium-rich iron oxide is S2, 7≤S1 / S2≤40. The particle size of lithium manganese iron phosphate is small, and its specific surface area is large, while the particle size of lithium-rich iron oxide is large, and its specific surface area is small. By setting the specific surface areas of the two to meet the condition of 7≤S1 / S2≤40, it is beneficial for the two to form a better stacking effect, thereby establishing a more effective conductive network and exerting a better effect.

[0044] Illustratively, S1 / S2 may be 7, 10, 12, 15, 17, 20, 22, 25, 28, 30, 32, 35, 38, or 40.

[0045] It is understandable 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 area is larger, it can provide more reaction sites, and when the specific surface area is smaller, it has relatively more stable properties. As the main material of the positive electrode active material, lithium manganese iron phosphate has a small particle size and a large specific surface area, which can provide more reaction sites and is conducive to improving the energy density. The addition ratio of lithium-rich lithium iron oxide is relatively small, and it is mainly to provide lithium supplementation and other functions. Its particle size is larger, the specific surface area is smaller, and it has a certain stability and can play a better role.

[0046] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate satisfies: 8m 2 / g≤S1≤18m 2 / g. If the specific surface area of ​​lithium manganese iron phosphate is too small, it is easy to affect the gram capacity, making the gram capacity less than expected. When its specific surface area is large, problems such as agglomeration are likely to occur during the preparation of positive electrode slurry using positive electrode active materials.

[0047] For example, the specific surface area of ​​lithium manganese iron phosphate can be 8m 2 / g, 10m 2 / g, 12m 2 / g, 14m 2 / g, 16m 2 / g or 18m 2 / g.

[0048] In some embodiments, the specific surface area of ​​lithium-rich lithium ferrite satisfies: 0.2 m 2 / g≤S2≤2.4m 2 / g. If the specific surface area of ​​lithium-rich lithium iron oxide is too small, it will also affect the gram capacity, making the gram capacity less than expected. If the specific surface area is too large, it will easily lead to a large amount of gas production in the battery, which is difficult to completely eliminate during the production process, thus affecting the performance of the battery.

[0049] For example, the specific surface area of ​​lithium-rich lithium ferrite may be 0.2 m2 / 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.

[0050] In some embodiments, in the positive electrode active material, the molar ratio of lithium to manganese is (1.5-2.9): 1, and the molar ratio of lithium to iron is (1.7-3.5): 1. That is to say, as a whole, the molar ratio of lithium to manganese in the positive electrode active material provided in the embodiment of the present application is (1.5-2.9): 1, and the molar ratio of lithium to iron is (1.7-3.5): 1. By controlling the molar ratio of lithium to manganese and the molar ratio of lithium to iron in the positive electrode active material within a suitable range, the advantages of the mixture of lithium iron manganese phosphate and lithium-rich lithium ferrate can be fully utilized, the cycle performance is better, the gram capacity can be increased while the voltage platform can be guaranteed, thereby having better comprehensive performance.

[0051] In some embodiments, the molecular formula of lithium manganese iron phosphate is Li a M b (PO4) c .

[0052] Where 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 lithium manganese iron phosphate material within a suitable range, the material can have a better crystal structure and thus better comprehensive performance.

[0053] In a second aspect, an embodiment of the present application provides a positive electrode slurry, comprising the positive electrode active material, a conductive agent, a binder and a solvent as described above.

[0054] The beneficial effects of the positive electrode slurry provided in the embodiments of the present application are basically consistent with those of the positive electrode active material, and will not be elaborated here.

[0055] In some embodiments, the positive electrode slurry further includes a dispersant. Adding the dispersant to the positive electrode slurry can improve fluidity, improve the dispersion performance of the positive electrode active material, and maintain the stability of the positive electrode slurry.

[0056] It is understandable that the conductive agent, binder, dispersant and solvent may be conventional materials in the art, and the embodiments of the present application do not impose any specific limitations thereon.

[0057] Exemplarily, 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 oily binders such as polyvinylidene fluoride or water-based binders such as carboxymethyl cellulose, polyacrylic acid, and polyamide. The dispersant can be selected from inorganic dispersants such as metal oxides or hydroxides or organic dispersants such as polymers and surfactants. The solvent can be selected from solvents such as N-methyl-2-pyrrolidone, dimethyl carbonate, ethylene carbonate, divinyl carbonate, and dimethyl sulfoxide.

[0058] In addition, the positive electrode active material, the conductive agent, the binder, the dispersant and the solvent may also be conventionally configured in the art, and the embodiments of the present application do not impose any specific limitation thereto.

[0059] In a third aspect, an embodiment of the present application provides a positive electrode sheet, which is prepared using the positive electrode active material as described above or the positive electrode slurry as described above.

[0060] The positive electrode slurry is coated on the current collector, and then dried, punched, and rolled to obtain a positive electrode sheet.

[0061] The beneficial effects of the positive electrode sheet provided in the embodiment of the present application are basically consistent with those of the positive electrode active material, and will not be elaborated here.

[0062] In some embodiments, the compaction density of the positive electrode sheet is 2.2 g / cm 3 -2.6g / cm 3 . Compacted density = surface density / thickness of material. Compacted density has a great influence on the performance of the battery. On the one hand, by increasing the compacted density of the positive electrode sheet, 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 compacted density, the conductive channels and bridges inside the positive electrode sheet can also be increased, making the transmission of electrons and ions smoother, so as to improve the conductivity of the battery. In addition, positive electrodes with high compacted density also have better structural stability. However, too high or too low compacted density of the positive electrode sheet can easily lead to increased internal resistance and polarization of the battery. In the embodiment of the present application, the compacted density of the positive electrode sheet is set to 2.2g / cm 3 -2.6g / cm 3 It can reduce or avoid the increase of battery internal resistance and polarization while improving the gram capacity and cycle performance, thereby improving the overall performance of the battery.

[0063] In a fourth aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet as described above.

[0064] The beneficial effects of the battery provided in the embodiments of the present application are basically consistent with those of the positive electrode active material and will not be elaborated here.

[0065] The following examples are further described in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods in the following examples that do not specify specific conditions are usually based on the conditions recommended by the manufacturer.

[0066] It should be noted that, except for the differences already described, the other conditions in the embodiments of the present application and the comparative examples remain the same.

[0067] The manufacturing process of the battery in the embodiments and comparative examples of the present application includes: compounding lithium iron manganese phosphate and lithium-rich lithium ferrite in proportion to form a positive electrode active material; mixing the positive electrode active material with a conductive agent, a binder, a dispersant and a solvent to prepare a positive electrode slurry; the positive electrode active material accounts for 97.5%, the conductive agent is carbon black, accounting for 0.7%, the binder is polyvinylidene fluoride, accounting for 1.5%, and the dispersant is a polyester substance, accounting for 0.3%. Artificial graphite is used as the negative electrode active material, mixed with a conductive agent and a binder to prepare a negative electrode slurry; the negative electrode active material accounts for 96.9%, the conductive agent is carbon black, accounting for 0.5%, and the binder is sodium carboxycellulose and styrene-butadiene rubber, among which sodium carboxycellulose accounts for 1.1% and styrene-butadiene rubber accounts for 1.5%. The positive electrode slurry and the negative electrode slurry are respectively obtained through coating and sheet making processes to obtain positive electrode sheets and negative electrode sheets; the positive electrode sheets, negative electrode sheets and separators, electrolytes, cover plates and aluminum shells are assembled together into square aluminum shell batteries, and then the finished batteries are obtained after chemical composition and capacity division.

[0068] Example 1

[0069] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 1.6%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0070] Example 2

[0071] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 2.9%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0072] Example 3

[0073] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 3.1%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0074] Example 4

[0075] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 0.5%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0076] Example 5

[0077] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 5.0%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0078] Example 6

[0079] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 2.9%. The particle size D50 of lithium iron manganese phosphate is 0.8 μm, and the specific surface area is 18 m 2 / g, the D50 of lithium-rich lithium iron oxide is 5μm, and the specific surface area is 2.4m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0080] Example 7

[0081] In this embodiment, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 2.9%. The particle size D50 of lithium iron manganese phosphate is 1.6 μm, and the specific surface area is 8 m 2 / g, the D50 of lithium-rich lithium iron oxide is 15μm, and the specific surface area is 0.2m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0082] Comparative Example 1

[0083] In this comparative example, the positive electrode active material is composed only of lithium manganese iron phosphate, the particle size D50 of lithium manganese iron phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0084] Comparative Example 2

[0085] In this comparative example, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium ferrate, wherein the addition ratio of lithium iron-rich lithium ferrate is 0.3%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0086] Comparative Example 3

[0087] In this comparative example, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium, wherein the addition ratio of lithium iron-rich lithium is 6%. The particle size D50 of lithium iron manganese phosphate is 1.2 μm, and the specific surface area is 13 m 2 / g, the D50 of lithium-rich lithium iron oxide is 10μm, and the specific surface area is 1.3m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0088] Comparative Example 4

[0089] In this comparative example, the positive electrode active material is composed of lithium iron manganese phosphate and lithium iron-rich lithium, wherein the addition ratio of lithium iron-rich lithium is 2.9%. The particle size D50 of lithium iron manganese phosphate is 1.8 μm, and the specific surface area is 6 m 2 / g, the D50 of lithium-rich lithium iron oxide is 3μm, and the specific surface area is 2.6m 2 / g. The battery was prepared according to the above-mentioned battery manufacturing process.

[0090] The batteries in Examples 1-7 and Comparative Examples 1-4 were subjected to gram capacity test, DCR test, cycle test, EOLMn 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 to 4.2V at 0.33C constant current and constant voltage, with a cut-off current of 0.05C; leaving for 30min; discharging at 0.33C constant current to 2.5V; the capacity of the discharge process / weight of active material is the gram capacity. The DCR test method includes: placing the battery in a 25±2℃ constant temperature box, charging to 4.2V at 0.33C constant current and constant voltage, with a cut-off current of 0.05C; leaving for 30min; discharging at 0.33C constant current to 80% SOC; leaving for 1h, the voltage is recorded as U1; then discharging at 1C (current is expressed as A) at a constant current for 10s, 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 cut-off current of 0.05C; leaving it for 30 minutes; discharging at 1C constant current to 2.5V; leaving it for 30 minutes; repeating the above steps until the discharge capacity / initial discharge capacity = 80%, which is recorded as the number of cycles. The EOL Mn deposition test method includes: discharging the battery that has been cycled to 80% SOH, disassembling and removing the negative electrode sheet, scraping the negative electrode powder on the current collector, and then performing an 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 to 4.2V at 0.1C constant current and constant voltage, with a cut-off current of 0.05C, leaving for 30 minutes, discharging to 2.5V at 0.1C constant current, and the discharge capacity is recorded as Q0; charging to 4.2V at 0.1C constant current and constant voltage, with a cut-off current of 0.05C, leaving for 30 minutes, discharging to 2.5V at 3C constant current, and the discharge capacity is recorded as Q; Q / Q0 is the 3C capacity retention rate. The results are shown in Table 1:

[0091] Table 1 Performance test results of different embodiments and comparative examples

[0092]

[0093] As can be seen from Table 1, compared with Comparative Example 1 in which no lithium-rich iron oxide is added to the positive electrode active material, after adding 1.6% of lithium-rich iron oxide to Example 1, the gram capacity and cycle performance are greatly improved, and compared with Comparative Example 1, the rate performance (3C capacity retention rate) of Example 1 is also maintained at a high level, there is no obvious change in manganese deposition at the negative electrode, and the DCR is reduced. Compared with Comparative Example 1, Example 2 adds 2.9% of lithium-rich iron oxide to the positive electrode active material, and the gram capacity and cycle times are greatly improved compared with Comparative Example 1, and the deposition of manganese at the negative electrode is reduced, and the DCR is also reduced, but due to the high proportion of lithium-rich iron oxide added, the rate performance is greatly reduced. In Examples 3-7, 3.1%, 0.5%, 5.0%, 2.9% and 2.9% of lithium-rich iron oxide were added to the positive electrode active material, respectively, and lithium iron manganese phosphate and lithium-rich iron oxide with different particle sizes and specific surface areas were used. The gram capacity and the number of cycles were improved to varying degrees compared with Comparative Example 1, and the deposition of manganese at the negative electrode was reduced, and the DCR was also reduced. Although lithium-rich iron oxide was added to the positive electrode active material in Comparative Example 2, due to the low amount of addition, there was no significant change in the gram capacity, the number of cycles, and the deposition of manganese at the negative electrode, i.e., DCR, relative to Comparative Example 1 without the addition of lithium-rich iron oxide, but the rate performance was reduced. In Comparative Example 3, an excessive amount of lithium-rich iron oxide was added, which easily led to excessive gas production, making its gram capacity basically the same as that of Comparative Example 1 without the addition of lithium-rich iron oxide, the number of cycles was reduced, the deposition of manganese at the negative electrode was reduced, and the DCR was increased. That is, although lithium-rich iron phosphate was added in Comparative Example 3, which reduced the deposition of manganese at the negative electrode and helped improve the cycle performance, the excessive amount of lithium-rich iron phosphate added led to a decrease in the final cycle performance, and the rate performance decreased significantly. In Comparative Example 4, the particle size of lithium manganese iron phosphate was too large and the specific surface area was small, while the particle size of lithium-rich iron phosphate was too small and the specific surface area was too large, resulting in the inability to form a good conductive network after the two were compounded. In addition, due to the excessive specific surface area of ​​lithium-rich iron phosphate, the gas production of the battery increased, the side reactions increased, and the cycle performance decreased, and the rate performance also decreased significantly.

[0094] In summary, the embodiments of the present application include lithium iron manganese phosphate and lithium-rich lithium iron oxide as the positive electrode active materials, and the lithium-rich lithium iron oxide can improve the problems of low gram capacity and poor cycle performance when lithium iron manganese phosphate is used alone, and maintain good rate performance. And by controlling the addition ratio of lithium-rich lithium iron oxide within an appropriate range, the gram capacity and cycle performance are improved to varying degrees, and a high rate performance is maintained. By compounding lithium iron manganese phosphate with a small particle size and lithium-rich lithium iron oxide with a large particle size, it is ensured that a stacking effect can be formed when the two are compounded, thereby establishing a more effective conductive network, and better exerting the effect of lithium-rich lithium iron oxide on improving gram capacity and cycle performance.

[0095] The embodiments of the present application are introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A positive electrode active material, characterized in that: Including lithium iron manganese phosphate and lithium iron oxide rich; Wherein, the mass ratio of the lithium-rich lithium iron oxide added to 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.

2. The positive electrode active material according to claim 1, characterized in that The particle size D50 of the lithium manganese iron phosphate satisfies: 0.8 μm≤d1≤1.6 μm; And / or, the particle size D50 of the lithium-rich lithium ferrite satisfies: 5 μm≤d2≤15 μm.

3. The positive electrode active material according to claim 1, characterized in that The lithium-rich lithium iron oxide is added to the positive electrode active material in a mass ratio of 0.5% to 2%.

4. The positive electrode active material according to claim 3, characterized in that The lithium-rich lithium ferrite is added to the positive electrode active material in a mass ratio of 1.5% to 1.8%.

5. The positive electrode active material according to claim 1, characterized in that The specific surface area of ​​the lithium manganese iron phosphate is S1, and the specific surface area of ​​the lithium-rich lithium iron oxide is S2, 7≤S1 / S2≤40.

6. The positive electrode active material according to claim 5, characterized in that The specific surface area of ​​the lithium manganese iron phosphate meets the following requirements: 8m 2 / g≤S1≤18m 2 / g; And / or, the specific surface area of ​​the lithium-rich lithium iron oxide satisfies: 0.2m 2 / g≤S2≤2.4m 2 / g.

7. The positive electrode active material according to claim 1, characterized in that 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.

8. The positive electrode active material according to any one of claims 1 to 7, characterized in that: The molecular formula of the lithium manganese iron 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.

9. A positive electrode slurry, characterized in that: The method comprises the positive electrode active material as claimed in any one of claims 1 to 8, a conductive agent, a binder and a solvent.

10. A positive electrode sheet, characterized in that: The positive electrode is prepared using the positive electrode active material according to any one of claims 1 to 8 or the positive electrode slurry according to claim 9.

11. The positive electrode sheet according to claim 10, characterized in that: The compaction density of the positive electrode sheet is 2.2 g / cm 3 -2.6g / cm 3 .

12. A battery, characterized in that: Comprising the positive electrode sheet as claimed in claim 10 or 11.

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