Positive electrode active material and preparation method thereof, positive electrode sheet and battery

Abstract

This application proposes a positive electrode active material and a preparation method thereof, a positive electrode sheet and a battery. The positive electrode active material includes Li y Fe a M b PO4, wherein 0.95≤a≤0.99, 0.001≤b≤0.1, 1≤y≤1.08, and M comprises at least one of Ti, Na, K, Mg, V, W, Nb, La, Cr, Mo, Ca, Zn, Y, Zr, Sm, Co, Ni, B, Cu, and Gd. The positive electrode active material comprises Li4P2O7, and the mass proportion of the Li4P2O7 based on the total mass of the positive electrode active material is less than or equal to 0.3%. This reduces the content of Li4P2O7 in the positive electrode active material, reduces iron dissolution, and thereby improves the capacity and cycle performance of the battery.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular to positive electrode active materials and preparation methods thereof, positive electrode sheets, and batteries. Background Art

[0002] Lithium-ion batteries have attracted considerable attention due to their high theoretical capacity (170 mAh / g), excellent stability, high safety, and environmental friendliness. They are currently widely used in energy storage, power tools, electric vehicles, military equipment, aerospace, and other fields. With the increasing application of energy storage, long-life lithium-ion batteries have become a research hotspot in the battery field in recent years. New energy storage systems have high requirements for the service life of lithium-ion batteries. Typically, home energy storage systems require a service life of more than 3,000 cycles, commercial energy storage systems require a service life of more than 6,000 cycles, and large-scale energy storage systems require a service life of more than 10,000 cycles. Among different types of lithium-ion batteries, olivine-structured lithium iron phosphate materials are a hot topic of research due to their excellent safety, low cost, and long cycle performance. However, lithium iron phosphate materials can leach iron, which reduces the battery's cycle performance. Summary of the Invention

[0003] The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

[0004] In a first aspect, the present application proposes a positive electrode active material, wherein the positive electrode active material comprises Li y Fe a M b PO4, wherein 0.95≤a≤0.99, 0.001≤b≤0.1, 1≤y≤1.08, and M comprises at least one of Ti, Na, K, Mg, V, W, Nb, La, Cr, Mo, Ca, Zn, Y, Zr, Sm, Co, Ni, B, Cu, and Gd. The positive electrode active material comprises Li4P2O7, and the mass proportion of the Li4P2O7 based on the total mass of the positive electrode active material is less than or equal to 0.3%. This reduces the content of Li4P2O7 in the positive electrode active material, reduces iron dissolution, and thereby improves the cycle performance of the battery.

[0005] According to some embodiments of the present application, the positive electrode active material is compacted using a pressure i, and the specific surface area of ​​the positive electrode active material before compaction is SSA0, and the specific surface area of ​​the positive electrode active material after compaction is SSA i , and satisfy: 0<(SSA i -SSA0) / SSA0≤(7.07ln(i)-5.9) / 100, where 6T≤i≤12T. Thus, after the positive electrode active material is compacted with different pressures, the specific surface area increase rate ((SSAi -SSA0) / SSA0) is small, indicating that the number of large particles in the positive electrode active material is small, which can reduce the risk of cracking of the positive electrode active material during battery cycling, reduce iron dissolution, and improve the capacity and cycle performance of the battery.

[0006] According to some embodiments of the present application, the positive electrode active material satisfies at least one of the following conditions: under the condition of i=6T, (SSA 6T -SSA0) / SSA0≤6.5%; Under the condition of i=9T, (SSA 9T -SSA0) / SSA0≤9%; Under the condition of i=12T, (SSA 12T -SSA0) / SSA0≤11%. This reduces the rate of increase in specific surface area of ​​the positive electrode active material under different pressures, reduces the risk of cracking of the positive electrode active material during battery cycling, reduces iron dissolution, and improves the capacity and cycle performance of the battery.

[0007] According to some embodiments of the present application, K 95 =(Dv 95 -Dv5) / Dv 50 , and satisfy 0.1≤K 95 ≤7, optional 1≤K 95 ≤5, wherein Dv5 is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 5%, Dv 50 Dv is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 50%, 95 The particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaches 95%. This improves the uniformity of the positive electrode active material particles, reduces the risk of cracking of the positive electrode active material during battery cycling, reduces iron dissolution, and improves the battery's cycling performance.

[0008] According to some embodiments of the present application, the iron dissolution rate of the positive electrode active material is less than or equal to 300 ppm, and optionally, the iron dissolution rate of the positive electrode active material is less than or equal to 150 ppm. This improves the structural stability of the positive electrode active material during battery cycling, thereby improving the capacity and cycle performance of the battery.

[0009] According to some embodiments of the present application, the positive electrode active material satisfies at least one of the following conditions: under a pressure of 3T, the compaction density of the positive electrode active material is 2.3 g / cm 3 -2.6g / cm 3 The resistance of the positive electrode active material is 1Ω·cm-20Ω·cm. This improves the energy density and rate performance of the battery.

[0010] According to some embodiments of the present application, at least a portion of the surface of the positive electrode active material has a carbon coating layer, thereby improving the electronic conductivity of the positive electrode sheet.

[0011] The second aspect of the present application provides a method for preparing the positive electrode active material provided in the first aspect of the present application, the method comprising: mixing a Li source, a P source, an Fe source, an M source, and a solvent to obtain a slurry, wherein the molar ratio of the P element in the P source to the Fe element in the Fe source is less than or equal to 1.053, grinding and drying the slurry to obtain precursor particles; and sequentially performing a first sintering and a second sintering on the precursor particles, wherein the temperature of the first sintering is T1, the temperature of the second sintering is T2, T1 < T2, and crushing to obtain the positive electrode active material. Thus, by controlling the molar ratio of the P element and the Fe element, the probability of generating pyrophosphate is reduced during the sintering process, the content of Li4P2O7 in the positive electrode active material is reduced, and thereby the iron dissolution is reduced, thereby improving the cycle performance and capacity of the battery.

[0012] According to some embodiments of the present application, the molar ratio of the P element in the P source to the Fe element in the Fe source is 1.015-1.045. This reduces the content of Li4P2O7 in the positive electrode active material, thereby reducing iron dissolution and improving the cycle performance and capacity of the battery.

[0013] According to some embodiments of the present application, the first sintering satisfies at least one of the following conditions: 300 ≤ T1 ≤ 500°C; the holding time for the first sintering is t1, and satisfies 1h ≤ t1 ≤ 7h, optionally 3h ≤ t1 ≤ 5h; and the heating rate for heating the precursor particles to T1 is 2°C / min-3°C / min. Thus, by keeping the temperature, heating rate, and holding time of the first sintering within this range, the crystallization process can be better controlled, forming a positive electrode active material with uniform particle distribution.

[0014] According to some embodiments of the present application, the second sintering process satisfies at least one of the following conditions: 700 ≤ T2 ≤ 900°C; the second sintering process has a holding time of t2, which satisfies 5h ≤ t2 ≤ 15h, and optionally 7h ≤ t2 ≤ 10h; and the heating rate from T1 to T2 is 0.5°C / min-3°C / min. Thus, by ensuring that the second sintering process has a temperature, heating rate, and holding time within the above ranges, the crystallization effect is enhanced, the number of large particles in the positive electrode active material is reduced, and the particle uniformity is improved.

[0015] According to some embodiments of the present application, the method further comprises: mixing the Li source, the P source, the Fe source, the M source, a carbon source, and the solvent, thereby improving the electronic conductivity of the positive electrode active material.

[0016] According to some embodiments of the present application, the method satisfies at least one of the following conditions: the Li source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium acetate, lithium oxalate, and lithium nitrate; the carbon source includes at least one of glucose, sucrose, starch, cellulose, citric acid, oxalic acid, polyethylene glycol, polyvinyl alcohol, and polyethylene; the P source and the Fe source include iron phosphate; and the M source includes an oxide of M.

[0017] The third aspect of the present application provides a positive electrode plate, which includes the positive electrode active material provided by the first aspect of the present application or the positive electrode active material prepared by the method provided by the second aspect of the present application.

[0018] The fourth aspect of the present application provides a battery comprising the positive electrode sheet provided in the third aspect of the present application, thereby having excellent cycle performance and high capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments with reference to the following drawings, in which:

[0020] Figure 1 A schematic flow chart of a method for preparing a positive electrode active material according to an embodiment of the present application is shown.

[0021] Figure 2 The XRD pattern of lithium iron phosphate prepared in Example 2 of the present application is shown.

[0022] Figure 3 The SEM image of the lithium iron phosphate prepared in Example 2 of the present application is shown. DETAILED DESCRIPTION

[0023] The following embodiments of the present invention are described in detail. The embodiments described below are exemplary and are only used to explain the present invention, and are not to be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this field or the product specifications are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be obtained commercially.

[0024] The quality of positive electrode active materials, crucial for further improving battery performance, directly impacts various lithium battery performance indicators. During long-term cycling and storage of olivine-structured positive electrode materials, iron dissolution occurs. The dissolved iron ions migrate to the negative electrode surface and deposit, accelerating the decomposition reaction of the electrolyte. This iron dissolution adversely affects the performance of both the positive and negative electrodes, as well as the electrolyte, exacerbating the capacity decay of the olivine-structured positive electrode active materials and reducing the battery's cycling performance.

[0025] The present application proposes a positive electrode active material, wherein the positive electrode active material comprises Li y Fe a M b PO4, wherein 0.95≤a≤0.99, 0.001≤b≤0.1, 1≤y≤1.08, and M comprises at least one of Ti, Na, K, Mg, V, W, Nb, La, Cr, Mo, Ca, Zn, Y, Zr, Sm, Co, Ni, B, Cu, and Gd. The positive electrode active material comprises Li4P2O7, and the mass proportion of the Li4P2O7 based on the total mass of the positive electrode active material is less than or equal to 0.3%. This reduces the content of Li4P2O7 in the positive electrode active material, reduces iron dissolution, and thereby improves the capacity and cycle performance of the battery.

[0026] According to some embodiments of the present application, the mass proportion of the Li4P2O7 is less than or equal to 0.3%, for example, it can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25% or 0.3%, or it can be a range consisting of any of the above values. In this way, the content of Li4P2O7 in the positive electrode active material is reduced, the consumption of Li by pyrophosphate is reduced, the structural stability of the lithium iron phosphate material is improved, and iron dissolution is reduced.

[0027] According to some embodiments of the present application, the positive electrode active material is compacted using a pressure i, and the specific surface area of ​​the positive electrode active material before compaction is SSA0, and the specific surface area of ​​the positive electrode active material after compaction is SSA i , and satisfy: 0<(SSA i -SSA0) / SSA0≤(7.07ln(i)-5.9) / 100, wherein 6T≤i≤12T. Thus, by increasing the specific surface area increase rate (SSA i -SSA0) / SSA0 is within the above range. After the positive electrode active material is compacted with different pressures i, the increase rate of the specific surface area of ​​the positive electrode active material is small, indicating that the number of cracked particles in the positive electrode active material is small and the particle uniformity is good. During the battery cycle, the structural stability of the positive electrode active material can be improved, the risk of cracking of the positive electrode active material can be reduced, the iron dissolution can be reduced, and the attenuation of the capacity of the positive electrode active material can be reduced. The influence of iron dissolution on the positive and negative electrodes and the electrolyte can be reduced, and the cycle performance and capacity of the battery can be improved.

[0028] In this application, the specific surface area of ​​the positive electrode active material can be measured by BET, using the nitrogen adsorption method and a Micromeritics Tristar II 3030 machine to measure the total surface area of ​​all particles in each gram of positive electrode active material. The unit is: m 2Specifically, the specific surface area SSA0 of the positive electrode active material before compaction is first tested, and then the positive electrode active material is compacted using pressure i, ground and sieved, and the specific surface area SSA of the positive electrode active material under the sieve is tested. i .

[0029] As an example, i may be 6T, 8T, 10T, or 12T, or may be a range consisting of any of the above values.

[0030] According to some embodiments of the present application, under the condition of i=6T, (SSA 6T -SSA0) / SSA0≤6.5%, for example, can be 1.5%, 2.5%, 3.5%, 4.5%, 5.5% or 6.5%, or can be a range consisting of any of the above values. Therefore, after compaction using a pressure of 6T, the increase rate of specific surface area of ​​the positive electrode active material is small, indicating that the positive electrode active material particles are uniformly distributed, which can reduce the risk of particle cracking during battery cycling and reduce iron dissolution, thereby improving the capacity and cycle performance of the battery.

[0031] According to some embodiments of the present application, under the condition of i=9T, (SSA 9T -SSA0) / SSA0≤9%, for example, can be 2%, 4%, 6%, 8% or 9%, or can be a range consisting of any of the above values. Therefore, after compaction using a pressure of 9T, the increase rate of the specific surface area of ​​the positive electrode active material is small, indicating that the positive electrode active material particles are evenly distributed, which can reduce the risk of particle cracking during battery cycling and reduce iron dissolution, thereby improving the battery capacity and cycle performance.

[0032] According to some embodiments of the present application, under the condition of i=12T, (SSA 12T -SSA0) / SSA0≤11%, for example, (SSA 12T -SSA0) / SSA0 can be 3%, 5%, 7%, 9%, or 11%, or can be a range consisting of any of the above values. Thus, after compaction using a pressure of 12T, the increase in the specific surface area of ​​the positive electrode active material is small, indicating that the positive electrode active material particles are evenly distributed, which can reduce the risk of particle cracking during battery cycling and reduce iron dissolution, thereby improving the battery capacity and cycle performance.

[0033] It can be seen from this that the positive electrode active material proposed in this application has a specific surface area increase rate (SSA) after being compacted at different pressures. i -SSA0) / SSA0) are both small, indicating that the positive electrode active material particles are evenly distributed, which can reduce the risk of particle cracking during battery cycling and reduce iron dissolution, thereby improving the battery capacity and cycle performance.

[0034] According to some embodiments of the present application, the K 95 =(Dv 95 -Dv5) / Dv 50 , and satisfy 0.1≤K 95 ≤7, wherein Dv5 is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 5%, Dv 50 Dv is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 50%, 95 is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 95%. 95 It can be 0.1, 1, 3, 4, 5, 6 or 7, or can be a range consisting of any of the above values. As a result, the uniformity of the positive electrode active material particles is better, the difference in particle size between particles is smaller, and the risk of cracking of the positive electrode active material during battery cycling can be reduced, reducing iron dissolution, thereby improving the capacity and cycle performance of the battery. According to some specific embodiments of the present application, 4.5≤K 95 ≤6.

[0035] As an example, the Dv of the positive electrode active material 50 The thickness may be 0.55 μm to 0.75 μm, for example, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm or 0.75 μm, or may be within a range consisting of any of the above values.

[0036] As an example, the Dv5 of the positive electrode active material may be 0.25 μm-0.35 μm, for example, 0.25 μm, 0.27 μm, 0.29 μm, 0.31 μm, 0.33 μm or 0.35 μm, or a range consisting of any of the above values.

[0037] As an example, the Dv of the positive electrode active material 95 The thickness may be 3 μm to 7 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm or 7 μm, or may be within a range consisting of any of the above values.

[0038] Thus, by making Dv 50 、Dv5、Dv 95 In the above range, the K of the positive electrode active material is regulated. 95 , thereby improving the particle uniformity of the lithium iron phosphate material. During the battery cycle, it can reduce the risk of cracking of the lithium iron phosphate material and reduce iron dissolution, thereby improving the capacity and cycle performance of the battery.

[0039] According to some embodiments of the present application, the iron dissolution rate of the positive electrode active material is less than or equal to 300 ppm. This reduces the iron dissolution rate of the positive electrode active material, which can reduce the capacity decay of the positive electrode active material. During battery cycling, the risk of iron dissolution and deposition at the negative electrode can be reduced, reducing electrolyte decomposition, thereby improving the battery's cycling performance and capacity. According to some specific embodiments of the present application, the iron dissolution rate of the positive electrode active material is less than or equal to 150 ppm.

[0040] According to some embodiments of the present application, under a pressure of 3T, the compaction density of the positive electrode active material can be 2.3 g / cm 3 -2.6g / cm 3 , for example, it can be 2.3 g / cm 3 , 2.35g / cm 3 , 2.4g / cm 3 , 2.45g / cm 3 , 2.5g / cm 3 , 2.55g / cm 3 or 2.6g / cm 3 The energy density of the battery is thereby increased.

[0041] According to some embodiments of the present application, the resistance of the positive electrode active material is 1 Ω·cm-20 Ω·cm, for example, 1 Ω·cm, 5 Ω·cm, 10 Ω·cm, 15 Ω·cm, or 20 Ω·cm, or a range of any of the above values. This improves the efficiency of lithium ion migration and the rate performance of the battery.

[0042] According to some embodiments of the present application, at least a portion of the surface of the positive electrode active material has a carbon coating layer, thereby improving the electronic conductivity of the positive electrode sheet.

[0043] The second aspect of the present application provides a method for preparing the positive electrode active material provided in the first aspect of the present application, the method comprising: mixing a Li source, a P source, an Fe source, an M source, and a solvent to obtain a slurry, wherein the molar ratio of the P element in the P source to the Fe element in the Fe source is less than or equal to 1.053, grinding and drying the slurry to obtain precursor particles; sequentially performing a first sintering and a second sintering on the precursor particles, the temperature of the first sintering being T1, the temperature of the second sintering being T2, T1<T2, and crushing to obtain the positive electrode active material.

[0044] The method for preparing positive electrode active materials proposed in this application can reduce the generation of pyrophosphate during the sintering process by controlling the molar ratio of P element and Fe element, reduce the consumption of Li by pyrophosphate, reduce iron dissolution, and thus reduce the capacity loss of the positive electrode active material, reduce the impact of iron dissolution on the positive and negative electrodes and the electrolyte, and improve the cycle performance and capacity of the battery.

[0045] The method for preparing the positive electrode active material proposed in this application is described in detail below. Figure 1 , the method comprising:

[0046] S10: Mixing a Li source, a P source, an Fe source, an M source, and a solvent to obtain a slurry, grinding and drying the slurry to obtain precursor particles.

[0047] According to some embodiments of the present application, the Li source, the P source, the Fe source, the M source, the carbon source, and the solvent are mixed. Thus, during the preparation of lithium iron phosphate, the temperature of the first sintering is the temperature at which the carbon source cracks. The first sintering allows the carbon source to be fully carbonized, thereby forming a uniform carbon coating on the surface of the lithium iron phosphate, further improving the uniformity of the carbon coating and improving the electronic conductivity of the positive electrode.

[0048] According to some embodiments of the present application, the carbon source includes at least one of glucose, sucrose, starch, cellulose, citric acid, oxalic acid, polyethylene glycol, polyvinyl alcohol, and polyethylene.

[0049] According to some embodiments of the present application, anhydrous ferric phosphate, the Li source, the M source, the carbon source, and a solvent are mixed.

[0050] According to some embodiments of the present application, the Dv of the anhydrous ferric phosphate is 50 The thickness may be 1 μm to 5 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm, or may be within a range consisting of any of the above values.

[0051] As an example, the Li source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium acetate, lithium oxalate, and lithium nitrate.

[0052] As an example, the P source and the Fe source include iron phosphate.

[0053] As an example, the M source includes an M oxide.

[0054] According to some embodiments of the present application, the molar ratio of the P element in the P source to the Fe element in the Fe source can be 1.010, 1.015, 1.020, 1.025, 1.030, 1.035, 1.040, 1.045 or 1.053, etc., or can be a range composed of any of the above values. Thus, by making the molar ratio of the P element and the Fe element within the above range, the content of the P element in the material is reduced, the generation of pyrophosphate is reduced during high-temperature sintering, the consumption of pyrophosphate for Li is reduced, and the dissolution of iron is reduced, thereby improving the capacity and cycle performance of the battery. At the same time, by making the molar ratio of the P element and the Fe element within the above range, the crystallization performance of the particles can be improved during high-temperature sintering, and the compaction density of the positive electrode active material can be improved. According to some specific embodiments of the present application, the molar ratio of the P element in the P source to the Fe element in the Fe source is 1.015-1.045.

[0055] According to some embodiments of the present application, the grinding method may include: coarse grinding the slurry, the zirconium ball size is 0.7mm-0.8mm, the coarse grinding time is 0.5h-1h, and the Dv of the particles in the slurry after coarse grinding is 50 The slurry is finely ground, the zirconium ball size is 0.3mm-0.4mm, the fine grinding time is 1h-5h, and the Dv of the slurry after fine grinding is 1μm-4μm. 50 The particle size can be 0.2 μm to 0.6 μm, thereby reducing the particle size of the particles in the slurry and improving the crystallization performance.

[0056] According to some embodiments of the present application, the drying method may include spray drying the finely ground slurry, with an atomization frequency of 20Hz-70Hz, a heated air inlet temperature of 100℃-300℃, and an outlet temperature of 50℃-150℃.

[0057] S20: performing a first sintering and a second sintering on the precursor particles in sequence

[0058] According to some embodiments of the present application, the precursor particles are subjected to a first sintering and a second sintering in sequence, the temperature of the first sintering is T1, the temperature of the second sintering is T2, T1<T2, and are crushed to obtain the positive electrode active material.

[0059] According to some embodiments of the present application, 300≤T1≤500°C, for example, it can be 300°C, 340°C, 380°C, 420°C, 460°C or 500°C, etc., or can be a range consisting of any of the above values.

[0060] The method for preparing positive electrode active materials proposed in this application can better control the growth process of particles, improve the crystallinity of positive electrode active materials, and make the reaction more complete by adding a first sintering step, and the temperature of the first sintering is 300°C-500°C. After the prepared positive electrode active materials are compacted with different pressures, the increase rate of specific surface area of ​​the positive electrode active materials is small, indicating that the particles of the positive electrode active materials are evenly distributed. During the battery cycle, the risk of particle cracking can be reduced, iron dissolution can be reduced, and the side reactions of the electrolyte can be reduced, thereby improving the cycle performance and capacity of the battery.

[0061] According to some embodiments of the present application, the precursor particles are sequentially sintered for a first time and a second time in an atmosphere of a protective gas, wherein the protective gas comprises at least one of nitrogen and an inert gas.

[0062] According to some embodiments of the present application, the holding time of the first sintering is t1, and satisfies 1h≤t1≤7h. For example, it can be 1h, 2h, 3h, 4h, 5h, 6h or 7h, or a range consisting of any of the above values. Thus, by making the holding time of the first sintering within this range, the crystallization process is better controlled to form a positive electrode active material with uniform particle distribution. According to some specific embodiments of the present application, 3h≤t1≤5h.

[0063] According to some embodiments of the present application, the heating rate of the precursor particles to T1 is 2°C / min-3°C / min, for example, 2°C / min, 2.2°C / min, 2.4°C / min, 2.6°C / min, 2.8°C / min, or 3°C / min, or a range consisting of any of the above values. Thus, by setting the heating rate of the first sintering within this range, the crystallization process is better controlled to form a positive electrode active material with uniform particle distribution.

[0064] According to some embodiments of the present application, 700 ≤ T2 ≤ 900°C, for example, 700°C, 800°C, or 900°C, or any range thereof. This allows for a more complete reaction and better crystallization, thereby increasing the compaction density of the positive electrode active material.

[0065] According to some embodiments of the present application, the heating rate from T1 to T2 is 0.5°C / min-3°C / min, for example, 0.5°C / min, 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min, or 3°C / min, or a range of any of the above values. This allows the reaction to proceed more fully, resulting in better crystallization and, in turn, an increase in the compaction density of the positive electrode active material.

[0066] According to some embodiments of the present application, the holding time of the second sintering is t2, and satisfies 5h≤t2≤15h, for example, 5h, 7h, 9h, 11h, 13h, or 15h, or a range consisting of any of the above values. According to some specific embodiments of the present application, 7h≤t2≤10h.

[0067] In summary, the positive electrode active material and preparation method thereof proposed in this application have the following advantages:

[0068] (1) In the process of preparing the positive electrode active material, by selecting a suitable P / Fe iron phosphate precursor, the content of P element in the slurry is reduced, the generation of pyrophosphate is reduced during high-temperature sintering, the consumption of Li by pyrophosphate is reduced, the content of Li4P2O7 in the positive electrode active material is reduced, and the iron dissolution is reduced, thereby improving the capacity and cycle performance of the battery.

[0069] (2) After compaction with different pressures i, the increase rate of the specific surface area of ​​the positive electrode active material is small, indicating that the positive electrode active material particles are evenly distributed. During the battery cycle, the probability of cracking of the positive electrode active material particles can be reduced, the iron dissolution can be reduced, and the capacity and cycle performance of the battery can be improved.

[0070] (3) In the process of preparing positive electrode active materials, adding the first sintering process can better control the crystallization process, improve the uniformity of the positive electrode active material particles, reduce the probability of cracking of the positive electrode active material particles during battery cycling, reduce iron dissolution, and improve the capacity and cycle performance of the battery.

[0071] The third aspect of the present application provides a positive electrode plate, comprising the positive electrode active material provided in the first aspect of the present application or the positive electrode active material prepared by the method provided in the second aspect of the present application.

[0072] The fourth aspect of the present application provides a battery, comprising the positive electrode sheet provided in the third aspect of the present application. Therefore, the battery provided in the present application has a high capacity and excellent cycle performance.

[0073] The following embodiments of the present invention are described in detail. The embodiments described below are exemplary and are only used to explain the present invention, and are not to be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this field or the product specifications are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be obtained commercially.

[0074] Example 1

[0075] 1. Preparation of positive electrode active materials

[0076] Fe 0.99The iron phosphate precursor of PO4 (Fe source and P source), lithium carbonate (Li source), and titanium dioxide are mixed with pure water in a molar ratio of Li:Fe:Ti:P=1.03:0.99:0.008:1. Glucose accounts for 8.5% of the mass of the iron phosphate, polyethylene glycol accounts for 3% of the mass of the iron phosphate, and the solid content of the slurry is 48% to obtain a first slurry.

[0077] The slurry is coarsely ground with zirconium ball size of 0.7mm-0.8mm and coarse grinding time of 0.5h, and then ultrafine ground with zirconium ball size of 0.3mm-0.4mm and fine grinding time of 2h. The slurry particle size Dv 50 0.42μm;

[0078] The slurry was spray dried to obtain precursor particles, with an atomization frequency of 20 Hz, a heated air inlet temperature of 120°C, and an air outlet temperature of 70°C;

[0079] The precursor particles were sintered in a nitrogen atmosphere. The sintering schedule was as follows: heating to 400°C at 2°C / min, holding for 3 h, then heating to 800°C at 2°C / min, holding for 8 h to obtain Li 1.03 Fe 0.99 Ti 0.008 PO4 positive electrode active material.

[0080] 2. Preparation of positive electrode sheet

[0081] The positive electrode active material Li 1.03 Fe 0.99 Ti 0.008 PO4, conductive agent carbon nanotubes, binder polyvinylidene fluoride (PVDF) are mixed with N-methylpyrrolidone (NMP) in a mass ratio of 92:5:3 to obtain the positive electrode slurry. The positive electrode slurry is evenly coated on the current collector aluminum foil, and then dried in a vacuum drying oven at 60°C for 20 hours. Then, it is stamped into a positive electrode sheet with a diameter of 11 mm and a thickness of 54 μm using a pressure of 100 MPa. The positive electrode sheet is placed in a vacuum drying oven at 120°C and dried for 12 hours.

[0082] 3. Negative electrode

[0083] A metal lithium sheet with a diameter of 15.6 mm and a thickness of 0.45 mm is used as the negative electrode.

[0084] 4. Isolation film

[0085] A polyethylene porous membrane with a thickness of 25 μm and coated with an alumina ceramic layer was used as the separator.

[0086] 5. Electrolyte

[0087] LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1, and the molar concentration of LiPF6 was 1 mol / L.

[0088] 6. Preparation of batteries

[0089] The positive electrode sheet, separator, negative electrode sheet and electrolyte were assembled into a 2025 button cell in an Ar gas glove box with a water content and an oxygen content of less than 5 ppm.

[0090] The preparation methods of lithium iron phosphate in Examples 2 to 15 and Comparative Example 1 are the same as those in Example 1, and the differences are detailed in Table 1.

[0091] Table 1

[0092]

[0093]

[0094] Performance Testing

[0095] 1. Specific surface area of ​​positive electrode active material

[0096] The total surface area of ​​all particles in each gram of positive electrode active material was measured using a Micromeritics Tristar II 3030 instrument using the nitrogen adsorption method. The unit is m 2 / g.

[0097] 2.Mass ratio of Li4P2O7

[0098] 1) dissolving the positive electrode active material in water, and filtering the obtained suspension to obtain a filtrate; 2) adjusting the pH of the filtrate to 11-13 with a standard alkaline solution, and performing a positive titration with a standard acid solution as an acid titrant to obtain a positive titration solution; the volume of the acid titrant consumed at the first hop point and the second hop point is respectively recorded as V 正1 mL and V 正2 mL; 3) the positive titration solution was heated and then cooled to room temperature, and the base standard solution was used as a base titrant for back titration to obtain a back titration solution; respectively, the first hop point 'and the second hop point 'consumption of the base titrant volume is V 反1 mL and V 反2 mL; 4) based on the amount of the acid titrant and the base titrant, determine the existence form and content of the residual pyrophosphate.

[0099] 3. Testing of Dv5, Dv50, and Dv95

[0100] Malvern 3000 particle size analyzer test, particle refractive index 1.74, absorption rate 1, ultrasonic intensity 100%, ultrasonic 45S test, unit μm.

[0101] 4. Iron dissolution

[0102] 1) Place a 50ml beaker on a ten-thousandth electronic balance, reset it to zero, and accurately weigh 5.0000±0.005g (target: close to 5.0000g) of lithium iron phosphate sample. Then, use a graduated cylinder to measure 50.0ml of pure water, pour it into the beaker, seal it with plastic wrap, shake it a few times, and let it stand at room temperature for 70 hours.

[0103] 2) After the standing period, take a clean centrifuge tube and place 3 layers of medium-speed filter paper on it. Pour the sample into the filter and rinse the inner wall of the beaker with a wash bottle. 3) Assemble the filtration device with 3 layers of 0.22μm filter membrane, connect it to the vacuum pump, turn on the vacuum pump, pour the filtrate into the filtration cup for filtration, and thoroughly wash the centrifuge tube and the inner wall of the beaker with a wash bottle. After the filtration is complete, all the filtrate is diluted to a 100ml volumetric flask, and then perform the inductively coupled plasma test (ICP test).

[0104] 5. Compaction density

[0105] Sansi Zongheng (UTM7305) compaction density meter, test value at a test force of 3T, unit: g / cm 3 。

[0106] 6. Resistance of positive electrode active material

[0107] D600 resistance meter, test pressure 1.57kN, test value under pressure holding for 10s.

[0108] 7. Capacity retention rate

[0109] The prepared battery was subjected to a 200-cycle charge-discharge test under the conditions of 3.75V-2.0V, 45°C, and 1C charge-discharge to evaluate the battery's cycle performance.

[0110] 8. Discharge capacity

[0111] The prepared battery was tested at 3.75V~2.0V, 25°C, and 0.1C / 0.1C+1C / 1C charge and discharge to evaluate the discharge capacity of the battery.

[0112] 9. Scanning electron microscope test

[0113] The model of the scanning electron microscope used was S-4800 (manufacturer: Hitachi, Japan), and the testing conditions of the scanning electron microscope were: acceleration voltage of 1 kV, magnification of 10K.

[0114] 10.XRD test

[0115] The XRD diffractometer used was an XRD-6000 X-ray powder diffractometer (Shimadzu, Japan). The XRD test conditions were as follows: Cu target, Kα ray (wavelength λ = 0.154 nm), tube voltage of 40 kV, tube current of 200 mA, and scanning speed of 10° (2θ) / min.

[0116] The test results of the positive electrode active materials and batteries in Examples 1 to 15 and Comparative Example 1 are shown in Tables 2 and 3.

[0117]

[0118]

[0119] It can be seen from the comparison of Examples 1 to 15 and Comparative Example 1 that the battery assembled with the positive electrode active material proposed in the present application has a higher discharge capacity and cycle capacity retention rate, indicating that by making the Li4P2O7 content in the positive electrode active material less than 0.3%, iron dissolution can be reduced and the capacity decay of the lithium iron phosphate material can be reduced. At the same time, due to the low iron dissolution, the impact on the electrolyte and the positive and negative electrodes can be reduced, thereby improving the cycle performance of the battery.

[0120] Comparing Examples 1-4 with Comparative Example 1 reveals that different P / Fe molar ratios during the preparation of lithium iron phosphate result in varying Li₄P₂Oₐ contents. By controlling the P / Fe molar ratio, lithium iron phosphate materials with high compaction density and low Li₄P₂Oₐ content can be obtained. The extent of iron dissolution varies; lower Li₄P₂Oₐ content leads to less iron dissolution and improved battery discharge capacity and cycling performance.

[0121] It can be seen from Examples 5 to 8 that the temperature of the first sintering is different, and the specific surface area increase rate (SSA i -SSA0) / SSA0) is different. The smaller the specific surface area increase rate of lithium iron phosphate, the less iron dissolution, and the better the discharge capacity and cycle performance of the battery. This shows that the smaller the specific surface area increase rate of lithium iron phosphate, the better the uniformity of the particles, and the probability of cracking of lithium iron phosphate particles can be reduced during the battery cycle.

[0122] It can be seen from Examples 2, 3, 6-8 that when the lithium iron phosphate material simultaneously meets the following conditions: low Li4P2O7 content, small specific surface area increase rate, less iron dissolution, and the battery has a higher discharge capacity and cycle capacity retention rate.

[0123] It can be seen from Examples 9 to 12 that by controlling the heating rate and holding time during the first sintering, the generation of pyrophosphate can be reduced, the content of Li4P2O7 and iron dissolution can be lowered, and the discharge capacity and cycle performance of the battery can be improved.

[0124] It can be seen from Example 14 and Example 15 that by using different temperatures for the second sintering, batteries with high powder compaction density, high battery discharge capacity and good cycle performance can be obtained.

[0125] By the attached Figure 2 It can be seen that the material prepared in Example 2 is lithium iron phosphate material.

[0126] By the attached Figure 3 It can be seen that the lithium iron phosphate particles prepared in Example 2 are evenly distributed.

[0127] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A positive electrode active material, characterized in that Including Li y Fe a M b PO4, wherein 0.95≤a≤0.99, 0.001≤b≤0.1, 1<y≤1.08, M includes at least one of Ti, Na, K, Mg, V, W, Nb, La, Cr, Mo, Ca, Zn, Y, Zr, Sm, Co, Ni, B, Cu, and Gd, and the positive electrode active material includes Li4P2O7, and the mass proportion of the Li4P2O7 based on the total mass of the positive electrode active material is less than or equal to 0.3%, The positive electrode active material is compacted using a pressure i. The specific surface area of ​​the positive electrode active material before compaction is SSA0, and the specific surface area of ​​the positive electrode active material after compaction is SSA i , and satisfy: 0<(SSA i -SSA0) / SSA0≤(7.07ln(i)-5.9) / 100, where 6T≤i≤12T, K 95 =(Dv 95 -Dv5) / Dv 50 , and satisfy 0.1≤K 95 ≤7, wherein Dv5 is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 5%, Dv 50 Dv is the particle size corresponding to the volume distribution percentage of the positive electrode active material particle size reaching 50%, 95 is the particle size corresponding to when the volume distribution percentage of the positive electrode active material particle size reaches 95%, The molar ratio of the P element in the P source to the Fe element in the Fe source of the positive electrode active material is 1.015-1.

045.

2. The positive electrode active material according to claim 1, characterized in that Meet at least one of the following conditions: Under the condition of i=6T, (SSA 6T -SSA0) / SSA0≤6.5%; Under the condition of i=9T, (SSA 9T -SSA0) / SSA0≤9%; Under the condition of i=12T, (SSA 12T -SSA0) / SSA0≤11%.

3. The positive electrode active material according to claim 2, characterized in that 4.5≤K 95 ≤6。 4. The positive electrode active material according to any one of claims 1 to 3, characterized in that The iron dissolution of the positive electrode active material is less than or equal to 300 ppm.

5. The positive electrode active material according to claim 4, characterized in that The iron dissolution of the positive electrode active material is less than or equal to 150 ppm.

6. The positive electrode active material according to claim 4, characterized in that Meet at least one of the following conditions: Under the pressure of 3T, the compaction density of the positive electrode active material is 2.3g / cm 3 -2.6g / cm 3 ; The positive electrode active material has a resistance of 1 Ω∙cm-20 Ω∙cm.

7. The positive electrode active material according to claim 4, characterized in that At least a portion of the surface of the positive electrode active material has a carbon coating layer.

8. A method for preparing the positive electrode active material according to any one of claims 1 to 7, characterized in that: include: Mixing a Li source, a P source, an Fe source, an M source, and a solvent to obtain a slurry, wherein the molar ratio of the P element in the P source to the Fe element in the Fe source is 1.015-1.045, and grinding and drying the slurry to obtain precursor particles; The precursor particles are sequentially subjected to a first sintering and a second sintering, wherein the temperature of the first sintering is T1 and the temperature of the second sintering is T2, T1<T2, and then crushed to obtain the positive electrode active material.

9. The method according to claim 8, characterized in that The first sintering satisfies at least one of the following conditions: 300≤T1≤500℃； The holding time of the first sintering is t1, and satisfies 1h≤t1≤7h; The precursor particles are heated to T1 at a heating rate of 2° C. / min-3° C. / min.

10. The method according to claim 9, characterized in that 3h≤t1≤5h.

11. The method according to claim 8, characterized in that The second sintering satisfies at least one of the following conditions: 700≤T2≤900℃； The holding time of the second sintering is t2, and satisfies 5h≤t2≤15h; The heating rate from T1 to T2 is 0.5°C / min-3°C / min.

12. The method according to claim 11, characterized in that 7h≤t2≤10h.

13. The method according to claim 8, characterized in that Also includes: The Li source, the P source, the Fe source, the M source, the carbon source, and the solvent are mixed.

14. The method according to claim 13, characterized in that Meet at least one of the following conditions: The Li source includes at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium acetate, lithium oxalate, and lithium nitrate; The carbon source comprises at least one of glucose, sucrose, starch, cellulose, citric acid, oxalic acid, polyethylene glycol, polyvinyl alcohol, and polyethylene; The P source and the Fe source include iron phosphate; The M source includes M oxide.

15. A positive electrode plate, characterized in that: The invention comprises the positive electrode active material according to any one of claims 1 to 7 or the positive electrode active material prepared by the method according to any one of claims 8 to 14.

16. A battery, characterized in that: Including the positive electrode sheet according to claim 15.

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