Lithium iron phosphate battery and hybrid vehicle

Abstract

A lithium iron phosphate battery and a hybrid vehicle. The lithium iron phosphate battery comprises a first positive electrode active material and a second positive electrode active material having different median particle sizes, and further comprises a first negative electrode active material and a second negative electrode active material having different median particle sizes. The lithium iron phosphate battery has both large particle positive and negative electrode active materials, as well as small particle positive and negative electrode active materials; the positive and negative electrodes both utilize mixed large and small particle solutions, thus solving the defects of poor rate capability and polarization when large particle positive and negative active materials are used alone, and also solving the defects of poor cycle performance caused by excessive side reactions of small particle positive and negative active materials; the battery combines the advantages of large and small particle positive and negative active materials serving as the active materials of a lithium iron phosphate battery, and improves the rate capability of the lithium iron phosphate battery while also ensuring that the lithium iron phosphate battery has excellent cycle performance and high energy efficiency.

Description

Lithium iron phosphate battery and hybrid vehicle

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 20, 2023, with application number 2023117659000. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of battery technology, for example, a lithium iron phosphate battery, and more specifically, to a lithium iron phosphate battery and a hybrid vehicle. Background Art

[0003] Currently, new energy vehicles face challenges with a lack of recharging facilities, low recharging efficiency, and long recharging times, which are significant pain points for end users. To address these issues, hybrid vehicles have been introduced to the market, alleviating both range anxiety and recharging anxiety while also achieving the goal of reducing pollution and carbon emissions. Hybrid vehicles require high power and long life in their battery cells, but lithium iron phosphate cathode materials suffer from poor conductivity, slow lithium ion diffusion, low platform voltage, and poor cycle life during high-rate charge and discharge, making them unable to meet the technical requirements of hybrid heavy-duty trucks.

[0004] CN116216679A discloses a high-density lithium iron phosphate, a preparation method thereof, a positive electrode sheet and a lithium-ion battery. The preparation method includes: mixing iron phosphate, lithium carbonate and an organic carbon source, then grinding the mixture to 200-500 nm in a deionized water medium, then spray drying, sintering and finally performing air flow crushing to form large-sized lithium iron phosphate particles with a particle size of 600-1000 nm, wherein the organic carbon source is one or more of sucrose, polybutadiene, polyethylene glycol, etc., and the organic carbon source accounts for 6%-9%wt of the total weight of the mixture; mixing iron phosphate, lithium carbonate and an inorganic carbon source, then grinding the mixture to 100-300 nm in a deionized water medium, then spray drying, sintering and finally performing air flow crushing to form small-sized lithium iron phosphate particles with a particle size of 300-500 nm, wherein the inorganic carbon source is carbon black, acetylene black and a mixture thereof, and the inorganic carbon source accounts for 4%-6%wt of the total weight of the mixture; mixing the large-sized lithium iron phosphate particles and the small-sized lithium iron phosphate particles in a mass ratio of 1:1.5-2.5 to form a lithium iron phosphate positive electrode material.

[0005] CN114709486A discloses a lithium iron phosphate battery and its manufacturing method, which includes: obtaining material parameters; the material parameters include: high-capacity, high-density, and small-particle-size lithium iron phosphate and graphite, a high-porosity separator, an electrolyte with a high lithium salt concentration, and a combined conductive agent; determining the ratio of the positive and negative electrode sheets; obtaining key design parameters of the battery; and manufacturing the lithium iron phosphate battery based on the material parameters, the ratio of the positive and negative electrode sheets, and the key design parameters. This application can improve the battery's high-rate charge and discharge performance while maintaining a constant energy density, while also controlling the battery's temperature rise during high-rate discharge to a low level and improving the battery's cycle life. Technical issues

[0006] Currently available lithium iron phosphate batteries all have certain drawbacks, including the inability to achieve both high power and long life. Therefore, it is crucial to develop a lithium iron phosphate battery with both high power and long cycle life.

[0007] The present application provides a lithium iron phosphate battery and a hybrid vehicle. The lithium iron phosphate battery provided in the present application has both large-particle positive electrode active materials and negative electrode active materials, and small-particle positive electrode active materials and negative electrode active materials. Both the positive and negative electrodes adopt a scheme of mixing large and small particles. This not only solves the shortcomings of large-particle positive and negative active materials having poor rate performance and polarization when used alone, but also solves the shortcomings of small-particle positive and negative active materials having poor cycle performance due to excessive side reactions. It combines the advantages of large and small-particle positive and negative active materials as active materials for lithium iron phosphate batteries, and while improving the rate performance of lithium iron phosphate batteries, it also ensures that the lithium iron phosphate batteries have excellent cycle performance and high energy efficiency. Solution

[0008] In a first aspect, the present application provides a lithium iron phosphate battery, which includes a first positive electrode active material and a second positive electrode active material with different median particle sizes, and also includes a first negative electrode active material and a second negative electrode active material with different median particle sizes.

[0009] In a second aspect, the present application provides a hybrid vehicle, comprising the lithium iron phosphate battery described in the first aspect. Beneficial effects

[0010] Compared with the related art, this application has the following beneficial effects:

[0011] The lithium iron phosphate battery provided in the present application has both large-particle positive and negative active materials, as well as small-particle positive and negative active materials. Both the positive and negative electrodes adopt a scheme of mixing large and small particles. This not only solves the shortcomings of poor rate performance and polarization when large-particle positive and negative active materials are used alone, but also solves the shortcomings of poor cycle performance caused by excessive side reactions of small-particle positive and negative active materials. It combines the advantages of large and small-particle positive and negative active materials as active materials for lithium iron phosphate batteries. While improving the rate performance of lithium iron phosphate batteries, it also ensures that lithium iron phosphate batteries have excellent cycle performance and high energy efficiency.

[0012] Still other aspects will become apparent upon reading and understanding the detailed description. Modes for Carrying Out the Invention

[0013] In a first aspect, the present application provides a lithium iron phosphate battery, which includes a first positive electrode active material and a second positive electrode active material with different median particle sizes, and also includes a first negative electrode active material and a second negative electrode active material with different median particle sizes.

[0014] Positive and negative electrode active materials with large particles (larger median particle size) have the disadvantages of low capacity and large polarization, while positive and negative electrode active materials with small particles (smaller median particle size) have the disadvantages of more side reactions due to their large specific surface area, which affects the cycle performance.

[0015] The lithium iron phosphate battery provided in the present application has both large-particle positive and negative active materials, as well as small-particle positive and negative active materials. Both the positive and negative electrodes adopt a scheme of mixing large and small particles. This not only solves the shortcomings of poor rate performance and polarization when large-particle positive and negative active materials are used alone, but also solves the shortcomings of poor cycle performance caused by excessive side reactions of small-particle positive and negative active materials. It combines the advantages of large and small-particle positive and negative active materials as active materials for lithium iron phosphate batteries. While improving the rate performance of lithium iron phosphate batteries, it also ensures that lithium iron phosphate batteries have excellent cycle performance and high energy efficiency.

[0016] Optionally, the median particle size of the first positive electrode active material is larger than the median particle size of the second positive electrode active material.

[0017] Optionally, the median particle size of the first positive electrode active material is 0.8-1.6 μm, and the median particle size of the second positive electrode active material is 0.3-1 μm.

[0018] In the present application, the median particle size of the first positive electrode active material is 0.8~1.6μm, for example, it can be 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm or 1.6μm, but is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0019] In the present application, the median particle size of the second positive electrode active material is 0.3~1μm, for example, it can be 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm or 1μm, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0020] Optionally, the mass ratio of the first positive electrode active material to the second positive electrode active material in the lithium iron phosphate battery is (4~7):(3~6), for example, it can be 4:3:, 4:4:, 4:5:, 4:6:, 5:3:, 5:4:, 5:6:, 6:3, 6:4, 6:5, 7:3:, 7:4:, 7:5: or 7:6, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] Optionally, the median particle size of the first negative electrode active material is larger than the median particle size of the second negative electrode active material.

[0022] Optionally, the median particle size of the first negative electrode active material is 10-15 μm, and the median particle size of the second negative electrode active material is 5-8 μm.

[0023] In the present application, the median particle size of the first negative electrode active material is 10-15 μm, for example, it can be 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 13.5 μm, 14 μm, 14.5 μm or 15 μm, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0024] In the present application, the median particle size of the second negative electrode active material is 5 to 8 μm, for example, it can be 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm or 8 μm, but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0025] Optionally, the mass ratio of the first negative electrode active material to the second negative electrode active material in the lithium iron phosphate battery is (2~6):(4~8), for example, it can be 2:4, 2:5, 2:6, 2:7, 2:8, 3:4, 3:5, 3:7, 3:8, 4:4, 4:5, 4:6, 4:7, 5:4, 5:6, 5:7, 5:8, 6:4, 6:5, 6:7, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0026] Optionally, the lithium iron phosphate battery includes a positive electrode sheet, a negative electrode sheet and a separator;

[0027] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes the first positive electrode active material, the second positive electrode active material, a positive electrode conductive agent and a positive electrode binder;

[0028] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes the first negative electrode active material, the second negative electrode active material, a negative electrode conductive agent and a negative electrode binder.

[0029] Optionally, the mass ratio of the first positive electrode active material, the positive electrode conductor and the positive electrode binder is (38.8~67.9):(1.5~2.0):(1.0~1.5), for example, it can be 38.8:2.0:1.0, 67.9:1.5:1.5, 42.3:1.7:1.3, 51.7:1.9:1.4 or 62.8:1.6:1.2, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0030] Optionally, the mass ratio of the first negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (19.2~57.6):(0.5~1.0):(3.0~3.5), for example, it can be 19.2:0.5:3.5, 57.6:1.0:3, 22.5:0.6:3.1, 35.7:0.7:3.4, 43.8:0.9:3.1 or 54.8:0.8:3.3, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] Optionally, the first positive electrode active material and the second positive electrode active material include lithium iron phosphate.

[0032] Optionally, the first negative electrode active material and the second negative electrode active material include graphite.

[0033] Optionally, the positive electrode conductive agent and the negative electrode conductive agent independently include any one or a combination of at least two of conductive graphite, carbon nanotubes or carbon fibers. Typical but non-limiting combinations include a combination of conductive graphite and carbon nanotubes, a combination of carbon nanotubes and carbon fibers, or a combination of conductive graphite, carbon nanotubes and carbon fibers.

[0034] Optionally, the positive electrode binder includes PVDF (polyvinylidene fluoride) and / or PMMA (polymethyl methacrylate).

[0035] Optionally, the negative electrode binder includes any one of CMC (carboxymethyl cellulose), SBR (styrene-butadiene rubber), PAA (polyacrylic acid) or PTFE (polytetrafluoroethylene), or a combination of at least two thereof. Typical but non-limiting combinations include a combination of CMC and SBR, a combination of SBR and PAA, a combination of PAA and PTFE, or a combination of CMC, SBR and PAA.

[0036] Optionally, the diaphragm comprises a polyethylene diaphragm.

[0037] In a second aspect, the present application provides a hybrid vehicle, comprising the lithium iron phosphate battery described in the first aspect.

[0038] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application. Example 1

[0039] This embodiment provides a lithium iron phosphate battery, which includes a positive electrode sheet, a negative electrode sheet and a polyethylene separator;

[0040] The positive electrode plate includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes a first lithium iron phosphate with a median particle size of 1.15 μm, a second lithium iron phosphate with a median particle size of 0.5 μm, conductive graphite and PVDF, wherein the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 6:4, and the mass ratio of the first lithium iron phosphate to the conductive graphite to the PVDF is 42.3:1.7:1.3;

[0041] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes a first graphite with a median particle size of 12 μm, a second graphite with a median particle size of 6 μm, a conductive graphite and CMC. The mass ratio of the first graphite to the second graphite is 5:5, and the mass ratio of the first graphite, the conductive graphite and the CMC is 22.5:0.6:3.1. Example 2

[0042] This embodiment provides a lithium iron phosphate battery, which includes a positive electrode sheet, a negative electrode sheet and a polyethylene separator;

[0043] The positive electrode plate includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes a first lithium iron phosphate with a median particle size of 1.3 μm, a second lithium iron phosphate with a median particle size of 0.45 μm, carbon nanotubes, and PMMA, wherein the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 6:4, and the mass ratio of the first lithium iron phosphate to the carbon nanotubes to the PMMA is 51.7:1.9:1.4;

[0044] The negative electrode plate includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes a first graphite with a median particle size of 13.5 μm, a second graphite with a median particle size of 5.5 μm, carbon fiber and SBR. The mass ratio of the first graphite to the second graphite is 5:5, and the mass ratio of the first graphite carbon fiber to SBR is 35.7:0.7:3.4. Example 3

[0045] This embodiment provides a lithium iron phosphate battery, which includes a positive electrode sheet, a negative electrode sheet and a polyethylene separator;

[0046] The positive electrode plate includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes a first lithium iron phosphate with a median particle size of 0.8 μm, a second lithium iron phosphate with a median particle size of 0.3 μm, carbon fiber and PVDF, the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 4:6, and the mass ratio of the first lithium iron phosphate, carbon fiber and PVDF is 38.8:2.0:1.0;

[0047] The negative electrode plate includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes a first graphite with a median particle size of 10 μm, a second graphite with a median particle size of 5 μm, conductive graphite and PTFE. The mass ratio of the first graphite to the second graphite is 2:8, and the mass ratio of the first graphite, conductive graphite and PTFE is 19.2:0.5:3.5. Example 4

[0048] This embodiment provides a lithium iron phosphate battery, which includes a positive electrode sheet, a negative electrode sheet and a polyethylene separator;

[0049] The positive electrode plate includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes a first lithium iron phosphate with a median particle size of 1.6 μm, a second lithium iron phosphate with a median particle size of 1 μm, carbon nanotubes, and PMMA, wherein the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 7:3, and the mass ratio of the first lithium iron phosphate to the carbon nanotubes to the PMMA is 67.9:1.5:1.5;

[0050] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes a first graphite with a median particle size of 15 μm, a second graphite with a median particle size of 8 μm, conductive graphite and PAA. The mass ratio of the first graphite to the second graphite is 6:4, and the mass ratio of the first graphite, conductive graphite and PAA is 57.6:1.0:3. Example 5

[0051] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the first lithium iron phosphate is 0.6 μm. Example 6

[0052] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the first lithium iron phosphate is 2 μm. Example 7

[0053] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the second lithium iron phosphate is 0.1 μm. Example 8

[0054] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the second lithium iron phosphate is 1.1 μm. Example 9

[0055] This embodiment provides a lithium iron phosphate battery, which is the same as that of Embodiment 1 except that the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 2:6. Example 10

[0056] This embodiment provides a lithium iron phosphate battery, which is the same as that of Embodiment 1 except that the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate is 9:3. Example 11

[0057] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the first graphite is 8 μm. Example 12

[0058] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the first graphite is 20 μm. Example 13

[0059] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the second graphite is 3 μm. Example 14

[0060] This embodiment provides a lithium iron phosphate battery, which is the same as that of embodiment 1 except that the median particle size of the second graphite is 10 μm. Example 15

[0061] This embodiment provides a lithium iron phosphate battery, which is the same as that of Embodiment 1 except that the mass ratio of the first graphite to the second graphite is 1:8. Example 16

[0062] This embodiment provides a lithium iron phosphate battery, which is the same as that of Example 1 except that the mass ratio of the first graphite to the second graphite is 8:4. Comparative Example 1

[0063] This comparative example provides a lithium iron phosphate battery, which is the same as Example 1 except that the second lithium iron phosphate with a median particle size of 0.5 μm is replaced by the first lithium iron phosphate with a median particle size of 1.15 μm of the same mass. Comparative Example 2

[0064] This comparative example provides a lithium iron phosphate battery, which is the same as Example 1 except that the second graphite with a median particle size of 6 μm is replaced by the first graphite with a median particle size of 12 μm of the same mass. Comparative Example 3

[0065] This comparative example provides a lithium iron phosphate battery, which is the same as Example 1 except that the second lithium iron phosphate with a median particle size of 0.5 μm is replaced by the first lithium iron phosphate with a median particle size of 1.15 μm of equal mass, and the second graphite with a median particle size of 6 μm is replaced by the first graphite with a median particle size of 12 μm of equal mass.

[0066] The lithium iron phosphate batteries provided in Examples 1 to 16 and Comparative Examples 1 to 3 were subjected to rate performance tests, cycle performance tests, and energy efficiency tests;

[0067] The rate performance test method is as follows: at an ambient temperature of 25°C, the battery cell is charged at a certain rate at a constant current and constant voltage, with a cut-off voltage of 3.65V and a cut-off current of 0.05C. The maximum charging current that the lithium iron phosphate battery can withstand without lithium deposition is shown in Table 1.

[0068] The method for the cycle performance test is as follows: at an ambient temperature of 25°C, charge at a constant current and constant voltage rate of 1C, with a cut-off voltage of 3.65V and a cut-off current of 0.05C, and leave for 30 minutes; discharge at a constant current of 1C, with a cut-off voltage of 2.5V, and cycle 500 times according to the above test method to confirm that the battery cell capacity retention rate is as shown in Table 1;

[0069] The energy efficiency test method is as follows: at an ambient temperature of 25°C, the battery cell is charged at a constant current and constant voltage of 1C, with a cut-off voltage of 3.65V and a cut-off current of 0.05C, and then left for 30 minutes. The battery cell is discharged at a constant current of 1C and a cut-off voltage of 2.5V. The energy efficiency is calculated as follows: energy during discharge / energy during charging. The energy efficiency of the lithium iron phosphate battery when charged and discharged at the maximum charging current is shown in Table 1.

[0070] Table 1

[0071]

[0072] From Table 1, we can get:

[0073] (1) The lithium iron phosphate batteries provided in Examples 1 to 4 have a higher maximum charging current, a higher capacity retention rate, and a higher energy efficiency;

[0074] (2) By comparing Example 1 with Examples 5 and 6, it can be seen that the median particle size of the first positive electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the median particle size of the first positive electrode active material is smaller, the capacity retention rate of the lithium iron phosphate battery will decrease; this is because when the particle size of the positive electrode first active material is smaller, the side reactions increase, so the cycle performance deteriorates and the capacity retention rate decreases; when the median particle size of the first positive electrode active material is larger, the maximum charging current will decrease, which is because the difference between the median particle size of the first positive electrode active material and the particle size of the second positive electrode active material is too large, which leads to increased material polarization and a decrease in the overall electrical performance of the battery cell;

[0075] (3) By comparing Example 1 with Examples 7 and 8, it can be seen that the median particle size of the second positive electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the median particle size of the second positive electrode active material is too small, there is a problem that the large particles of the positive electrode active material are extremely large and the small particles are extremely small, and the material uniformity is poor, resulting in large material polarization and a decrease in the overall electrical performance of the lithium iron phosphate battery; when the median particle size of the second positive electrode active material is too large, it will lead to a decrease in the maximum charging current, poor cycle performance and reduced energy efficiency. This is because the difference between the median particle size of the second positive electrode active material and the particle size of the first positive electrode active material is too small, and the positive electrode active materials are all particles with larger particle sizes, which makes it difficult to solve the problem of poor rate performance when using large-particle positive electrode active materials alone;

[0076] (4) By comparing Example 1 with Examples 9 and 10, it can be seen that the mass ratio of the first positive electrode active material to the second positive electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the mass ratio of the first positive electrode active material to the second positive electrode active material is low, the capacity retention rate decreases and the energy efficiency decreases. This is because the proportion of small particles increases. Small particles have a small particle size and a large specific surface area, and the contact area with the electrolyte is large, so the side reactions increase, resulting in a decrease in cycle performance and a decrease in capacity retention rate; when the mass ratio of the first positive electrode active material to the second positive electrode active material is high, the maximum charging current will decrease. This is because the number of large particles increases, the particle size of the large particle active material is large, and the lithium ion transmission path is long during charging and discharging, so the rate performance decreases;

[0077] (5) By comparing Example 1 with Examples 11 and 12, it can be seen that the median particle size of the first negative electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the median particle size of the first negative electrode active material is too small, the capacity retention rate will decrease, the energy efficiency will decrease, and the first efficiency will decrease. This is because the negative electrode particles are reduced, the overall particle size of the negative electrode active material is reduced, the specific surface area is increased, the lithium ions used to form the SEI film during the first charge increase, and the reversible active lithium ions decrease, so the first efficiency decreases and the energy efficiency also decreases accordingly; when the median particle size of the first negative electrode active material is too large, the maximum charging current will decrease. This is because the negative electrode particles increase, which is the same as the positive electrode, resulting in a decrease in rate performance;

[0078] (6) By comparing Example 1 with Examples 13 and 14, it can be seen that the median particle size of the first negative electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the median particle size of the second negative electrode active material is too small, the capacity retention rate will be reduced and the energy efficiency will be reduced. This is because the number of small particles increases, and the active lithium ions used to form the SEI film on the negative electrode surface during the first charge and discharge are larger, and the reversible lithium ions decrease, so the first efficiency is reduced and the energy efficiency is reduced. Due to the increase in side reactions, the cycle performance decreases and the capacity retention rate is reduced; when the median particle size of the second negative positive electrode active material is too large, the maximum charging current will be reduced, the cycle performance will be worse, and the energy efficiency will be reduced. This is because the difference between the median particle size of the second negative positive electrode active material and the particle size of the first negative electrode active material is too small, and the negative electrode active materials are all particles with larger particle sizes, which makes it difficult to solve the problem of poor rate performance when using large-particle negative electrode active materials alone;

[0079] (7) By comparing Example 1 with Examples 15 and 16, it can be seen that the mass ratio of the first negative electrode active material to the second negative electrode active material in the present application will affect the performance of the lithium iron phosphate battery; when the mass ratio of the first negative electrode active material to the second negative electrode active material is low, the maximum charging current will be reduced, the capacity retention rate will be reduced, and the energy efficiency will remain unchanged. This is because the number of small particles in the negative electrode increases, the lithium ion transmission path is shortened during the charge and discharge process, but large particles still exist, resulting in a large difference between D99 and D10, so the rate performance does not increase but decreases. At the same time, as the number of small particles increases, the side reactions increase, so During the cycle, the capacity retention rate decreases and the energy efficiency is close. When the mass ratio of the first negative electrode active material to the second negative electrode active material is too high, the maximum charge rate will drop significantly, the capacity retention rate will be close, and the energy efficiency will decrease. This is because the proportion of large particles is too large, and the lithium ion transmission path increases during the charge and discharge process. Therefore, the maximum rate charging performance will drop significantly. However, as the number of large particles increases, the specific surface area of ​​the material decreases, and the contact area with the electrolyte decreases, so the side reactions are reduced, and the capacity retention rate is close to that of Example 1. However, as the number of large particles increases, the polarization increases, and the temperature rise of the battery cell increases, so the energy efficiency decreases.

[0080] (8) By comparing Example 1 with Comparative Examples 1 to 3, it can be seen that the lithium iron phosphate battery provided in the present application has both large-particle positive electrode active materials and negative electrode active materials, and small-particle positive electrode active materials and negative electrode active materials. Both the positive and negative electrodes adopt a scheme of mixing large and small particles. This not only solves the shortcomings of large-particle positive and negative active materials having poor rate performance and polarization when used alone, but also solves the shortcomings of small-particle positive and negative active materials having poor cycle performance due to excessive side reactions. It combines the advantages of large and small-particle positive and negative active materials as active materials for lithium iron phosphate batteries, and while improving the rate performance of lithium iron phosphate batteries, it also ensures that lithium iron phosphate batteries have excellent cycle performance and high energy efficiency.

Claims

1. A lithium iron phosphate battery, comprising a first positive electrode active material and a second positive electrode active material having different median particle sizes, and further comprising a first negative electrode active material and a second negative electrode active material having different median particle sizes.

2. The lithium iron phosphate battery according to claim 1, wherein: The median particle size of the first positive electrode active material is larger than the median particle size of the second positive electrode active material.

3. The lithium iron phosphate battery according to claim 1 or 2, wherein: The median particle size of the first positive electrode active material is 0.8-1.6 μm, and the median particle size of the second positive electrode active material is 0.3-1 μm.

4. The lithium iron phosphate battery according to any one of claims 1 to 3, wherein: The mass ratio of the first positive electrode active material to the second positive electrode active material in the lithium iron phosphate battery is (4-7):(3-6).

5. The lithium iron phosphate battery according to any one of claims 1 to 4, wherein: The median particle size of the first negative electrode active material is greater than the median particle size of the second negative electrode active material.

6. The lithium iron phosphate battery according to any one of claims 1 to 5, wherein: The median particle size of the first negative electrode active material is 10-15 μm, and the median particle size of the second negative electrode active material is 5-8 μm.

7. The lithium iron phosphate battery according to any one of claims 1 to 6, wherein: The mass ratio of the first negative electrode active material to the second negative electrode active material in the lithium iron phosphate battery is (2-6):(4-8).

8. The lithium iron phosphate battery according to any one of claims 1 to 7, wherein: The lithium iron phosphate battery comprises a positive electrode sheet, a negative electrode sheet and a separator; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material on the surface of the positive electrode current collector, wherein the positive electrode active material includes the first positive electrode active material, the second positive electrode active material, a positive electrode conductive agent and a positive electrode adhesive; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material on the surface of the negative electrode current collector. The negative electrode active material includes the first negative electrode active material, the second negative electrode active material, a negative electrode conductive agent and a negative electrode adhesive.

9. The lithium iron phosphate battery according to claim 8, wherein: The mass ratio of the first positive electrode active material, the positive electrode conductive agent and the positive electrode binder is (38.8~67.9):(1.5~2.0):(1.0~1.5).

10. The lithium iron phosphate battery according to claim 8 or 9, wherein: The mass ratio of the first negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (19.2~57.6):(0.5~1.0):(3.0~3.5).

11. The lithium iron phosphate battery according to any one of claims 1 to 10, wherein: The first positive electrode active material and the second positive electrode active material include lithium iron phosphate.

12. The lithium iron phosphate battery according to any one of claims 1 to 11, wherein: The first negative electrode active material and the second negative electrode active material include graphite.

13. The lithium iron phosphate battery according to any one of claims 8 to 10, wherein: The positive electrode conductive agent and the negative electrode conductive agent independently include any one of conductive graphite, carbon nanotubes or carbon fibers, or a combination of at least two of them.

14. The lithium iron phosphate battery according to any one of claims 8 to 10 or claim 13, wherein: The positive electrode adhesive includes PVDF and / or PMMA; Optionally, the negative electrode binder includes any one of CMC, SBR, PAA or PTFE or a combination of at least two thereof; Optionally, the diaphragm comprises a polyethylene diaphragm.

15. A hybrid vehicle, wherein: The hybrid vehicle comprises the lithium iron phosphate battery according to any one of claims 1-14.

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