Positive plate and lithium ion battery applying same

Abstract

The invention provides a positive plate and a lithium ion battery using the same, the positive plate comprises a current collector and a positive active coating containing a positive active material, the positive active material comprises at least three of particles N1, particles N2, particles N3, particles N4 and particles N5, the particle size of the particles N1 is not more than 100 nm, the particle size of the particles N2 is 100-150 nm, the particle size of the particles N3 is 100-150 nm, the particle size of the particles N4 is 100-150 nm, and the particle size of the particles N5 is 100-150 nm. The particle N3 is the positive electrode active material with the particle size of 150-250nm, the particle N4 is the positive electrode active material with the particle size of 250-500nm, and the particle N5 is the positive electrode active material with the particle size of more than 500nm; in the positive electrode active material, according to the mass ratio, the ratio of the particle N1 is X1, the ratio of the particle N2 is X2, the ratio of the particle N3 is X3, the ratio of the particle N4 is X4, and the ratio of the particle N5 is X5, the X1, the X2, the X3, the X4 and the X5 are subjected to power normalization processing to obtain a normalization function, and R2 corresponding to the normalization function is larger than or equal to 0.9. The positive plate has relatively high compaction density and excellent lithium ion transmission performance at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and specifically relates to a positive electrode sheet and a lithium ion battery using the same. Background Art

[0002] Lithium ion batteries have the advantages of high power density, high energy density, long cycle life, low cost and safety, and are considered as ideal choices for portable electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). With the progress of technology and the expansion of application scenarios, battery manufacturers' requirements for battery endurance have also increased. By increasing the compaction density of the electrode sheet, the ineffective space in the electrode sheet can be reduced, enabling the battery to accommodate more active materials in the same volume, thereby improving the energy density of the battery. A battery with a higher energy density can store more electrical energy in the same volume, which means a longer cycle life of the battery. The electric vehicle using the battery can travel a longer distance after a single charge, reducing the number of charging times and improving practicality and convenience.

[0003] However, if the compaction density of the electrode sheet is too high, it will lead to an increase in the contact resistance between active material particles and a reduction in lithium ion diffusion channels. As a result, the transmission path of lithium ions inside the electrode sheet will become unsmooth, thus affecting the charge and discharge performance of the battery, manifested as a decrease in battery capacity, an increase in internal resistance, and a decline in cycle stability.

[0004] Therefore, it is necessary to develop a positive electrode sheet that can maintain a high lithium shuttle and has a high compaction density. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a positive electrode sheet and a lithium ion battery using the same, which simultaneously have a high compaction density and excellent lithium ion transmission performance.

[0006] According to one aspect of the present invention, a positive electrode sheet is provided, which includes a current collector and a positive electrode active coating containing a positive electrode active material. The positive electrode active material includes at least three of particles N1, N2, N3, N4, and N5. Particle N1 is a positive electrode active material with a particle size not exceeding 100 nm, particle N2 is a positive electrode active material with a particle size reaching 100 - 150 nm, particle N3 is a positive electrode active material with a particle size reaching 150 - 250 nm, particle N4 is the above positive electrode active material with a particle size reaching 250 - 500 nm, and particle N5 is a positive electrode active material with a particle size exceeding 500 nm; in the positive electrode active material, calculated by mass ratio, the proportion of particle N1 is X1, the proportion of particle N2 is X2, the proportion of particle N3 is X3, the proportion of particle N4 is X4, and the proportion of particle N5 is X5. Power normalization processing is performed on X1, X2, X3, X4, and X5 to obtain a normalization function, and the R corresponding to the normalization function 2 ≥0.9.

[0007] By comprehensively regulating the mass ratios X1 - X5 of particles N1 - N5 in different particle size ranges in the positive electrode active material, so that the coefficient of determination R corresponding to the normalization function obtained by normalizing X1 - X5 2≥0.9, thus effectively optimizing the packing mode of the cathode active material, simultaneously achieving a high tap density of the cathode sheet and increasing the migration speed of lithium ions, thereby enhancing the energy density and cycle stability of the battery. On the one hand, the mass ratios of particles N1, N2, N3, N4, and N5 in a specific particle size range are comprehensively regulated so that the cathode active material with a smaller particle size can fill the pores between the larger particle sizes, effectively improving the tap density of the cathode sheet, enabling the cathode sheet to have more active substances per unit volume, increasing the volumetric energy density of the battery using this cathode sheet, and extending the cycle life of the battery. On the other hand, the cathode active coating including the above-mentioned particles N1, N2, N3, N4, and N5 not only has a high tap density, but also the electrolyte can infiltrate into the pores between the cathode active material particles, ensuring smooth transmission paths for lithium ions in the cathode sheet, facilitating the reduction of the internal transmission impedance of the cathode sheet, increasing the lithium ion insertion / extraction speed, and improving the rate performance and cycle stability of the battery using this cathode sheet. Specifically, particles N1, N2, N3, N4, and N5 are respectively marked with 1, 2, 3, 4, and 5 as particle size classification identifiers. This normalization method uses the particle size classification identifier as the abscissa, the mass ratio of the particles in the cathode active material as the ordinate, and a power function as the function model for fitting, that is, fitting with the point values of (X1, 1), (X2, 2), (X3, 3), (X4, 4), and (X5, 5) respectively. When it is necessary to test the particle size distribution of the cathode active material in the cathode electrode sheet, methods such as laser scattering method or dynamic particle imaging analysis system can be used. Taking the laser scattering method as an example, its principle is to disperse the particle sample at an appropriate concentration in a suitable liquid or gas, make it pass through a monochromatic light beam, and when the light encounters the particles, it scatters at different angles, and the scattered light is measured by a multi-element detector. Through appropriate optical models and mathematical processes, these quantified scattering data are converted to obtain the particle size volume distribution.

[0008] Preferably, the normalization equation satisfies the coefficient of determination R 2 ≥0.99. When the particle size range and mass ratio of the cathode active coating particles make the above normalization equation satisfy the coefficient of determination R 2 ≥0.99, the particle size grading of the cathode active material particles is conducive to improving the efficient transmission of lithium ions, optimizing the transmission path of lithium ions in the cathode sheet, thereby enhancing the lithium ion migration efficiency, and being able to further improve the tap density of the cathode sheet while ensuring smooth lithium ion transmission.

[0009] Preferably, let Y represent the normalization function, Y = aX 3 +bX 2+cX + d; Y satisfies that at least one of a, b, and c is not 0. Compared with the positive electrode sheet processed by using other normalization functions, the positive electrode sheet that fits X1 to X5 by using a normalization function with specific coefficients in the above manner has a higher tap density, and the contact between the positive active material particles is closer. It can not only improve the mechanical stability and cycle life of the positive electrode sheet, but also help to improve the energy density of the battery, optimize the cycle life of the battery, and improve the portability and practicality of the battery device.

[0010] Preferably, in the normalization function, the value range of a is -0.1 to 0.1, the value range of b is -0.5 to 0.5, the value range of c is -1.0 to 1.0, and the value range of d is -0.97 to 0.97. When the particle size distribution of the positive active material particles satisfies that the value ranges of the coefficients a to e of the normalization function respectively fall into the above ranges, the positive electrode sheet has a high tap density and can achieve higher energy storage under the same volume, thereby prolonging the battery cycle life.

[0011] Preferably, the positive active material includes at least four of particle N1, particle N2, particle N3, particle N4, and particle N5. Selecting at least four of N1 to N5 as the positive active material can more flexibly provide a positive electrode sheet with a high tap density.

[0012] Preferably, calculated by mass percentage, the total proportion of particle N1, particle N2, particle N3, and particle N4 in the positive active material is not less than 60%. By regulating the total mass proportion of N1 to N4 in the positive active material, it is possible to provide a larger contact area for the interfacial reaction between the positive active material and the electrolyte on the premise of ensuring a high tap density of the positive electrode sheet, promote the rapid diffusion of lithium ions, improve the ion transport rate, and help to improve the rate performance and cycle stability of the battery using this positive electrode sheet.

[0013] Preferably, the positive active material includes particle N1, particle N2, particle N3, and particle N4.

[0014] Preferably, calculated by mass percentage: the proportion X1 of particle N1 in the positive electrode active material is 5% to 15%; the proportion X2 of particle N2 in the positive electrode active material is 20% to 40%; the proportion X3 of particle N3 in the positive electrode active material is 25% to 40%; the proportion X4 of particle N4 in the positive electrode active material is 10% to 40%; the proportion X5 of particle N5 in the positive electrode active material is 0% to 15%. When the mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material satisfy the above value ranges, the compaction density of the positive electrode sheet can be increased without sacrificing the lithium-ion transmission efficiency. It can not only optimize the migration path of lithium ions in the positive electrode sheet, reduce the obstacles in the lithium-ion migration path, improve the rate performance and long-term cycle stability of the battery, but also enhance the energy density and power output performance of the battery, and improve the overall performance of the battery.

[0015] Preferably, the positive electrode active material includes a phosphate material, and the general formula of the phosphate material is Li 1+x Fe y M z P α O β , where -0.05 ≤ x ≤ 0.15; 0.95 ≤ y + z ≤ 1.0, y ≠ 0, 0.98 ≤ α ≤ 1.0, 3.95 ≤ β ≤ 4.0, and M includes at least one of Mn, Al, Mg, Ti, and Na. The above phosphate material has good thermal stability, high safety, and long cycle life. Since the phosphate material was developed for use in lithium battery positive electrode materials, good results have been achieved in the field of positive electrode battery materials. However, commercially available phosphate materials have the problem of low energy density. Using the phosphate material to prepare the positive electrode sheet according to the above particle size grading principle, the positive electrode sheet not only has the advantages of the above lithium positive electrode active material, but also overcomes the defect of low volumetric energy density. The battery using this positive electrode sheet can not only maintain cycle stability in high-temperature environments or other extreme conditions, but also has a large discharge capacity, strong endurance, and high cycle retention rate.

[0016] Preferably, the positive electrode active material includes a phosphate composite material, and the phosphate composite material includes a core containing a phosphate material and an outer coating layer covering the core. The outer coating layer contains Me element and / or C element, where the Me element is selected from at least one of Fe element, Mn element, Al element, Mg element, Ti element, and Na element.

[0017] The above phosphate composite material has more lithium-ion storage sites, good structural stability, and is not easily eroded by the electrolyte. The positive electrode sheet using this phosphate composite material has higher ionic conductivity and electrical conductivity, and the battery using this positive electrode sheet has excellent cycle life and rate performance.

[0018] Preferably, the mass ratio of the outer coating layer in the phosphate composite material is not higher than 3 wt%.

[0019] Preferably, when the outer coating layer contains C element, the outer coating layer comprises a carbon material, and the graphitization degree of the carbon material is 8-12%.

[0020] Preferably, the mass ratio of the carbon material in the positive electrode active material is ≤ 10 wt%.

[0021] Preferably, when the outer coating layer contains Me element, the mass ratio of the Me element in the positive electrode active material is ≤ 1 wt%.

[0022] According to another aspect of the present invention, there is provided a lithium ion battery, which includes the above positive electrode sheet.

[0023] Preferably, the tap density of the positive electrode active coating is 2.65-2.75 g / cm 3 . The lithium ion battery using the positive electrode sheet meeting the above requirements has excellent rate performance and cycle capacity retention rate, and the lithium ion battery has small polarization and is not easily attenuated during the cycling process. Specific embodiments

[0024] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0025] Example 1

[0026] The present invention provides a positive electrode sheet, which includes a current collector and a positive electrode active coating. The positive electrode active coating includes a positive electrode active material, and the positive electrode active material is selected as lithium iron phosphate (LFP), and the tap density of the positive electrode active coating is 2.70.

[0027] In this embodiment, the positive electrode active material includes particles N1, N2, N3, N4, and N5. Among them, particle N1 is a positive electrode active material with a particle size not exceeding 100 nm, particle N2 is a positive electrode active material with a particle size reaching 100-150 nm, particle N3 is a positive electrode active material with a particle size reaching 150-250 nm, particle N4 is the above positive electrode active material with a particle size reaching 250-500 nm, and particle N5 is the above positive electrode active material with a particle size exceeding 500 nm.

[0028] In the positive electrode active material, by mass ratio, the proportion of particle N1 is X1, the proportion of particle N2 is X2, the proportion of particle N3 is X3, the proportion of particle N4 is X4, and the proportion of particle N5 is X5. X1, X2, X3, X4, and X5 are subjected to power normalization to obtain a normalization function. Among them, X1 = 6%, X2 = 22%, X3 = 32%, X4 = 28%, X5 = 12%, such that X1 + X2 + X3 + X4 = 95% ≥ 60%.

[0029] Let Y represent the normalization function, Y = aX 3 + bX 2 + cX + d, where a = -0.005, b = -0.011, c = 0.234, d = -0.160, and the coefficient of determination R 2 = 0.997 ≥ 0.99.

[0030] In other embodiments, according to the actual situation, the type of the positive electrode material can be adjusted. A phosphate material with the general formula Li 1+ x Fe y M z P α O β can be selected, where -0.05 ≤ x ≤ 0.15; 0.95 ≤ y + z ≤ 1.0, y ≠ 0, 0.98 ≤ α ≤ 1.0, 3.95 ≤ β ≤ 4.0, and M includes at least one of Mn, Al, Mg, Ti, and Na. Or a phosphate composite material can be selected. The phosphate composite material includes a core containing a phosphate material and an outer coating layer covering the core. The outer coating layer contains Me element and / or C element, where the Me element is selected from at least one of Fe element, Mn element, Al element, Mg element, Ti element, and Na element.

[0031] Preparation of the electrode sheet: Using the above positive electrode active material, using carbon black as the conductive agent and polyvinylidene fluoride as the binder, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active coating is 97:1.5:2. The above positive electrode active material is sequentially mixed with a solvent. The positive electrode active material includes particles N1, N2, N3, N4, and N5. Then, the conductive agent and the binder are added to the solvent, and they are mixed evenly at a rotation speed of 1000 ± 100 in a stirring tank. A solvent is added to prepare a positive electrode slurry. The viscosity of the discharged positive electrode slurry is controlled to be 10000 ± 500, and the solid content is 70 - 80%. A positive electrode slurry is formed. Then, the positive electrode slurry is coated on an aluminum foil and vacuum dried at 120°C for 5 minutes, and cold pressed at 25 ± 5°C to obtain a positive electrode sheet.

[0032] Example 2

[0033] This example refers to the preparation method provided in Example 1 to prepare a positive electrode sheet. The difference between this example and Example 1 is as follows: The mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted so that X1 = 7%, X2 = 22.5%, X3 = 34%, X4 = 22.5%, and X5 = 14%. Then, the normalization function Y = aX obtained by power normalization processing of X1, X2, X3, X4, and X5 3 + bX 2 + cX + d, where a = 0.006, b = -0.103, c = 0.456, d = -0.295, and the corresponding coefficient of determination satisfies: R 2 = 0.93 ≥ 0.9. The other raw material ratios and preparation methods are strictly the same as those in Example 1.

[0034] Example 3

[0035] This example refers to the preparation method provided in Example 1 to prepare a positive electrode sheet. The difference between this example and Example 1 is as follows: The mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted so that X1 = 6%, X2 = 22%, X3 = 32%, X4 = 28%, and X5 = 12%. Then, the functional formula of the normalization function Y obtained by power normalization processing of X1, X2, X3, X4, and X5 is: Y = aX 3 + bX 2 + cX + d, where a = 0, b = -0.056, c = 0.352, d = -0.244. The other raw material ratios and preparation methods are strictly the same as those in Example 1. In particular, the normalization function satisfies its corresponding R 2 = 0.989 ≥ 0.9.

[0036] Example 4

[0037] This example refers to the preparation method provided in Example 1 to prepare a positive electrode sheet. The difference between this example and Example 1 is as follows: The mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted so that X1 = 13%, X2 = 35%, X3 = 35%, X4 = 17%, and X5 = 0%. Then, the functional formula of the normalization function Y obtained by power normalization processing of X1, X2, X3, X4, and X5 is: Y = aX 3 + bX 2 + cX + d, where a = 0.019, b = -0.241, c = 0.82, d = -0.47. The other raw material ratios and preparation methods are strictly the same as those in Example 1. In particular, the normalization function satisfies its corresponding R 2 = 0.997 ≥ 0.9.

[0038] Example 5

[0039] In this example, referring to the preparation method provided in Example 1, a positive electrode sheet is prepared. The difference between this example and Example 1 is that: the mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted such that X1 = 0%, X2 = 23%, X3 = 35%, X4 = 30%, and X5 = 12%. Then the functional formula of the normalized function Y obtained by power normalization of X1, X2, X3, X4, and X5 is: Y = aX 3 + bX 2 + cX + d, where a = -0.002, b = -0.056, c = 0.416, and d = -0.36. The other raw material ratios and preparation methods are strictly the same as those in Example 1. In particular, the normalized function satisfies its corresponding R 2 = 0.998 ≥ 0.9.

[0040] Example 6

[0041] In this example, referring to the preparation method provided in Example 1, a positive electrode sheet is prepared. The difference between this example and Example 1 is that: the mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted such that X1 = 0%, X2 = 26%, X3 = 40%, X4 = 34%, and X5 = 0%. Then the functional formula of the normalized function Y obtained by power normalization of X1, X2, X3, X4, and X5 is: Y = aX 3 + bX 2 + cX + d, where a = 0, b = -0.1, c = 0.64, and d = -0.62. The other raw material ratios and preparation methods are strictly the same as those in Example 1. In particular, the normalized function satisfies its corresponding R 2 = 0.999 ≥ 0.9.

[0042] Example 7

[0043] In this example, referring to the preparation method provided in Example 1, a positive electrode sheet is prepared. The difference between this example and Example 1 is that: the mass ratios of particles N1, N2, N3, N4, and N5 in the positive electrode active material are adjusted such that X1 = 20%, X2 = 20%, X3 = 20%, X4 = 20%, and X5 = 20%. At this time, the functional formula of the normalized function Y is: Y = 0.2. The other raw material ratios and preparation methods are strictly the same as those in Example 1. In particular, the normalized function satisfies its corresponding R 2 = 1 ≥ 0.9.

[0044] Example 8

[0045] This example refers to the preparation method provided in Example 1 to prepare a positive electrode sheet. The difference between this example and Example 1 is that lithium iron manganese phosphate (LMFP) with equal particle size and equal mass ratio is used to replace the positive electrode active material in Example 1. The other raw material ratios and preparation methods are strictly the same as those in Example 1, especially the normalization function satisfies its corresponding R 2 ≥0.9.

[0046] Example 9

[0047] This example refers to the preparation method provided in Example 7 to prepare a positive electrode sheet. The difference between this example and Example 7 is that lithium iron manganese phosphate (LMFP) with equal particle size and equal mass ratio is used to replace the positive electrode active material in Example 7. The other raw material ratios and preparation methods are strictly the same as those in Example 7, especially the normalization function satisfies its corresponding R 2 ≥0.9.

[0048] Comparative Example 1

[0049] This comparative example refers to the preparation method provided in Example 1 to prepare a positive electrode sheet. The difference between this comparative example and Example 1 is that in this comparative example, the positive electrode active material includes particles N1, N2, N3, N4, N5, and N6. Among them, particle N1 is a positive electrode active material with a particle size not exceeding 100 nm, particle N2 is a positive electrode active material with a particle size reaching 100 - 150 nm, particle N3 is a positive electrode active material with a particle size reaching 150 - 250 nm, particle N4 is the above-mentioned positive electrode active material with a particle size reaching 250 - 500 nm, particle N5 is the above-mentioned positive electrode active material with a particle size reaching 500 - 1000 nm, and particle N6 is a positive electrode active material with a particle size exceeding 1000 nm. In the positive electrode active material, calculated by mass ratio, the proportion of particle N1 is X1, the proportion of particle N2 is X2, the proportion of particle N3 is X3, the proportion of particle N4 is X4, the proportion of particle N5 is X5, and the proportion of particle N6 is X6, such that X1 = 1%, X2 = 5%, X3 = 22%, X4 = 12%, X5 = 15%, X6 = 45%. Then the functional formula of the normalization function Y obtained by power normalization of X1, X2, X3, X4, X5, and X6 is Y = aX 3 + bX 2 + cX + d, where a = 0.018, b = -0.172, c = 0.534, d = -0.393. To make the coefficient of determination corresponding to the normalization function obtained by power normalization of X1, X2, X3, X4, X5, and X6 satisfy: R 2 = 0.880 < 0.9. The other raw material ratios and preparation methods are strictly the same as those in Example 1.

[0050] Preparation Example

[0051] (1) Positive electrode sheet

[0052] The positive electrode sheets provided in Examples 1-9 and Comparative Example 1 above were respectively used.

[0053] (2) Preparation of negative electrode sheet

[0054] Graphite was selected as the negative electrode active material for the negative electrode, SP and CNT were used as conductive agents, and CMC, SBR, and PAA were used as binders to prepare the negative electrode sheet. The above-mentioned negative electrode active material, conductive agent, and binder were respectively mixed according to a mass ratio of 96:2:2 to form a negative electrode slurry, and then the negative electrode slurry was coated on a copper foil and vacuum-dried at 90 °C for 5 min to obtain the negative electrode sheet.

[0055] (3) Preparation of lithium-ion battery

[0056] A PE+OBS separator was selected. The positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, and the battery cell was obtained through the stacking process. The battery cell was placed in an outer packaging case, dried, and an organic electrolyte containing lithium ions was injected through the liquid injection hole. After vacuum packaging, standing, formation, and grading processes, a lithium-ion battery was obtained.

[0057] Test Example 1

[0058] Test objects: Lithium-ion batteries were respectively prepared using the positive electrode sheets provided in Examples 1-9 and Comparative Example 1. The specific preparation method can be seen in the preparation examples.

[0059] Test items and test methods:

[0060] (1) Compaction density: Using Sansi Zongheng equipment, 0.8 g of the positive electrode active material sample was weighed and loaded into a mold, and a compaction test was carried out with a force of 3 T, and the powder compaction density was recorded. The thickness and weight of the positive electrode sheet were measured, and the compaction density of the electrode sheet was calculated.

[0061] (2) 25 °C 1C cycle test: A lithium-ion battery was prepared using the above test objects as the positive electrode active material. At 25 °C, the lithium-ion battery was charged at a constant current and constant voltage to 2.5 V-3.65 V (LFP) / 2.5-4.2 V (LMFP) at a 1C current, the cut-off current was 0.05C, and it was left standing for 10 min.

[0062] Then it was discharged at a constant current of 1C to 2.5 V, and charged at a constant current and constant voltage to 3.65 V (LFP) / 4.2 V (LMFP) at a 1C current at 25 °C, the cut-off current was 0.05C, and it was left standing for 10 min. This was used as 1 charge-discharge cycle; the number of cycles when the capacity of the lithium-ion battery decreased to 80% of the initial capacity after cycling was recorded.

[0063] (3) 1C discharge specific capacity: Charge the lithium-ion battery at a constant current of 1C at 25°C with constant current and constant voltage to 2.5V - 3.65V (LFP) / 2.5 - 4.2V (LMFP), with a cut-off current of 0.05C, and let it stand for 10 min; then discharge it at a constant current of 1C to 2.5V, and record its discharge capacity Q. 1C Initially, it is used as the initial discharge capacity. Then, charge the battery at a constant current of 1C at 25°C with constant current and constant voltage to 3.65V (LFP) / 4.2V (LMFP), with a cut-off current of 0.05C, and let it stand for 10 min; then discharge the fully charged battery at a constant current of 1C to 2.5V, and record its discharge capacity Q. 1C Then, based on the discharge capacity Q 1C and the thickness of the positive electrode sheet, calculate the 1C energy density (Wh / L) of the battery cell.

[0064] Test results: The particle size parameters and function parameters of each test object are shown in Table 1, and the test results are shown in Table 2.

[0065] Table 1. Particle size parameters and function parameters of each test object

[0066] Group X1 X2 X3 X4 X5 a b c d <![CDATA[R 2 > Example 1 6 22 32 28 12 -0.005 -0.011 0.234 -0.16 0.997 Example 2 7 22.5 34 22.5 14 0.006 -0.103 0.456 -0.295 0.93 Example 3 6 22 32 28 12 0 -0.056 0.352 -0.244 0.989 Example 4 13 35 35 17 0 0.019 -0.241 0.82 -0.47 0.997 Example 5 0 23 35 30 12 -0.002 -0.056 0.416 -0.36 0.998 Example 6 0 26 40 34 0 0 -0.1 0.64 -0.62 0.999 Example 7 20 20 20 20 20 0 0 0 0.2 1 Example 8 6 22 32 28 12 -0.005 -0.011 0.234 -0.16 0.997 Example 9 20 20 20 20 20 0 0 0 0.2 1 Comparative Example 1 1 5 22 12 15 0.018 -0.172 0.534 -0.393 0.880

[0067] Table 2. Test results measured for each test object

[0068]

[0069] Result analysis:

[0070] Compare the positive electrode sheets and the test performance of the lithium-ion batteries provided in Examples 1 - 9 with those in Comparative Example 1 in Table 2. It can be found that, compared with the lithium-ion battery in Comparative Example 1, the lithium-ion batteries provided in Examples 1 - 9 have excellent energy density and better cycle stability. This is because the positive electrode active material in the positive electrode sheets provided in Examples 1 - 9 has good tap density, and the migration speed of lithium ions in the positive electrode sheets is fast.

[0071] Compare the performance data provided in Example 1, Examples 4 - 6 with those in Comparative Example 1. It can be found that the 25°C - 1C cycle number @80% SOC of the lithium-ion batteries provided in Example 1 and Examples 4 - 6 is greater than that of the lithium-ion battery provided in Comparative Example 1. This is because the positive electrode active material in the positive electrode sheets in Example 1 and Examples 4 - 6 includes particles N1 - N5, and after power normalization processing with particles N1 - N5, the R 2 ≥0.9, and the R corresponding to the normalization function obtained from the positive electrode active material in Comparative Example 1 2≥ 0.9, the particle packing mode of the positive active material in the positive electrode sheet of Comparative Example 1 is different from that of Examples 1 to 9. Thus, in terms of performance indicators, the electrochemical performance of the lithium-ion batteries provided by Example 1 and Examples 4 to 6 is superior to that of the lithium-ion battery provided by Comparative Example 1.

[0072] Moreover, the electrochemical performance of the lithium-ion batteries of Example 1 and Examples 4 to 5 is superior to that of the lithium-ion battery of Example 6. This shows that selecting at least four kinds of particles among N1 to N5 as the positive active material can more flexibly provide a positive electrode with a high tap density. The electrochemical performance of the lithium-ion batteries of Example 1 and Example 5 is superior to that of Example 4. This shows that when the positive active material includes Particle N1, Particle N2, Particle N3, and Particle N4, the corresponding lithium-ion battery has better cycle performance.

[0073] Comparing the performance data provided by Examples 1 to 3, it can be found that as the R corresponding to the obtained normalization function 2 gets closer to 1, the cycle stability and energy density of the corresponding lithium-ion battery show an increasing trend. Among them, the 25°C-1C cycle number @80% SOC of the lithium-ion battery of Example 1 is greater than that of the lithium-ion batteries provided by Example 2 and Example 3. This shows that when the particle size range and mass ratio of the positive electrode active coating particles make the above normalization equation satisfy the coefficient of determination R 2 ≥ 0.99, the particle size grading of the positive active material particles is beneficial to improving the efficient transmission of lithium ions in the positive electrode sheet and optimizing the cycle stability of the lithium-ion battery.

[0074] Comparing the lithium-ion batteries provided by Example 1 and Examples 7 to 9, it can be found that the electrochemical performance of the lithium-ion battery of Example 1 is superior to that of the lithium-ion battery of Example 7, and the electrochemical performance of the lithium-ion battery of Example 8 is superior to that of the lithium-ion battery of Example 9. This shows that when the particle size distribution mode of the positive active material in the positive electrode sheet makes the above normalization equation meet the range requirements of the coefficient of determination, the types of positive active materials can be flexibly selected according to actual applications.

[0075] The above examples are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A positive electrode sheet, characterized in that: A current collector and a positive electrode active coating containing a positive electrode active material, wherein the positive electrode active material includes at least three of particles N1, particles N2, particles N3, particles N4, and particles N5, wherein the particle N1 is a positive electrode active material having a particle size not exceeding 100 nm, the particle N2 is a positive electrode active material having a particle size of 100 to 150 nm, the particle N3 is a positive electrode active material having a particle size of 150 to 250 nm, the particle N4 is the above positive electrode active material having a particle size of 250 to 500 nm, and the particle N5 is the positive electrode active material having a particle size exceeding 500 nm; In the positive electrode active material, according to the mass percentage, the proportion of the particle N1 is X1, the proportion of the particle N2 is X2, the proportion of the particle N3 is X3, the proportion of the particle N4 is X4, and the proportion of the particle N5 is X5. The X1, the X2, the X3, the X4, and the X5 are subjected to power normalization to obtain a normalized function, and the normalized function corresponds to R 2 ≥0.

9.

2. The positive electrode sheet according to claim 1, characterized in that: The normalized equation satisfies the coefficient of determination R 2 ≥0.

99.

3. The positive electrode sheet according to claim 1, characterized in that: Let Y represent the normalized function, Y=aX 3 +bX 2 +cX+d; Y satisfies that at least one of a, b, and c is not 0.

4. The positive electrode sheet according to claim 3, characterized in that: In the normalization function, the value range of a is -0.1 to 0.1, the value range of b is -0.5 to 0.5, the value range of c is -1.0 to 1.0, and the value range of d is -0.97 to 0.

97.

5. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active material includes at least four of the particles N1, the particles N2, the particles N3, the particles N4, and the particles N5.

6. The positive electrode sheet according to claim 5, characterized in that: Calculated by mass percentage, the total proportion of the particles N1, the particles N2, the particles N3, and the particles N4 in the positive electrode active material is not less than 60%.

7. The positive electrode sheet according to claim 6, characterized in that: The positive electrode active material includes the particles N1, the particles N2, the particles N3, and the particles N4.

8. The positive electrode sheet according to claim 7, characterized in that: Calculated by mass percentage: The proportion X1 of the particles N1 in the positive electrode active material is 5% to 15%; The proportion X2 of the particles N2 in the positive electrode active material is 20% to 40%; The proportion X3 of the particles N3 in the positive electrode active material is 25% to 40%; The proportion X4 of the particles N4 in the positive electrode active material is 10% to 40%; The proportion X5 of the particles N5 in the positive electrode active material is 0% to 15%.

9. The positive electrode sheet according to any one of claims 1 to 8, characterized in that: The positive electrode active material includes a phosphate material, and the general formula of the phosphate material is Li 1+x Fe y M z P α O β , wherein, -0.05≤x≤0.15; 0.95≤y+z≤1.0, y≠0, 0.98≤α≤1.0, 3.95≤β≤4.0, and M includes at least one of Mn, Al, Mg, Ti, and Na.

10. The positive electrode sheet according to claim 9, characterized in that: The positive electrode active material includes a phosphate composite material, which includes an inner core containing the phosphate material and an outer coating layer coating the inner core, wherein the outer coating layer contains Me element and / or C element, wherein the Me element is selected from at least one of Fe element, Mn element, Al element, Mg element, Ti element, and Na element.

11. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet as claimed in any one of claims 1 to 10.

12. The lithium ion battery according to claim 11, characterized in that: The compaction density of the positive electrode active coating is 2.55-2.85 g / cm 3 .

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