High-safety lithium ion battery and preparation method thereof

Abstract

The invention provides a high-safety lithium ion battery and a preparation method thereof, and belongs to the technical field of lithium batteries. The battery comprises a positive plate, a negative plate, a diaphragm and an electrolyte, the positive plate comprises a positive current collector and a coating which is arranged on the surface of the positive current collector and contains a positive active material, and the negative plate comprises a negative current collector and a coating which is arranged on the surface of the negative current collector and contains a negative active material. The positive active material comprises a composition of three or more of lithium manganese iron phosphate, lithium-rich manganese base, nickel cobalt lithium manganate and lithium iron phosphate, the chemical formula of the lithium manganese iron phosphate is LiMnxFe (1-x) PO4, x is more than 0.4 and less than 0.7, the chemical formula of the lithium-rich manganese base is yLi2MnO3. (1-y) LiMO2, y is more than 0.2 and less than 0.6, and M is Ni or Mn. According to the invention, high energy density and long circulation are realized, and at the same time, higher safety and excellent needling performance are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a high-safety lithium-ion battery and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are a new type of secondary energy storage battery, which have the advantages of high volumetric energy density, long cycle life, environmental friendliness, etc., and have broad prospects in the applications of large-scale energy storage and electric vehicles.

[0003] In recent years, electric bicycles have caused many fire accidents. The Ministry of Industry and Information Technology, together with multiple departments, publicly solicited opinions on the newly revised mandatory national standard "Safety Technical Specification for Electric Bicycles", and made modifications and improvements in many aspects such as product safety, travel convenience, and industry norms. In particular, clear requirements are put forward for the needle-punching performance of lithium batteries.

[0004] Currently, the mainstream NCM ternary layered material has a relatively high energy density, with a conductivity of 10 -3 s / cm and a lithium-ion diffusion rate of 10 -9 cm 2 / s. It has good rate performance and long cycle life, and is suitable for 3C digital and power fields with high energy density. However, the oxygen release temperature is about 180°C, and the safety performance is poor, especially for NCM ternary materials with a high nickel content.

[0005] The olivine-structured lithium iron phosphate material has a low energy density, with a conductivity of 10 -9 s / cm and a lithium-ion diffusion rate of 10 -14 cm 2 / s. After modification, it has relatively good rate performance, stable structure, and long cycle life. At the same time, the oxygen release temperature of its material is about 400°C, with good safety, and is suitable for electric vehicles with low requirements for energy density and energy storage fields with long-life requirements.

[0006] Lithium manganese iron phosphate material has an olivine structure similar to that of lithium iron phosphate material, a similar specific capacity, and a higher plateau voltage (4.1V), so it can greatly improve the energy density. The conductivity of its material is 10 -13 s / cm and the lithium-ion diffusion rate is 10 -15 cm 2 / s, with relatively poor kinetic performance and good safety. At the same time, manganese ions in lithium manganese iron phosphate have the Jahn-Teller effect during the conversion process, and there is a problem of manganese dissolution. Manganese ions deposit on the surface of the negative electrode, which will damage the SEI film, resulting in a voltage drop of the lithium manganese iron phosphate battery, a phase change in the structure, and the cycle performance not meeting the expectations. Therefore, it is very difficult to be used alone.

[0007] The lithium-rich manganese-based material with a layered structure can be regarded as being formed by the uniform atomic-scale composite of two components: lithium-rich layered lithium manganate and layered lithium-ion transition metal oxide. In the transition metal / lithium mixed layer, lithium and transition metal atoms are arranged in an orderly manner to form a superlattice structure, with a specific capacity of up to 320 mAh / g and a relatively high energy density. The conductivity is 10 -7 S / cm, and the lithium-ion diffusion rate is 10 -14 cm 2 / s. The rate performance of the material is relatively poor. The working voltage window is relatively wide, and operating at high voltages easily causes the layered structure to transform into a spinel structure, and the resulting phase change leads to poor cycling performance. Therefore, it is very difficult to use the lithium-rich manganese-based material alone.

[0008] When the above-mentioned cathode materials for lithium batteries are used alone, there are many drawbacks. How to maintain high energy density and long cycling while having high safety, especially solving the problem of needle puncture, is a pain point that the industry urgently needs to solve. Summary of the Invention

[0009] In view of this, the technical problem to be solved by the present invention is to provide a high-safety lithium-ion battery with high energy density, long cycling, higher safety and excellent needle puncture performance in view of the deficiencies of the prior art.

[0010] To solve the above technical problem, the technical solution adopted by the present invention is: a high-safety lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a coating provided on the surface of the positive electrode current collector and containing positive electrode active materials. The negative electrode sheet includes a negative electrode current collector and a coating provided on the surface of the negative electrode current collector and containing negative electrode active materials. The positive electrode active materials include a combination of three or more of lithium manganese iron phosphate, lithium-rich manganese-based, lithium nickel cobalt manganese oxide and lithium iron phosphate. The chemical formula of the lithium manganese iron phosphate is LiMn x Fe （1-x） PO 4 , where 0.4 < X < 0.7. The chemical formula of the lithium-rich manganese-based is y Li 2 MnO 3 · （1-y） LiMO 2 , where 0.2 < y < 0.6, and M is Ni or Mn.

[0011] By combining materials with different particle sizes and different proportions of lithium manganese iron phosphate, lithium nickel cobalt manganese oxide (NCM), lithium iron phosphate (LFP) and lithium-rich manganese-based, the present invention achieves high safety of the lithium battery while maintaining excellent performance of high energy density and long cycling.

[0012] Through a large number of experiments, it is found that the chemical formula of lithium manganese iron phosphate is LiMn x Fe （1-x） PO 4 , when 0.4 < x < 0.7, the manganese content is appropriate, thus overcoming the Jahn-Teller effect and maintaining the stability of the structure during the cycling process. The chemical formula of the lithium-rich manganese-based material is y Li 2 MnO 3 · （1-y） LiMO 2 , when 0.2 < y < 0.6 and M is Ni or Mn, it can reduce the over-oxidation of Li 2 O, reduce the gradual transformation of the material structure from the layered phase to the spinel phase, maintain the stability of its structure, and improve its cycling performance. The lithium-rich manganese-based material is suitable for use in combination with lithium manganese iron phosphate below 4.2V. At the same time, due to the lithium-rich phase, it can continuously supplement lithium to lithium manganese iron phosphate.

[0013] Optionally, the D50 particle size of the lithium manganese iron phosphate is 0.6 - 1.8 µm, and the specific surface area is 13 - 18 m 2 / g; the D50 particle size of the lithium iron phosphate is 0.6 - 2.0 µm, and the specific surface area is 9.5 - 13 m 2 / g; the D50 particle size of the lithium-rich manganese-based material is 3.5 - 9 µm, and the specific surface area is 0.3 - 0.7 m 2 / g; the D50 particle size of the lithium nickel cobalt manganese oxide is 6 - 11 µm, and the specific surface area is 0.25 - 0.7 m 2 / g.

[0014] The D50 of the positive electrode active material needs to be controlled within a suitable range to balance the charge-discharge performance, cycling stability, and safety performance of the battery. However, the selection of D50 is not just a single numerical adjustment, but also involves the optimization of the entire particle size distribution. Too narrow or too wide a particle size distribution will reduce the tap density of the material and affect the battery performance. The smaller the D50 of the positive electrode active material, the larger the specific surface area, the smaller or wider the particle size distribution, the larger the contact area with the electrolyte, and the more reaction active sites. However, the first efficiency is relatively low, and the Jahn-Teller effect is more serious, which is not conducive to the improvement of the cycling performance and the enhancement of the safety performance. And the larger the D50, it may be not conducive to the insertion and extraction of Li + , affecting the charge-discharge efficiency of the battery, may lead to an increase in the internal resistance of the lithium-ion battery, and affect the discharge performance and capacity of the battery. In addition, differences in the raw material processing technology result in different particle sizes of the materials. Therefore, in the process of compounding and using various materials in the present invention, considering various factors comprehensively and filling with a reasonable particle size grading can improve the tap density of the material itself, obtain a high tap density during the processing of the positive electrode sheet, and achieve a high energy density for processing lithium batteries.

[0015] Optionally, the mass percentage of lithium iron manganese phosphate in the positive electrode active material is 70-92%, the mass percentage of lithium iron phosphate ≤ 15%, the mass percentage of lithium-rich manganese-based material ≤ 6%, the mass percentage of lithium nickel cobalt manganese oxide ≤ 15%, and at least two of lithium iron phosphate, lithium-rich manganese-based material and lithium nickel cobalt manganese oxide are included.

[0016] Through the matching and compounding of different specific surface areas and different particle sizes, the present invention can improve the tap density of lithium iron manganese phosphate, achieving the effect that the grading "1 + 1" > 2; meanwhile, the safety, energy density and cycle performance are all greatly improved.

[0017] Optionally, the positive electrode active material contains lithium iron manganese phosphate, lithium iron phosphate, lithium-rich manganese-based material and lithium nickel cobalt manganese oxide. Among them, the mass percentage of lithium iron manganese phosphate is 70-88%, the mass percentage of lithium iron phosphate is 5-15%, the mass percentage of lithium-rich manganese-based material is 2-6%, and the mass percentage of lithium nickel cobalt manganese oxide is 5-15%.

[0018] Optionally, the coating containing the positive electrode active material further contains a binder, and the binder is polyvinylidene fluoride, and its mass percentage is 1.8-3.5%.

[0019] In this way, the binder can have a good binding effect on the active material and the current collector, and at the same time is greatly helpful for safety, especially improving the needle-punching performance.

[0020] Optionally, the coating containing the positive electrode active material further contains a conductive agent, and the conductive agent is a composition of conductive carbon black and carbon nanotubes. The mass ratio of conductive carbon black to carbon nanotubes is (1-3):1, preferably 2:1. The mass percentage of the conductive agent is 1-3%, and the mass ratio of the binder to the conductive agent is (1.5-3):1.

[0021] Optionally, the coating containing the positive electrode active material further contains a dispersant, and the mass percentage of the dispersant is 0.1-0.3%, preferably 0.15%; the dispersant is a polyacrylate dispersant, which mainly reduces the interaction force between particles to achieve uniform dispersion of particles in the solution; it can also form a protective film on the particle surface to prevent particle agglomeration and sedimentation.

[0022] Optionally, the negative electrode active material is artificial graphite secondary particles with a graphitization degree of 91.5-93%; the D50 particle size is 8-12 µm, and the specific surface area is 1.2-2.2 m 2 / g; through verification, the graphite particles are relatively dense, with a large interlayer spacing, which is beneficial to the insertion and extraction of ions, and there are relatively few charge-discharge side reactions, which has a positive effect on improving the safety performance and cycle performance.

[0023] Optionally, the positive current collector is a carbon-coated aluminum foil with a thickness of 15-20 microns, thereby improving the electronic conductivity between the positive active material particles and the aluminum foil; The negative current collector is a copper foil with a thickness of 10-15 microns; thereby, during the charge-discharge and safety test processes, heat can be dissipated in a timely manner to prevent thermal runaway caused by heat accumulation during puncture.

[0024] The present invention also provides a method for preparing the above high-safety lithium-ion battery, comprising the following steps: S1. Provide positive active material, binder, conductive agent, and dispersant. Stir the above materials evenly by dry mixing, add a solvent, and stir evenly to make a positive electrode paste; evenly coat the positive electrode paste on the surface of the positive current collector, dry and compact it, then cut it, weld the positive electrode tab, and stick a high-temperature adhesive tape to obtain a positive electrode sheet; S2. Provide negative active material, thickening agent, and conductive agent. Stir the above materials evenly by dry mixing, add deionized water and continuously stir. After being uniform, add the binder and stir evenly to make a negative electrode paste; evenly coat the negative electrode paste on the surface of the negative current collector, dry and compact it, then cut the sheet, weld the negative electrode tab, and stick a high-temperature adhesive tape to obtain a negative electrode sheet; S3. Provide a base film, and coat a functional coating on the surface layer of the base film to make a separator; S4. Provide a lithium salt, a solvent, and an additive. Dissolve the lithium salt in the solvent, then add the additive and mix evenly to obtain an electrolyte; S5. Stack the negative electrode sheet, positive electrode sheet, and separator in sequence, then obtain an electrode core through a winding process. Transfer the electrode core into a battery packaging case, inject the electrolyte into it, and perform aging at a high temperature to obtain a lithium-ion battery.

[0025] Optionally, in the positive electrode sheet, the mass percentage of the binder is 1.8-3.5%, the mass percentage of the conductive agent is 1-3%, the mass percentage of the dispersant is 0.1-0.3%, and the balance is the positive active material; in the negative electrode sheet, the mass percentage of the thickening agent is 0.5-2.5%, the mass percentage of the conductive agent is 0.3-1.5%, the mass percentage of the binder is 1.0-3.5%, and the balance is the negative active material.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: Based on the above technical solutions, the present invention provides a high-safety lithium-ion battery. By combining materials with different particle sizes and different ratios of lithium iron phosphate, nickel cobalt manganese oxide (NCM), lithium iron phosphate (LFP), and lithium-rich manganese-based materials, while achieving high safety of the lithium battery, excellent performance of high energy density and long cycle life is maintained. Further, the present invention optimizes the ratio of Mn and Fe in the lithium iron phosphate material of the positive active material in the positive electrode sheet, and Li in the lithium-rich manganese-based material 2MnO 3 and LiMO 2 ratio to improve the deterioration of battery performance caused by material phase change during charge and discharge; at the same time, on the basis of the conventional industry blending of lithium iron manganese phosphate and nickel cobalt manganese ternary materials, lithium iron phosphate and lithium-rich manganese-based are added. Lithium iron phosphate with better thermal stability is blended into the other three materials, which plays a positive role in improving its safety, especially in solving the nail penetration performance.

[0027] The present invention conducts long-term tracking research on the particle size, specific surface area, and blending ratio of the cathode active material. Through grading, the tap density of the lithium-ion battery is increased from 2.35 g / cm 3 to 2.72 g / cm 3 , achieving the purpose of improving the process processing performance and increasing the energy density.

[0028] In the preparation of the cathode active material slurry, a dispersant with a mass content of 0.1 - 0.3% is added, achieving uniform dispersion between particles, ensuring the flow stability of the slurry, and improving the quality of the electrode sheet.

[0029] Under experimental conditions, compared with the battery made of traditional single lithium iron manganese phosphate, the energy density of the lithium battery of the present invention is increased by more than 13.5%, the cycle performance is increased by more than 35%, and the nail penetration performance is 100% passed, with remarkable improvement effects. Detailed Embodiments

[0030] To better understand the present invention, the content of the present invention will be further clearly described below in conjunction with embodiments. However, the protected content of the present invention is not limited to the following embodiments. In the following description, a large number of specific details are given to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details.

[0031] Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0032] In all the examples shown and discussed here, any specific value should be interpreted as merely exemplary, not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0033] Unless otherwise specified, all raw materials are commercially available products, and unless otherwise specified, they do not contain other components not explicitly stated except for inevitable impurities.

[0034] Lithium-ion batteries with different component compositions were prepared according to the ratios shown in Examples 1 to 5, and evaluation tests were carried out. The specific content of the evaluation tests will be described later. In addition, lithium-ion batteries with different component compositions and manufacturing methods from those of the present invention were prepared according to the ratios or methods shown in Comparative Examples 1 to 9. Then, the evaluation tests were carried out in the same way. Next, the details of the ratios and manufacturing methods of Examples 1 to 5 and Comparative Examples 1 to 9 will be described.

[0035] Examples 1-5: Combined with Table 1-4, a preparation method of the above high-safety lithium-ion battery includes the following steps: S1. Provide a positive electrode active material as shown in Table 2-3. The positive electrode active material is selected from three or four of lithium manganese iron phosphate (LiMn x Fe （1-x） PO 4 ), lithium-rich manganese-based ( y Li 2 MnO 3 · （1-y） LiMO 2 ), lithium nickel cobalt manganese oxide (NCM), and lithium iron phosphate (LFP). Using polyvinylidene fluoride (PVDF5130) as the binder, carbon black and carbon nanotubes as the conductive agents with a mass ratio of 2:1, and polyacrylate as the dispersant, the above materials are stirred evenly by dry mixing, and then the solvent N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode slurry with a solid content of 64.5%. The positive electrode slurry is evenly coated on the surface of an 18-micron carbon-coated aluminum foil, dried, compacted, cut, welded with a positive electrode tab, and a high-temperature adhesive tape is pasted to obtain a positive electrode sheet; S2. Using artificial graphite secondary particles as the negative electrode active material (parameter values are shown in Table 4), sodium carboxymethyl cellulose (CMC) as the thickener, and carbon black (S-P) as the conductive agent, the above materials are stirred evenly by dry mixing, deionized water is added for continuous stirring, and after being evenly mixed, the binder styrene-butadiene rubber latex (SBR) is added and stirred evenly to prepare a negative electrode slurry with a solid content of 45.6%. The negative electrode slurry is evenly coated on the surface of a 12-micron copper foil, dried, compacted, cut, welded with a negative electrode tab, and a high-temperature adhesive tape is pasted to obtain a negative electrode sheet; S3. Using a polyethylene microporous membrane as the base film, coating a slurry of aluminum oxide on the surface layer to prepare a separator; S4. Using lithium hexafluorophosphate as the lithium salt and a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate as the mixed solvent with a weight ratio of DEC:DMC:EMC = 1:1:1; dissolving lithium hexafluorophosphate in the mixed solvent (concentration is 1 mol / L) to obtain a mixture; then adding 0.5% of fluoroethylene carbonate, 0.5% of vinyl sulfate, 0.5% of vinylene carbonate, 0.8% of lithium difluorooxalate borate, and 0.5% of 1,3 - propane sultone based on the mass of the aforementioned mixture, and stirring evenly to obtain the electrolyte; S5. After sequentially stacking the negative electrode sheet, the positive electrode sheet, and the separator, a battery core is prepared through a winding process. The battery core is transferred into a battery packaging case, the electrolyte is injected therein, and it is left standing at 45 °C for 48 h for formation to obtain a lithium - ion battery.

[0036] Table 1 The ratio of each raw material in Examples 1 - 5

[0037] Table 2 The composition of the positive active material in Examples 1 - 5

[0038] Table 3 The parameter values of the positive active material in Examples 1 - 5

[0039] Table 4 The parameter values of the negative active material in Examples 1 - 5

[0040] The following are comparative implementation schemes.

[0041] Comparative Example 1: Different from Example 1: The value of x in lithium iron manganese phosphate (LiMn x Fe （1-x） PO 4 ) is 0.7.

[0042] Comparative Example 2: Different from Example 1: The value of y in lithium - rich manganese - based (yLi 2 MnO 3 · （1-y） LiMO 2 ) is 0.6.

[0043] Comparative Example 3: Different from Example 1: The value of y in lithium - rich manganese - based (yLi 2 MnO 3 · （1-y） LiMO 2 ) is 0.2.

[0044] Comparative Example 4: Different from Example 3: The D50 of lithium nickel cobalt manganese oxide is 5.5 μm, and the specific surface area is 0.83 m2 ·g -1 。

[0045] Comparative Example 5: Different from Example 4, the D50 of lithium iron phosphate is 2.3 μm, and the specific surface area is 8.7 m 2 ·g -1 。

[0046] Comparative Example 6: Only lithium iron manganese phosphate is used as the positive active material, and other parameter values and processes are the same as those in Example 1.

[0047] Comparative Example 7: Lithium iron manganese phosphate and lithium-rich manganese-based are used as the positive active materials, and the mass ratio of the two is 95:5. Other parameter values and processes are the same as those in Example 1.

[0048] Comparative Example 8: Lithium iron manganese phosphate and lithium iron phosphate are used as the positive active materials, and the mass ratio of the two is 85:15. Other parameter values and processes are the same as those in Example 1.

[0049] Comparative Example 9: Lithium iron manganese phosphate and lithium nickel cobalt manganese oxide are used as the positive active materials, and the mass ratio of the two is 85:15. Other parameter values and processes are the same as those in Example 1.

[0050] Next, the content of the evaluation test will be described.

[0051] 1.1 Battery cycle performance test method: (1) Charge the above-prepared battery cells at a constant current of 0.5C to 4.2V at room temperature of 25°C ± 3°C, and then charge at a constant voltage of 4.2V until the cut-off current of 0.05C; (2) Leave it standing for 30 min; (3) Discharge at a constant current of 1C to 2.5V at room temperature of 25°C ± 3°C, and record the capacity D1 at this time; (4) Leave it standing for 30 min; (5) Repeat steps (1) to (4) for N times, and record the capacity after N cycles, denoted as D2.

[0052] Calculate the capacity retention rate after N cycles according to the following formula: Capacity retention rate D = D2 / D1 × 100%.

[0053] 1.2 Pinprick performance test method: (1) Charge at a constant current and voltage of 0.5C to 4.2V with a cut-off current of 0.01C for 180 min; (2) Let it stand at room temperature for 120 min; (3) Use a high-temperature resistant steel needle with a diameter of Φ5 mm (tungsten steel, the cone angle of the needle tip is 45°) to penetrate the geometric center of the battery from a direction perpendicular to the battery electrode plate at a speed of (25 ± 5) mm / s. The steel needle stays in the battery and is observed for 1 h; (4) Standard requirements: No fire and no explosion.

[0054] 1.3 The test data are shown in the following table:

[0055] It can be seen from the above data that Examples 1-5 and Comparative Example 1 all passed the needle penetration test 100%. Their safety performances are relatively excellent. However, compared with Comparative Example 1, Examples 1-5 have a greater improvement in terms of compaction density and energy density. Especially for Example 1 compared with Comparative Example 1, the energy density increased by 12.2%, and the compaction density increased by 0.37 g / cm 3 , achieving the purpose of "1 + 1 > 2" and having great application value.

[0056] In terms of cycling, in Examples 1-5, by mixing more than three kinds of positive electrode materials such as lithium iron manganese phosphate, lithium-rich manganese-based, lithium nickel cobalt manganese oxide, and lithium iron phosphate in the material system, not only the electronic conductivity is improved, but also the structural stability and lithium supplement characteristics of the low voltage section of the lithium-rich manganese-based are utilized. Compared with Comparative Example 1, the number of cycles increased by 34 - 45%, and the service life was greatly improved.

[0057] In Comparative Examples 2-9, by adjusting the parameters or types of the positive electrode active substances respectively, the phenomenon of needle penetration failure occurred to varying degrees, which poses a certain safety risk for the battery when used under abnormal terminal conditions.

[0058] It can be seen that the lithium iron manganese phosphate battery manufactured by the present invention not only has a high energy density, but also has good safety and cycling performance.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Any other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention shall be covered within the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-safety lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a coating layer disposed on the surface of the positive electrode current collector and containing a positive electrode active material, and the negative electrode sheet comprises a negative electrode current collector and a coating layer disposed on the surface of the negative electrode current collector and containing a negative electrode active material, characterized in that: The positive electrode active material comprises a combination of three or more of lithium iron manganese phosphate, lithium-rich manganese-based, lithium nickel cobalt manganese oxide and lithium iron phosphate, and the chemical formula of the lithium iron manganese phosphate is LiMn x Fe （1-x） PO4, wherein 0.4<X<0.7, the chemical formula of the lithium-rich manganese-based y Li2MnO3 （1-y） LiMO2, wherein 0.2<y<0.6, M is Ni or Mn.

2. A high-safety lithium-ion battery as claimed in claim 1, characterized in that: The D50 particle size of the lithium manganese iron phosphate is 0.6-1.8 μm, and the specific surface area is 13-18 m 2 / g; the D50 particle size of the lithium iron phosphate is 0.6~2.0µm, and the specific surface area is 9.5-13m 2 / g; the D50 particle size of the lithium-rich manganese-based is 3.5~9µm, and the specific surface area is 0.3-0.7m 2 / g; the D50 particle size of the lithium nickel cobalt manganese oxide is 6-11µm, and the specific surface area is 0.25-0.7m 2 / g.

3. A high-safety lithium-ion battery as claimed in claim 2, characterized in that: The mass percentage of lithium iron manganese phosphate in the positive electrode active material is 70-92%, the mass percentage of the lithium iron phosphate is ≤15%, the mass percentage of the lithium-rich manganese base is ≤6%, and the mass percentage of the lithium nickel cobalt manganese oxide is ≤15%, and the lithium iron phosphate, lithium-rich manganese base and lithium nickel cobalt manganese oxide contain at least two.

4. A high-safety lithium-ion battery as claimed in claim 3, characterized in that: The positive electrode active material contains lithium iron manganese phosphate, lithium iron phosphate, lithium-rich manganese base and nickel cobalt manganese oxide, wherein the mass percentage of the lithium iron manganese phosphate is 70-88%, the mass percentage of the lithium iron phosphate is 5-15%, the mass percentage of the lithium-rich manganese base is 2-6%, and the mass percentage of the nickel cobalt manganese oxide is 5-15%.

5. A high-safety lithium-ion battery as claimed in claim 4, characterized in that: The coating containing the positive electrode active material also contains a binder, and the binder is polyvinylidene fluoride, and the mass percentage of the binder is 1.8-3.5%.

6. A high-safety lithium-ion battery as claimed in claim 5, characterized in that: The coating containing the positive electrode active material also contains a conductive agent, which is a composition of conductive carbon black and carbon nanotubes, the mass ratio of the conductive carbon black to the carbon nanotubes is (1-3):1, the mass percentage of the conductive agent is 1-3%, and the mass ratio of the binder to the conductive agent is (1.5-3):

1.

7. A high-safety lithium-ion battery as claimed in claim 6, characterized in that: The coating layer containing the positive electrode active material further contains a dispersant, and the mass percentage of the dispersant is 0.1-0.3%, preferably 0.15%.

8. A high-safety lithium-ion battery as claimed in claim 7, characterized in that: The negative electrode active material is artificial graphite secondary particles with a graphitization degree of 91.5-93%, a D50 particle size of 8-12 μm, and a specific surface area of ​​1.2-2.2 m 2 / g.

9. A high-safety lithium-ion battery as claimed in claim 8, characterized in that: The positive electrode current collector is a 15-20 micron carbon-coated aluminum foil, and the negative electrode current collector is a 10-15 micron copper foil.

10. The method for preparing a high-safety lithium-ion battery according to claim 9, characterized in that: The following steps are involved: S1. Provide a positive electrode active material, a binder, a conductive agent and a dispersant, stir the above materials evenly by dry mixing, add a solvent, and stir evenly to form a positive electrode slurry; evenly apply the positive electrode slurry on the surface of the positive electrode collector, dry and compact it, cut it, weld the positive electrode ears, and stick high-temperature adhesive tape to obtain a positive electrode sheet; S2, providing a negative electrode active material, a thickener and a conductive agent, stirring the above materials evenly by dry mixing, adding deionized water and stirring continuously, adding a binder after being evenly mixed, stirring evenly to form a negative electrode slurry; coating the negative electrode slurry evenly on the surface of the negative electrode collector, drying and compacting, cutting into pieces, welding the negative electrode tabs, and sticking high-temperature adhesive tape to obtain a negative electrode sheet; S3, providing a base film, and coating a functional coating on the surface of the base film to form a diaphragm; S4, providing lithium salt, solvent and additive, dissolving the lithium salt in the solvent, adding the additive, mixing evenly, and obtaining an electrolyte; S5, stacking the negative electrode sheet, the positive electrode sheet and the separator in sequence, and then winding the battery cell to obtain a battery cell, transferring the battery cell into a battery packaging shell, injecting an electrolyte into the shell, and placing the shell at a high temperature for formation to obtain a lithium-ion battery; In the positive electrode sheet, the mass percentage of the binder is 1.8-3.5%, the mass percentage of the conductive agent is 1-3%, the mass percentage of the dispersant is 0.1-0.3%, and the balance is the positive electrode active material; in the negative electrode sheet, the mass percentage of the thickener is 0.5-2.5%, the mass percentage of the conductive agent is 0.3-1.5%, the mass percentage of the binder is 1.0-3.5%, and the balance is the negative electrode active material.

Images