Lithium manganese iron phosphate and lithium nickel cobalt manganese oxide composite cathode materials and their preparation methods, cathodes, and batteries

Abstract

This application discloses a composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, its preparation method, cathode, and battery. The preparation method includes: preparing a first solution by mixing a binder and a solvent to obtain a first solution; preparing a first mixture by adding carbon nanotubes to the first solution and mixing to obtain a first mixture; preparing a second mixture by mixing lithium manganese iron phosphate and lithium nickel cobalt manganese oxide to obtain a second mixture; preparing a third mixture by mixing the second mixture and SP to obtain a third mixture; and preparing a fourth mixture by adding the first mixture to the third mixture and mixing them evenly to obtain the fourth mixture, i.e., the cathode material; wherein, in at least one step, the mixing is carried out in a vacuum environment and / or by ultrasonic treatment. The cathode material of this application is used in a battery, and the battery has good energy density and safety.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, its preparation method, cathode, and battery. Background Technology

[0002] Cathode materials in lithium-ion batteries are classified into lithium iron phosphate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide. Among these, lithium manganese iron phosphate cathode materials exhibit structural stability, are less prone to structural collapse during charge and discharge, offer better safety, lower cost, and longer lifespan (over 2000 cycles, exceeding that of ternary lithium batteries). However, lithium manganese iron phosphate has a higher cost and lower energy density. Ternary materials possess higher energy density and rate performance, but suffer from lower safety and have limited nickel and cobalt resources, resulting in higher material costs.

[0003] Mixing lithium nickel manganese cobalt oxide (LCO) and lithium manganese iron phosphate (LFP) as cathode materials for lithium-ion batteries combines the advantages of both materials, optimizing overall battery performance, improving energy density and cycle stability, and providing a solution to problems such as low conductivity and slow lithium-ion diffusion rate of LFP. In existing technologies, the mixing of LCO and LFP as cathode materials is typically achieved through conventional mechanical stirring. However, this method carries the risk of poor stability, which can negatively impact battery performance. Summary of the Invention

[0004] To address the aforementioned problems, this application provides a stable composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, its preparation method, the cathode material, and a battery. The technical solution adopted in this application is as follows:

[0005] The first aspect of this application discloses a method for preparing a composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide. The method includes: preparing raw materials, wherein the raw materials include an active material, a conductive agent, a binder, and a solvent; the active material includes lithium manganese iron phosphate and lithium nickel cobalt manganese oxide; the conductive agent includes carbon nanotubes and SP; preparing a first solution by mixing the binder and solvent to obtain a first solution; preparing a first mixture by adding the carbon nanotubes to the first solution and mixing to obtain a first mixture; preparing a second mixture by mixing lithium manganese iron phosphate and lithium nickel cobalt manganese oxide to obtain a second mixture; and preparing a third mixture by mixing the second mixture with the first solution. The mixture and SP are mixed to obtain the third mixture; the fourth mixture is prepared by adding the first mixture to the third mixture and mixing them evenly to obtain the fourth mixture, which is the positive electrode material; wherein: at least one step of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture and preparing the fourth mixture is carried out in a vacuum environment; at least one step of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture and preparing the fourth mixture includes mixing by ultrasonic treatment.

[0006] In one implementation of this application, the vacuum level of the vacuum environment is -5MPa to -1MPa.

[0007] In one implementation of this application, the frequency of the ultrasound is 10kHz to 20kHz.

[0008] In one implementation of this application, in the step of preparing the fourth mixture, the first mixture is added to the third mixture in 2 to 4 separate additions.

[0009] In one implementation of this application, at least one step of preparing the first solution, the first mixture, the second mixture, the third mixture, and the fourth mixture further includes mixing by stirring, wherein the stirring rate is 1000 rpm to 4000 rpm and the processing time is 0.1 h to 3.5 h.

[0010] In one implementation of this application, a dual planetary stirrer is used for stirring, with a revolution speed of 5 to 50 rpm and a rotation speed of 1000 to 4000 rpm.

[0011] In one implementation of this application, the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide is 1:9 to 9:1, the D50 of the lithium manganese iron phosphate is 1 to 5 μm, and the D50 of the lithium nickel cobalt manganese oxide is 5 to 20 μm.

[0012] The second aspect of this application discloses a cathode material, which is prepared by the preparation method disclosed in the first aspect of this application.

[0013] A third aspect of this application discloses a positive electrode comprising a positive electrode current collector and a positive electrode material as disclosed in the second aspect of this application attached to the positive electrode current collector.

[0014] The fourth aspect of this application discloses a battery, said battery including the positive electrode disclosed in the third aspect of this application.

[0015] The beneficial effects of this application are as follows:

[0016] The composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide in this application exhibits no agglomeration, can be uniformly coated on the electrode surface, has good adhesion, and the battery has good energy density and safety. Attached Figure Description

[0017] Figure 1 This is a surface morphology diagram of the positive electrode sheet of Embodiment 1 of this application. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings. In the following embodiments, many details are described to facilitate a better understanding of the present application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other materials or methods. In some cases, certain operations related to the present application are not shown or described in the specification. This is to avoid obscuring the core parts of the present application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0019] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification and drawings are only for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0020] The serial numbers assigned to components in this article, such as "first" and "second", are used only to distinguish the objects being described and have no sequential or technical meaning.

[0021] Mixing lithium nickel cobalt manganese oxide (LiCO) and lithium manganese iron phosphate (LFP) as a cathode material for lithium-ion batteries combines the advantages of both materials, optimizing overall battery performance, improving energy density and cycle stability, and providing a solution to the problems of low conductivity and slow lithium-ion diffusion rate inherent in LiFP. In existing technologies, the mixing of LiCO and LFP as cathode materials is typically achieved through conventional mechanical stirring. However, this method carries the risk of poor stability, which can negatively impact battery performance. Therefore, this application creatively proposes a composite cathode material of LiFP and LiCO, and its preparation method. This application offers the following advantages:

[0022] (1) Adding the first mixture to the third mixture in several steps can stabilize the separated particles, prevent them from agglomerating again, and make the dispersion more uniform. The high-speed rotating impeller uses the friction generated between the inclined surface and the material to make the material move tangentially along the impeller surface. At the same time, due to the centrifugal force, the material is thrown against the wall of the mixing chamber and rises along the wall. When it rises to a certain height, it falls back to the center of the impeller due to gravity and is then thrown up again. This combination of upward motion and tangential motion makes the material actually in a continuous spiral motion state. Because the impeller speed is very high, the material movement speed is also very fast. The collision and friction between the rapidly moving particles break up the particles or agglomerated clumps. At the same time, the temperature of the material also rises accordingly, which is beneficial to the adsorption of various additives by the powder.

[0023] (2) By adding the first mixture (liquid) to the third mixture (solid), the amount of the first mixture added in steps can be adjusted, and the appropriate amount can play a better kneading role.

[0024] (3) The addition of ultrasonic technology during the preparation process allows the growth and rupture of bubbles generated by ultrasound to impact and disperse agglomerates in the mixture, and also induces material flow on a macroscopic level, resulting in more uniform mixing. Specifically, on a microscopic level, continuous ultrasonic vibration is used during the preparation process, causing a large number of bubbles to be generated inside the material during mixing. The bubbles float upward and grow. When the bubble size reaches a certain critical value, the bubble growth rate increases rapidly and then bursts instantly, forming a shock wave to disperse the agglomerates in the material. On a macroscopic level, the salt environment during the material mixing process is conducive to the discharge of gas from the material, preventing the gas in the material from affecting the subsequent coating quality.

[0025] (5) The preparation process uses a vacuum environment, which is beneficial to remove gas from the material and prevent gas from affecting the subsequent coating quality.

[0026] (6) The positive and negative electrode slurries do not agglomerate and adhere to the electrode sheets relatively uniformly, with good adhesion. The DCR is small during battery cycling, resulting in good battery safety performance.

[0027] The present application will now be described in conjunction with the accompanying drawings and specific embodiments.

[0028] This application first relates to a method for preparing a composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide.

[0029] In one specific embodiment, the preparation method of the lithium manganese iron phosphate and lithium nickel cobalt manganese oxide composite cathode material includes the following steps:

[0030] Step S1: Prepare raw materials, including active substances, conductive agents, binders and solvents.

[0031] Step S2: Preparation of the first solution: Mix the adhesive and solvent to obtain the first solution.

[0032] Step S3: Preparation of the first mixture: Carbon nanotubes are added to the first solution and mixed.

[0033] Step S4: Prepare the second mixture: Mix lithium manganese iron phosphate and lithium nickel cobalt manganese oxide.

[0034] Step S5: Prepare the third mixture: Mix the second mixture and SP.

[0035] Step S6: Prepare the fourth mixture: Add the first mixture to the third mixture and mix evenly to obtain the fourth mixture, which is the cathode material.

[0036] In one specific embodiment, the steps of preparing the first mixture and preparing the third mixture may be performed in any order.

[0037] In one specific embodiment, the raw materials for the positive electrode material include active material, conductive agent, binder and solvent.

[0038] In one specific embodiment, the active material includes lithium manganese iron phosphate and lithium nickel cobalt manganese oxide.

[0039] In one specific embodiment, the lithium manganese iron phosphate material has a particle size D50 of 1–5 μm and a specific surface area of ​​15–20 μm. 2 / g, density is 0.5-1g / cm³ 3 .

[0040] In one specific embodiment, the particle size D50 of the lithium nickel cobalt manganese oxide material is 5–20 μm.

[0041] In one specific embodiment, in the lithium nickel cobalt manganese oxide material, the proportion of nickel to cobalt and lithium is 60% to 95%. For example, in lithium nickel cobalt manganese oxide, the proportion of nickel to the three elements is 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%. Preferably, in the lithium nickel cobalt manganese oxide material, the proportion of nickel to the three elements is 80% to 95%.

[0042] In one specific embodiment, the ratio of nickel, cobalt, and lithium in the lithium nickel cobalt manganese oxide material is 9:0.5:0.5.

[0043] In one specific embodiment, the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide can be 1:9 to 9:1. For example, the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide can be 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 or 9:1.

[0044] In one specific embodiment, the binder can be PVDF (polyvinylidene fluoride), the conductive agent can be CNT (carbon nanotubes) or SP (SuperP), and the solvent can be NMP (N-methylpyrrolidone).

[0045] In one specific embodiment, at least one step of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture is performed in a vacuum environment.

[0046] In one specific embodiment, the steps of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture are all performed in a vacuum environment.

[0047] In one specific embodiment, the vacuum level of the vacuum environment is -5 MPa to 0 MPa. For example, the vacuum level of the vacuum environment can be -5 MPa, -4 MPa, -3 MPa, -2 MPa, -1 MPa, -0.5 MPa, or 0 MPa. Preferably, the vacuum level of the vacuum environment is -5 MPa to -1 MPa. It should be noted that MPa is megapascal; a vacuum level of -1 MPa is approximately equal to -10 standard atmospheres.

[0048] In one specific embodiment, the steps of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture include mixing by ultrasonic treatment.

[0049] In one specific embodiment, the steps of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture all include mixing by ultrasonic treatment.

[0050] In one specific embodiment, the frequency of the ultrasound is 10 kHz to 20 kHz. For example, the frequency of the ultrasound can be 10 kHz, 11 kHz, 12 kHz, 13 kHz, 14 kHz, 15 kHz, 16 kHz, 17 kHz, 18 kHz, 19 kHz or 20 kHz.

[0051] In one specific embodiment, at least one step of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture further includes mixing by stirring.

[0052] In one specific embodiment, the stirring rate is 1000 rpm to 4000 rpm, and the processing time is 0.1 h to 3.5 h.

[0053] In one specific embodiment, a dual planetary mixer is used for stirring, with an orbital speed of 5 to 50 rpm and a rotational speed of 1000 to 4000 rpm.

[0054] In one specific embodiment, an ultrasonic dual planetary mixer is used to mix the materials.

[0055] In one specific embodiment, in the step of preparing the fourth mixture, the first mixture is added to the third mixture in multiple portions. In another specific embodiment, in the step of preparing the fourth mixture, the first mixture is added to the third mixture in 2 to 4 portions. The mass of the first mixture added each time is 20% to 80% of its own mass.

[0056] In one specific embodiment, the method for preparing the positive electrode material further includes: adding a binder to the fourth mixture and adjusting the viscosity to obtain the positive electrode material.

[0057] In one specific embodiment, the viscosity of the positive electrode material is 2500 MPa·s to 4000 MPa·s.

[0058] In one specific embodiment, the method for preparing the positive electrode material further includes: sieving and / or removing iron from the fourth mixture after adjusting its viscosity.

[0059] In one specific embodiment, the sieving process includes filtration using a 200-mesh sieve.

[0060] In one specific embodiment, the iron removal process includes stirring the slurry using an iron removal device to remove magnetic materials from the slurry.

[0061] In one specific embodiment, the steps of preparing the first solution, preparing the first mixture, preparing the second mixture, preparing the third mixture, and preparing the fourth mixture are all carried out in an environment of 20°C to 35°C.

[0062] In one specific embodiment, in the step of preparing the first solution, dispersion is carried out using an ultrasonic dual planetary stirrer at a revolution speed of 30-50 rpm, a rotation speed of 2100-2300 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 2.5-3.5 h.

[0063] In one specific embodiment, in the step of preparing the first mixture, an ultrasonic-double planetary stirrer is used for dispersion, with a revolution speed of 20-40 rpm, a rotation speed of 2500-3500 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 0.5-1.5 h.

[0064] In one specific embodiment, in the step of preparing the second mixture, an ultrasonic-double planetary stirrer is used for dispersion, with a revolution speed of 5-15 rpm, a rotation speed of 100-200 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 5-30 min.

[0065] In one specific embodiment, in the step of preparing the third mixture, an ultrasonic-double planetary stirrer is used for dispersion, with a revolution speed of 10-30 rpm, a rotation speed of 200-300 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 1-2 hours.

[0066] In one specific embodiment, in the step of preparing the fourth mixture, an ultrasonic-double planetary stirrer is used for dispersion, with a revolution speed of 30-50 rpm, a rotation speed of 1000-2000 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 1.5-4 h.

[0067] In one specific embodiment, in the step of preparing the fourth mixture, 60-80% of the first mixture is first added to the third mixture, and dispersed using an ultrasonic-double planetary stirrer at a revolution speed of 30-50 rpm, a rotation speed of 1400-1600 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 1.5-2.5 h. Then, 20-40% of the first mixture is added to the third mixture, and dispersed using an ultrasonic-double planetary stirrer at a revolution speed of 30-50 rpm, a rotation speed of 1100-1300 rpm, an ultrasonic frequency of 10-20 kHz, and a time of 0.5-1.5 h, to obtain the fourth mixture.

[0068] In one specific embodiment, the solid content of the first mixture is 3% to 5%.

[0069] In one specific embodiment, the solid content of the positive electrode material is 55% to 60%, the viscosity is 2500 to 4000 MPa.s, and the fineness is ≤15 μm.

[0070] In one specific embodiment, the positive electrode material, by weight parts, is PVDF:NMP:CNT:SP:Lithium manganese iron phosphate:Lithium nickel cobalt manganese oxide = 0.1~1:10~30:10~30:0.5~3:30~80:20~70.

[0071] This application also relates to a cathode material prepared by the above-described preparation method.

[0072] This application also relates to a positive electrode, which has a positive electrode current collector and a positive electrode material attached to the positive electrode current collector, wherein the positive electrode material is a positive electrode material prepared by the above-described preparation method.

[0073] This application also relates to a battery, which includes the aforementioned positive electrode.

[0074] The present application will be further described in detail below through specific embodiments. These embodiments are only for further illustration and should not be construed as limiting the present application. It should be noted that the reagents, kits, instruments, etc., used in the embodiments are all commercially available, and the operating steps are performed according to the instruction manual or general operating steps in the art.

[0075] Example 1:

[0076] 1) Mix PVDF and NMP and disperse them using an ultrasonic dual planetary stirrer at a speed of 40 rpm revolution and 2200 rpm rotation, an ultrasonic frequency of 15 kHz, a time of 3 h, a temperature of 25 ℃, and a vacuum of -1 MPa to obtain the material.

[0077] 2) Add CNTs to material A, disperse them at a revolution speed of 30 rpm, a rotation speed of 3000 rpm, an ultrasonic frequency of 15 kHz, a time of 1 h, a temperature of 25 ℃, and a vacuum degree of -1 MPa to obtain material B.

[0078] 3) Mix lithium manganese iron phosphate and lithium nickel cobalt manganese oxide of series 9, disperse at a revolution speed of 10 rpm, a rotation speed of 150 rpm, an ultrasonic frequency of 15 kHz, a time of 30 min, a temperature of 25 ℃, and a vacuum degree of -1 MPa to obtain material C.

[0079] 4) Add SP to material C, disperse at a revolution speed of 20 rpm and a rotation speed of 200 rpm, an ultrasonic frequency of 15 kHz, a time of 1.5 h, a temperature of 25 ℃, and a vacuum degree of -1 MPa to obtain material D.

[0080] 5) Add 70% of material B to material D, disperse at a revolution speed of 40 rpm, a rotation speed of 1500 rpm, an ultrasonic frequency of 15 kHz, a time of 2 h, a temperature of 25 ℃, and a vacuum degree of -1 MPa to obtain material E.

[0081] 6) Add 30% of material B to material E, disperse at a revolution speed of 40 rpm, a rotation speed of 1200 rpm, an ultrasonic frequency of 15 kHz, a time of 1 h, a temperature of 25 ℃, and a vacuum degree of -1 MPa to obtain material F.

[0082] 7) Add appropriate NMP to material F to adjust the viscosity of material F to 2500-4000 mPa·s, and obtain material G.

[0083] 8) Use a 200-mesh sieve to sieve material G under a vacuum of -0.01mPa, and then use an iron removal device to stir the slurry.

[0084] In the cathode material, by weight, the ratio of PVDF:NMP:CNT:SP:Lithium manganese iron phosphate:Lithium nickel cobalt manganese oxide (the ratio of nickel, cobalt and lithium is 9:0.5:0.5) is 0.55:15.5:15:1:60:40.

[0085] Subsequently, the negative electrode material was prepared: CMC, water, CNT, conductive adhesive, graphite, SP and SBR were mixed evenly to obtain the negative electrode material, wherein, by weight parts, CMC:water:CNT:conductive adhesive:graphite:SP:SBR = 0.45:82:15:2:100:1:1.2.

[0086] Mix 0.45 parts CMC, 82 parts deionized water, 15 parts CNT, 2 parts conductive adhesive, 100 parts graphite, 1 part SP, and 1.2 parts SBR until homogeneous.

[0087] Electrolyte preparation: Lithium salt LiPF6 and solvents (including DMC, EMC, EC and PC) and additives (including VC, PE, FEC, DTD, PS and TVSi) are mixed evenly to obtain the electrolyte. The electrolyte is prepared by weight in the following proportions: LiPF6:DMC:EMC:EC:PC:VC:PE:FEC:DTD:PS:TVSi = 6.5:50:9:2.8:13.8:0.2:9:12:0.5:1.2:0.4.

[0088] Battery assembly:

[0089] The positive electrode material is coated onto the positive electrode sheet (aluminum foil), and the negative electrode paste is coated onto the negative electrode sheet (copper foil). The coating parameters for both the positive and negative electrodes are: areal density 17.3 mg / cm³. 2 Compacted density 1.6 g / cm³ 3 Thickness 0.047mm. Figure 1 This is a surface morphology diagram of the positive electrode sheet in Embodiment 1 of this application. Subsequently, the positive electrode sheet, negative electrode sheet, and electrolyte are wound into a cylindrical battery.

[0090] Comparative Example 1: Instead of adding material B to material D in steps for mixing, material B is added to material D all at once for mixing. That is, steps 5) and 6) are combined into one step. Other conditions are the same as in Example 1.

[0091] Comparative Example 2: Instead of mixing lithium manganese iron phosphate and lithium nickel cobalt manganese oxide first and then mixing with SP, we mixed lithium manganese iron phosphate, lithium nickel cobalt manganese oxide and SP together, that is, we combined steps 3) and 4) into one step, and other conditions were the same as in Example 1.

[0092] Comparative Example 3: Instead of mixing PVDF and NMP first and then mixing them with CNT, PVDF, NMP and CNT were mixed together. That is, steps 1) and 2) were combined into one step. Other conditions were the same as in Example 1.

[0093] Comparative Example 4: In step 5, the dispersion speed was 30 rpm for revolution and 1000 rpm for rotation, and other conditions were the same as in Example 1.

[0094] Comparative Example 5: The ultrasonic frequency during the preparation of the cathode material was changed to 5kHz, and other conditions were the same as in Example 1.

[0095] In Comparative Example 1, the cathode material exhibits poor composite performance of lithium manganese iron phosphate with lithium nickel cobalt manganese oxide, resulting in slurry agglomeration and poor electrode adhesion. In Comparative Example 2, the cathode material also shows poor composite performance of lithium manganese iron phosphate with lithium nickel cobalt manganese oxide, with slurry agglomeration and high internal resistance. In Comparative Example 3, the slurry material B exhibits poor dispersion, leading to a high drain-cement ratio (DCR) during battery cycling. In Comparative Example 4, the cathode material contains large particles in the slurry, and the electrode surface shows particles and pitting, causing the battery to fail overcharge and over-discharge safety tests. In Comparative Example 5, the cathode material exhibits slurry agglomeration and poor dispersion, resulting in poor rate performance.

[0096]

[0097] The steps for each test item are as follows:

[0098] Peel force test: The peeling of the electrode surface material was tested using an electronic tensile testing machine at a speed of 250 mm / min, a test time of 36.2 s, and a maximum deformation of 150 mm.

[0099] Battery AC internal resistance (DCR) test: The battery was charged to 100% SOC using the Nebula lithium-ion battery test cabinet, then discharged to 50% SOC and left to stand for 1 hour. It was then discharged at 1C for 30 seconds, and the battery AC internal resistance was calculated.

[0100] Overcharge and over-discharge test: According to the national standard GB31241-2022, the battery passes the overcharge test if it does not catch fire or explode when charged to 6.3V at 1C, and passes the over-discharge test if it does not catch fire or explode when discharged to 2V at 1C; the battery fails the overcharge and over-discharge test if it catches fire or explodes.

[0101] 5C rate performance test: According to the national standard GBT31486-2015, at room temperature, it is charged to 4.2V at 2C, left to stand for 1 hour, and discharged to 2.5V at 5C. The discharge capacity is not less than 80% of the initial capacity to pass the test.

[0102] The above description, in conjunction with specific embodiments, provides a further detailed explanation of this application and should not be construed as limiting the specific implementation of this application to these descriptions. Those skilled in the art to which this application pertains can make several simple deductions or substitutions without departing from the concept of this application.

Claims

1. A method for preparing a composite cathode material of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, characterized in that, The preparation method includes: Prepare raw materials, which include active substances, conductive agents, binders and solvents. The active substances include lithium manganese iron phosphate and lithium nickel cobalt manganese oxide. The conductive agents include carbon nanotubes and SP. To prepare a first solution, the adhesive and solvent are mixed to obtain the first solution; To prepare the first mixture, the carbon nanotubes are added to the first solution and mixed to obtain the first mixture. To prepare the second mixture, lithium manganese iron phosphate and lithium nickel cobalt manganese oxide are mixed to obtain the second mixture. To prepare the third mixture, the second mixture and SP are mixed to obtain the third mixture. Preparation of the fourth mixture: The first mixture is added to the third mixture in two batches and mixed evenly to obtain the fourth mixture, which is the positive electrode material; wherein, when the first mixture is added to the third mixture for mixing, the mixing is carried out by stirring at a speed of 1500 rpm to 4000 rpm. in: The steps of preparing the first solution, the first mixture, the second mixture, the third mixture, and the fourth mixture are all performed in a vacuum environment. In the steps of preparing the first solution, the first mixture, the second mixture, the third mixture, and the fourth mixture, ultrasonic treatment is used for mixing, and the frequency of the ultrasonic treatment is 10kHz~20kHz.

2. The preparation method according to claim 1, characterized in that, The vacuum level of the vacuum environment is -5MPa to -1MPa.

3. The preparation method according to claim 1, characterized in that, In at least one step of preparing the first solution, the first mixture, the second mixture, and the third mixture, mixing is further performed by stirring, wherein the stirring rate is 1000 rpm to 4000 rpm and the processing time is 0.1 h to 3.5 h.

4. The preparation method according to claim 3, characterized in that, The mixing is performed using a dual planetary mixer with a revolution speed of 5~50 rpm and a rotation speed of 1000~4000 rpm.

5. The preparation method according to claim 1, characterized in that, The mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide is 1:9 to 9:1, the D50 of lithium manganese iron phosphate is 1 to 5 μm, and the D50 of lithium nickel cobalt manganese oxide is 5 to 20 μm.

6. A cathode material prepared by the preparation method according to any one of claims 1 to 5.

7. A positive electrode, characterized in that, The positive electrode includes a positive electrode current collector and the positive electrode material as described in claim 6 attached to the positive electrode current collector.

8. A battery, characterized in that, The battery includes the positive electrode as described in claim 7.

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