A positive electrode material, battery and preparation method of a mixture of lithium iron phosphate and lithium iron manganese phosphate

Abstract

The present invention discloses a cathode material, a battery and a preparation method of a combination of lithium iron phosphate and lithium iron manganese phosphate; lithium iron phosphate, lithium iron manganese phosphate and a polymer are mixed and then pyrolyzed at low temperature to obtain a cathode material of a combination of lithium iron phosphate and lithium iron manganese phosphate. At present, although LFP and LMFP are used in lithium-ion batteries, the objective defects of the two have limited their application. The existing technology cannot solve the contradiction between energy density and processing performance, and the existing technical solutions have a complex process that makes them unfavorable for industrial production and application. The present invention adopts a simple technical solution and creatively proposes a cathode material of a combination of lithium iron phosphate and lithium iron manganese phosphate, which unexpectedly achieves good cycle stability, safety and processing performance, and can improve the energy density of the battery, thereby realizing the promotion of the cathode in high energy density application fields.

Description

Technical Field

[0001] The present invention belongs to battery technology and relates to lithium-ion batteries, and in particular to a positive electrode material blending lithium iron phosphate and lithium iron manganese phosphate, a battery and a preparation method. Background Art

[0002] Lithium iron phosphate (LiFePO4, LFP) and lithium iron manganese phosphate (LiMn x Fe 1-x Lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) are two common cathode materials for lithium-ion batteries. Currently, single-material systems have limitations in battery performance, making it difficult to balance processability and energy density. This makes large-scale application of lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) difficult in actual production. Existing solutions primarily focus on optimizing single-material systems. For example, improving the performance of LFP or LMFP through doping and coating still struggles to resolve the conflict between energy density and processability. Furthermore, existing technologies, such as nano-scaling and carbon coating, can reduce battery energy density. Furthermore, single-material systems struggle to meet the diverse battery performance requirements of diverse application scenarios.

[0003] The existing technology grinds, sinters and crushes a mixture of lithium iron manganese phosphate, a lithium iron phosphate precursor and water to form a lithium iron manganese phosphate composite material, wherein the lithium iron manganese phosphate forms an inner core with an average particle size of 600nm~1200nm, and the lithium iron phosphate precursor forms a lithium iron phosphate coating layer and a carbon coating layer. This technical solution still uses a traditional coating method, and the preparation process is complicated. The product performance is obviously dependent on the preparation process and control, which makes it difficult to apply to industrial production, especially without improving the processing performance of the material.

[0004] Therefore, it is necessary to develop new positive electrode materials based on the existing industrial products lithium manganese iron phosphate and lithium iron phosphate, hoping to obtain positive electrode materials with improved electrical properties and processing properties, thereby improving the performance of lithium-ion batteries; in particular, the material can be conventionally industrialized and is conducive to commercial application. Summary of the Invention

[0005] Currently, although LFP and LMFP have been used in lithium-ion batteries, their objective defects have limited their application, such as low voltage platform, relatively low energy density, and poor processing performance. These factors lead to problems such as agglomeration and uneven dispersion during electrode preparation, which in turn affect battery performance. Existing technologies cannot resolve the contradiction between energy density and processing performance, and existing technical solutions have complex process steps that make them unsuitable for industrial production and application. The present invention adopts a simple technical solution and creatively proposes a cathode material that combines lithium iron phosphate and lithium manganese iron phosphate. For the first time, lithium iron phosphate and lithium manganese iron phosphate are used as the main raw materials. Through simple physical mixing, preferably combined with a low-temperature pyrolysis polymer, the material unexpectedly achieves good cycle stability, safety, and processing performance, and can improve the energy density of the battery, thus promoting the promotion of this cathode in high-energy-density applications.

[0006] The present invention adopts the following technical solutions.

[0007] A lithium iron phosphate and lithium iron manganese phosphate cathode material comprises a mixture of lithium iron phosphate and lithium iron manganese phosphate. The resistance of the lithium iron phosphate and lithium iron manganese phosphate cathode material is ≤ 20Ω·cm. This resistance is the powder resistance, measured using a conventional four-probe method.

[0008] In the present invention, the weight ratio of lithium iron phosphate to lithium manganese iron phosphate is 1: (0.25-4); preferably, the weight ratio of lithium iron phosphate to lithium manganese iron phosphate is 1: (0.4-2.5); further preferably, the weight ratio of lithium iron phosphate to lithium manganese iron phosphate is 1: (0.5-2), such as 3:2, 1:1, 1:1.5 or any ratio within the range.

[0009] In the present invention, the particle size of lithium iron phosphate is larger than that of lithium iron manganese phosphate. Preferably, the particle size of lithium iron phosphate is 300nm to 700nm, and the particle size of lithium iron manganese phosphate is 100nm to 300nm, but not both are 300nm. More preferably, the particle size of lithium iron phosphate is 350nm to 500nm, and the particle size of lithium iron manganese phosphate is 150nm to 300nm. Unless otherwise specified, particle size refers to D50 particle size.

[0010] In the present invention, the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material also includes a low-temperature pyrolysis polymer.

[0011] Preferably, the temperature of the low-temperature pyrolysis is 200-400°C; more preferably, the temperature of the low-temperature pyrolysis is 250-350°C, and even more preferably, 280-320°C.

[0012] Preferably, the polymer includes one or more of polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyvinylidene fluoride (PVDF).

[0013] Preferably, the low-temperature pyrolysis is carried out in an inert gas, such as nitrogen or argon.

[0014] In the present invention, the polymer is pyrolyzed at a low temperature of about 300°C to form an amorphous carbon layer with both electrical conductivity and adhesiveness. The carbon layer is evenly filled between the lithium iron phosphate and lithium manganese iron phosphate mixtures, and can provide an additional electron conduction path for the LFP and LMFP particles, thereby improving the conductivity of the material. In particular, no additional conductive agent needs to be added during the later production of the positive electrode and the battery, thereby reducing the process flow and the difficulty of dispersing the conductive agent. In addition, the low-temperature pyrolyzed polymer has liquid retention and adhesiveness and can absorb electrolyte, thereby improving the liquid retention of the electrode. The adhesiveness can better contact the lithium iron phosphate and the lithium manganese iron phosphate, thereby optimizing the electronic conduction and interface contact of the electrode.

[0015] The present invention discloses a preparation method of the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material, comprising the following steps: mixing lithium iron phosphate and lithium iron manganese phosphate to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material.

[0016] Furthermore, the preparation method of the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material includes the following steps: mixing lithium iron phosphate, lithium iron manganese phosphate and a polymer, and then heat-insulating to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material.

[0017] Preferably, the temperature of the heat preservation treatment is 250 to 350° C., preferably 280 to 320° C.; the time of the heat preservation treatment is 15 to 120 minutes, preferably 25 to 80 minutes, and more preferably 35 to 60 minutes.

[0018] Preferably, the mass of the polymer is 0.5-5% of the sum of the mass of lithium iron phosphate and lithium iron manganese phosphate, and further preferably, the mass of the polymer is 1-4% of the sum of the mass of lithium iron phosphate and lithium iron manganese phosphate; for example, the mass of the polymer is 1%, 1.5%, 2%, 2.5%, 3%, 3.5% of the sum of the mass of lithium iron phosphate and lithium iron manganese phosphate, or any data within the range.

[0019] The invention discloses a mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry, comprising the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material, an adhesive and / or a solvent.

[0020] In the present invention, the rheological recovery rate of the positive electrode slurry blended with lithium iron phosphate and lithium manganese iron phosphate is ≥80%, which is obtained by a conventional 3ITT test method.

[0021] The present invention discloses a method for preparing the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry, comprising the following steps: mixing the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode materials and a binder to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry; or mixing the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode materials, a binder and a solvent to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry.

[0022] The present invention discloses a hybrid lithium iron phosphate and lithium iron manganese phosphate positive electrode plate, comprising the aforementioned hybrid lithium iron phosphate and lithium iron manganese phosphate positive electrode material. The hybrid lithium iron phosphate and lithium iron manganese phosphate positive electrode plate has a sheet resistance of 0.30 Ω·cm² or less and a peel force of 38 N / m or more. The sheet resistance refers to the single-surface sheet resistance of the hybrid lithium iron phosphate and lithium iron manganese phosphate positive electrode plate, as measured by a conventional four-probe method; the peel force refers to the result after roller pressing, as measured by the adhesive tape peel strength test method.

[0023] Furthermore, the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode plate also includes a current collector; preferably, the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material is located on the current collector.

[0024] In the present invention, the current collector includes a two-dimensional current collector or a three-dimensional current collector, such as copper foil.

[0025] In the present invention, a positive electrode sheet of mixed lithium iron phosphate and lithium manganese iron phosphate is obtained by coating a positive electrode slurry of mixed lithium iron phosphate and lithium manganese iron phosphate on the surface of a current collector, drying, rolling and slicing.

[0026] Preferably, the coated single-sided surface density is 200-300 g / m²; further preferably, the coated single-sided surface density is 220-260 g / m².

[0027] In the present invention, the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry includes the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material, a binder and / or a solvent.

[0028] Preferably, the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode slurry contains a conductive agent or does not contain a conductive agent, and the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet prepared therefrom contains a conductive agent or does not contain a conductive agent. Preferably, the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet does not contain a conductive agent.

[0029] The prior art generally adds conductive agents (such as carbon black and carbon nanotubes) to the positive electrode slurry. Otherwise, the electrode has poor conductivity, which also brings difficulties in process operation because carbon black, carbon nanotubes, etc. are well-known to be difficult to disperse. The present invention overcomes the prejudice of the prior art and discloses a positive electrode plate of a blend of lithium iron phosphate and lithium iron manganese phosphate that does not contain a conductive agent, unexpectedly achieving good electrical properties and overcoming the problem of difficult dispersion of conductive agents in the existing slurry, which leads to a complicated preparation process.

[0030] Preferably, the sum of the weight of the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode materials and the binder is 100%, wherein the weight percentage of the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode materials is 95% to 100%, excluding 100%; further preferably, the weight percentage of the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode materials is 96% to 99.5%, and even more preferably, the weight percentage of the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode materials is 97% to 99%, such as 96.5%, 97.5%, 98%, 98.5%, or any data within the range. The amount of binder used in the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode slurry of the present invention is less than the conventional amount used. Unexpectedly, the combined lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet prepared thereby has good peeling performance.

[0031] In the present invention, the solid content of the positive electrode slurry of the mixed lithium iron phosphate and lithium iron manganese phosphate is 50% to 90%; preferably, the solid content of the positive electrode slurry of the mixed lithium iron phosphate and lithium iron manganese phosphate is 55% to 80%; further preferably, the solid content of the positive electrode slurry of the mixed lithium iron phosphate and lithium iron manganese phosphate is 60% to 75%; for example, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69% or any data within the range.

[0032] The present invention discloses a battery comprising the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material or the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode plate.

[0033] In the present invention, the battery is a lithium-ion battery.

[0034] As common sense, lithium-ion batteries also include conventional components such as negative electrode plates, separators, and electrolytes.

[0035] The present invention discloses the application of the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material, mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry, and mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode sheet in the preparation of batteries; specifically, the battery is a lithium ion battery.

[0036] Existing technologies usually adopt a single material system, which makes it difficult to resolve the contradiction between energy density and processing performance. Although the conductivity and processing performance of the material can be improved by nano-sizing, carbon coating and other means, it will lead to a decrease in the energy density of the battery. The present invention provides a lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) mixed positive electrode material and a preparation method thereof, which have the following beneficial effects: the process flow is simple, which improves the energy density of the battery, while improving the processing performance of the electrode, and achieving a good balance between energy density and processing performance; the weight ratio of lithium iron phosphate and lithium manganese iron phosphate is 1: (0.25~4), and further by controlling the particle size distribution of LFP and LMFP, the mixed positive electrode material has good processing performance and maintains the advantage of high energy density, which is beneficial to improving the capacity and energy density of the battery; low temperature The addition of pyrolytic polymers can improve the conductivity and liquid retention of the material, optimize the contact between LFP and LMFP particles, reduce the interfacial impedance, and help improve the capacity and energy density of the battery; further, low-temperature pyrolytic polymers can reduce or even avoid the addition of conductive carbon black or CNT slurry in the slurry (this is a technical means considered necessary by the existing technology), ensuring that LFP and LMFP are evenly dispersed in the electrode slurry, avoiding the occurrence of agglomeration, and further improving the processing performance of the electrode; the use of mixed materials can increase the discharge equalization voltage of the battery, making the voltage output more stable; extending the cycle life of the battery and improving the cycle stability of the battery; meeting the diverse needs of battery performance in different application scenarios, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a scanning electron microscope image of a positive electrode material that blends lithium iron phosphate and lithium iron manganese phosphate.

[0038] Figure 2 This is the rheological curve of the slurry made by mixing lithium iron phosphate and lithium manganese iron phosphate positive electrode materials - 3ITT. DETAILED DESCRIPTION

[0039] Lithium iron manganese phosphate is considered an "upgraded version of lithium iron phosphate," with cycling performance and safety comparable to lithium iron phosphate. Its theoretical energy density is approximately 20% higher than that of lithium iron phosphate, addressing the pain point of lithium iron phosphate's insufficient energy density. However, lithium iron phosphate also suffers from issues such as low conductivity, poor battery cycling stability, and poor processing performance. Carbon coating and metal ion doping are common methods for modifying lithium iron phosphate in existing technologies, but these methods offer limited improvements and still fail to fully leverage the advantages of lithium iron phosphate's high energy density, resulting in poor cycling performance.

[0040] The present invention provides a lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) mixed positive electrode material and a battery thereof. The process flow is simple, and the energy density of the battery is increased while improving the processing performance of the electrode, thereby achieving a good balance between energy density and processing performance.

[0041] The present invention mixes lithium iron phosphate, lithium iron manganese phosphate, and a polymer, and then heat-treats the mixture to obtain a mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material. Preferably, the present invention mixes lithium iron phosphate and lithium iron manganese phosphate to obtain a lithium iron phosphate and lithium iron manganese phosphate mixture, and then mixes the mixture with a polymer, and then heat-treats the mixture to obtain a mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material.

[0042] Wherein, mixing is physical mixing, such as stirring and mixing. The present invention stirs and mixes three solid materials, lithium iron phosphate, lithium iron manganese phosphate, and polymer, without the need to add liquid, which reduces the difficulty of preparation. The polymer can decompose to produce a carbon layer that is evenly filled between the lithium iron phosphate and lithium iron manganese phosphate mixtures, and can provide additional electron conduction paths for LFP and LMFP particles, thereby improving the conductivity of the material. As an example, the present invention stirs and mixes lithium iron phosphate powder and lithium iron manganese phosphate powder to obtain a mixture of lithium iron phosphate and lithium iron manganese phosphate; then adds the polymer, stirs and mixes again, and then performs low-temperature pyrolysis treatment to obtain a mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material. The present invention preferably has a resistance of ≤20Ω·cm for the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material. This resistance is powder resistance, tested by a conventional four-probe method.

[0043] Preferably, the temperature of the heat preservation treatment is 250 to 350° C., preferably 280 to 320° C.; the time of the heat preservation treatment is 15 to 120 minutes, preferably 25 to 80 minutes, and more preferably 35 to 60 minutes.

[0044] Preferably, the polymer includes one or more of polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyvinylidene fluoride (PVDF). The polymer is pyrolyzed at 300°C to form an amorphous carbon layer with both electrical conductivity and adhesive properties. This layer is evenly filled between the lithium iron phosphate and lithium manganese iron phosphate mixtures, providing an additional electron conduction path for the LFP and LMFP particles, thereby improving the conductivity of the material. In particular, the addition of a conductive agent is not required during the subsequent battery production process, reducing process flow and carbon black dispersion issues. Furthermore, the low-temperature pyrolysis polymer has liquid retention and adhesive properties, allowing it to absorb electrolyte and improve the liquid retention of the electrode. The adhesive property allows for better contact between the lithium iron phosphate and lithium manganese iron phosphate, optimizing the electronic conduction and interfacial contact of the electrodes. Furthermore, the binder content can be reduced, increasing the proportion of active material in the material, thereby increasing the energy density of the battery cell within a limited space and reducing the material cost of the battery cell.

[0045] The present invention preferably has a weight ratio of lithium iron phosphate to lithium manganese iron phosphate of 1: (0.25-4); further preferably 1: (0.4-2.5); further preferably 1: (0.5-2), and further preferably 1: 1.5-1.5: 1, such as 1: 1.2, 1: 1, 1.2: 1, or any ratio within the range; the present invention preferably has a particle size of lithium iron phosphate of 350nm-500nm, and a particle size of lithium manganese iron phosphate of 150nm-300nm. Furthermore, the powder compaction density of the lithium iron phosphate is 2.5g / cm 3 The powder compaction density of lithium manganese iron phosphate is 2.2g / cm 3 The mixed cathode material has good processing performance while maintaining the advantage of high energy density, which is beneficial to improving the capacity and energy density of the battery.

[0046] Based on the above-mentioned blended lithium iron phosphate and lithium iron manganese phosphate positive electrode material, the present invention further discloses a blended lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry, a blended lithium iron phosphate and lithium iron manganese phosphate positive electrode sheet and a battery containing the blended lithium iron phosphate and lithium iron manganese phosphate positive electrode material. Preferably, the battery is a lithium ion battery.

[0047] The rheological recovery rate of the positive electrode slurry of the present invention blending lithium iron phosphate and lithium iron manganese phosphate is ≥80%, which is obtained by the conventional 3ITT test method. For specific test conditions, please refer to the test examples below. These test conditions are for those skilled in the art to understand the technical effects of the present invention, rather than limiting the technical solution of the present invention; wherein, the sum of the weight of the blended lithium iron phosphate and lithium iron manganese phosphate positive electrode materials and the binder is 100%, wherein the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate positive electrode materials is 95% to 100%, excluding 100%; further preferably, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate positive electrode materials is 96% to 99.5%, and even more preferably, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate positive electrode materials is 97% to 99%, such as 96.5%, 97.5%, 98%, 98.5% or any data within the range. The amount of binder used in the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode slurry of the present invention is less than the conventional amount. Unexpectedly, the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet prepared thereby has good peeling performance.

[0048] The sheet of the present invention's combined lithium iron phosphate and lithium iron manganese phosphate positive electrode has a sheet resistance of ≤0.30 Ω·cm² and a peel strength of ≥38 N / m. Sheet resistance refers to the single-surface resistance of the combined lithium iron phosphate and lithium iron manganese phosphate positive electrode, as measured using a conventional four-probe test method; peel strength refers to the result after roller pressing, as measured using the adhesive tape peel strength test method.

[0049] The following specific experiments illustrate the technological advancements of the present invention. The raw materials used are all existing products that meet the general requirements of lithium-ion batteries. The specific preparation operations and performance tests are all conventional techniques. Unless otherwise specified, all operations are performed at room temperature and normal pressure. The obtained data are the average value of three parallel tests. Unless otherwise specified, the particle size in the present invention refers to the D50 particle size. Example 1

[0050] The preparation method of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material is as follows:

[0051] A lithium iron phosphate material with a D50 of 400 nm and a lithium manganese iron phosphate with a D50 of 200 nm were mixed in a weight ratio of 1:1 to obtain a mixture; the mixture was mixed with polyethylene glycol (PEG-6000), and then kept at 300°C in nitrogen for 40 minutes to obtain a mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material; the amount of polyethylene glycol used was 2% by weight of the mixture.

[0052] Figure 1 This is a scanning electron microscope image of the above-mentioned mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material. It can be seen that the processing performance is good, and no problems such as agglomeration and uneven dispersion occur during the electrode preparation process; there is an obvious particle size gradient between the particles, and the large particles (LFP) fill the gaps between the small particles (LMFP) to form a dense stacking without obvious agglomeration. Moreover, unlike the carbon coating in the existing technology, the above-mentioned low-temperature pyrolysis polymer forms an amorphous carbon layer that fills between the two particles, increasing the conductivity and also building an additional electron conduction path between the materials, improving the contact between the LFP and LMFP particles, and reducing the interface impedance.

[0053] Comparative Example 1

[0054] Refer to Example 1, with the difference being that only lithium iron phosphate material is mixed with polyethylene glycol, i.e. lithium iron manganese phosphate is omitted; the rest is the same, benchmarked against commercially available lithium iron phosphate positive electrode products.

[0055] Comparative Example 2

[0056] Refer to Example 1, with the difference being that only lithium manganese iron phosphate material is mixed with polyethylene glycol, i.e. lithium iron phosphate is omitted; the rest is the same, and the commercially available lithium manganese iron phosphate positive electrode product is benchmarked.

[0057] Comparative Example 3

[0058] Referring to Example 1, the difference is that the D50 of the lithium iron phosphate material is 150-300nm, and the D50 of the lithium iron manganese phosphate is 350-500nm; the rest are the same.

[0059] Comparative Example 4

[0060] A lithium iron phosphate material with a D50 of 350-500 nm and a lithium manganese iron phosphate with a D50 of 150-300 nm were mixed in a weight ratio of 1:1 to obtain a mixture, which was then heated at 300° C. in nitrogen for 40 minutes to serve as a positive electrode material. The difference from Example 1 is that the low-temperature pyrolysis polymer was omitted.

[0061] Comparative Example 5

[0062] Refer to Example 1, except that: the temperature is kept at 400° C. for 40 minutes in nitrogen to obtain a positive electrode material comprising a mixture of lithium iron phosphate and lithium manganese iron phosphate.

[0063] Comparative Example 6

[0064] Refer to Example 1, except that 2% of carbon black is used instead of the low-temperature pyrolysis polymer to obtain a positive electrode material of a mixture of lithium iron phosphate and lithium manganese iron phosphate. Example 2

[0065] Referring to Example 1, the difference is that the lithium iron phosphate material and the lithium manganese iron phosphate are mixed in a weight ratio of 1:1.5; the rest are the same. Example 3

[0066] Referring to Example 1, the difference is that the lithium iron phosphate material and the lithium iron manganese phosphate are mixed in a weight ratio of 1.5:1; the rest are the same. Example 4

[0067] Referring to Example 1, the difference is that the lithium iron phosphate material and the lithium manganese iron phosphate are mixed in a weight ratio of 1:4; the rest are the same. Example 5

[0068] Referring to Example 1, the difference is that the lithium iron phosphate material and the lithium manganese iron phosphate are mixed in a weight ratio of 4:1; the rest are the same. Example 6

[0069] Refer to Example 1, except that polyethylene glycol is replaced by polyvinyl alcohol (PVA-1788); the rest are the same. Example 7

[0070] Refer to Example 1, except that polyethylene glycol is replaced by polyvinylidene fluoride (PVDF-5130); the rest are the same. Example 8

[0071] Refer to Example 1, except that the amount of polyethylene glycol used is 3% of the weight of the mixture; the rest are the same. Embodiment 9

[0072] Refer to Example 1, except that the amount of polyethylene glycol used is 1% by weight of the mixture; the rest are the same. Example 10

[0073] Referring to Example 1, the difference is that the amount of polyethylene glycol used is 4% of the weight of the mixture, and a positive electrode material of a mixture of lithium iron phosphate and lithium manganese iron phosphate is obtained. Example 11

[0074] The positive electrode materials obtained above were mixed with a binder, and a solvent was added and stirred to form a positive electrode slurry. The weight ratio of the positive electrode material to the binder (polyvinylidene fluoride PVDF, HEVER 701) was 98.95:1.05, and no additional conductive agent was added. The solvent was N-methylpyrrolidone, and the solid content of the positive electrode slurry was 68%.

[0075] The blended lithium iron phosphate and lithium iron manganese phosphate positive electrode material disclosed in the present invention can reduce the amount of binder used in the positive electrode slurry due to the addition of a low-temperature pyrolysis polymer. Excellent bonding effect can be achieved with only 1.05%. For comparison, the mass ratio of phosphate, conductive agent, and binder in the current positive electrode slurry is (95-97): (1.4-2.6): (1.6-2.3), which not only requires more binder, but also significantly causes processing difficulties due to the addition of the conductive agent.

[0076] The positive electrode slurry is coated on the surface of the current collector aluminum foil (the coated single-side surface density is 245g / m²), and after drying, it is rolled and sliced to obtain the positive electrode sheet.

[0077] The weight ratio of graphite material, conductive agent (conductive carbon black SP) and binder (polyacrylic acid) is 95.03:0.49:4.48; the solvent is deionized water, and the solid content of the negative electrode slurry is 53%; the slurry after uniform stirring is coated on the surface of the current collector copper foil (the coated single-side surface density is 88g / m²), and after drying, it is rolled and sliced to obtain the negative electrode sheet. This is a conventional technique and does not affect the understanding of the technical effects of the present invention by those skilled in the art.

[0078] A square full battery is assembled from a positive electrode sheet, a negative electrode sheet, a separator, and a lithium hexafluorophosphate electrolyte (the concentration of LiPF6 is 1 mol / L); the negative electrode sheet, the separator, and the lithium hexafluorophosphate electrolyte are all existing technologies, and the specific assembly is conventional technology, which does not affect the understanding of the technical effects of the present invention by those skilled in the art.

[0079] Test Examples

[0080] Figure 2This is the 3ITT rheological curve of the positive electrode slurry made by blending lithium iron phosphate and lithium iron manganese phosphate positive electrode materials in Example 1. The 3ITT test method is widely used in the lithium battery industry to characterize the rheological properties and processing performance of battery slurries. By applying a certain shear rate to the fluid and measuring the change in viscosity during shear, it characterizes the fluid's structural destruction, structural recovery, and structural stability. The test process and test parameter settings use a step-by-step process: 1) In the static phase, a constant, small shear load is applied. At this time, the viscosity change is small and can be considered the initial viscosity; 2) In the shear phase, a constant, large shear load is applied; 3) In the structural recovery phase, the same constant shear load as in the first step is used. For the industrial production of positive electrode slurry for power batteries: a destruction rate of 70-80% has good fluidity, too high (>80%) leads to pumping difficulties, and too low (<70%) is easy to settle; a recovery rate of 80-90% has good fluidity and good anti-sagging ability, but exceeding 90% is considered to recover too quickly, which will cause the slurry to lose fluidity prematurely, which is not conducive to coating control and causes roller sticking during rolling; the slurry with a stability index of 98-102% has excellent recycling performance, and the slurry exceeding 102% has a tendency to gel and has the problem of hardening and mold blocking; the product of the present invention has good destruction rate, recovery rate and stability, and has good initial viscosity. It belongs to the optimal performance range in the positive electrode slurry system of power lithium batteries, and is even slightly better than lithium iron phosphate products, which is beyond people's imagination.

[0081] Test conditions:

[0082]

[0083] Test data:

[0084]

[0085] 1T means: first stage structural destructiveness: high shear rate is applied to destroy the structure, and the viscosity drops rapidly. Algorithm: Destruction rate = (( or 0− or 1) / or 0) × 100% = 77.11%, excellent fluidity and easy pumping and processing.

[0086] 2T means: Second stage structural reconstruction: reflects the structural recovery characteristics after the slurry removes the high shear rate. Algorithm: Recovery rate = ( or 2 / or 0) × 100% = 83.28%; Analysis: The recovery rate > 80% belongs to the fast recovery type, with good anti-settling and anti-sagging capabilities.

[0087] 3T meaning: The third stage (storage stability): reflects the stability of the slurry during storage, the reversibility of the slurry structure after recovery, and long-term stability; algorithm: stability index = ( or3 / or 0) × 100% = 101.21%. Analysis: Good stability indicates that the structure is reversible and suitable for long-term storage and recycling.

[0088] The slurries prepared in the examples and comparative examples and the battery test results are as follows:

[0089]

[0090] Compared to LFP (Comparative Example 1) and LMFP (Comparative Example 2), this invention achieves significant technological advancements, boasting superior energy density and processability, facilitating practical application. Unexpectedly, it improves the high recovery rate (89.66%) of lithium iron phosphate slurry alone, making it more compatible with industrial production requirements. Comparative Examples 1 and 2 exhibited insufficient adhesion, hindering later cycling. Increasing the binder dosage to 2 wt% during slurry preparation yielded positive electrode sheets with post-roll peel strengths of 38 N / m and 34 N / m, respectively. However, these formulations yielded batteries with low energy density and, in particular, high cost.

[0091] As a comparison, in the case of less adhesive, the positive electrode sheet prepared with the existing active material, conductive agent, and adhesive formula has poor peeling performance and cannot be used, so no further testing is performed.

[0092] Further particle size selection improves the product's electrochemical performance, leveraging its high voltage advantage and increasing the average voltage. In particular, polymer treatment significantly enhances conductivity, increasing the resistance from 15.20 Ω·cm² and the capacity from 120 mAh / g to 0.28 Ω·cm² and 133 mAh / g. In summary, this invention (LFP:LMFP = 1:1.5 to 3:2) achieves breakthroughs in both processability and energy density through particle size grading and 2-3% PEG modification.

[0093] The test method involved in the present invention is a conventional method, which is briefly described as follows. The test conditions are intended to enable those skilled in the art to understand the technical effects of the present invention, but are not intended to limit the technical solution of the present invention:

[0094] Viscosity test method: According to GB / T 22235-2008 (liquid viscosity determination method); for lithium battery positive electrode slurry with solid content of 65%-70%, the viscosity is generally considered to be less than 10 4 mPa·s has processing properties that can be used in industrial applications, especially below 9000 mPa·s, which is the basis of good processing properties.

[0095] Solid content test method: According to GB / T 6284-2016 (Determination of moisture content in chemical products);

[0096] Electrode compaction density test method: in accordance with GB / T 24533-2019 (lithium-ion battery electrode test method);

[0097] Electrode sheet resistance test method: Use a diaphragm resistance tester, Yuanneng Technology BER 1300, to cut a 50 mm × 50 mm electrode sample (flat and wrinkle-free). Measure three times at a constant pressure of 25 MPa and take the average value.

[0098] Peel force test method: Servo material testing machine, Haida HD-B609B-S, positive electrode: 300mm (L) × 25mm (W), double-sided tape is attached to a stainless steel plate, the electrode active material layer and the tape are laminated; 0.5MPa pressure roller is pressed three times (speed 10mm / s); one end of the tape is attached to the stainless steel plate, and the stainless steel plate is fixed to the clamp of the tensile testing machine. The other clamp of the testing machine clamps the free end of the tape at a 90° angle to the stainless steel plate, and the tape is pulled apart at 100mm / min; after the tensile testing machine clamp is running, the value obtained when mechanically peeling the first 25mm of the tape is ignored, and the average force obtained by the next 50mm of the tape is used as the peel force, which is converted to peel strength: Peel strength (N / m) = average force (F) × 1000 / sample width (25mm). Three parallel measurements are taken and the average value is calculated.

[0099] Powder resistance test method: Four-probe method, using Lattice Electronics ST2742B, take 3g of dry powder (vacuum dried at 105℃ for 12h), test pressure: 30MPa, molded into a diameter of 13mm; parallel measurement three times, take the average value;

[0100] The test shows that the present invention has improved standard capacity, surface resistance and discharge voltage compared with the same type of battery.

[0101] The embodiments described above are merely preferred embodiments of the present invention, intended to facilitate understanding and application of the present invention by those skilled in the art. Obviously, anyone skilled in the art can modify or alter these embodiments without inventive effort and apply them to other embodiments. Therefore, the present invention is not limited to the embodiments described above; any equivalent variations, simple modifications, and modifications made within the scope of the present invention remain within the scope of the present invention.

Claims

1. A method for preparing a positive electrode material comprising lithium iron phosphate and lithium iron manganese phosphate, characterized in that: The method comprises the following steps: mixing lithium iron phosphate, lithium iron manganese phosphate and a polymer, and then performing heat preservation treatment to obtain a mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material; the heat preservation temperature is 250-350°C; the heat preservation time is 25-80 minutes; the weight ratio of lithium iron phosphate to lithium iron manganese phosphate is 1:(0.4-2.5); the particle size of lithium iron phosphate is 300nm-700nm, and the particle size of lithium iron manganese phosphate is 100nm-300nm, and the particle size of both is not 300nm at the same time; the mass of the polymer is 1.5-5% of the total mass of the lithium iron phosphate and the lithium iron manganese phosphate; the polymer comprises one or two of polyethylene glycol and polyvinylidene fluoride.

2. The method for preparing a cathode material comprising a mixture of lithium iron phosphate and lithium iron manganese phosphate according to claim 1, wherein: The resistance of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material is ≤20Ω·cm.

3. A mixed lithium iron phosphate and lithium manganese phosphate positive electrode material prepared according to the method for preparing a mixed lithium iron phosphate and lithium manganese phosphate positive electrode material according to claim 1.

4. A positive electrode slurry combining lithium iron phosphate and lithium iron manganese phosphate, characterized in that: Comprising the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material and adhesive as described in claim 3; the sum of the weight of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material and the adhesive is 100%, wherein the weight percentage of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material is 95% to 100%, excluding 100%.

5. The positive electrode slurry of lithium iron phosphate and lithium manganese iron phosphate according to claim 4, characterized in that: The weight percentage of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode material is 98.5% to 99.5%.

6. A mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode sheet, comprising the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material according to claim 3, characterized in that: The surface resistance of the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet is ≤0.30Ω·cm², and the peeling force is ≥38N / m.

7. The method for preparing a positive electrode sheet of a mixture of lithium iron phosphate and lithium iron manganese phosphate according to claim 6, characterized in that: The method comprises the following steps: coating the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode slurry according to claim 4 on the surface of the current collector, then drying, and rolling to obtain the mixed lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet.

8. A battery, characterized in that: It includes the positive electrode material of the combination of lithium iron phosphate and lithium iron manganese phosphate as described in claim 3 or the positive electrode sheet of the combination of lithium iron phosphate and lithium iron manganese phosphate as described in claim 6.

9. Use of the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material according to claim 3, the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode slurry according to claim 4, or the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode sheet according to claim 6 in the preparation of a battery.

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