Lithium iron phosphate and lithium manganese iron phosphate mixed positive electrode material, battery and preparation method of lithium iron phosphate and lithium manganese iron phosphate mixed positive electrode material

Abstract

The invention discloses a lithium iron phosphate and lithium manganese phosphate mixed positive electrode material, a battery and a preparation method. And mixing lithium iron phosphate, lithium iron manganese phosphate and the polymer, and then performing low-temperature pyrolysis to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate positive electrode material. At present, although LFP and LMFP are applied to lithium ion batteries, the application of the LFP and the LMFP is limited due to objective defects of the LFP and the LMFP, the contradiction between energy density and processability cannot be solved in the prior art, and the technical scheme in the prior art has the problems that the technical process is complicated, and the LFP and the LMFP are not beneficial to industrial production and application. By adopting a simple technical scheme, the invention creatively provides the positive electrode material mixing lithium iron phosphate and lithium manganese iron phosphate, unexpectedly, good cycling stability, safety and processing performance are obtained, the energy density of the battery can be improved, and the popularization of the positive electrode in the field of high-energy density application is realized.

Description

Technical Field

[0001] The present invention belongs to the field of battery technology, relates to lithium-ion batteries, and particularly relates to a cathode material, a battery and a preparation method thereof, which combine lithium iron phosphate and lithium manganese iron phosphate. Background Art

[0002] Lithium iron phosphate (LiFePO4, LFP) and lithium manganese iron phosphate (LiMn x Fe 1-x PO4, LMFP) are two common cathode materials for lithium-ion batteries. At present, there are certain limitations in the battery performance of a single material system, and it is difficult to balance the processing performance and energy density, making it difficult for lithium iron phosphate or lithium manganese iron phosphate to be widely applied in actual production. The existing solutions mainly focus on the optimization of a single material system. For example, the performance of LFP or LMFP is improved by means of doping, coating, etc., but it is still difficult to solve the contradiction between energy density and processing performance; moreover, the existing technology will reduce the energy density of the battery by means of nanosizing, carbon coating, etc. At the same time, a single material system is also difficult to meet the diverse requirements for battery performance in different application scenarios.

[0003] The prior art grinds, sinters and crushes a mixture including lithium manganese iron phosphate, a lithium iron phosphate precursor and water to form a lithium manganese iron phosphate composite material. The lithium manganese iron phosphate forms a core, and the average particle size of the core is 600 nm to 1200 nm. The lithium iron phosphate precursor forms a lithium iron phosphate coating layer and a carbon coating layer; this technical solution is still a traditional coating method, and the preparation process is complex. The product performance significantly depends on the preparation process and control, and it is difficult to be applied to industrial production, especially the processing performance of the material is not improved.

[0004] Therefore, it is necessary to develop a new cathode material based on the existing industrial products lithium manganese iron phosphate and lithium iron phosphate, hoping to obtain a cathode material with improved electrical performance and processing performance, so as to improve the performance of lithium-ion batteries; in particular, this material can be produced by conventional industrial methods, which is beneficial to commercial application. Summary of the Invention

[0005] At present, although LFP and LMFP are used in lithium-ion batteries, the objective defects of the two limit their applications. For example, they have a relatively low voltage platform, relatively low energy density, and poor processing performance, which easily lead to problems such as agglomeration and uneven dispersion during the electrode preparation process, affecting the performance of the battery. The prior art cannot solve the contradiction between energy density and processing performance, and the existing technical solutions have complex process procedures, making them not conducive to industrial production applications. The present invention adopts a simple technical solution and creatively proposes a composite cathode material of 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, and through simple physical mixing, preferably combined with low-temperature pyrolysis polymers, unexpectedly good cycle stability, safety and processing performance are obtained, and the energy density of the battery can be improved, realizing the popularization of this cathode in the field of high-energy density applications.

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

[0007] A composite cathode material of lithium iron phosphate and lithium manganese iron phosphate, comprising a mixture of lithium iron phosphate and lithium manganese iron phosphate, and the resistance of the composite cathode material of lithium iron phosphate and lithium manganese iron phosphate ≤ 20 Ω·cm. This resistance is the powder resistance, tested by the 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); more 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 manganese iron phosphate. Preferably, the particle size of lithium iron phosphate is 300 nm - 700 nm, and the particle size of lithium manganese iron phosphate is 100 nm - 300 nm, and the two do not take 300 nm at the same time; more preferably, the particle size of lithium iron phosphate is 350 nm - 500 nm, and the particle size of lithium manganese iron phosphate is 150 nm - 300 nm. Unless otherwise specified in the present invention, the particle size refers to the D50 particle size.

[0010] In the present invention, the composite cathode material of lithium iron phosphate and lithium manganese iron phosphate further comprises a low-temperature pyrolysis polymer.

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

[0012] Preferably, the polymer comprises 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 undergoes low-temperature pyrolysis at about 300 °C to form an amorphous carbon layer with both conductivity and adhesiveness, which is uniformly filled between the lithium iron phosphate and lithium manganese iron phosphate mixture, and can provide an additional electron conduction path for LFP and LMFP particles, improving the conductivity of the material. In particular, during the later production of the positive electrode and the battery, no additional conductive agent needs to be added, reducing the process flow and the problem of difficult dispersion of the conductive agent. Moreover, the low-temperature pyrolyzed polymer has liquid retention and adhesiveness, which can adsorb the electrolyte, enhancing the liquid retention of the electrode. The adhesiveness can better bring the lithium iron phosphate and lithium manganese iron phosphate into contact, optimizing the electron conduction and interface contact of the electrode.

[0015] The present invention discloses a method for preparing the above-mentioned composite lithium iron phosphate and lithium manganese iron phosphate cathode material, which includes the following steps: mixing lithium iron phosphate and lithium manganese iron phosphate to obtain the composite lithium iron phosphate and lithium manganese iron phosphate cathode material.

[0016] Further, the method for preparing the above-mentioned composite lithium iron phosphate and lithium manganese iron phosphate cathode material includes the following steps: mixing lithium iron phosphate, lithium manganese iron phosphate, and a polymer, and then performing heat preservation treatment to obtain the composite lithium iron phosphate and lithium manganese iron phosphate cathode material.

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

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

[0019] The present invention discloses a composite lithium iron phosphate and lithium manganese iron phosphate cathode slurry, which includes the above-mentioned composite lithium iron phosphate and lithium manganese iron phosphate cathode material, a binder, and / or a solvent.

[0020] In the present invention, the rheological recovery rate of the composite lithium iron phosphate and lithium manganese iron phosphate cathode slurry ≥ 80%, obtained by the 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 cathode slurry, which includes the following steps: mixing the mixed lithium iron phosphate and lithium iron manganese phosphate cathode material and a binder to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate cathode slurry; or mixing the mixed lithium iron phosphate and lithium iron manganese phosphate cathode material, a binder and a solvent to obtain the mixed lithium iron phosphate and lithium iron manganese phosphate cathode slurry.

[0022] The present invention discloses a mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate, which includes the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate cathode material. The surface resistance of the mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate is ≤0.30 Ω·cm², and the peel strength is ≥38 N / m. Here, the surface resistance is the single-sided surface resistance of the mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate, which is tested by the conventional four-probe method; the peel strength is the result after rolling, which is tested by the test method of the peel strength of the adhesive tape.

[0023] Furthermore, the mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate further includes a current collector; preferably, the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate cathode 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 a copper foil.

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

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

[0027] In the present invention, the mixed lithium iron phosphate and lithium iron manganese phosphate cathode slurry includes the above-mentioned mixed lithium iron phosphate and lithium iron manganese phosphate cathode material, a binder and / or a solvent.

[0028] Preferably, the mixed lithium iron phosphate and lithium iron manganese phosphate cathode slurry contains a conductive agent or does not contain a conductive agent. Thus, the prepared mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate contains a conductive agent or does not contain a conductive agent. Preferably, the mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate does not contain a conductive agent.

[0029] In the prior art, a conductive agent (such as carbon black, carbon nanotubes) is generally added to the cathode slurry. Otherwise, the conductivity of the cathode plate is poor, which also brings difficulties in process operation because carbon black, carbon nanotubes, etc. have the well-known characteristic of being difficult to disperse; the present invention overcomes the prejudice of the prior art, and the disclosed mixed lithium iron phosphate and lithium iron manganese phosphate cathode plate does not contain a conductive agent, and unexpectedly obtains good electrical properties, and overcomes the problem that the conductive agent in the existing slurry is difficult to disperse, resulting in a complex preparation process.

[0030] Preferably, taking the total weight of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode materials and the binder as 100%, wherein the weight percentage of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode materials is 95% - 100%, excluding 100%; more preferably, the weight percentage of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode materials is 96% - 99.5%; still more preferably, the weight percentage of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode materials is 97% - 99%, such as 96.5%, 97.5%, 98%, 98.5% or any data within the range. In the mixed lithium iron phosphate and lithium manganese iron phosphate cathode slurry of the present invention, the dosage of the binder is less than the conventional dosage. Unexpectedly, the mixed lithium iron phosphate and lithium manganese iron phosphate cathode sheet prepared therefrom has good peeling performance.

[0031] In the present invention, the solid content of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode slurry is 50% - 90%; preferably, the solid content of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode slurry is 55% - 80%; more preferably, the solid content of the mixed lithium iron phosphate and lithium manganese iron phosphate cathode slurry is 60% - 75%; such as 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 manganese iron phosphate cathode material or the mixed lithium iron phosphate and lithium manganese iron phosphate cathode sheet.

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

[0034] As common knowledge, a lithium-ion battery also includes conventional components such as a negative electrode sheet, a separator, and an electrolyte.

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

[0036] The prior art usually adopts a single material system, which is difficult to solve the contradiction between energy density and processing performance. Although the conductivity and processing performance of materials can be improved by means of nanosizing, carbon coating, etc., the energy density of the battery will decrease. A lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) mixed cathode material and a preparation method thereof provided by the present invention have the following beneficial effects: the process flow is simple, the energy density of the battery is improved, and at the same time, the processing performance of the electrode is improved, realizing a good balance between energy density and processing performance; the weight ratio of lithium iron phosphate to lithium manganese iron phosphate is 1∶(0.25-4). Further, by controlling the particle size distribution of LFP and LMFP, the mixed cathode material has both good processing performance and maintains the high energy density advantage, which is beneficial to improving the capacity and energy density of the battery; the addition of the low-temperature pyrolyzed polymer can improve the conductivity and liquid retention of the material, optimize the contact between LFP and LMFP particles, reduce the interfacial impedance, and is beneficial to improving the capacity and energy density of the battery; further, the low-temperature pyrolyzed polymer also reduces or even avoids the addition of conductive carbon black or CNT slurry in the slurry (which is considered a necessary technical means in the prior art), ensures the uniform dispersion of LFP and LMFP in the electrode slurry, avoids the occurrence of agglomeration phenomenon, and further improves the processing performance of the electrode; the use of the mixed material can improve the discharge average voltage of the battery, making the voltage output more stable; it prolongs the cycle life of the battery and improves the cycle stability of the battery; it meets the diverse requirements of different application scenarios for battery performance and has a broad application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a scanning electron microscope image of the blended lithium iron phosphate and lithium manganese iron phosphate cathode material.

[0038] Figure 2 It is the rheological curve - 3ITT of the blended lithium iron phosphate and lithium manganese iron phosphate cathode material made into a slurry. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] Lithium manganese iron phosphate is considered to be an "upgraded version of lithium iron phosphate". Its cycle performance and safety can be comparable to those of lithium iron phosphate, and its theoretical energy density is about 20% higher than that of lithium iron phosphate, solving the pain point of insufficient energy density of lithium iron phosphate. However, lithium manganese iron phosphate also has problems such as low conductivity, poor battery cycle stability, and poor processing performance. In the prior art, carbon coating and metal ion doping are relatively common methods for modifying lithium manganese iron phosphate, but the improvement degree of this method is limited, and it still cannot give full play to the high energy density advantage of lithium manganese iron phosphate, and the cycle performance is poor.

[0040] The present invention provides a mixed cathode material of lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP) and a battery thereof. The process flow is simple, the energy density of the battery is improved, the processing performance of the electrode is improved at the same time, and a good balance between the energy density and the processing performance is achieved.

[0041] In the present invention, lithium iron phosphate, lithium manganese iron phosphate and a polymer are mixed and then subjected to heat preservation treatment to obtain a fused lithium iron phosphate and lithium manganese iron phosphate cathode material. Preferably, in the present invention, lithium iron phosphate and lithium manganese iron phosphate are mixed to obtain a mixture of lithium iron phosphate and lithium manganese iron phosphate, and then mixed with a polymer, and then subjected to heat preservation treatment to obtain a fused lithium iron phosphate and lithium manganese iron phosphate cathode material.

[0042] Among them, the mixing is physical mixing, such as stirring and mixing. In the present invention, three solid materials of lithium iron phosphate, lithium manganese iron phosphate and a polymer are stirred and mixed without adding liquid, which reduces the preparation difficulty. The polymer can decompose to form a carbon layer that is uniformly filled between the lithium iron phosphate and lithium manganese iron phosphate mixtures, and can provide an additional electron conduction path for LFP and LMFP particles, improving the conductivity of the material. As an example, in the present invention, lithium iron phosphate powder and lithium manganese iron phosphate powder are stirred and mixed to obtain a mixture of lithium iron phosphate and lithium manganese iron phosphate; then a polymer is added, and then stirred and mixed, and then subjected to low-temperature pyrolysis treatment to obtain a fused lithium iron phosphate and lithium manganese iron phosphate cathode material. The present invention preferably has the resistance of the fused lithium iron phosphate and lithium manganese iron phosphate cathode material ≤20 Ω·cm. This resistance is the powder resistance, and is tested by the conventional four-probe method.

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

[0044] Preferably, the polymer includes one or more of polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyvinylidene fluoride (PVDF). The polymer undergoes low-temperature pyrolysis at 300 °C to form an amorphous carbon layer with both conductivity and adhesiveness, which is uniformly filled between the lithium iron phosphate and lithium manganese iron phosphate mixtures, and can provide an additional electron conduction path for LFP and LMFP particles, improving the conductivity of the material. In particular, during the later process of battery production, no additional conductive agent needs to be added, reducing the process flow and the problem of difficult carbon black dispersion. And the low-temperature pyrolyzed polymer has liquid retention and adhesiveness, can adsorb the electrolyte, improve the liquid retention of the electrode, and the adhesiveness can make lithium iron phosphate and lithium manganese iron phosphate better contact with each other, optimizing the electron conduction and interface contact of the electrode, and can reduce the content of the binder, increase the proportion of the active material in the material, thereby improving the energy density of the battery cell in a limited space, and further reducing the material cost of the battery cell.

[0045] In the present invention, the weight ratio of lithium iron phosphate and lithium iron manganese phosphate is preferably 1:(0.25 - 4); more preferably 1:(0.4 - 2.5); further preferably 1:(0.5 - 2), and still more preferably 1:1.5 - 1.5:1, such as 1:1.2, 1:1, 1.2:1 or any ratio within the range; in the present invention, the particle size of lithium iron phosphate is 350nm - 500nm, and the particle size of lithium iron manganese phosphate is 150nm - 300nm. Further, the powder tap density of lithium iron phosphate is 2.5g / cm 3 , and the powder tap density of lithium iron manganese phosphate is 2.2g / cm 3 . The thus mixed cathode material not only has good processability but also maintains 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 cathode material, the present invention further discloses a blended lithium iron phosphate and lithium iron manganese phosphate cathode slurry, a blended lithium iron phosphate and lithium iron manganese phosphate cathode plate, and a battery containing the blended lithium iron phosphate and lithium iron manganese phosphate cathode material. Preferably, the battery is a lithium-ion battery.

[0047] The rheological recovery rate of the blended lithium iron phosphate and lithium iron manganese phosphate cathode slurry of the present invention is ≥80%, which is obtained by the conventional 3ITT test method. The specific test conditions are described in 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 a limitation on the technical solution of the present invention; among them, taking the total weight of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material and the binder as 100%, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material is 95% - 100%, excluding 100%; further preferably, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material is 96% - 99.5%, and still more preferably, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material is 97% - 99%, such as 96.5%, 97.5%, 98%, 98.5% or any data within the range. In the blended lithium iron phosphate and lithium iron manganese phosphate cathode slurry of the present invention, the amount of the binder is less than the existing conventional amount. Unexpectedly, the thus prepared blended lithium iron phosphate and lithium iron manganese phosphate cathode plate has good peeling performance.

[0048] The surface resistance of the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate of the present invention is ≤0.30Ω·cm², and the peel force is ≥38N / m. Here, the surface resistance is the single-sided surface resistance of the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate, which is measured by the conventional four-probe method; the peel force is the result after rolling, which is measured by the test method of the adhesive tape peeling strength.

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

[0050] The preparation method of the composite lithium iron phosphate and lithium manganese iron phosphate cathode material is as follows: Mix lithium iron phosphate material with D50 of 400 nm and lithium manganese iron phosphate with D50 of 200 nm in a weight ratio of 1:1 to obtain a mixture; mix the mixture with polyethylene glycol (PEG-6000), and then keep it at 300 °C in nitrogen for 40 min to obtain the composite lithium iron phosphate and lithium manganese iron phosphate cathode material; the dosage of polyethylene glycol is 2% of the weight of the mixture.

[0051] Figure 1 Figure [Figure number not provided in the original] is the scanning electron microscope image of the above composite lithium iron phosphate and lithium manganese iron phosphate cathode material. It can be seen that the processing performance is good, and there are no problems such as agglomeration and uneven dispersion during the electrode preparation process; there is an obvious particle size gradient between the particles. The large particles (LFP) fill the gaps between the small particles (LMFP) to form a close packing without obvious agglomeration. Different from the existing carbon coating technology, the polymer formed by the above low-temperature pyrolysis forms an amorphous carbon layer filled between the two kinds of particles, increasing the conductivity, and also building an additional electron conduction path between the materials, improving the contact between LFP and LMFP particles and reducing the interfacial impedance.

[0052] Comparative Example 1 Referring to Example 1, the difference is that only lithium iron phosphate material is mixed with polyethylene glycol, that is, lithium manganese iron phosphate is omitted; the rest is the same, and it is compared with a commercially available lithium iron phosphate cathode product.

[0053] Comparative Example 2 Referring to Example 1, the difference is that only lithium manganese iron phosphate material is mixed with polyethylene glycol, that is, lithium iron phosphate is omitted; the rest is the same, and it is compared with a commercially available lithium manganese iron phosphate cathode product.

[0054] Comparative Example 3 Referring to Example 1, the difference is that the D50 of the lithium iron phosphate material is 150 - 300 nm, and the D50 of the lithium manganese iron phosphate is 350 - 500 nm; the rest is the same.

[0055] Comparative Example 4 Mix lithium iron phosphate material with D50 of 350 - 500 nm and lithium iron manganese phosphate with D50 of 150 - 300 nm in a weight ratio of 1:1 to obtain a mixed material, and then keep it at 300 °C for 40 min in nitrogen as the cathode material; the difference from Example 1 is that the low-temperature pyrolysis polymer is omitted.

[0056] Comparative Example 5 Referring to Example 1, the difference is that: keep it at 400 °C for 40 min in nitrogen to obtain a fused lithium iron phosphate and lithium iron manganese phosphate cathode material.

[0057] Comparative Example 6 Referring to Example 1, the difference is that: replace the low-temperature pyrolysis polymer with 2% carbon black to obtain a fused lithium iron phosphate and lithium iron manganese phosphate cathode material. Example 2

[0058] Referring to Example 1, the difference is that: lithium iron phosphate material and lithium iron manganese phosphate are mixed in a weight ratio of 1:1.5; the rest is the same. Example 3

[0059] Referring to Example 1, the difference is that: lithium iron phosphate material and lithium iron manganese phosphate are mixed in a weight ratio of 1.5:1; the rest is the same. Example 4

[0060] Referring to Example 1, the difference is that: lithium iron phosphate material and lithium iron manganese phosphate are mixed in a weight ratio of 1:4; the rest is the same. Example 5

[0061] Referring to Example 1, the difference is that: lithium iron phosphate material and lithium iron manganese phosphate are mixed in a weight ratio of 4:1; the rest is the same. Example 6

[0062] Referring to Example 1, the difference is that: replace polyethylene glycol with polyvinyl alcohol (PVA - 1788); the rest is the same. Example 7

[0063] Referring to Example 1, the difference is that: replace polyethylene glycol with polyvinylidene fluoride (PVDF - 5130); the rest is the same. Example 8

[0064] Referring to Example 1, the difference is that: the dosage of polyethylene glycol is 3% of the weight of the mixed material; the rest is the same. Example 9

[0065] Referring to Example 1, the difference is that: the dosage of polyethylene glycol is 1% of the weight of the mixed material; the rest is the same. Example 10

[0066] Referring to Example 1, the difference is that the dosage of polyethylene glycol is 4% of the weight of the mixture, and the blended lithium iron phosphate and lithium manganese iron phosphate cathode materials are obtained. Example 11

[0067] The above-obtained cathode materials are respectively mixed with a binder, and a solvent is added and stirred to prepare a cathode slurry; among them, the weight ratio of the cathode material to the binder (polyvinylidene fluoride PVDF, HEVER 701) is 98.95:1.05, and no additional conductive agent is added; the solvent is N-methylpyrrolidone, and the solid content of the cathode slurry is 68%.

[0068] Due to the addition of the low-temperature pyrolysis polymer in the blended lithium iron phosphate and lithium manganese iron phosphate cathode materials disclosed in the present invention, the dosage of the binder in the cathode slurry can be reduced, and only 1.05% can achieve excellent adhesion effect. As a comparison, in the current cathode slurry, the mass ratio of phosphate, conductive agent, and binder is (95-97):(1.4-2.6):(1.6-2.3). Not only is more binder required, but also the addition of the conductive agent significantly causes processing difficulties.

[0069] The cathode slurry is coated on the surface of the current collector aluminum foil (the coated single-sided areal density is 245 g / m²), and after drying, it is roll-pressed and sliced to obtain a cathode pole piece.

[0070] The weight ratio of the 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 anode slurry is 53%; the uniformly stirred slurry is coated on the surface of the current collector copper foil (the coated single-sided areal density is 88 g / m²), and after drying, it is roll-pressed and sliced to obtain an anode pole piece; this is a conventional technology and does not affect the understanding of the technical effects of the present invention by those skilled in the art.

[0071] A square full cell assembled from the cathode pole piece, anode pole piece, separator, and lithium hexafluorophosphate electrolyte (the concentration of LiPF6 is 1 mol / L); the anode pole piece, separator, and lithium hexafluorophosphate electrolyte are all prior arts, and the specific assembly is a conventional technology, which does not affect the understanding of the technical effects of the present invention by those skilled in the art. Test Example

[0072] Figure 2Rheological curve of the cathode slurry prepared by blending lithium iron phosphate and lithium iron manganese phosphate cathode materials in Example 1 - 3ITT. The 3ITT test method has been 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 viscosity change during the shear process of the fluid, the destruction, recovery, and structural stability of the fluid structure are characterized. The test process and test parameter settings adopt a step - by - step process: 1) Static stage: Apply a constant and relatively small shear load. At this time, the viscosity change is small, which can be considered as the initial viscosity; 2) Shear stage: Apply a constant and relatively large shear load; 3) Structure recovery stage: Apply the same constant shear load as in the first process. For the industrial production of power battery cathode slurries: A destruction rate of 70 - 80% indicates good fluidity. If it is too high (>80%), it will cause difficult pumping, and if it is too low (<70%), it is prone to sedimentation; A recovery rate of 80 - 90% indicates good fluidity and good anti - sagging ability. However, if it exceeds 90%, it is considered that the recovery is too fast, which will cause the slurry to lose fluidity prematurely and is not conducive to coating control, and there will be sticking to the roller during rolling; A stability index of 98 - 102% indicates excellent slurry recycling performance. If the slurry exceeds 102%, it has a tendency to gel and there will be problems of hardening and die blockage; The product of the present invention has good effects in terms of destruction rate, recovery rate, and stability, and has good initial viscosity, belonging to the optimal performance range in the power lithium battery cathode slurry system, and is even slightly better than the lithium iron phosphate product, beyond people's imagination.

[0073] Test conditions:

[0074] Test data:

[0075] Significance of 1T: Structural destructiveness in the first stage: Applying a high shear rate to destroy the structure, and the viscosity drops rapidly. Algorithm: Destruction rate = (( η 0− η 1) / η 0)×100% = 77.11%. The fluidity is excellent, pumping is easy, and it is easy to process.

[0076] Significance of 2T: Structural reconstructibility in the second stage: Reflects the structure recovery characteristics of the slurry after removing the high shear rate. Algorithm: Recovery rate = ( η 2 / η 0)×100% = 83.28%; Analysis: A recovery rate > 80% belongs to the rapid recovery type, with good anti - sedimentation and anti - sagging abilities.

[0077] Significance of 3T: Third stage (storage stability): Reflects the stability of the slurry during the storage stage, the reversibility and long - term stability of the slurry structure after recovery; Algorithm: Stability index = ( η 3 / η0) × 100% = 101.21%. Analysis: Good stability indicates a reversible structure, suitable for long-term storage and cyclic use.

[0078] The slurries prepared in the examples and comparative examples and the battery test results are as follows:

[0079] Compared with the separate LFP (Comparative Example 1) and separate LMFP (Comparative Example 2), the present invention has made significant technological progress, with good energy density and processability, which is conducive to practical application. Unexpectedly, the problem of the relatively high recovery rate of the separate lithium iron phosphate slurry (89.66%) has been improved, which better meets the requirements of industrial production. The adhesion of Comparative Example 1 and Comparative Example 2 is insufficient, affecting the later cycle. After increasing the amount of the binder to 2 wt% during the preparation of the slurry, the peel strengths of the positive electrode sheets prepared are 38 N / m and 34 N / m respectively after rolling, but the energy density of the batteries obtained by this formulation is low, especially the cost is high.

[0080] As a control, in the case of less binder, the peel performance of the positive electrode sheets prepared by the existing active material, conductive agent, and binder formulation is poor and cannot be applied, so no further testing is done.

[0081] By further selecting the particle size, the electrochemical performance of the product is good, the high-voltage advantage is exerted, and the average voltage increases; especially the polymer treatment significantly improves the conductivity, and the resistance is increased from 15.20 Ω·cm² and the capacity is 120 mAh / g to 0.28 Ω·cm² and 133 mAh / g. In short, the present invention (LFP:LMFP = 1:1.5 - 3:2) achieves a double breakthrough in processability and energy density through particle size grading and 2 - 3% PEG modification.

[0082] The test methods involved in the present invention are conventional methods, which are briefly described as follows. These test conditions are for those skilled in the art to understand the technical effects of the present invention, rather than a limitation on the technical solution of the present invention: Viscosity test method: According to GB / T 22235-2008 (Determination method of liquid viscosity); for lithium-ion battery positive electrode slurries with a solid content of 65% - 70%, those skilled in the art generally believe that a viscosity lower than 10 4 mPa·s has processability for industrial application, especially a viscosity lower than 9000 mPa·s is the basis for good processability.

[0083] Solid content test method: According to GB / T 6284-2016 (Determination of water content in chemical products); Positive electrode sheet compaction density test method: According to GB / T 24533-2019 (Test method for lithium-ion battery electrode sheets); Test method for the surface resistance of the electrode: Use a diaphragm resistance tester, Yuaneng Technology BER 1300. Cut the electrode sample: 50 mm × 50 mm (flat and without wrinkles); apply a constant pressure of 25 MPa, measure three times, and take the average value; Test method for the peel force: Use a servo material testing machine, Haida HD-B609B-S. For the positive electrode plate: 300 mm (L) × 25 mm (W). Paste the double-sided tape on the stainless steel plate, and laminate the active material layer of the electrode plate with the tape; roll three times under a pressure of 0.5 MPa (speed 10 mm / s); stick one end of the adhesive tape on the stainless steel plate, fix the stainless steel plate on the fixture of the tensile testing machine, and the other fixture of the testing machine clamps the free end of the adhesive tape, at a 90° angle to the stainless steel plate, and pull the adhesive tape at a speed of 100 mm / min; after the fixture of the tensile testing machine starts to operate, ignore the value obtained during the mechanical peeling of the first 25 mm of the adhesive tape, and take the average force value obtained from the next 50 mm of the adhesive tape as the peel force, and convert it to the peel strength: Peel strength (N / m) = average force value (F) × 1000 / sample width (25 mm). Measure three times in parallel and take the average value; Test method for the powder resistance: Measure by the four-probe method, using Lattice Electronics ST2742B. Take 3 g of dry powder (vacuum dried at 105 °C for 12 h), test pressure: 30 MPa, and form it into a diameter of 13 mm; measure three times in parallel and take the average value; The test shows that compared with the same model of battery, the present invention has improvements in the target capacity, surface resistance, and discharge average voltage.

[0084] The above-described embodiments are only preferred embodiments of the present invention, which are convenient for those skilled in the art to understand and use the present invention. Obviously, any person skilled in the art can make a slight modification or variation to this embodiment without creative labor and apply it to other embodiments. Therefore, the present invention is not limited to the above embodiments, and any equal changes, simple modifications, and decorations made within the scope of the present invention still fall within the scope covered by the present invention.

Claims

1. A composite lithium iron phosphate and lithium iron manganese phosphate cathode material, comprising a mixture of lithium iron phosphate and lithium iron manganese phosphate, characterized in that, The resistance of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material ≤ 20 Ω·cm.

2. The lithium iron phosphate and lithium iron manganese phosphate composite cathode material according to claim 1, wherein The weight ratio of lithium iron phosphate to lithium iron manganese phosphate is 1∶(0.25 - 4); the particle size of lithium iron phosphate is larger than that of lithium iron manganese phosphate.

3. The lithium iron phosphate and lithium iron manganese phosphate composite cathode material according to claim 2, wherein 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 300 nm - 700 nm, and the particle size of lithium iron manganese phosphate is 100 nm - 300 nm. They do not both take 300 nm simultaneously.

4. The lithium iron phosphate and lithium iron manganese phosphate composite cathode material according to claim 1, characterized in that, The blended lithium iron phosphate and lithium iron manganese phosphate cathode material also includes a low-temperature pyrolyzed polymer.

5. The lithium iron phosphate and lithium iron manganese phosphate composite cathode material according to claim 4, wherein, The mass of the polymer is 0.5% - 5% of the sum of the masses of lithium iron phosphate and lithium iron manganese phosphate; the low-temperature pyrolysis temperature is 200 - 400 °C.

6. The preparation method of the fused lithium iron phosphate and lithium iron manganese phosphate cathode material according to any one of claims 1 to 5, characterized in that, It includes the following steps: mixing lithium iron phosphate and lithium iron manganese phosphate to obtain the blended lithium iron phosphate and lithium iron manganese phosphate cathode material; or mixing lithium iron phosphate, lithium iron manganese phosphate, and the polymer, and then performing heat preservation treatment to obtain the blended lithium iron phosphate and lithium iron manganese phosphate cathode material.

7. The preparation method of the composite lithium iron phosphate and lithium iron manganese phosphate cathode material according to claim 6, wherein, The temperature of the heat preservation treatment is 250 - 350 °C; the time of the heat preservation treatment is 15 - 120 minutes.

8. A positive electrode slurry incorporating lithium iron phosphate and lithium iron manganese phosphate, characterized in that, It includes the blended lithium iron phosphate and lithium iron manganese phosphate cathode material according to any one of claims 1 to 5 and a binder.

9. A cathode slurry incorporating lithium iron phosphate and lithium iron manganese phosphate, characterized in that, Taking the sum of the weights of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material and the binder as 100%, among which, the weight percentage of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material is 95% - 100%, excluding 100%.

10. A blended lithium iron phosphate and lithium manganese iron phosphate positive electrode sheet, comprising the blended lithium iron phosphate and lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 5, characterized in that, The surface resistance of the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate ≤ 0.30 Ω·cm², and the peel strength ≥ 38 N / m.

11. The preparation method of the fused lithium iron phosphate and lithium iron manganese phosphate positive electrode sheet according to claim 10, characterized in that, It includes the following steps: coating the blended lithium iron phosphate and lithium iron manganese phosphate cathode slurry according to claim 8 on the surface of the current collector, then drying, and obtaining the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate through rolling.

12. A battery, characterized in that, It includes the blended lithium iron phosphate and lithium iron manganese phosphate cathode material according to any one of claims 1 to 5 or the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate according to claim 10.

13. The application of the blended lithium iron phosphate and lithium iron manganese phosphate cathode material according to claim 1, the blended lithium iron phosphate and lithium iron manganese phosphate cathode slurry according to claim 8, or the blended lithium iron phosphate and lithium iron manganese phosphate cathode plate according to claim 10 in the preparation of a battery.

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