Positive pole piece, preparation method thereof and secondary battery

Abstract

The invention relates to a positive pole piece and a preparation method thereof and a secondary battery, the positive pole piece comprises a current collector and an active coating located on at least one side surface of the current collector, the active coating comprises a lithium-containing phosphate material, a conductive agent and a binder, and the delta Dv50 of the lithium-containing phosphate material is less than or equal to 0.30 [mu] m. According to the invention, by regulating the delta Dv50 of the lithium-containing phosphate material in the active coating of the positive pole piece to be less than or equal to 0.30 [mu] m, the positive pole piece has good processability, that is, the lithium-containing phosphate material has relatively high dispersity and is not easy to agglomerate in the positive pole slurry, and the positive pole slurry has relatively high solid content when the viscosity of the positive pole slurry is in a proper range; therefore, the content of the positive active material in the positive pole piece is further improved, the stripping force of the positive pole piece is increased, the energy density of the battery is improved, and the cycle life is longer.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode sheet, its preparation method, and a secondary battery. Background Technology

[0002] In secondary batteries using lithium phosphate materials as the positive electrode active material, with increasing charge-discharge cycles, metal-oxygen (MO) chemical bonds break due to lattice structure damage, electrolyte corrosion, or material defects. This causes metal ions to detach from the crystal lattice and dissolve into the electrolyte. Specifically: First, during charge-discharge cycles, the insertion / extraction of lithium ions into the crystal lattice is accompanied by slight changes in lattice volume and metal ion valence states. The accumulated lattice stress after repeated cycles disrupts the bond between the metal and the lattice. Second, the oxidative decomposition of the electrolyte produces acidic substances that corrode the positive electrode material, further dissolving metal ions. Furthermore, lattice defects in the lithium phosphate material accelerate metal detachment. These factors combined lead to a significant reduction in battery cycle life due to metal ion detachment and dissolution in the electrolyte.

[0003] To address the aforementioned issues, the widely adopted improvement method is to coat lithium phosphate materials to achieve physical barrier properties. However, the amount of coating and the type of coating material have a significant impact on performance improvement. The most direct way to achieve superior performance of lithium phosphate materials as positive electrode active materials in secondary batteries is to increase the carbon coating amount or optimize the type of coating agent, such as using multiple coating agents synergistically. However, excessive carbon coating or differences in the composition of the coating materials can lead to poor processing performance of the positive electrode sheet. For example, the slurry dispersibility may deteriorate, and the positive electrode active material may easily agglomerate, making it difficult to disperse. This can also negatively affect the stability or viscosity of the slurry, leading to subsequent coating failure, poor coating uniformity, or cracking or breakage during rolling. Consequently, it cannot be used to prepare the positive electrode sheet, and even if it can be used to assemble batteries, it cannot effectively improve the electrochemical performance of the secondary battery. In addition, existing technologies also involve attaching modified groups to the carbon coating layer to effectively reduce the agglomeration of composite cathode material particles, thereby improving the dispersibility of the composite cathode material. At the same time, the hydrophilicity and / or oleophilicity of the modified groups improve the hydrophilicity and oleophilicity of the composite cathode material particle surface, making the composite cathode material particle surface amphiphilic. When the composite cathode material is used to prepare slurry, it improves the affinity between the composite cathode material particles and the solvent, effectively promotes the contact between the solvent and solid particles, and improves the processing performance of the slurry. It achieves a significant improvement in the slurry flowability while meeting the requirement of high solid content. However, although the above processes improve the processing performance of cathode sheets to some extent, the improvement effect is limited, and the energy density of the battery is still relatively low.

[0004] Therefore, how to provide a lithium phosphate material that can be used as a positive electrode active material while also ensuring good processing performance of the positive electrode sheet and further improving the energy density and cycle life of the secondary battery has become an urgent technical problem to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a positive electrode sheet, its preparation method, and a secondary battery. By controlling the ΔDv50 of the lithium phosphate material in the active coating of the positive electrode sheet to be ≤0.30μm, this invention enables the positive electrode sheet to possess excellent processing performance. Specifically, the lithium phosphate material exhibits high dispersibility in the positive electrode slurry, is less prone to agglomeration, and the viscosity of the positive electrode slurry maintains a high solid content within a suitable range. This further increases the content of the positive electrode active material in the positive electrode sheet, enhances the peeling force of the positive electrode sheet, improves the energy density of the battery, and extends its cycle life.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a current collector and an active coating located on at least one side surface of the current collector, the active coating comprising a lithium phosphate material, a conductive agent and a binder, wherein the lithium phosphate material has a ΔDv50 ≤ 0.30 μm, for example 0.30 μm, 0.28 μm, 0.25 μm, 0.22 μm, 0.20 μm, 0.18 μm, 0.15 μm, 0.12 μm, 0.10 μm, 0.08 μm, 0.05 μm, 0.02 μm or 0.01 μm, etc.;

[0008] The test method for ΔDv50 is as follows:

[0009] Four sets of Dv50 values ​​were continuously tested on the lithium phosphate material, and the average values ​​were recorded as Ave[Dv50(1)], Ave[Dv50(2)], Ave[Dv50(3)], and Ave[Dv50(4)], respectively.

[0010] When the absolute value of the difference between Ave[Dv50(1)] and Ave[Dv50(2)] is greater than 1μm, then ΔDv50 is the range of Ave[Dv50(2)], Ave[Dv50(3)] and Ave[D50(4)].

[0011] When the absolute value of the difference between Ave[Dv50(1)] and Ave[Dv50(2)] is ≤1μm, then ΔDv50 is the range of Ave[Dv50(1)], Ave[Dv50(2)] and Ave[Dv50(3)].

[0012] The positive electrode active material used in the active coating of the positive electrode sheet of this invention is a lithium phosphate material with a ΔDv50 ≤ 0.30 μm. Controlling the particle size of the lithium phosphate material within this range ensures high dispersibility of the lithium phosphate material in the positive electrode slurry, preventing agglomeration. This results in a higher solid content in the positive electrode slurry viscosity within a suitable range, further increasing the content of the positive electrode active material in the positive electrode sheet, increasing the peel strength of the positive electrode sheet, improving the energy density of the battery, and extending the cycle life. If the ΔDv50 of the lithium phosphate material is greater than 0.30 μm, i.e., ΔDv50 is too large, agglomeration will occur due to poor dispersion of the positive electrode slurry, leading to low electrode peel strength, reduced battery energy density, and shorter cycle life.

[0013] It should be noted that a laser particle size analyzer (Malvin 3000) was used to test Dv50 in this invention. Dv50 represents the particle size corresponding to a cumulative volume distribution percentage of 50%, and was determined with reference to the standard procedure GB / T19077-2016 / ISO13320:2009.

[0014] Specific test method: Take an appropriate amount of the sample to be tested, ensure the sample concentration is 8%~12% opacity, add 50mL of deionized water, and sonicate for 5min, then magnetically stir for 1min (250rpm) to ensure complete dispersion of the sample. Then, measure the sample according to the standard GB / T19077-2016 / ISO13320:2009.

[0015] It should be noted that the present invention does not impose specific requirements or limitations on the type of adhesive. Commonly used adhesives in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, it can be polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) or polyvinyl butyral (PVB), or a combination of at least two of the above.

[0016] It should be noted that the present invention does not impose specific requirements or special limitations on the type of conductive agent. Commonly used conductive agents in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, it can be acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene or carbon nanofibers, or a combination of at least two of the above.

[0017] It should be noted that the present invention does not impose specific requirements or special limitations on the type of dispersant, as long as it can assist in the dispersion of the positive electrode slurry. Commonly used dispersants in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, it can be polyether, polyester or polyvinylpyrrolidone (PVP), or a combination of at least two of the above.

[0018] As a preferred embodiment of the present invention, the lithium phosphate material comprises a lithium phosphate core and a carbon coating layer located on the surface of the lithium phosphate core.

[0019] As a preferred embodiment of the present invention, the general chemical formula of the lithium phosphate core is Li. x Mn y Fe 1- y M z PO4, wherein 1.0≤x≤1.05, for example, x can be 1.0, 1.01, 1.02, 1.03, 1.04 or 1.05, etc.; 0≤y≤1, for example, y can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, etc.; 0≤z≤0.2, for example, z can be 0, 0.03, 0.05, 0.08, 0.1, 0.13, 0.15, 0.18 or 0.2, etc.; and M includes any one or at least two combinations of Be, Ca, Mg, Ba, Sr, Ti, V, W, Ni or B.

[0020] As a preferred embodiment of the present invention, the mass of the carbon coating layer is 1wt% to 3wt% of the mass of the lithium phosphate core, for example, 1wt%, 1.2wt%, 1.5wt%, 1.8wt%, 2wt%, 2.2wt%, 2.5wt%, 2.8wt%, or 3wt%.

[0021] Preferably, the carbon coating agent used in the carbon coating layer includes any one or a combination of at least two of glucose, polyethylene glycol (PEG), graphite, asphalt, carbon nanotubes, or graphene.

[0022] As a preferred technical solution of the present invention, the preparation method of the lithium phosphate material includes the following steps:

[0023] S1. The lithium source, iron source, phosphorus source and solvent are mixed in the first step to obtain mixture A;

[0024] S2. The mixture A described in step S1 is sequentially spray-dried and subjected to a first sintering process to obtain a sintered material;

[0025] S3. Mix the calcined material, carbon source and solvent described in step S2 for the second time to obtain mixture B;

[0026] S4. The mixture B described in step S3 is sequentially spray-dried and then sintered to obtain the second sintered material;

[0027] S5. The two sintered materials described in step S4 are crushed, screened, and demagnetized to obtain lithium phosphate material with ΔDv50≤0.30μm.

[0028] Preferably, the raw materials for the first mixture in step S1 further include a manganese source.

[0029] Preferably, the raw materials for the second mixture in step S3 further include source M.

[0030] Preferably, step S1, the first mixing, and step S3, the second mixing, each independently include grinding.

[0031] Preferably, the temperature of the first sintering in step S2 is 400℃~730℃, such as 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃ or 730℃.

[0032] Preferably, the sintering time in step S2 is 2h to 10h, for example, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.

[0033] Preferably, the primary particle size of the sintering material in step S2 is 100nm~500nm, for example 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm or 500nm.

[0034] Preferably, the second sintering temperature in step S4 is 600℃~850℃, for example 600℃, 650℃, 700℃, 750℃, 800℃ or 850℃.

[0035] Preferably, the second sintering time in step S4 is 2h to 14h, for example, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h or 14h.

[0036] Preferably, the primary particle size of the secondary sintering material in step S4 is 100nm~500nm, for example, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm or 500nm.

[0037] Preferably, during the crushing process described in step S5, the feeding frequency is 10Hz~50Hz, such as 10Hz, 20Hz, 30Hz, 40Hz or 50Hz.

[0038] Preferably, during the crushing process described in step S5, the frequency of the grading wheel is 10Hz~60Hz, such as 10Hz, 20Hz, 30Hz, 40Hz, 50Hz or 60Hz.

[0039] Preferably, during the crushing process described in step S5, the pressure is 200KPa to 600KPa, such as 200KPa, 300KPa, 400KPa, 500KPa, or 600KPa.

[0040] In this invention, by adjusting the feeding frequency to 10Hz~50Hz, the classifying wheel frequency to 10Hz~60Hz, and the pressure to 200KPa~600KPa during the crushing process, the ΔDv50 of the crushed lithium phosphate material is made to be ≤0.30μm.

[0041] It should be noted that the present invention does not impose specific requirements or special limitations on the types of lithium, manganese, iron, and phosphorus sources. Any type commonly used in the art is applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, the lithium source can be lithium carbonate and / or lithium dihydrogen phosphate, the manganese source can be manganese carbonate, manganese sulfate, manganese tetroxide, or manganese dioxide, or a combination of at least two of the above, the phosphorus source can be phosphoric acid and / or ammonium dihydrogen phosphate, and the iron source can be iron phosphate and / or iron oxalate, etc.

[0042] As a preferred technical solution of the present invention, the thickness of the active coating is 170mm~210mm, such as 170mm, 175mm, 180mm, 185mm, 190mm, 195mm, 200mm, 205mm or 210mm.

[0043] As a preferred embodiment of the present invention, the areal density of the active coating is 360 g / m³. 2 ~440g / m 2 For example, 360g / m 2 370g / m 2 380g / m 2 390g / m 2 400g / m 2 410g / m 2 420g / m 2 430g / m 2 Or 440g / m 2 wait.

[0044] As a preferred embodiment of the present invention, the lithium phosphate material in the active coating has a mass ratio of 94wt% to 98wt%, such as 94wt%, 94.5wt%, 95wt%, 95.5wt%, 96wt%, 96.5wt%, 97wt%, 97.5wt%, or 98wt%.

[0045] Preferably, the conductive agent in the active coating has a mass percentage of 0.5wt% to 1.5wt%, such as 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, or 1.5wt%.

[0046] Preferably, the binder accounts for 1.5wt% to 3.5wt% of the active coating by mass, for example, 1.5wt%, 1.8wt%, 2.0wt%, 2.2wt%, 2.5wt%, 2.8wt%, 3.0wt%, 3.2wt%, or 3.5wt%.

[0047] Preferably, the active coating also includes a dispersant.

[0048] Preferably, the dispersant in the active coating has a mass percentage of 0wt% to 0.5wt%, excluding 0wt%, for example 0.01wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, or 0.5wt%.

[0049] In a second aspect, the present invention also provides a method for preparing a positive electrode sheet as described in the first aspect, the method comprising the following steps:

[0050] A positive electrode slurry is obtained by uniformly mixing a lithium phosphate material, a conductive agent, a binder, and an organic solvent. The positive electrode slurry is then coated onto at least one side of a current collector to obtain a positive electrode sheet.

[0051] As a preferred embodiment of the present invention, the viscosity of the positive electrode slurry is 8000 mPa·s to 20000 mPa·s, for example, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s, 11000 mPa·s, 12000 mPa·s, 13000 mPa·s, 14000 mPa·s, 15000 mPa·s, 16000 mPa·s, 17000 mPa·s, 18000 mPa·s, 19000 mPa·s, or 20000 mPa·s.

[0052] Preferably, the solid content of the positive electrode slurry is 50% to 70%, such as 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, or 70%.

[0053] Preferably, the mixed raw materials also include a dispersant.

[0054] It should be noted that the present invention does not impose specific requirements or special limitations on the type of organic solvent. Commonly used organic solvents in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, it can be N-methylpyrrolidone.

[0055] Preferably, the coating process further includes a drying and rolling step performed sequentially.

[0056] It should be noted that the present invention does not impose specific requirements or special limitations on the type of material of the current collector. Commonly used current collector materials in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions. For example, it can be aluminum foil or carbon-coated aluminum foil.

[0057] It should be noted that the present invention does not impose specific requirements or special limitations on the thickness of the current collector. Current collectors with commonly used thicknesses in the art are applicable to the present invention. Those skilled in the art can make adaptive selections and adjustments according to actual conditions, for example, it can be 12μm~18μm.

[0058] As a preferred technical solution of the present invention, the preparation method includes the following steps:

[0059] A positive electrode slurry is prepared by uniformly mixing lithium phosphate material, conductive agent, binder, dispersant and organic solvent. The positive electrode slurry has a viscosity of 8000 mPa·s to 20000 mPa·s and a solid content of 50% to 70%. The positive electrode slurry is coated on at least one side of the current collector and then dried and rolled to obtain a positive electrode sheet.

[0060] Thirdly, the present invention also provides a secondary battery, the secondary battery comprising a positive electrode sheet as described in the first aspect, or a positive electrode sheet prepared by the preparation method described in the second aspect.

[0061] Compared with the prior art, the present invention has at least the following beneficial effects:

[0062] This invention achieves good processing performance of the positive electrode by controlling the ΔDv50 of the lithium phosphate material in the active coating of the positive electrode to be ≤0.30μm. Specifically, the lithium phosphate material has high dispersibility in the positive electrode slurry and is not prone to agglomeration. The viscosity of the positive electrode slurry is within a suitable range and has a high solid content. This further increases the content of positive active material in the positive electrode, increases the peeling force of the positive electrode, improves the energy density of the battery, and extends the cycle life. Attached Figure Description

[0063] Figure 1 This is a scan image of lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention.

[0064] Figure 2 This is an elemental composition diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Example 1 of the present invention.

[0065] Figure 3 This is a Mn element distribution diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention.

[0066] Figure 4 This is a Fe element distribution diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention.

[0067] Figure 5 This is a transmission image of lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention.

[0068] Figure 6 This is a scan image of lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention.

[0069] Figure 7 This is an elemental content diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Example 4 of the present invention.

[0070] Figure 8 This is the Mn element distribution diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention.

[0071] Figure 9 This is a Fe element distribution diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention. Detailed Implementation

[0072] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0073] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0074] Example 1

[0075] This embodiment provides a positive electrode sheet and its preparation method, wherein the positive electrode sheet comprises 97 wt% lithium manganese iron phosphate material (core LiMn). 0.6 Fe 0.4 PO4, the mass of the carbon coating is equal to that of the core LiMn 0.6 Fe 0.4 Four sets of Dv50 values ​​were continuously tested on lithium manganese iron phosphate material using 2 wt% PO4, 0.8 wt% carbon nanotubes, 2 wt% PVDF, and 0.2 wt% polyvinylpyrrolidone, and the average values ​​were taken respectively. Among them, Ave[Dv50(1)]=0.86μm, Ave[Dv50(2)]=0.83μm, Ave[Dv50(3)]=0.73μm, and Ave[Dv50(4)]=0.75μm. Since the absolute value of the difference between Ave[Dv50(1)] and Ave[Dv50(2)] is ≤1μm, ΔDv50 is the range of Ave[Dv50(1)]-Ave[Dv50(3)], that is, ΔDv50=0.13μm.

[0076] The preparation method includes the following steps:

[0077] 97 wt% lithium manganese iron phosphate, 0.8 wt% carbon nanotubes, 2 wt% PVDF, 0.2 wt% polyvinylpyrrolidone, and N-methylpyrrolidone were mixed uniformly to obtain a positive electrode slurry with a viscosity of 10100 mPa·s and a solid content of 65.4%. The positive electrode slurry was coated onto one side of an aluminum foil (15 μm thick), and after drying and rolling, a thickness of 190 mm and an areal density of 400 g / m³ were formed. 2 The active coating is cut to obtain the positive electrode sheet.

[0078] The preparation method of the lithium manganese iron phosphate material includes the following steps:

[0079] S1, according to kernel LiMn 0.6 Fe 0.4 The chemical formula of PO4 is obtained by weighing manganese tetroxide, iron phosphate, lithium carbonate, lithium dihydrogen phosphate and deionized water and grinding them to obtain mixture A.

[0080] S2. After spray drying, the mixture A is put into the furnace for the first sintering at 600℃ for 8 hours to obtain a primary sintered material with a particle size of 200nm.

[0081] S3, based on the mass of the carbon coating layer as the core LiMn 0.6 Fe 0.4 Weigh out 2 wt% of PO4, glucose + PEG (mass ratio 10:1), add deionized water, mix and grind to obtain mixture B;

[0082] S4. After spray drying, the mixture B is put into the furnace for a second sintering at 700℃ for 10 hours to obtain a second sintered material with a primary particle size of 300nm.

[0083] S5. The secondary sintered material is crushed. During the crushing process, the feeding frequency is 25Hz, the classifier wheel frequency is 30Hz, and the pressure is 600KPa. After screening and demagnetization, lithium manganese iron phosphate material (LiMn core) with a ΔDv50 of 0.13μm is obtained. 0.6 Fe 0.4 PO4, the mass of the carbon coating is equal to that of the core LiMn 0.6 Fe 0.4 (2wt% of PO4 by mass).

[0084] Figure 1 The scan image of lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention is shown. Figure 2 The elemental composition diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention is shown. Figure 3 The diagram shows the Mn elemental distribution of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention. Figure 4 The figure shows the Fe element distribution of the lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Embodiment 1 of the present invention. Figure 5 The image shows a transmission spectrum of lithium manganese iron phosphate material with a ΔDv50 of 0.13 μm provided in Example 1 of this invention. As can be seen from the image, the lithium manganese iron phosphate cathode material (core LiMn) with a target ΔDv50 of 0.13 μm was successfully prepared. 0.6 Fe 0.4 PO4 has a carbon coating on its surface, and the elements are evenly distributed.

[0085] Examples 2-12 and Comparative Examples 1-13 are all based on Example 1 with parameter adjustments, and the specific parameter changes are shown in Table 1. The preparation of the lithium phosphate-containing materials provided in Examples 2-12 and Comparative Examples 1-13 were all adaptively adjusted based on the core chemical formula and the quality of the carbon coating layer. Furthermore, the corresponding ΔDv50 was obtained by adaptively adjusting and controlling the crushing parameters of the secondary calcined material, namely the feeding frequency, classifier frequency, and pressure in step S5.

[0086] Table 1

[0087]

[0088] Figure 6 The scan image of lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention is shown. Figure 7 The elemental composition diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Example 4 of the present invention is shown. Figure 8 The Mn elemental distribution diagram of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention is shown. Figure 9 The figure shows the Fe element distribution of the lithium manganese iron phosphate material with a ΔDv50 of 0.11 μm provided in Embodiment 4 of the present invention. As can be seen from the figure, the lithium manganese iron phosphate cathode material with a target ΔDv50 of 0.11 μm (core LiMn) was successfully prepared. 0.5 Fe 0.5 PO4 has a carbon coating on its surface, and the elements are evenly distributed.

[0089] The viscosity and solid content of the positive electrode slurry provided in Examples 1-12 and Comparative Examples 1-13 were tested. The peel force of the positive electrode sheet was tested. The specific peel force test method is as follows: the positive electrode sheet was fixed, and the tape was used to bond the active layer of the positive electrode sheet. The active layer bonded by the tape was peeled off at a 180-degree angle on a tensile testing machine. The peel force was calculated and given. The specific test results are shown in Table 2.

[0090] Table 2

[0091]

[0092] Note: "N / A" indicates that coating / rolling could not be performed normally, or that the coating / rolling process was abnormal, therefore no test results were obtained.

[0093] Application Examples 1-12 and Comparative Application Examples 1-4

[0094] Soft-pack full cells were assembled using the positive electrode, negative electrode graphite, separator PE, and 1.0 mol / L LiPF6 solution (solvents in a volume ratio of 1:1:1, EC, DMC, and EMC) provided in Examples 1-12 and Comparative Application Examples 1-4, respectively.

[0095] Since positive electrode sheets could not be prepared in comparative examples 5-13, battery assembly was not possible.

[0096] After the pouch cells provided in Application Examples 1-12 and Comparative Application Examples 1-4 were left to stand for 24 hours, their electrochemical performance was tested under the following conditions: capacity retention after 1000 cycles at 1C under a voltage range of 2.5V to 4.2V, and energy density in the initial state (below the capacity limit of the battery). The specific test results are shown in Table 3.

[0097] Table 3

[0098]

[0099] The test results in Tables 2 and 3 show that:

[0100] (1) As can be seen from Examples 1 to 12 and Application Examples 1 to 12, the present invention, by controlling the ΔDv50 of the lithium phosphate material in the active coating of the positive electrode sheet to be ≤0.30μm, has high dispersibility of the lithium phosphate material in the positive electrode slurry, superior performance of the positive electrode slurry, and high solid content in the slurry viscosity within a suitable range, thereby further increasing the content of positive electrode active material in the positive electrode sheet, resulting in higher peeling force of the positive electrode sheet, higher energy density of the battery, and longer cycle life. Specifically, the solid content is 58.2%~66.2%, the peeling force is 0.47N~0.95N, the initial state (battery capacity test line) energy density is 186.7Wh / kg~196.2Wh / kg, and the capacity retention rate after 1000 cycles at 1C is 94.2%~98.0%. Moreover, the present invention has high universality and can be adapted to a variety of lithium phosphate materials.

[0101] (2) By comparing Comparative Examples 1 to 13 with Comparative Application Examples 1 to 4, it can be seen that when the ΔDv50 of the lithium phosphate material is greater than 0.30 μm, the solid content is greatly reduced when the slurry viscosity is within a suitable range. This indicates that the lithium phosphate material has a serious agglomeration phenomenon in the positive electrode slurry and poor dispersibility, which in turn makes the processing performance of the positive electrode sheet worse. The resulting positive electrode sheet is assembled into a battery, and the electrochemical performance of the battery is also significantly reduced (Comparative Application Examples 1 to 4). Furthermore, the positive electrode slurry provided by Comparative Examples 5 to 13 cannot be properly coated / rolled to prepare the positive electrode sheet. That is, the lithium phosphate material provided by Comparative Examples 5 to 13 cannot be used as a positive electrode active material.

[0102] In summary, this invention achieves good processing performance for the positive electrode by controlling the ΔDv50 of the lithium phosphate material in the active coating of the positive electrode to be ≤0.30μm. Specifically, the lithium phosphate material has high dispersibility in the positive electrode slurry and is not prone to agglomeration. The viscosity of the positive electrode slurry is within a suitable range and has a high solid content. This further increases the content of positive active material in the positive electrode, increases the peeling force of the positive electrode, improves the energy density of the battery, and extends the cycle life.

[0103] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A positive electrode sheet, characterized by, The positive electrode plate comprises a current collector and an active coating layer on at least one side surface of the current collector, wherein the active coating layer comprises a lithium-containing phosphoric acid salt material, a conductive agent and a binder, and the lithium-containing phosphoric acid salt material has a ΔDv50≤0.30 μm. The test method of the ΔDv50 is as follows: Four groups of Dv50 are continuously tested on the lithium-containing phosphoric acid salt material, and the average values are respectively recorded as Ave[Dv50(1)], Ave[Dv50(2)], Ave[Dv50(3)] and Ave[Dv50(4)]; When the absolute value of the difference between Ave[Dv50(1)] and Ave[Dv50(2)] is >1 μm, the ΔDv50 is the range value of Ave[Dv50(2)], Ave[Dv50(3)] and Ave[Dv50(4)]; When the absolute value of the difference between Ave[Dv50(1)] and Ave[Dv50(2)] is ≤1 μm, the ΔDv50 is the range value of Ave[Dv50(1)], Ave[Dv50(2)] and Ave[Dv50(3)].

2. The cathode electrode of claim 1, wherein, The lithium-containing phosphoric acid salt material comprises a lithium-containing phosphoric acid salt core and a carbon coating layer on the surface of the lithium-containing phosphoric acid salt core.

3. The positive electrode sheet according to claim 1 or 2, characterized by, The chemical general formula of the lithium-containing phosphate inner core is Li x Mn y Fe 1-y M z PO4, wherein 1.0≤x≤1.05, 0≤y≤1, 0≤z≤0.2, and M includes any one or a combination of at least two of Be, Ca, Mg, Ba, Sr, Ti, V, W, Ni, or B.

4. The cathode electrode of claim 2, wherein, The mass of the carbon coating layer is 1wt%-3wt% of the mass of the lithium-containing phosphoric acid salt core.

5. The positive electrode plate according to any one of claims 1 to 4, characterized by, The thickness of the active coating layer is 170 mm-210 mm.

6. The positive electrode plate according to any one of claims 1 to 5, characterized by, The areal density of the active coating is 360 g / m 2 440 g / m 2 .

7. The positive electrode plate according to any one of claims 1 to 6, characterized by, The mass ratio of the lithium-containing phosphoric acid salt material in the active coating layer is 94wt%-98wt%; Preferably, the mass ratio of the conductive agent in the active coating layer is 0.5wt%-1.5wt%; Preferably, the mass ratio of the binder in the active coating layer is 1.5wt%-3.5wt%; Preferably, the active coating layer further comprises a dispersant. Preferably, the mass ratio of the dispersant in the active coating layer is 0wt%-0.5wt%, and 0wt% is not included.

8. A method of producing the positive electrode sheet according to any one of claims 1 to 7, characterized by, The preparation method comprises the following steps: Mixing the lithium-containing phosphoric acid salt material, the conductive agent, the binder and the organic solvent uniformly to obtain a positive electrode slurry, and coating the positive electrode slurry on at least one side surface of the current collector to obtain the positive electrode plate.

9. The production method according to claim 8, characterized by, The viscosity of the positive electrode slurry is 8000 mPa.s-20000 mPa.s; Preferably, the solid content of the positive electrode slurry is 50%-70%; Preferably, the mixed raw materials further comprise a dispersant.

10. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode plate according to any one of claims 1-7, or the positive electrode plate prepared by the preparation method according to claim 8 or 9.

Images