Lithium manganese iron phosphate composite electrode sheet, and preparation method therefor and use thereof

Abstract

The present application relates to the technical field of lithium-ion batteries, relates to a lithium manganese iron phosphate composite electrode sheet, and further discloses a preparation method therefor and a use thereof. The lithium manganese iron phosphate composite electrode sheet forms a double-coating structure by means of a lithium manganese iron phosphate material layer and a carbon-coated lithium iron phosphate material layer which are compositely arranged, wherein an upper coating is the carbon-coated lithium iron phosphate material layer, which can better improve the overall ion mobility of a composite lithium manganese iron phosphate system and optimize the dynamic performance; and a lower coating is a nano lithium manganese iron phosphate material layer, which not only reduces direct contact between an electrolyte and a material surface, thus reducing side reactions, but also provides a voltage platform for manganese, thereby increasing an average voltage platform of the material and obtaining a higher energy density.

Description

Lithium manganese iron phosphate composite electrode plate and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on July 14, 2023, with application number 202310870083.9 and invention name “A lithium manganese iron phosphate composite electrode plate and its preparation method and application”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the technical field of lithium-ion batteries, relates to a lithium iron manganese phosphate composite electrode plate, and further discloses its preparation method and application. Background Art

[0004] Lithium-ion batteries are energy storage devices widely used in energy storage, power batteries, and portable electronic devices. They offer advantages such as high operating voltage, low self-discharge, and excellent safety. Lithium-ion batteries are primarily composed of positive electrode materials, negative electrode materials, electrolytes, separators, and casings. Positive electrode materials primarily include lithium cobalt oxide, lithium manganese oxide, nickel-manganese binary systems, nickel-cobalt-manganese ternary systems, nickel-cobalt-aluminum ternary systems, lithium iron phosphate, and lithium manganese iron phosphate. Among these, lithium iron phosphate (LiFePO4) is a common positive electrode material in this field, offering advantages such as low cost, high safety, and a long cycle life.

[0005] Lithium manganese iron phosphate (LMFP, LiMn 1-x Fe x Lithium iron phosphate (LiFePO4) is a material developed based on the modification of lithium iron phosphate. It is a solid solution material composed of lithium iron phosphate and lithium manganese phosphate. LiFePO4 has similar properties to lithium iron phosphate and lithium manganese phosphate, with excellent thermal and chemical stability. Moreover, compared to lithium iron phosphate, LiFePO4 has a higher voltage platform and improved electrical conductivity and lithium ion conductivity, making it a highly noteworthy positive electrode material. However, the conductivity of pure LiFePO4 is very poor, and pure LiFePO4 is almost an insulator, making it difficult to fully utilize the material's electrochemical properties.

[0006] At present, the method based on the composite of lithium iron manganese phosphate and lithium iron phosphate is a solution for composite molding of lithium iron manganese phosphate and lithium iron phosphate. Lithium iron phosphate powder material is often added to lithium iron manganese phosphate for composite use as an additive to improve compaction. With the continuous development of science and technology, people have higher and higher requirements for the preparation process of the composite method of lithium iron manganese phosphate and lithium iron phosphate. However, the existing composite method of lithium iron manganese phosphate and lithium iron phosphate has certain disadvantages when used, and because the lithium iron phosphate material particles have large and small particle gradations, it is not easy to composite well with lithium iron manganese phosphate, resulting in low cycle life and low compaction density. It cannot be well composited, affecting the application performance and user experience.

[0007] Summary of the Invention

[0008] To this end, the technical problem to be solved by this application is to provide a lithium iron manganese phosphate composite electrode plate, which is directly based on a double-coating structure formed by lithium iron manganese phosphate and lithium iron phosphate, effectively improving its electrochemical performance;

[0009] The second technical problem to be solved by this application is to provide a method for preparing the above-mentioned lithium manganese iron phosphate composite electrode plate.

[0010] In order to solve the above technical problems, the present application discloses a lithium iron manganese phosphate composite electrode plate, comprising a lithium iron manganese phosphate material layer and a lithium iron phosphate material layer sequentially arranged on the surface of a current collector;

[0011] The maximum compaction density U of the lithium manganese iron phosphate composite electrode sheet satisfies the following relationship:

[0012] U=1.02+0.065S1*D1+0.0086S2*D2;wherein,

[0013] S1 and D1 are respectively the surface density of the lithium iron phosphate material layer and the median particle size D of the lithium iron phosphate material forming the lithium iron phosphate material layer. 50 ;

[0014] S2 and D2 are respectively the surface density of the lithium manganese iron phosphate material layer and the median particle size D of the nano-lithium manganese iron phosphate material forming the lithium manganese iron phosphate material layer. 50 .

[0015] Specifically, the lithium manganese iron phosphate composite electrode plate:

[0016] In the lithium manganese iron phosphate composite electrode plate, the mass ratio of the nano-lithium manganese iron phosphate material to the lithium iron phosphate material is (1-x): (x), wherein 1≤x≤4;

[0017] Optionally, the compaction density of the lithium manganese iron phosphate composite electrode plate is greater than 2.00 g / cc;

[0018] Optionally, the compaction density of the lithium iron manganese phosphate composite electrode plate is 2.00-2.60 g / cc; specifically, the compaction density of the lithium iron manganese phosphate composite electrode plate can be 2.00 g / cc, 2.10 g / cc, 2.20 g / cc, 2.30 g / cc, 2.40 g / cc, 2.50 g / cc, or 2.60 g / cc.

[0019] Specifically, the lithium manganese iron phosphate composite electrode plate:

[0020] The surface density S1 of the lithium iron phosphate material layer is 18-20 g / m 2 , for example: 18g / m 2 , 18.5g / m 2 , 19g / m 2 , 19.5g / m 2 or 20g / m 2 etc.; and / or,

[0021] The surface density S2 of the lithium manganese iron phosphate material layer is 15-18 g / m 2 , for example, 15g / m 2 , 16g / m 2 , 17g / m 2 , 18g / m 2 wait.

[0022] Specifically, the lithium manganese iron phosphate composite electrode plate:

[0023] The median particle size D of the nano-lithium manganese iron phosphate material forming the lithium manganese iron phosphate material layer 50 0.4-0.6 μm, for example, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm or 0.6 μm, etc.; and / or,

[0024] The median particle size D of the lithium iron phosphate material forming the lithium iron phosphate material layer 50 It is 0.8-1.2μm, for example: 0.8μm, 0.9μm, 1.0μm, 1.1μm or 1.2μm, etc.

[0025] Specifically, the lithium manganese iron phosphate composite electrode plate:

[0026] The specific surface area of ​​the nano-manganese iron lithium phosphate material is 18-20m 2 / g, for example: 18m 2 / g, 18.5m 2 / g、19m 2 / g, 19.5m 2 / g or 20m 2 / g, etc.; and / or,

[0027] The grain size of the nano-lithium manganese iron phosphate material is 75-85 nm, for example, 75 nm, 78 nm, 80 nm, 83 nm or 85 nm; and / or,

[0028] The unit cell volume of the nano-manganese iron phosphate lithium material is For example: or etc.; and / or,

[0029] The compaction density (2T pressure) of the nano-lithium manganese iron phosphate material is 2.1-2.3 g / cc, for example, 2.10 g / cc, 2.15 g / cc, 2.20 g / cc, 2.25 g / cc or 2.30 g / cc; and / or,

[0030] The tap density of the nano-manganese iron lithium phosphate material is 1.0-1.2 g / cm 3 , for example: 1.0g / cm 3 , 1.05g / cm 3 , 1.1g / cm 3 , 1.15g / cm 3 or 1.2 g / cm 3 wait.

[0031] Specifically, in the lithium iron phosphate composite electrode plate, the lithium iron phosphate material is a carbon-coated lithium iron phosphate material;

[0032] Optionally, the mass content of carbon element in the carbon-coated lithium iron phosphate material is 1.2-1.4 wt%.

[0033] The present application also discloses a method for preparing the lithium manganese iron phosphate composite electrode sheet, comprising the following steps:

[0034] (1) mixing a nano-lithium manganese iron phosphate material, a conductive agent, and a binder, and applying the mixture to the surface of the current collector to form the lithium manganese iron phosphate material layer;

[0035] (2) dispersing the lithium iron phosphate material in an organic solvent to obtain a suspension, and then applying the suspension to the surface of the lithium manganese iron phosphate material layer to form the lithium iron phosphate material layer;

[0036] (3) The electrode plates after the above treatment are subjected to roller pressing to obtain the desired lithium manganese iron phosphate composite electrode plates.

[0037] Specifically, the method for producing the lithium manganese iron phosphate composite electrode plate:

[0038] In step (1), the current collector includes a carbon-coated aluminum foil; optionally, the thickness of the carbon-coated aluminum foil is 12-16 μm, for example, 12 μm, 13 μm, 14 μm, 15 μm or 16 μm; and / or,

[0039] In the step (1), the conductive agent includes at least one of acetylene black, Ketjen black, carbon nanotubes (CNTs), 350G or carbon fiber (VGCF); and / or,

[0040] In the step (1), the binder comprises polyvinylidene fluoride (PVDF); and / or,

[0041] In the step (1), the mass ratio of the nano-manganese iron phosphate material, the conductive agent and the binder is 95.5-96.5:1.5-2.5:2; and / or,

[0042] In the step (1), the coating speed of the coating step is 5-10 m / min; and / or,

[0043] In the step (2), the organic solvent includes n-hexane and / or isopropanol; and / or,

[0044] The step (2) further comprises dispersing the carbon-coated lithium iron phosphate material in mineral oil and adding ethanol for washing. Preferably, the ethanol is anhydrous ethanol, more preferably analytical grade (AR, 99.9%); and / or,

[0045] In the step (3), the speed of the rolling step is 2-3 m / min; and / or,

[0046] The step (3) further includes a step of drying the electrode sheet before the rolling step; optionally, the atmosphere of the drying step includes at least one of hydrogen, nitrogen, helium, neon, argon, krypton or xenon;

[0047] Optionally, the temperature of the drying step is 100-130°C, for example, 100°C, 115°C, 120°C, 125°C or 130°C, etc.; the drying time is 2-4h, for example, 2h, 2.5h, 3h, 3.5h or 4h, etc.

[0048] The present application also discloses the use of the lithium iron manganese phosphate composite electrode sheet or the lithium iron manganese phosphate composite electrode sheet prepared by the method for preparing a lithium ion battery positive electrode sheet or a lithium ion battery.

[0049] The present application also discloses a lithium-ion battery positive electrode sheet or a lithium-ion battery prepared from the lithium iron manganese phosphate composite electrode sheet or the lithium iron manganese phosphate composite electrode sheet prepared by the method.

[0050] The present application also discloses a positive electrode plate, which includes the lithium manganese iron phosphate composite positive electrode plate.

[0051] The present application also discloses a lithium-ion battery, which comprises the positive electrode sheet.

[0052] The lithium iron phosphate composite electrode plate described in the present application forms a double-coating structure by compositely setting a lithium iron phosphate material layer and a carbon-coated lithium iron phosphate material layer, wherein the upper coating layer is a lithium iron phosphate material layer coated with a carbon material, which can better improve the overall ion mobility of the composite lithium iron phosphate system and play a role in optimizing the kinetic performance; the lower coating layer is a nano lithium iron phosphate material layer, which can not only reduce the direct contact between the electrolyte and the material surface, thereby reducing side reactions, but also provide a manganese voltage platform, thereby improving the average voltage platform of the material and obtaining a higher energy density.

[0053] The lithium iron manganese phosphate composite electrode plate described in this application has a unique double-coating structure design. The full battery prepared based on the lithium iron manganese phosphate composite electrode plate can achieve a mass energy density of 225Wh / Kg and a volume energy density of 495Wh / L, which is about 12% higher than the energy density of commercial lithium iron phosphate materials. Its high-temperature cycle (2.5-4.35V 45℃ 1C / 1C) has a capacity retention rate of >80% after 2000 cycles, and a high-temperature storage (55℃) has a capacity retention rate of >94% after 30 days, which is basically the same as that of commercial lithium iron phosphate materials. The lithium iron manganese phosphate composite electrode plate described in this application has more excellent electrochemical performance.

[0054] The lithium iron phosphate composite electrode plate of the present application has a double-coating structure, and further by regulating the surface density and material particle size parameters of the lithium iron phosphate material layer and the carbon-coated lithium iron phosphate material layer, the maximum compaction density U of the lithium iron phosphate composite electrode plate is adjusted to satisfy the following relationship: U = 1.02 + 0.065S1*D1 + 0.0086S2*D2; wherein S1 and D1 represent the surface density of the carbon-coated lithium iron phosphate material layer and the median particle size D of the carbon-coated lithium iron phosphate material, respectively. 50 , S2 and D2 represent the surface density of the lithium manganese iron phosphate material layer and the median particle size D of the nano-lithium manganese iron phosphate material, respectively. 50 , the electrochemical performance of the composite electrode sheet is better. Moreover, the fitted value C of the compaction density of the obtained composite lithium iron manganese phosphate electrode sheet determined based on the above relationship is very close to the measured value, which can achieve the controllable preparation of the required high compaction density electrode sheet without having to perform compaction density testing on the positive electrode sheet made of the obtained positive electrode material each time, saving materials and time.

[0055] The lithium manganese iron phosphate composite electrode plate of the present application is further prepared by regulating the median particle size D of the upper and lower coating materials during the composite electrode preparation process. 50 and coating surface density, which can make the compaction density of the lithium manganese iron phosphate composite electrode pole piece in an optimal state; at the same time, it is necessary to take into account the coating surface density of the upper carbon-coated lithium iron phosphate and the coating surface density design of the lower lithium manganese iron phosphate. If the surface density value is too high, cracks are likely to appear on the surface of the pole piece during coating, and broken belts will appear during rolling, which will affect processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to make the content of this application easier to understand, the following further describes this application in detail based on the specific embodiments of this application and in conjunction with the accompanying drawings, wherein:

[0057] FIG1 is a SEM image of the lithium manganese iron phosphate composite electrode sheet described in Example 1 of the present application. DETAILED DESCRIPTION

[0058] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0059] Example 1

[0060] This embodiment provides a method for preparing the lithium manganese iron phosphate composite electrode sheet, comprising the following steps:

[0061] (1) Nano-manganese iron phosphate lithium material (average primary particle size of 80 nm), acetylene black and binder polyvinylidene fluoride (PVDF) were mixed and homogenized in a mass ratio of 95.5:2.5:2, and then coated on the surface of 12 μm carbon-coated aluminum foil to form a manganese iron phosphate lithium material layer;

[0062] (2) using pulsed ultrasound to disperse carbon-coated lithium iron phosphate material powder (average primary particle size of 400 nm, carbon coating amount of 1.2 wt%, average thickness of carbon coating layer of 2 nm) in mineral oil, and the dispersed lithium iron phosphate powder solution is washed and filtered with analytical grade (AR, 99.9%) anhydrous ethanol, and then the wet lithium iron phosphate powder after filtration is suspended in n-hexane; the suspension in n-hexane is continuously applied to the surface of the lithium manganese iron phosphate material layer, and then dried in an oven at 100° C. for 2 h under a nitrogen atmosphere to form a carbon-coated lithium iron phosphate material layer;

[0063] (3) The above-mentioned electrode sheet is subjected to a rolling process, and the rolling speed is controlled to be 2m / min, so that the lithium iron phosphate material layer and the carbon-coated lithium iron phosphate material layer are adhered and composited to obtain the required lithium iron phosphate composite electrode sheet.

[0064] In the lithium manganese iron phosphate composite electrode plate described in this embodiment:

[0065] The surface density S1 of the carbon-coated lithium iron phosphate material layer is 18 g / m 2 The median particle size D of the carbon-coated lithium iron phosphate material 50 (i.e. D1) is 0.8 μm;

[0066] The surface density S2 of the lithium manganese iron phosphate material layer is 15g / m 2 The median particle size D of the nano-manganese iron phosphate lithium material 50 (ie D2) is 0.4 μm.

[0067] After calculation, the maximum compaction density of the lithium manganese iron phosphate composite electrode plate described in this embodiment is U=1.02+0.065S1*D1+0.0086S2*D2=2.0076g / cc.

[0068] It has been measured that the compaction density of the lithium iron manganese phosphate composite electrode plate described in this embodiment is 2.0076 g / cc.

[0069] An SEM image of the lithium manganese iron phosphate composite electrode plate described in this embodiment is shown in Figure 1. As can be seen from the SEM image, the electrode plate prepared in this embodiment has a double-coating structure with sparsely distributed and porous particles, which reduces the polarization effect of thick electrode plates. The lower coating is a layer of nano-lithium manganese iron phosphate material with relatively dense particles, which reduces direct contact between the electrolyte and the material surface, thereby reducing side reactions.

[0070] Example 2

[0071] This embodiment provides a method for preparing the lithium manganese iron phosphate composite electrode sheet, comprising the following steps:

[0072] (1) Nano-manganese iron phosphate lithium material (average primary particle size of 120 nm), carbon nanotubes (CNTs) and binder polyvinylidene fluoride (PVDF) were mixed and homogenized in a mass ratio of 95.5:2.5:2, and then coated on the surface of 13 μm carbon-coated aluminum foil to form a manganese iron phosphate lithium material layer;

[0073] (2) using pulsed ultrasound to disperse carbon-coated lithium iron phosphate material powder (average primary particle size of 500 nm, carbon coating amount of 1.4 wt%, average thickness of carbon coating layer of 4 nm) in mineral oil, and the dispersed lithium iron phosphate powder solution is washed and filtered with analytical grade (AR, 99.9%) anhydrous ethanol, and then the wet lithium iron phosphate powder after filtration is suspended in n-hexane; the suspension in n-hexane is continuously applied to the surface of the lithium manganese iron phosphate material layer, and then dried in an oven at 110° C. for 3 h under a nitrogen atmosphere to form a carbon-coated lithium iron phosphate material layer;

[0074] (3) The above-mentioned electrode sheet is subjected to a rolling process, and the rolling speed is controlled to be 3m / min, so that the lithium manganese iron phosphate material layer and the carbon-coated lithium iron phosphate material layer are adhered and composited to obtain the required lithium manganese iron phosphate composite electrode sheet.

[0075] In the lithium manganese iron phosphate composite electrode plate described in this embodiment:

[0076] The surface density S1 of the carbon-coated lithium iron phosphate material layer is 20 g / m 2 The median particle size D of the carbon-coated lithium iron phosphate material is 50 (i.e. D1) is 1.2 μm;

[0077] The surface density S2 of the lithium manganese iron phosphate material layer is 18g / m 2 The median particle size D of the nano-manganese iron phosphate lithium material 50 (ie D2) is 0.6 μm.

[0078] After calculation, the maximum compaction density of the lithium manganese iron phosphate composite electrode plate described in this embodiment is U=1.02+0.065S1*D1+0.0086S2*D2=2.67288 g / cc.

[0079] It has been determined that the compaction density of the lithium iron manganese phosphate composite electrode plate described in this embodiment is 2.67288 g / cc.

[0080] Example 3

[0081] This embodiment provides a method for preparing the lithium manganese iron phosphate composite electrode sheet, comprising the following steps:

[0082] (1) Nano-lithium manganese iron phosphate material (average primary particle size of 80 nm), carbon fiber (VGCF) and binder polyvinylidene fluoride (PVDF) were mixed and homogenized in a mass ratio of 95.5:2.5:2, and then coated on the surface of 14 μm carbon-coated aluminum foil to form a lithium manganese iron phosphate material layer;

[0083] (2) using pulsed ultrasound to disperse carbon-coated lithium iron phosphate material powder (average primary particle size of 500 nm, carbon coating amount of 1.4 wt%, average thickness of carbon coating layer of 2 nm) in mineral oil, and the dispersed lithium iron phosphate powder solution is washed and filtered with analytical grade (AR, 99.9%) anhydrous ethanol, and then the wet lithium iron phosphate powder after filtration is suspended in n-hexane; the suspension in n-hexane is continuously applied to the surface of the lithium manganese iron phosphate material layer, and then dried in an oven at 130° C. for 4 h under a nitrogen atmosphere to form a carbon-coated lithium iron phosphate material layer;

[0084] (3) The above-mentioned electrode sheet is subjected to a rolling process, and the rolling speed is controlled to be 3m / min, so that the lithium manganese iron phosphate material layer and the carbon-coated lithium iron phosphate material layer are adhered and composited to obtain the required lithium manganese iron phosphate composite electrode sheet.

[0085] In the lithium manganese iron phosphate composite electrode plate described in this embodiment:

[0086] The surface density S1 of the carbon-coated lithium iron phosphate material layer is 18 g / m 2 The median particle size D of the carbon-coated lithium iron phosphate material is 50 (i.e. D1) is 0.8 μm;

[0087] The surface density S2 of the lithium manganese iron phosphate material layer is 18g / m 2 The median particle size D of the nano-manganese iron phosphate lithium material 50 (ie D2) is 0.4 μm.

[0088] After calculation, the maximum compaction density of the lithium manganese iron phosphate composite electrode plate described in this embodiment is U=1.02+0.065S1*D1+0.0086S2*D2=2.01792 g / cc.

[0089] It has been determined that the compacted density of the lithium iron manganese phosphate composite electrode plate described in this embodiment is 2.01792 g / cc.

[0090] Example 4

[0091] This embodiment provides a method for preparing the lithium manganese iron phosphate composite electrode sheet, comprising the following steps:

[0092] (1) Nano-manganese iron phosphate lithium material (average primary particle size of 100 nm), acetylene black and binder polyvinylidene fluoride (PVDF) were mixed and homogenized in a mass ratio of 95.5:2.5:2, and then coated on the surface of 16 μm carbon-coated aluminum foil to form a manganese iron phosphate lithium material layer;

[0093] (2) using pulsed ultrasound to disperse carbon-coated lithium iron phosphate material powder (average primary particle size of 450 nm, carbon coating amount of 1.3 wt%, average thickness of carbon coating layer of 3 nm) in mineral oil, and the dispersed lithium iron phosphate powder solution is washed and filtered with analytical grade (AR, 99.9%) anhydrous ethanol, and then the wet lithium iron phosphate powder after filtration is suspended in n-hexane; the suspension in n-hexane is continuously applied to the surface of the lithium manganese iron phosphate material layer, and then dried in an oven at 130° C. for 4 h under a nitrogen atmosphere to form a carbon-coated lithium iron phosphate material layer;

[0094] (3) The above-mentioned electrode sheet is subjected to a rolling process, and the rolling speed is controlled to be 2m / min, so that the lithium iron phosphate material layer and the carbon-coated lithium iron phosphate material layer are adhered and composited to obtain the required lithium iron phosphate composite electrode sheet.

[0095] In the lithium manganese iron phosphate composite electrode plate described in this embodiment:

[0096] The surface density S1 of the carbon-coated lithium iron phosphate material layer is 20 g / m 2 The median particle size D of the carbon-coated lithium iron phosphate material 50 (i.e. D1) is 0.8 μm;

[0097] The surface density S2 of the lithium manganese iron phosphate material layer is 18g / m 2 The median particle size D of the nano-manganese iron phosphate lithium material 50 (ie D2) is 0.4 μm.

[0098] After calculation, the maximum compaction density of the lithium manganese iron phosphate composite electrode plate described in this embodiment is U=1.02+0.065S1*D1+0.0086S2*D2=2.12192 g / cc.

[0099] It has been determined that the compacted density of the lithium iron manganese phosphate composite electrode plate described in this embodiment is 2.12192 g / cc.

[0100] Example 5

[0101] The preparation method of the lithium manganese iron phosphate composite electrode plate in this embodiment is the same as that in embodiment 1, except that the surface density of the lithium manganese iron phosphate material layer is 17g / m 2 The surface density of the carbon-coated lithium iron phosphate material layer is 14 g / m 2 .

[0102] The compacted density of the lithium iron manganese phosphate composite electrode plate described in this example was measured to be 1.8344 g / cc. Due to the low compacted density of the plate, the battery core could not be assembled into the housing, thus failing to meet the design requirements.

[0103] Example 6

[0104] The preparation method of the lithium manganese iron phosphate composite electrode plate in this embodiment is the same as that in embodiment 1, the only difference is that the surface density of the lithium manganese iron phosphate material layer is 20g / m 2 The surface density of the carbon-coated lithium iron phosphate material layer is 25g / m 2 .

[0105] The compacted density of the lithium iron phosphate composite electrode sheet described in this embodiment was measured to be 2.17736 g / cc. Because both the surface densities of lithium iron phosphate and lithium iron phosphate exceeded the upper limit, the lower layer of lithium iron phosphate was prone to cracking when coated with 2% binder. Furthermore, the upper layer of lithium iron phosphate was also prone to cracking if it was too thick, affecting the quality of the electrode sheet.

[0106] Example 7

[0107] The preparation method of the lithium manganese iron phosphate composite electrode plate in this embodiment is the same as that in embodiment 1, the only difference is that the median particle size D of the carbon-coated lithium iron phosphate material is 50 (ie D1) is 0.6 μm, and the median particle size D 50 (ie D2) is 0.2 μm.

[0108] The compacted density of the lithium iron manganese phosphate composite electrode plate described in this embodiment was measured to be 1.7478 g / cc. Because the median particle size D50 of the lithium iron manganese phosphate exceeded the lower limit, the lower layer of lithium iron manganese phosphate was prone to cracking when using a 2% binder, affecting the plate quality.

[0109] Example 8

[0110] The preparation method of the lithium manganese iron phosphate composite electrode plate in this embodiment is the same as that in embodiment 1, the only difference is that the median particle size D of the carbon-coated lithium iron phosphate material is 50 (ie D1) is 1.5 μm, and the median particle size D 50 (ie D2) is 0.8 μm.

[0111] The compacted density of the lithium iron manganese phosphate composite electrode plate described in this embodiment was measured to be 2.8782 g / cc. Because the median particle size D50 of the lithium iron manganese phosphate exceeded the upper limit, large particles and pitting easily formed on the lithium iron phosphate coating, resulting in numerous defects on the electrode surface and affecting the quality of the electrode.

[0112] Comparative Example 1

[0113] The electrode plate in this comparative example is directly coated on the surface of the carbon-coated aluminum foil using lithium iron manganese phosphate according to the method in Example 1. The electrode plate is only a single-coating structure of lithium iron manganese phosphate.

[0114] The electrode plate of this comparative example cannot effectively inhibit direct contact between the electrolyte and the lithium iron manganese phosphate, resulting in metal dissolution and destruction of the lithium iron manganese phosphate structure, which affects the quality of the electrode plate.

[0115] Comparative Example 2

[0116] The electrode plate described in this comparative example is prepared according to the method in Example 1, with the only difference being that the nano-lithium manganese iron phosphate material and the carbon-coated lithium iron phosphate material are homogenized together and directly coated on the surface of the carbon-coated aluminum foil. The electrode plate is a single-coating structure containing both lithium manganese iron phosphate and carbon-coated lithium iron phosphate.

[0117] In this comparative method, since the nano-lithium iron manganese phosphate material and the carbon-coated lithium iron phosphate material are homogenized together and then coated into pole pieces, part of the lithium iron manganese phosphate is on the uppermost surface of the pole piece, which cannot effectively inhibit the direct contact between the electrolyte and the lithium iron manganese phosphate, causing metal dissolution and destruction of the lithium iron manganese phosphate structure, affecting the quality of the pole piece; moreover, since the homogenization treatment is carried out at the same time, the raw materials will be unevenly dispersed, and the pole piece will experience coating cracking, material falling off, etc., which seriously affects the quality of the pole piece.

[0118] Comparative Example 3

[0119] The electrode plate in this comparative example is prepared according to the method in Example 1, with the only difference being that the carbon-coated lithium iron phosphate material is homogenized according to the same system as the nano-lithium manganese iron phosphate material and coated on the surface of the lithium manganese iron phosphate material layer. The electrode plate has a double-coating structure containing both lithium manganese iron phosphate and carbon-coated lithium iron phosphate.

[0120] In this comparative method, the lithium iron phosphate was homogenized according to the same system formula and coated on the surface of the lithium iron manganese phosphate. On the one hand, the simultaneous homogenization process resulted in uneven dispersion of the raw materials. Moreover, direct coating on the lithium iron manganese phosphate coating was prone to cracking and material shedding, seriously affecting the quality of the electrode. This is mainly because the wet coating process causes the solvent to evaporate at the same rate during the drying process of the upper and lower layers, which can cause cracking and even material shedding of the electrode.

[0121] However, the method described in the aforementioned embodiment of the present application can solve the problem of cracking and falling of the electrode piece by controlling the inconsistent evaporation speed of the two solvents in the upper and lower layers, thereby ensuring the quality of the electrode piece.

[0122] Experimental example

[0123] 1. Charge and discharge performance test

[0124] The electrochemical and physical and chemical indicators of the lithium iron manganese phosphate composite electrode plates prepared in the above Examples 1-4 (button-type half-cell, size 2032, negative electrode is lithium plate) were tested by conventional technical means, and the test results are shown in Table 1 below.

[0125] Table 1 Physical and chemical index test results

[0126] It can be seen from the data in Table 1 above that there is no obvious difference in the surface state of the electrode pieces of Example 5 and Examples 1-4. The only difference is that the compaction density is lower, which does not affect the normal performance. However, the electrode pieces of Examples 6-8 have abnormalities such as cracking and material falling, resulting in abnormal performance. Comparative Examples 1-3 cannot effectively suppress the metal dissolution caused by the side reaction of lithium manganese iron phosphate and the electrolyte, resulting in serious structural damage and low performance.

[0127] 2. Discharge capacity test

[0128] The lithium iron manganese phosphate composite electrode plates prepared in Examples 1-6 and Comparative Example 1 were respectively used to perform full battery assembly verification on the positive electrode plates and the graphite negative electrode plates. The initial efficiency and gram capacity were tested. The test results are shown in Table 2 below.

[0129] Table 2 Discharge capacity test results

[0130] It can be seen from the data in Table 2 above that the results of Examples 1-4 show that the battery made of the positive electrode material described in the present application has a discharge capacity of more than 36.89 Ah at -40 ° C 1C, a discharge capacity of more than 56.49 Ah at -20 ° C 1C, a discharge capacity of more than 55.57 Ah at 25 ° C 1C, and a discharge capacity of more than 51.64 Ah at 25 ° C 2C, while the performance of the electrode plates in Comparative Examples 1-3 is significantly worse.

[0131] From the comparison between Example 1 and Examples 5-8, it can be seen that in the lithium manganese iron phosphate composite electrode sheet described in this application, the surface density of the carbon-coated lithium iron phosphate material layer and the surface density of the lithium manganese iron phosphate material layer will affect its performance. The surface density of the lithium manganese iron phosphate material layer is controlled at 15-18 g / m 2 The surface density of the carbon-coated lithium iron phosphate material layer is controlled at 18-20g / m 2 , the performance of the lithium manganese iron phosphate composite electrode plate is better. If the surface density of the carbon-coated lithium iron phosphate material layer and the surface density of the lithium manganese iron phosphate material layer are too large, the Li + The deintercalation of ions affects Li + The migration rate of ions. If the surface density of the carbon-coated lithium iron phosphate material layer and the surface density of the lithium manganese iron phosphate material layer are too small, the direct contact between the electrolyte and the material surface cannot be effectively reduced during the charge and discharge process, and the side reactions cannot be reduced.

[0132] In summary, the composite lithium iron manganese phosphate electrode plates described in this application meet battery-grade requirements and exhibit excellent rate and low-temperature performance, indicating that this composite lithium iron manganese phosphate electrode plate is suitable for high-energy-density battery systems.

[0133] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A lithium manganese iron phosphate composite electrode plate, characterized in that: It includes a lithium iron manganese phosphate material layer and a lithium iron phosphate material layer sequentially arranged on the surface of the current collector; The maximum compaction density U of the lithium manganese iron phosphate composite electrode plate satisfies the following relationship: U=1.02+0.065S1*D1+0.0086S2*D2; S1 and D1 are respectively the surface density of the lithium iron phosphate material layer and the median particle size D of the lithium iron phosphate material forming the lithium iron phosphate material layer. 50 ; S2 and D2 are respectively the surface density of the lithium manganese iron phosphate material layer and the median particle size D of the nano-lithium manganese iron phosphate material forming the lithium manganese iron phosphate material layer. 50 ; The maximum compaction density of the lithium iron manganese phosphate composite electrode plate is greater than 2.00 g / cc.

2. The lithium manganese iron phosphate composite electrode plate according to claim 1, characterized in that: In the lithium manganese iron phosphate composite electrode plate, the mass ratio of the nano lithium manganese iron phosphate material to the lithium iron phosphate material is (1-x): (x), wherein 1≤x≤4; Preferably, the compaction density of the lithium iron manganese phosphate composite electrode plate is 2.00-2.60 g / cc.

3. The lithium manganese iron phosphate composite electrode plate according to claim 1 or 2, characterized in that: The surface density S1 of the lithium iron phosphate material layer is 18-20 g / m 2 and / or, The surface density S2 of the lithium manganese iron phosphate material layer is 15-18 g / m 2 .

4. The lithium manganese iron phosphate composite electrode plate according to any one of claims 1 to 3, characterized in that: The median particle size D of the nano-lithium manganese iron phosphate material forming the lithium manganese iron phosphate material layer 50 0.4-0.6 μm; and / or, The median particle size D of the lithium iron phosphate material forming the lithium iron phosphate material layer 50 0.8-1.2μm.

5. The lithium manganese iron phosphate composite electrode plate according to any one of claims 1 to 4, characterized in that: The specific surface area of ​​the nano-manganese iron lithium phosphate material is 18-20m 2 / g; and / or, The grain size of the nano-lithium manganese iron phosphate material is 75-85nm; and / or, The unit cell volume of the nano-manganese iron lithium phosphate material is and / or, The compaction density (2T pressure) of the nano-lithium manganese iron phosphate material is 2.1-2.3 g / cc; and / or, The tap density of the nano-manganese iron lithium phosphate material is 1.0-1.2 g / cm 3 .

6. The lithium manganese iron phosphate composite electrode plate according to any one of claims 1 to 5, characterized in that: The lithium iron phosphate material is a carbon-coated lithium iron phosphate material; Preferably, in the carbon-coated lithium iron phosphate material, the mass content of carbon element is 1.2-1.4wt%.

7. A method for preparing the lithium manganese iron phosphate composite electrode sheet according to any one of claims 1 to 6, characterized in that: The steps include: (1) mixing nano-manganese iron phosphate lithium material, a conductive agent and a binder, and applying the mixture on the surface of the current collector to form the manganese iron phosphate lithium material layer; (2) dispersing the lithium iron phosphate material in an organic solvent to obtain a suspension, and applying the suspension to the surface of the lithium iron manganese phosphate material layer to form the lithium iron phosphate material layer; (3) The electrode plates after the above treatment are subjected to roll pressing to obtain the desired lithium manganese iron phosphate composite electrode plates.

8. The method for producing a lithium iron manganese phosphate composite electrode sheet according to claim 7, characterized in that: In the step (1), the current collector comprises a carbon-coated aluminum foil; preferably, the thickness of the carbon-coated aluminum foil is 12-16 μm; and / or, In the step (1), the conductive agent includes at least one of acetylene black, Ketjen black, carbon nanotubes (CNTs), 350G or carbon fiber (VGCF); and / or, In the step (1), the binder comprises polyvinylidene fluoride (PVDF); and / or, In the step (1), the mass ratio of the nano-manganese iron lithium phosphate material, the conductive agent and the binder is 95.5-96.5:1.5-2.5:2; and / or, In the step (1), the coating speed of the coating step is 5-10 m / min; and / or, In the step (2), the organic solvent comprises n-ethane and / or isopropanol; and / or The step (2) further comprises dispersing the carbon-coated lithium iron phosphate material in mineral oil and adding ethanol for washing; and / or, In the step (3), the speed of the rolling step is 2-3 m / min; and / or, The step (3) further comprises a step of drying the electrode sheet before the rolling step; preferably, the atmosphere of the drying step comprises at least one of hydrogen, nitrogen, helium, neon, argon, krypton or xenon; Preferably, the temperature of the drying step is 100-130° C., and the drying time is 2-4 hours.

9. Use of the lithium iron manganese phosphate composite electrode sheet according to any one of claims 1 to 6 or the lithium iron manganese phosphate composite electrode sheet prepared by the method according to claim 7 or 8 for preparing a positive electrode sheet for a lithium ion battery or a lithium ion battery.

10. A lithium-ion battery positive electrode sheet or a lithium-ion battery prepared by the lithium iron manganese phosphate composite electrode sheet according to any one of claims 1 to 6 or the lithium iron manganese phosphate composite electrode sheet prepared by the method according to claim 7 or 8.

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