A method for improving the conductivity of the positive electrode of a lithium iron manganese phosphate battery

Abstract

The application discloses a method for improving the conductivity of a lithium iron manganese phosphate battery positive electrode, and belongs to the field of lithium ion battery design and manufacturing processes. The method comprises the following steps: preparing lithium iron manganese phosphate positive electrode slurry by fully mixing and uniformly mixing conductive organic matter and positive electrode material, coating the slurry on an aluminum foil, assembling a full battery by laminating the slurry after rolling, cutting and sheeting, and obtaining the lithium iron manganese phosphate battery after small-current formation. The battery forms a layer of elastic conductive polymer during formation, can improve the conductivity of the lithium iron manganese phosphate positive electrode, thereby improving the cycle performance of the battery, and reducing the DCR increase during the cycle process. The whole preparation process of the application matches the production process of mass-produced batteries, and has a high application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery design and manufacturing process, in particular to a method for improving the positive electrode conductivity of lithium iron manganese phosphate battery. BACKGROUND

[0002] With the increase of electric vehicles and the gradual release of energy storage, the safety of the battery is also gradually improved. Lithium iron phosphate battery has gradually become the main positive electrode material of electric vehicles or energy storage battery due to its high safety. With the progress of technology and the upgrading of materials, the capacity of lithium iron phosphate is almost at the limit, and there is little room for performance improvement. The crystal structure of lithium iron manganese phosphate is similar to that of lithium iron phosphate, and it also has the characteristics of stable chemical properties and excellent safety performance. Moreover, the doping of manganese element in lithium iron manganese phosphate can improve the charging voltage of the material, and the theoretical energy density is increased by 15% to 20% compared with lithium iron phosphate, which can further expand the cruising range.

[0003] However, due to the doping of manganese element, the conductivity of lithium iron manganese phosphate material is poor, only 10 -13 S / cm. The conductivity of lithium manganese iron phosphate material can be improved by surface carbon coating, but the surface carbon coating of the positive electrode material requires special equipment and process, which will increase the cost of raw material manufacturing. In addition, the surface carbon coating of lithium manganese iron phosphate material can only solve the conductivity of single particle, and more conductive agent needs to be added in the preparation of the electrode sheet to reduce the film resistance of the electrode sheet, improve the electrochemical performance of the electrode sheet, and improve the cycle performance and direct current resistance (DCR) growth of lithium manganese iron phosphate battery. SUMMARY

[0004] Based on the problems existing in the prior art, the present application provides a method for improving the positive electrode conductivity of lithium iron manganese phosphate battery, which aims to reduce the carbon coating amount of lithium iron manganese phosphate material, reduce the amount of conductive agent in the electrode sheet, and reduce the film resistivity and electrode resistance, and improve the cycle performance and DCR growth problem of lithium iron manganese phosphate.

[0005] The present application first provides a method for improving the positive electrode conductivity of lithium iron manganese phosphate battery, comprising the following steps:

[0006] (1) preparing a slurry by using conventional slurry preparation process with lithium iron manganese phosphate, PVDF, composite conductive agent and organic matter;

[0007] (2) coating the slurry on both sides of aluminum foil or carbon-coated aluminum foil, and drying to obtain lithium iron manganese phosphate positive electrode roll;

[0008] (3) obtaining lithium iron manganese phosphate positive electrode sheet by rolling the positive electrode roll obtained in step (2);

[0009] (4) The lithium manganese iron phosphate positive electrode sheet is matched with the negative electrode to assemble a full battery, and after baking and passing the moisture test, liquid injection is carried out for infiltration;

[0010] (5) The infiltrated battery is subjected to formation, and the finished battery is obtained through subsequent trial production.

[0011] As preferred: in the lithium manganese iron phosphate, Mn accounts for 10% to 90% of the total molar content of Mn and Fe; the lithium manganese iron phosphate primary particles have carbon coating or no carbon coating on the surface.

[0012] As preferred: the composite conductive agent is composed of single-walled carbon nanotubes, multi-walled carbon nanotubes and SP conductive agent, and the mass ratio of the three is 0.1% to 2.0%: 33.3% to 19.6%: 66.6% to 78.4%.

[0013] As preferred: the organic matter includes at least one of aromatic, benzene, non-benzene and heterocyclic compound organic matters, wherein: the aromatic organic matter is aromatic amine or styrene, etc., the benzene organic matter is aniline, etc., the non-benzene organic matter is 1,8-diaminonaphthalene or 1-aminoanthracene, etc., and the heterocyclic compound organic matter is pyrrole, thiophene or indole, etc.

[0014] As preferred: when the lithium manganese iron phosphate primary particles have carbon coating on the surface, the dry matter mass ratio of the carbon-coated lithium manganese iron phosphate, PVDF, composite conductive agent and organic matter is (97 to 98):(1.0 to 1.5):(0 to 1):(0.2 to 0.8), and specifically can be 97:1.2:1:0.8 or 98:1.0:0.5:0.5. When the lithium manganese iron phosphate primary particles have no carbon coating on the surface, the dry matter mass ratio of the lithium manganese iron phosphate without carbon coating, PVDF, composite conductive agent and organic matter is (95 to 97):(1.0 to 1.5):(0.5 to 2):(0.5 to 2), and specifically can be 95:1.4:1.8:1.8 or 97:1.2:0.3:1.5.

[0015] As preferred, in step (1), when preparing the slurry, PVDF and organic matter are added to NMP in proportion, glue is made, the solid content of the slurry is adjusted to 6% to 10%, then the composite conductive agent is added for high-speed dispersion, and then the lithium manganese iron phosphate material is added to the solvent for slurry preparation. The solid content of the slurry is 50% to 65%, and the viscosity is 4000 to 7000 mpas.

[0016] As preferred, in step (2), the coating double-sided area density of the slurry coated on the surface of the aluminum foil or carbon-coated aluminum foil is 320 to 430 g / m 2 .

[0017] As preferred, in step (3), the compaction density of the roll-pressed is 2.3 to 2.5 g / cm 3 .

[0018] Preferably, in step (4), the formation process includes a low-current activation process to induce the electro-oxidation polymerization of organic matter on the positive electrode surface to generate a conductive polymer. Specifically, this can be a constant current charge of 0.01C to 1.8V and a constant current charge of 0.02C to 2.7V. The subsequent formation process is consistent with the formation process of conventional lithium iron manganese phosphate batteries.

[0019] The method for improving the positive electrode conductivity of lithium iron manganese phosphate batteries of the present invention introduces organic matter that can be adsorbed on the surface of the positive electrode material. After formation by a small current, an elastic conductive polymer is formed on the surface of the positive electrode material. This can effectively improve the problem of the positive electrode material peeling off from the traditional conductive agent caused by the volume change during the lithium insertion / extraction process. It can effectively improve the conductivity of the positive electrode material during cycling, reduce the DCR growth during cycling, and improve cycle performance.

[0020] The present invention has the following beneficial effects:

[0021] This invention addresses the low conductivity of lithium iron manganese phosphate (LMP) cathode materials by providing a solution to improve conductivity at the cell level. Due to the incorporation of manganese, the conductivity of LMP cathode materials is very low. By thoroughly mixing a conductive organic material with the cathode material and then electro-oxidizing and polymerizing it during the initial formation phase, an elastic conductive polymer layer is formed on the cathode surface. This effectively improves the resistivity of the cathode film, thereby optimizing the film resistivity and improving the cycle performance of the LMP cell while reducing DCR growth during cycling. The entire production process of this invention is fully compatible with mass-produced cell processes and has high application prospects. Attached Figure Description

[0022] Figure 1 Comparison of room temperature cycling capacity retention rates of the battery cells prepared in Examples 1 and 2 and Comparative Examples 2 and 3;

[0023] Figure 2 The comparison of the room temperature cycling DCR growth rate of the cells prepared in Examples 1 and 2 and Comparative Examples 2 and 3. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0025] Unless otherwise specified, the experimental methods described in the following examples are conventional methods.

[0026] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0027] Unless otherwise specified, all percentages below refer to mass percentages.

[0028] Example 1

[0029] Using NMP as solvent, carbon-coated lithium iron manganese phosphate (commercially available, with a Mn molar ratio of 60%), PVDF, a composite conductive agent (composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, and SP conductive agent in a mass percentage ratio of 1.0%:32.4%:66.6%), and 1,8-diaminonaphthalene were wet-mixed in a mass ratio of 98:1.0:0.5:0.5. The resulting cathode slurry had a solid content of 55.1% and a viscosity of 4583 mPa·s. The slurry was filtered using a 200-mesh slurry filter and then coated onto both sides of aluminum foil. The coating speed was set at 15 m / min, the maximum baking temperature in the coating oven was 110℃, and the overall surface density of the coating was 400 g / m³. 2 The coated positive electrode roll is transferred to a roller press. After rolling, the compacted density of the positive electrode is 2.35 g / cm³. 3 The positive electrode after roll forming is slit and laser-cut to obtain electrode sheets, which are then stacked with matching graphite negative electrodes to prepare a square-shell battery with a capacity of 86Ah. After vacuum baking, the moisture content of the battery cells is <200ppm. The battery cells are then injected with electrolyte and impregnated, followed by formation. The formation process involves charging to 1.8V at 0.01C, to 2.7V at 0.02C, to 3.0V at 0.03C, to 3.4V at 0.08C, and to 3.7V at 0.2C. After formation, the battery cells undergo aging, electrolyte replenishment, and sealing, followed by a 0.33C charge-discharge cycle for 3 weeks to obtain the finished battery cells.

[0030] Example 2

[0031] Using NMP as solvent, a wet slurry was prepared by mixing uncoated lithium iron manganese phosphate (commercially available, with a Mn molar ratio of 60%), PVDF, a composite conductive agent (composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, and SP conductive agent in a mass percentage ratio of 1.0%:32.4%:66.6%), and 1,8-diaminonaphthalene in a mass ratio of 97:1.2:0.3:1.5. The resulting cathode slurry had a solid content of 56.2% and a viscosity of 4123 mPa·s. The slurry was filtered using a 200-mesh stencil and then coated onto both sides of an aluminum foil at a coating speed of 15 m / min. The maximum baking temperature in the coating oven was 110 °C, and the overall surface density of the coating was 400 g / m³. 2 The coated positive electrode roll is transferred to a roller press. After rolling, the compacted density of the positive electrode is 2.35 g / cm³. 3The positive electrode after roll forming is slit and laser-cut to obtain electrode sheets, which are then stacked with matching graphite negative electrodes to prepare a square-shell battery with a capacity of 86Ah. After vacuum baking, the moisture content of the battery cells is <200ppm. The battery cells are then injected with electrolyte and impregnated, followed by formation. The formation process involves charging to 1.8V at 0.01C, to 2.7V at 0.02C, to 3.0V at 0.03C, to 3.4V at 0.08C, and to 3.7V at 0.2C. After formation, the battery cells undergo aging, electrolyte replenishment, and sealing, followed by a 0.33C charge-discharge cycle for 3 weeks to obtain the finished battery cells.

[0032] Comparative Example 1

[0033] The difference between Comparative Example 1 and Example 1 is that no organic matter was added. Specifically, carbon-coated lithium manganese iron phosphate, PVDF, and composite conductive agent were wet-mixed in a mass ratio of 98:1.0:1.0. The mixing and volume separation process was the same as in Example 1.

[0034] Comparative Example 2

[0035] The difference between Comparative Example 2 and Example 1 lies in the formation process. Specifically, the formula and operation from slurry mixing to liquid injection are the same as in Example 1. The formation process uses 0.03C charging to 3.0V, 0.08C charging to 3.4V, and 0.2C charging to 3.7V. After formation, the battery cell is aged, replenished with liquid, and sealed, and then charged and discharged at 0.33C for 3 weeks to obtain the finished battery cell.

[0036] Comparative Example 3

[0037] The difference between Comparative Example 3 and Example 2 is that no organic matter was added. Specifically, lithium manganese iron phosphate without carbon coating, PVDF, and composite conductive agent were wet-mixed in a mass ratio of 97:1.2:1.8. The mixing and volume separation operations were the same as in Example 2.

[0038] Comparative Example 4

[0039] The difference between Comparative Example 4 and Example 2 lies in the formation process. Specifically, the formula and operation from slurry mixing to liquid injection are the same as in Example 2. The formation process uses 0.03C charging to 3.0V, 0.08C charging to 3.4V, and 0.2C charging to 3.7V. After formation, the battery cell is aged, replenished with liquid, and sealed, and then charged and discharged at 0.33C for 3 weeks to obtain the finished battery cell.

[0040] Table 1 shows a comparison of the film resistivity and full cell internal resistance of the electrode sheets prepared in each embodiment and comparative example.

[0041] Table 1

[0042] Grouping Membrane resistivity Full cell resistance Example 1 0.335 Ω-cm 0.403 mΩ Example 2 0.392 Ω-cm 0.421 mΩ Comparative Example 1 0.409 Ω-cm 0.455 mΩ Comparative Example 2 0.367 Ω-cm 0.412 mΩ Comparative Example 3 0.432 Ω-cm 0.464 mΩ Comparative Example 4 0.401 Ω-cm 0.442 mΩ

[0043] The comparison between the film resistance and the full cell internal resistance shows that adding organic matter during the slurry mixing process and oxidizing it with a small current during the formation stage to form an elastic conductive polymer layer on the surface can effectively improve the film resistivity of the lithium iron manganese phosphate cathode and the full cell internal resistance.

[0044] Figure 1 and Figure 2 By comparing the cell's capacity retention and DCR growth rate at room temperature (50% SOC, 1.5C discharge for 10s) in the examples and two comparative examples, it can be seen that the introduction of the elastic conductive polymer effectively improves the cell's cycle life and reduces the DCR growth.

[0045] The above are merely exemplary embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving the conductivity of a lithium iron manganese phosphate battery anode, characterized in that: (1) a slurry is prepared by mixing lithium iron manganese phosphate, PVDF, a composite conductive agent, and an organic substance 1,8-diaminonaphthalene using a slurry mixing process; in the lithium iron manganese phosphate, Mn accounts for 10-90% of the total molar content of Mn and Fe; the surface of the primary particles of the lithium iron manganese phosphate is coated with carbon or is not coated with carbon; the composite conductive agent is composed of single-walled carbon nanotubes, multi-walled carbon nanotubes, and SP conductive agent; when the surface of the primary particles of the lithium iron manganese phosphate used is coated with carbon, the dry mass ratio of the carbon-coated lithium iron manganese phosphate, PVDF, composite conductive agent, and organic substance is 97-98: 1.0-1.5: 0-1: 0.2-0.8; when the surface of the primary particles of the lithium iron manganese phosphate used is not coated with carbon, the dry mass ratio of the carbon-uncoated lithium iron manganese phosphate, PVDF, composite conductive agent, and organic substance is 95-97: 1.0-1.5: 0.5-2: 0.5-2; (2) the slurry is applied to both sides of an aluminum foil or a carbon-coated aluminum foil and dried to obtain a lithium iron manganese phosphate anode roll; (3) the lithium iron manganese phosphate anode roll obtained in step (2) is rolled to obtain a lithium iron manganese phosphate anode sheet; (4) the lithium iron manganese phosphate anode sheet is matched with a negative electrode to assemble a full cell, which is subjected to liquid injection and soaking after being baked to pass the moisture test; (5) the soaked cell is subjected to formation and subsequent trial production to obtain a finished cell; the formation process is 0.01C charging to 1.8V, 0.02C charging to 2.7V, 0.03C charging to 3.0V, 0.08C charging to 3.4V, and 0.2C charging to 3.7V. In step (1), the solid content of the slurry is 50-65%, and the viscosity is 4000-7000 mPa·s. ​ ​ ​ ​ 2. The method of improving the conductivity of the positive electrode of lithium iron manganese phosphate battery as claimed in claim 1, wherein: ​ 3. The method of improving the conductivity of the positive electrode of lithium iron manganese phosphate battery as claimed in claim 1, wherein: In step (2), the paste is coated on both sides of the aluminum foil or carbon-coated aluminum foil, and the coating double-sided area density is 320-430 g / m 2 ; in step (3), the compaction density of the roll-pressed product is 2.3-2.5 g / cm 3 .

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