Composite positive plate, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of batteries, and particularly discloses a composite positive plate, a preparation method thereof and a lithium ion battery. Comprising the following steps: (1) dissolving a binder in a solvent, and stirring for the first time to prepare a glue solution; (2) mixing a positive electrode material, a flame retardant and a conductive agent, and stirring for the second time to prepare dry powder; (3) adding the dry powder obtained in the step (2) into the glue solution obtained in the step (1), then adding a solvent, and stirring for the third time to obtain slurry; (4) sieving and baking the slurry obtained in the step (3) to obtain a composite positive plate; the mass ratio of the positive electrode material to the flame retardant to the conductive agent to the binder is (84-96): (1-10): (1.5-3): 1.5; in the step (2), the flame retardant comprises at least one of ammonium polyphosphate, melamine polyphosphate, calcium hypophosphite and aluminum hypophosphite. When the prepared composite positive plate is prepared, the flame retardant is doped into the positive slurry, and the assembled lithium battery effectively inhibits thermal runaway.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a composite positive electrode sheet and a preparation method thereof, and a lithium ion battery. Background Art

[0002] With the rapid development of new energy vehicles, energy storage systems and other fields and emerging industries such as the Internet, smart phones, and electric vehicles, batteries, as one of the core technologies for power storage, have become the focus of attention. Lithium-ion batteries, as a battery technology with high energy density, long life, and environmental protection, are widely understood and applied. Positive electrode materials are mainly used in the field of lithium-ion batteries. They are the key components for storing and releasing energy in lithium-ion batteries, which in turn affect the various performance indicators of the battery. Therefore, positive electrode materials occupy a core position in lithium-ion batteries. However, lithium-ion positive electrode materials will release oxygen during the decomposition process. As the gas rapidly gathers and expands, the pressure inside the battery will rise rapidly, resulting in battery volume expansion, pole piece / diaphragm dislocation, and battery polarization. Once the internal pressure exceeds the limit that the battery safety valve can withstand, a jetting phenomenon will occur, which seriously threatens the safety and stability of the battery and is an important reason for the battery life decay and even safety problems.

[0003] The layered metal oxide positive electrode material of lithium-ion batteries, lithium nickel cobalt manganese oxide (NCM), is developed on the basis of transition metal oxides. NCM undergoes an irreversible phase transition during use, that is, from a layered structure to a spinel phase, and finally to a rock salt phase, and precipitates O 2 and heat; at the same time, the positive electrode material undergoes a decomposition reaction when heated, releasing heat and oxygen; on this basis, the oxygen reacts with the electrolyte and precipitates a large amount of carbon monoxide, carbon dioxide and heat; as the amount of heat released increases, the internal temperature of the lithium battery increases significantly, the positive electrode active material further decomposes, and more oxygen is precipitated; in addition, diethyl carbonate (DEC) reacts with LiPF 6 PF 5 Reactions occur, accelerating oxygen consumption, releasing carbon dioxide and heat, as well as smoke C 2 H 5 F. A large amount of heat and harmful gases will be released during the entire chain reaction, causing thermal runaway of the battery, which will lead to battery fire and explosion.

[0004] Therefore, in order to address the problem of thermal runaway of lithium batteries caused by oxygen production in the positive electrode material, it is necessary to provide a composite positive electrode sheet and its preparation method and a lithium-ion battery to effectively inhibit thermal runaway, improve the thermal stability and safety performance of the battery, and at the same time improve the efficiency and life of the battery. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention proposes a composite positive electrode sheet and a preparation method thereof and a lithium-ion battery, which can effectively suppress thermal runaway, improve the thermal stability and safety performance of the battery, and at the same time improve the efficiency and life of the battery.

[0006] A first aspect of the present invention provides a method for preparing a composite positive electrode sheet.

[0007] Specifically, a method for preparing a composite positive electrode sheet comprises the following steps:

[0008] (1) dissolving the binder in a solvent and stirring for the first time to obtain a glue solution;

[0009] (2) mixing the positive electrode material, the flame retardant, and the conductive agent, and stirring for the second time to obtain a dry powder;

[0010] (3) adding the dry powder of step (2) to the glue of step (1), and then adding a solvent and stirring for a third time to obtain a slurry;

[0011] (4) screening and baking the slurry in step (3) to obtain the composite positive electrode sheet;

[0012] The mass ratio of the positive electrode material, the flame retardant, the conductive agent, and the binder is 84-96:1-10:1.5-3:1.5;

[0013] In step (2), the flame retardant includes at least one of ammonium polyphosphate, melamine polyphosphate, calcium hypophosphite, and aluminum hypophosphite;

[0014] Among them, ammonium polyphosphate APP first decomposes to produce ammonia (NH 3 ) and polyphosphoric acid (H x P y O z ), ammonia can dilute oxygen to achieve the purpose of inhibiting combustion, and the carbon layer further formed by polyphosphoric acid on the surface of the positive electrode inhibits the reaction between the positive electrode material and the electrolyte. If the amount of APP added is too low, the flame retardant effect cannot be achieved. If the amount of APP added is too high, the electrochemical performance and energy density of the battery will be affected.

[0015] Further preferably, the mass ratio of the positive electrode material, the flame retardant, the conductive agent and the binder is 87-96:1-10:1.5-2:1.5.

[0016] Preferably, in step (1), the solid content of the glue solution is 6 to 8 wt %.

[0017] Further preferably, in step (1), the solid content of the glue solution is 7 to 8 wt %.

[0018] Preferably, in step (1), the first stirring is performed at a speed of 1000 to 2000 r / min for 2 to 4 hours.

[0019] Further preferably, in step (1), the rotation speed of the first stirring is 1000-1500 r / min, and the time is 2-3 h.

[0020] Preferably, in step (2), the positive electrode material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt aluminum oxide.

[0021] Preferably, in step (1), the binder comprises polyvinylidene fluoride.

[0022] Further preferably, the polyvinylidene fluoride includes at least one of HSV 900 and PVDF 5130.

[0023] Preferably, the molecular weight of the polyvinylidene fluoride is 100-110W.

[0024] More preferably, the molecular weight of the polyvinylidene fluoride is 110W.

[0025] Preferably, in step (2), the second stirring speed is 1000-2000 r / min, and the time is 0.5-1 h.

[0026] Further preferably, in step (2), the second stirring speed is 1000-1500 r / min, and the time is 0.5-1 h.

[0027] Preferably, in step (3), the solid content of the slurry is 78-85 wt %.

[0028] Further preferably, in step (3), the solid content of the slurry is 80-85 wt %.

[0029] Preferably, in step (3), at 25° C., the viscosity of the slurry is 4000 to 6000 Pa·s.

[0030] Preferably, in step (3), at 25° C., the viscosity of the slurry is 4000 to 5000 Pa·s.

[0031] Preferably, in step (3), at 25° C., the viscosity of the slurry is 5000 Pa·s.

[0032] Preferably, in step (3), the rotation speed of the third stirring is 1000-2000 r / min, and the time is 0.5-1 h.

[0033] Further preferably, in step (3), the rotation speed of the third stirring is 1000-1500 r / min, and the time is 0.5-1 h.

[0034] Preferably, the surface density of the composite positive electrode sheet is 155-190 g / m 2 .

[0035] Further preferably, the surface density of the composite positive electrode sheet is 155-165 g / m 2 .

[0036] A second aspect of the present invention provides a composite positive electrode sheet.

[0037] Specifically, the composite positive electrode sheet is prepared by the preparation method provided by the first aspect.

[0038] A third aspect of the present invention provides a lithium ion battery.

[0039] Specifically, the lithium-ion battery includes the composite positive electrode sheet provided in the second aspect.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] The composite positive electrode sheet prepared by the present invention is mixed with a flame retardant in the positive electrode slurry during preparation. The flame retardant will be more firmly combined with the solid electrode. The assembled lithium battery can more directly inhibit the violent exothermic decomposition reaction of the positive electrode, effectively inhibit thermal runaway, optimize the internal structure of the battery, and improve the efficiency and life of the battery; it can prevent ion migration from being blocked and battery performance from being reduced, and significantly improve the rate performance, thermal stability and safety performance of the positive electrode material. And the present invention finds that only specific flame retardants can inhibit thermal runaway, such as the general flame retardant triphenyl phosphate, because it can be miscible with the electrolyte, after the positive electrode sheet contacts the electrolyte, the triphenyl phosphate detaches from the positive electrode sheet and enters the electrolyte; and the flame retardant effect of triphenyl phosphate in the electrolyte only acts to extend the runaway time, and cannot change the runaway temperature of the battery. After a certain period of time, the battery will still have thermal runaway. DETAILED DESCRIPTION

[0042] In order to make the technical scheme of the present invention more clearly understood by those skilled in the art, the following embodiments are listed for illustration. It should be pointed out that the following embodiments do not limit the protection scope of the present invention.

[0043] Unless otherwise specified, the raw materials, reagents or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods.

[0044] Example 1

[0045] A composite positive electrode sheet and a preparation method thereof, comprising the following steps:

[0046] (1) Dissolving and dispersing the binder PVDF 5130 (purchased from Solvay) in N-methylpyrrolidone, and performing the first stirring at a stirring speed of 1200 r / min for 2 h until the adhesive solution is completely transparent, thereby obtaining an adhesive solution having a solid content of 8 wt %;

[0047] (2) mixing lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, and a conductive agent (three conductive agents are used: conductive carbon black SP, carbon nanotubes CNT, and carbon nanofibers VGCF), and stirring for a second time at a stirring speed of 1200 r / min for 0.5 h to obtain a dry powder;

[0048] (3) adding the dry powder of step (2) to the glue of step (1), and then adding N-methylpyrrolidone to adjust the viscosity of the slurry to 5000 Pa·s, and stirring for the third time at a stirring speed of 1200 r / min for 0.5 h to obtain a slurry with a solid content of 82 wt%;

[0049] (4) The slurry in step (3) is sieved with a 200-mesh screen, and then the sieved slurry is coated on an aluminum foil, and then baked, rolled, slit, and die-cut to obtain the composite positive electrode sheet, wherein the surface density of the composite positive electrode sheet is 165 g / m 2 .

[0050] The mass ratio of lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, conductive agent, and PVDF 5130 is 96:1:1.5:1.5.

[0051] Example 2

[0052] A composite positive electrode sheet and a preparation method thereof.

[0053] The difference from Example 1 is that the mass ratio of lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, conductive agent, and PVDF 5130 is 92:5:1.5:1.5, and the remaining preparation steps are the same as those in Example 1.

[0054] Example 3

[0055] A composite positive electrode sheet and a preparation method thereof.

[0056] The difference from Example 1 is that the mass ratio of lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, conductive agent, and PVDF 5130 is 87:10:1.5:1.5, and the remaining preparation steps are the same as those in Example 1.

[0057] Comparative Example 1

[0058] A composite positive electrode sheet and a preparation method thereof.

[0059] The difference from Example 1 is that no flame retardant is added, and the mass ratio of lithium nickel cobalt manganese oxide NCM811, conductive agent, and PVDF5130 is 97:1.5:1.5, and the remaining preparation steps are the same as Example 1.

[0060] Comparative Example 2

[0061] A composite positive electrode sheet and a preparation method thereof.

[0062] The difference from Example 2 is that ammonium polyphosphate is replaced by triphenyl phosphate, and the remaining preparation steps are the same as those of Example 2.

[0063] Comparative Example 3

[0064] A composite positive electrode sheet and a preparation method thereof.

[0065] The difference from Example 1 is that the mass ratio of lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, conductive agent, and PVDF 5130 is 96.5:0.5:1.5:1.5, and the remaining preparation steps are the same as those in Example 1.

[0066] Comparative Example 4

[0067] A composite positive electrode sheet and a preparation method thereof.

[0068] The difference from Example 1 is that the mass ratio of lithium nickel cobalt manganese oxide NCM811, ammonium polyphosphate, conductive agent, and PVDF 5130 is 82:15:1.5:1.5, and the remaining preparation steps are the same as those in Example 1.

[0069] Lithium-ion battery assembly:

[0070] Preparation of negative electrode sheet: Add graphite negative electrode material, ultrafine carbon powder SP, styrene butadiene rubber SBR, and sodium carboxymethyl cellulose CMC in a mass ratio of 96:1.5:1:1.5 to the solvent and mix thoroughly to prepare negative electrode slurry, and control the solid content to 50%. Evenly coat the negative electrode slurry on the copper foil, and obtain the negative electrode sheet through baking, rolling, striping, and die-cutting. The diaphragm is a commercial ceramic diaphragm, and the electrolyte is 1M LiPF 6 The volume ratio of / EC+DEC+DMC is 1:1:1. The composite positive electrode sheets, separators and negative electrode sheets prepared in Examples 1 to 3 and Comparative Examples 1 to 4 are laminated to form soft-pack batteries for experiments.

[0071] Test method:

[0072] First coulomb efficiency: The soft-pack battery is charged and discharged at a rate of 0.1C. The first coulomb efficiency = first discharge capacity / first charge capacity × 100%;

[0073] Cycling performance: At room temperature, the soft-pack battery is cycled at 0.3C. When the discharge capacity is less than 80% of the initial discharge capacity, the battery reaches the end of life.

[0074] 150℃ hot box test: 100% state of charge (SOC state), the hot box is heated from the test environment temperature to 150℃±2℃ at a rate of 5℃ / min, and this temperature is maintained for 30 minutes before stopping heating. Observe for 1 hour to monitor whether the battery cell catches fire or explodes.

[0075] Table 1 Performance test of the composite positive electrode sheets prepared in each embodiment and comparative example after being assembled into lithium batteries

[0076]

[0077] As can be seen from Table 1, the composite positive electrode sheets prepared in Examples 1 to 3 of the present invention are prepared by adding flame retardants to the positive electrode slurry. The flame retardants will be more firmly combined with the solid electrode, and the assembled lithium battery can more directly inhibit the violent exothermic decomposition reaction of the positive electrode, effectively inhibit thermal runaway, optimize the internal structure of the battery, and improve the efficiency and life of the battery; it can prevent the obstruction of ion migration and the decline of battery performance, and significantly improve the rate performance, thermal stability and safety performance of the positive electrode material. In Comparative Example 1, since no flame retardant is added, even if the battery efficiency is high, the thermal stability is poor, which will lead to thermal runaway. In Comparative Example 2, since the flame retardant ammonium polyphosphate is replaced by triphenyl phosphate TPP, although TPP is also used as a flame retardant, it is often used as an electrolyte additive, and TPP and electrolyte are miscible. When TPP is added to the positive electrode sheet and assembled into a soft-pack battery, after the positive electrode sheet contacts the electrolyte, TPP detaches from the positive electrode sheet and enters the electrolyte; and the flame retardant effect of TPP in the electrolyte only acts to prolong the runaway time, and cannot change the runaway temperature of the battery. After a certain period of time, the battery will still thermally runaway. In Comparative Example 3, the amount of flame retardant ammonium polyphosphate added was reduced, resulting in the inability to effectively suppress the violent exothermic decomposition reaction of the positive electrode, which ultimately led to thermal runaway of the battery. In Comparative Example 4, in the case of excessive flame retardant, although thermal runaway can be effectively suppressed, the efficiency and life of the battery are reduced.

[0078] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution obtained by any modification, equivalent replacement, improvement, etc. made by a person skilled in the art based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art shall be within the scope of protection determined by the claims.

Claims

1. A method for preparing a composite positive electrode sheet, characterized in that: The following steps are involved: (1) dissolving the binder in a solvent and stirring for the first time to obtain a glue solution; (2) mixing the positive electrode material, the flame retardant, and the conductive agent, and stirring for the second time to obtain a dry powder; (3) adding the dry powder of step (2) to the glue of step (1), and then adding a solvent and stirring for a third time to obtain a slurry; (4) screening and baking the slurry in step (3) to obtain the composite positive electrode sheet; The mass ratio of the positive electrode material, the flame retardant, the conductive agent, and the binder is 84-96:1-10:1.5-3:1.5; In step (2), the flame retardant includes at least one of ammonium polyphosphate, melamine polyphosphate, calcium hypophosphite, and aluminum hypophosphite.

2. The preparation method according to claim 1, characterized in that: In step (1), the solid content of the glue solution is 6 to 8 wt %.

3. The preparation method according to claim 1, characterized in that: In step (1), the binder includes polyvinylidene fluoride.

4. The preparation method according to claim 1, characterized in that: In step (2), the positive electrode material includes at least one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt aluminum oxide.

5. The preparation method according to claim 1, characterized in that: In step (2), the second stirring is carried out at a speed of 1000 to 2000 r / min and for a time of 0.5 to 1 h.

6. The preparation method according to claim 1, characterized in that: In step (3), the solid content of the slurry is 78-85wt%.

7. The preparation method according to claim 1, characterized in that: In step (3), at 25° C., the viscosity of the slurry is 4000 to 6000 Pa·s.

8. The preparation method according to claim 1, characterized in that: The surface density of the composite positive electrode sheet is 155-190 g / m 2 .

9. A composite positive electrode sheet, characterized in that: The composite positive electrode sheet is prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the composite positive electrode sheet according to claim 9.

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