CN116093268A - A kind of positive electrode sheet and its preparation method and application - Google Patents

Abstract

The invention provides a positive pole piece, a preparation method and application thereof, wherein the preparation method comprises the following steps: (1) Mixing a first active material with biochar powder, and calcining to obtain a calcined material; (2) Mixing a calcined material, a first part of conductive agent and a solvent, and adding a second active material to obtain mixed slurry; (3) The second part of the conductive agent, the binder and the mixed slurry are mixed and stirred, the viscosity is regulated to obtain positive electrode slurry, the positive electrode slurry is coated on the surface of a positive electrode current collector to obtain the positive electrode plate, the transmission path of lithium ions is shortened, interface impedance is reduced, interface conductivity is improved through matching of positive electrode powder materials with different particle sizes and a biochar material, the biochar material has higher specific surface area, excellent conductivity and rich surface functional groups, and the lithium iron phosphate battery prepared by the positive electrode plate has better cycle performance and the low-temperature discharge performance is also remarkably improved.

Description

A kind of positive electrode sheet and its preparation method and application

technical field

The invention belongs to the technical field of lithium ion batteries, and relates to a positive pole piece, a preparation method and application thereof.

Background technique

Due to the low intrinsic conductivity of lithium iron phosphate and the low diffusion coefficient of lithium ions in the solid phase, the cycle performance and discharge performance of lithium iron phosphate batteries under low temperature conditions are significantly affected. In order to improve the conductivity of lithium iron phosphate batteries, currently more methods are to develop electrolytes with good conductivity, select components with low viscosity and high conductivity, enhance the fluidity and conductivity of the electrolyte, and improve the electrolyte in phosphoric acid. The wetting effect in lithium iron improves its cycle and discharge performance under low temperature conditions. The discharge process of a lithium-ion battery involves deintercalation of lithium ions from the negative electrode and intercalation back to the positive electrode (lithium iron phosphate).

CN108878797A discloses a high compacted density lithium iron phosphate positive electrode material and positive electrode sheet, which is prepared by processing lithium iron phosphate into a nanoparticle dispersion, then adding a coating agent and a conductive nano-carbon material, mixing the slurry and drying it. Lithium iron phosphate secondary particles. The obtained lithium iron phosphate positive electrode material has high compaction density but poor low-temperature performance.

CN103715452A discloses a low-temperature lithium iron phosphate lithium-ion power battery. The positive electrode active material it adopts is nanometerized lithium iron phosphate coated with a discontinuous graphene structure, wherein the median particle size of the nanometerized lithium iron phosphate is 5 -10nm, graphene is 3-8 layers of multilayer graphene, the coating area accounts for 40-70% of the total surface area of lithium iron phosphate material, and the positive electrode sheet made by it has a low compaction density, resulting in low energy density.

At present, the existing technology is difficult to balance the cycle performance and low-temperature performance of lithium iron phosphate batteries, which greatly limits the practical application of lithium iron phosphate batteries.

Contents of the invention

The purpose of the present invention is to provide a positive electrode sheet and its preparation method and application. The present invention shortens the transmission path of lithium ions, reduces the interface impedance, and improves the interface conductivity through the matching of positive electrode powders with different particle sizes and biochar materials. The biochar material has a high specific surface area, excellent electrical conductivity and abundant surface functional groups, and the lithium iron phosphate battery prepared by using the positive electrode sheet has better cycle performance, and the low-temperature discharge performance has also been significantly improved.

To achieve this purpose of the invention, the present invention adopts the following technical solutions:

In a first aspect, the present invention provides a method for preparing a positive electrode sheet, the preparation method comprising the following steps:

(1) mixing the first active material and biochar powder, and calcining to obtain a fired material;

(2) mixing the one-calcined material, the first part of the conductive agent and the solvent, and adding the second active material to obtain a mixed slurry;

(3) mixing and stirring the second part of the conductive agent and the binder with the mixed slurry, adjusting the viscosity to obtain a positive electrode slurry, and coating the positive electrode slurry on the surface of the positive electrode current collector to obtain the positive electrode sheet;

Wherein, the first active material includes lithium iron phosphate with a median diameter D50 of 400-950 nm, and the second active material includes lithium iron phosphate with a median diameter D50 of 100-200 nm.

In the preparation process of the positive pole piece, the present invention adds a biochar material with a large specific surface area and conductivity, which can significantly improve the conductivity of the positive active material, and at the same time fully promote the mixing uniformity of two active materials with different particle sizes , shorten the transmission path of lithium ions, which is conducive to deintercalation, further improves the cycle and rate performance of the battery, and the low-temperature discharge performance is simultaneously significantly improved. To allow the electrolyte to be fully infiltrated.

Preferably, the biochar powder in step (1) includes bamboo charcoal powder.

Preferably, the mass ratio of the first active material to the biochar powder is 1:(0.08-0.12), for example: 1:0.08, 1:0.09, 1:0.1, 1:0.11 or 1:0.12.

Preferably, the calcination temperature in step (1) is 250-350°C, for example: 250°C, 280°C, 300°C, 320°C or 350°C.

Preferably, the calcination time is 5-8 hours, for example: 5 hours, 5.5 hours, 6 hours, 7 hours or 8 hours.

Preferably, based on the total mass of the conductive agent in the positive pole piece as 100%, the mass fraction of the first part of the conductive agent in step (2) is 30-50%, for example: 30%, 35%, 40%, 45% or 50% etc.

Preferably, the mass ratio of the monocalcined material to the first conductive agent is 100:(0.03-0.4), for example: 100:0.03, 100:0.05, 100:0.1, 100:0.2 or 100:0.4, etc.

Preferably, the mass ratio of the second active material to the first active material in step (2) is 1:(0.8-1.2), for example: 1:0.8, 1:0.9, 1:1, 1:1.1 or 1: 1.2 etc.

Preferably, based on the total mass of the conductive agent in the positive pole piece as 100%, the mass fraction of the first part of the conductive agent in step (3) is 50-70%, for example: 50%, 55%, 60%, 65% or 70% etc.

Preferably, the mass ratio of the binder to the total mass of the first active material and the second active material is (1-5):45, for example: 1:45, 2:45, 3:45, 4:45 or 5:45 etc.

Preferably, the mixing and stirring in step (3) includes vacuum stirring.

Preferably, the vacuum degree of the vacuum stirring is -0.09～-0.08MPa, for example: -0.09MPa, -0.088MPa, -0.085MPa, -0.082MPa or -0.08MPa and so on.

Preferably, the mixing and stirring time is 3-6 hours, for example: 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

Preferably, the viscosity in step (3) is 8000-15000mpa.s, for example: 8000mpa.s, 9000mpa.s, 10000mpa.s, 12000mpa.s or 15000mpa.s, etc.

In a second aspect, the present invention provides a positive electrode sheet, which is manufactured by the method described in the first aspect.

In a third aspect, the present invention provides a lithium ion battery, which comprises the positive electrode sheet as described in the second aspect.

Compared with the prior art, the present invention has the following beneficial effects:

(1) In the preparation process of the positive electrode sheet, the present invention adds a biochar material with a larger specific surface and conductivity, which can significantly improve the conductivity of the positive active material, and fully promote the formation of two different particle size active materials simultaneously. The uniformity of mixing shortens the transmission path of lithium ions and facilitates deintercalation.

(2) The present invention shortens the transmission path of lithium ions, reduces interfacial impedance, and improves interfacial conductivity by matching positive electrode powders of different particle sizes and adding conductive active substances. The additive has a higher specific surface area and excellent conductivity and rich surface functional groups, the lithium iron phosphate battery prepared by using the positive electrode sheet of the present invention has better cycle performance, and the low-temperature discharge performance has also been significantly improved.

Detailed ways

The technical solutions of the present invention will be further described below through specific embodiments. It should be clear to those skilled in the art that the embodiments are only for helping to understand the present invention, and should not be regarded as specific limitations on the present invention.

Unless otherwise specified in the examples of the present invention and comparative examples, the stated parts are parts by mass.

Example 1

This embodiment provides a positive pole piece, and the preparation method of the positive pole piece is as follows:

(1) 45 parts of lithium iron phosphate with a median particle size D50 of 600nm and 5 parts of bamboo charcoal were mixed, and calcined at 300°C for 6h under a nitrogen atmosphere to obtain a one-fired material;

(2) Mix 1 part of conductive carbon black with N-methylpyrrolidone, add the one-fired material obtained in step (1) and mix evenly, then add 45 parts of lithium iron phosphate whose median particle diameter D50 is 150nm to obtain a mixed slurry;

(3) Mix 2 parts of conductive carbon black and 2 parts of polyvinylidene fluoride with the mixed slurry obtained in step (2), stir for 4 hours in a vacuum environment (-0.085MPa), adjust the viscosity to 10000mpa.s, and obtain the positive electrode slurry The positive electrode slurry is coated on the surface of the aluminum foil to obtain the positive electrode sheet.

Example 2

This embodiment provides a positive pole piece, and the preparation method of the positive pole piece is as follows:

(1) 48 parts of lithium iron phosphate with a median particle size D50 of 800nm and 5 parts of bamboo charcoal were mixed, and calcined at 320° C. for 6 hours under a nitrogen atmosphere to obtain a one-fired material;

(2) 1 part of conductive carbon black is mixed with N-methylpyrrolidone, and the one-fired material obtained in step (1) is added and mixed evenly, and then 42 parts of lithium iron phosphate with a median particle diameter D50 of 130nm are added to obtain a mixed slurry;

(3) Mix 2 parts of conductive carbon black and 2 parts of polyvinylidene fluoride with the mixed slurry obtained in step (2), stir for 4 hours in a vacuum environment (-0.085MPa), adjust the viscosity to 12000mpa.s, and obtain the positive electrode slurry The positive electrode slurry is coated on the surface of the aluminum foil to obtain the positive electrode sheet.

Example 3

The only difference between this embodiment and Example 1 is that the amount of bamboo charcoal added is 3 parts (the mass ratio of the first active material and biochar powder is 1:0.067), and other conditions and parameters are exactly the same as in Example 1.

Example 4

The only difference between this example and Example 1 is that the amount of bamboo charcoal added is 6 parts (the mass ratio of the first active material to the biochar powder is 1:0.13), and other conditions and parameters are exactly the same as in Example 1.

Example 5

The only difference between this example and Example 1 is that the calcination temperature in step (1) is 200° C., and other conditions and parameters are exactly the same as those in Example 1.

Example 6

The only difference between this example and Example 1 is that the calcination temperature in step (1) is 400° C., and other conditions and parameters are exactly the same as those in Example 1.

Comparative example 1

The only difference between this comparative example and Example 1 is that no bamboo charcoal is added, and other conditions and parameters are identical to Example 1.

Comparative example 2

The only difference between this comparative example and Example 1 is that lithium iron phosphate with a single particle size is used, and other conditions and parameters are exactly the same as those of Example 1.

Performance Testing:

Take the positive pole pieces obtained in Examples 1-6 and Comparative Examples 1-2 and match them with graphite negative pole pieces, 12 μm diaphragm, and 1.0MLiPF ₆ EC+EMC electrolyte. Battery cells, put the bare cells into the aluminum shell to assemble the cells, inject the electrolyte into the dry cells, and make lithium-ion batteries through aging, chemical formation, shaping, packaging and other processes. The prepared lithium-ion battery is then subjected to cycle test and low-temperature discharge test. The cycle test method is: charge to 3.65V with 0.5C constant current at 25°C, turn to constant voltage charge until the current reaches 0.05C, and 0.5C constant current Discharge to 2.5V, repeat the above charging and discharging steps, record the discharge capacity of the 500th cycle as C1, take the maximum value of the discharge capacity as the initial capacity C0, and calculate the capacity retention rate of the 500th cycle as R1=C1/C0* 100%. The low-temperature discharge test method is: charge at 25°C with a constant current of 0.5C to 3.65V, turn to constant voltage charging until the current reaches 0.05C, discharge at a constant current of 0.5C to 2.5V, and record the discharge capacity at room temperature as C2; then use 0.5 C constant current charge to 3.65V, turn to constant voltage charge until the current reaches 0.05C, then place the battery in an environment of -20°C for 12 hours, then discharge it to 2.5V with a constant current of 0.5C, and record the discharge capacity as C3. Calculation of discharge capacity retention at -20°C: R2=C3/C2*100%. The test results are shown in Table 1:

Table 1

As can be seen from Table 1, it can be obtained from Examples 1-2 that the positive electrode sheet of the present invention has a capacity retention rate of more than 96.05% after 500 cycles of battery cycles, and a discharge capacity retention rate of -20°C 0.5C can reach 90.89%. %above.

From the comparison of Example 1 and Examples 3-4, it can be seen that in the preparation process of the positive electrode sheet of the present invention, the mass ratio of the first active material to the biochar powder will affect the performance of the positive electrode sheet. The mass ratio of active material and biochar powder is controlled at 1: (0.08-0.12), and the performance of the positive electrode sheet is better. If the amount of biochar powder added is too large, the capacity of the electrochemical active material will be affected, and the battery capacity will be biased. Low; if the amount of biochar powder added is too small, it is not conducive to promoting the full infiltration of the electrolyte, and the electrical performance of the battery will be affected.

From the comparison of Example 1 and Examples 5-6, it can be seen that in the preparation process of the positive pole piece of the present invention, the temperature of the calcination in step (1) will affect the performance of the positive pole piece, and the temperature of the calcination is controlled At 250-350 ° C, the performance of the positive electrode sheet is better. If the calcination temperature is too high, the surface functional groups of the carbonized bamboo charcoal will be damaged due to high temperature, which is not conducive to the infiltration of the electrolyte; if the calcination temperature is too low, the carbonization degree of the bamboo charcoal Not enough, the conductivity will be affected.

From the comparison of Example 1 and Comparative Example 1, it can be obtained that in the preparation process of the positive electrode sheet, the present invention adds a biochar material with a large specific surface area and electrical conductivity, which can significantly improve the electrical conductivity of the positive active material, while fully promoting The mixing uniformity of two active materials with different particle sizes is improved, the ohmic internal resistance is reduced, and it is beneficial to cycle and low-temperature discharge performance.

From the comparison of Example 1 and Comparative Example 2, it can be obtained that the present invention shortens the transmission path of lithium ions, reduces the interface impedance, and improves the interface conductivity through the matching of positive electrode powders with different particle sizes and the addition of conductive active materials. With high specific surface area, excellent electrical conductivity and abundant surface functional groups, the lithium iron phosphate battery prepared by using the positive electrode sheet of the present invention has more excellent cycle performance, and the low-temperature discharge performance has also been significantly improved.

The applicant declares that the above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto, and those skilled in the art should understand that any person skilled in the art should be aware of any disclosure in the present invention Within the technical scope, easily conceivable changes or substitutions all fall within the scope of protection and disclosure of the present invention.

Claims (10)

1. a preparation method of positive electrode sheet, is characterized in that, described preparation method comprises the following steps: (1) mixing the first active material and biochar powder, and calcining to obtain a fired material; (2) mixing the one-calcined material, the first part of the conductive agent and the solvent, and adding the second active material to obtain a mixed slurry; (3) mixing and stirring the second part of the conductive agent and the binder with the mixed slurry, adjusting the viscosity to obtain a positive electrode slurry, and coating the positive electrode slurry on the surface of the positive electrode current collector to obtain the positive electrode sheet; Wherein, the first active material includes lithium iron phosphate with a median particle diameter D50 of 400-950 nm, and the second active material includes lithium iron phosphate with a median particle diameter D50 of 100-200 nm. 2. preparation method as claimed in claim 1, is characterized in that, step (1) described biochar powder comprises bamboo charcoal powder; Preferably, the mass ratio of the first active material to the biochar powder is 1:(0.08-0.12). 3. The preparation method according to claim 1 or 2, characterized in that the calcining temperature in step (1) is 250-350°C; Preferably, the calcination time is 5-8 hours. 4. the preparation method as described in any one of claim 1-3 is characterized in that, with the total mass of conducting agent in the positive pole sheet being 100%, the mass fraction of the first part of conducting agent described in step (2) is 30-50%; Preferably, the mass ratio of the mono-fired material to the first part of the conductive agent is 100:(0.03˜0.4). 5. The preparation method according to any one of claims 1-4, characterized in that the mass ratio of the second active material to the first active material in step (2) is 1:(0.8-1.2). 6. the preparation method as described in any one of claim 1-5 is characterized in that, with the total mass of conducting agent in the positive pole sheet being 100%, the mass fraction of the first part of conducting agent described in step (3) is 50-70%; Preferably, the mass ratio of the binder to the total mass of the first active material and the second active material is (1-5):45. 7. The preparation method according to any one of claims 1-6, wherein the mixing and stirring in step (3) comprises vacuum stirring; Preferably, the vacuum degree of the vacuum stirring is -0.09～-0.08MPa; Preferably, the mixing and stirring time is 3-6 hours. 8. The preparation method according to any one of claims 1-7, characterized in that the viscosity in step (3) is 8000-15000 mpa.s. 9. A positive pole piece, characterized in that the positive pole piece is made by the method according to any one of claims 1-8. 10. A lithium-ion battery, characterized in that the lithium-ion battery comprises the positive pole piece as claimed in claim 9.