Composite positive electrode material based on KB-coated LFP core-shell structure and PVP interface modification and application thereof

Abstract

The invention discloses a KB-LFP composite material for a lithium ion battery and a preparation method of the KB-LFP composite material. Aiming at the problems that lithium iron phosphate (LFP) is poor in conductivity, easy to agglomerate and the like, Ketjen black (KB) is used for constructing a three-dimensional conductive network to coat the LFP, a core-shell structure is formed, and polyvinylpyrrolidone (PVP) is introduced for interface modification. According to the invention, the high conductivity and fibrous structure of KB are utilized to replace the traditional conductive agent, and the use amount of the conductive agent is reduced from 3-5 wt% to 1.5-2.5 wt%, so that the proportion of active substances is increased, the resistance of the pole piece is effectively reduced, the cycling stability is enhanced, and the dispersity of the slurry is remarkably improved at the same time.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery cathode materials, and particularly relates to a composite material based on a ketchen black (KB) conductive network coated lithium iron phosphate (LFP) core-shell structure and interface modified by polyvinylpyrrolidone (PVP), and application of the composite material in improving cathode slurry dispersion uniformity and battery cycle stability. BACKGROUND

[0002] Lithium iron phosphate material occupies an important position in the power battery and energy storage market due to good thermal stability, excellent cycle performance and low price. Lithium iron phosphate (LiFePO4) has low electronic conductivity and lithium ion diffusion coefficient due to the olivine structure. Although the cycle performance can be improved by nanocrystallization and surface carbon coating, the following problems are caused: 1. difficult slurry dispersion: the specific surface area of nanoparticles is large, and van der Waals force causes slurry agglomeration; 2. excessive addition of conductive agent: traditional conductive carbon black (such as Super P) needs to be added by 3-5wt%, which reduces the proportion of active material; 3. insufficient cycle stability: the electrode volume changes greatly during charging and discharging, the conductive network is broken, the internal resistance of the battery is increased, and the cycle performance is reduced.

[0003] Compared with the isolated particle structure of Super P, ketchen black (KB) has a branched conductive path, which can form a three-dimensional conductive network with less addition amount. This structural characteristic gives it three core advantages: 1) conductive efficiency is doubled, the branched structure shortens the electron hopping distance; 2) excellent mechanical stability, the three-dimensional network can effectively buffer the volume change of LFP material during charging and discharging; 3) interface contact optimization, the branched structure forms multiple-point contact, improving the cycle performance; 4) the branched structure of KB also creates greater porosity, providing more channels for electrolyte infiltration and lithium ion transmission. This structure-performance synergistic effect makes it a key material for breaking through the intrinsic conductivity bottleneck of LFP. However, its strong hydrophobicity easily causes slurry stratification, and a dispersant is needed to achieve uniform compounding. SUMMARY

[0004] To solve the problems of low active material proportion, poor dispersion of nanoparticles, and breakage of conductive network during the cycle process of traditional lithium iron phosphate material caused by excessive addition of conductive agent.

[0005] A preparation method of a composite cathode material based on KB coated LFP core-shell structure and PVP interface modification, specifically comprising the following steps: Step 1: lithium iron phosphate (LFP) and ketchen black (KB) are added to a planetary ball mill in a certain mass ratio, and dry grinding is performed to form a primary composite; Step 2: high-temperature argon atmosphere sintering is performed to form a carbon thermal reduction enhanced LFP-KB core-shell structure; Step 3: Add an ethanol solution containing polyvinylpyrrolidone (PVP) dispersant, and ultrasonic auxiliary stirring for 1h to make the PVP agent molecular chain uniformly coat the surface of the particles; Step 4: After the slurry is vacuum dried for 12h, it is transferred to an argon furnace for heat treatment to form a PVP-coated KB-LFP conductive composite structure.

[0006] Further, in step 1, the mass ratio of LFP to KB is 40-60:1.

[0007] Further, in step 1, the ball-to-material ratio in the planetary ball mill is 10-15:1.

[0008] Further, in step 1, the dry grinding time of the planetary ball mill is 0.5-3h.

[0009] Further, in step 2, the sintering temperature in the high-temperature argon atmosphere is 450-650℃, and the time is 2-8h.

[0010] Further, in step 3, the concentration of the ethanol solution containing PVP dispersant is 2-8wt%.

[0011] Further, in step 3, the solid-liquid mass ratio of the LFP-KB sintered product to the ethanol solution containing PVP dispersant is 1:8-15.

[0012] Further, in step 4, the heat treatment temperature in the argon furnace is 80-150℃, and the time is 1-3h.

[0013] The application of a KB-coated LFP core-shell structure and a PVP interface-modified composite positive electrode material prepared by the above preparation method, the KB-coated LFP core-shell structure and the PVP interface-modified composite material are used as a positive electrode slurry in a lithium ion battery; in the positive electrode slurry, the proportion of the KB-coated LFP core-shell structure and the PVP interface-modified composite material to the binder is 94.5%-96.5%:2.5%.

[0014] The preparation method of the present application has simple steps, convenient operation, and stable performance of the prepared composite material. At the same time, the prepared conductive slurry has good conductivity and dispersion stability, and is suitable for various application scenarios requiring conductivity.

[0015] The beneficial effects of the present application relative to the prior art are: ‌Improve slurry dispersion stability: by KB coating combined with dispersant interface modification, the agglomeration problem of nano lithium iron phosphate particles caused by van der Waals force is effectively solved. The dispersant molecular chain selectively inserts into the gap between the particles, forming a steric hindrance effect, which significantly improves the stability of the slurry.

[0016] Optimization of electrode conductive network: the high conductivity and fibrous structure of Ketjen Black (compared to traditional Super P) form a three-dimensional conductive network, and the amount of conductive agent can be reduced to 1.5-2.5wt% (originally 3-5wt%), and the proportion of active material is increased; the core-shell structure enhanced by carbon thermal reduction (LFP-KB) and the synergistic effect of the dispersant layer can alleviate the fracture of the conductive network caused by the volume change of the electrode during charging and discharging, and reduce the resistance of the electrode.

[0017] Prolonging the cycle life of the battery: the use of 80-150℃ argon heat treatment makes PVP-KB-LFP form a stable composite interface, which not only retains its dispersion function, but also achieves a "soft coating" effect through molecular chain winding. Moreover, the KB-LFP conductive composite structure coated with a dispersant inhibits the side reaction of active material and electrolyte, and improves the cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0018] Table 1 is the viscosity, coating state and specific capacity of the positive electrode slurry prepared in the examples and comparative examples.

[0019] Figure 1 The volume resistance of the electrode sheet of the examples and comparative examples.

[0020] Figure 2 The room temperature cycle curve of Example 3 and Comparative Example 1. DETAILED DESCRIPTION

[0021] The technical solutions in the present application will be described below in conjunction with examples. Obviously, the described examples are only a part of the examples of the present application, rather than all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor are within the protection scope of the present application. If the specific conditions are not specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market. Example 1

[0022] 1. Weigh 96.5g lithium iron phosphate and 1g Ketjen Black (KB), and add it to a planetary ball mill. Add zirconium oxide grinding balls (ball-to-material ratio 10:1), and under the protection of argon circulation, planetary ball mill at 400rpm for 2h, with a 10min pause every 30min.

[0023] 2. Transfer the ball-milled product into a tube furnace, and raise the temperature to 600℃ at a rate of 10℃ / min, and sinter for 4h under an argon flow of 50sccm.

[0024] 3. Prepare an ethanol solution containing 3wt% PVP, and add the sintered product according to the solid-liquid mass ratio of 1:10, and ultrasonic treatment with simultaneous mechanical stirring.

[0025] 4. Maintain the system temperature at 25±2℃, vacuum dry the slurry for 12h, then transfer to the argon furnace for heat treatment, increase the temperature to 100℃ at 5℃ / min in the argon furnace, and keep for 1h.

[0026] Dissolve the sample lithium iron phosphate composite material and the binder in the solvent NMP at a ratio of 0.96:0.025, use a stirrer to fully stir the mixture, and ensure that the solid powder is uniformly dispersed in the NMP. Example 2

[0027] 1. Weigh 96g of lithium iron phosphate and 1.5g of KB, add them to a planetary ball mill, add zirconium oxide grinding balls (ball-to-material ratio of 10:1), and planetary ball mill at 400rpm under argon circulation protection for 2h, with a 10min pause every 30min.

[0028] 2. Transfer the ball-milled product to a tube furnace, increase the temperature to 600℃ at 10℃ / min, and sinter for 4h under an argon flow of 50sccm.

[0029] 3. Prepare an ethanol solution containing 3wt% PVP, add the sintered product at a solid-to-liquid ratio of 1:10, and ultrasonically treat it while mechanically stirring.

[0030] 4. Maintain the system temperature at 25±2℃, vacuum dry the slurry for 12h, then transfer to the argon furnace for heat treatment, increase the temperature to 100℃ at 5℃ / min in the argon furnace, and keep for 1h.

[0031] Dissolve the sample lithium iron phosphate composite material and the binder in the solvent NMP at a ratio of 0.96:0.025, use a stirrer to fully stir the mixture, and ensure that the solid powder is uniformly dispersed in the NMP.

[0032] Finally, adjust the slurry viscosity for coating, prepare a button cell, and test its electrochemical performance. Example 3

[0033] 1. Weigh 95.5g of lithium iron phosphate and 2g of KB, add them to a planetary ball mill, add zirconium oxide grinding balls (ball-to-material ratio of 20:1), and planetary ball mill at 400rpm under argon circulation protection for 2h, with a 10min pause every 30min.

[0034] 2. Transfer the ball-milled product to a tube furnace, increase the temperature to 600℃ at 10℃ / min, and sinter for 4h under an argon flow of 50sccm.

[0035] 3. Prepare an ethanol solution containing 3wt% PVP, add the sintered product at a solid-to-liquid ratio of 1:10, and ultrasonically treat it while mechanically stirring.

[0036] 4. Maintain the system temperature at 25±2℃, vacuum dry the slurry for 12h, then transfer it to the argon furnace for heat treatment, increase the temperature to 100℃ at 5℃ / min in the argon furnace, and keep it for 1h.

[0037] Dissolve the sample lithium iron phosphate composite material and the binder in the solvent NMP at a ratio of 0.955:0.025, use a stirrer to fully stir the mixture, and ensure that the solid powder is uniformly dispersed in the NMP.

[0038] Finally, adjust the viscosity of the slurry for coating, prepare a button cell, and test its electrochemical performance. Example 4

[0039] 1. Weigh 95g of lithium iron phosphate and 2.5g of KB, add them to a planetary ball mill, add zirconium oxide grinding balls (ball-to-material ratio 10:1), and planetary ball mill at 400rpm under argon circulation protection for 2h, with a 10min pause every 30min.

[0040] 2. Transfer the ball-milled product to a tube furnace, increase the temperature to 600℃ at 10℃ / min, and sinter it for 4h under an argon flow of 50sccm.

[0041] 3. Prepare an ethanol solution containing 3wt% PVP, add the sintered product at a solid-to-liquid ratio of 1:10, and ultrasonically treat it while mechanically stirring.

[0042] 4. Maintain the system temperature at 25±2℃, vacuum dry the slurry for 12h, then transfer it to the argon furnace for heat treatment, increase the temperature to 100℃ at 5℃ / min in the argon furnace, and keep it for 1h.

[0043] Dissolve the sample lithium iron phosphate composite material and the binder in the solvent NMP at a ratio of 0.955:0.025, use a stirrer to fully stir the mixture, and ensure that the solid powder is uniformly dispersed in the NMP.

[0044] Finally, adjust the viscosity of the slurry for coating, prepare a button cell, and test its electrochemical performance. Comparative Example 1

[0045] Dissolve the sample lithium iron phosphate, the binder, and KB in the solvent NMP at a ratio of 0.955:0.025:0.02, use a stirrer to fully stir the mixture, and ensure that the solid powder is uniformly dispersed in the NMP.

[0046] Finally, adjust the viscosity of the slurry for coating, prepare a button cell, and test its electrochemical performance. Comparative Example 2

[0047] Replace the dispersant PVP with PAA, and the rest remains the same as in Example 3.

[0048] Table 1. Viscosity, coating state and specific capacity of positive electrode slurry prepared by examples and comparative examples Serial number Sample Coating state Slurry viscosity / mPa-s Slurry viscosity / mPa-s after standing for 12h 0.1C specific capacity / (mAh / g) 1 Example 1 slightly particulate good 7300 16795 159.63 2 Example 2 good 7100 15364 159.85 3 Example 3 good 6640 14251 160.51 4 Example 4 good 6600 14575 160.25 5 Comparative sample 1 slightly particulate 8600 17521 159.33 6 Comparative sample 2 slightly particulate 8220 17345 160.18 As shown in Table 1, examples 1-4 all show effective reduction of slurry viscosity and improvement of slurry stability, and the coating process of examples 2-4 is significantly optimized. It is particularly noteworthy that the specific capacity of example 3 is improved compared with comparative example 1 at 0.1C discharge rate, which also confirms that the designed coating structure has certain advantages in improving the electrochemical performance of electrode materials.

[0049] As shown in Table 1, examples 1-4 all show effective reduction of slurry viscosity and improvement of slurry stability, and the coating process of examples 2-4 is significantly optimized. It is particularly noteworthy that the specific capacity of example 3 is improved compared with comparative example 1 at 0.1C discharge rate, which also confirms that the designed coating structure has certain advantages in improving the electrochemical performance of electrode materials. Figure 1 、 Figure 2 As shown in Table 1, examples 1-4 all show effective reduction of slurry viscosity and improvement of slurry stability, and the coating process of examples 2-4 is significantly optimized. It is particularly noteworthy that the specific capacity of example 3 is improved compared with comparative example 1 at 0.1C discharge rate, which also confirms that the designed coating structure has certain advantages in improving the electrochemical performance of electrode materials.

[0051] Figure 1 Volume resistance of electrode sheet of examples and comparative examples

[0052] Figure 2 Cycle curve of example 3 and comparative example 1 at room temperature.

Claims

1. A method for preparing a composite cathode material based on a KB-coated LFP core-shell structure and PVP interface modification, characterized in that, The method is as follows: Step 1: Add lithium iron phosphate (LFP) and Ketjen black (KB) to a planetary ball mill at a certain mass ratio and dry grind to form a primary complex; Step 2: Sintering in a high-temperature argon atmosphere to form a carbothermic reduction-enhanced LFP-KB core-shell structure; Step 3: Add an ethanol solution containing polyvinylpyrrolidone (PVP) and stir with ultrasonic assistance for 1 hour to ensure that the PVP molecular chains uniformly coat the surface of the particles. Step 4: After vacuum drying for 12 hours, the slurry is transferred to an argon furnace for heat treatment to form a KB-LFP conductive composite structure coated with dispersant.

2. The preparation method according to claim 1, characterized in that, In step 1, the lithium iron phosphate and Ketjen Black are mixed in a mass ratio of 40~60:

1.

3. The preparation method according to claim 1, characterized in that, In step 1, the ball-to-material ratio in the planetary ball mill is 10-15:

1.

4. The preparation method according to claim 1, characterized in that, The dry grinding time of the planetary ball mill in step 1 is 0.5~3h.

5. The preparation method according to claim 1, characterized in that, The high-temperature argon atmosphere sintering temperature in step 2 is 450~650℃, and the time is 2~8h.

6. The preparation method according to claim 1, characterized in that, The concentration of the ethanol solution containing the dispersant in step 3 is 2-8 wt%.

7. The preparation method according to claim 1, characterized in that, The solid-liquid mass ratio of the LFP-KB sintered product to the ethanol solution of the dispersant in step 3 is 1:8~15.

8. The preparation method according to claim 1, characterized in that, In step 4, the argon furnace is used for heat treatment at a temperature of 80~150℃ for 1~3 hours.

9. An application of a composite cathode material based on a KB-coated LFP core-shell structure and PVP interface modification prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The composite material with KB-coated LFP core-shell structure and PVP interface modification is used as the positive electrode slurry in lithium-ion batteries; in the positive electrode slurry, the ratio of the composite material with KB-coated LFP core-shell structure and PVP interface modification to the binder is 94.5%~96.5%:2.5%.

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