CN102368543A - A lithium ion battery negative pole and a lithium ion battery using the negative pole - Google Patents

Abstract

The present invention belongs to the field of lithium ion battery materials, and provides a negative electrode for a lithium ion battery, including a current collector and a negative electrode material coated on the current collector, wherein the negative electrode material includes a carbon material or an alloy material, and the binder is sodium carboxymethyl cellulose. The degree of substitution of sodium carboxymethyl cellulose is 0.6-1.2, the number average molecular weight is 50,000-1,000,000, and the viscosity of its 1% aqueous solution at 25°C is 50-1200 mPa.s. And the mass percentage of sodium carboxymethyl cellulose in the negative electrode material is 1%-5%wt. The lithium ion battery using the negative electrode solves the defect of large thickness expansion during battery cycling, and the prepared battery has the characteristics of safety, reliability and long cycle life.

Description

A lithium ion battery negative pole and a lithium ion battery using the negative pole

technical field

The invention belongs to the field of lithium-ion battery materials, and more specifically relates to a composition and a method of a lithium-ion battery with improved performance.

Background technique

Since its commercialization, lithium-ion batteries have been widely used as power sources for various mobile devices due to their high energy density, high working voltage, no memory effect, long cycle life, and no pollution to the environment. The practical phase of the scale.

The characteristics of lithium-ion batteries can be developed by improving the components of the battery. The performance of the battery depends on the electrodes, electrolyte and battery materials contained therein, while the characteristics of the electrodes depend on the active materials, current collectors and binders of the battery. The agent provides the adhesion between the battery active material and the current collector. When the binder provides a strong adhesive force between the active material or between the active material and the current collector, electrons and lithium ions move smoothly in the electrode and the electrode The resistance is lowered.

At present, the water-based negative electrode sheet of lithium-ion batteries generally uses styrene-butadiene rubber (SBR) as the binder, but the modulus of this binder is small, resulting in a large change in the thickness of the battery during the cycle. This is more obvious in pouch batteries, which obviously limits the application of lithium batteries; on the other hand, SBR has poor resistance to electrolytes, and once soaked in electrolytes, the bonding force is significantly weakened, so the anode during the cycle Sheet adhesion deteriorates, causing lithium batteries to exhibit poor cycle life characteristics.

Contents of the invention

The invention aims at providing a kind of adhesive agent of lithium ion battery with strong rigidity and low expansion performance aiming at the deficiencies of the prior art, and the lithium ion battery using the adhesive agent has good anti-expansion performance.

In order to achieve the above-mentioned requirements, the present invention adopts the following technical solutions:

The invention provides a negative electrode of a lithium ion battery. The negative electrode includes a current collector and a negative electrode material coated on the current collector. The negative electrode material mainly includes an active material, a conductive agent and a binder, wherein the binder is made of cellulose Binder, the amount of the binder accounts for 1%-5%wt% of the total weight of the negative electrode material.

In the slurry formula, if the amount of binder added is too small (mass fraction is less than 1%), the dispersion effect of the active material in the slurry is poor, and the stability of the prepared slurry is poor, resulting in uneven distribution of the active material in the pole piece , which leads to overcharge and overdischarge of some active materials, which in turn affects the cycle performance of the battery. In addition, if the binder is too small, the adhesion of the pole piece is poor, and the negative electrode material is easy to fall off during the production process of the pole piece. On the contrary (mass fraction greater than 5%), the binder may cover most of the surface of the active material, resulting in an increase in anode impedance, and lithium is likely to be separated during battery charging, affecting battery performance.

Described cellulose binder is sodium carboxymethyl cellulose, xanthan gum and hydroxyethyl cellulose, because these materials have relatively high modulus, lower extensibility, in circulation process, can effectively Inhibits the expansion of graphite. On the other hand, the activity between the binder and the electrolyte is lower than that of SBR and PVDF, thus ensuring the long-term cycle performance of the battery. In addition, due to a certain amount of carboxyl groups in its molecular structure, it can significantly improve the interaction with active materials and Current collector bonding.

When the aqueous solution concentration of the cellulose binder is 1%, the viscosity at 25° C. is 50-1200 mPa.s. The viscosity of the binder is too high, and the solid content in the slurry preparation process is low, resulting in a large load during the drying process of the pole piece and a waste of energy.

It is found in the research that the suitable degree of substitution of carboxymethyl cellulose sodium is 0.6-1.2, and the degree of substitution should not be too large, otherwise the viscosity will be too high, resulting in poor adhesion and even affecting the capacity of the battery. As a result, the hydrophilicity of sodium carboxymethyl cellulose will deteriorate, and the insoluble matter will increase significantly, which will affect the dispersion effect of the slurry. A large number of studies have found that the appropriate number average molecular weight of sodium carboxymethyl cellulose is 50,000-1 million, the bonding is relatively moderate, and the performance of the prepared battery is the best.

The present invention also provides a lithium-ion battery negative electrode, comprising a current collector, an active material attached to the current collector, a conductive agent, and a cellulose-based adhesive, wherein the adhesive is the above-mentioned adhesive, wherein, Active material: conductive agent: cellulose binder = 92-99:0-3:1-5. The active material is carbon material or alloy material.

In addition, the present invention also provides a lithium ion battery, including a positive electrode, a negative electrode, a separator and an electrolyte, wherein the negative electrode is the above-mentioned negative electrode.

Beneficial effects of the present invention:

After the present invention uses cellulose-based binders instead of traditional sodium carboxymethylcellulose and styrene-butadiene rubber as lithium battery binders, it achieves superior bonding performance compared to traditional binders, and the prepared batteries have outstanding The excellent performance is mainly reflected in the following aspects:

First, low battery expansion rate. Because the cellulose binder has the excellent characteristics of high modulus and low expansion elongation, it can obviously inhibit the expansion of the anode sheet during the cycle of the battery, thereby reducing the expansion rate of the battery.

Second, low impedance, high efficiency. During the slurry preparation process, because it significantly improves the dispersion performance of the active material and conductive carbon, the active material and conductive carbon in the electrode are more uniformly dispersed, reducing the electronic conductivity of the battery, thereby improving the efficiency of the battery.

Third, good long cycle characteristics. On the one hand, the carboxyl group in the cellulose binder can form chemical bonds with the active material and the hydroxyl group of the conductive carbon, thereby enhancing the binding force between the active material and the conductive carbon. When the electrode is partially destroyed, this strong polar carboxyl group And the hydroxyl group of the active material exhibits a good self-healing effect, thus ensuring the stability of the electrode. On the other hand, due to the low reactivity between the binder and the electrolyte and its good resistance to the electrolyte, part of the surface of the active material is well covered by the binder during the cycle, so that the exposed electrolyte The surface of the battery is greatly reduced, thereby reducing the irreversible consumption of the electrolyte on the surface of the active material particles and ensuring the long-term cycle performance of the battery.

Description of drawings

Fig. 1 is the rebound curve diagram of negative electrode plate in the manufacturing process of the embodiment 1 of the present invention and comparative example 1 electric core;

Fig. 2 is the AC impedance spectrogram of the batteries prepared in Example 1 of the present invention and Comparative Example 1;

Fig. 3 is the cycle diagram of charging and discharging at 0.7C and 0.5C at 45°C for lithium-ion batteries made in Example 1 and 2 of the present invention and Comparative Example 1.

Fig. 4 is a diagram of the variation of the battery expansion rate during the cycling process of the Li-ion battery made in Embodiment 1 of the present invention and Comparative Example 1 at 45°C at 0.7C and 0.5C discharge.

Detailed ways

The content of the present invention will be further described below in conjunction with specific embodiments and drawings, but the protection scope of the present invention is not limited only to the content described in the embodiments.

Example 1

Negative sheet preparation process

Negative electrode slurry formula is based on dry material weight percentage, is made up of three parts: 2.5% sodium carboxymethyl cellulose (CMC-Na), 1.5% conductive carbon black, 96% negative electrode active material, wherein negative electrode active material adopts artificial graphite, carboxy Sodium methylcellulose has a number average molecular weight of about 500,000, a 1% aqueous solution viscosity of 150 mPa.s at 25°C, and a degree of substitution of 0.8. During the preparation of the slurry, the solvent is water, and water accounts for 60% of the total slurry.

First, add water and sodium carboxymethyl cellulose into the stirring grinder according to the above formula, and dissolve completely in a vacuum state, then add conductive carbon black into the dissolved water-based polymer solution according to the formula, and quickly stir and grind Until the fineness is below 5μm, finally add artificial graphite according to the formula, and stir evenly under vacuum at a slow speed. Filter with a 150-mesh stainless steel screen to obtain the required negative electrode slurry.

The slurry was evenly coated on both sides of an 8 μm copper foil, and the pole piece was compacted with a roller press to obtain a water-based negative pole piece.

The preparation process of the positive electrode sheet

The positive electrode uses lithium cobaltate (LiCoO ₂ ) as an active material. Prepare the positive electrode slurry containing the active material according to the formula, the solid content of the slurry includes 90% of the positive electrode active material, 5% of PVDF (polyvinylidene fluoride) as a binder, and 5% of conductive carbon as electrical conduction Auxiliary. The slurry uses NMP as a solvent, and the solid content of the slurry is 40%.

According to the above formula, add N-methylpyrrolidone (NMP) and PVDF into the stirring grinder, dissolve completely in a vacuum state, then add conductive carbon black into the dissolved oily polymer solution according to the formula, and stir quickly Grind to a fineness of less than 5 μm, and finally add lithium cobaltate according to the formula, and stir evenly under vacuum. Filter with a 200-mesh stainless steel screen to obtain the desired positive electrode slurry.

The slurry was evenly coated on both sides of a 12 μm aluminum foil, and the pole piece was compacted with a roller press to obtain an oily positive pole piece.

Assembly of the battery:

Weld the conductive lugs on the positive and negative pole pieces so that the positive and negative electrodes are overlapped with a 14um PP/PE composite separator between them, wind them to form a bare cell, then package it with aluminum-plastic film, and inject non-aqueous electrolysis The electrolyte of the battery is 1mol/L LiPF ₆ solution, the main solvent is a mixture of EC, PC, and DMC. After packaging, the battery is subjected to chemical formation and aging tests to obtain a square soft-packed battery with a length, width, and thickness of 32mm×82mm×42mm. .

Example 2

The difference from Example 1 is that: the negative electrode formula contains 97.5% of the negative electrode active material, 1% of sodium carboxymethylcellulose, and 1.5% of conductive carbon black as an electrical conduction aid. Wherein the sodium carboxymethyl cellulose substitution degree that adopts is 1.0, molecular weight 1,000,000, its 1% aqueous solution 25 ℃ viscosity 1200mPa.s, other is identical with embodiment 1, repeats no more here.

Example 3

The difference from Example 1 is that: the negative electrode formula contains 93.5% of the negative electrode active material, 5% sodium carboxymethyl cellulose, and 1.5% conductive carbon black as an electrical conduction aid. Wherein the sodium carboxymethylcellulose that adopts is 0.6, molecular weight 50,000, its 1% aqueous solution 25 ℃ viscosity 50mPa.s, other is identical with embodiment 1, repeats no more here.

Example 4

The difference from Example 1 is that: the negative electrode formula contains 96.5% of the negative electrode active material, 1% sodium carboxymethyl cellulose, 1% xanthan gum, and 1.5% conductive carbon black as an electrical conduction aid. Others are the same as in Embodiment 1, and will not be repeated here.

Example 5

The difference from Example 1 is that: the negative electrode formula contains 96.5% of the negative electrode active material, 1% sodium carboxymethylcellulose, 1% sodium hydroxyethylcellulose, and 1.5% conductive carbon black as an electrical conduction aid. Others are the same as in Embodiment 1, and will not be repeated here.

Example 6

The difference from Example 1 is that an alloy is used as the active material, and the solid content of the slurry contains 93.5% of the negative electrode active material, 5% of sodium carboxymethyl cellulose, and 1.5% of conductive carbon black as an electrical conduction aid. Others are the same as in Embodiment 1, and will not be repeated here.

Comparative example 1

Negative electrode slurry formula is based on dry material weight percentage, is made up of 1.0% carboxymethylcellulose sodium (CMC-Na), 1.5% conductive carbon, 96.0% artificial graphite, 1.5% styrene-butadiene rubber (SBR) four parts, wherein negative electrode The active material adopts artificial graphite, sodium carboxymethyl cellulose, its number average molecular weight is about 500,000, its 1% aqueous solution has a viscosity of 150 mPa.s at 25° C., and its degree of substitution is 0.8. During the preparation of the slurry, the solvent is water, and water accounts for 60% of the total slurry.

According to the above formula, add water and sodium carboxymethyl cellulose into the stirring grinder, dissolve completely in a vacuum state, then add conductive carbon black into the dissolved water-based polymer solution according to the formula, and quickly stir and grind until The fineness is less than 5μm, and finally add artificial graphite according to the formula, and stir evenly in a vacuum at a slow speed. Use a 150-mesh stainless steel screen to test the filtration. After the filtration performance is good, slow down the stirring speed, then add styrene-butadiene rubber, and stir slowly in vacuum to prevent the styrene-butadiene rubber from demulsifying. After the dispersion is uniform, the required negative electrode slurry is obtained.

The remaining electrodes are provided in the same manner as described in Example 1.

Performance test method:

Pole piece adhesion test:

Take the cold-pressed anode diaphragm, cut it into a small rectangular piece with a size of 20mm*10cm, and stick it on a clean stainless steel plate with a 20mm wide double-sided adhesive. Use a tensile machine to test the 180-degree peeling force of the pole piece, and the tensile speed of the tensile machine is 50mm/min. In order to characterize the electrolyte immersion resistance of the pole piece, take the cut anode piece, place it in the electrolyte solution, seal it and place it in a 60-degree oven, take out the diaphragm after 96 hours, and use the same method to test the adhesion of the pole piece.

Electrode rebound test:

After the pole piece is cold-pressed, measure the thickness of the pole piece with a micrometer, and use it as the initial thickness of the pole piece. Subsequently, in the process of cell production, after the anode sheet is baked and before winding, the cell fixture is shaped, the battery is fully charged to 4.2V for the first time, the battery is discharged to 3.0V for the first time, and the voltage is 3.8V. Test the thickness of the anode piece, and calculate the rebound rate of the anode piece under different states.

Battery cycle expansion rate test:

Before the battery cycle, use a height gauge to test the thickness of the 3.8V battery, and record it as the original thickness of the battery. During the cycle, use the same method to test the thickness of the battery after 10, 20, 30, 40, 60, 80, 100, 200, 300, and 400 cycles with 4.2V full charge.

Battery expansion rate=(4.2V battery thickness/3.8V battery original thickness-1)*100%

After the negative electrode sheet prepared in the above embodiment was cold-pressed and soaked in the electrolyte, the results of the adhesion test of the electrode sheet are shown in Table 1:

Table 1 Electrode adhesion force and resistance of different embodiments/comparative examples

The results of the electrode adhesion test show that the negative electrode sheet prepared by carboxymethyl cellulose binder has the characteristics of strong adhesion and small sheet resistance compared with the electrode sheet prepared by styrene-butadiene rubber in the comparative example. . Moreover, under the same conditions, after the pole piece prepared by the patent of the present invention is soaked in the electrolyte, the adhesive force is obviously higher than that of the comparative example.

In order to compare the performance of sodium carboxymethyl cellulose and conventional SBR binder, during the battery manufacturing process, the rebound of the negative electrode sheet during the manufacturing process was monitored, and the results are shown in Figure 1. The rebound of the negative electrode sheet made of sodium carboxymethylcellulose binder is significantly lower than that of SBR binder at the same stage.

The AC impedance spectra of the batteries prepared in Example 1 and Comparative Example 1 were tested at room temperature at 3.85V, and the results are shown in Figure 2. The impedance of batteries prepared with carboxymethylcellulose binder was significantly lower than that of conventional SBR batteries.

The full battery made in Example 1 and Example 2 was charged at 45°C at a rate of 0.7, and after 400 discharge cycles at a rate of 0.5C, the capacity retention rate was still 88%, which was higher than that of Comparative Example 1, as shown in Figure 3, which is safe Reliable, long cycle life characteristics.

Figure 4 is a diagram of the thickness expansion of the battery in Example 1 and Comparative Example 1 under the condition of 45°C, charging at a rate of 0.7, and discharging at a rate of 0.5C. Under the same number of cycles, the battery prepared by pure sodium carboxymethyl cellulose Significantly lower than batteries made of traditional SBR.

Based on the above data, sodium carboxymethyl cellulose, as a binder, can indeed enhance the bonding performance between the current collector and the active material film layer, reduce the contact resistance, and inhibit the rebound of the battery during cycling. The manufactured lithium-ion battery has the characteristics of low internal resistance, long cycle life and so on.

It should be noted that, according to the disclosure and elaboration of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above implementation manners. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some equivalent modifications and changes to the present invention should also be within the protection scope of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims (9)

1. lithium ion battery negative; Comprise collector, be coated in the negative material on the collector; Described negative material comprises active material, conductive agent and binding agent; It is characterized in that: described binding agent is a cellulosic binder, and said cellulose family bonding agent accounts for the 1%-5%wt of negative material total weight.

2. lithium ion battery negative according to claim 1 is characterized in that: described cellulosic binder is at least a in sodium carboxymethylcellulose, xanthans or the hydroxyethylcellulose.

3. lithium ion battery negative according to claim 1 and 2 is characterized in that: the concentration of aqueous solution of described cellulosic binder is 1% o'clock, and the viscosity under 25 ℃ is 50-1200mPa.s.

4. lithium ion battery negative according to claim 2 is characterized in that: the substitution value of said sodium carboxymethylcellulose is 0.6-1.2.

5. lithium ion battery negative according to claim 2 is characterized in that: the number-average molecular weight of said sodium carboxymethylcellulose is 50,000-1,000,000.

6. lithium ion battery negative according to claim 1 is characterized in that: the mass ratio of said active material, conductive agent, cellulosic binder is 92～99: 0～3: 1～5.

7. according to claim 1 or 6 described lithium ion battery negatives, it is characterized in that: said active material is material with carbon element or alloy material.

8. lithium ion battery negative according to claim 1 is characterized in that: said conductive agent is a conductive black.

9. a lithium ion battery comprises positive pole, negative pole, barrier film and electrolyte, it is characterized in that: described negative pole is each described negative pole of claim 1 to 8.

Images