KR102478877B1 - Composition for forming cathode active material layer, cathode prepared by using the composition, and lithium ion secondary battery comprising the cathode - Google Patents

Abstract

The present invention relates to a composition for forming a positive electrode active material layer having improved OCV characteristics, a positive electrode prepared using the composition, and a lithium ion secondary battery including the positive electrode, comprising a lithium composite oxide, a CNT conductive material containing iron, A composition for forming a cathode active material layer for a lithium secondary battery including a PVDF binder and tannic acid, a cathode made of the composition, and a lithium ion secondary battery including the cathode are provided.

Description

A composition for forming a positive electrode active material layer having excellent OCV, a positive electrode prepared from the composition, and a lithium ion secondary battery including the positive electrode }

The present invention relates to a composition for forming a cathode active material layer having improved OCV characteristics, a cathode manufactured using the composition, and a lithium ion secondary battery including the cathode.

A lithium ion secondary battery generally includes a positive electrode current collector and a positive electrode having a positive electrode active material layer formed on both sides of the positive electrode current collector, a negative electrode current collector and a negative electrode having negative electrode active material layers formed on both sides of the negative electrode current collector, a separator, and an electrolyte solution. do.

In order to form the positive electrode active material layer, a Li-rich positive electrode active material, a conductive material, and a binder are included. For example, PVDF (polyvinylidene fluoride) is used as the binder. At this time, in order to improve the adhesion of the positive electrode active material layer to the current collector, an attempt was made to increase the molecular weight of PVDF as a binder or to apply it as a binder by additionally introducing a functional group into PVDF.

However, there is a limit to the increase in the molecular weight of PVDF, and the more polymerized it is, the more difficult it is to mix with additives in the process of preparing the slurry for forming the positive electrode active material layer. In the process, there was a problem that it was difficult to increase the electrode density by acting as a load.

On the other hand, when an additional functional group is introduced into PVDF, a problem of gelation of the positive electrode slurry also occurs.

Furthermore, CNTs are manufactured using Fe as a seed material during the CNT manufacturing process. Therefore, the produced CNTs contain about 1-10% by weight of Fe. However, when CNTs containing Fe are used as a conductive material in the positive electrode active material layer, there is a problem in that Fe is eluted from the positive electrode and acts as a cause of OCV defects.

The present invention improves workability in the mixing process of the active material composition while improving adhesion without increasing the molecular weight or introducing additional functional groups in preparing a cathode active material using a polymeric PVDF binder, thereby improving the stability of the slurry And, furthermore, to provide a composition for forming a positive electrode active material layer capable of smoothly performing a rolling process for producing a high-density active material layer.

In addition, the present invention is intended to improve the problem of OCV defects due to elution of Fe contained in CNTs even when CNTs are included as a conductive agent.

The present invention is to provide a composition for forming a cathode active material layer of a lithium ion secondary battery, and according to one embodiment of the present invention, lithium ion containing a lithium composite oxide, a CNT conductive material containing iron, a PVDF binder, and tannic acid. A composition for forming a secondary battery positive active material layer is provided.

Provided is a composition for forming a positive active material layer for a lithium secondary battery in which the iron is 15 ppm or more and the tannic acid is 0.03 to 0.5% by weight based on the total weight of the composition.

The iron may be derived from catalysts used during the CNT manufacturing process.

The composition is a composition for forming a positive electrode active material layer for a lithium secondary battery, comprising 1 to 7% by weight of PVDF, 0.2 to 1.5% by weight of a CNT conductive material, and the balance of lithium composite oxide.

The cathode active material may be at least one selected from the group consisting of NMC, LCO, NCA, LFP, and LMO.

The present invention is to provide a positive electrode for a lithium ion secondary battery, and according to one embodiment of the present invention, a positive electrode current collector and a positive electrode active material made of the composition of any one of claims 1 to 5 on both surfaces of the positive electrode current collector A positive electrode for a lithium ion secondary battery comprising a layer is provided.

The cathode current collector may be a metal selected from the group consisting of aluminum and aluminum alloys.

The present invention is also intended to provide a lithium ion secondary battery, and according to one embodiment of the present invention, a negative electrode and the positive electrode are alternately stacked with a separator as a boundary, and a lithium ion secondary battery including an electrolyte solution is provided.

The lithium ion secondary battery has an OCV defect rate represented by Formula (1) of 0%.

According to the present invention, when using PVDF as a binder, adhesion to a current collector can be improved without increasing the molecular weight of the binder or introducing additional functional groups.

Therefore, it is possible to reduce the amount of the PVDF binder, and through this, the characteristics of the electrode mixing and pressing process can be improved.

Furthermore, it is possible to increase the amount of the positive electrode active material as much as the reduced amount of the binder, thereby increasing the capacity of the battery.

1 is a diagram schematically showing a mechanism for improving adhesion by adding tannic acid to a PVDF binder.

The present invention is to provide a composition for forming a positive electrode active material layer of a lithium ion secondary battery. The composition for forming a positive electrode active material layer provided by the present invention includes a lithium composite oxide, a CNT conductive material, and a binder.

The lithium composite oxide may be suitably used in the present invention as long as it is commonly used for a cathode material of a lithium secondary battery. Although not limited thereto, for example, lithium cobaltate (LiCoO ₂ ), modified lithium cobaltate, lithium nickelate (LiNiO ₂ ), modified lithium nickelate, lithium manganate (LiMn ₂ 0 ₄ ), It may be a modified product of lithium manganate, a product in which Co, Mn or Ni of these oxides is partially substituted with another transition metal element. Each of the above metamorphic substances may contain elements such as aluminum, magnesium, and iron. Moreover, there are also those containing at least ₂ types of cobalt, nickel, and manganese like LiNiCoMnO2. Examples include NMC, LCO, NCA, LFP, and LMO.

In the present invention, the cathode active material composition includes a binder. The binder binds the positive electrode active material composition to each other and further binds the positive electrode active material layer to the current collector, and it is preferable to use PVDF as such a binder. In general, when a PVDF binder is used, an increase in molecular weight or an additional introduction of a functional group is required to secure the adhesive strength required in a secondary battery. However, according to the present invention, by adding tannic acid, it is possible to sufficiently secure the adhesive strength (about 0.5 N) required for a secondary battery without increasing molecular weight or introducing a functional group. This will be explained in more detail later.

The PVDF binder is preferably included in an amount of 1 to 7% by weight based on the total weight of the positive electrode active material composition. If it is less than 1% by weight, the coating layer may be detached due to lack of adhesion between the current collector and the coating layer, and if it exceeds 7% by weight, the amount of positive electrode active material is relatively reduced, resulting in a decrease in battery capacity, and the movement of lithium ions There is a problem in that the cell resistance increases by acting as a resistance.

The composition for forming a positive electrode active material layer of the present invention (based on solid content, the same unless otherwise specified) includes CNT as a conductive material. As the conductive material, other carbon materials such as carbon black may be used in addition to CNT. However, the present invention is intended to solve the problem of using CNT as a conductive material.

The CNT is manufactured by using Fe as a seed during the manufacturing process. Therefore, the prepared CNTs contain Fe, which is present in the range of approximately 0.1 to 10% by weight. When CNTs containing Fe are used as the conductive material of the positive electrode, Fe is eluted from the positive electrode during the charging and discharging reaction of the secondary battery, and such Fe causes an open-circuit voltage (OCV) defect.

In the present invention, when CNTs containing Fe are used as a conductive material, the occurrence of OCV defects due to the elution of Fe can be suppressed by the addition of tannic acid. A more detailed description of this will be given later.

The CNT is preferably included in an amount of 0.2 to 1.5% by weight based on the total weight of the cathode active material composition. If it is less than 0.2% by weight, there is a problem that the power density of the cell at room temperature is significantly lowered, and if it exceeds 1.5% by weight, there is a problem that the adhesive strength is significantly lowered.

At this time, the present invention can work more effectively when Fe contained in the CNT is included in an amount of 15 ppm or more based on the total weight of the composition. When the Fe content is less than 15 ppm, OCV defects due to iron elution hardly occur. Therefore, it is more preferable to apply the present invention to the case with an iron content of 15 ppm or more. Although not particularly limited, the Fe may be 500 ppm or less.

The present invention includes tannic acid as an additive. The tannic acid has the following structure, and as shown in FIG. 1, a hydrogen bond is formed between the F of PVDF and the OH group of tannic acid to enable multiple physical cross-links between PVDF molecules, thereby enabling a secondary battery It can provide the adhesive strength required in

Therefore, it is possible to reduce the amount of the binder and increase the amount of the positive electrode active material accordingly, so that the capacity of the battery can be increased with the same amount of active material layer attached. Furthermore, since a separate process for increasing the molecular weight of PVDF is not required to increase adhesion, the process for mixing the composition can be performed more smoothly, and the composition slurry can be stabilized. In addition, there is no need to introduce a separate functional group into PVDF, and thus gelation of the composition can be suppressed. Conventionally, an additive such as maleic acid was also added for neutralization to suppress gelation, but since the tannic acid according to the present invention is acidic in itself, gelation can be suppressed without using such maleic acid.

Furthermore, tannic acid added in the present invention serves as a scavenger for Fe ions. As the gallic acid (reducing agent) portion of the terminal group of tannic acid is partially decomposed and falls off, a complex is formed while reducing Fe ^{2+ and} Fe ^{3+ .} Through this, Fe ions are precipitated as Fe metal on the surface of the negative electrode having a low potential to cause an internal short circuit, thereby suppressing the occurrence of OCV defects. Therefore, in the case of using CNTs containing Fe as a conductive material, the elution of Fe in the secondary battery can be prevented by scavenging iron contained in the CNTs by tannic acid, and thus, the OCV defect problem can be significantly improved. can

In the present invention, the tannic acid is preferably included in an amount of 0.03 to 0.5% by weight of the total weight of the composition. If it contains less than 0.03% by weight, the effect of collecting Fe ions is low, so the probability of occurrence of OCV defects increases, and if it exceeds 0.5% by weight, there is a problem in that the output of the cell decreases.

The composition for forming a positive electrode active material of the present invention includes a solvent. The solvent is not particularly limited, but examples thereof include NMP and DMSO.

The solvent may be used in the range of 2 to 8:8 to 2 in the weight ratio of the solid content to the solvent. In the process of mixing the cathode active material and coating the current collector, it is preferable to secure a viscosity of 1000 cps to 15000 cps, so it is preferable to include the solvent within the above range. More preferably, the weight ratio of the solid content to the solvent may be 3 to 7:7 to 3.

As described above, a positive electrode for a lithium ion secondary battery may be prepared by applying the composition for active use of a positive electrode of a lithium ion secondary battery provided in the present invention on the surface of a positive electrode current collector and drying the composition.

The positive electrode current collector is not particularly limited, and if it is commonly used, it can be suitably used in the present invention. For example, a foil of aluminum or an aluminum alloy may be mentioned.

Furthermore, a secondary battery can be obtained using the positive electrode. As described above, the secondary battery may have a conventional secondary battery structure in which the positive electrode provided in the present invention and the negative electrode commonly used are alternately stacked with a separator as a boundary and may include an electrolyte solution.

The lithium ion secondary battery of the present invention may have an OCV defect rate of 1% or less represented by the following formula (1) by including the positive electrode provided in the present invention. The defect rate is determined by the difference between the OCV value immediately after maximum charging (CC-CV method, 1C-4.2V, 0.02C cut-off charging) of the cell at an environmental temperature of 60 ° C. and the OCV value after resting for 92 hours. The difference If is 50 mV or more, the OCV is determined to be defective. Expressing this as an equation, the following equation (1) is obtained.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, but merely represent one embodiment of the present invention.

Example

A positive electrode active material composition with the composition shown in Table 1 below was applied to both sides of an aluminum metal plate and dried to prepare a positive electrode. The contents in Table 1 are by weight.

Example No. One 2 3 4 5 6 7 8 sheep
pole cathode active material
(NCM622) 96.80% 96.50% 96.00% 95.50% 95.47% 95.35% 94.60% 94.00% conductive material
(CNT) 0.2% 0.5% 1.0% 1.5% 1.5% 1.5% 1.5% 1.5% in conductive material
Fe content 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% Total Fe (ppm) 6 15 30 45 45 45 45 45 bookbinder
(PVDF) 3% 3% 3% 3% 3% 3% 3% 3% tannin 0% 0% 0% 0% 0.03% 0.15% 0.90% 1.50% electrolyte 1M LiPF6, EC/EMC/DEC/PC separator PE Separator 20um cathode Graphite 97%, SBR 1.5%, CMC 1.5%

Furthermore, negative electrodes and separators were manufactured under the conditions shown in Table 1.

A lithium ion secondary battery was manufactured using the positive electrode, the negative electrode, the separator, and the electrolyte solution.

For the obtained lithium ion secondary battery, the cell nominal capacity, electrode adhesive strength, power density at room temperature, and OCV defect rate were respectively measured, and the results are shown in Table 2.

The evaluation method of each physical property is as follows.

-Cell Nominal Capacity-

Cell capacity is measured by the following method using a charge/discharger and a constant temperature chamber.

Under the environmental conditions of temperature 25±1 and humidity 65±20%, 1/3C, 2.5V cut-off discharge, and after a 30-minute rest period, 1/3C, 4.2V / 0.02C cut-off charge, 30 minutes After a rest period of 1/3C, 2.5V cut-off discharge.

-Electrode Adhesion-

After sticking to the electrode using an adhesive tape manufactured by 3M with a width of 18 mm, the adhesive strength between the current collector and the coating layer is measured using a tensile strength tester.

-room temperature power density-

Measure using the HPPC power measurement method (see FreedomCAR Battery Test Manual for Power-Assist Hybrid Electric Vehicles, DOE/ID-11069, Octoberr 2003), and calculate the power. At this time, the temperature is measured at 25°C, and the HPPC output is based on a current of 60A, a lower limit voltage of 2.5V, and an SOC of 50%.

-OCV defect rate-

After manufacturing a total of 100 cells, set the environment temperature to 60 ℃, measure the OCV value immediately after maximum charging of the cells (CC-CV method, 1C-4.2V, 0.02C cut-off charging), and rest for 92 hours After measuring the subsequent OCV values, if the difference between the OCV values is 50 mV or more, it is determined that the OCV is defective.

Example No. One 2 3 4 5 6 7 8 cell nominal capacity 60Ah Electrode Adhesion (N/18mm) 1.1 0.8 0.5 0.3 0.5 1.0 1.1 1.3 Room temperature power density (W/kg) 1532 2184 2321 2474 2423 2213 2105 2012 OCV defect rate (%) 0 3 5 10 One 0 0 0 note reference example comparative example comparative example comparative example example of invention example of invention example of invention example of invention

As can be seen from Table 2, when the content of tannic acid is 0.03%, which is out of the range of the present invention, the OCV defect rate is 3% or more due to the elution of Fe contained in the cathode active material, resulting in a high defect rate.

However, in the case of Example 5 containing 0.03% of tannic acid according to the present invention, the OCV defect rate was only 1%, indicating a good level, and in the case of Examples 6 to 8 containing a higher amount of tannic acid, the OCV defect rate This 0% showed very good results.

On the other hand, when the iron content is included in a small amount of 6ppm as in Example 1, it was found that the occurrence of OCV defects due to Fe elution was not affected.

Claims (8)

Including lithium composite oxide, CNT conductive material containing iron, PVDF binder and tannic acid,
Based on the total weight of the composition, the iron is 15 ppm or more, and the tannic acid is 0.03 to 0.5% by weight, a composition for forming a cathode active material layer for a lithium secondary battery. delete The composition for forming a cathode active material layer of a lithium secondary battery according to claim 1, wherein the iron is used as a catalyst during the CNT manufacturing process. The composition for forming a positive active material layer for a lithium secondary battery according to claim 1, wherein the PVDF binder is 1 to 7% by weight, the CNT conductive material is 0.2 to 1.5% by weight, and the balance is a lithium composite oxide. The composition for forming a cathode active material layer of a lithium secondary battery according to claim 1, wherein the lithium composite oxide is at least one selected from the group consisting of NMC, LCO, NCA, LFP and LMO. positive current collector; and
A cathode active material layer made of the composition of any one of claims 1 and 3 to 5 on both sides of the cathode current collector.
Anode comprising a. A secondary battery comprising an electrolyte solution in which the negative electrode and the positive electrode of claim 6 are alternately laminated with a separator as a boundary. The secondary battery according to claim 7, wherein the OCV defect rate represented by Formula (1) is 0%.

Images