KR20240061126A - An aqueous binder for silicon-based anode materials and its manufacturing method - Google Patents

Abstract

The present invention provides a polymer crosslinking binder for silicon anode materials.
The present invention relates to a polymer binder for negative electrodes in which acrylic acid is grafted onto synthetic rubber used as a binder. It improves the performance of lithium-ion secondary batteries by strengthening the binding force of negative electrode materials containing silicon, improving mechanical properties, and improving the performance of lithium-ion secondary batteries. By forming a polymer network, it is possible to manufacture a polymer binder for a negative electrode that can improve electrochemical performance through multiple contacts.

Description

An aqueous binder for silicon-based anode materials and its manufacturing method}

The present invention relates to a water-based binder that has good dispersibility and excellent adhesion when used in silicon-based anode materials, and a method for manufacturing the same.

Silicon has a capacity more than 10 times that of graphite, the current main anode material. However, unlike carbon-based anode materials that use the intercalation-deintercalation reaction of lithium ions, silicon-based anode materials use a charge-discharge mechanism through an alloy reaction with lithium. This causes the problem of volume expansion of more than 300%, causing the negative electrode active material to be detached from the current collector, or cracks occurring within the electrode, causing an increase in resistance between negative electrode active materials, and as charging and discharging progresses, the capacity of the electrode rapidly decreases. It causes serious degradation of electrochemical performance.

Among lithium-ion battery materials, an area in which the role of polymer materials is becoming more important is electrode binder. The binder binds electrode active materials of several to tens of micrometers and electrically conductive materials of tens of nanometers, and helps the electrode coating layer adhere well to the metal current collector. In particular, in order to improve the energy density of batteries, the thickness and density of the electrode coating layer are continuously increased, and the role of the polymer binder is becoming more important.

To date, polyvinylidene fluoride (PVDF), a non-aqueous binder, and styrene-butadiene rubber [SBR]/Carboxymethyl cellulose [CMC], an aqueous binder, are the most common emulsions. It is being used as. However, when using electrode active materials with large volume changes such as silicon, the existing binders mentioned above have limitations in application due to their relatively weak adhesion.

Secondary battery research and development is focused on developing high-capacity and high-output technologies, and high-capacity development is focused on non-carbon anode materials based on silicon, one of the anode materials.

As a water-based binder for carbon-based anode materials, carboxymethyl cellulose (CMC) is used as a thickener, as well as styrene-butadiene rubber (SBR). In the case of carbon-based anode materials, not only are they excellent in conductivity, but there is no significant change in volume during charging and discharging, so there is no problem in using styrene-butadiene rubber (SBR)/carboxymethyl cellulose (CMC) binder.

However, in the case of high-capacity cathode materials such as silicon materials, the volume change during charging and discharging is very large, exceeding 300%, so electrode adhesion becomes an important factor in cathode performance, and cathode performance can be improved depending on the type of binder. When applying styrene-butadiene rubber (SBR)/carboxymethyl cellulose (CMC) or polyvinylidene fluoride (PVDF) binder to a silicon material cathode, excessive amounts are used due to weak adhesion, and in this case, conductivity is lowered and Delamination occurs between the entire body and the active material layer, causing a significant decrease in cycle life.

Silicone high-capacity anode materials have the advantage of having a very large specific capacity value compared to existing carbon-based anode materials, but they also have the fatal disadvantage of serious volume changes as charging and discharging progress. To solve this, the binder must effectively bind the electrode particles. do.

Applying a polyacrylic acid (PAA) binder improves lifespan because it has excellent binding properties and strength to withstand volume expansion, and has the advantage of good processability due to the low sintering temperature. However, polyacrylic acid (PAA) is difficult to achieve uniform electrode density because the active material and conductive material easily precipitate during slurry production. This is acting as an obstacle to the widespread application of polyacrylic acid (PAA) to silicon anode materials.

To solve this problem, the active material particles must be controlled at the nano level while the binder must effectively bind the electrode particles. Therefore, there is a need to develop a high-adhesion and high-stability binder that can enable the lifespan characteristics and high capacity of silicon anode materials with large volume expansion.

Registered Patent Publication No. 10-2147727 (registration date: 2020. 08. 19); Registered Patent Publication No. 10-2321261 (registration date: October 28, 2021); Registered Patent Publication No. 10-2278996 (registration date: July 13, 2021); Public Patent Publication No. 10-2021-25919 (Publication date: 2021. 03. 10.)

The present invention is intended to solve the problems of the prior art. When used in a silicon anode material, the present invention uses styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) as the main raw material and retains strength according to volume expansion. The purpose is to provide a high-adhesion and high-stability binder that can enable the lifespan characteristics and high capacity of silicon anode materials.

In order to solve the above problem, the present invention reacts styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) with a specific polymer monomer to produce the styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR). An aqueous binder for a silicon anode material that can respond to changes in volume of the silicon anode material and improve the lifespan characteristics of the silicon anode material by cross-linking a specific polymer monomer to the polymer chain and a method for manufacturing the same are provided.

The method for producing an aqueous binder for a silicon anode material according to the present invention is prepared by selecting styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) as the main raw material of the polymer binder for the silicon anode material and selecting an acrylic monomer as a secondary raw material. Step (S 210) and; The acrylic monomer is added to 1 mole of the main raw material, styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) monomer. A polymer reaction step (S 220) of adding 5 to 30 mol and crosslinking at a reaction temperature of 60°C to 95°C for 1 to 4 hours; Completing an aqueous binder for a silicone anode material in which the acrylic monomer is crosslinked to the polymer chain of the styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) (S 230); It includes.

The present invention relates to a polymer crosslinking binder for silicon anode materials. Although it uses styrene-butadiene rubber (SBR) as the main raw material, it has the advantage of providing strong bonding force by overcoming the weak bonding force, which is a disadvantage of styrene-butadiene rubber binder.

In addition, the present invention has the advantage of further improving the mechanical properties and cycle life characteristics of lithium secondary batteries while using a binder based on nitrile-butadiene rubber (NBR).

In addition, the present invention has the advantage of not only improving mechanical properties and cycle life characteristics, but also further improving electrical performance when nitrile-butadiene rubber (NBR) is used as the main raw material.

Figure 1a shows the chemical formula in which a specific polymer monomer causes a crosslinking reaction in the polymer chain of styrene-butadiene rubber (SBR),
Figure 1b shows the chemical formula in which a specific polymer monomer causes a crosslinking reaction in the polymer chain of nitrile-butadiene rubber (NBR),
Figure 2 schematically illustrates the three-dimensional polymer network structure of the binder by causing a crosslinking reaction of a polymer monomer specific to the rubber polymer structure;
Figure 3 is a graph showing the results of evaluating the bonding strength of silicon negative electrode plates when using binders according to comparative examples and embodiments of the present invention;
Figure 4 is a graph showing the results of cycle life characteristics of each coin cell when using binders according to comparative examples and embodiments of the present invention.

Hereinafter, the present invention will be described in more detail and detail. It is clear that the specific numerical values or specific examples provided in the present invention are preferred embodiments of the present invention, and are only intended to explain the technical idea of the present invention in more detail, and the present invention is not limited thereto.

In addition, in the specification of the present invention, detailed description of parts that are known in the technical field and can be easily created by those skilled in the art will be omitted.

The present invention provides a method for manufacturing a polymer crosslinking binder for silicon anode materials.

The present invention includes the step (S 210) of selecting styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) as the main raw material of the polymer binder for the silicon anode material, and selecting and preparing an acrylic monomer as a secondary raw material. .

The present invention selects styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) as the main raw material of the polymer binder of the silicon anode material. This is not much different from conventional binder ingredients. However, the present invention differs in that it crosslinks the chain of the secondary raw material to the chain of the rubber polymer, which is the main raw material.

The present invention selects acrylic monomer as an auxiliary material. This is to form crosslinks in the polymer chain of the rubber, which is the main raw material, when the acrylic monomer is polymer bonded. When the acrylic monomer forms a cross-link to the polymer chain of the rubber, the chain of the rubber molecule, which is the main raw material, and the acrylic molecule, which is the secondary raw material, form cross-links to form an overall network structure. In this respect, the present invention shows a significant difference from conventional technologies that use polyacrylic as the main or secondary raw material.

In the present invention, 5 to 30 moles of the acrylic monomer are added to 1 mole of the main raw material, styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) monomer, and the reaction is carried out at a reaction temperature of 60°C to 95°C for 1 to 3 hours. It includes a polymer reaction step (S 220) of crosslinking over time.

The present invention uses the styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) as the main raw material, but rather than using it as is or kneading or mixing it with other binder components, the main chain for the acrylic monomer It is used to provide structure. In addition, the present invention uses the acrylic monomer as a secondary raw material, but rather than using it as is or simply polymerizing the component itself, it is used in the polymer chain structure of the styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR). It is used for crosslinking.

In the present invention, 5 to 30 moles of the acrylic monomer are added to 1 mole of the styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR) monomer and crosslinked. When the molar number of the acrylic monomer is 5 times or less, it is not preferable because the chains of the grafted PAA are relatively short and a three-dimensional network is not properly formed, and when the molar number of the acrylic monomer is used at 30 times or more, the grafted PAA chains are too long. It is undesirable because it becomes long and the chains become tangled and do not function properly as a binder. More preferably, it is good to use 10 to 20 moles of acrylic monomer per 1 mole of monomer of the rubber component. The cross-linking reaction is carried out at a temperature of 60°C to 95°C for 1 to 4 hours under elevated temperature and constant dosage conditions. It is preferable to use a persulfate-based reaction initiator.

Figure 1a conceptually shows the chemical formula for crosslinking the acrylic monomer to the styrene-butadiene rubber (SBR). In this chemical formula,

Figure 1b conceptually shows the chemical formula for crosslinking the acrylic monomer to the nitrile-butadiene rubber (NBR). In this chemical formula, again,

The present invention includes a step (S 230) of completing an aqueous binder for a silicone anode material in which the acrylic monomer is cross-linked to the polymer chain of styrene-butadiene rubber (SBR) or nitrile-butadiene rubber (NBR).

In the present invention, after going through the polymer reaction step (S 220), a crosslink is formed in which the acrylic monomer is bonded to the polymer chain structure of the rubber molecule while forming a branch chain, thereby completing the overall network-structured polymer chain structure. I do it.

Figure 2 conceptually shows the polymer chain structure according to the present invention and the bonding state of acrylic molecules that exist as branch chains in the main chain by forming crosslinks in the chain structure.

The polymer chain having this network structure according to the present invention provides at least three functions. This includes the function as an elastic body, as a binder, and as a life extender, as shown below. To explain this in more detail, the double bonds of the polymer chains present in the network structure still function as an elastic body, allowing it to flexibly cope with the expansion of the silicon anode material, while the acrylic molecules have strong binding properties in aqueous solutions. It functions as a binder that provides bonding force, preventing the cathode active material from detaching from the current collector or cracking occurring within the electrode despite repeated charge-discharge mechanisms, and the polymer chain And the network structure due to the acrylic molecules doubles the function as the elastic body and the function as the binder, and in addition, it functions as a life extender of the silicon anode material by the binder component.

Hereinafter, the present invention will be described in more detail through the following examples. However, the following examples are only intended to aid understanding of the present invention, and the scope of the present invention is not limited by these examples in any way.

<Preparation Example 1>: Preparation of polyacrylic acid (PAA) binder

In order to compare the binder of the present invention and its performance, a previously used polyacrylic acid binder was manufactured in the following manner.

The reactor was prepared by attaching a temperature sensor, nitrogen tube, dropping funnel, and cooler. 80 g of distilled water and 1 g of initiator were added to the reactor and the temperature was raised to 80°C. Then, while slowly stirring the reaction solution, a mixture of 10 g of acrylic acid, 10 g of distilled water, and 0.1 g of a chain transfer agent was slowly added using a dropping funnel. After the addition was completed, the mixture was stirred for 3 hours, then the temperature was raised to 95°C and stirred for 1 hour to prepare a 10% polyacrylic acid (PAA) solution.

<Preparation Example 2 1 to 3>: SBR-g - PAA binder production

In order to measure the performance of the binder based on styrene-butadiene rubber (SBR) according to the present invention, it was prepared in the following manner.

A temperature sensor, nitrogen tube, dropping funnel, and cooler were attached to the reactor. The number of moles of reaction between styrene-butadiene rubber (SBR) polymer and acrylic acid monomer was calculated and added respectively based on weight ratio.

Styrene-butadiene rubber (SBR) dispersion in reactor 500 g and an additional 100 g of distilled water were added, and 1 g of initiator sodium persulfate (SPS) was added. The reaction temperature was raised to 80°C, and while the reaction solution was slowly stirred, a mixture of acrylic acid and distilled water at a weight ratio of 1:1 was slowly added using a dropping funnel. At this time, the amount of acrylic acid is 10 times each for 1 mole of butadiene monomer; 20 times; It was added 30 times, and the parts by weight were 1.00; 1.98; It was 2.97 g. After the addition was completed and stirred for 3 hours, the temperature was raised to 95°C and stirred for 1 hour to prepare PAA-g-SBR crosslinked binders.

<Preparation Example 3 1 to 3>: NBR-PAA binder production

In order to measure the performance of the binder based on nitrile-butadiene rubber (NBR) according to the present invention, it was prepared in the following manner.

A temperature sensor, nitrogen tube, dropping funnel, and cooler were attached to the reactor. The number of moles of reaction between nitrile-butadiene rubber (NBR) polymer and acrylic acid monomer was calculated and added respectively based on weight ratio.

Nitrile-butadiene rubber (NBR) dispersion in reactor 500 g and 100 g of additional distilled water were added, and 1 g of initiator sodium persulfate (SPS) was added. The reaction temperature was raised to 80°C, and while the reaction solution was slowly stirred, a mixture of acrylic acid and distilled water at a weight ratio of 1:1 was slowly added using a dropping funnel. At this time, the amount of acrylic acid is 10 times each for 1 mole of butadiene monomer; 20 times; It was added 30 times, and the weight part was 0.77; 1.54; It was 2.32 g. After the addition was completed and stirred for 3 hours, the temperature was raised to 95°C and stirred for 1 hour to prepare PAA-g-NBR crosslinked binders.

<Comparative Example 1>: Preparation of Si anode and binder used (Preparation Example 1 binder)

In order to compare the binder of the present invention and its performance, a silicon anode material and a conventional polyacrylic acid binder were prepared and used in the following manner.

The cathode composition consists of active material, conductive material, and binder in a weight ratio of 95:2:3. The anode active materials used at this time were SiC and graphite, and commercially available carbon black was used as the conductive material.

As a binder, a mixture of the polyacrylic acid (PAA) binder prepared in Preparation Example 1 and 1.5 wt% CMC aqueous solution was used.

For electrode production, a slurry was prepared by uniformly stirring and dispersing the active material, conductive material, and binder using pre-mixing and a Thinky mixer.

For electrode coating, the previously prepared slurry was coated on the current collector with a constant thickness of 50 to 60 ㎛ using a doctor blade. The coated electrode was dried in a dryer at 80°C for more than 3 hours and then vacuum dried for more than 6 hours. Rolling was performed so that the composite density was 1.2 to 1.6 g/cm ³ , and the loading level of the electrode at this time was 5 to 8 g/mm ² . The manufactured electrode was dried in a vacuum oven for more than 12 hours.

<Example 1 1 to 3>: Preparation of Si anode and binder used (Preparation Example 2 binder)

To measure the performance of the styrene-butadiene rubber (SBR)-based binder according to the present invention, it was prepared and used in the following manner.

The cathode composition ratio was the same as in Comparative Example 1.

The binder used was each of the PAA-g-SBR crosslinking binders prepared in 1 to 3 of Preparation Example 2 instead of the polyacrylic acid (PAA) binder.

The electrode manufacturing and electrode coating methods were carried out in the same manner as in Comparative Example 1 above.

<Example 2 1 to 3>: Preparation of Si anode and binder used (Preparation Example 3 binder)

To measure the performance of the nitrile-butadiene rubber (NBR)-based binder according to the present invention, it was prepared and used in the following manner.

The cathode composition ratio was the same as in Comparative Example 1.

The binder used was each of the PAA-g-NBR crosslinking binders prepared in 1 to 3 of Preparation Example 3 instead of the polyacrylic acid (PAA) binder.

The electrode manufacturing and electrode coating methods were carried out in the same manner as in Comparative Example 1 above.

<Test Example 1>: Evaluation of binding force

To evaluate the mechanical properties of the electrode according to the binder material, the manufactured electrode was cut into a wide rectangular shape. 3M's double-sided tape was attached to a 12 mm wide slide glass, and the binding force was measured using a UTM (Universal Testing Machine) at a test speed of 50 mm/min.

<Table 1> below shows the results of each electrode plate using each binder manufactured in Comparative Example 1, Example 1 1 to Example 1 3, and Example 2 1 to Example 2 3. This shows the results of evaluating cohesion.

Cohesion evaluation result table Binder Type Loading level (mg/cm ² ) Mean stress (gf/mm) Comparative Example 1 PAA/CMC 6.0 2.090 8.0 1.710 Example 1-1 PAA-g-SBR(10:1)
/CMC 6.0 2.249 8.0 1.772 Example 1-2 PAA-g-SBR(20:1)
/CMC 6.1 2.294 8.1 1.862 Example 1-3 PAA-g-SBR(30:1)
/CMC 6.0 1.991 8.0 1.707 Example 2-1 PAA-g-NBR(10:1)
/CMC 6.1 2.241 8.0 1.781 Example 2-2 PAA-g-NBR(20:1)
/CMC 6.0 2.290 8.0 1.842 Example 2-3 PAA-g-NBR(30:1)
/CMC 6.0 2.004 8.0 1.709

As confirmed in the above binding force evaluation result table, the molar ratio of acrylic acid monomer to butadiene monomer in the SBR polymer gradually increased from 10 times, and then decreased when the molar ratio reached 30 times.

The more acrylic acid monomers there are, the more evenly they are grafted onto the main chain, SBR, forming a more robust three-dimensional network. However, if the amount exceeds the appropriate amount, the grafted PAA chain becomes too long and its own binding force becomes stronger, which causes It shows that the binding force decreases.

In addition, as can be seen from the binding force evaluation result table, the molar ratio of the acrylic acid monomer to the butadiene monomer in the NBR polymer gradually increases from 10 times, but when the molar ratio reaches 30 times, it actually decreases. there is. This was interpreted as showing this trend for the same reason as the change in the molar ratio of butadiene monomer in the SBR polymer.

Figure 3 graphically shows the results of evaluating the binding force of each electrode plate.

<Test Example 2>: Evaluation of cycle life characteristics and expansion rate

In order to evaluate the cycle life characteristics, electrodes were prepared using the electrode plates manufactured in Comparative Example 1 and Examples 1 and 2 using a 14pi puncher, and Li metal (0.5T) was added to 16pi in a glove box filled with Ar. A coin cell was manufactured using a punching machine, a solution of 1M LiPF6 dissolved in EC (ethylene carbonate):DMC (dimethyl carbonate)=1:2 as the electrolyte, and commercial PE (polyethylene) as a separator.

Using the coin cell manufactured above, lifespan was evaluated in the voltage range of 0.01 to 2.0 V at a current application rate of 0.5 C at room temperature, and the lifespan maintenance rate was determined based on the capacity upon lithium delithiation after 100 cycles of charge and discharge. was calculated based on the charging capacity after formation, which is the first charge/discharge operation in the coin cell.

Additionally, the expansion rate was calculated from the electrode thickness by measuring the cross-section of the electrode using a scanning electron microscope (SEM). At this time, expansion rate (%) = [(electrode thickness after charging - electrode thickness before charging) / electrode thickness before charging]

<Table 2> below shows that for each coin cell manufactured in Test Example 2, each binder manufactured in Comparative Example 1, Example 1 1 to 3, and Example 2 1 to 3 was used. , shows the results of measuring the cycle life characteristics and expansion rate of each coin cell.

Cycle life characteristics evaluation results Binder Type Cycle life (%)
<100 times> Expansion rate (%) Comparative Example 1 PAA/CMC 80.73 101 Example 1-1 PAA-g-SBR(10:1)/CMC 82.80 87 Example 1-2 PAA-g-SBR(20:1)/CMC 86.58 80 Example 1-3 PAA-g-SBR(30:1)/CMC 64.34 69 Example 2-1 PAA-g-NBR(10:1)/CMC 84.33 71 Example 2-2 PAA-g-NBR(20:1)/CMC 88.23 64 Example 2-3 PAA-g-NBR(30:1)
/CMC 66.35 60

As confirmed by the result table of the cycle life characteristic evaluation, the molar ratio of the acrylic acid monomer to the butadiene monomer in the SBR polymer gradually increased from 10 times, and then decreased when the molar ratio reached 30 times.

Within the above range, the more acrylic acid monomers are evenly grafted onto the main chain, SBR, forming a more robust three-dimensional network, which can reduce the volume expansion rate of the silicon anode material. However, outside the range, the grafted PAA chains become too large. As it gets longer, its own binding force becomes stronger, and as a result, the binding force at the electrode decreases, which is thought to actually lower the cycle life characteristics.

In addition, the NBR rubber polymer showed the same tendency as the BR polymer. In other words, when NBR was used as a cross-linking binder as the main chain compared to SBR, the cycle life characteristics were improved and the volume expansion rate was relatively significantly reduced. This was interpreted to be because NBR not only has a sufficiently excellent elongation rate to accommodate large volume changes of silicon particles when they expand and contract, but also maintains stable bonding through three-dimensional multi-point contact.

Figure 4 graphically shows the cycle life characteristics of each coin cell.

In this way, the PAA-g-SBR and PAA-g-NBR crosslinking binders manufactured by the present invention showed superior adhesion to the anode mixture containing silicon compared to existing binders, and had excellent strength and elongation, making them suitable for styrene-butadiene. It has been confirmed that rubber (SBR) and polyacrylic acid (PAA) binders improve the performance of lithium-ion secondary batteries by improving mechanical properties that are problematic when used alone.

In addition, the cross-linking binder produced by the present invention can easily form a three-dimensional polymer network around Si particles through hydrogen bonding and van der Waals action, and provides multi-point contact with the Si surface to enable bonding to Si particles. Improve your abilities. In addition, it can be seen that this multi-point contact is advantageous in preserving electrode stability during charge and discharge even if a large volume change of Si particles occurs.

Although the manufacturing method of the polymer cross-linking binder for silicon anode material according to the present invention has been specifically presented above, this is only specified in the process of explaining the embodiments of the present invention, and all features of the present invention apply only to the items mentioned above. It should not be interpreted as limited to what is possible.

In addition, anyone skilled in the art will be able to make various modifications and imitations based on the contents of the specification of the present invention, but it will be clear that these are not beyond the scope of the present invention.

Claims (6)

A method for producing a cross-linked binder, characterized in that polyacrylic acid (PAA) is grafted onto the polymer chain of styrene-butadiene rubber (SBR) through radical polymerization by reacting an initiator with an acrylic acid monomer to styrene-butadiene rubber (SBR).
According to clause 1,
A method for producing a cross-linked binder, characterized in that the styrene content of the styrene-butadiene rubber (SBR) is 20 to 50 wt%.
According to clause 1,
A method for producing a cross-linked binder, characterized in that the number of moles of acrylic acid is 5 to 30 times that of 1 mole of butadiene monomer in the styrene-butadiene rubber (SBR).
A method for producing a crosslinking binder, characterized in that polyacrylic acid (PAA) is grafted onto the polymer chain of nitrile-butadiene rubber (NBR) through radical polymerization by reacting an initiator with an acrylic acid monomer to nitrile-butadiene rubber (NBR).
According to clause 4,
A method for producing a cross-linked binder, characterized in that the acrylonitrile content of the nitrile-butadiene rubber (NBR) is 18 to 48 wt%.
According to clause 4,
A method for producing a cross-linked binder, characterized in that the number of moles of acrylic acid is 5 to 30 times that of 1 mole of acrylonitrile in the nitrile-butadiene rubber (NBR).