KR102253133B1 - Binder for electrode, anode including the same and lithium secondary battery including the anode - Google Patents

Abstract

The present invention is a boronic acid crosslinking agent; And two or more guar gum molecules, wherein the boric acid crosslinking agent is capable of chemical bonding with a hydroxyl group present in each of the two or more guar gum molecules, and the two or more guar gum molecules are crosslinked with each other by the boronic acid crosslinking agent. The present invention relates to a binder for electrodes having a network structure, an anode including the same, and a lithium secondary battery including the anode.
The electrode binder according to an embodiment of the present invention serves to improve the durability of the silicon-based anode, and the initial charge/discharge performance and life characteristics of a lithium secondary battery including the anode to which the electrode binder according to the present invention is applied are improved. .

Description

TECHNICAL FIELD [Binder for electrode, anode including the same and lithium secondary battery including the anode]

The present invention relates to a binder for an electrode, an anode comprising the same, and a lithium secondary battery comprising the anode, and in more detail, a binder for an electrode comprising guar gum having a network structure, an anode comprising the same, and the anode comprising the It relates to a lithium secondary battery.

Lithium secondary batteries have a high energy density, and as the application field expands from small IT devices to mid- to large-sized power sources such as electric vehicles, the demand for high energy density is increasing.The anode using conventional graphite as an active material meets these needs. I can't.

Silicon-based active materials have long been considered as the next-generation anode active material, but extreme volume expansion (~ 300%) due to occlusion and desorption of lithium ions during charging/discharging promotes electrode deterioration, becoming the biggest obstacle in commercialization of silicon-based materials. .

This volume change causes crushing of the silicon-based active material to expose a new interface, and the exposed interface forms a new SEI (passive film) due to a side reaction with the electrolyte, resulting in continuous consumption of lithium ions and electrolyte.

The pieces of silicon thus crushed form a dead phase that cannot participate in charging/discharging by being separated from the original conductive structure, which causes life to deteriorate.

In addition, externally, the extreme volume expansion increases the thickness of the electrode, thereby increasing the physical thickness of the battery, thereby causing a serious problem in the design of the battery pack.

There have been many attempts to solve these problems, but most of them are being developed as a concept that accommodates volume change in the active material itself by manufacturing an active material in the form of a composite or structure.

A binder is a very important material that imparts the integrity of an electrode, and although it plays a large role in maintaining the conductive structure of an active material, there is still a lack of aqueous binders that can be applied in addition to the existing binders (SBR/CMC, etc.).

In addition, the SBR/CMC binder is an aqueous binder used for a graphite-based anode active material, and is not suitable for an occlusion and desorption system having an extreme volume change like a silicon-based anode, so a new binder system is required.

Therefore, the problem to be solved by the present invention is an electrode binder used with a high-capacity electrode active material having a large volume change according to the occlusion and release of lithium ions, and an electrode binder including guar gum having a network structure, It is to provide an anode including and a lithium secondary battery including the anode.

In order to solve the above problems, according to an aspect of the present invention, a boric acid crosslinking agent; And two or more guar gum molecules, wherein the boric acid crosslinking agent is capable of chemical bonding with a hydroxyl group present in each of the two or more guar gum molecules, and the two or more guar gum molecules are crosslinked with each other by the boronic acid crosslinking agent. Thus, a binder for electrodes having a network structure is provided.

Here, the boronic acid crosslinking agent may include a polyethylene glycol unit.

In addition, the weight ratio of the boronic acid crosslinking agent and the guar gum molecule may be 0.5:14.5 to 2:13.

Meanwhile, the weight average molecular weight of the boronic acid crosslinking agent may be 10,551 g/mol to 15,678 g/mol.

In addition, the binder for the electrode may be a binder for an anode of a lithium secondary battery.

On the other hand, according to another aspect of the present invention, the current collector; And an anode active material layer formed on at least one surface of the current collector and including a silicon-based anode active material, a conductive material, and a binder for an electrode, wherein the binder for the electrode is the binder for the electrode of the present invention. An anode is provided.

Meanwhile, according to another aspect of the present invention, in a lithium secondary battery including a cathode, an anode, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte, the anode is the anode of the present invention. A lithium secondary battery is provided.

The binder for an electrode according to an embodiment of the present invention serves to improve the durability of the silicon-based anode.

In addition, initial charge/discharge performance and life characteristics of a lithium secondary battery including an anode to which an electrode binder according to the present invention is applied are improved.

The following drawings attached to the present specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention together with the contents of the above-described invention, so the present invention is limited to the matters described in such drawings. It is limited and should not be interpreted.
1 is a graph showing initial charge/discharge characteristics of lithium secondary batteries according to Examples and Comparative Examples of the present invention.
2 is a graph showing the life characteristics of lithium secondary batteries according to Examples and Comparative Examples of the present invention.

Hereinafter, the present invention will be described in detail. The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The binder for an electrode according to the present invention includes a boric acid crosslinking agent; And two or more guar gum molecules, wherein the boric acid crosslinking agent is capable of chemical bonding with a hydroxyl group present in each of the two or more guar gum molecules, and the two or more guar gum molecules are crosslinked with each other by the boronic acid crosslinking agent. To form a network structure.

Guar gum is a natural nonionic polysaccharide extracted from nature. It is well soluble in water, environmentally friendly, and easy to form a film, so research is being conducted as a novel binder. Since this guar gum has a polysaccharide backbone, it has a characteristic similar in structure to carboxymethyl cellulose (CMC).

Conventionally, studies have been conducted to evaluate the performance of a silicon-based anode in which guar gum itself is applied as a binder for electrodes.

However, in the present invention, by using a boronic acid crosslinking agent capable of chemically binding to a vicinal hydroxyl group (-OH) present in many polysaccharide rings of guar gum, the two or more guar gum molecules are It provides a binder for electrodes having a network structure formed by crosslinking each other.

The lithium secondary battery to which the binder for an electrode according to an embodiment of the present invention is applied has an effect of improving initial charge/discharge performance and life characteristics compared to the case of using non-crosslinked guar gum as a binder, which is for the electrode. This is because the durability of the electrode is improved by using the binder.

At this time, the boronic acid crosslinking agent may include a polyethylene glycol unit, and there is an advantage of increasing water solubility by including the polyethylene glycol unit.

In addition, the weight ratio of the boronic acid crosslinking agent and the guar gum molecule may be 0.5:14.5 to 2:13. If this numerical range is satisfied, the density of the crosslinking is not too high, and by adjusting the crosslinking to an appropriate density, the electrode binder can exhibit an effect of buffering a large volume change of the electrode active material.

Meanwhile, the weight average molecular weight of the boronic acid crosslinking agent may be 10,551 g/mol to 15,678 g/mol. As such, the boronic acid crosslinking agent corresponding to the polymer has an abundant crosslinking functional group unlike other crosslinking agents, so that even a single molecule can crosslink a large amount of guar gum molecules. In addition, since the boronic acid crosslinking agent having such a weight average molecular weight has excellent durability against external tension, even after crosslinking, it may play a role of effectively binding the electrode active material compared to general crosslinking agents.

Here, the electrode binder may be a binder for a lithium secondary battery anode, but is not limited thereto, and may also be used as a binder for an anode or a cathode of an electrochemical device other than a lithium secondary battery.

On the other hand, according to another aspect of the present invention, the current collector; And an anode active material layer formed on at least one surface of the current collector and including a silicon-based anode active material, a conductive material, and a binder for an electrode, wherein the binder for the electrode is the binder for the electrode of the present invention. An anode is provided.

In addition, in a lithium secondary battery comprising a cathode, an anode, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte, the anode is provided with a lithium secondary battery, characterized in that the anode of the present invention described above.

The anode active material applied and used for the anode of a lithium secondary battery causes volume expansion according to the occlusion and release of lithium ions. In particular, when a silicon-based anode active material is used as the anode active material, volume expansion may be further increased. Due to this volume expansion, as the cycle of the lithium secondary battery progresses, the size of the pores formed on the surface of the anode active material layer increases, and cracks may be formed. , As the conductivity between the active material and the current collector decreases as well as the conductivity between the anode active material, the charge/discharge characteristics of the lithium secondary battery are deteriorated, resulting in a decrease in the life characteristics of the lithium secondary battery.

Here, non-limiting examples of the current collector include a foil manufactured by copper, gold, nickel, or a copper alloy, or a combination thereof.

In addition, in the lithium secondary battery according to the present invention, a stack, lamination, folding, and stack/folding process of a separator and an electrode is possible in addition to winding, which is a general process.

In addition, the external shape of the lithium secondary battery is not particularly limited, but may be a cylindrical shape using a can, a square shape, a pouch type, or a coin type.

Meanwhile, the cathode to be applied to the lithium secondary battery according to the present invention is not particularly limited, and a cathode active material may be manufactured in a form bound to a current collector according to a conventional method known in the art.

As a non-limiting example of the cathode active material, a conventional cathode active material that can be used for the cathode of a conventional lithium secondary battery can be used, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination of these lithium composites Oxide can be used. In addition, non-limiting examples of the cathode current collector include a foil manufactured by aluminum, nickel, or a combination thereof.

Meanwhile, the separator used in the present invention can be used as long as it is a porous polymer substrate commonly used in the art, and for example, a polyolefin-based porous polymer membrane or a nonwoven fabric may be used, but is not particularly limited thereto. .

Examples of the polyolefin-based porous polymer membrane include polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polyolefin-based polymers such as polypropylene, polybutylene, and polypentene, respectively, alone or as a mixture of them. The formed film (membrane) is mentioned.

As the nonwoven fabric, in addition to the polyolefin nonwoven fabric, for example, polyethyleneterephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate ), polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalene, etc., either alone or Nonwoven fabrics formed of polymers obtained by mixing them are exemplified. The structure of the nonwoven fabric may be a sponbond nonwoven fabric composed of long fibers or a melt blown nonwoven fabric.

The thickness of the porous polymer substrate is not particularly limited, but may be 5 μm to 50 μm, and the pore size and porosity present in the porous polymer substrate are also not particularly limited, but may be 0.01 μm to 50 μm and 10 to 95%, respectively. have.

In addition, in order to improve the mechanical strength of the separator and the safety of the lithium secondary battery, a porous coating layer including inorganic particles and a polymer binder may be further included on at least one surface of the porous polymer substrate.

The polymeric binder included in the porous coating layer is coated on a part or all of the surfaces of the inorganic particles, and the inorganic particles are connected and fixed to each other by the polymeric binder in an intimate state, and exist between the inorganic particles. It is preferable that pores are formed due to an interstitial volume. That is, the inorganic particles of the porous coating layer exist in close contact with each other, and the interstitial volume generated when the inorganic particles are in close contact may become pores of the porous coating layer.

On the other hand, the electrolyte salt contained in the non-aqueous electrolyte solution that can be used in the present invention is a lithium salt. As the lithium salt, those commonly used in an electrolyte solution for a lithium secondary battery may be used without limitation. For example is the above lithium salt anion ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF _{^{4 -, (CF 3) 3}} PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 - _{, (CF 3 SO 2) 2} N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, ( ^{_{CF 3 SO 2) 3 C -}} , CF 3 (CF 2) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 -, It may be any one selected from the group consisting of ^SCN- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^-.

As organic solvents included in the above-described non-aqueous electrolyte, those commonly used in electrolytes for lithium secondary batteries can be used without limitation, and for example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. can be used alone or in two or more types. It can be mixed and used.

Among them, representatively, a cyclic carbonate, a linear carbonate, or a carbonate compound that is a mixture thereof may be included.

Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, Any one selected from the group consisting of 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, or a mixture of two or more thereof. Examples of these halides include, but are not limited to, fluoroethylene carbonate (FEC).

In addition, specific examples of the linear carbonate compound include any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, or these A mixture of two or more of them may be representatively used, but is not limited thereto.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are organic solvents of high viscosity and have a high dielectric constant, so that lithium salts in the electrolyte can be more easily dissociated, such as dimethyl carbonate and diethyl carbonate. If a low viscosity, low dielectric constant linear carbonate is mixed in an appropriate ratio and used, an electrolyte solution having a higher electrical conductivity can be prepared.

In addition, as the ether of the organic solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, and ethylpropyl ether, or a mixture of two or more thereof may be used. , But is not limited thereto.

And esters in the organic solvent include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and Any one selected from the group consisting of ε-caprolactone, or a mixture of two or more of them may be used, but is not limited thereto.

The injection of the non-aqueous electrolyte may be performed at an appropriate step in the manufacturing process of the lithium secondary battery according to the manufacturing process and required physical properties of the final product. That is, it can be applied before assembling the lithium secondary battery or in the final stage of assembling the lithium secondary battery.

Hereinafter, examples will be described in detail in order to describe the present invention in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art.

Preparation of silicon-based electrode active material slurry

(1) Example 1-1

Silicon nanoparticles of 50 nm or less were used as an electrode active material, Super-P was used as a conductive material, and guar gum having a network structure was mixed as a binder to prepare an electrode active material slurry. Water served as a solvent for dissolving the binder. At this time, the anode active material, the conductive material, and the binder were mixed in a weight ratio of 70:15:15, and the guar gum binder of the network structure was crosslinked by mixing guar gum and a boronic acid crosslinking agent in a weight ratio of 14.25:0.75. .

(2) Example 2-1

An electrode active material slurry was prepared in the same manner as in Example 1-1, except that a guar gum binder having a network structure formed by crosslinking the guar gum and boronic acid crosslinking agent was mixed at a weight ratio of 13.5:1.5.

(3) Comparative Example 1-1

An electrode active material slurry was prepared in the same manner as in Example 1-1, except that the same content of SBR/CMC (a weight ratio of SBR and CMC was 7:3) was used instead of a network structured guar gum binder.

(4) Comparative Example 2-1

An electrode active material slurry was prepared in the same manner as in Example 1-1, except that an uncrosslinked guar gum of the same amount was used instead of the network structured guar gum binder.

Preparation of half cell

(1) Example 1-2

The silicon-based electrode active material slurry prepared in Example 1-1 was coated on a copper (Cu) foil current collector by a conventional method to prepare an electrode. The thus-prepared electrode was set as a positive electrode and lithium metal as a negative electrode, and a coin-shaped half cell was manufactured using an electrode assembly made by interposing a polyethylene porous polymer membrane between the electrodes. _{At this time, as the non-aqueous electrolyte, a mixture of 1.3M LiPF 6} lithium salt and 10% by weight FEC as an additive was used in a non-aqueous solvent mixed so as to have a volume ratio of EC:DEC = 3:7.

(2) Example 2-2

A coin-shaped half-cell was manufactured in the same manner as in Example 1-2, except that a positive electrode prepared using the electrode active material slurry prepared in Example 2-1 was used as the positive electrode.

(3) Comparative Example 1-2

A coin-shaped half-cell was manufactured in the same manner as in Example 1-2, except that a positive electrode prepared using the electrode active material slurry prepared in Comparative Example 1-1 was used as the positive electrode.

(4) Comparative Example 2-2

A coin-shaped half-cell was manufactured in the same manner as in Example 1-2, except that a positive electrode prepared using the electrode active material slurry prepared in Comparative Example 2-1 was used as the positive electrode.

Evaluation of the cycle characteristics of the half cell

Using the coin-shaped half-cell prepared above, initial charge/discharge characteristics were measured and shown in Table 1 and FIG. 1 below.

Example 1-2 Example 2-2 Comparative Example 1-2 Comparative Example 2-2 Charging capacity (mAh/g) 3,286 3,096 3,336 2,958 Discharge capacity (mAh/g) 2,958 2,655 2,702 2,432 1 ^st Efficiency (%) 86.5 85.7 81.0 82.2

Referring to Table 1 and FIG. 1, it can be seen that in the case of Comparative Example 2-2 using a general guar gum instead of a network structure, the charge/discharge capacity is the smallest.

In addition, in the case of Comparative Example 1-2 using a general aqueous binder, although the charging capacity is the largest, it can be seen that the initial charging and discharging efficiency is lower than that of the examples.

Further, using the coin-shaped half-cell prepared above, after a continuous charge/discharge cycle was performed, the capacity retention rate was measured and shown in Table 2 and FIG. 2 below.

Example 1-2 Example 2-2 Comparative Example 1-2 Comparative Example 2-2 First capacity (mAh/g) 2,842 2,655 2,702 2,432 40th capacity (mAh/g) 1,789 1,604 1,669 1,218 Capacity retention rate (%) 63.0 60.4 61.8 50.1

Referring to Table 2 and FIG. 2, in the case of Comparative Example 2-2 using a general guar gum instead of a network structure, the capacity retention rate was the lowest at about 50%.

On the other hand, in the case of Example 1-2, the capacity retention rate was measured to be about 63%, but it can be seen that the performance is the best compared to other experimental examples.

For reference, in the case of Example 2-2, compared to the case of Comparative Example 2-2, the capacity and capacity retention rate were significantly improved, but compared to the case of Comparative Example 1-2, the capacity and capacity retention rate were lower. The reason for this is that due to the relatively high crosslinking density, the hardness of the electrode binder is also increased, and it is predicted that the ductility is somewhat insufficient to withstand a large volume change of the electrode active material.

The above description is merely illustrative of the technical idea of the present invention, and those of ordinary skill in the art to which the present invention pertains will be able to make various modifications and variations without departing from the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are not intended to limit the technical idea of the present invention, but to explain the technical idea, and the scope of the technical idea of the present invention is not limited by these embodiments. The scope of protection of the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

Claims (7)

Boronic acid crosslinking agents; And two or more guar gum molecules,
The boric acid crosslinking agent is capable of chemical bonding with a hydroxyl group present in each of the two or more guar gum molecules,
A binder for an electrode in which the hydroxyl groups of the two or more guar gum molecules are crosslinked with each other by the boric acid crosslinking agent to form a network structure. The method of claim 1,
The boronic acid crosslinking agent is a binder for an electrode, characterized in that it contains a polyethylene glycol unit. The method of claim 1,
The binder for an electrode, characterized in that the weight ratio of the boronic acid crosslinking agent and the guar gum molecule is 0.5:14.5 to 2:13. The method of claim 1,
The weight average molecular weight of the boronic acid crosslinking agent is 10,551 g/mol to 15,678 g/mol. The method of claim 1,
The electrode binder is a binder for an electrode, characterized in that the binder for a lithium secondary battery anode. Current collector; And
An anode formed on at least one surface of the current collector and including an anode active material layer including a silicon-based anode active material, a conductive material, and a binder for an electrode,
The anode for the electrode, characterized in that the binder for the electrode of any one of claims 1 to 5. In the lithium secondary battery comprising a cathode, an anode, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte,
The anode is a lithium secondary battery, characterized in that the anode of claim 6.

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