KR20120121623A - Method for improving capacity retention of lithium secondary cell, separator for the same, manufacturing method for the separator, and secondary cell comprising the same - Google Patents

Abstract

PURPOSE: A method for improving the charging capacity of a lithium secondary battery is provided to remarkably improve the capacity retention of a lithium metal electrode by using a microporous separator on which polydopamine polymers are coated. CONSTITUTION: A method for improving the charging capacity of a lithium secondary battery, which comprises a lithium metal electrode as a negative electrode and a separator, comprises a step of surface-treating the separator of the lithium secondary battery by a compound represented by chemical formula 1. In chemical formula 1, at least one of R1, R2, R3, R4 and R5 is selected from thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide, halide, polyhexamethylene dithiocarbonate, hydroxy, carboxylic acid, carboxylic ester or carboxamide.

Description

Method for improving discharge capacity of lithium secondary battery Separation membrane and surface treatment method thereof, Lithium secondary battery comprising same {Method for improving capacity retention of lithium secondary cell, separator for the same, manufacturing method for the separator, and secondary cell comprising the same }

The present invention relates to a method for improving the discharge capacity of a lithium secondary battery, a separator and a surface treatment method thereof, and a lithium secondary battery including the same. More specifically, the ion conductivity and the electrolyte impregnation amount are lower than those of a conventional polymer surface coating separator. In spite of being small, the present invention relates to a method for improving discharge capacity of a lithium secondary battery, which exhibits excellent discharge capacity retention characteristics.

Recently, as the use of mobile communication and portable electronic devices continues to increase, and with the rapid development of portable electronic devices, the demand for secondary batteries is gradually increasing, and the functions required for the secondary batteries are also diversified. Light weight, miniaturization and high capacity are required. In accordance with such demands, lithium ion secondary batteries have been in the spotlight for their advantages of higher operating voltage and significantly higher energy density than conventional batteries. The lithium secondary battery is prepared by using a metal oxide such as LiCoO ₂ as a positive electrode active material and a carbon material as a negative electrode active material, inserting a porous separator between the negative electrode and the positive electrode, and adding a non-aqueous electrolyte with lithium salt such as LiPF ₆ . .

During charging, lithium ions of the positive electrode active material are released and inserted into the carbon layer of the negative electrode, and during discharge, lithium ions of the negative electrode carbon layer are released and inserted into the positive electrode active material, wherein the non-aqueous electrolyte solution is lithium ions between the negative electrode and the positive electrode. It acts as a medium for moving the. Such a lithium secondary battery should basically be stable in the operating voltage range of the battery and have a performance capable of transferring ions at a sufficiently high speed.

Lithium, which is used as an anode in such a lithium secondary battery, is the most popular material as an electrode material of a high energy density battery because of its low density (0.54 g / cm 3) and very low standard reduction potential (−3.045 VSHE). However, due to its high chemical activity, a passivation film is formed by reaction with an organic electrolyte, and the passivation of the lithium metal surface during charging and discharging disproportionately occurs due to uneven repetition of oxidation (dissolution) and reduction (deposition). There is a problem that the formation and growth of the film is severe. As a result, the battery capacity decreases during charging and discharging, and dendrite (lithium resin phase) is formed on the surface of lithium metal, causing safety problems such as short circuits. Not even commercialized. However, as the demand for the development of high-energy density secondary batteries such as Li / air secondary batteries is increasing with the continuous increase and rapid development of the use of mobile communication and portable electronic devices, the necessity of using lithium metal electrodes is increasing. It is emerging.

The present invention has been made to solve the above problems and to solve the technical problems required in the prior art, the problem to be solved by the present invention is to provide a method that can improve the characteristics of the lithium metal electrode.

Another object of the present invention is to provide a lithium secondary battery including a gap metal electrode and having excellent discharge capacity retention characteristics.

In order to solve the above problems, the present invention is a method for improving the discharge capacity retention characteristics of a lithium secondary battery comprising a lithium metal electrode and a separator as a negative electrode, the method

It provides a lithium secondary battery discharge capacity improving method comprising the step of surface treatment of the separator of the lithium secondary battery with a compound of the formula 1.

(1)

(One)

In which R ₁ , R ₂ , R ₃ , R ₄ and R ₅ At least one of thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide and halide, respectively ), Polyhexamethylene dithiocarbonate, polyhydroxymethylene dithiocarbonate, hydroxyl, carboxylic acid, carboxylic ester, carboxylic ester or carboxamide. R ₁ , R ₂ , R ₃ , R _4, and R ₅ except for these are hydrogen)

In one embodiment of the present invention, the separator is an olefin resin, fluorine resin, polyester resin, cellulose resin, polyamide resin, polyimide resin, polysulfone resin, polyacrylonitrile resin, polyacetal Resin, polycarbonate-based resin, vinylidene fluoride-based resin, glass fiber to inorganic composites include a porous substrate consisting of a single or multilayer, the pore size of the porous substrate ranges from 0.001 to 1000㎛, porosity is 5 to 95% range, thickness is in the range of 1-1000 μm.

In addition, in one embodiment of the present invention, the coating thickness of the separator is in the range of 0.001 to 1㎛.

The present invention is a membrane surface treatment method used in the lithium secondary battery discharge capacity improving method to solve the above-mentioned another problem, the method comprises the steps of dissolving a compound of formula 1 in a solution of pH 7 to 11; And it provides a membrane surface treatment method comprising the step of immersing the porous substrate in the solution.

(1)

(One)

In which R ₁ , R ₂ , R ₃ , R ₄ and R ₅ At least one of thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide and halide, respectively ), Polyhexamethylene dithiocarbonate, polyhydroxymethylene dithiocarbonate, hydroxyl, carboxylic acid, carboxylic ester, carboxylic ester or carboxamide. R ₁ , R ₂ , R ₃ , R _4, and R ₅ except for these are hydrogen)

In one embodiment of the present invention, the compound is a polymerization reaction in the solution, the method of impregnating the porous substrate in the solution is any one of immersion coating method, pressure coating method, spin coating method, spray coating method or roller coating method.

In one embodiment of the invention the solvent of the solution is water or alcohol or a mixture thereof

The present invention provides a separation membrane surface-treated by the above-described method in order to solve the above another problem.

In addition, the present invention (a) a positive electrode; (b) a negative electrode made of lithium metal; (c) a separator provided between the anode and the cathode and surface treated by the above-described method; And (d) provides a lithium secondary battery comprising an electrolyte.

In one embodiment of the present invention, the lithium secondary battery has improved discharge capacity retention characteristics according to the use of the separator.

The present invention significantly improves the capacity retention characteristics of lithium metal electrodes using a microporous separator coated with a polydopamine polymer. In particular, the polydopamine-coated microporous separator according to the present invention exhibits excellent discharge capacity retention characteristics in a lithium secondary battery, despite the low or low ion conductivity and electrolyte impregnation amount compared to the conventional polymer surface coating membrane.

1 is a schematic diagram of a method for producing a porous polyethylene separator coated with polydopamine according to an embodiment of the present invention.
Figure 2 is a schematic diagram of the polymerization mechanism (mechanism) of the polydopamine polymer formed from dopamine in accordance with an embodiment of the present invention.
3 is a graph showing discharge capacity retention characteristics that change with charge and discharge cycles.

In order to fully understand the present invention, operational advantages of the present invention, and objects achieved by the practice of the present invention, reference should be made to the accompanying drawings and the accompanying drawings which illustrate preferred embodiments of the present invention.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the accompanying drawings. However, the present invention can be implemented in various different forms, and is not limited to the embodiments described. In order to clearly describe the present invention, parts that are not related to the description are omitted, and the same reference numerals in the drawings denote the same members.

Throughout the specification, when a part is said to "include" a certain component, it means that it may further include other components, without excluding the other components unless otherwise stated.

The present invention provides a microporous separator coated with polydopamine in order to improve the above-described problem of the lithium metal electrode, that is, the problem of discharge capacity deteriorated with the cycle. As a result, despite the progress of the cycle, a lithium secondary battery having a discharge capacity deteriorated at a low speed is possible. That is, the polydopamine-coated microporous separator according to the present invention may be cycled by the surface characteristics of excellent hydrophilicity, or the solvent may be evenly spread throughout the separator even over time. It is found that the current distribution is also maintained very uniformly, thereby generating an effect of improving the discharge capacity retention characteristics.

The microporous separator, which is mainly used as a separator for secondary batteries, has a problem in that the impregnating ability of the electrolyte is poor due to the hydrophobic surface characteristic, making it unsuitable for high output and high capacity secondary batteries. Therefore, the present invention uses a mussel-derived polymer as a coating agent to improve the surface properties on the surface of the conventional microporous membrane substrate, and by coating this, the impregnating ability of the electrolyte is improved, thereby realizing a secondary battery capable of high output and high capacity. In addition, the discharge capacity retention characteristics were also improved due to the uniform current distribution.

Mussels produce and secrete special water-insoluble adhesives and have been studied as potential sources of effective water-resistant bio-adhesives. The mussel is firmly attached to the water surface through the footpath extending from the foot, which includes a water-resistant adhesive at the end of each foot, so that the plaque can be fixed on a wet solid surface (Waite et al., Biology Review 58: 209-231 (1983)). In addition, the mussel-derived adhesive polymer is harmless to the human body and does not cause an immune response, so that it can be used as an adhesive for medical use (Dove et al., Journal of American Dental Association, 112: 879 (1986)).

Therefore, the present invention uses the mussel-derived polymer as a membrane coating agent, thereby improving both the containing property and the wettability of the electrolyte solution, thereby improving the discharge capacity retention characteristics of the secondary battery using the lithium metal electrode as a negative electrode. .

Formula 1 is a chemical structure of the membrane coating agent using the mussel-derived polymer according to the present invention.

R ₁ , R ₂ , R ₃ , R ₄ and R ₅ in Chemical Formula 1 At least one of thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide and halide, respectively ), Polyhexamethylene dithiocarbonate, polyhydroxymethylene dithiocarbonate, hydroxyl, carboxylic acid, carboxylic ester, carboxylic ester or carboxamide. The remaining R ₁ , R ₂ , R ₃ , R ₄ and R ₅ are hydrogen.

The present invention provides a hydrophilic property to the separator without damaging the pores of the hydrophobic porous separator by coating a coating agent, such as the formula (1) in a simple method. In addition, the method of manufacturing a hydrophilized porous separator capable of producing a high output battery and a high capacity battery by improving the impregnation ability of the electrolyte by improving the surface properties such as compatibility with the electrolyte and a low contact angle, and a separator for a lithium secondary battery, and The used lithium secondary battery can be provided. In addition, the discharge capacity retention characteristics of the recessed metal electrode were also improved by the separator having excellent surface properties.

The membrane coating agent according to the present invention includes a distilled water-based buffer and the compound represented by the formula (1), alcohol may be optionally added according to the description of the porous membrane. In particular, the compound represented by Chemical Formula 1 is a dopamine-based material, and the dopamine-based material is spontaneously polymerized into polydopamine, a mussel-derived polymer, in a weak base environment (pH 8.5). Form a thin polymer layer on the surface of the, the coating thickness of the separator may range from 0.001 to 1㎛.

In particular, the polydopamine formed in the separator according to the present invention not only possesses excellent chemical stability, but also effectively converts the hydrophobic surface property into hydrophilicity without damaging the pores of the microporous membrane substrate due to the thin polymer coating thickness of about 50 nm. You can expect. In addition, dopamine has been used as a cost-effective, eco-friendly, distilled water-based buffer solution (10 mM tris buffer solution, pH 8.5) in place of the costly and environmentally harmful everyday organic solvents, In order to form a polydopamine coating layer, which is a polymer derived from mussel, the solution must be maintained in a weak base (pH 8.5) state constantly.

The porous substrate of the separator according to the present invention is an olefin resin, a fluorine resin, a polyester resin, a cellulose resin, a polyamide resin, a polyimide resin, a polysulfone resin, a polyacrylonitrile resin, a polyacetal system Resin, polycarbonate-based resin, vinylidene fluoride-based resin, glass fiber to the inorganic composite may be in the form of a single or multiple layers, the pore size of the porous substrate is in the range of 0.001 to 1000㎛, porosity is 5 to 95% Range, thickness may range from 1 to 1000 μm.

Hereinafter, the present invention will be described in more detail with reference to Examples and drawings. The following embodiments are provided by way of example so that those skilled in the art can fully understand the spirit of the present invention. Therefore, the scope of the present invention is not limited thereto.

Example  One

Example  1-1

Polydopamine  Coating Membrane Manufacturing

Dopamine (2 mg / ml) was dissolved in a buffer solution (10 mM tris buffer solution, pH 8.5): methanol in a ratio of 1: 1 according to the following formula (2). After stirring for 30 seconds, the solution was impregnated with a porous polyethylene membrane (Asahi Kasei, 20 μm, porosity: 40%). After 24 hours, the separator was taken out, washed thoroughly three times with distilled water, once with acetone, and then dried in a vacuum oven at room temperature for 24 hours.

Figure 1 is a schematic diagram of a manufacturing procedure of the hydrophilic membrane of the present invention.

First, after impregnating the porous membrane with the above-mentioned dopamine-membrane coating, and then removing it after a predetermined time, a porous membrane having a hydrophilic property may be obtained. The impregnation method may be a variety of methods such as pressure coating method, spin coating method, spray method or roller coating method in addition to the general immersion coating method, all belong to the scope of the present invention.

Figure 2 shows the chemical reaction characteristics occurring in the membrane coating using dopamine. As can be seen in Figure 2, the membrane coating agent using the dopamine may be spontaneous polymerization reaction under weak base conditions (pH 8.5) to form a polydopamine polymer derived from mussels. Therefore, the pH condition of the solution in which the compound of Formula 1 is dissolved according to the present invention is preferably 7 to 11, and more preferably 7 to 9, which is a weak base condition in which the compound of Formula 1 is spontaneously polymerized.

Example  1-2

Lithium secondary battery  Produce

To prepare a lithium secondary battery consisting of a lithium metal anode / electrolyte (EC / PC = 1/1 mass ratio, 1M LiClO4), a polydopamine-surface treated polyethylene porous separator / LiCoO2 positive electrode prepared in Example 1-1, the lithium secondary The battery was charged and discharged at a charge rate of 0.5C-rate (CC / CV charge, CC discharge).

Comparative example  One

Lithium secondary battery composed of lithium metal anode / electrolyte (EC / DEC = 1/1 mass ratio, 1M LiClO4), polyethylene porous separator / LiCoO2 anode, 0.5C-rate charge and discharge conditions (CC / CV charging, CC discharge) Charge and discharge cycle was performed. At this time, the hydrophilicity of the polyethylene porous membrane is so lacking, when there is no surface treatment using the EC / PC electrolyte was not able to drive the charge and discharge at all, compared with the EC / DEC electrolyte.

Comparative example  2

0.5C-rate charge for lithium secondary battery consisting of polyethylene porous separator / LiCoO2 positive electrode coated with lithium metal negative electrode / electrolyte (EC / PC = 1/1 mass ratio, 1M LiClO4), P (VdF-co-HFP) polymer Charge and discharge cycles were performed under discharge conditions (CC / CV charging, CC discharge).

Analysis

Table 1 below is an analysis result of the electrolyte impregnation amount and the ion conductivity of the separation membrane of Example 1 and Comparative Examples 1 and 2.

Example 1 Comparative Example 1 Comparative Example 2 Electrolyte Impregnation Volume (%) 126 96 170 Ion Conductivity (mS / cm) 0.41 0.23 1.4

Referring to the results of Table 1, it can be seen that the separation membrane according to Example 1, the impregnation amount and ionic conductivity of the electrolyte is particularly smaller than the separation membrane according to Comparative Example 2.

3 is a graph showing discharge capacity retention characteristics that change as the charge and discharge cycle progresses.

Referring to FIG. 3, it can be seen that the lithium secondary battery according to Example 1 exhibits excellent discharge capacity retention characteristics compared to Comparative Examples 1 and 2. In particular, the polydopamine-treated porous separator according to the present invention exhibits excellent discharge capacity retention characteristics in spite of the lower electrolyte impregnation amount and ion conductivity than the separator of Comparative Example 2.

The results shown in FIG. 3 are new results that cannot be predicted in the prior art. In the case of microporous separators surface-modified with polydopamine according to the present invention, the hydrophilic surface characteristics by the polydopamine substrate uniformly and thinly formed throughout the separator are shown. Indicates. This indicates that the solvent may be evenly spread throughout the separator even over time or with repeated charge and discharge, which means that the current distribution in the charge and discharge process can be maintained very uniformly, even though the polydopamine surface is modified. Even though the separator has a smaller ion conductivity and electrolyte impregnation amount than the polymer surface coating membrane, the discharge capacity retention characteristics can be improved more effectively by controlling the dendrite of lithium.

Furthermore, the liquid electrolytes generally used have safety concerns in batteries with high energy density due to their flammable properties. When using the polydopa only treatment membrane according to the present invention, the discharge capacity is excellent with a minimum amount of electrolyte. You can implement the property.

Claims (11)

The method of improving the discharge capacity retention characteristics of a lithium secondary battery comprising a lithium metal electrode and a separator as a negative electrode, the method
Method for improving the discharge capacity of the lithium secondary battery, comprising the step of surface treatment of the separator of the lithium secondary battery with a compound of the formula (1).

(One)
In which R ₁ , R ₂ , R ₃ , R ₄ and R ₅ At least one of thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide and halide, respectively ), Polyhexamethylene dithiocarbonate, polyhydroxymethylene dithiocarbonate, hydroxyl, carboxylic acid, carboxylic ester, carboxylic ester or carboxamide. R ₁ , R ₂ , R ₃ , R _4, and R ₅ except for these are hydrogen) The method of claim 1,
The separator is an olefin resin, fluorine resin, polyester resin, cellulose resin, polyamide resin, polyimide resin, polysulfone resin, polyacrylonitrile resin, polyacetal resin, polycarbonate resin , Vinylidene fluoride-based resin, glass fiber to inorganic composites comprising a porous substrate comprising a single or multiple layers, lithium secondary battery discharge capacity improvement method. The method of claim 2,
The pore size of the porous substrate is 0.001 to 1000㎛ range, porosity is 5 to 95% range, thickness is a lithium secondary battery discharge capacity improvement method, characterized in that 1 to 1000㎛ range. The method of claim 3, wherein
Coating thickness of the separator is a lithium secondary battery discharge capacity improvement method characterized in that the range of 0.001 to 1㎛. The separator surface treatment method used in the lithium secondary battery discharge capacity improving method according to any one of claims 1 to 4, wherein the method
Dissolving the compound of Formula 1 in a solution of pH 7-11; And
Separation membrane surface treatment method comprising the step of immersing the porous substrate in the solution.

(One)
In which R ₁ , R ₂ , R ₃ , R ₄ and R ₅ At least one of thiol, primary amine, secondary amine, nitrile, aldehyde, imidazole, azide and halide, respectively ), Polyhexamethylene dithiocarbonate, polyhydroxymethylene dithiocarbonate, hydroxyl, carboxylic acid, carboxylic ester, carboxylic ester or carboxamide. R ₁ , R ₂ , R ₃ , R _4, and R ₅ except for these are hydrogen) 6. The method of claim 5,
The compound surface treatment method of the membrane, characterized in that to cause a polymerization reaction in the solution. The method according to claim 6,
Separation coating method, characterized in that any one of immersion coating method, pressure coating method, spin coating method, spray method or roller coating method by impregnating the porous substrate in the solution. 8. The method of claim 7,
Separator surface treatment method characterized in that the solvent of the solution is water or alcohol or a mixture thereof. Separation membrane surface treated by the method according to claim 5. (a) an anode; (b) a negative electrode made of lithium metal; (c) a separator provided between the positive electrode and the negative electrode according to claim 9; And (d) an electrolytic solution. The method of claim 10,
The lithium secondary battery is a lithium secondary battery, characterized in that the discharge capacity retention characteristics improved by using the separator according to claim 9.

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