KR101098377B1 - Method for pre-doping with lithium ion for lithium ion capacitors - Google Patents

Abstract

The present invention relates to a high efficiency lithium ion predoping method for a lithium ion capacitor, and more particularly, a high efficiency lithium ion predoping method for imparting functionality to a graphite surface by reacting graphite with a lithium ion reagent and predoped by the above method. It relates to graphite for lithium ion capacitors.
According to the lithium ion pre-doping method of the present invention, an optimal pre-doping amount can be realized by a simpler method than the conventional method, and the control of the pre-doping amount is easy and uniform by introducing lithium quantitatively through chemical bonding to the graphite electrode surface. To be distributed. In addition, there is an advantage that there is no lithium metal remaining after the oxide film is formed on the graphite surface.

Description

High Efficiency Lithium Ion Pre-doping Method for Lithium-ion Capacitors {METHOD FOR PRE-DOPING WITH LITHIUM ION FOR LITHIUM ION CAPACITORS}

The present invention relates to a high efficiency lithium ion predoping method for a lithium ion capacitor, and more particularly, a high efficiency lithium ion predoping method for imparting functionality to a graphite surface by reacting graphite with a lithium ion reagent and predoped by the above method. It relates to graphite for lithium ion capacitors.

Recently, electric double-layer capacitors (EDLCs) and lithium-ion secondary batteries (LIB, Lithium ion battery) among power sources with excellent energy density and output characteristics such as household electronic devices, personal information communication portable devices and hybrid electric vehicles ) Has been adopted and widely used.

In addition, as interest in environmental and alternative energy-related issues increases, facilities for hybrid electric vehicles that replace fuel from existing vehicles or environmentally friendly energy storage systems by photovoltaic and wind power generation are becoming more efficient and functional, and power output increases. As demand increases, new power supplies with improved energy and power densities are needed.

In order to satisfy these conditions, lithium ion secondary batteries (LIB) having advantages in high energy density and stable output characteristics as well as using carbon materials other than graphite and activated carbon as electrode active materials or performing surface treatment on the electrode active materials (LIB) ) And the use of power doubled capacitors (EDLCs) with high power and semi-permanent lifetime in parallel are increasing.

However, there is a disadvantage in that the voltage must be matched for parallel connection by using the voltage of the electric double layer capacitors (EDLCs) and the rated voltage of the lithium ion secondary battery (LIB) used as the hybrid power source, and through this process, the internal resistance is doubled. There is a growing problem.

Recently, a new power source device, called a hybrid capacitor, which combines electric double layer capacitors (EDLCs) and lithium ion secondary batteries (LIB) with different mechanisms for positive and negative electrodes, has attracted attention. Compared to symmetrical electrodes of EDLCs, electrodes of lithium ion capacitors (LICs), which are a kind of hybrid capacitors, have an asymmetric structure, and carbon materials are used for the electrodes. In this case, the energy density can be greatly increased by lowering the negative electrode potential after doping lithium ions using a chemical method and an electrochemical method to a carbon material which can continuously occlude and desorb lithium ions. In addition, since the active material of the positive electrode and the negative electrode can be used differently and each active material can be adjusted, it is possible to maximize the advantage that the use voltage and capacity can be adjusted.

As described above, various forms have been developed as high power and high density power supply devices, but are not technically perfect at present. Li-ion capacitors proposed as such alternatives also require further improvements in energy density, power density, cycle life and stability.

Particularly, graphite, which is mainly used as a negative electrode active material of a lithium ion capacitor, has a low potential of 0.1 to 0.3 V compared to lithium and is an excellent electrode material having good cycle characteristics and economy, but has a disadvantage in that an initial irreversible specific cost is large.

This high initial irreversible specific amount is mainly caused by the solid electrolyte interphase (SEI) formed on the graphite surface as the organic solvent decomposes between the electrolyte and the electrode active material interface, which is essential for the charge and discharge mechanism. However, it is a factor that reduces the energy density of lithium ion capacitors. In addition, since the lithium ions consumed to form such a film do not substantially participate in the capacitance, the function, and the electrochemical characteristics of the power storage device, when the amount of lithium silver related to the irreversible capacity is more than necessary, the capacity of the power storage device as a whole is increased. It will adversely affect the improvement. Therefore, it is necessary to control the pre-doping amount of lithium through an appropriate pre-doping method.

Accordingly, the present invention has been made to solve the above problems and by the necessity of the above, an object of the present invention is a method of pre-doping lithium ions in an optimal way to the negative electrode active material in order to obtain a lithium ion capacitor having excellent electrochemical properties In providing. In addition, the pre-doping does not occur for the extra lithium ions not actually involved in charging and discharging to achieve high energy and high density.

In order to achieve the above object, the present invention provides a high efficiency lithium ion pre-doping method.

The present invention provides a highly efficient lithium ion pre-doping method comprising preparing a lithium ion reagent and immersing the graphite in the lithium ion reagent, followed by ultrasonic dispersion treatment.

In the present invention, the lithium ion reagent is characterized in that any one of a LiCl solution or Li ₂ CO ₃ solution, the solution is LiCl or Li ₂ CO ₃ concentration in the solution is preferably 0.5 ~ 2.0 M.

The ultrasonic dispersion step of the present invention is performed for 10 to 60 minutes, the immersion step is characterized in that for 30 minutes to 3 hours.

In addition, the present invention may further comprise a step of purifying the graphite, the step of purifying the graphite by-products by attaching the natural graphite to the acid solution mixed with sulfuric acid and nitric acid in a volume ratio of 1: 3 at room temperature for 5 to 10 hours. It characterized in that to remove.

In another aspect, the present invention provides a graphite for lithium ion capacitor doped with lithium ions by the high efficiency lithium ion pre-doped method.

In the present invention, the graphite for lithium ion capacitor is characterized in that the lithium ion doping amount in the graphite is 0.1 ~ 1.0wt.%, The discharge capacity is 300 ~ 500 mAh / g.

The high efficiency lithium ion pre-doping method of the present invention is a technique for improving the electrochemical characteristics of power storage devices, and is applicable to electrode materials introduced into power storage devices such as lithium ion secondary batteries and lithium ion capacitors. Especially, it is applicable to the graphite mainly used for a negative electrode material.

According to the lithium ion pre-doping method of the present invention, an optimal pre-doping amount can be realized by a simpler method than the conventional method, and the control of the pre-doping amount is easy and uniform by introducing lithium quantitatively through chemical bonding to the graphite electrode surface. To be distributed. In addition, there is an advantage that there is no lithium metal remaining after the oxide film (SEI, Solid electrolyte interphase) is formed on the graphite surface.

1 is a flow chart of a high efficiency lithium ion predoping method according to the present invention.
FIG. 2 is a graph showing XRD characteristics (lithium salt solution: 0.1 M LiCl) of lithium ion-doped graphite according to one embodiment of the present invention.
3 is a graph showing the impedance change (lithium salt solution: 0.1 M LiCl) of the lithium ion-doped graphite according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The present invention provides a high efficiency lithium ion predoping method for a lithium ion capacitor.

Specifically, the high-efficiency lithium ion pre-doping method of the present invention includes the steps of preparing a lithium ion reagent and the step of immersing the graphite after the ultrasonic dispersion treatment by putting graphite in the lithium ion reagent.

In the present invention, the lithium ion reagent uses LiCl solution or Li ₂ CO ₃ solution as the lithium metal salt reagent. In one embodiment of the present invention, the LiCl solution was prepared by putting LiCl powder in tetrahydrofuran (THF, tetrahydrofuran) and stirring for 1 hour, Li ₂ CO ₃ solution was put Li ₂ CO ₃ powder in distilled water for 1 hour Prepared by stirring. In the present invention, the concentration of the LiCl solution or Li ₂ CO ₃ solution is characterized in that 0.5 ~ 2.0 M.

In the present invention, the ultrasonic dispersion step is performed for 10 to 60 minutes, the immersion step is characterized in that for 30 minutes to 3 hours.

In the present invention, the graphite is preferably subjected to a purification step before the pre-doping. The purification step of graphite is to remove by-products by adding natural graphite to an acid solution containing sulfuric acid (H ₂ SO ₄ ) and nitric acid (HNO ₃ ) in a volume ratio of 3: 1 for at least 5 hours. Structural defects occurring during the natural production of graphite and defects within the graphite may be removed, and lithium ion predoping may be more easily induced by inducing negative charges on the graphite surface.

In another aspect, the present invention provides a graphite for lithium ion capacitor doped with lithium ions by the high efficiency lithium ion pre-doped method. The lithium ion doping amount in the graphite for the lithium ion capacitor is 0.1 ~ 1.0wt.%, The discharge capacity of the graphite is preferably 300 ~ 500 mAh / g.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention and the scope of the present invention is not limited thereto.

Example 1.

The graphite was sonicated for 30 minutes in LiCl solution prepared at 0.1 M concentration, and then immersed for 2 hours to pre-dope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 2.

The graphite was sonicated for 30 minutes in LiCl solution prepared at 0.5 M concentration, and then immersed for 2 hours to pre-dope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 3.

The graphite was sonicated for 30 minutes in LiCl solution prepared at 1.0 M concentration, and then immersed for 2 hours to predope lithium. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 4.

The graphite was sonicated for 30 minutes in LiCl solution prepared at a concentration of 1.5 M, and then immersed for 2 hours to pre-dope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 5.

The LiCl solution prepared at 2.0 M concentration was ultrasonically dispersed for 30 minutes, and then immersed for 2 hours to predope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 6.

Graphite was sonicated for 30 minutes in LiCl solution prepared at 1.5 M concentration, and then immersed for 1 hour to predope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 7.

The graphite was sonicated for 30 minutes in a Li ₂ CO ₃ solution prepared at 0.1 M concentration, and then immersed for 2 hours to predope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 8.

The graphite was sonicated for 30 minutes in a Li ₂ CO ₃ solution prepared at a concentration of 1.0 M, and then immersed for 2 hours to predope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

Example 9.

The graphite was sonicated for 30 minutes in a Li ₂ CO ₃ solution prepared at a concentration of 2.0 M, and then immersed for 2 hours to predope lithium ions. Finally, the filtered lithium-doped graphite was dried in a vacuum oven at 120 ° C. for 24 hours and then used as a negative electrode active material for a lithium ion capacitor to prepare a cell described above.

In order to investigate the electrochemical property change of the graphite which was not pre-doped, the following comparative example was prepared.

Comparative Example 1.

Graphite without lithium ion predoping was used as a negative electrode active material for a lithium ion capacitor, and the above-described cell was manufactured.

The changes in the crystal structure (XRD curve) and impedance (Z ′: real part) values of the untreated graphite and the lithium ion pre-doped graphite as described above are shown in FIGS. 2 and 3, respectively. Table 1 shows the lithium doping amount of the treated graphite and the resulting capacitance of the cell.

TABLE 1

Experimental Example 1. Fabrication of lithium ion capacitor half cell

The lithium ion capacitor cell for evaluating the electrochemical properties of the present invention was fabricated with a half cell and is composed of a positive electrode, a negative electrode, and an electrolyte.

As an electrolyte, an aprotic organic solvent electrolyte of lithium salt can be used, and preferably ethylene carbonate, propylene carbonate, acetonitrile, tetrahydrofuran (THF) and methylene chloride ( It is preferable to use at least one organic solvent solution selected from the group consisting of methylene chloride). In an embodiment of the present invention, a solution of diethyl carbonate (DEC) and ethylene carbonate (ethylene carbonate, EC) in a volume ratio of 1: 1 was used in a solution of 1.0 M LiPF ₆ .

As the positive electrode active material, it is preferable to use a material capable of reversible doping and dedoping of lithium ions and anions. Preferably, it is best to use activated carbon, a conductive polymer, and a polyacene-based material, and more preferably, activated carbon having an excellent specific surface area and well-developed pore structure.

As a separator, a polyethylene (PE) film was used, and the positive electrode and the negative electrode were kept short at 2.0 V after shorting.

[Production of Anode Electrode]

Activated carbon (RP-20, Kurade Academical, specific surface area 1,400 ~ 1,600 m ² / g) was used as the positive electrode active material, PVdF (Polyvinylidenefluoride) was used as the binder, and the active material and the binder were composed of a weight ratio of 85: 15 to form an electrode. It was. The components of the electrode were stirred with NMP (N-methylpyrrolidone) solvent to prepare a slurry, and then the solvent was dried to remove. The dried slurry was rolled, coated on aluminum foil, cut to a size of 2 × 2 cm, and dried in a vacuum oven at 120 ° C. for 24 hours to prepare an electrode.

[Production of Cathode Electrode]

In the present invention, graphite (Aldrich Co.) shown in Table 2 was used as the negative electrode active material, and each cell was fabricated using lithium pre-doped graphite as the negative electrode active material, and the electrochemical characteristics thereof were evaluated. .

 TABLE 2

The above-mentioned graphite was mixed with PVdF (Polyvinylidenefluoride) as a binder so that each weight ratio might be 85:15, and the electrode was comprised. A slurry prepared by stirring an active material and a binder with an N-methylpyrrolidone (NMP) solvent was prepared and coated on an aluminum foil, and then the electrode was cut into a size of 2 × 2 cm and dried in a vacuum oven at 120 ° C. for 24 hours. Was prepared.

[Production of Laminated Half Cells]

A lithium electrode was used as a reference electrode corresponding to each electrode, and a separator was inserted between the electrodes, and then immersed in the electrolyte to finally produce a half cell. Each electrode manufactured as described above was carried out in a dry room in which a vacuum state was maintained, and the measurement was performed after being immersed in the prepared electrolyte solution for 24 hours prior to the charge and discharge test.

In the present invention, the capacity of the cell is defined as in Equation 1 below.

[Equation 1]

The capacitance of the cathode cell is the product of the voltage change and the capacitance of the cell, and the unit is C (coulomb), but 1 C is the amount of charge when a current of 1 A flows for 1 second. It was described by the value (mAh / g) divided by the weight (g) of the active material which comprises the following.

Experimental Example 2 Measurement of Characteristic Values of Graphite Doped with Lithium-ion

In the present invention, each characteristic value was measured by the following method.

1. Observation of Crystal Structure Change of Lithium Ion Predoping

The surface crystal properties of the lithium ion predoped graphite prepared according to the present invention were observed through XRD (X-ray diffractometer, D / MAX 2200V / PC, Rigaku), and the crystal structure of graphite when lithium ion was predoped The purpose of this study was to determine the effect and the doping form.

2. Measurement of Lithium-Ion Pre-Doping of Graphite by Pre-doping Method

AAS (Atomic Absorption Spectroscopy, AA-SCAN, Thermo Jarrell Ash) analysis method was used to measure the lithium ion pre-doping amount of the graphite prepared according to the present invention, which was described in wt.% Units by weight.

3. Evaluation of Discharge Capacity Characteristics of Graphite with Li-Ion Pre-doping

Finally, the pre-doping amount according to the pre-doping method of lithium ions was confirmed, and the electrochemical properties of the graphite were evaluated through the charge-discharge test using an electronic-chemical analyzer (IVIUMSTAT, HS Technology). The capacity of the cell was confirmed. The driving voltage was 0 to 3 V, and the current density was measured at 0.3 mA / cm ² within the range of 0.1 C-rate to 10 C-rate, and the discharge capacity was calculated as described above.

4. Evaluation of Impedance Characteristics of Graphite by Li-ion Pre-doping

Impedance measurement of lithium-ion pre-doped graphite was performed using an Electronic-Chemical Analyzer (IVIUMSTAT, HS Technology) with an amplitude value of 50 mV in the frequency range of 10 mHz to 100 kHz.

Claims (10)

In the high efficiency lithium ion predoping method,
Preparing a lithium ion reagent with a LiCl solution or a Li ₂ CO ₃ solution at a concentration of 0.5 to 2.0 M; High efficiency lithium ion predoping method. delete delete delete The method of claim 1,
The immersion step is a high efficiency lithium ion pre-doping method, characterized in that for 30 minutes to 3 hours. The method of claim 1,
A high efficiency lithium ion predoping method further comprising the step of purifying graphite. The method of claim 6,
The step of purifying the graphite is a high efficiency lithium ion pre-doping method characterized in that by removing the by-products by attaching natural graphite in an acid solution of sulfuric acid and nitric acid in a volume ratio of 1: 3 at room temperature for 5 to 10 hours. delete Lithium ion doping amount in the graphite is 0.1 ~ 1.0wt.%, The graphite discharge capacity, characterized in that the discharge capacity of the graphite is 300 ~ 500 mAh / g. delete

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