KR102311173B1 - Self-standing gelled polymer compound utilizing LiFSI and use as gelled polymer electrolyte thereof for flexible lithium-ion battery - Google Patents

Abstract

The present invention provides a self-supporting gelling polymer compound composed of lithium bis(fluorosulfonyl)imide (LiFSI) and poly(ethylene glycol) dimethacrylate (PEGDMA) and use of the same as a gel polymer electrolyte for flexible lithium ion batteries.
The free-standing gelling polymer compound of the present invention exhibits excellent thermal stability at 150 ° C. or less, a very low glass transition temperature ( T _g ) (about -75 ° C), and plastic crystal behavior, resulting in excellent room temperature ionic conductivity. It has been confirmed that it can be usefully used as an electrolyte applied to a flexible lithium-ion battery in the lithium-ion battery field.

Description

Self-standing gelled polymer compound utilizing LiFSI and its use as a gel polymer electrolyte for a flexible lithium-ion battery

The present invention relates to a self-supporting gelling polymer compound utilizing LiFSI and its use as a flexible lithium ion battery gel polymer electrolyte, and more particularly, lithium bis(fluorosulfonyl)imide [LiFSI] And poly (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA] It relates to a self-supporting gelling polymer compound and its use as a gel polymer electrolyte for flexible lithium ion batteries.

Lithium-ion batteries (LIBs) have received a lot of attention for decades (since 1991) due to their high energy density, long lifespan, and high efficiency, and have been widely applied in various mobile devices, electric vehicles, and energy storage fields. Like other types of batteries, LIBs have three essential components: an anode, a cathode, and an electrolyte. Here, the electrolyte plays an important role in transporting lithium ions between a pair of electrodes and determines the rate of energy released. Organic liquid electrolytes have been commercialized for several decades, and now occupy a large portion of the battery market due to their excellent room temperature ionic conductivity. However, safety issues arising from leakage and flammability of flammable organic liquids are issues that must be addressed as quickly as possible. Recently, a solid polymer electrolyte (SPE) has been introduced as a strategy to replace the conventional organic liquid electrolyte in order to solve the above-mentioned problem. Extensive studies have shown that SPE not only has excellent safety, but also inhibits the growth of lithium dendrites. In addition, SPE can be used as a separator in LIB because of its excellent mechanical stability. In general, SPEs are composed of a polymer and a lithium salt, where the polymer acts as a host, allowing lithium ions to migrate within the free volume provided by the polymer. poly (ethylene oxide), (PEO)], poly (acrylonitrile) [poly (acrylonitrile), (PAN)], poly (methyl methacrylate) [poly (methyl methacrylate), (PMMA) )], poly (vinylidene fluoride-hexafluoro propylene) [poly (vinylidene fluoride-hexafluoro propylene), (PVdF-HFP)], etc. have been reported and studied as polymer hosts. ^{However, the poor room temperature ionic conductivity (10 −5} cm/s) of the SPE caused by the large interfacial resistance between the electrode and the SPE is a major barrier limiting the widespread application of LIBs. To overcome this barrier, a high-performance gelled polymer electrolyte (GPE) has gained great interest and has been investigated by some researchers. Traditional GPEs are usually composed of SPEs used in liquid electrolytes and plasticizers or organic solvents. That is, GPE can provide high ionic conductivity (10 ^-3 cm/s) similar to organic liquid electrolytes at room temperature, and at the same time maintain mechanical stability similar to SPE. However, as the concentration of the plasticizer or organic solvent increases, it is generally recognized that the mechanical stability of the GPE decreases even though the ionic conductivity is improved. Therefore, the same problem with organic liquid electrolytes must be considered again. As a result, a demand for a high-performance electrolyte with secured safety is currently emerging.

Korean Patent Publication No. 10-2018-0121391 (2018.11.07.) Korean Patent Publication No. 10-2016-0140211 (2016.12.07.)

While the inventors of the present invention were studying a high-performance electrolyte with excellent ionic conductivity while maintaining the mechanical stability of the gelled polymer electrolyte, lithium bis(fluorosulfonyl)imide [LiFSI] was used as a methacrylic-based electrolyte. A novel self-standing gelled polymer without solvent or plasticizer by mixing it with the monomer poly (ethylene glycol) dimethacrylate [PEGDMA], homogenizing it, and curing it with UV. electrolyte, SGPE) was prepared, and it was confirmed that the prepared SGPE exhibited high flexibility and strong mechanical stability, as well as excellent physicochemical and electrochemical properties.

Accordingly, the present invention provides a self-supporting gelling polymer compound composed of lithium bis(fluorosulfonyl)imide [LiFSI] and poly(ethylene glycol) dimethacrylate [PEGDMA] and use of the same as a gel polymer electrolyte for flexible lithium ion batteries aim to do

According to one aspect of the present invention, lithium bis(fluorosulfonyl)imide [LiFSI] and poly (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA] are constituted Provided is a self-supporting gelling polymer compound containing as a unit.

In one embodiment, LiFSI and PEGDMA may be included in the polymer compound in a weight ratio of 15:85 to 45:55.

In one embodiment, the PEGDMA may have a number average molecular weight (Mn) of 100 to 1000.

In one embodiment, the polymer compound may be in the form of a film, and may have a thickness of less than 500 μm.

According to another aspect of the present invention, (a) dissolving LiFSI in PEGDMA to prepare a precursor solution; (b) adding a photoinitiator to the precursor solution and stirring by adding an organic solvent; (c) casting the stirred precursor solution on a plate and evaporating the organic solvent to obtain a precursor electrolyte modeling; and (d) irradiating a UV lamp for modeling the precursor electrolyte to obtain an electrolyte film.

In one embodiment, the LiFSI and PEGDMA of step (a) may be mixed in a weight ratio of 15:85 to 45:55.

In an embodiment, the photoinitiator of step (b) may be 2-hydroxy-2-methylpropiophenone (HMPP), and may be added in an amount of 0.1 to 5% by weight of PEGDMA. have.

In one embodiment, the organic solvent of step (b) may be acetone, and may be added in a ratio of organic solvent: precursor solution = 30: 1 to 1:1 (v/w).

In one embodiment, the UV lamp of step (d) ^{may have an irradiation peak intensity of 100 to 300 mW/cm 2} , and the irradiation may be performed for 10 to 100 minutes.

According to another aspect of the present invention, there is provided a self-supporting gel polymer electrolyte for a lithium ion battery comprising the polymer compound.

According to another aspect of the present invention, there is provided a lithium ion battery including the self-supporting gel polymer electrolyte.

The self-standing gelling polymer compound utilizing LiFSI of the present invention exhibits excellent thermal stability at 150 ° C or less, a very low glass transition temperature ( T _g ) (about -75 ° C), and calcination crystal behavior, resulting in excellent room temperature. It was confirmed that it exhibits ionic conductivity.

Therefore, the self-supporting gelling polymer compound of the present invention can be usefully used as an electrolyte applied to a flexible lithium-ion battery in the lithium-ion battery field.

1 is a photograph of the physical appearance of SGPE at room temperature.
2 is an FT-IR spectrum of SGPE before/after UV-curing.
3 is a graph showing the thermal stability _{of 1M LiPF 6} in pure PEGDMA, SGPE and EC/DMC.
4 is a graph showing the glass transition temperature of (a) PEGDMA and (b) SGPE.
5 is an XRD pattern of pure PEGDMA, PEGDMA film and SGPE.
6 is a Nyquist plot of (a) SGPE30 and (b) SGPE40.
7 is a graph showing the temperature-dependent ionic conductivity of SGPE30 and SGPE40.
8 is a schematic diagram showing a process for preparing SGPE by UV-curing.

The present invention is a self-supporting type comprising lithium bis(fluorosulfonyl)imide (LiFSI) and poly (ethylene glycol) dimethacrylate (PEGDMA) as constituent units. A gelled polymer compound is provided.

In the present invention, a series of high ion conductivity self-standing gelled polymer electrolytes (SGPE) were prepared through UV-curing. UV-curing is a technique applied in the coating industry to protect the surface of various substrates. This is an easy and fast polymerization method, which is more environmentally friendly and can be formulated without a solvent. In addition, the smooth, smooth appearance produced by UV curing can provide good interfacial contact with the electrode. At the time of the polymer matrix, low molecular weight UV-curable PEGDMA [(poly (ethylene glycol) dimethacrylate)] was introduced as a new polymer matrix. These PEOs and their derivatives (PEGDMA) have very high dielectric constants and thus high solubility of lithium salts. The ethylene oxide unit of PEGDMA binds lithium ions and then transports the ions by hopping in the polymer matrix. Therefore, the high chain flexibility of PEGDMA is expected to enhance ion transport and ion conductivity. In the case of lithium salts, LiFSI has been extensively studied as an electrolyte salt because of its lower viscosity and better properties such as T _{g than LiTFSI or LiPF 6 .} In addition, LiFSI has a significant effect on reducing the crystallinity of the electrolyte. Anhydrous acetone is added during the manufacturing process to obtain a homogeneous precursor electrolyte solution, but is completely removed before the UV-curing process. Thus, SGPE is prepared without solvents and plasticizers.

To compare the effect of LiFSI on SGPE properties, various weight ratios of SGPE were prepared. Polymerization of SGPE was all performed by Fourier transformation infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (X-ray). ray diffraction, XRD), and ion conductivity was measured by electrochemical impedance spectroscopy (EIS).

In one embodiment, LiFSI and PEGDMA may be included in the polymer compound in a weight ratio of 15:85 to 45:55, preferably 20:80 to 40:60, more preferably 30:70 to 40:60 by weight.

In one embodiment, the PEGDMA may have a number average molecular weight (Mn) of 100 to 1000, preferably 200 to 800, more preferably 400 to 600.

In one embodiment, the polymer compound may be in the form of a film, and may have a thickness of less than 500 μm, preferably 400 μm, more preferably 300 μm.

The present invention also comprises the steps of (a) dissolving LiFSI in PEGDMA to prepare a precursor solution; (b) adding a photoinitiator to the precursor solution and stirring by adding an organic solvent; (c) casting the stirred precursor solution on a plate and evaporating the organic solvent to obtain a precursor electrolyte modeling; and (d) irradiating a UV lamp for modeling the precursor electrolyte to obtain an electrolyte film.

The preparation method of the present invention comprises dissolving LiFSI in PEGDMA to prepare a precursor solution [ie, step (a)]. In step (a), the mixing ratio of LiFSI and PEGDMA is as described above.

The preparation method of the present invention includes the step of adding a photoinitiator to the precursor solution obtained in step (a) and stirring by adding an organic solvent [ie, step (b)]. In step (b), the photoinitiator may be 2-hydroxy-2-methylpropiophenone (HMPP), 0.1 to 5% by weight of PEGDMA, preferably 0.5 to 3% by weight may be added. In addition, the organic solvent used in step (b) may be used without limitation as long as it is an organic solvent capable of homogeneously dissolving LiFSI and PEGDMA, preferably from the group consisting of acetone, dimethyl carbonate, ethylene carbonate, and ethylmethyl carbonate. It may be one or more selected, more preferably anhydrous acetone, but is not limited thereto. The organic solvent may be added in a ratio of organic solvent:precursor solution = 30:1 to 1:1 (v/w), preferably 10:1 to 2:1 (v/w). All the above starting materials are mixed with an organic solvent and stirred at room temperature for 3 to 4 hours until dissolved to prepare a homogeneous solution.

The preparation method of the present invention includes casting the stirred precursor solution obtained in step (b) on a plate and evaporating the organic solvent to obtain precursor electrolyte modeling [ie, step (c)]. The plate used in step (c) may be used without limitation as long as it is a plate capable of forming a precursor solution in which all of the starting materials are dissolved in a film form, and preferably a glass plate may be used.

The manufacturing method of the present invention includes a step of obtaining an electrolyte film by irradiating a UV lamp to the precursor electrolyte modeling obtained in step (c) [ie, step (d)]. The UV lamp used in step (d) ^{may have an irradiation peak intensity of 100 to 300 mW/cm 2} , and the irradiation may be performed for 10 to 100 minutes.

The present invention also provides a self-supporting gel polymer electrolyte for a lithium ion battery comprising the polymer compound.

The present invention also provides a lithium ion battery comprising the self-supporting gel polymer electrolyte.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are intended to illustrate the present invention, and the scope of the present invention is not limited thereto.

<Example>

1. Experiment

1.1 Materials

Polyethylene glycol dimethacrylate (PEGDMA, average Mn 550) and photoinitiators 2-hydroxy-2-methylpropiophenone (HMPP, 99%), ethylene carbonate and dimethyl carbonate (50v/50v) ) solution of lithium hexafluorophosphate (1.0 M LiPF ₆ EC/DMC) was purchased from Aldrich. LiFSI was obtained from Chunbo Co., Ltd. The Hg UV lamp was purchased from Lichtzen as having an irradiation peak intensity of ^{200 mW/cm 2 .} The acetone solvent was purified by distillation to remove water. The UV polymerization process and chemical structure are shown in FIG. 8 .

1.2 Preparation of Freestanding Gelling Polyelectrolyte

Different weight ratios of LiFSI were dissolved in PEGDMA to prepare precursor solutions of SGPE. Then, the photoinitiator HMPP was added to the solution, wherein the concentration of HMPP was 1-2% by weight of the amount of PEGDMA. Anhydrous acetone was finally added to obtain a homogeneous solution, and the ratio of acetone to the mixed solution was 3 : 1 (v/w). Then, it was stirred at room temperature for 3-4 hours until all starting materials were dissolved. The precursor solution was cast on a glass plate (8 cm x 8 cm) attached with a silicone model (6 cm x 6 cm inside), and then anhydrous acetone was removed. Finally, the precursor electrolyte modeling was exposed to Hg UV lamp irradiation (Lichtzen) for 20 min to obtain a freestanding gelling polyelectrolyte. Here, 'n' in sample SGPEn means wt% of LiFSI in the electrolyte.

1.3 Characteristics of Freestanding Polyelectrolyte

Polymerization of SGPE was confirmed by FT-IR (Nicolet iS5 FTIR spectrometer) with a spectral resolution of ^{4 cm -1} at room temperature. The thermal stability behavior was measured with a thermogravimetric analyzer (Perkin-Elmer TGA-7) from 25° C. to 500° C. at a heating rate of 10° C./min under a nitrogen flow rate of 10 ml/min. The weight of the sample was adjusted to 3 mg to 5 mg. The glass transition temperature ( T _g ) of SGPE was detected with DSC (Perkin-Elmer DSC 6000) at -80 °C to 120 °C at a heating rate of 10 °C/min under a nitrogen gas atmosphere. The plastic crystal behavior of SGPE and PEGDMA was verified using XRD (Dmax2500/PC). Samples were scanned from 10° to 80° at room temperature at a rate of 2°/min. To calculate the ionic conductivity (σ, mS/cm), EIS was measured using a home-made two-electrode cell from 30 °C to 90 °C in the frequency range of 0.1 Hz to 1 MHz with an amplitude of 5 mV. To assemble the home-made cell, the SGPE was sandwiched between two conductive nickel foil disks and placed between two FTO glass electrodes. Conductivity was calculated by Equation 1 below.

<Formula 1>

σ = l / (R Χ A)

Here, ‘l’ and ‘A’ represent the thickness and area of SGPE, respectively.

2. Results and Discussion

A series of SGPEs were prepared from the precursor mixture solution through UV-curing polymerization, and the appearance of the prepared SGPEs is shown in FIG. 1 . The precursor solution turned into a transparent and flexible electrolyte film that could be easily peeled off the glass plate. The thickness of SGPE was controlled to be less than 300 μm by adjusting the silicon model size and the amount of precursor solution. However, SGPE50 could not be prepared due to the excessive lithium salt content and it was not possible to produce a complete electrolyte film. The chemical structure and UV polymerization of SGPE were confirmed by FT-IR. Before UV-curing, as shown in FIG. 2 , PEGDMA characteristic absorption peaks of C=C and C=O double bonds exist ^{at 1640 cm −1} and 1725 cm ^{−1 , respectively.} After UV-curing, only the C=O double bond remained at 1725 cm ⁻¹ and the peak of the C=C double bond disappeared. In addition, typical S=O stretch peaks of LiFSI are still located at 1376 cm ⁻¹ and 1172 cm ⁻¹ before or after UV-curing. FT-IR graphs suggest that polymerization of SGPE was achieved after UV-curing and LiFSI was still in the curing process.

Overheating is one of the most common causes of LIB accidents, and heat is generated during battery operation. As a result, all components included in batteries, with or without electrolytes or separators, are required to have good thermal stability. Thermal properties were detected by TGA and DSC, respectively. As shown in Fig. 3, it is evident that pure PEGDMA has the best thermal stability, and the weight loss starts at 175°C. In contrast, the commercial liquid electrolyte (1.0 M LiPF ₆ in EC/DMC) exhibited the lowest thermal durability and rapidly decomposed in weight from the start. For SGPE, the weight loss decreased slightly at 150 °C and decomposed significantly at 210 °C, which corresponds to the thermal stability of LiFSI and PEGDMA. From Fig. 3, it is indicated that, compared with commercial liquid electrolytes, SGPE has more promising thermal stability, which can improve battery thermal stability and safety.

According to the Vogel-Tammann-Fulcher behavior, the T _g of a polymer is related to its free volume and ionic conductivity. In general, the ionic conductivity correlates with the T _g , so increasing the ionic conductivity of the electrolyte should decrease the T _{g of the material.} In Figure 4, SGPE exhibits very low T _g of -75.33 °C (SGPE20), -74.47 °C (SGPE30) and -74.02 °C (SGPE40), respectively, which is much lower than pure PEGDMA (-69.54 °C). A slight increase is observed with decreasing PEGDMA concentration, but the T _g is still maintained at around -74 °C. In addition, SGPE has no endothermic peak observed with increasing temperature, indicating that it has no melting point. The DSC graph demonstrates that after UV-curing, amorphous SGPE with very good T _{g is obtained.} Due to different concentrations of LiFSI in SGPE, it is observed that the T _g fluctuates slightly, but the plastic crystal behavior of SGPE does not disappear.

As mentioned above, the crystalline phase is considered to be the main reason for retarding polymer chain dynamics and causing low ion transport. On the other hand, this phenomenon can be mitigated by the amorphous phase with activated chain segments. Based on these points, the plastic crystal behavior of PEGDMA and SGPE is confirmed by XRD analysis. In Fig. 5, it was clearly seen that the pure PEGDMA monomer had a large and broad peak at 20°, which corresponds to the crystal properties of PEGDMA. In contrast, after UV-curing, this typical peak was reduced to a sharply small peak for SGPE, which could be explained by the acquisition of amorphous properties for SGPE. Moreover, SGPE exhibited fewer crystalline phases than the pure PEGMDA film, suggesting that the formation of crystalline phases can be significantly alleviated because the structure of FSI in LiFSI extends the distance between main chains by being between PEGMDA. do.

The ionic conductivity of the electrolyte is one of the most important factors of LIB indicating the mobility of lithium ions in the electrolyte. It is also a major barrier to commercialization of polyelectrolytes in LIBs. To study the ionic conductivity of SGPE, EIS was first measured at different temperatures of 30 °C to 90 °C, and the Nyquist plot is shown in FIG. 6 . However, the impedance of SGPE20 is so high that only SGPE30 and SGPE40 are shown in FIG. 6 . It is clear that SGPE40 has a lower resistance compared to SGPE30, and this trend applies over the entire temperature measurement range. According to Equation 1, this result can directly affect the ionic conductivity of SGPE. As a result, in FIG. 7 , it was observed that SGPE40 exhibited higher ionic conductivity than SGPE30 over the entire irradiation temperature range. In addition, SGPE30 (σ = 1.11 mS/cm) and SGPE40 (σ = 5.21 mS/cm) both reached a good level of ionic conductivity and were very close to liquid electrolytes even at room temperature, suggesting that SGEP as electrolyte candidates could be applied to LIBs. indicated that there is

3. Conclusion

A series of flexible freestanding gelling polyelectrolytes with different LiFSI weight ratios were prepared by UV-curing. Polymerization was confirmed by FT-IR by monitoring the change of functional group. However, SGPE50 was not discussed in the present invention as it was difficult to prepare an intact electrolyte film due to the high lithium salt concentration in the precursor solution. On the other hand, when SGPE is used as an electrolyte, sufficient thermal stability may increase the safety of LIB. In addition, the plastic crystal behavior (amorphous) and low glass transition temperature (about -75 °C) of SGEP means high ionic conductivity, resulting in excellent ionic conductivity in both SGPE30 (1.11 mS/cm) and SGPE40 (5.21 mS). observed at room temperature. However, SGPE20 could not be tested in the present invention because the impedance was too high. Meanwhile, in-depth cell tests should be systematically studied to confirm the electrochemical properties of SGPE. However, given the good properties of SGPEs observed above, these novel SGPEs demonstrate promise as electrolyte candidates for LIBs.

4. Summary

A series of novel self-standing gelled polymer electrolytes (SGPEs) were prepared through ultraviolet (UV)-curing for application to flexible lithium-ion batteries. Unlike the conventional gel polymer electrolyte combined with a solvent or plasticizer, the novel SGPE of the present invention contains only lithium bis(fluorosulfonyl)imide [LiFSI] as a methacrylic monomer, poly It was prepared by curing (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA] in a different weight ratio. In particular, there are no solvents or plasticizers associated with the final SGPE. Due to this, SGPE exhibits high flexibility and strong mechanical stability. In addition, the SGPE exhibits excellent physicochemical properties and electrochemical properties. The SGPE exhibits excellent thermal stability below 150° C. and a very low glass transition temperature ( T _g ) (about -75° C.). In addition, the plastic crystal behavior of SGPE was confirmed in this study. As a result, SGPE exhibits excellent room temperature ionic conductivity, demonstrating that this novel series of SGPEs can be widely applied in flexible lithium-ion batteries as promising electrolyte candidates.

Claims (15)

A self-supporting gelling polymer compound containing lithium bis(fluorosulfonyl)imide [LiFSI] and poly (ethylene glycol) dimethacrylate [PEGDMA] as constituent units . The polymer compound according to claim 1, wherein the LiFSI and PEGDMA are included in a weight ratio of 15:85 to 45:55. The polymer compound according to claim 1, wherein the PEGDMA has a number average molecular weight (Mn) of 100 to 1000. The polymer compound according to claim 1, wherein the polymer compound is in the form of a film. 5. The polymer compound according to claim 4, wherein the film has a thickness of less than 500 μm. (a) dissolving LiFSI in PEGDMA to prepare a precursor solution;
(b) adding a photoinitiator to the precursor solution and stirring by adding an organic solvent;
(c) casting the stirred precursor solution on a plate and evaporating the organic solvent to obtain a precursor electrolyte modeling; and
(d) obtaining an electrolyte film by irradiating a UV lamp to the precursor electrolyte modeling
A method for producing the polymer compound of claim 1 comprising a. The method according to claim 6, wherein the LiFSI and PEGDMA of step (a) are mixed in a weight ratio of 15:85 to 45:55. The method according to claim 6, wherein the photoinitiator of step (b) is 2-hydroxy-2-methylpropiophenone (HMPP). The method according to claim 6, wherein the photoinitiator of step (b) is added in an amount of 0.1 to 5% by weight of PEGDMA. The method according to claim 6, wherein the organic solvent in step (b) is acetone. The method according to claim 6, wherein the organic solvent of step (b) is added in a ratio of organic solvent:precursor solution=30:1 to 1:1 (v/w). The method according to claim 6, wherein the UV lamp of step (d) has an irradiation peak intensity of ^{100 to 300 mW/cm 2 .} The method according to claim 6, wherein the irradiation in step (d) is performed for 10 to 100 minutes. A self-supporting gel polymer electrolyte for a lithium ion battery comprising the polymer compound of any one of claims 1 to 5. A lithium ion battery comprising the free-standing gel polymer electrolyte of claim 14 .

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