KR20210046336A - 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-reliant gelled polymer compound composed of lithium bis(fluorosulfonyl)imide (LiFSI) and poly(ethylene glycol) dimethacrylate (PEGDMA) and use of a gel polymer electrolyte for a flexible lithium ion battery thereof. The self-reliant gelled polymer compound of the present invention exhibits excellent thermal stability at 150℃ or less, a very low glass transition temperature (Tg) (about -75℃), and exhibits plastic crystal behavior. As a result, it was confirmed that the self-reliant gelled polymer compound exhibits excellent room temperature ionic conductivity, so the self-reliant gelled polymer compound can be usefully used as an electrolyte applied to flexible lithium-ion batteries in the field of lithium-ion batteries.

Description

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

The present invention relates to a self-supporting gelling polymer compound using LiFSI and a use of a gel polymer electrolyte for flexible lithium ion batteries thereof, and more particularly, lithium bis (fluorosulfonyl) imide (LiFSI) And a self-supporting gelling polymer compound composed of poly (ethylene glycol) dimethacrylate (PEGDMA), and a use of a gel polymer electrolyte for flexible lithium ion batteries thereof.

Lithium-ion batteries (LIBs) have attracted much 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 areas. Like other types of batteries, LIB has three essential components: positive, negative, and 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 decades, and now occupy a large part of the battery market due to excellent room temperature ion conductivity. However, the leakage of flammable organic liquids and safety issues arising from flammability are issues that need to be resolved as quickly as possible. Recently, in order to solve the above problems, a solid organic polymer electrolyte (SPE) has been introduced as a strategy to replace the conventional organic liquid electrolyte. Extensive studies have shown that SPE not only possesses excellent safety, but also inhibits the growth of lithium dendrites. In addition, because SPE has excellent mechanical stability, it can be used as a separator in LIB. In general, SPE consists of a polymer and a lithium salt, where the polymer acts as a host so that lithium ions can move 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 of SPE (10 ^-5 cm/s) caused by the large interfacial resistance between the electrode and the SPE is a major barrier limiting the wide application of LIB. To overcome this barrier, a high-performance gelled polymer electrolyte (GPE) has gained great interest and has been investigated by some researchers. Traditional GPE consists of SPE and plasticizers or organic solvents, which are commonly used in liquid electrolytes. In other words, GPE can provide high ionic conductivity (10 ^-3 cm/s) similar to organic liquid electrolytes at room temperature, while maintaining mechanical stability similar to SPE. However, it is a general recognition that the mechanical stability of GPE decreases even though the ionic conductivity increases as the concentration of the plasticizer or organic solvent increases. Therefore, the same problem as the organic liquid electrolyte must be considered again. As a result, the 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.)

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

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

According to an aspect of the present invention, lithium bis (fluorosulfonyl) imide [lithium bis (fluorosulfonyl) imide, LiFSI] and poly (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA] are constructed. There is provided 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 onto a plate and evaporating an organic solvent to obtain a precursor electrolyte modeling; And (d) irradiating a UV lamp to model the precursor electrolyte to obtain an electrolyte film.

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

In one 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 in 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-standing gel polymer electrolyte.

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

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

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 temperatures 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 of manufacturing SGPE by UV-curing.

The present invention is a self-supporting type comprising lithium bis (fluorosulfonyl) imide [lithium bis (fluorosulfonyl) imide, LiFSI] and poly (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA] as structural units It provides a gelling polymer compound.

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 and 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)] is introduced as a new polymer matrix. Such PEO and its derivatives (PEGDMA) have a very high dielectric constant and thus have high lithium salt solubility. The ethylene oxide units of PEGDMA are bound with lithium ions and then transported by hopping in a polymer matrix. Therefore, the high chain flexibility of PEGDMA is expected to improve 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 than _{LiTFSI or LiPF 6} and better properties such as T _g. In addition, LiFSI has a significant effect on lowering the crystallinity of the electrolyte. Anhydrous acetone is added in 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.

In order to compare the effect of LiFSI on SGPE properties, various weight ratios of SGPE were prepared. All polymerizations of SGPE are Fourier transformation infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (X-ray diffraction). 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 in a weight ratio of 20:80 to 40:60, and more preferably in a weight ratio of 30:70 to 40:60.

In one embodiment, the PEGDMA may be 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, and more preferably 300 μm.

The present invention also includes 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 onto a plate and evaporating an organic solvent to obtain a precursor electrolyte modeling; And (d) irradiating a UV lamp to model the precursor electrolyte to obtain an electrolyte film.

The manufacturing method of the present invention includes a step of preparing a precursor solution by dissolving LiFSI in PEGDMA (ie, step (a)). In step (a), the mixing ratio of LiFSI and PEGDMA is as described above.

The manufacturing method of the present invention includes a step of adding a photoinitiator to the precursor solution obtained in step (a), adding an organic solvent, and stirring [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 5% by weight of PEGDMA. It can be added in 3% by weight. 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, and preferably from the group consisting of acetone, dimethyl carbonate, ethylene carbonate, and ethylmethyl carbonate. At least one selected, more preferably anhydrous acetone may be used, 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 of the 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 manufacturing method of the present invention includes a step of casting the stirred precursor solution obtained in step (b) onto a plate and evaporating an organic solvent to obtain a 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 making a precursor solution in which all of the starting materials are dissolved into a film.

The manufacturing method of the present invention includes a step (ie, step (d)) of obtaining an electrolyte film by irradiating a UV lamp on the precursor electrolyte modeling obtained in step (c). 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-standing gel polymer electrolyte.

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

<Example>

1. Experiment

1.1 material

Polyethylene glycol dimethacrylate (PEGDMA, average Mn 550) and photoinitiator 2-hydroxy-2-methylpropiophenone (HMPP, 99%), ethylene carbonate and dimethyl carbonate (50v/50v) ) In a 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 self-supporting gelled polymer electrolyte

Different weight ratios of LiFSI were dissolved in PEGDMA to prepare a precursor solution of SGPE. Then, the photoinitiator HMPP was added to the solution, at which time 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 the acetone and 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 silicon model (6 cm x 6 cm inside), and anhydrous acetone was removed. Finally, the precursor electrolyte modeling was exposed to Hg UV lamp irradiation (Lichtzen) for 20 minutes to obtain a self-standing gelled polymer electrolyte. Here,'n' in the sample SGPEn means the wt% of LiFSI in the electrolyte.

1.3 Characteristics of self-supporting polymer electrolyte

The polymerization of SGPE was confirmed by FT-IR (Nicolet iS5 FTIR spectrometer) having a spectral resolution of ^{4 cm -1 at room temperature.} The thermal stability behavior was measured with a thermal gravimetric 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 by DSC (Perkin-Elmer DSC 6000) at -80°C to 120°C at a heating rate of 10°C/min in a nitrogen gas atmosphere. The firing 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 self-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 a self-made cell, SGPE was sandwiched between two conductive nickel foil disks and placed between two FTO glass electrodes. Conductivity was calculated by the following formula 1.

<Calculation 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 SGPE was prepared through UV-curing polymerization from the precursor mixture solution, and the appearance of the prepared SGPE is shown in FIG. 1. The precursor solution turned into a transparent and flexible electrolyte film that could be easily peeled off from the glass plate. The thickness of the SGPE was controlled to be less than 300 μm by adjusting the silicon model size and the amount of the precursor solution. However, SGPE50 could not be prepared due to an excessive lithium salt content, and a complete electrolyte film could not be produced. The chemical structure and UV polymerization of SGPE was confirmed by FT-IR. Before UV-curing, as shown in FIG. 2, the absorption peaks of the PEGDMA characteristics of the 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, the typical S=O stretch peaks of LiFSI are still located at 1376 cm ^-1 and 1172 cm ^-1 before or after UV-curing. The FT-IR graph suggests that polymerization of SGPE was achieved after UV-curing and LiFSI is still in the curing process.

Overheating is one of the most common reasons for LIB accidents, and heat is generated when the battery is operated. As a result, good thermal stability is required for all components, including or not including an electrolyte or a separator, or included in a battery. 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) exhibits the lowest thermal endurance and its weight is rapidly decomposed from the beginning. In the case of SGPE, the weight loss decreased slightly at 150° C. and largely decomposed at 210° C., corresponding to the thermal stability of LiFSI and PEGDMA. From FIG. 3, it is shown that SGPE has more promising thermal stability, and thus battery thermal stability and safety can be improved as compared with a commercial liquid electrolyte.

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 has a correlation with T _g , so increasing the ionic conductivity of the electrolyte should reduce the T _{g of the material.} In FIG. 4, SGPE shows very low T _g of -75.33 °C (SGPE20), -74.47 °C (SGPE30) and -74.02 °C (SGPE40), respectively, which is much lower than that of pure PEGDMA (-69.54 °C). A slight increase is observed as the PEGDMA concentration decreases, 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 does not have a melting point. The DSC graph demonstrates that after UV-curing, amorphous SGPE with very good T _{g is obtained.} Due to the 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 believed to be the main reason for delaying the polymer chain dynamics and causing low ion transport. On the other hand, this phenomenon can be alleviated by the amorphous phase with activated chain segments. Based on this point, the firing crystal behavior of PEGDMA and SGPE is confirmed by XRD analysis. In Fig. 5, it was clearly shown that the pure PEGDMA monomer had a large and broad peak at 20°, which corresponds to the crystalline properties of PEGDMA. On the contrary, after UV-curing, this typical peak was reduced to a sharply small peak for SGPE, which can be explained as SGPE acquired amorphous properties. Moreover, SGPE showed less crystalline phase than pure PEGMDA film, suggesting that the FSI structure in LiFSI can significantly alleviate the formation of crystal phase because it widens the distance between main chains by being present between PEGMDA. do.

The ionic conductivity of the electrolyte is one of the most important factors of LIB, which indicates the mobility of lithium ions in the electrolyte. In addition, this is a major barrier to commercialization of polymer electrolytes in LIB. In order 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, since the impedance of SGPE20 is too high, only SGPE30 and SGPE40 are shown in FIG. 6. It is clear that the SGPE40 has a lower resistance compared to the SGPE30, and this trend applies to the entire temperature measurement range. According to Equation 1, these results can directly affect the ionic conductivity of SGPE. As a result, in FIG. 7, it was observed that SGPE40 exhibits 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 good levels of ionic conductivity, and they are very close to the liquid electrolyte even at room temperature, so SGEP can be applied to LIB as an electrolyte candidate. Suggests that there is.

3. Conclusion

A series of flexible freestanding gelled polymer electrolytes with different LiFSI weight ratios were prepared by UV-curing. The change in functional groups was monitored and polymerization was confirmed by FT-IR. However, SGPE50 has not been discussed in the present invention because it is 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 can increase the safety of LIB. In addition, the sintering crystal behavior (amorphous) and low glass transition temperature (about -75 °C) of SGEP mean 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 was too high in impedance to be tested in the present invention. Meanwhile, in-depth cell tests should be systematically studied to confirm the electrochemical properties of SGPE. However, considering the good properties of SGPE observed above, these new SGPEs demonstrate the prospect of LIB as an electrolyte candidate.

4. Summary

For application to flexible lithium-ion batteries, a series of novel self-standing gelled polymer electrolytes (SGPE) were prepared through ultraviolet (UV)-curing. Unlike conventional gel polymer electrolytes combined with solvents or plasticizers, the novel SGPE of the present invention only uses lithium bis(fluorosulfonyl)imide [LiFSI] as a polymethacrylic monomer. It was prepared by curing at a different weight ratio than (ethylene glycol) dimethacrylate [poly (ethylene glycol) dimethacrylate, PEGDMA]. 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 exhibits 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 new series of SGPE can be widely applied to 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 [poly (ethylene glycol) dimethacrylate, PEGDMA] as structural units . The polymer compound according to claim 1, wherein the LiFSI and PEGDMA are contained 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. 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 onto a plate and evaporating an organic solvent to obtain a precursor electrolyte modeling; And
(d) obtaining an electrolyte film by irradiating a UV lamp on the precursor electrolyte modeling
The method for producing the polymer compound of claim 1 comprising a. The method of claim 6, wherein the LiFSI and PEGDMA in step (a) are mixed in a weight ratio of 15:85 to 45:55. The method of claim 6, wherein the photoinitiator in step (b) is 2-hydroxy-2-methylpropiophenone (HMPP). The method of 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 of claim 6, wherein the UV lamp of step (d) has an irradiation peak intensity of ^{100 to 300 mW/cm 2.} The method of 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 self-standing gel polymer electrolyte of claim 14.

Images