KR102394800B1 - Crosslinked polyelectrolyte gel and battery comprising the same - Google Patents

Abstract

Provided are a cross-linked polyelectrolyte gel and a battery comprising the same. The cross-linked polyelectrolyte gel is prepared by combining a polymer with a polymerizable lithium salt. The polymerizable lithium salt may include LiTVI (Lithium (trifluoromethanesulfonyl)-(vinylsulfonyl)imide) having Formula 1 below.
[Formula 1]

Description

Cross-linked polyelectrolyte gel and battery comprising same

The present invention relates to a cross-linked polyelectrolyte gel and a battery comprising the same.

Currently commercialized lithium secondary batteries are based on liquid electrolytes, and liquid electrolytes have the advantage of high lithium ion conductivity. However, the liquid electrolyte has safety problems such as leakage or explosion, and studies have been conducted to develop a solid electrolyte to solve this problem. However, the solid electrolyte has a disadvantage in that ionic conductivity is significantly lower than that of the liquid electrolyte, so a gel electrolyte in which the liquid electrolyte is impregnated in the solid electrolyte has been proposed. The gel electrolyte can improve the safety problem while supplementing the ionic conductivity through the contained liquid electrolyte.

Lithium salts such as LiPF ₆ and LiClO ₄ are currently used in commercially available lithium secondary batteries. Such lithium salts dissociate into positive and negative ions in the electrolyte, and move in opposite directions during charging and discharging. At this time, a concentration polarization phenomenon occurs in which negative ions not participating in the actual electrochemical reaction are accumulated on one side, and this concentration polarization phenomenon increases internal resistance of the battery and causes side reactions, thereby deteriorating battery performance.

In order to solve the above problems, the present invention provides a cross-linked polymer electrolyte gel having excellent performance.

The present invention provides a battery comprising the cross-linked polymer electrolyte gel.

Other objects of the present invention will become apparent from the following detailed description and accompanying drawings.

The cross-linked polymer electrolyte gel according to embodiments of the present invention is prepared by combining a polymer and a polymerizable lithium salt.

The polymerizable lithium salt may include LiTVI (Lithium (trifluoromethanesulfonyl)-(vinylsulfonyl)imide) having Formula 1 below.

[Formula 1]

The polymer may include at least one of PEGMA and PEGDMA.

The crosslinked polyelectrolyte gel may be prepared by performing a thermally initiated radical polymerization reaction on a mixture containing PEGMA, PEGDMA and LiTVI.

The thermally initiated radical polymerization reaction may be performed using AIBN.

A battery according to embodiments of the present invention includes the cross-linked polymer electrolyte gel.

A cross-linked polymer electrolyte gel according to embodiments of the present invention and a battery including the same may have excellent performance. For example, the cross-linked polyelectrolyte gel may have excellent electrochemical properties such as high ionic conductivity and a wide electrochemical stability window, and the battery can be stored at high temperature and ambient temperature based on the excellent electrochemical properties of the cross-linked polyelectrolyte gel. It can have excellent cycle stability.

1 shows a method for preparing a cross-linked polymer electrolyte gel according to embodiments of the present invention.
2 shows the ionic conductivity according to the temperature of the cross-linked polymer electrolyte gel according to the embodiments of the present invention.
3 shows an Arrhenius plot of the ionic conductivity of a cross-linked polyelectrolyte gel according to embodiments of the present invention.
4 shows a linear sweep voltammogram (LSV) of CPG4 obtained at 25° C. at a scan rate of 1 mV/s.
5 shows a chronoamperometric curve of a Li/CPG4/Li cell for lithium ion transition measurement.
FIG. 6 shows electrochemical impedance spectra (EIS) of a LiFePO ₄ /CPG4/Li battery stored under open circuit conditions at 25° C. FIG.
7 shows electrochemical impedance spectra (EIS) of a LiFePO ₄ /CPG4/Li battery stored under open circuit conditions at 60° C. FIG.
8 shows the discharge capacity and coulombic efficiency of a LiFePO ₄ /CPG4/Li battery cycled at a rate of 0.1C and 60°C.
9 shows the discharge capacity and coulombic efficiency of a LiFePO ₄ /CPG4/Li battery cycled at a rate of 0.1C and 25°C.
10 shows the discharge capacity of a LiFePO ₄ /CPG4/Li battery cycled at various discharge rates.

Hereinafter, the present invention will be described in detail through examples. Objects, features, and advantages of the present invention will be readily understood through the following examples. The present invention is not limited to the embodiments described herein, and may be embodied in other forms. The embodiments introduced here are provided so that the disclosed content can be thorough and complete, and so that the spirit of the present invention can be sufficiently conveyed to those of ordinary skill in the art to which the present invention pertains. Therefore, the present invention should not be limited by the following examples.

The cross-linked polymer electrolyte gel according to embodiments of the present invention is prepared by combining a polymer and a polymerizable lithium salt.

The polymerizable lithium salt may include LiTVI (Lithium (trifluoromethanesulfonyl)-(vinylsulfonyl)imide) having Formula 1 below.

[Formula 1]

The polymer may include at least one of PEGMA and PEGDMA.

The crosslinked polyelectrolyte gel may be prepared by performing a thermally initiated radical polymerization reaction using AIBN on a mixture containing PEGMA, PEGDMA and LiTVI.

A battery according to embodiments of the present invention includes the cross-linked polymer electrolyte gel.

[ LiTVI's Synthesis example ]

Lithium (trifluoromethanesulfonyl)-(vinylsulfonyl)imide (LiTVI), which is a polymerizable lithium salt according to an embodiment of the present invention, may be prepared by the following method.

Trifluoromethanesulfonamide (18.0 g, 0.121 mol), triethylamine (36.65 g, 0.362 mol), and tetrahydrofuran (THF) (150 mL) were mixed in 500 mL equipped with a funnel and magnetic stir bar. A first solution was formed by placing it in a round bottom flask. 2-chloroethanesulfonyl chloride (20.67 g, 0.127 mol) mixed in 80 mL of THF was added to the first solution through a funnel at 0° C. for 1 hour to form a first mixture. The first mixture was stirred at 25° C. for 21 hours. The precipitated salt was filtered off and the solution was concentrated under reduced pressure. A second solution formed by dissolving the crude product in acetonitrile (200 mL) was placed in a 500 mL round bottom flask equipped with a magnetic stir bar.

Potassium carbonate (33.5 g, 0.242 mol) was added to the second solution to form a second mixture. The second mixture was stirred at 0° C. for 4 h, then filtered and dried in vacuo at ambient temperature. After washing with ether and recrystallizing from acetone, an intermediate product, KTVI (potassium (trifluoromethanesulfonyl)(vinylsulfonyl)imide), was further purified. Lithium perchlorate trihydrate (11.6 g, 0.072 mol) was added to KTVI (20.0 g, 0.072 mol) mixed in 50 mL of acetonitrile to form a third mixture. The third mixture was stirred at 25° C. for 12 hours, insoluble matter was filtered off, and freeze-dried for 1 week to obtain a product. The final product was recrystallized in acetonitrile, dried under high vacuum at 50° C. for 48 hours, and stored in an Ar-filled glovebox prior to use.

[Production Example of Crosslinked Polyelectrolyte Gels (CPG)]

A solution consisting of predetermined amounts of PEGMA, PEGDMA, LiTVI, PC and AIBN was prepared in a glass bottle in an argon-filled glovebox. The sample was stirred overnight to obtain a homogeneous and clear solution. The sample was then transferred to a glass mold and held in an oven set at 70° C. for 3 hours. The prepared crosslinked polymer electrolyte gel was denoted by CPG#, and is shown in Table 1 below.

[Table 1]

[Cell Manufacturing and Electrochemical Analysis]

The ionic conductivity of the crosslinked polyelectrolyte gel (CPG) was measured by electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 mHz to 1 MHz with an applied alternating current (AC) voltage of 10 mV. Symmetrical cells for ionic conductivity measurements were assembled using stainless steel spacers as electrodes in a 2032 coin cell. Lithium ion transference number (t _Li ⁺ ) measurements were performed by EIS and direct current (DC) polarization methods. The Li/CPG4/Li symmetric cell was polarized by a constant DC voltage of 10 mV. The current was recorded until a steady state was reached. The initial and steady-state impedances of the cells were measured by EIS. t _Li ⁺ was calculated using Equation 1 below

[Equation 1]

In Equation 1 above, V is a constant DC voltage applied to the cell, i ₀ and _iss are initial and steady-state currents, and R ₀ and R _ss are initial and steady-state interface resistances, respectively.

The electrochemical stability of the crosslinked polyelectrolyte gel (CPG) was analyzed by linear sweep voltammetry (LSV) using a 2032 coin cell with a stainless steel working electrode and a lithium metal reference electrode. The cell was swept over a potential range of 2.95 to 7.0 V (vs Li/Li ⁺ ) with a scan rate of 1.0 mV/s. Potential difference charge/discharge cycle test was performed with lithium metal anode and LiFePO ₄ cathode (80 wt% LiFePO ₄ , 10 wt% carbon black and 10 wt% poly(vinylidene difluoride) binder) in the potential range of 2.5 to 4.0 V (vs Li/Li ⁺ ). It was performed using a 2032 coin cell consisting of All cells were assembled in an argon-filled glovebox. In each cell, 100 μL of the prepared electrolyte solution was impregnated in a glass fiber separator, and the assembled cell was maintained at 70° C. for 3 hours to induce a thermal crosslinking reaction. All electrochemical measurements were recorded with a WBCS3000 battery cycler (WonATech) at 60°C.

1H and 19F nuclear magnetic resonance (NMR) spectroscopy was performed on an Ascend 400 spectrometer (300 MHz), and Fourier transform infrared (FT-IR) spectra were obtained from Perkin Elmer FT- in the wavenumber range from 4000 to 650 cm ^-1 with a resolution of 1 cm ^-1 . Recorded with an IR system. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was measured with a PerkinElmer Optima-4300 DV ICP system, and differential scanning calorimetry (DSC) was performed using a TA Instruments DSC 2910 differential scanning calorimeter under a nitrogen atmosphere.

Samples were encapsulated in sealed aluminum pans for DSC measurements. The sample was first heated to 150 °C and then quenched to -80 °C. A second heating scan was recorded from −80 to 100° C. at a heating rate of 10° C./min. Thermogravimetric analysis (TGA) was evaluated using a TA Instruments TGA Q-5000IR thermogravimetric analyzer under a nitrogen atmosphere. The sample was heated from 25 to 700°C at a heating rate of 10°C/min.

LiTVI is synthesized by a reaction between 2-chloroethanesulfonyl chloride and trifluoromethanesulfonamide followed by an ion exchange reaction. The 1H NMR spectrum of LiTVI accurately shows the chemical structure of LiTVI without any peak due to impurities, and the synthesis and purification was found to be well done. The synthesis of LiTVI differs from the 19F NMR spectrum of LiTVI by 19F NMR spectrum with a single term at -79.47 ppm different from the 19F NMR spectrum of the starting compound trifluoromethanesulfonamide and the peak at -81.38 ppm. The composition of LiTVI was characterized by ICP-AES. The concentration of lithium ions is 3.1 wt%, which is in good agreement with the theoretical composition (2.83 wt%). Potassium ion was not detected by ICP, indicating that the ion exchange was successful.

A crosslinked polyelectrolyte gel (CPG) can be prepared by thermally initiated radical polymerization of a mixture composed of PEGMA, PEGDMA and LiTVI using propylene carbonate (PC) as the liquid electrolyte and AIBN as the thermal initiator (Fig. 1). A series of cross-linked polyelectrolyte gels (CPG) were obtained by varying the ratios of PEGMA and LiTVI, and the contents of PEGDMA, PC and AIBN were kept constant. The composition of the mixture for preparing the crosslinked polyelectrolyte gel (CPG) is listed in Table 1 above. All the prepared mixtures are clear and homogeneous solutions, except for the mixture for CPG6, which contains the highest amount of LiTVI because it is not completely dissolved. Therefore, the characteristics of CPG1 to CPG5 obtained from a homogeneous solution were analyzed. When these mixtures were heated at 70° C. for 3 hours, independent and flexible transparent gels were obtained, and FT-IR spectra of LiTVI, PEGMA, PEGDMA and CPG were analyzed to determine gel formation by thermal crosslinking reaction. investigated. The vibrational peak at 1176 cm ⁻¹ appears in the spectra of LiTVI and CPG4 and is due to SO elongation at the sulfonylimide group, which is not present in the spectra of PEGMA and PEGDMA, indicating that the LiTVI unit is well incorporated into CPG4. The C=C vibrational peak in the region of 1680 ~ 1600 cm ^-1 was used to observe the polymerization and/or crosslinking behavior of the monomer, and PEGMA and PEGDMA show characteristic peaks at 1638 cm ^-1 .

LiTVI having CC groups confirmed by NMR spectroscopy did not show any peak in the region of 1680 ~ 1600 cm ^-1 . The C=C vibrational peak is not for a monomer having a vinylsulfonyl group. While the C=C groups of PEGMA and PEGDMA are completely consumed by thermal polymerization, the degree of reaction of the C=C groups of LiTVI cannot be fully estimated. Since the reactivity of the C=C group in LiTVI must be lower than that of PEGMA and PEGDMA, LiTVI may not react completely.

The ionic conductivity of the crosslinked polyelectrolyte gel (CPG) was measured by electrochemical impedance spectroscopy (EIS) at various temperatures (Fig. 2). Among the crosslinked polyelectrolyte gels (CPG), the highest ionic conductivity value of CPG4 was 6.7×10 ⁻⁴ S/cm at 25° C. and 1.8×10 ⁻³ S/cm at 60° C. The high ionic conductivity of CPG4 corresponds to practical applications in lithium batteries at ambient temperature.

The increase in the ionic conductivity of the cross-linked polyelectrolyte gel (CPG) as the content of LiTVI increased up to 12.5 w/w could be attributed to the increase in the concentration of charge carriers in the electrolyte system. When the LiTVI content is greater than 12.5 w/w, the decrease in the ionic conductivity of the crosslinked polyelectrolyte gel (CPG) results in increased charge density and polymer backbone or agglomeration leading to increased charge-charge interaction in the CPG with more LiTVI. This may be due to the increased fraction of anions immobilized on the ionic species.

The temperature dependence of the ionic conductivity for the crosslinked polyelectrolyte gel (CPG) was analyzed. A linear relationship was observed for all prepared cross-linked polyelectrolyte gels (CPG) (Fig. 3). The activation energy was calculated from the slope of the Arrhenius plot and is shown in Table 2 below. CPGs with higher LiTVI content show slightly higher activation energies, which also corresponds to a decrease in ionic conductivity of CPGs with LiTVI content greater than 12.5 w/w.

[Table 2]

Differential scanning calorimetry (DSC) was performed to investigate the thermal transition of cross-linked polyelectrolyte gels (CPG). Because PC acts as a plasticizer to lower the glass transition temperature (Tg), the cross-linked polyelectrolyte gel (CPG) does not show a glass transition at -80 to 100°C. On the other hand, the cross-linked polyelectrolyte gel (CPG) vacuum-dried at 60 °C with PC removed shows clear glass transition behavior. The crosslinked polyelectrolyte gel (CPG) with higher LiTVI content exhibited a higher Tg, indicating a decrease in chain mobility as the charge-charge interaction increased.

The thermal stability of the crosslinked polyelectrolyte gel (CPG) was investigated by thermogravimetric analysis (TGA). The temperature for the 5 wt% weight loss of the cross-linked polyelectrolyte gel (CPG) was in the range of 95 to 110 °C, indicating that the cross-linked polyelectrolyte gel (CPG) is thermally stable at ambient conditions (<70 °C). The weight loss at about 140° C. is due to vaporization of PC, and the weight loss at about 350° C. is due to degradation of the polymer network. All crosslinked polyelectrolyte gels (CPG) exhibited a PC weight loss of about 50% by weight, indicating that a negligible amount of PC was lost during the thermal crosslinking process. Since the char yield of the cross-linked polyelectrolyte gel (CPG) increases as the content of LiTVI in the mixture for preparing the cross-linked polyelectrolyte gel (CPG) increases, the various char yields can be explained by the lithium remaining in the char.

The electrochemical stability of CPG4 was investigated by linear sweep voltammetry (LSV) at 60 °C (Fig. 4). There was no significant anode current peak below 5.3 V (vs Li/Li ⁺ ), indicating that CPG4 has a wide electrochemical window applicable to high voltage lithium batteries.

The lithium ion transition number (t _Li ⁺ ) was measured by combining the methods of DC polarization and AC impedance using a Li/CPG4/Li symmetric cell (Fig. 5). The t _Li ⁺ of CPG4 was calculated to be 0.52, which is larger than that of poly(ethylene glycol)-based polyelectrolyte (<0.5). The larger t _Li ⁺ value of CPG4 could be attributed to the incorporation of LiTVI into the cross-linked polymer structure so that anions were attached to the polymer without migration.

Although LiTVI is included in the polymer to increase t _Li ⁺ , not all LiTVI is polymerized due to the low reactivity of the vinylsulfonyl group. To investigate the conversion of LiTVI, a series of 1 H NMR analyzes of CPG were performed. The conversion of LiTVI to CPG4 is 0.62, and the t _Li ⁺ value can be further increased by increasing the conversion of LiTVI. In addition, since the t _Li ⁺ value of CPG4 is larger than that of other polymer electrolytes, it can be applied to a lithium battery.

To investigate the stability of CPG4 over time, the impedance of LiFePO ₄ /CPG4/Li batteries stored under open circuit conditions at 25 and 60 °C was measured using EIS. The semicircle is due to the combined impedance of the bulk resistance of CPG4 and the interface resistance between CPG4 and the electrode. At 25°C, the impedance of the battery stabilized after 2 days (FIG. 6), and CPG4 shows excellent stability in both the lithium metal positive electrode and the LiFePO ₄ negative electrode. At 60°C, the impedance gradually increased up to 10 days ( FIG. 7 ). This may be due to the reaction of LiTVI remaining in CPG4. The stable operation of LiFePO ₄ /CPG4/Li shows that CPG 4 can be applied to the electrolyte of lithium metal batteries.

A constant current charge-discharge test of the LiFePO ₄ /CPG4/Li battery was performed at 60° C. to evaluate the cycle stability of high-temperature applications through charging and discharging at a current density of 0.1 C ( FIG. 8 ). It shows stable cycling performance maintaining a discharge capacity of 131.0 mAh/g after 50 cycles. This is a capacity of 89.2% of the maximum discharge capacity during cycling. Coulombic efficiency was maintained above 98% except for the cell activation step. The stable operation of LiFePO ₄ /CPG4/Li batteries shows that CPG4 is highly compatible with lithium polymer batteries operating at a high temperature of 60°C. This is generally not possible in organic liquid electrolyte systems.

The feasibility of CPG4 as an electrolyte for batteries (LiFePO ₄ /CPG4/Li) operating at ambient temperature was also tested by cycling at 25°C at 0.1C rate ( FIG. 9 ). Cell activation requires about 20 or more cycles, but the battery can operate at 25°C without significantly reducing capacity. Cell activation can be attributed to the formation of SEI and penetration of the liquid electrolyte into the cathode. The discharge capacity after 50 cycles was found to be 118.4 mAh/g.

CPG4 is not a perfect single ion conductive polyelectrolyte, but stable capacity maintenance with a reasonable discharge capacity value in long-term cycling at room temperature can be achieved by using a polyelectrolyte containing a polymerizable lithium salt without using a conventional lithium salt. . 10 shows the discharge capacity at 25 and 60°C. A battery at 60°C can provide discharge capacities of 148.2, 143.5, 133.4, 98.4 and 35.6 mAh/g at rates of 0.1, 0.2, 0.5, 1.0 and 2.0C, respectively. In addition, the battery can provide discharge capacities of 109.4, 101.2, 89.9 and 32.4 mAh/g at 25°C at rates of 0.1, 0.2, 0.5 and 1.0C.

So far, specific embodiments of the present invention have been described. Those of ordinary skill in the art to which the present invention pertains will understand that the present invention can be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments are to be considered in an illustrative rather than a restrictive sense. The scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

Claims (6)

It is prepared by combining a polymer and a polymerizable lithium salt,
The polymerizable lithium salt is a cross-linked polymer electrolyte gel, characterized in that it contains LiTVI (Lithium (trifluoromethanesulfonyl)-(vinylsulfonyl)imide) having the following formula (1).
[Formula 1]

delete The method of claim 1,
The polymer is a cross-linked polyelectrolyte gel, characterized in that it comprises at least one of PEGMA and PEGDMA. The method of claim 1,
The crosslinked polyelectrolyte gel is a crosslinked polyelectrolyte gel, characterized in that it is prepared by performing a thermally initiated radical polymerization reaction on a mixture containing PEGMA, PEGDMA and LiTVI. 5. The method of claim 4,
The thermally initiated radical polymerization reaction is a crosslinked polyelectrolyte gel, characterized in that carried out using AIBN. A battery comprising the crosslinked polyelectrolyte gel of any one of claims 1 and 3 to 5.

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