Polymeric ionic liquid and process for producing a polymer membrane including the same

Phenolic epoxy resin (PNE 177, Chang Chun Plastics, average molecular weight 1600 g mol −1 , EEW 172-182 g eq −1 ) was dissolved in chloroform (Tedia, 99.9%) at room temperature to obtain a chloroform solution. To the chloroform solution, hexane (Seedchem, 95%) was added dropwise to dissolve impurities in an upper hexane layer. The upper hexane layer was removed, and then hexane was further added dropwise to the chloroform solution. The steps of addition and removal of hexane were repeated several times. Thereafter, the chloroform solvent in the chloroform solution (a lower layer solution) was removed using a rotary evaporator to obtain a purified phenolic epoxy resin.

The purified phenolic epoxy resin (10 g) was placed in a pear-shaped flask, and was dissolved in chloroform (Tedia, 99.9%) at room temperature. Then, the pear-shaped flask was disposed in an ice bath, and 10 ml of hydrobromic acid (Acros Organics, 48.0%) was slowly added into the pear-shaped flask to obtain a reaction mixture. A ring-opening bromination reaction of the purified phenolic epoxy resin was allowed to proceed, with stirring of the reaction mixture for 6 hours. Thereafter, the reaction mixture was washed using water to remove unreacted hydrobromic acid, and then the solvent (chloroform) in the reaction mixture was removed using a rotary evaporator (Rotavapor) to obtain a brominated intermediate polymer.

In another pear-shaped flask, the brominated intermediate polymer (5 g) was dissolved in dimethyl sulfoxide (DMSO) at room temperature, and then mixed with 7 ml of 1-methylimidazole (Alfa Aesar, 99.0%) at 80° C. to obtain a mixture. The mixture was stirred for 24 hours. To the mixture, ethyl acetate (a solvent) was added to dissolve unreacted reactants in the mixture. Next, the solvent was removed using a rotary evaporator so as to obtain a polymeric ionic liquid with counter-ions of bromide ions (PIL-Br anion).

The PIL-Br anion was dissolved in deionized water at room temperature to obtain a PIL solution. Potassium hexafluorophosphate (KPF 6 ) was dissolved in deionized water at room temperature to obtain a KPF 6 solution. The KPF 6 solution was added dropwise to the PIL solution and mixed therewith for 6 hours to obtain a precipitate. The precipitate was washed using water to remove unreacted reactants, was dried using a rotary evaporator, and then further dried in a vacuum oven so as to obtain a polymeric ionic liquid with counter-ions of hexafluorophosphate anions (PIL-PF 6 anion).

The scheme for synthesizing the polymer ionic liquid of Example 1 is shown below.

The PIL-Br anion obtained in Example 1 was dissolved in deionized water at room temperature to obtain a PIL solution. Bis(trifluoromethane) sulfonimide lithium salt (Solvay) was dissolved in deionized water at room temperature to obtain a LiTFSI solution. The LiTFSI solution was added dropwise to the PIL solution and mixed therewith for 6 hours to obtain a precipitate. The precipitate was washed using water to remove unreacted reactants, was dried using a rotary evaporator, and then further dried in a vacuum oven so as to obtain a polymeric ionic liquid with counter-ions of TFSI anions (PIL-TFSI anion).

The scheme for synthesizing the polymer ionic liquid of Example 2 is shown below.

In a pear-shaped flask, the purified phenolic epoxy resin (6 g) obtained in Example 1 was dissolved in acetonitrile, and then mixed with 8 ml of 1-methylimidazole (Alfa Aesar, 99.0%) at room temperature to allow a ring-opening reaction of the purified phenolic epoxy resin to proceed, with stirring for 24 hours, to thereby obtain an intermediate polymer.

The intermediate polymer was dissolved in deionized water at room temperature to obtain an intermediate solution. Potassium hexafluorophosphate (KPF 6 ) was dissolved in deionized water at room temperature to obtain a KPF 6 solution. The KPF 6 solution was added dropwise to the intermediate solution and mixed therewith for 6 hours to obtain a precipitate. The precipitate was washed using water to remove unreacted reactants, was dried using a rotary evaporator, and then further dried in a vacuum oven so as to obtain a polymeric ionic liquid with counter-ions of hexafluorophosphate anions (PIL-PF 6 anion).

The scheme for synthesizing the polymer ionic liquid of Example 3 is shown below.

The intermediate polymer obtained in Example 3 was dissolved in deionized water at room temperature to obtain an intermediate solution. Bis(trifluoromethane)sulfonimide lithium salt (Solvay) was dissolved in deionized water at room temperature to obtain a LiTFSI solution. The LiTFSI solution was added dropwise to the intermediate solution and mixed therewith for 6 hours to obtain a precipitate. The precipitate was washed using water to remove unreacted reactants, dried using a rotary evaporator, and then further dried in a vacuum oven so as to obtain a polymeric ionic liquid with counter-ions of TFSI anions (PIL-TFSI anion).

The scheme for synthesizing the polymer ionic liquid of Example 4 is shown below.

Polymer Membrane

The polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 was dissolved in N-methyl-2-pyrrolidone (solvent) and stirred for 2 hours, poured onto a film forming plate, and then disposed in an oven set at 80° C. for 24 hours to remove the solvent therefrom, thereby obtaining a polymer membrane.

A polymer membrane of Example 6 was prepared according to a procedure similar to that described in Example 5, except that the polymeric ionic liquid (PIL-TFSI anion) obtained in Example 2 was used to replace the polymeric ionic liquid (PIL-PF 6 anion) of Example 1.

COMPARATIVE EXAMPLE 1 (CE 1)

A polymer membrane of Comparative Example 1 was prepared according to a procedure similar to that described in Example 5, except that the PVdF-co-HFP (poly(vinylidenefluoride-co-hexafluoropropylene)) was used to replace the polymeric ionic liquid (PIL-PF 6 anion) of Example 1.

The polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 and PVdF-co-HFP (poly(vinylidenefluoride-co-hexafluoropropylene)) were mixed and dissolved in N-methyl-2-pyrrolidone (solvent) to obtain a mixture in which a weight ratio of PIL-PF 6 anion to PVdF-co-HFP was 3:7. The mixture was stirred for 2 hours, poured onto a film forming plate, and then disposed in an oven set at 80° C. for 24 hours to remove the solvent therefrom, thereby obtaining a polymer membrane.

A polymer membrane of Example 8 was prepared according to a procedure similar to that described in Example 7, except that a weight ratio of PIL-PF 6 anion to PVdF-co-HFP was 1:1.

A polymer membrane of Example 9 was prepared according to a procedure similar to that described in Example 7, except that a weight ratio of PIL-PF 6 anion to PVdF-co-HFP was 7:3.

A polymer membrane of Example 10 was prepared according to a procedure similar to that described in Example 7, except that the polymeric ionic liquid (PIL-TFSI anion) obtained in Example 2 was used to replace the polymeric ionic liquid (PIL-PF 6 anion) of Example 1.

A polymer membrane of Example 11 was prepared according to a procedure similar to that described in Example 10, except that a weight ratio of PIL-TFSI anion to PVdF-co-HFP was 1:1.

A polymer membrane of Example 12 was prepared according to a procedure similar to that described in Example 10, except that a weight ratio of PIL-TFSI anion to PVdF-co-HFP was 7:3.

0.197 g of the purified phenolic epoxy resin obtained in Example 1 and 0.553 g of polyetheramine (Jeffamine® ED-2003) were mixed and stirred in acetone (solvent) at 80° C. for 2 hours, poured onto a film forming plate made of polypropylene, and then disposed in an oven set at 80° C. for 24 hours so as to permit a crosslinking reaction to occur and to permit removal of the solvent therefrom, thereby obtaining a polymer membrane.

0.1773 g of the purified phenolic epoxy resin obtained in Example 1, 0.498 g of polyetheramine (Jeffamine® ED-2003), and 0.075 g of the polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 were stirred in acetone (solvent) at 80° C. for 2 hours, poured onto a film forming plate made of polypropylene, and then disposed in an oven set at 80° C. for 24 hours so as to permit a crosslinking reaction to occur and to permit removal of the solvent therefrom, thereby obtaining a polymer membrane.

0.1379 g of the purified phenolic epoxy resin obtained in Example 1, 0.3873 g of polyetheramine (Jeffamine® ED-2003), and 0.2251 g of the polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 were mixed and stirred in acetone (solvent) at 80° C. for 2 hours, poured onto a film forming plate made of polypropylene, and then disposed in an oven set at 80° C. for 24 hours so as to permit a crosslinking reaction to occur and to permit removal of the solvent therefrom, thereby obtaining a polymer membrane. A molar ratio of epoxy group to —CH 2 CH 2 O— chain (EO) in the mixture of this example was the same as that of Example 14.

0.0985 g of the purified phenolic epoxy resin obtained in Example 1, 0.2767 g of polyetheramine (Jeffamine® ED-2003), and 0.3752 g of the polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 were mixed and stirred in acetone (solvent) at 80° C. for 2 hours, poured onto a film forming plate made of polypropylene, and then disposed in an oven set at 80° C. for 24 hours so as to permit a crosslinking reaction to occur and to permit removal of the solvent therefrom, thereby obtaining a polymer membrane. A molar ratio of epoxy group to —CH 2 CH 2 O— chain (EO) in the mixture of this example was the same as that of Example 14.

0.0591 g of the purified phenolic epoxy resin obtained in Example 1, 0.166 g of polyetheramine (Jeffamine® ED-2003), and 0.5253 g of the polymeric ionic liquid (PIL-PF 6 anion) obtained in Example 1 were mixed and stirred in acetone at 80° C. for 2 hours, poured onto a film forming plate made of polypropylene, and then disposed in an oven set at 80° C. for 24 hours so as to permit a crosslinking reaction to occur and to permit removal of the solvent therefrom, thereby obtaining a polymer membrane. A molar ratio of epoxy group to —CH 2 CH 2 O— chain (EO) in the mixture of this example was the same as that of Example 14.

Coin Cell Type Battery for Electrochemical Stability Test

A circular piece (diameter Φ=15 mm) was cut from the polymer membrane obtained in Example 5, and soaked in a LiPF 6 electrolyte solution (Ubiq Technology Co., Ltd.) for 24 hours in an argon-filled glovebox to obtain a gel polymer electrolyte. The LiPF 6 electrolyte solution had a LiPF 6 concentration of 1.0M in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1 in V/V). A coin cell type battery was assembled in a dry, oxygen free glovebox. In the coin cell type battery, the gel polymer electrolyte served as a separator, a lithium foil served as a counter and reference electrode, and a stainless steel disc served as a working electrode.

A coin cell type battery of Example 19 was prepared according to a procedure similar to that described in Example 18, except that the polymer membrane obtained in Example 6 was used to replace the polymer membrane of Example 5.

A coin cell type battery of Example 20 was prepared according to a procedure similar to that described in Example 18, except that the polymer membrane obtained in Example 10 was used to replace the polymer membrane of Example 5.

A coin cell type battery of Example 21 was prepared according to a procedure similar to that described in Example 18, except that the polymer membrane obtained in Example 11 was used to replace the polymer membrane of Example 5.

A coin cell type battery of Example 22 was prepared according to a procedure similar to that described in Example 18, except that the polymer membrane obtained in Example 12 was used to replace the polymer membrane of Example 5.

Coin Cell Type Battery for Ionic Conductivity Test

A circular piece (diameter Φ=15 mm) was cut from the polymer membrane obtained in Example 7, and soaked in the above-mentioned LiPF 6 electrolyte solution for 24 hours in an argon-filled glovebox to obtain a gel polymer electrolyte. A coin cell type battery having the gel polymer electrolyte sandwiched between two parallel stainless steel discs was assembled in the glovebox.

A coin cell type battery of Example 24 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Example 8 was used to replace the polymer membrane of Example 7.

A coin cell type battery of Example 25 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Example 9 was used to replace the polymer membrane of Example 7.

A coin cell type battery of Example 26 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Example 10 was used to replace the polymer membrane of Example 7.

A coin cell type battery of Example 27 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Example 11 was used to replace the polymer membrane of Example 7.

A coin cell type battery of Example 28 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Example 12 was used to replace the polymer membrane of Example 7.

COMPARATIVE EXAMPLE 2 (CE 2)

A coin cell type battery of Comparative Example 2 was prepared according to a procedure similar to that described in Example 23, except that the polymer membrane obtained in Comparative Example 1 was used to replace the polymer membrane of Example 7.

Coin Cell Type Battery for Battery Capacity Test

A circular piece (diameter Φ=15 mm) was cut from the polymer membrane obtained in Example 9, and soaked in the above-mentioned LiPF 6 electrolyte solution for 24 hours in an argon-filled glovebox to obtain a gel polymer electrolyte.

A slurry mixture was prepared by adding 80 wt % LiFePO 4 powder (Aleees Advanced Lithium Electrochemistry Co. Ltd., Taiwan), 10 wt % conductive carbon black (Super P from Timcal), and 10 wt % polyvinylidene fluoride (PVDF) to N-methyl-2-pyrrolidone (NMP), followed by stirring for 24 hours. The LiFePO 4 powder and the conductive carbon black were ground in advance. A cathode was prepared by coating the slurry mixture evenly onto a high purity aluminum foil, drying at 90° C. for 24 hours in vacuum to obtain a cathode film, pressing the cathode film evenly using a rolling machine, and then cutting the cathode film into a cathode disc. A high purity lithium metal served as an anode.

A coin cell type battery having the gel polymer electrolyte sandwiched between the cathode and the anode was assembled in the glovebox.

A coin cell type battery of Example 30 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 10 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 31 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 11 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 32 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 12 was used to replace the polymer membrane of Example 9.

COMPARATIVE EXAMPLE 3 (CE 3)

A coin cell type battery of Comparative Example 3 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Comparative Example 1 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 33 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 13 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 34 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 14 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 35 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 15 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 36 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 16 was used to replace the polymer membrane of Example 9.

A coin cell type battery of Example 37 was prepared according to a procedure similar to that described in Example 29, except that the polymer membrane obtained in Example 17 was used to replace the polymer membrane of Example 9.

Coin Cell Type Battery for Positive Electrode Test

A slurry mixture was prepared by adding 80 wt % LiFePO 4 powder (Aleees Advanced Lithium Electrochemistry Co. Ltd., Taiwan), 10 wt % conductive carbon black (Super P from Timcal), and 10 wt % binder to N-methyl-2-pyrrolidone (NMP), followed by stirring for 24 hours. The binder included the polymeric ionic liquid (the PIL-TFSI anion obtained in Example 2) and polyvinylidene fluoride (PVDF), in which a weight ratio of the PIL-TFSI anion to PVDF was 3:7.

A positive electrode was prepared by coating the slurry mixture evenly onto a high purity aluminum foil, drying at 100° C. for 24 hours in vacuum to obtain an electrode film, pressing the electrode film evenly using a rolling machine, and then cutting the electrode film into a positive electrode disc (diameter Φ=13 mm). A high purity lithium metal served as a negative electrode.

A commercial separator (Celgard M824) was soaked in the above-mentioned LiPF 6 electrolyte solution for 24 hours in an argon-filled glovebox to obtain a polymer electrolyte.

A coin cell type battery having the polymer electrolyte sandwiched between the positive and negative electrodes was assembled in the glovebox.

A coin cell type battery of Example 39 was prepared according to a procedure similar to that described in Example 38, except that in the binder, a weight ratio of the PIL-TFSI anion to PVDF was 1:1.

COMPARATIVE EXAMPLE 4 (CE 4)

A coin cell type battery of Comparative Example 4 was prepared according to a procedure similar to that described in Example 38, except that the binder included only the PVDF.

Coin Cell Type Battery Having an Electrospun Polymer Membrane

The polymeric ionic liquid (PIL-TFSI anion) obtained in Example 2 and PVdF-co-HFP were dissolved in dimethylformamide (DMF) to obtain a homogeneous solution (10 wt %), in which a weight ratio of PIL-TFSI anion to PVdF-co-HFP was 1:1. The homogeneous solution was subjected to electrospinning at 20 kV using an electrospinning device. The homogeneous solution was accelerated to flow out from a nozzle tip of the electrospinning device toward a collector at a flow rate of 0.25 ml/hr to obtain polymer nanofibers. A distance from the nozzle tip to the collector was set to be 20 cm. After a period of electrospinning time, an electrospun polymer membrane having a thickness of 20 μm was obtained. The electrospun polymer membrane was dried overnight at 60° C. to remove residual DMF.

A coin cell type battery was prepared according to a procedure similar to that described in Example 18, except that the electrospun polymer membrane was used to replace the polymer membrane obtained in Example 5.

<Evaluation>

1 H-NMR Analysis

A test sample was placed in an oven (80° C.) for 12 hours and then placed in a quartz tube. Deuterated acetone was used as a solvent for the test sample. 1 H-NMR spectra were recorded in a Bruker AV-600 spectrometer. The 1 H-NMR spectrum for the purified phenolic epoxy resin and the brominated intermediate polymer obtained in Example 1 is shown in FIG. 1 . The 1 H-NMR spectrum for the PIL-TFSI anion obtained in Example 2 is shown in FIG. 2 .

It can be noted from FIG. 1 that the purified phenolic epoxy resin had chemical shifts (δ=2.5˜2.8 ppm, and δ=3.1˜3.3 ppm) due to hydrogen atoms in the epoxy group (near the positions 1, 1′, and 2). The brominated intermediate polymer did not have the above chemical shifts. Instead, the brominated intermediate polymer exhibited relatively high chemical shifts (δ=3.4˜3.7 ppm, and δ=4.1 ppm) due to a reaction of hydrogen bromine and the epoxy group. Therefore, the reaction between the purified phenolic epoxy resin and the hydrobromic acid was confirmed to be a ring opening reaction.

It can be noted from the spectrum of the PIL-TFSI anion shown in FIG. 2 that a ratio of an integrated intensity of the chemical shift (δ=8.6˜9.1 ppm) to an integrated intensity of the chemical shift (δ=7.3˜7.8 ppm) was 1/2. This confirmed that an anion of 1-methylimidazole did exist in the PIL-TFSI anion of Example 2.

FT-IR Analysis

A test sample and an IR transparent crystal (potassium bromide, KBr) were mixed and ground into fine powders, and then the mixture was pressed into a KBr disk. The KBr disk was placed in an oven (80° C.) for 12 hours. Thereafter, the KBr disk was analyzed by FT-IR (NICOLET 5700) at room temperature in a wavelength range between 4000 cm −1 and 400 cm −1 . The IR spectrum for the PIL-Br anion and the PIL-PF 6 anion obtained in Example 1 are shown in FIG. 3(A) , and the IR spectrum for the PIL-Br anion obtained in Example 1 and the PIL-TFSI anion obtained in Example 2 are shown in FIG. 3(B) .

From the IR spectrum shown in FIG. 3(A) , the characteristic peaks of the PF 6 anion at 563 cm −1 and 882 cm −1 were found. This confirmed that the PIL-PF 6 anion was obtained through an anion replacement reaction.

From the IR spectrum shown in FIG. 3(B) , the characteristic peaks of TFSI anion listed in Table 1 were found. This confirmed that the PIL-TFSI anion was obtained through an anion replacement reaction.

Heat Stability Test

A polymer membrane (3˜5 mg) was analyzed using a thermogravimetric analyzer (Perkin Elmer TGA 7 ) under a nitrogen atmosphere. The polymer membrane in the thermogravimetric analyzer was heated at a heating rate of 20° C./min to 120° C. to remove moisture therefrom. After the temperature of the polymer membrane was down to 50° C., the polymer membrane was further heated in the thermogravimetric analyzer at a heating rate of 20° C./min to 800° C. for the heat stability test. The TGA test results for the polymer membranes of Examples 5 and 6 are shown in FIG. 4 . The TGA test results for the polymer membranes of Examples 6 and 10˜12 and Comparative Example 1 are shown in FIG. 5 . The temperature when the polymer membrane had a weight loss of 5% was defined as a degradation temperature (Td). The degradation temperatures for the polymer membranes of Examples 5, 6, and 10˜12 and Comparative Example 1 are shown in Table 2.

As shown in Table 2, the degradation temperatures of the polymer membranes of Examples 5, 6, and 10˜12 and Comparative Example 1 are all higher than 250° C. Under a working temperature (25° C.˜90° C.) of a lithium-ion battery, the polymer membranes of Examples 5, 6, and 10˜12 and Comparative Example 1 have good heat stability.

Thermal Transition Property Test

A thermal transition property of a dried polymer membrane (3˜5 mg) was measured under a nitrogen atmosphere using a TA Instruments DSC 2010 in a temperature range from −150° C. to 150° C. with the temperature rising at a rate of 10° C./min. The nitrogen flow rate was controlled at 55 ml/min. DSC curves for the polymer membranes of Examples 5 and 6 are shown in FIG. 6 . DSC curves for the polymer membranes of Examples 10˜12 and Comparative Example 1 are shown in FIG. 7 .

From the DSC curves, melting enthalpy (ΔH) of PVdF-co-HFP for each polymer membrane were calculated, and glass transition temperature (Tg) and melting temperature (Tm) were observed. The data of ΔH, Tg, and Tm are summarized in Table 3.

From the data of ΔH shown in Table 3, it was noted that the crystallinity of the PVdF-co-HFP deteriorated due to the existence of the polymer ionic liquid.

Electrochemical Stability Test

An electrochemical stability window of a coin cell type battery was determined by linear sweep voltammetry using an electrochemical instrument (CHI604A, CH Instrument, Inc.). The potential was swept from an open circuit voltage to 6 V at a scanning rate of 5 mVs −1 . The electrochemical stability results (linear sweep voltammetry curves) for the coin cell type batteries of Examples 18 and 19 are shown in FIG. 8 , and the electrochemical stability results for the coin cell type batteries of Examples 20 to 22 are shown in FIG. 9 .

It can be seen from FIG. 8 that the coin cell type battery of Example 18 (which had the polymeric ionic liquid (PIL-PF 6 anion) of Example 1) had an oxidation voltage of about 4.5 V, and that the coin cell type battery of Example 19 (which had the polymeric ionic liquid (PIL-TFSI anion) of Example 2) had an oxidation voltage of about 4.7 V.

It can be seen from FIG. 9 that the coin cell type battery of Examples 20 to 22 (which had the polymeric ionic liquid (PIL-TFSI anion) of Example 2 in different weight ratios) had oxidation voltages of about 4.5 V.

From the oxidation voltages of the coin cell type battery of Examples 18˜22, it can be anticipated that when the polymer membranes including the polymeric ionic liquids of Example 1 or 2 are applied to the gel polymer electrolytes of lithium-ion batteries, the lithium-ion batteries will have high electrochemical stability under an operating voltage (2.5 V˜4.2 V).

Ionic Conductivity Test

Ionic conductivities of a gel polymer electrolyte in a coin cell type battery at different temperatures were determined by electrochemical impedance spectroscopy on an electrochemical instrument (CHI604A, CH Instrument, Inc.) using alternative current signals with a potential amplitude of 10 mV and frequencies from 100 kHz to 10 Hz.

Ionic conductivity (σ)=l/RS

where l is the thickness of the gel polymer electrolyte, S is the contact area between the gel polymer electrolyte and the stainless steel discs, and R is the bulk resistance of the gel polymer electrolyte.

The ionic conductivities at different temperatures (30° C.˜80° C.) for the gel polymer electrolytes of the coin cell type batteries of Examples 23 to 25 and Comparative Example 2 are shown in FIG. 10 . The ionic conductivities at different temperatures (30° C.˜80° C.) for the gel polymer electrolytes of the coin cell type batteries of Examples 26 to 28 and Comparative Example 2 are shown in FIG. 11 .

The gel polymer electrolyte in Comparative Example 2 had poor ionic conductivities between 30° C. and 80° C.

At 30° C., the ionic conductivities of the gel polymer electrolytes (each including PIL-PF 6 anion) in Examples 23 to 25 were 0.57×10 −3 mScm −1 , 1.09×10 −3 mScm −1 , and 1.13×10 −3 mScm −1 , respectively, and the ionic conductivities of the gel polymer electrolytes (each including PIL-TFSI anion) in Examples 26 to 28 were 0.92×10 −3 mScm −1 , 1.16×10 −3 mScm −1 , and 1.95×10 −3 mScm −1 , respectively.

At 80° C., the ionic conductivities of the gel polymer electrolytes (each including PIL-PF 6 anion) in Examples 23 to 25 were 1.57×10 −3 mScm −1 , 4.07×10 −3 mScm −1 , and 5.17×10 −3 mScm −1 , respectively, and the ionic conductivities of the gel polymer electrolytes (each including PIL-TFSI anion) in Examples 26 to 28 were 1.26×10 −3 mScm −1 , 3.40×10 −3 mScm −1 , and 6.66×10 −3 mScm −1 , respectively.

It can be concluded that the gel polymer electrolytes in the coin cell type batteries of Examples 23 to 28 had ionic conductivities higher than that of Comparative Example 2.

Battery Capacity Test—1

A coin cell type battery was charged at 0.1 C, followed by discharging at different discharging rates. This test was performed galvanostatically between 2.5 V and 4.0 V at room temperature on a Battery Automation Test system (Acu Tech Systems, BAT-750B).

The battery capacity test result for the coin cell type battery of Comparative Example 3 is shown in FIG. 12 . It can be seen from FIG. 12 that the coin cell type battery of Comparative Example 3 had a specific discharge capacity close to zero, and could not function as a lithium-ion battery.

The battery capacity test for the coin cell type batteries of Examples 29 to 32 are shown in FIGS. 13(A) to 13(D) , respectively. The specific discharge capacities for the coin cell type batteries of Examples 29 to 32 are summarized in Table 4.

The battery capacity test results for the coin cell type batteries of Examples 33 to 37 are shown in Table 5.

It is known in the art that a specific discharge capacity of a lithium-ion battery will decrease with an increase in discharging rate. It can be seen from Table 4 and Table 5 that when the coin cell type batteries of Examples 29 to 37, especially the coin cell type batteries of Examples 29 to 32, discharged at a relatively high discharging rate (1 C or 3 C), the specific discharge capacities thereof were not greatly reduced.

Limiting Oxygen Index Test

Two polymer membranes which were substantially the same were prepared. One of the polymer membranes was soaked in the above-mentioned LiPF 6 electrolyte solution for 24 hours in an argon-filled glovebox to obtain a gel polymer electrolyte, but the other was not. For each of the polymer membranes, a limiting oxygen index (LOI), which is the minimum concentration of oxygen that will support combustion of the polymer membrane, was measured using a limiting oxygen index tester (ALTAS). In this test, volume flow rates of oxygen and nitrogen were adjusted until the flame on the polymer membrane just went out. The ratio of the volume flow rate of oxygen to the sum of the volume flow rates of oxygen and nitrogen at this point was then determined as the LOI value.

The LOI values for the polymer membranes of Examples 9 and 12 and the gel polymer electrolytes made therefrom are shown in Table 6.

The LOI values shown in Table 6 are all greater than 20%, which is equivalent to the oxygen content in the air. This proves the polymer membranes including the polymeric ionic liquid of Example 1 or 2 are less likely to burn in the air. Especially, the gel polymer electrolyte made by soaking the polymer membrane in a flammable electrolyte still had the LOI value greater than 20%. Therefore, a lithium ion battery having the gel polymer electrolyte can have improved safety.

Positive Electrode Test

A coin cell type battery was charged at 0.1 C and discharged at different discharging rates (0.1 C, 0.3 C, 0.5 C, 1 C, 2 C, 3 C, 4 C, and 5 C). This test was performed galvanostatically between 2.5 V and 4.0 V at room temperature on a Battery Automation Test system (Acu Tech Systems, BAT-750B). FIGS. 14(A), 14(B) , and 14 (C) are charge-discharge curves for the coin cell type batteries of Examples 38 and 39 and Comparative Example 4, respectively. The specific discharge capacities for the coin cell type batteries are summarized in Table 7.

It can be noted from Table 7 that when the coin cell type batteries were discharged at a relatively high rate (5 C), the specific discharge capacity of the coin cell type battery of Example 38 was much higher than that of Comparative Example 4. This means that the polymeric ionic liquid (the PIL-TFSI anion) can be used as a binder of a positive electrode for improving the specific discharging capacity of a lithium-ion battery.

Battery Capacity Test—2

A coin cell type battery of Example 40 was charged at 0.1 C, followed by discharging at 0.1 C. The battery capacity test result for the coin cell type battery of Example 40 is shown in FIG. 15 . As shown in FIG. 15 , the specific discharge capacity of the coin cell type battery of Example 40 was about 150 mAhg −1 .

While the present invention has been described in connection with what are considered the most practical embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretations and equivalent arrangements.