Abstract

In this study, a lithium ion conducting polymerized ionic liquid pentablock terpolymer (PILPTP) was investigated as a battery electrolyte (Xiamen Tmaxcn Inc.) for lithium ion batteries. The ABCBA pentablock terpolymer, poly(tbS-b-EP-b-MS-b-EP-b-tbS) (tbS = tert-butyl-styrene; EP = ethylene-r-propylene; MS = 4-methylstyrene), was brominated and quaternized to covalently attach two different cations (methylimidazolium and methylpyrrolidinium) to the C block and subsequently ion exchanged to form two different TFSI-exchanged PILPTPs (MPyr-TFSI and MIm-TFSI; TFSI = bis(trifluoromethane)sulfonimide). Free standing, mechanically stable, transparent SPE films were produced with MPyr-TFSI and MIm-TFSI containing 1 M LiTFSI/ionic liquid (IL) (IL = EMIm-TFSI or PYR14-TFSI; EMIm = 1-ethyl-3-methylimidazolium, PYR14 = 1-butyl-1-methylpyrrolidinium), referred to as MPyr-TFSI+Li-TFSI/PYR14-TFSI and MIm-TFSI+Li-TFSI/EMIm-TFSI. Both SPEs show promising ionic conductivities,electrochemical stabilities, and stripping and plating stabilities. Specifically, the MIm-TFSI+LiTFSI/EMIm-TFSI SPE possessed an ionic conductivity of 0.1 mS cm-1 at 28 °C; the MPyrTFSI+Li-TFSI/PYR14-TFSI SPE possessed an electrochemical stability window of 4.2 V versusLi/Li+ at room temperature; the MPyr-TFSI+Li-TFSI/PYR14-TFSI SPE exhibited stable stripping and platting overvoltage profiles over 500 cycles at 70 °C. These results demonstrate the feasibility of a PIL multiblock polymer as an SPE for lithium ion batteries.

Keywords: multiblock polymer; ionic liquid; battery

2. Experimental

2.1. Materials

 Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%), and lithium ribbon (0.38 mm

× 23 mm, 99.9%) were used as received from Sigma-Aldrich. 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI, 99%, IoLiTec) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14-TFSI, 99%, IoLiTec) were dried under dynamic vacuum for 24 h and stored in argon-filled vacuum glove box (Xiamen Tmaxcn Inc.) before use. Conductive carbon-coated aluminum foil (0.05 mg cm-2),coin cell cases with O-rings for battery research, stainless steel spacers for CR2032 cells (15.5 mm diameter × 0.5 mm thickness, 15.5 mm diameter × 0.2 mm thickness), and stainless steel wave springs for CR2032 cases were used as received from Xiamen TMAX Battery Equipments Limited.Mylar PET release liner substrates (Grade 26965, 0.0762 mm) were used as received from LOPAREX. Deionized water with resistivity > 18 MΩ cm was used as appropriate.

2.4. Electrochemical tests

All the electrochemical test cells were prepared and assembled in an argon-purged glovebox (both water and oxygen concentrations < 5 ppm). Ionic conductivity and linear voltammetry were measured with an impedance analyzer (Solartron 1260) and potentiostat/galvanostat (Solartron 1287), respectively. A two-electrode cell was used for ionic conductivity measurements, where SPEs were sandwiched between two stainless steel solid blocking electrodes (surface area = 1.2161 ± 0.0015 cm2) within a sealable Telfon custom-made cell [28]. Impedance scans (Nyquist plots) were measured at 10 mV amplitude over a frequency range of 1 MHz to 1 Hz at open circuit potential at a temperature range of 28 to 105 °C controlled by heating tape (BriskHeat; XtremeFLEXSDC) and a digital temperature controller with type thermocouple (Model 650, OMEGA). SPEs were equilibrated for at least 1.5 h at each temperature. Ionic conductivity was calculated by using the following equation:  = L/AR, where L and A is the thickness and cross-section area of the SPE, respectively; resistance, R, was determined from the semi-circle regression of high x-intercept from the Nyquist plot.

Electrochemical stability was determined via linear sweep voltammetry (LSV) with conductive carbon as working electrode and lithium metal as counter and reference electrodes. The test cell

was assembled in an argon-filled glove box by sandwiching the SPE films between lithium ribbon (counter and reference electrode, 12 mm diameter) and conductive carbon electrode (working electrode, 12 mm diameter) in a CR2032 coin cell. Additional drops of 1.0 M LiTFSI/IL (80 mg) were added to each electrode during assembly to improve the contact between electrodes and SPE. Cells were then pressed twice using an electric coin cell crimper. The cell was examined at a voltage rate of 1 mV s-1 from -1 to 6 V (vs. Li/Li+) under ambient temperature.

The SPE cyclability and stability with lithium metal was evaluated using a battery tester (Xiamen Tmaxcn Inc.) by stripping and plating. The test cell was assembled by sandwiching the lithium ion conducting SPE between two lithium ribbons (12 mm dia.) using similar assembly process as described above. Symmetrical lithium metal/SPE/lithium metal cells were examined under constant current (0.02 mA cm-2, reversed polarization every 1h) at 70 °C controlled by a temperature chamber (MTC-020, MACCOR). Impedance scans were collected with an impedance analyzer every 10th polarization cycle at 10 mV amplitude at a frequency range of 100 kHz to 1 Hz.