Highly conductive, oriented polymer electrolytes for lithium batteries

Abstract

In semicrystalline complexes of poly(ethylene oxide) (PEO) with different salts, such as lithium iodide, lithium trifluoromethanesulfonate (LiTF) and lithium trifluoromethanesulfonimide (LiTFSI), stretching induced longitudinal DC conductivity enhancement was observed, in spite of the formation of more ordered polymer electrolyte (PE) structure. It was found that the more amorphous the PE, the less its lengthwise conductivity is influenced by stretching. The results of our investigation suggest that ionic transport occurs preferentially along the PEO helical axis, at least in the crystalline phase, and that the rate‐determining step of the lithium ion conduction in LiI:P(EO)20, LiTF:­P(EO)20 polymer electrolytes below Tm is “interchain” hopping. Understanding ion transport processes is clearly a fertile field for research and development in the synthesis of new rigid polymers with ordered channels and composition appropriate for enhanced ionic conductivity. Copyright © 2003 John Wiley & Sons, Ltd.

New Generation of Ordered Polymer Electrolytes for Lithium Batteries

“…1. As was found previously, [19][20][21][22][23][24][25] there is some planar orientation of helices in as-cast polymer electrolytes. Under the magnetic field gradient ͑GMF͒ directed perpendicularly to the plane of the cast film a considerable number of the bundles of helices are aligned in the Z direction.…”

“…The ac conductivity in the perpendicular direction was measured ex situ with the use of a computer-interfaced Solartron 1260 frequency-response analyzer over the frequency range 1 MHz to 0.1 Hz. [22][23][24] The polymer electrolyte sample of 1.8 cm 2 area was placed between two lithium electrodes and hermetically sealed in a 2032 coin cell. A typical complex impedance plot of a symmetrical Li/ PE/Li cell is represented by two partially depressed semicircles ͑where X and Y axes represent real and imaginary resistances, respectively, plot not shown here͒.…”

“…Recently, we found a way [19][20][21][22][23][24] to increase the ionic conduction of PEs in one direction by a longitudinal alignment of the helices with the use of a stretching procedure. Stretching-induced longitudinaldc-conductivity enhancement, observed in crystalline LiI-P͑EO) n and Li triflate-P͑EO) 20 electrolytes, was about 5-to 40-fold and reached 0.1 mS/cm at room temperature, one order of magnitude higher that the best reported value.…”

“…Fourier transform infrared spectrometry ͑FTIR͒ tests showed that stretched polymer electrolyte adopts an aligned helical conformation, in which the CH 2 groups all face outward ͑favoring CH 2 wagging vibrations͒ and the atoms of oxygen are directed inward, lining the tunnel cavity, and thus favoring cation transport. [19][20][21][22][23][24][25] Recognition that aligned crystalline PEO chains are energetically more favorable for fast ion transport than are tangled chains [19][20][21][22][23][24][25][26][27] prompted our attempts to achieve orientation of the PEO helices perpendicular to the plane of the film, and doing so without mechanical means ͑i.e., stretching͒.In this study we have examined the possibility of producing a rational design of novel polymer materials with extraordinary electrical properties on the basis of structural ordering. The effect of PE casting under a magnetic field and incorporation of diamagnetic and paramagnetic nanosize inorganic fillers is addressed.…”

“…10 Theoretical models [11][12][13] have been proposed to explain the mechanism responsible for ionic conductivity in these systems, and molecular dynamic ͑MD͒ and Monte Carlo ͑MC͒ simulations have been carried out for PEO:salt complexes. [14][15][16][17][18] Recently, we found a way [19][20][21][22][23][24] to increase the ionic conduction of PEs in one direction by a longitudinal alignment of the helices with the use of a stretching procedure. Stretching-induced longitudinaldc-conductivity enhancement, observed in crystalline LiI-P͑EO) n and Li triflate-P͑EO) 20 electrolytes, was about 5-to 40-fold and reached 0.1 mS/cm at room temperature, one order of magnitude higher that the best reported value.…”