Fast Ion Transport Phenomena in Oriented Semicrystalline LiI−P(EO)_n-Based Polymer Electrolytes

Abstract

We have employed a variety of experimental methods, including DC and AC conductivity, scanning electron microscopy (SEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, and pulsed field gradient nuclear magnetic resonance (NMR), to investigate the poly(ethylene oxide):LiI system. The effect of stretching the polymer electrolyte on its DC conductivity is dramatic, resulting in up to a 40-fold increase in the LiI P(EO) 7 composition. Structural ordering imposed by the stretching is observed in SEM and AFM images, and the cation solvation sheath (i.e., the helical PEO structure) is also affected by stretching in a manner believed to favor enhanced transport, according to the FTIR results. The NMR results demonstrate unambiguously that Li + diffusivity is anisotropic and enhanced along the stretch direction. Although the cation transport mechanism in polyether-salt polymer electrolytes is believed to rely heavily on polymer segmental mobility, this investigation suggests that other factors also contribute significantly. Such factors which can be augmented by stretching are modest changes in the cation solvation sheath and alignment of the helical structural units characteristic of PEO and its salt complexes.

New Generation of Ordered Polymer Electrolytes for Lithium Batteries

“…22 All handling of PEs took place under an argon atmosphere in a VAC glove box with water content Ͻ10 ppm. LiI:P͑EO) n polymer electrolytes with n ϭ 3, 7, and 20 were cast under a strong ͑0.8 T͒ magnetic field, created by samarium-cobalt permanent magnets in the standard dipole arrangement in a home-made device.…”

“…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.…”