Structural investigations of polymer electrolyte poly(propylene oxide)-LiClO4 using diffraction experiments and reverse Monte Carlo simulation

Abstract

The structure of an amorphous polymer electrolyte, poly(propylene oxide) (PPO) complexed with LiClO4, has been studied using reverse Monte Carlo (RMC) simulations. The simulations require no force field but are based on experimental data only, in this case from x-ray and neutron diffraction experiments. Excellent agreement between the experimental data and the structures resulting from the RMC simulation is obtained. Samples with ether-oxygen to lithium concentrations (molar ratios) O:Li=16:1 and 5:1 were studied and compared to results of pure PPO from a previous study. We focus on the effects of the solvated salt on the structure of the polymer matrix, the spatial distribution of ions, and the correlations between the anions and the polymer chains. Analyzing the structures produced in the simulations, we find that for a concentration 16:1, the interchain distance is approximately the same as in pure PPO but more well defined. For a concentration 5:1, we find a larger and less well-defined interchain distance compared to the 16:1 concentration. This signifies that at the 16:1 salt concentration, there is enough free volume in the polymer host to accommodate the ions, and that the solvation of salt induces ordering of the polymer matrix. At the higher salt concentration 5:1, the polymer network must expand and become less ordered to host the ions. We also note, in accordance with previous studies, that the solvation of salt changes the conformation of the polymer chain towards more gauche states. The simulations furthermore reveal marked correlations between the polymer chains and the anions, which we suggest arise predominantly from an interaction mediated via cations, which can simultaneously coordinate both ether oxygens in the polymer chains and anions. Interanionic distances at 5 A, which are consistent with two or more anions being coordinated around the same cation, are also observed. On a larger scale, the RMC structure of PPO-LiClO4 16:1 clearly indicates the presence of salt-rich and salt-depleted domains having a length scale of <20 A. In view of such a heterogeneous structure of PPO-LiClO4 16:1, it is plausible that the increased ordering of the polymer matrix is due to rather well-defined structural arrangements within the salt-rich domains, and that the characteristic interchain distance in the salt-rich domains is similar to that of the pure polymer.

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4. I. Insights from NMR line-shape analysis

“…8,[10][11][12][13][14][15][16] Given that the rate of the polymer segmental motion is related to the local salt concentration, the existence of well defined microphases should manifest itself in the existence of a bimodal distribution G͑log ͒. Thus, the width of G͑log ͒ implies strong fluctuations in the local salt concentration for the intermediate 15:1 composition, while the structural diversity is reduced for both high and low salt contents.…”

“…Following the common nomenclature in the literature, 8,[13][14][15]21 we disregard the difference between PPO and PPG and refer to the studied polymer as PPO. PPO ͑M w = 6510 g / mol͒ and LiClO 4 were purchased from Polymer Source Inc. and Aldrich, respectively.…”

“…The electric conductivity dc at room temperature shows a broad maximum at intermediate O:Li ratios ranging from about 10:1 to 30:1. 15 Specifically, the RMC model of PPO-LiClO 4 features salt-rich and salt-depleted regions on a small length scale of about 1 nm. Likewise, an inhomogeneous structure in the intermediate composition range was proposed to explain results from dielectric spectroscopy ͑DS͒, 8 which give evidence for the existence of two processes related to fast and slow polymer dynamics.…”

Dinâmica molecular de eletrólitos poliméricos

“…A estrutura local de eletrólitos poliméricos mostrada na Figura 1.3, em que os cátions são solvatados pelos átomos de oxigênio, neste caso um poliéter, tem sido verificada pelas técnicas de espalhamento de raios X [22,23] e de espalhamento de nêutrons [24][25][26][27][28]. A espectroscopia vibracional Raman e de absorção no infravermelho têm sido utilizadas para estudar o efeito de formação de par iônico nos eletrólitos poliméricos [29][30][31].…”

“…O autotermo do fator de espalhamento intermediário calculado para o POE puro em 0,50 A-I, 1,00 k l e 1,50 k l está apresentado na Figura 4 28…”