Josefina Adebahr

The effect of nano-particle TiO2 fillers on structure and transport in polymer electrolytes

Nano-particle oxide fillers including TiO2, SiO2 and Al2O3 have previously been shown to have a significant affect on the properties of polymer electrolytes, especially those based on polyether–lithium salt systems. In some cases, conductivity increases of more than one order of magnitude have been reported in crystalline PEO-based complexes. In this work, we report on the effects of TiO2 on a completely amorphous polyether-based system to remove the complication of multiple phases presented by the semi-crystalline nature of PEO. Multinuclear magnetic resonance spectroscopy has shown that the lithium ion environment is changed by the addition of filler. Vibrational spectroscopy shows that the filler influences the disordered-longitudinal acoustic modes (DLAM) in the case of an amorphous polyether and suggests an interaction between the filler surface and the polymer. Positron annihilation lifetime spectroscopy indicates an increase in free volume upon addition of filler to an amorphous polyether–salt complex, coinciding with an apparent increase in polymer mobility as determined from 1H T2 NMR measurements. Impedance spectroscopy has shown clear evidence of an inter-phase region that may be more or less conductive than the bulk polymer electrolyte itself. The data support a model which includes conduction through an interfacial region in addition to the bulk polymer

Enhancement of ion dynamics in PMMA-based gels with addition of TiO2 nano-particles

Solvent and ion dynamics in PMMA based gels have been investigated as a function of the loading of nanosized TiO2 particles. The gels have a molar ratio of 46.5:19:4.5:30 of ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate and PMMA, respectively. A series of samples with 0, 4, 6 and 8 wt.% TiO2 filler were investigated. The diffusion coefficients for the lithium ions and for the two solvents (EC and PC) were investigated by pfg-NMR. It was shown that the addition of filler to the gel electrolytes enhances the diffusion of the cations, while the diffusion of the solvents remains constant. Raman measurements show no significant changes in ion–ion interactions with the addition of fillers, while the ionic conductivity is seen to decrease. However, the sample with 8 wt.% TiO2 had a conductivity close to that of the unfilled sample.

Ion transport in polymer electrolytes containing nanoparticulate TiO2: The influence of polymer morphology

Recent studies have shown that composite polymer electrolytes, formed by dispersing nanosized ceramic particles in polyether-based electrolytes, have improved ion transport properties as compared to their unfilled analogues. In the present study polymer electrolytes with different loadings of nano-sized ceramic particles (TiO2) and different polymer chemistry and morphology have been investigated. Of special interest are filler induced effects on polymer, solvent and cationic mobility. Partly crystalline polymer electrolytes based on poly(ethylene oxide) have been compared to fully amorphous polymer electrolytes based on a polyether urethane, as well as gel electrolytes based on PMMA. 7Li pfg-NMR, linewidth and spin–spin relaxation times as well as 1H pfg-NMR and spin–spin relaxation times, were measured as a function of temperature and composition. The 1H spin–spin relaxation measurements reveal increased average polymer mobility with the addition of filler up to a maximum at 4 and 8 wt.% TiO2 for the fully amorphous and the partly crystalline electrolytes, respectively. The 7Li linewidth measurements for the fully amorphous system show a broadening of the linewidth with addition of filler. Based on variable temperature measurements this broadening is interpreted as a result of the inhomogeneity introduced by the filler particles. Pulsed field gradient (pfg) diffusion measurements were employed to determine ion and solvent self-diffusion coefficients. In the case of the PMMA-based gel electrolyte and the fully amorphous electrolytes enhanced cation self-diffusion was observed upon addition of TiO2.

Diffusive and segmental dynamics in polymer gel electrolytes

Dynamic light scattering data on a polymer gel electrolyte with a complex relaxation behavior is presented. The electrolyte consists of lithium perchlorate dissolved in an ethylene carbonate and propylene carbonate solution that is immobilized with poly(methyl methacrylate). We attribute the observed relaxation processes to two diffusive and one segmental relaxation processes based on the form of the time decay of the intermediate scattering function and the corresponding temperature and wave vector dependencies. The dynamic light scattering results are compared with the ionic conductivity, which reveals a close connection between the fast diffusive motion of the low molecular weight solvent within the gel and the ionic conductivity. This motion is strongly decoupled from and considerably faster than the segmental motion of the polymer matrix. The results indicate that the ionic transport occurs mainly within the low molecular weight solvent.

7Li NMR measurements of polymer gel electrolytes

Ion conducting polymer gels prepared from (ethylene oxide)n grafted methacrylates, ethylene carbonate (EC), gamma butyrolactone (gBL), and lithium hexafluorophosphate are studied by means of nuclear magnetic resonance spectroscopy. This study shows that there are at least two possible lithium sites with different mobility. The lithium-ions with lower mobility dominate at room temperature, but this is changed as the temperature is increased. The NMR results also show that the 7Li spin–spin relaxation time decreases with increasing length of the grafted ethylene oxide side chains, indicating a stronger interaction between the polymer and the Li-ions, and hence, a lower mobility of the Li-ions.

Cation coordination in ion-conducting gels based on PEO-grafted polymers

Abstract Ionic conducting polymer gels prepared from PEO-grafted acrylates, ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF 6 are studied by means of infrared and Raman spectroscopy. It is found that the presence of grafted PEO chains substantially changes the coordination of lithium cations from ‘cation–solvent’ to ‘cation–polymer’. Spectroscopic studies show that Li + –PEO coordination is strongly favored in competition with the solvent molecules and dominates completely at ether oxygen to Li–cation ratios (O:Li) of 10:1 and 5:1. At an O:M of 4:1 a solvent–cation interaction arises, in agreement with previously reported MD calculations of a preferred coordination number of 5 in similar systems. The results suggest that the complex interactions which determine long time stability and ionic transport properties can be designed by functionalization of the polymer backbone in PMMA-based gel electrolytes.

N-Methyl-N-Alkylpyrrolidinium Bis(perfluoroethylsulfonyl)amide ([NPf2]–) and Tris(trifluoromethanesulfonyl)methide ([CTf3]–) Salts: Synthesis and Characterization

Novel salts based the pyrrolidinium cation [Cnmpyr]+ (where n denote the number of carbons in the straight alkyl chain) and either the [NPf2]– or [CTf3]– anions have been synthesized and characterized to determine their thermal behaviour, stability, and conductivity. [C1mpyr][NPf2], [C2mpyr][NPf2], and [C1mpyr][CTf3] exhibit behaviour indicative of a plastic crystal phase. Both [C3mpyr][NPf2] and [C4mpyr][NPf2] are RTILs, while all of the [CTf3]– salts, have melting points above 60°C. [C3mpyr][NPf2] exhibited the widest electrochemical window of 5.5 V. The [NPf2]– salt exhibited similar reductive limits to the [NTf2]– anion, –3.2 V versus Fc+|Fc, while [CTf3]– had lower reductive stability. The [CTf3]– salts were more stable towards oxidation, +2.5 V versus Fc+|Fc, compared to the [NPf2]– and [NTf2]– salts.

Polymer concentration dependence of the dynamics in gel electrolytes

A polymer gel electrolyte sample with a gradually varying concentration of polymer have been examined by Raman and photon correlation spectroscopy. The gel consisted of poly(methyl methacrylate) complexed with a liquid electrolyte of lithium perchlorate salt in an ethylene carbonate and propylene carbonate solution. The mole ratio composition was determined throughout the sample by means of Raman spectroscopy, which reveal a gradual vertical decrease of the polymer concentration. The photon correlation experiments show three dynamical processes of which two are attributed to diffusive processes, related to the solvent, and one to segmental motion, due to the relaxation of the polymer matrix. As the polymer concentration is increased the fast diffusive process gets slower, while there is no or a very small effect on the segmental motion. The implications for the overall performance in applications concerning ionic conductivity is also briefly discussed.

Thermal analysis, nuclear magnetic resonance spectroscopy, and impedance spectroscopy of N,N-dimethyl-pyrrolidinium iodide: An ionic solid exhibiting rotator phases

N,N-dimethyl-pyrrolidinium iodide has been investigated using differential scanning calorimetry, nuclear magnetic resonance (NMR) spectroscopy, second moment calculations, and impedance spectroscopy. This pyrrolidinium salt exhibits two solid-solid phase transitions, one at 373 K having an entropy change, Delta S, of 38 J mol(-1) K-1 and one at 478 K having Delta S of 5.7 J mol(-1) K-1. The second moment calculations relate the lower temperature transition to a homogenization of the sample in terms of the mobility of the cations, while the high temperature phase transition is within the temperature region of isotropic tumbling of the cations. At higher temperatures a further decrease in the H-1 NMR linewidth is observed which is suggested to be due to diffusion of the cations

Concentration Polarization of a Polymer Electrolyte

In this study, the salt concentration in a concentrated binary polymer electrolyte was measured in situ by means of confocal Raman spectroscopy during galvanostatic polarization experiments. The electrolyte studied was 0.8 M lithium bis(trifluoromethanesulfone)imide in a copolymer of ethylene- and propylene oxide at 25degreesC. Recent work with a transport model and characterization of the transport properties, for the same electrolyte, was verified with the spectroscopic results of this study. A good agreement between modeled and measured results was found. The spectroscopic method suited well for these studies. The possibilities of using a transport model are briefly demonstrated and discussed.