Anders Ferry

Slow recrystallization in the polymer electrolyte system poly(ethylene oxide) n –LiN(CF 3 SO 2 ) 2

Thermal and ion-transport properties of the salt-in-polymer system poly(ethylene oxide) n –LiN(CF 3 SO 2 ) 2 [P(EO) n LiTFSI] were investigated for compositions ranging from n = 5 to n = 50. Particular attention was paid to the region n = 8 to 10 where a crystallinity gap previously had been reported. We concluded that the absence of distinct melting transitions for salt-rich compositions ( n = 5 to 12) was attributable to the extremely slow kinetics of recrystallization of this system following a heat treatment. The results further indicated that it was primarily the nucleation process that was inhibited by the [(bis)trifluoromethanesulfonate imide] (TFSI) anion. As a corollary, the ionic conductivity was strongly dependent on the thermal history of samples, and an enhancement of up to 300% was observed in the ambient temperature ionic conductivity for pre-heated salt-rich samples.

The molar conductivity behavior in polymer electrolytes at low salt concentrations; A Raman study of poly(propylene glycol) complexed with LiCF3SO3

Raman scattering measurements have been carried out on poly(propylene glycol) complexed with LiCF 3 SO 3 salt over a wide salt concentration range, the ether oxygen to alkali metal cation ratios (O : M) ranging from 4820 : to 12 :1. The relative concentrations of solvated anions, anion-cation pairs and ionic aggregates have been calculated from an analysis of the symmetric anion stretch. The degree of association is found to be almost constant in the O : M range 4820 : 1-40 : 1 whereafter it increases with increasing salt concentration. The results show that the dramatic increase reported for the molar conductivity in the O : M range 1000 : 1-40 : 1 cannot be explained by the redissociation of contact ion pairs or the formation of conducting triplets. Instead the major contribution to the conductivity increase seems to be a concentration dependent enhancement of the ionic mobility. A percolation based ionic hopping process involving exchange between ions in pairs and dissolved ions, either free or coordinated to ether oxygen sites, is advanced as a possible microscopic mechanism.

Ionic interactions and transport in a low-molecular-weight model polymer electrolyte

AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically “free” anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D−(19F) and D+(7Li) are very similar for all concentrations (i.e., O:Li⩾53:1) and temperatures (25–80 °C) investigated. Most notably, the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases. Predicted values f...

NMR and Raman studies of a novel fast-ion-conducting polymer-in-salt electrolyte based on LiCF3SO3 and PAN

We report spectroscopic results from investigations of a novel solid polymeric fast-ion-conductor based on poly(acrylonitrile), (PAN, of repeat unit [CH 2 CH(CN)] n ), and the salt LiCF 3 SO 3 , From NMR studies of the temperature and concentration dependencies of 7 Li- and 1 H-NMR linewidths, we conclude that significant ionic motion occurs at temperatures close to the glass transition temperature of these polymer-in-salt electrolytes, in accordance with a recent report on the ionic conductivity. In the dilute salt-in-polymer regime, however, ionic motion appears mainly to be confined to local salt-rich domains, as determined from the dramatic composition dependence of the ionic conductivity. FT-Raman spectroscopy is used to directly probe the local chemical anionic environment. as well as the Li + - PAN interaction. The characteristic δ s (CF 3 ) mode of the CF 3 SO 3 anion at ∼ 750- 780 cm shows that the ionic substructure is highly complex. Notably, no spectroscopic evidence of free anions is found even at relatively salt-depleted compositions (e.g. N:Li ∼ 60 10:1). A strong Li + - PAN interaction is manifested as a pronounced shift of the characteristic polymer C=N stretching mode, found at ∼ 2244 cm -1 in pure PAN, to ∼ 2275 cm -1 for Li + -coordinated C=N moieties. Our proton-NMR data suggest that upon complexation of PAN with LiCF 3 SO 3 , the glass transition occurs at progressively lower temperatures.

Connectivity, ionic interactions, and migration in a fast-ion-conducting polymer-in-salt electrolyte based on poly(acrylonitrile) and LiCF3SO3

The ionic conductivity of a polymeric fast-ion-conductor based on LiCF3SO3 salt and poly(acrylonitrile), [CH2CH(CN)]n), is enhanced by ∼5 orders of magnitude when the composition approaches the “polymer-in-salt” regime; i.e., when the salt content increases from N:Li=12:1 to 1.2:1 (or ∼70 wt % of salt). This is in contrast to common salt-in-polymer electrolytes where a conductivity maximum typically is encountered at intermediate compositions. We suggest that connectivity effects in a microscopically phase segregated material may influence the long-range migration of charge carriers. Conductivity data are augmented with Raman spectroscopic investigations, thus probing microscopic details regarding the state of the dissolved salt.

Transport Property Measurements of Polymer Electrolytes

Abstract Straightforward electrochemical methods for determining the transport properties (bulk ionic conductivity, salt diffusion coefficient, and cation transference number) of polymer electrolytes are described herein. The new technique for measuring t 0 + is based on concentrated solution theory, and requires no assumptions to be made concerning ideality of the solution. The experimental methods are described and results on representative polymer electrolyte systems are presented in this paper. Cation transference numbers are found to be salt-concentration dependent and considerably less than unity or even negative over a wide concentration range. This implies that the cationic current is mainly carried by complexed ions. The presence of associated ionic species in the present systems is confirmed by Raman spectroscopic evidence. Finally, the practical effects of a negative t 0 + on cell operation and implications for device design are discussed.

Diffusion : A comparison between liquid and solid polymer LiTFSI electrolytes

From careful analyses of pfg-NMR data, it is demonstrated that the size of the diffusing Li+·xH2O complex in an aqueous solution of LiTFSI is strongly dependent on salt concentration, with the numb ...

Effect of Electrolyte Composition on the Performance of Sodium/Polymer Cells

The dependence on Na/P(EO){sub n}NaX/Na{sub x}MnO{sub 2} (P(EO) = poly(ethylene oxide), X = CF{sub 3}SO{sub 3} or (CF{sub 3}SO{sub 2}){sub 2}N) cell cycle life and rate capability on polymer electrolyte composition is described. Transition time experiments and mathematical modeling indicate that failure due to salt precipitation occurs at it{sup 1/2} = 10.5 to 21.4 mA s{sup 0.5}/cm{sup 2}, when high initial concentrations of NaCF{sub 3}SO{sub 3} are used in operating cells. Evidence for large ionic clusters in concentrated PEO/NaCF{sub 3}SO{sub 3} solutions is also seen in the Raman spectroscopic data. Salt precipitation is a direct consequence of the concentration gradients that arise during operation, due to the negative cationic transference numbers (t{sub +}{sup 0}) of the binary salt/polymer electrolyte. By decreasing the initial salt concentration, t{sub +}{sup 0} is increased, cell rate capability is doubled, and the cycle life is enhanced nearly threefold. Similar improvements are obtained when PEO/NaN(CF{sub 3}SO{sub 2}){sub 2} electrolytes are used.

Effects of dynamic spatial disorder on ionic transport properties in polymer electrolytes based on poly(propylene glycol)(4000)

The equivalent ionic dc conductivity (Λ) generally exhibits a dramatic concentration dependence in electrolytic systems based on the host polymer poly(propylene glycol) of molecular weight 4000 (PPG4000). In particular, Λ typically increases rapidly with increasing salt concentration passing through a temperature dependent maximum at high concentration. Prompted by recent reports on a microscopic phase separation occurring in these electrolytes, we here report vibrational spectroscopic, ionic conductivity, and restricted diffusion data for ion-conductors based on PPG4000 complexed with the lithium salts LiCF3SO3 and LiN(CF3SO2)2, in an attempt to resolve seemingly contradictory results concerning ionic transport phenomena in these complexes. We find that the differential salt diffusion coefficient Ds, describing bulk salt motion over long time scales, exhibits a qualitatively similar concentration dependence as Λ. This is contrary to recent 19F pfg-NMR diffusion results for the PPG4000-LiCF3SO3 system whi...

Time resolved luminescence and vibrational spectroscopic studies on complexes of poly(ethylene oxide) oligomers and EuTFSI3 salt

Abstract AC impedance, FT-Raman/IR, DSC, continuous and time resolved luminescence measurements have been conducted on solutions of poly(ethylene glycol) (PEG), MW 400, and poly(ethylene glycol)-dimethyl ether (DME), MW 425, complexed with Eu[N(CF 3 SO 2 ) 2 ] 3 salt, EuTFSI 3 . Ion-polymer interactions are manifested as changes in characteristic vibrational modes of the polymer, including CH 2 and –OH stretching motions at ∼2700–3700 cm −1 , and also in cation-induced polymer modes at ∼865–910 cm −1 . Comparing the vibrational features of the TFSI anion (i.e., both Raman and IR), we find no modes that are substantially changing with increasing salt concentration, or upon change of cation (i.e., M=Li + , Na + or Eu 3+ ). This observation suggests that TFSI-salts are highly dissociated in PEO oligomer solvents even up to relatively high salt concentrations (i.e., O:M=26: 1). Clear evidence of –OH end-group coordination in the PEG systems emerges from IR spectra and the strong dependence of T g upon salt concentration, and also from the pronounced temperature dependence of the ionic conductivity. Despite of this, however, few distinct differences could be observed in the luminescence spectra between the PEG and the DME host materials. Luminescence spectra of Eu 3+ show a relatively small distribution of energies (30 cm −1 FWHM in 5 D 0 – 7 F 0 ) in a low-symmetry site throughout the entire concentration range investigated for both PEG and DME solvents. The population decays of the 5 D 0 excited state, measured by exciting to the degenerate state 5 D 1 with a pulsed dye laser, are also very similar for the PEG and DME hosts (lifetimes=0.35 ms).