Relating Structure, Viscoelasticity, and Local Mobility to Conductivity in PEO/LiTf Electrolytes

Abstract

The phase state, local structure, local mobility, and viscoelastic response have been studied in the archetypal polymer electrolyte (PEO) x LiCF 3 SO 3 with ether oxygen to lithium ion ratio of 2 ≤ [EO]/[Li] ≤12 over a broad temperature range in an effort to explore the factors controlling ionic conduction. We confirm that the crystal structure of the complex is identical to the (PEO) 3 LiCF 3 SO 3 polymer electrolyte independent of the [EO]:[Li] content. Heating the nonstoichiometric compositions result in progressive melting of the complex, whereas the complex formed at or near the stoichiometric composition remains stable up to the liquidus temperature. The temperature dependence of dc conductivity is neither Arrhenius nor VFT. Its temperature dependence is more complex reflecting the underlying structural changes. Surprisingly, ionic conduction takes place both within the crystalline complex and in the amorphous phase with the latter having the major contribution. The (PEO) 12 LiCF 3 SO 3 polymer electrolyte is the one with the highest conductivity at all temperatures investigated. The linear viscoelastic properties were studied as a function of temperature at two compositions. The different phases have distinct viscoelastic signatures. The complex formed at or near stoichiometric composition has a predominantly elastic response, whereas the more dilute compositions (consisting of the crystalline complex and an ion-containing amorphous phase) have a viscoelastic response and an ultraslow relaxation. Local polymer relaxation and ionic mobility are completely coupled. It is suggested that local ion jumps at subsegmental level are responsible for the measured conductivity.

Poly(ethylene oxide)-based electrolytes for lithium-ion batteries

“…The local relaxation and segmental motion of PEO host are needed for efficient lithium ion transport in the polymer electrolyte, and ionic mobility occurs predominantly in the amorphous phase of PEO. 79,85,90,94,99,122 The perfluoroalkyl sulfonic-type conducting salts like lithium trifluoromethanesulfonate (LiTf), 38,79,99,102,103,109,110,116,[123][124][125][126][127][128][129] lithium bis(trifluoromethanesulfonimidate) (LiTFSI), 103,110,123,[130][131][132][133][134][135] lithium bis(trifluoromethanesulfonimide) (LiBETI), 103,[136][137][138][139][140][141][142] and lithium bis(fluorosulfonyl)amide (LiFSI) 143,144 have high solubility, high ionic conductivity, and high electrochemical stability. These lithium salts with large anions have attracted much attention.…”

Rheology and Microscopic Heterogeneity of Poly(ethylene oxide) Solid Polymer Electrolytes

“…A number of rheological studies on SPEs shows that the variation of viscoelastic parameters in solid state resembles the results of Tg where the maximum of storage modulus is reached when the polymer reaches saturation limit with salt. [21,22] In melt state, the incorporation of salt may cause less effect on the viscoelastic parameters as a function of frequency [23][24][25] and longer polymer chain relaxation time. [26] In other words, addition of salt in polymer melt may result in the transition of liquid-like to solid-like viscoelastic behavior.…”

Ionic Conductivity, Self-Assembly, and Viscoelasticity in Poly(styrene-b-ethylene oxide) Electrolytes Doped with LiTf

“…6 The viscoelastic properties of the homopolymer electrolytes, PEO/LiTf, with the same LiTf compositions have been examined earlier. 7 It was found at the stoichiometric composition the crystalline complex shows a predominantly elastic response whereas the structure formed at more dilute concentrations exhibits a viscoelastic response. Another feature of the crystalline complex in the polymer electrolytes (PEO/LiTf electrolyte and SEO/LiTf electrolytes) is that they are very susceptible to strain.…”