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Abstract
Poly(ethylene oxide)/lithium perchlorate (PEO/LiClO4) complexes are widely studied as a prototype solid polymer electrolyte in rechargeable lithium-polymer batteries. Characterizing the structure and dynamics of the system in its molten state is important for understanding the role of the polymer environment in lithium ion transport and conductivity. A fiber-optic coupled Fabry-Perot interferometer is employed in the investigation of the electrolyte viscoelastic and dynamic properties, which are both related to the intrachain local mobility and therefore to ion diffusion. The properties of the system are studied as a function of composition, temperature, and frequency. Structural relaxation processes are observed both in the neat polymer melt and in the salt containing electrolytes. For the neat PEO-1K melt the relaxation is identified as Maxwell-Debye single-exponential relaxation (β = 1). The relaxation time follows Arrhenius temperature dependence with activation energy of the order of 10-11 kJ/mol. Upon addition of salt, the character of the relaxation persists with β = 1, while the characteristic relaxation time slows down and the activation energy increases slightly. The slowdown of the dynamics is more pronounced at lower temperatures. In addition, with increasing salt concentration the elastic modulus increases significantly making the system stiffer at all temperatures, while the maximum of the storage modulus is shifted to higher temperatures. These effects result in a decrease in polymer segmental mobility and consequently in reduction of lithium ion diffusivity, with increased salt concentration. A unique q-dependent measurement is performed, allowing the investigation of the Brillouin frequency and linewidth as a function of frequency. It revealed a double-step relaxation in the electrolyte. The two relaxations are identified as secondary relaxations with Maxwell-Debye character (β=1). The lower-frequency relaxation is stronger and has Arrhenius temperature behavior of the relaxation time. It is attributed to conformal fluctuations of the chain segments between transient crosslinks formed by the EO-Li+ complexation in the melt. The higher-frequency relaxation is weaker, especially at higher temperatures and more difficult to resolve. It is possible it results from librational motions or conformal rearrangements of the uncomplexed polymer dihedrals and seems to be strongly affected by the specifics of the local chain conformation and the EO-Li+ complexation in the melt.
