A study of the relationship between lithium ion transport and structure and dynamic behavior in polyethylene oxide-melt/LiClO4 battery electrolytes.
- Univ. of Nevada, Las Vegas, NV (United States)
An experimental study of the canonical SPE (“solid” polymer electrolyte) for rechargeable “rocking chair” lithium/polymer batteries, viz. LiClO4 dissolved in molten poly(ethylene oxide) (PEO), was carried out under DOE grant FG02-04ER15573. In this study, an improved understanding was obtained of the relationship between lithium ion transport and polymer behavior in these SPEs. Among other applications, these sturdy temperature-tolerant and powerful light-weight batteries would be used in electric and electric-hybrid vehicles to reduce greenhouse gas emissions, to store unused electrical energy for peak demand loads and as compact, light-weight energy sources for aircraft and spacecraft. During the period of the grant, the American/Canadian partnership company “Avestor” fabricated and successfully demonstrated a telecommunications application of shoe-box sized batteries and representatives from Avestor visited our research lab at UNLV. They found our results interesting and relevant to their work and invited us to visit Avestor and present a talk about our efforts at UNLV. Unfortunately Avestor (who was scheduled to build a battery production facility in Apex, Nevada just North of Las Vegas) folded before the visit could be made. In the grant work, two well characterized PEO samples having molar masses distinctly below and distinctly above the melt entanglement molar mass were used and three laser light scattering techniques employed as the principal noninvasive methods of investigating liquid poly(ethylene oxide) (PEO)/LiClO4 SPEs. These investigations considered the effects of temperature, dissolved salt concentration and scattering wavevector on SPE behavior. Classical or “static” light scattering and the dynamic light scattering techniques of photon correlation spectroscopy (PCS) and Fabry-Perot interferometry (FPI) were used to study SPE static, low frequency and high frequency dynamic behaviors, respectively. Static measurements provided information about system structure while low frequency results provided information about slower (0.1-10s) more global behavior and high frequency results provided information about faster (~10-11s) more local behavior. In addition, viscometry, rheometry and thermal analysis provided vital complementary results. It was found that liquid PEO/lithium salt solutions for both PEO molar masses are random transient physical networks with measureable network and intra-network relaxation times using PCS and FPI. Thus novel and informative results addressing both large scale and small scale behavior were obtained. For example, the high sensitivity of liquid PEO electrolytes to the presence of presumably undesirable trace amounts of residual water and/or methanol was clearly evident in PCS measurements. In “unentangled” melts the activation energies for diffusive relaxation in liquid PEO/lithium salt electrolytes measured using PCS and the activation energies for viscous flow in these systems determined by viscometry were identical while thermal analyses detected no phase transitions for these systems. These results reinforced an earlier assumption that the liquid PEO/LiClO4 system is a liquid polymer “bimorph” (to our knowledge the first of its kind to be reported) with the network comprising one form of the polymer while the second form corresponds to that of a viscous damping liquid. At a given temperature, FPI characteristic relaxation times for local, between-chain motions were consistent with PCS results so that increases with increasing salt concentration were accompanied by increases in the elastic modulus and corresponding increases in system stiffness. Note that corresponding decreases in polymer segmental mobility are accompanied by reduced ion diffusivity. For entangled melts, PCS network relaxations were again observed and these systems were also considered to be bimorphs even though diffusion activation energies were distinctly larger than viscous flow activation energies - a difference attributed to the effects of entanglement. Moreover, SLS measurements revealed entangled melts to be “percolation gels” with a measured fractal dimension of about 2 and static correlation lengths that increased gradually from about 500 nm to about 700 nm with increasing salt concentration. Meanwhile, rheological measurements found these polymer/salt melts to behave simply as liquids. FPI measurements have not yet been made on these entangled PEO melts.
- Research Organization:
- Univ. of Nevada, Las Vegas, NV (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- FG02-04ER15573
- OSTI ID:
- 958302
- Report Number(s):
- DOE/ER15573- Final Report
- Country of Publication:
- United States
- Language:
- English
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