Dale Teeters

Study of the ion conduction of polymer electrolytes confined in micro and nanopores

Abstract This work investigates the ion conduction of polymer electrolytes confined in cylindrical pores having diameters ranging from 400 to 30 nm. The confinement of PEO polymer electrolytes in nanopores increases ionic conduction as the pore size decreases with the greatest conductivity occurring in pores that were 30 nm in diameter where a specific conductivity of 2.43×10 −4 S cm −1 was observed. This is almost two orders of magnitude higher than a polymer electrolyte film of the same composition but not confined in nanopores. This enhanced conduction may occur because of an orientation of the polymer chains in the pores. In addition to the advantage of enhanced conduction, these membranes have enhanced conduction perpendicular to the plane of the thin electrolyte film, which is the configuration desired for the construction of polymer electrolyte films for use in batteries.

Temperature-dependent spectroscopic studies of poly(propylene oxide) and poly(propylene oxide)-inorganic salt complexes

Abstract Raman spectra of low molecular weight poly(propylene oxide), PPO, and PPO containing dissolved sodium thiocyanate, NaSCN, have been measured from 25°C to 225°C. A low frequency mode involving torsional or bending motion of the polymer backbone at 239 cm −1 in PPO is the most sensitive to the presence of NaSCN. Frequency shifts in the anion stretching vibrations suggest the presence of ion pairs. At temperatures above 165°C microcrystalline NaSCN is observed to precipitate out of the liquid PPO.

Resistance measurements at the nanoscale: scanning probe ac impedance spectroscopy

We report a novel nanoscale method for measuring the alternating current (AC) impedance of conducting polymer electrolyte films using an atomic force microscope (AFM). The use of a conductive AFM probe provides direct impedance measurements at localized sites allowing interfaces and surface areas the size of the cantilever tip to be examined. This proves useful in the examination of polymer electrolytes where conduction is known to be heterogeneous across the surface of the film. Using the AFM we are able to sequentially image the surface then measure the resistance across various regions of the surface. The localized nature of this technique allows clear differentiation between highly conductive amorphous regions and less conductive crystalline regions of the film. Direct comparison of these impedances is consistent with bulk polymer film measurements. The results show the AFM to be a powerful method for ac impedance measurements at the nanoscale.

Characterization of PVdF-HFP polymer membranes prepared by phase inversion techniques I. Morphology and charge–discharge studies

Abstract A novel nanoporous polymer membrane comprised of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) co-polymer was prepared by phase inversion techniques using two different non-solvents. The films were subjected to scanning electron microscope (SEM) and nitrogen adsorption/desorption analysis. The morphology and porosity of the membranes are correlated with the chemical structure of the non-solvents used. Also, nanoparticle LiCr 0.01 Mn 1.99 O 4 cathode material was prepared by solid-state reaction followed by ball milling. The prepared samples were analyzed using X-ray diffraction (XRD) and atomic force microscopy (AFM). The charge–discharge behavior of lithium cells made from the coupling of the nanoporous polymer membrane with the LiCr 0.01 Mn 1.99 O 4 nanoparticle cathode was studied. The cathode nanoparticles resulting from ball milling were found to be more important in enhanced cell performance than the nanostructure of the nanoporous polymer membranes.

Raman and infrared reflectivity spectra of 6LiNaSO4 and 7LiNaSO4

The polarized Raman and infrared reflectivity spectra of single crystal LiNaSO4 are reported. Symmetry‐based vibrational band assignments are given for both the internal and external optic models. The spectra of 6LiNaSO4 clearly identify higher frequency external modes as originating in lithium ion motion essentially decoupled from the lower frequency external optic modes.

Charge-discharge studies on a lithium cell composed of PVdF-HFP polymer membranes prepared by phase inversion technique with a nanocomposite cathode

Abstract A novel polymer membrane of poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) co-polymer was prepared by the phase inversion technique with two different non-solvents, 1-butanol or hexane. The prepared films were analyzed by scanning electron microscope (SEM) and nitrogen absorption/desorption techniques. The change in the morphology and pore diameter of the films prepared with different non-solvents correlates with the structure of the non-solvents used. This electrolyte membrane was coupled with a nanocomposite LiAl 0.01 Co 0.99 O 2 cathode which was prepared by a solid-state reaction method and subsequently by ball-milling. Lithium cells consisting of LiAl 0.01 Co 0.99 O 2 /polymer electrolyte/Li were assembled and their charge–discharge studies were investigated.

The concentration behavior of lithium triflate at the surface of polymer electrolyte materials

Surface tension measurements were made on tetraglyme-lithium triflate complexes of varying concentrations. The surface tensions of the liquid glymes were found to initially decrease with respect to the pure glyme. This was attributed to a surface excess concentration of the free triflate ion. These results prompted the investigation of poly(ethylene oxide)-lithium triflate films by the use of ATR FT-IR for the film surface and FT-IR transmission for bulk film studies. These data indicated that the free ion is the dominant species at the surface while ion pairs are more prevalent in the bulk.

Using self-assembled monolayers to inhibit passivation at the lithium electrode/polymer electrolyte interface

Abstract This work investigates the use of surface chemistry to modify the lithium electrode/polymer electrolyte interface by placing a molecular layer, most likely in the form of a self-assembled monolayer (SAM), of H–(CH 2 ) 32 –(CH 2 –CH 2 –O) 10 –H onto the surface of PEO, poly(ethylene oxide), electrolyte films. It is proposed that the PEO-like “head” of the molecule above preferentially orients itself to absorb onto the PEO electrolyte, leaving the hydrocarbon CH 2 “tail” to self-assemble. SAM placement was confirmed using attenuated total reflection FTIR spectroscopy and wetting studies, and AC impedance measurements were used to investigate the passivating layer development. Extended time period studies of untreated polymer films in contact with lithium exhibited the rapid rise of an interfacial passivating layer whose resistance overtook that of the bulk electrolyte. Similar studies demonstrated that samples with SAMs actually had a small increase in ion conductivity and developed interfacial passivation much slower, supporting the assertion that SAMs could be used to deter the formation of a barrier to Li + transport during cycling of lithium polymer batteries.

Characterization of the passivation layer at the polymer electrolyte/lithium electrode interface

Abstract This work investigates the formation of a passivation layer at the lithium/poly(ethylene oxide)-lithium triflate electrolyte interface using attenuated total reflection FTIR spectroscopy, atomic force microscopy (AFM) and a.c. impedance spectroscopy. Self-assembled molecular layer technology was used to generate interfaces having no passivation layer so that comparisons could be made between passivated and non-passivated surfaces. AFM data show the formation of a passivation layer composed of what appears to be crystallites and ATR-FTIR data indicate the presence of CF3 radicals in this passivation layer with possible polymer chain scission. The formation of Li–O–R compounds is also observed.

Characterization of self-assembled molecular layers at the polymer electrolyte/lithium electrode interface

Formation of a passivation layer at the lithium electrode/electrolyte interface is a major concern for lithium polymer batteries. This work investigates the formation of self-assembled molecular layers on the polymer electrolyte interface. Previous work in our laboratory has shown that these molecular layers can greatly slow the passivation process. The molecular layers are placed onto the surface of the poly(ethylene oxide). PEO. electrolyte films via adsorption from hexane solution and are formed from molecules of the general form H (CH 2 ) n (CH 2 CH 2 O) m H. We have studied molecular layers formed from molecules where n equals 29, 32 or 40 and m is 0, 3, 10 or 41. Based on ac impedance spectroscopy, all molecular layers studied appear to slow or even inhibit interfacial passivation from occurring in lithium symmetric cells under an open circuit potential. ATR-FTIR, light microscopy, atomic force microscopy (AFM) and alternating current (ac) impedance spectroscopy have been used to characterize the molecular layers. AFM data indicate that after adsorption, multiple layers having an average single layer thickness of 5.5 ± 0.5 nm are present on the PEO electrolyte surface.