Ionic Transport in Poly(ethylene oxide) (PEO)-LiX Polymeric Solid Electrolyte

Abstract

ABSTRACT:The ionic conductivity behavior of poly(ethylene oxide) complexed with lithium salts was investigated using an a.c. impedance technique. The composition range of the salt was from 0.01 to 0.5 mole fraction. The result showed that anions strongly influence the conductivity behavior of the polymer electrolyte. A simple model is proposed whereby the conductivity decrease at higher salt concentration is due to ion-pair formation. This results in an increase in the activation energy with increasing salt concentration and a reduction in the ionic mobility of Li+ ions. KEY WORDSPolymer Electrolytes/ Poly(ethylene oxide)/ Conductivity/ Ionic Transport / Ion-Pair / Fast ion transport was first demonstrated by Fenton et al.,1 on polymer solid electrolytes based on poly(ethylene oxide) (PEO) and an alkali metal salt at about I00°C. This was followed by Armand's proposal 2 for an allsolid state lithium battery. Since then there has been considerable interest in developing electrolytes based on these and other types of polymeric materials 3 -20 for use in high energy denisty batteries. PEO is by far the most widely studied polymer electrolyte. The pure polymer is highly crystalline ( > 60%) with a glass transition temperature of 213°K. The complex formed between PEO and an alkali metal salt imparts a high ionic conductivity in the elastomeric phase at tempertures above the melting point of PEO. However, the mechanism of ionic transport is not well understood. This is mainly due to a difference in opinion concerning the structure of the complex. The general occurrence of the complex at a stoichiometry of 4: 1 PEO: Li salt (termed O: Li ratio) led to the proposal of a helical strucute of the PEO strands with the lithium ions placed at the core of the helix 2 .However, this has recently been shown 21 to be in disagreement with experimental findings. The stoichiometry of the complex has been found to vary and O: Li ratios as low as 2.5: 1 have been found to exist. 22 X-ray analyses have shown the Li+ ions occupy positions outside the helical PEO strands. This would explain tbe observed stoichiometries lower than 4: 1, 0: Li. This conformation would result in a minimum coulombic repulsion 23 from the oxygen lone pairs.Phase diagrams of the PEO-LiX systems have shown that the complex formed is not single-phase but exists in two or more phases. At room temperature, three phases coexist; the pure PEO crystals, the salt rich stoichiometric crystalline complex and the amorphous phase. As the temperature is raised above 65°C, the crystalline PEO melts in the amorphous phase. It is at these elevated temperatures that the conductivity of the electrolyte becomes significant.Numerous studies have been made to eluci-577

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.…”

Measurement of Li+ Ion Transport Numbers in Poly(ethylene oxide)–LiX Complexes

“…8 The salts under study were LiCF 3 SO 3 , LiAIC1 4 , LiAsF 6 , LiBF 4 , and LiPF 6 and the PEO-LiX concentrations were 50:1, 25:1, 8:1, 4:1, 2:1, and 1:1. All subsequent handling of the films and cell manipulation were carried out in the glove-box.…”

“…In a previous study, 8 it was found that the ionic conductivity of the PEO-LiX system was strongly influenced by the salt concentration and anion type. A model was proposed whereby the decrease in the ionic conductivity with increasing salt concentration was due to ion-pair formation.…”

Poly(ammonioalkanesulfonate) Blends with Polar Organic Species and Alkali Metal Salts: Structure, Glass Transition and Ionic Conductivity

“…More specifically, a lamellar structure is generally favored for a more symmetrical composition in segregated systems, but it is not possible to ascertain that this type of structure, observed in the case of NaI only, is actually correlated with the maximum volume fraction of the salt in the various blends. On the other hand, the development of longrange order and of a given type of structure may probably depend on more subtle parameters than the overall composition of the system such as the intrinsic properties of the ionic species (shape, size, polarizability [32][33][34][35], trend towards ion-pairing). Here again rationalization of the experimental results remains difficult, as shown by the following examples.…”

“…Antiplasticization effects are observed in all the other cases, but to highly varying degrees and they are barely significant for trifluoromethanesulfonate, boron tetrafluoride and thiocyanate (⌬T g ≤ 5°C). The size and the polarizability of the anion [32][33][34][35] may contribute in a complex way to the blend behaviors.…”