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

Abstract

ABSTRACT:The transport number for Li+ ions in Poly(ethylene oxide)-LiX complexes was measured using a potentiostatic polarization technique. The results indicate that the Li+ 10n transport is strongly influenced by the salt concentration and the type of counter ions.KEY WORDS Transport Number / Poly(ethylene oxide) / Salt / Polarization / Lithium / Lithium ion conducting polymer materials are being intensively investigated in a number of research laboratories because of their potential use as solid electrolytes in high energy density solid state lithium batteries. The most widely used polymer has been based on poly(ethylene oxide) (PEO). The conduction mechanism in these polymer electrolytes has generally been assumed to be due to Li+ ion transport from one lone pair site to another along the helical PEO strands. However, recent studies have shown that the Li+ transport number is less than unity. 1 -7 The mechanism of ion transport in these polymer electrolytes remains an important issue and despite numerous studies its modus operandi therein is not well understood.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. In addition, there was an empirical relationship between the anion size and conductivity. The present study involving the measurement of transport numbers of the cations and anions in these polymer electrolytes was undertaken to clarify some of the addressed issues.A number of methods have been used to measure the transport number of Li+ ions in polymer electrolytes with quite diverse results. This may be due to the technique not being sound for this type of electrolyte. The transport of cations and anions through the electrolyte involves a number of parameter changes, viz, ionic mobility of cations and anions, conductivity of electrolyte, cation-anion interactions, and motions of PEO molecules in the electrolyte. These parameters vary for different salt concentrations, temperature and molecular weight of the PEO electrolyte. Hence it is not surprising to find that conventional methods of measuring transport numbers in such polymer electrolytes appear to be inapplicable. The use of dilute solution theory to explain ion transport phenomena is inadequate for such complex systems. In addition, concentrated solution theory is not well developed for polymer electrolytes. Until these phenomena are better understood, a compromise has to be made on the measurement techniques.

Ion-Conductive Properties of a Polymer Electrolyte Based on Ethylene Carbonate/Ethylene Oxide Random Copolymer

“…The P(EC/EO)/LiFSI electrolyte (160 mol%) also had a very high t + of 0.66 (see also Figure S6, Supporting Information). In contrast, the PEO electrolyte (5 mol%) had a very low t + value of 0.13, matching the values reported by many researchers . Based on values of the ionic conductivity and t + , the Li + conductivities (σ × t + at 60 °C) of P(EC/EO)‐, PEC‐, and PEO‐based electrolytes were estimated to be, respectively, 0.32, 0.041, and 0.045 mS cm −1 for the concentration of the best performing electrolytes.…”

“…[5] PEObased electrolytes exhibit relatively high ionic conductivity. Nevertheless, PEO-based electrolytes have very low Li transference numbers (t + ), [6][7][8] because cations form a quasi-cross-linking structure with polar ethereal oxygen atoms. [5] The resulting slow Li + conduction should be improved for use in various battery applications.We focus on polycarbonates as a novel matrix for SPEs.…”

Ionic conduction in poly(ethylene carbonate)-based rubbery electrolytes including lithium salts

“…This value is very high relative to the typical polyether-based electrolytes, for example, t þ B0.1 for PEO 2 LiBF 4 (B51.6 wt%) at 100 1C. 18 According to a previous report, the t þ drastically decreases with increasing salt concentration in PEO. 18 Those authors suggested that the decreases in t þ are caused by the decrease in the number of free PEO segments and that the motion of Li þ is hindered by the subsequent formation of ion-pairs.…”

“…18 According to a previous report, the t þ drastically decreases with increasing salt concentration in PEO. 18 Those authors suggested that the decreases in t þ are caused by the decrease in the number of free PEO segments and that the motion of Li þ is hindered by the subsequent formation of ion-pairs. However, the PEC electrolyte has a very high t þ value, in spite of the high salt concentration.…”

Effect of Anions on Lithium Ion Conduction in Poly(ethylene carbonate)-based Polymer Electrolytes

“…5 Munshi et al have also reported that crystalline PEO-based electrolytes also show low values less than 0.2 at Li salt concentration higher than 30 mol%. 12 It is well known that, in the polyether electrolyte, dissociated Li + stable interact with dipoles of ether oxygens in the polymer chains where the transference number is lowered. Figure 2 shows the relation between transference numbers and the radius of anions dissociated into PEC electrolytes.…”