Peter G. Bruce

Electrochemical measurement of transference numbers in polymer electrolytes

Abstract Electrochemical methods for the determination of transference numbers in polymer electrolytes are considered and a new technique which overcomes some of the problems associated with other methods in current use is described. Results are given of measurements of the transference numbers of lithium and trifluoromethanesulphonate ions in poly(ethylene oxide) at 90°C. A mean value of 0.46 ± 0.02 is reported for lithium.

Steady state current flow in solid binary electrolyte cells

The steady state current flow following the application of a dc voltage to cells of the form M|M+X−|M, where M+X− is a binary solid electrolyte and M is an electrode electroactive towards the M+ ions, is discussed. Consideration is given to three important cases: (i) where the electrolyte is ideal and the electrode kinetics are infinitely fast, (ii) where the electrolyte is ideal, but the electrodes exhibit finite kinetics, and (iii) where the electrolyte is non-ideal, but the electrode kinetics are again infinitely fast. It is shown that in (i) a linear relationship exists between the steady state current and the applied potential difference, only for small values of the latter. Under such conditions the transport numbers of the ions may be obtained directly from current—voltage measurements. Even in the presence of ion-ion interactions, the ionic transport may still be characterised by current—voltage measurements alone under certain well-defined conditions. The theory presented is of particular relevance to binary polymer, ceramic and immobilised electrolytes.

Conductivity and transference number measurements on polymer electrolytes

Abstract Straightforward methods based on dc techniques are described which permit reliable evaluation of both the total conductivity and transference numbers of binary polymer electrolytes. In theory, under restricted conditions, transference numbers may be obtained from polarisation data on cells of the type M(s) ƒM + X − (polymer)ƒM(s). In practice such measurements are frequently complicated by electrode resistances. Here methods are demonstrated for reliably extracting transference numbers in the presence of such effects; measurements are presented for PEOLiCF 3 SO 3 (9:1) electrolyte as a function of temperature. Use of a simple four-electrode cell for the measurement of polymer electrolyte conductivity by dc methods is also described and results are given for the same electrolyte.

Characterisation of the electrode/electrolyte interfaces in cells of the type Li/PEOLiCF3SO3/V6O13 by ac impedance methods

Abstract A combination of ac impedance methods and 3-electrode cells were employed to study separately the V 6 O 13 and lithium interface with, solid, high molecular weight PEO (5 × 10 6 ), and liquid, methoxy end-caped low molecular weight PEO (222). Surface layers appear to be present on both electrodes. Generally both the lithium and V 6 O 13 interfacial impedances develop until they are significantly larger than the bulk electrolyte resistance.

An anhydrous polymer electrolyte containing trivalent cations: Poly(ethylene oxide):La(ClO4)3

Abstract The polymer electrolyte PEO:La(ClO 4 ) 3 has been prepared and investigated. In addition to crystalline PEO, X-ray and DSC results indicate the existence of a crystalline complex between the salt and polymer. Transference number meaurements indicate that the lanthanum is immobile. The conductivities, though lower than poly(ethylene oxide) based electrolytes containing monovalent cations or Hg 2+ , are similar to those of other anhydrous divalent cation-based polymer electrolytes.

Steady state current flow in solid binary electrolyte cells: Part 2. The effect of ion association

Abstract The steady state current flow following the application of a constant voltage to cells of the form M(s) + X − (s)¦M(s) where M + X − (s) is a binary solid electrolyte, such as a polymer electrolyte, containing mobile M + cations, X − anions and MX ion pairs, is discussed. General equations are derived for the effective conductivity of the electrolyte in terms of the diffusion coefficients of the three species and the association constant. A number of special cases which are of particular importance to the study of polymer electrolytes are considered briefly. It is shown (i) that measurements of steady state currents can indicate when significant concentrations of ion pairs are present, and (ii) that this technique may provide an excellent method of evaluating the practical merit of these materials for applications such as power sources or electrochromic devices. It is also noted that transference numbers cannot be obtained from this or the related ac technique when mobile ion pairs are present.

AC impedance of the Li(1-x)CoO2 electrode

Abstract An equivalent circuit for a porous polycrystalline insertion electrode is presented. This model is found to be inadequate to describe the ac response of the Li(1−x)CoO2/LiBF4 in propylene carbonate interface because of the formation of a Li+-ion conducting layer on the surface of the electrode. Lithium diffusion in the electrode is determined by three methods which all indicate a value grater than that reported for TiS2.

Preparation and characterisation of PEOHg(ClO4)2 complexes and some thoughts on ion transport in polymer electrolytes

Abstract A new solid polymer electrolyte, PEOHg(ClO 4 ) 2 has been prepared and characterised over a composition range 100 ⩽ x ⩽ 4, where x represents the molar ratio of EO units to salt. Films of this material are amorphous at high salt concentrations and there is no evidence for the formation of a crystalline polymer-salt complex over the entire composition range studied. Electrical measurements indicate that both cations and anions are mobile, with a cation transference number of 0.25±0.05 at 52°C for an EO/salt molar ratio of 20:1. The overall conductivity of PEOHg(ClO 4 ) 2 is significantly higher than that of other strictly anhydrous polymer electrolytes containing mobile divalent cations, previously reported. The variation of conductivity with temperature and composition is discussed, and cation mobility in general is considered in terms of hard and soft character, and in relation to the lability of the coordinated etheroxygens.

Transport in associated polymer electrolytes

Abstract The influence of ion association on the measurement of transport in polymer electrolytes by the Hittorf, EMF, centrifugal, radio-tracer, PFGNMR, ac and dc polarisation, methods is described so that the information which may be extracted from measurements using each technique may be understood. The response of a binary solid electrolyte to a dc polarisation is described in some detail, followed by a discussion of why such measurements are amongst the most valuable for evaluating the practical utility of a polymer electrolyte in electrochemical devices and are capable of revealing the fundamental nature of ion association. Finally dc polarisation results are presented for the poly (ethylene oxide): LiClO 4 system and a possible model of ion association is presented.

Defect clustering in lisicon solid electrolytes

LISICON, Li (2+2x) Zn (1-x) GeO 4 , is a lithium ion conducting solid solution in which the interstial ions are ordered into defect clusters. Changes in the nature of the defect clusters with temperature for one member of the solid solution series Li 3 Zn 0.5 GeO 4 (x = 0.5), have been determined by powder neutron diffraction uo to 500 o C. These changes may explain curvature in the log ο versus 1/T plots and the anomalously high pre-exponential factors observed at low temperatures