Colin A. Vincent

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.

An investigation of the conducting species in polymer electrolytes

Abstract The conductances of LiClO 4 and LiCF 3 SO 3 solutions in a low mol. wt polyether have been measured as a function of salt concentration. The results have been interpreted in terms of ion pair and triplet formation. It is concluded that, at the electrolyte concentrations normally used to form “polymer electrolytes”, most of the salt is present as ion pairs and that the conducting species are likely to be triplets and higher charged aggregates rather than single ions.

Lithium batteries: a 50-year perspective, 1959–2009

The principles for realising commercially successful lithium secondary batteries are now well established. What is necessary during the next decade is the application of sophisticated solid state chemistry and materials science in order to find optimised solutions to the many conflicting requirements placed on the battery materials.

The effect of molecular weight on cation mobility in polymer electrolytes

Abstract Transport of cations in a polymer electrolyte is controlled by the dynamic relaxational modes of the host polymer. For high molecular weight systems, it has been established that the most probable mechanism involves short-range segmental motion of the polymer chains together with occasional transfer of coordinated cations from one polymer strand to another. In this study, we have investigated cation mobility in poly (ethylene oxide) hosts with molecular weights ranging from 400 to 4 × 10 6 , using electrochemical and pulsed field gradient NMR techniques. Above a critical limit of 3200 it is shown that the molecular weight has no significant effect on cation mobility, but that for lower values a mechanism involving molecular weight-dependent polymer chain diffusion becomes important. The mechanisms proposed are discussed in terms of the Rouse-Zimm and the de Gennes theories of polymer diffusion.

The effects of radiation-induced crosslinking on the conductance of LiClO4·PEO electrolytes

The conductance/temperature behaviour of poly(ethylene oxide)/LiClO4 electrolyte prepared under anhydrous conditions has been studied. Amorphous electrolyte samples have been crosslinked using γ-radiation from a Co-60 source. It is shown that light crosslinking cause a small reduction in the conductivity, probably due to restricted backbone segmental motion; however the tendency of electrolyte, treated in this way, to transform to a mixed crystalline/amorphous state of much lower conductivity, seems to have been comletely eliminated.

Correlating Capacity Loss of Stoichiometric and Nonstoichiometric Lithium Manganese Oxide Spinel Electrodes with Their Structural Integrity

It is known that stoichiometric spinel, , when used as a lithium‐ion battery cathode, exhibits significant capacity fade on cycling at room temperature, whereas by making the spinel slightly nonstoichiometric the capacity retention on cycling is improved to a great extent. To help understand this difference in performance, X‐ray diffraction (XRD) was used to investigate the spinel structure during lithium extraction and reinsertion. The stoichiometric spinel shows degradation during the first lithium extraction which becomes more severe at high rates. The XRD pattern measured at the end of 50 cycles showed significant loss of structural integrity, with several prominent peaks that were not present prior to cycling. In contrast, the nonstoichiometric spinel showed no evidence of degradation up to at least 300 cycles, even at high rates. It is suggested that in the case of stoichiometric spinel, , which forms upon extraction of lithium, accumulates during cycling and, because it becomes disconnected from the active electrode, capacity fade is significant.

Ion transport in polymer electrolytes

Abstract Conductivity data provide only limited insight into transport mechanisms in polymer electrolytes. Because the cation-polymer and anion-polymer interactions are fundamentally different, it is likely that the motion of cations on an atomic scale takes place by a different mechanism from that of anions. There is evidence that ion pairs may be mobile in some circumstances. It has also become apparent that it is necessary to consider the effect of varying the relative molar mass of the host polymer on the ionic motion. In this paper, we discuss recent electrochemical measurements and results from ion diffusion studies using pulsed field gradient nuclear magnetic resonance, and attempt to analyse these data in terms of possible ion transport mechanisms.

Capacity Loss of Lithium Manganese Oxide Spinel in LiPF6 / Ethylene Carbonate‐Dimethyl Carbonate Electrolytes

The stability of lithium manganese oxide spinel in LiPF{sub 6}/EC:DMC (ethylene carbonate-dimethyl carbonate) based electrolytes has been investigated at different states of charge and at temperatures between 21 and 55 C. Reversible capacity loss due to reinsertion of lithium takes place at all states of charge and temperature. This reaction can proceed to the extent of inserting more than one Li per Mn{sub 2}O{sub 4} at 55 C resulting in the formation of tetragonal Li{sub 1+x}Mn{sub 2}O{sub 4}. The second process, irreversible capacity loss due to dissolution of the cathode, makes little contribution to capacity loss in highly charged electrodes, but has an effect comparable to reinsertion in discharged electrodes.

Synthesis and characterization of ABA block copolymer-based polymer electrolytes

Abstract The synthesis and some of the properties of a novel ion-conducting polymer are described. The polymer, based on a styrene-butadiene-styrene ABA triblock copolymer has pendant, short-chain poly(ethylene oxide) (PEO) grafted onto the B block. The concentration of PEO in the polymer can be controlled by varying either the number of pendant groups per molecule or the molecular weight of the PEO chain. The polymers were combined with LiCF 3 SO 3 to form ion-conducting phases. The conductivities of films of these materials were found to be sensitive to preparation technique, and especially to casting solvent. The best conductivities, typically 10 −5 S cm −1 at ambient temperatures, were obtained using solvents that were likely to induce microphase separation (with concomitant improvement in the mechanical properties of the system). The temperature dependence of the conductivity suggested that the materials were essentially amorphous over the range studied.