Microelectrode studies of the lithium/electrolyte interface and ion transport in polyether electrolytes

Jun Xu, University of Pennsylvania

Abstract

Polymer electrolytes have received extensive attention worldwide due to their promising applications in all solid-state, rechargeable lithium batteries. This thesis concerns itself with three aspects of lithium electrochemistry, namely, lithium electrode kinetics, passivation, and ion transport, in model polyether electrolyte systems based on oligomeric poly(ethylene glycol dimethyl ether) (PEGM) (MW = 400). Electrochemical techniques with ultramicroelectrodes of micrometer dimensions were the main experimental techniques employed for the investigations. A systematic characterization of the lithium electrode kinetics found that the exchange current densities, i$\sb0$, for the Li/Li$\sp+$ couple are an order of magnitude smaller in PEGM than in its corresponding monomer 1,2-dimethoxyethane (DME) between 296 and 338 K. The exchange current density in PEGM is strongly temperature dependent with a high activation energy of 66 KJ/mole. The concentration dependence of i$\sb0$ in PEGM is unusual, with a linear relationship between i$\sb0$ and C$\rm\sb{salt}\sp{1/2}$ from 0.02 to 0.6 M after which the i$\sb0$ drops off sharply. The results are interpreted microscopically in light of recent advances concerning the influence of solvent dynamics on electron transfer, and insight gained into the factors affecting lithium electrode kinetics in polymer electrolytes. Lithium passivation and interfacial stability were investigated by cyclic voltammetry and specially designed potential step experiments in PEGM solutions containing different salts. It was found that lithium reactivity in PEGM solutions containing LiBr is much higher than in those containing LiAsF$\sb6$ or LiCF$\rm\sb3SO\sb3$. Lithium passivation appears to result from its reaction with the reducible anion, AsF$\sb6\sp-$ or CF$\rm\sb3SO\sb3\sp-$, and a passivation mechanism, in which the LiF formed in the reaction plays a key role, is proposed. A novel electrochemical method was proposed and demonstrated for measuring salt diffusion coefficients and ion transference numbers. By obtaining respectively the steady-state ion-transport-limited current at a microbic electrode in spherical diffusion geometry and the transient ion-transport-limited current at a macrodisc electrode in planar diffusion geometry in ionic solutions containing no supporting electrolyte, the salt diffusion coefficient and the transference number of the electroactive ion can be determined simultaneously. Among the advantages of the method are conceptual straightforwardness, experimental simplicity, and relative accuracy. Application of the method to PEGM electrolytes concluded that ion pairs are mobile in the polyether electrolytes and contribute to the ion transport limited currents. The salt diffusion coefficient in the polyether electrolytes was found to decrease with salt concentration and the Li$\sp+$ ion transference number increase with temperature. The results are explained in light of the unique ion transport mechanisms in polyether electrolytes.

Subject Area

Materials science|Chemical engineering|Analytical chemistry

Recommended Citation

Xu, Jun, "Microelectrode studies of the lithium/electrolyte interface and ion transport in polyether electrolytes" (1996). Dissertations available from ProQuest. AAI9628032.
https://repository.upenn.edu/dissertations/AAI9628032

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