Kim, Jong-Chul; (1998) Lithium deposition in solid polymer electrolyte batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with lithium deposition in solid polymer electrolyte batteries. Passivating layers are developed on the electrode even in polymer electrolytes as the cycling proceeds and perturbation of the morphology of the lithium deposited may be expected. In this project, cyclic voltammetry was used as a probe for the presence of electroactive impurities in polymer electrolytes. The main electroactive impurity present was water. It was demonstrated that oxygen as well as water could affect the voltammetric behavior of polymer electrolyte test cells. A galvanostatic experiment and theoretical analysis identified the presence of a critical current where the concentration of Li+ at the cathode surface approached zero. The expected dependence of the critical current on the polymer electrolyte thickness, the Li+ concentration and the Li+ diffusion coefficient was found. It was shown that the observed critical current density gradually decreased with the number of cycles because side reactions might produce the passivating layers during cycling. In-situ microscopic observation of the electrochemical deposition of lithium on a Ni substrate was carried out in poly(ethylene oxide) based electrolytes containing LiClO4 and LiCF3SO3. Globular lithium deposits were typically observed at a current density lower than the corresponding critical current density. A transition to dendritic growth was seen at a current density far higher than the corresponding critical current density. It was shown by the in-situ observation that dendrites grew via fine particles in the polymer electrolytes. These fine particles could accelerate the rate of side reaction between lithium and impurities by generating a high surface area and reactivity. They also changed the morphology of the lithium deposited in subsequent cycles towards dendrites. The presence of a passivating layer of LiOH was identified by FT-IR, in-situ.

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