Electrochemical Impedance Spectroscopy Study of Surface Film Formation on Lithium Anodes and the Role of Chain Termination on Poly(Ethylene Oxide) Electrolytes

The contact between solid polymer electrolytes and electrodes is a hot topic of investigation in lithium-ion batteries since the structure and composition of the interfaces is the limiting factor to both cell capacitance and life; a small change at the molecular level can have a dramatic effect on the cell behavior and performance. In this study, low-molecular polyethylene glycol electrolytes with two different chain termination groups are examined in terms of their electrochemical stability in contact with metallic lithium as a potential means of surface wetting. Using impedance spectroscopy, the ion transport mechanisms and time evolution of different phases are probed. It is shown that the electrolyte with hydroxy chain ends presents a very poor behavior with a high degree of electrolyte decomposition, which leads to a weak passivation layer and a significant inorganic phase development. This extreme case of electrolyte decomposition highlights the underlying mechanisms of film formation and the local environment of lithium ion transport. In contrast, methoxy chain ends lead to a stable electrolytic system that forms a robust passivating film accompanied by the development of a thin inorganic second phase.