The Impedance of Lithium Electrodes in LiPF6 ‐ Based Electrolytes

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Abstract
The impedance spectrum of a lithium electrode in an electrolyte exhibits one or two semicircles. The variation of the two semicircles with immersion time indicates the presence of two kinds of passivating surface films. The change in resistance (Δ R1) associated with the first film during the first 24 h immersion time increased with increasing concentration. The large value of Δ R1 in a degraded electrolyte indicated that the decomposition of would lead to the formation of a thick passivating film. Δ R1 was well correlated with the oxidation potential of the solvent, which measures the stability of the solvent against lithium. The change in resistance (Δ R2) of the second film over 24 h was smaller than for the first film. The most effective solvent for decreasing Δ R1 was an ethylene carbonate (EC)/2‐methyl‐tetrahydrofuran (2MeTHF) mixed solvent. The lithium cycling efficiency in various solvent systems was demonstrated to increase with a decrease in Δ R1. The efficiency of the electrolyte varied significantly with concentration.

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