Characterization of the passivation layer on disordered carbons in lithium-ion cells

Intercalation anodes of graphite or disordered carbon in rechargeable Li-ion batteries (based on aprotic organic solvents) develop a passivating film during the first intercalation of Li⁺ . The formation of this film reduces the cycling efficiency and results in excessive consumption of Li⁺. The exact nature of this film is not well defined, although there are many similarities in properties to the films that form on Li anodes under similar cycling conditions. In this study, the authors report on characterization studies of films formed during galvanostatic cycling of disordered carbons derived from polymethylacrylonitrile (PMAN) in a 1M LiPF₆ solution in ethylene carbonate/dimethyl carbonate solution (1:1 by vol). Complementary tests were also conducted with glass carbon where intercalation cannot occur. Complex-impedance spectroscopy was the primary measurement technique, supplemented by cyclic voltammetry. The passivation process was associated with two irreversible reduction peaks at ~0.75 V and ~1.1 V vs. Li/Li⁺ in the dC/dV-V plot of the galvanostatic data. Similar peaks were displayed in cyclic voltammograms of glassy carbon, but shifted to lower potentials. The PMAN impedance spectra showed inductive behavior during the first intercalation at potentials below 0.5 V. This inductive behavior was related to formation of nonequilibrium reactive intermediates associated with solvent reduction. It was not observed after several intercalation/deintercalation cycles at these potentials. Instead, the impedance spectra exhibited two semicircles and a Warburg-type tail