Filming mechanism of lithium-carbon anodes in organic and inorganic electrolytes

To study the filming mechanism of graphite-based LiCn electrodes, electrochemical reduction of graphite materials was carried out in 1 M LiClO4/ethylene carbonate (EC)-1,2-dimethoxyethane (DME) (1:1 by volume). Due to film forming a peak at potentials around 0.8 V versus Li/Li+ was observed during the first reduction. The reversibility of this peak was examined by cyclic voltammetry; in addition, the crystal expansion/contraction was checked by means of dilatometry. The results indicate that ternary solvated graphite-intercalation compounds (GICs) were formed at those potentials leading to drastic expansion of the graphite matrix (>150%). These Li(EC)y1(DME)y2Cn-GICs decompose and build up a protective layer on the graphite that prevents further solvent co-intercalation. The beneficial effect of EC-containing electrolytes on the stability of lithium-carbon anodes seems to be related to inorganic films formed via secondary chemical decomposition of electrochemically formed EC-GICs. The key-role of inorganic films is also demonstrated by the fact that inorganic additives, such as carbon dioxide, suppress the formation of solvated GICs. Furthermore, it can be seen that lithium-carbon negatives can even be operated in inorganic electrolytes such as SO2 and SOCl2.