Intercalation compounds from lih and graphite: Relative stability of metastable stages and thermodynamic stability of dilute stage I_d

The intercalation of lithium into graphite was studied at temperatures between 400 and 550 °C by heating mixtures of LiH and graphite powders with molar ratios 4:1, 1:1, and 1:6 under dynamic vacuum for periods between 1 and 72 h. These conditions probe the formation and thermal stability of metastable staged Li-graphite intercalation compounds (Li-GICs) close to the competing formation of the thermodynamically stable carbide Li₂C₂. Li-GICs of stages I (LiC₆, Aα), II_a (Li_0.5C₆, AαA), II_b (Li_∼0.33C₆, AαABβB), III (Li_∼0.22C₆, AαAB), IV (Li_∼0.167C₆), and dilute stage lithium I_d have been identified and characterized by powder X-ray diffraction and Raman spectroscopy. The rate and extent of intercalation (i.e., the achieved stage of Li-GIC) depends on LiH activity and temperature. Stage I was only observed for temperatures above 500 °C. At 400 °C, the highest intercalation corresponded to stage II_b, which was obtained after 2 and 24 h for 4:1 and 1:1 reaction mixtures, respectively. Lower-staged Li-GICs attained at temperatures below 500 °C deintercalate upon prolonged dwelling with the exception of stage II_a, which can be maintained for very long periods (several days) in the presence of LiH. At temperatures above 500 °C, the kinetically controlled formation of Li-GICs is followed by Li₂C₂ carbide formation. It is shown that the Li-GIC I_d coexists with Li₂C₂ at temperatures up to 800 °C and that the Li content of I_d (solubility of Li in graphite) increases between 550 and 800 °C. Consequently, I_d with a temperature-dependent homogeneity range should be added as a stable phase in the Li-C phase diagram. A sketch of a revised Li-C phase diagram is provided.