Intercalation Compounds from LiH and Graphite: Relative Stability of Metastable Stages and Thermodynamic Stability of Dilute Stage Id

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), IIa (Li0.5C6, AαA), IIb (Li∼0.33C6, AαABβB), III (Li∼0.22C6, AαAB), IV (Li∼0.167C6), and dilute stage lithium Id 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 IIb, 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 IIa, 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 Id coexists with Li2C2 at temperatures up to 800 °C and that the Li content of Id (solubility of Li in graphite) increases between 550 and 800 °C. Consequently, Id 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.