On the Mechanisms of Chemical Intercalation of Lithium in Electrode Materials

Abstract

Reaction with n-butyl lithium (BuLi) is commonly used to study the kinetics of intercalation of lithium in electrode materials for batteries. We performed lithium isotope exchange experiments on TiS2 single crystals as model system to determine the irreversible step in the intercalation process. Single crystals of TiS2 were prepared by chemical vapor transport and intercalated by lithium with natural isotopic signature (7% 6Li 93% 7Li) using a 2.5 molar solution of BuLi in hexane. Crystals were homogenized at 313 K in a dry argon atmosphere for 1-2 months and then exposed for several days to ∼1 molar BuLi solution enriched in the light lithium isotope (95% 6Li, 5% 7Li). After the isotopic exchange experiments concentration profiles of the isotopes were measured parallel to the a/b-plane of the crystal using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Profiles show that already intercalated lithium isotopes can be completely exchanged without significant increase in total lithium content. This observation implies that, after adsorption of BuLi at the crystal surface, the butyl radical can freely jump from one lithium atom to a neighbouring one. The jump sequence is either finished by desorption of a BuLi molecule or by combination of two adjacent butyl radicals forming an octane molecule or other kinds of deactivation of butyl radicals. The latter step is irreversible. Self diffusion coefficients determined by fitting of the lithium isotope profiles are in good agreement with chemical diffusivities determined by lithium intercalation experiments.