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A careful study of electrochemical lithiation of a composite graphite electrode at elevated temperatures was performed using a potentiostatic intermittent titration technique. Special emphasis was placed on the stage 4 to stage 3 phase transition, occurring in the range of x (in Li xC 6) between 0.12 and 0.22 (this corresponds to the partial dimensionless intercalation level ? ranging from 0 to 1). An abrupt increase in the Li chemical diffusion coefficient, D chem, when approaching the relative coverage level ?=1/3, and a specific non-Arrhenius shape of the plot of D chem vs. the inverse of the absolute temperature were observed. These two features of the phase transition seem to be in agreement with a recent thermodynamic model for adatoms diffusing on a surface with two considerably different barrier energy sites. [ABSTRACT FROM AUTHOR]