The mechanism of precipitation for substances produced by the reductive decomposition of a nonaqueous electrolyte solution (NES) at a graphite electrode (GE) was investigated with the help of classical thermodynamics in this paper. It was found that the substances tend to precipitate from a supersaturated NES by overcoming a Gibbs free energy barrier. This requires the atoms or molecules of the substances to initially aggregate into a stable solid nucleus, which will grow continuously into a macroscopic solid particle subsequently. When the Gibbs free energy barrier is reduced to zero under a certain condition, the substances will precipitate from the supersaturated NES as a two-dimensional (2D) solid layer on the GE surface in a nucleationless manner. Moreover, when the sum of the substance–NES interaction and the substance–GE interaction is stronger than the GE–NES interaction, the substances will even precipitate from an unsaturated NES as a 2D solid layer on the GE surface, provided the unsaturation potential is not too high.