The energy density of thin-film lithium batteries (TFLBs) is predominantly determined by the average voltage and specific capacity, however, the mechanism of regulating the voltage plateaus of the film electrodes is not well understood. In this study, three boride films (Co–B, Fe–B, and Co–Fe–B alloys) with different thicknesses were fabricated to enhance the specific capacity and voltage stability of TFLBs. By analyzing the cycling performance, redox peak evolution, and capacitive contribution, the thickness-dependent lithiation behavior of the thin/thick films was elucidated. Theoretical simulations and electrochemical analysis were conducted to investigate how the lithiation behaviors affected the voltage profiles of the film electrodes. In addition, the various-thickness CoB films were compared in all-solid-state TFLBs, demonstrating the universality and practicability of this simple regulation strategy to develop high-performance energy storage devices.