A lithium-ion battery containing methacrylate compounds as "additives" in the gel electrolyte has been developed such that an increase in its voltage greater than normal does not cause a thermal runaway. This phenomenon was studied by analysis of the positive active material, lithium cobaltate (LiCoO2), by measurement of the species on the negative electrode and the composition of the generated gases, and by estimating the abundance of the species in the electrolyte after/during charging the battery. The potential of the positive electrode maintained a constant value, the crystals of the positive material did not change, metallic Li and oxides of Li were produced on the negative electrode, CO2 and CO were generated, and the relative abundance of the species from the additives did not change during overcharging of the battery. We postulated that, in the lithium-ion battery sample with the additives, oxygen gas and/or carbon dioxide gas is generated at the positive electrode during overcharge, moves to the negative electrode, and oxidizes the Li deposited on the negative electrode. This reaction process has been suggested to be similar to that observed on overcharging of Ni-Cd or Ni-MH sealed batteries, and the battery with the additives remained safe during the overcharge. (C) 2006 The Electrochemical Society.