We have found that certain triple-bonded compounds show a very interesting behavior in Li-ion batteries. These novel types of additives have proven to improve battery performance, especially in cycleability. Propargyl methanesulfonate and propargyl methyl carbonate show good performance among several triple-bonded compounds. These triple-bonded compounds are investigated in contradistinction with previously known double-bonded compounds (allyl methanesulfonate and allyl methyl carbonate). We used molecular orbital calculations for the selection of the additives and have proved that the calculated lowest unoccupied molecular orbital and highest occupied molecular orbital values agree well with the measured reduction and oxidation potentials, respectively. To clarify the performance of the triple-bonded compounds, electrochemical properties and cycleability were investigated. The triple-bonded compounds were found to be deliberately decomposed on the negative electrode to produce a dense solid electrolyte interphase (SEI), showing an excellent improvement of cycleability. The nature and the component of the derived SEI were studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy. It was concluded that these triple-bonded compounds contribute to the improved cycleability, because the SEI derived from the triple-bonded compounds has a thinner and denser morphology than previously known additives. (c) 2007 The Electrochemical Society.