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 carbonate͒. 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 ͑SEI͒, 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.Since 1991, when the lithium-ion battery ͑LIB͒ appeared on the market, the use of small-sized electronic devices such as portable phones and notebook PCs has quickly expanded. Presently, the use of the LIB is spreading to power tools such as the hybrid electric vehicle ͑HEV͒. The energy density of the LIB has improved three times in capacity to that in 1991. However, a capacity increase is still required for future uses. It is obvious that the capacity increase of the LIB is due not only to the improvement of the battery manufacturing technology and the electrode material, but also to the development of its electrolyte technology.As an electrolyte development technology, old examples such as the addition of CO 2 , 1-3 SO 2 , 4,5 N 2 O, 6 and 12-crown-4 ether 7,8 are known. In contrast to the conventional technology, in 1999 we proposed the concept of "functional electrolytes," based on highly purified electrolyte to which only a slight amount of electrolyte additives is introduced. 9,10 Due to the high purity of the electrolyte, electrolyte decomposition itself is inhibited. Consequently, a slight amount of electrolyte additives is deliberately decomposed on the negative electrode surface to produce the solid electrolyte interphase ͑SEI͒, which improves the battery performance. We commercialized the functional electrolytes in 1997, and several kinds of additives for negative electrode surface modification have been reported such as cathecohol carbonate, 11,12 imide compounds, 13 and double-bonded compounds like vinyl acetate. 14-20 Since the emergence of functional electrolytes, it would not be an exaggera...
