The solid electrolyte interface ͑SEI͒ formation on composite graphite and highly oriented pyrolytic graphite in a vinylene carbonate ͑VC͒-containing electrolyte was analyzed using evolved gas analysis, Fourier transform infrared analysis, twodimensional nuclear magnetic resonance, X-ray photoelectron spectroscopy, time of flight-secondary-ion mass spectrometry, and scanning electron microscopy. We found that the SEI layers derived from VC-containing electrolytes consist of polymer species such as poly ͑vinylene carbonate͒ ͑poly͑VC͒͒, an oligomer of VC, a ring-opening polymer of VC, and polyacetylene. Moreover, lithium vinylene dicarbonate, (CHOCO 2 Li) 2 , lithium divinylene dicarbonate, (CHvCHOCO 2 Li) 2 , lithium divinylene dialkoxide, (CHvCHOLi) 2 , and lithium carboxylate, RCOOLi, were formed on graphite as VC reduction products. The presence of VC in the ethylene carbonate ͑EC͒-based electrolyte caused a decrease in the reductive gases of the EC dimethyl carbonate solvent such as C 2 H 4 , CH 4 , and CO. The VC-derived SEI layer was formed at a potential more positive than 1.0 V vs. Li/Li ϩ . Effective SEI formation by reduction of VC progresses before that of EC. The thermal decomposition temperature of the SEI layer derived from VC shifted to a higher temperature compared to that derived from the VC-free electrolytes. We concluded that the thermal stability of the VC-derived SEI layer has a close relation to high-temperature storage characteristics at elevated temperatures.

In order to elucidate the mechanism of gas evolution in lithium-ion batteries, we fabricated carbon-LiNi x Co y Al 1−x−y O 2 cells employing 13 C-labeled ethylene carbonate ͑ 13 C-EC͒ and diethyl carbonate ͑ 13 C-DEC͒ as solvent components and then stored them at 85°C. The gas species evolved during storage tests were analyzed by gas chromatography/atomic emission detector to determine the isotopic ratio of CO 2 and CO. The relative proportions of the CO 2 derived from EC, DEC, and nonsolvent components were determined to be 52, 11, and 37%, respectively. The main source of CO 2 was found to be EC. Further storage tests with either cathode or anode electrodes showed that the cathode components were a source of CO 2 , but anode components were not. As for evolved CO, the main source was found to be EC. Moreover, we also examined the gas-evolution behavior on the initial charge. The evolved gas species were mainly composed of H 2 , C 2 H 4 , and CO. A minor amount of C 2 H 6 was also detected.From our isotopic analysis it was shown that C 2 H 4 was exclusively formed from EC, while C 2 H 6 derived from DEC. In the case of CO, EC and nonsolvent components were found to be its sources. CO derived from DEC was not detected.

The lithium cycling efficiencies of the lithium anode in the ethylene carbonate ͑EC͒-based electrolytes were improved by adding vinylene carbonate ͑VC͒ to the electrolyte. We analyzed the surface films of deposited lithium on a nickel substrate in a VC-containing electrolyte with scanning electron microscopy, Fourier transform infrared spectroscopy, two-dimensional nuclear magnetic resonance, gel permeation chromatography, and X-ray photoelectron spectroscopy. The corresponding surface films comprise various polymeric species including poly-͑vinylene carbonate͒ ͓poly-͑VC͔͒, oligomeric VC, and a ring-opened polymer of VC. Furthermore, the surface film of carbon double bonds (C ϭ C-O) and lithium carboxylate ͑RCOOLi͒ as reduction products of VC were formed on deposited lithium. These structures of the surface film on the lithium anode were similar to those on the graphite anode. At elevated temperatures, the VC-containing electrolyte led to the formation of surface films comprising poly-͑VC͒. The VC-derived polymeric surface film, which exhibited gel-like morphology, could prevent the deleterious reaction which occurs between deposited lithium and the electrolyte, resulting in an enhanced lithium cycling efficiency.

Chemical components of surface films of deposited lithium on nickel substrates in electrolytes with LiN (SO 2 CF 3 ) 2 ) ͑LiTFSI͒, LiN (SO 2 C 2 F 5 ) 2 ͑LiBETI͒, LiPF 6 solutes, and tetrahydrofuran solvents were characterized by Fourier-transform infrared, twodimensional nuclear magnetic resonance ͑2D NMR͒, X-ray photoelectron spectroscopy, evolved gas analysis, and ion chromatograph in order to understand the electrochemical performance of lithium imide/cyclic ether-based electrolytes. The top layers of the surface film were ROCO 2 Li, Li 2 CO 3 , polymer constituents, and LiF. The inner layers of the surface film consisted of Li 2 O and carbide species. In imide/cyclic ether-based electrolytes, Li 2 S 2 O 4 and Li 2 SO 3 as outer layers, and Li 2 S as the inner layer were formed on a nickel substrate as reductive constituents of imide solute. We found that organic surface layers consisted of lithium etoxides, lithium ethylene dicarbonate (CH 2 OCO 2 Li) 2 , polyethylene oxide, and lithium ethylene dicarbonate containing an oxyethylene unit by 1 H, 13 C, and 2D NMR. Li cycling efficiency affects not only the deposited lithium morphology but also chemical components.

Gas evolution in lithium-ion batteries at elevated temperature is a big problem to be solved for practical usage. In order to elucidate the mechanism of gas evolution, we employed 13C-labeled ethylene carbonate (13C-EC) and diethyl carbonate (13C-DEC) as solvent components. Evolved CO2 during storage were analyzed by GC/AED technique to determine the isotopic ratio. The relative proportions of the CO2 derived from EC, DEC, and non-solvent were determined to be 52%, 11%, and 37%, respectively. The main source of CO2 was found to be EC. Further storage tests with a single electrode showed that CO2 evolution also occurs from cathode components.