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.

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.

The thermal behavior of ionic liquids in lithium-ion cells was examined by a Calvet-type calorimeter using typical ionic liquid electrolytes and the charged electrodes (a Li x C 6 anode and a Li y Ni 1/3 Mn 1/3 Co 1/3 O 2 cathode) obtained by disassembling the lithium-ion cells with a conventional carbonate solvent electrolyte. Although ionic liquid electrolytes showed a little bit different thermal behavior compared with the conventional electrolyte, general features were almost the same. Cell safety does not directly connect to non-flammability of electrolytes based on the thermal runaway mechanism. Due to the lack of carbonate structure, ionic liquids produce much less CO 2 , which might be an advantage of ionic liquids for the cell safety. IntroductionLithium-ion cells are now widely used as power sources for portable electronic devices such as laptop computers and cellular phones due to their high energy density. Large-sized lithium-ion cells are also very attractive for pure or hybrid electric vehicle applications. However, lithium-ion cells intrinsically have serious safety problems, and the cell safety becomes the most important issue for the large batteries. When they are abused, several exothermic reactions occur inside the cell and the cell temperature increases rapidly, which results in fire or explosion. Because the heat generation and dissipation are related to the volume and surface area of the cell, respectively, the heat generation surpasses the heat dissipation as the cell size increases. Based on this thermal runaway mechanism, cell safety can be controlled by the suppression of the heat generation. Therefore, it is very important to understand the thermal behavior of nonaqueous electrolytes in lithium-ion cells.There are many reports on the thermal behavior of lithium-ion cells and their components by using accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC). We also have investigated quantitatively the thermal behavior of a graphite/LiCoO 2 cell, its components, and their combinations by a Calvet-type calorimeter, and confirmed that the following four reactions are important as main exothermic factors (1):(1) Electrolyte reduction by the anode (2) Electrolyte oxidation by the cathode (3) Combustion reactions of electrolyte and separator based on the released oxygen from the cathode. (4) Chemical shuttle reactions between the anode and the cathode.Since flammable organic solvents such as ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are generally used for their nonaqueous electrolyte solutions, alternative non-flammable solvents are desired to enhance the cell safety. Ionic liquids have been expected as promising electrolyte materials due to their non-flammability, nonvolatility, and high thermal stability (2).It has long been believed that no ionic liquid is compatible with graphite anodes, because ammonium cations can destroy the graphite layer structure due to the cointercalation during charge. Therefore, the research using ionic liquids was oriented t...

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