Reductive decomposition mechanisms for ethylene carbonate (EC) molecule in electrolyte solutions for lithium-ion batteries are comprehensively investigated using density functional theory. In gas phase the reduction of EC is thermodynamically forbidden, whereas in bulk solvent it is likely to undergo one- as well as two-electron reduction processes. The presence of Li cation considerably stabilizes the EC reduction intermediates. The adiabatic electron affinities of the supermolecule Li(+)(EC)n (n = 1-4) successively decrease with the number of EC molecules, independently of EC or Li(+) being reduced. Regarding the reductive decomposition mechanism, Li(+)(EC)n is initially reduced to an ion-pair intermediate that will undergo homolytic C-O bond cleavage via an approximately 11.0 kcal/mol barrier, bringing up a radical anion coordinated with Li(+). Among the possible termination pathways of the radical anion, thermodynamically the most favorable is the formation of lithium butylene dicarbonate, (CH2CH2OCO2Li)2, followed by the formation of one O-Li bond compound containing an ester group, LiO(CH2)2CO2(CH2)2OCO2Li, then two very competitive reactions of the further reduction of the radical anion and the formation of lithium ethylene dicarbonate, (CH2OCO2Li)2, and the least favorable is the formation of a C-Li bond compound (Li carbides), Li(CH2)2OCO2Li. The products show a weak EC concentration dependence as has also been revealed for the reactions of LiCO3(-) with Li(+)(EC)n; that is, the formation of Li2CO3 is slightly more favorable at low EC concentrations, whereas (CH2OCO2Li)2 is favored at high EC concentrations. On the basis of the results presented here, in line with some experimental findings, we find that a two-electron reduction process indeed takes place by a stepwise path. Regarding the composition of the surface films resulting from solvent reduction, for which experiments usually indicate that (CH2OCO2Li)2 is a dominant component, we conclude that they comprise two leading lithium alkyl bicarbonates, (CH2CH2OCO2Li)2 and (CH2OCO2Li)2, together with LiO(CH2)2CO2(CH2)2OCO2Li, Li(CH2)2OCO2Li and Li2CO3.

The conductivities of lithium and quaternary ammonium salts have been measured in propylene carbonate and ~-butyrolactone at 25~ over the concentration range of I0 2 to i0 3 mol dm -3. The limiting molar conductivities and the ionic association constants were calculated by the expanded Fuoss-Hsia equation. The mobility of anions in both solvents decreased in the following order: BF4 > CIO~ > PF~ > AsF~ > CF3SO3 > (CF3SO~)2N-> C4F9SO~ > BPh~. The association constants increased in the following order: Li(CF3SQ)~N, LiAsF6 < LiPF6 < LiCIO~ < LiBF4 < LiC4FgSQ < LiCF3SQ, while those of the corresponding tetraethylammonium salts were leveled by electrostatic shielding effect of alkyl groups. These results were correlated with ionic radii obtained by MM2 calculation.

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The electrolytic conductivities and limiting reduction and oxidation potentials for various organic liquid electrolytes based on quaternary onium salts have been measured to find better electrolytes for electrical double-layer capacitors. An electrolyte composed of tetraethylammonium cation, tetrafluoroborate anion, and propylene carbonate solvent showed well-balanced performance of high electrolytic conductivity, a wide stable potential window and resistance to hydrolysis. Among quaternary onium salts, triethylmethylammonium, ethylmethylpyrrolidinium, and tetramethylenepyrrolidinium tetrafluoroborate salts exhibited higher electrolytic conductivity than the conventional tetraethylammonium salt due to their much greater solubility.Ref. 25.

The performance of a double-layer capacitor ͑DLC͒ composed of activated carbon electrodes and 1-ethyl-3-methylimidazolium fluoride ͑EMIF͒•2.3HF, which has extremely high conductivity with low viscosity, was examined and compared with those using the popular ionic liquid EMIBF 4 , conventional aqueous electrolyte 35 wt % H 2 SO 4 , and nonaqueous electrolyte 1 M Et 3 MeNBF 4 /propylene carbonate. The DLC using EMIF•2.3HF showed an intermediate capacitance and internal resistance between the aqueous and nonaqueous electrolyte systems due to its intermediate double-layer capacitance and electrolytic conductivity. EMIF•2.3HF afforded much higher capacitance than EMIBF 4 even at low temperatures, however, it had a lower decomposition voltage ͑ϳ2 V͒, resulting in lower energy density. The capacitance of EMIF•2.3HF was extremely dependent on the applied voltage.

The limiting reduction and oxidation potentials and electrolytic conductivities of new quaternary ammonium salts were examined for electrochemical capacitor applications, whose anions have already been tested as lithium salts for lithium battery applications. The anodic stability was in the following order

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.

Ion size is an important parameter in interpreting the electrochemical properties of the electrolyte materials. There is difficulty in selecting a representative value for ionic radius when the ion has a shape far from spherical. Since the van der Waals volume becomes a good parameter for ion size, it was calculated by numerical integral based on a simple overlapped spherical model using the optimized structures by ab initio molecular orbital calculation. We have examined the extent of errors affected by the coordinates, and given the van der Waals radii of each atom, and the integral grid size, and give the van der Waals volumes of various ions applied for the electrolyte materials of lithium batteries and double-layer capacitors. © 2002 The Electrochemical Society. All rights reserved.