Theoretical studies to understand surface chemistry on carbon anodes for lithium-ion batteries: Reduction mechanisms of ethylene carbonate

Reductive decomposition mechanisms for ethylene carbonate (EC) molecule in electrolyte solutions for lithium-ion batteries are comprehensively investi gated using density functional theory. In gas phase the reduction of EC is thermodynamically forbidden, whereas in bulk solvent it is likely to underg o one- as well as two-electron reduction processes. The presence of Li cati on considerably stabilizes the EC reduction intermediates. The adiabatic el ectron affinities of the supermolecule Li+(EC)(n) (n = 1-4) successively de crease with the number of EC molecules, independently of EC or Li+ being re duced. Regarding the reductive decomposition mechanism, Li+(EC), is initial ly 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 li thium butylene dicarbonate. (CH2CH2OCO2Li)(2), followed by the formation of one O-Li bond compound containing an ester group, LiO(CH2)(2)CO2(CH2)(2)OC O2Li, then two very competitive reactions of the further reduction of the r adical anion and the formation of lithium ethylene dicarbonate, (CH2OCO2Li) (2), and the least favorable is the formation of a C-Li bond compound (Li c arbides), Li(CH2)(2)OCO2Li. The products show a weak EC concentration depen dence 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 con centrations. whereas (CH2OCO2Li)(2) is favored at high EC concentrations. O n the basis of the results presented here, in line with some experimental f indings. we find that a two-electron reduction process indeed takes place b y a stepwise path. Regarding the composition of the surface films resulting from solvent reduction, for which experiments usually indicate that (CH2OC O2Li)(2) is a dominant component, we conclude that they comprise two leadin g lithium alkyl bicarbonates, (CH2CH2OCO2Li)(2) and (CH2OCO2Li)(2), togethe r with LiO(CH2)(2)CO2(CH2)(2)OCO2Li. Li(CH2)(2)OCO2Li and Li2CO3.