| Abstract
| - The electroreductive decompositions of propylene carbonate (PC) and vinylene carbonate (VC) in lithium-ion battery-electrolyte solutions have been investigated with density functional theory using Li+(PC)n(n = 2,3) and (PC)nLi+(VC) (n = 1, 2) cluster models. The objective is to help in understanding the experimentallyobserved irreversible capacity of graphite anodes when PC alone is used as a solvent versus the reportedreversible capacity found in the presence of small amounts of VC in PC-based electrolyte solutions. Theresults indicate that PC solvates Li+ more strongly than ethylene carbonate (EC) and VC do, which impliesthat, if it exists, the cointercalation of PC with Li ion into graphite layers is preferred; however, PC is moredifficult to be reduced than the other two carbonates. On the other hand, the reaction kinetics for the reductivedecomposition of PC is very similar to that of EC. Besides the problem caused by the possible cointercalation,the nature of the PC reduction products may also be responsible for its disability to passivate the graphitesurface. The theoretical results suggest the following explanations for the role of VC as an additive: A solvatedVC in the (PC)nLi+(VC) complex is initially reduced to a more stable ion-pair intermediate, which will undergoa ring-opening reaction by a homolytic C (carbonyl carbon)−O rupture in two ways. One is that the reducedVC decomposes to form a radical anion via a barrier of about 20 kcal/mol and the subsequent radical aniontermination would generate the proper products, dominated by unsaturated lithium alkyl dicarbonates, suchas lithium vinylene dicarbonate (CHOCO2Li)2 and lithium divinylene dicarbonate (CHCHOCO2Li)2, tobuild up an effective solid electrolyte interfacial (SEI) film. The unsaturated lithium dicarbonates may befurther polymerized, forming lithium polyvinylene dicarbonate or oligomers with several repeated CHCHunits that would considerably improve the passivation of graphite electrode in the presence of VC because offormation of a more cohesive and flexible film. The alternative way is that, starting from the VC-reductionintermediate, a ring opening occurs on the unreduced PC moiety rather than on the reduced VC, via a lowerbarrier. Briefly, VC plays an important role in terms of thermodynamic factors by somehow weakening thePC cointercalation into graphite, stabilizing the reduction ion-pair intermediate, and generating more properfilm-forming agents.
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