Theoretical Insights into the Reductive Decompositions of Propylene Carbonate and Vinylene Carbonate: Density Functional Theory Studies
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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) n Li + (VC) (n = 1, 2) cluster models. The objective is to help in understanding the experimentally observed irreversible capacity of graphite anodes when PC alone is used as a solvent versus the reported reversible capacity found in the presence of small amounts of VC in PC-based electrolyte solutions. The results indicate that PC solvates Li + more strongly than ethylene carbonate (EC) and VC do, which implies that, if it exists, the cointercalation of PC with Li ion into graphite layers is preferred; however, PC is more difficult to be reduced than the other two carbonates. On the other hand, the reaction kinetics for the reductive decomposition 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 graphite surface. The theoretical results suggest the following explanations for the role of VC as an additive: A solvated VC in the (PC) n Li + (VC) complex is initially reduced to a more stable ion-pair intermediate, which will undergo a ring-opening reaction by a homolytic C (carbonyl carbon)-O rupture in two ways. One is that the reduced VC decomposes to form a radical anion via a barrier of about 20 kcal/mol and the subsequent radical anion termination would generate the proper products, dominated by unsaturated lithium alkyl dicarbonates, such as lithium vinylene dicarbonate (CHOCO 2 Li) 2 and lithium divinylene dicarbonate (CH=CHOCO 2 Li) 2 , to build up an effective solid electrolyte interfacial (SEI) film. The unsaturated lithium dicarbonates may be further polymerized, forming lithium polyvinylene dicarbonate or oligomers with several repeated CH=CH units that would considerably improve the passivation of graphite electrode in the presence of VC because of formation of a more cohesive and flexible film. The alternative way is that, starting from the VC-reduction intermediate, a ring opening occurs on the unreduced PC moiety rather than on the reduced VC, via a lower barrier. Briefly, VC plays an important role in terms of thermodynamic factors by somehow weakening the PC cointercalation into graphite, stabilizing the reduction ion-pair intermediate, and generating more proper film-forming agents.
