Theoretical Studies To Understand Surface Chemistry on Carbon Anodes for Lithium-Ion Batteries:  How Does Vinylene Carbonate Play Its Role as an Electrolyte Additive?

Abstract

To elucidate the role of vinylene carbonate (VC) as a solvent additive in organic polar solutions for lithium-ion batteries, reductive decompositions for vinylene carbonate (VC) and ethylene carbonate (EC) molecules have been comprehensively investigated both in the gas phase and in solution by means of density functional theory calculations. The salt and solvent effects are incorporated with the clusters (EC)nLi+(VC) (n = 0-3), and further corrections that account for bulk solvent effects are added using the polarized continuum model (PCM). The electron affinities of (EC)nLi+(VC) (n = 0-3) monotonically decrease when the number of EC molecules increases; a sharp decrease of about 20.0 kcal/mol is found from n = 0 to 1 and a more gentle variation for n > 1. For (EC)nLi+(VC) (n = 1-3), the reduction of VC brings about more stable ion-pair intermediates than those due to reduction of the EC molecule by 3.1, 6.1, and 5.3 kcal/mol, respectively. This finding qualitatively agrees with the experimental fact that the reduction potential of VC in the presence of Li salt is more negative than that of EC. The calculated reduction potentials corresponding to radical anion formation are close to the experimental potentials determined with cyclic voltammetry on a gold electrode surface (-2.67, -3.19 eV on the physical scale for VC and EC respectively vs experimental values -2.96 and -2.94 eV). Regarding the decomposition mechanisms, the VC and EC moieties undergo homolytic ring opening from their respective reduction intermediates, and the energy barrier of VC is about one time higher than that of EC (e.g., 20.1 vs 8.8 kcal/mol for (EC)2Li+(VC)); both are weakly affected by the explicit solvent molecules and by a bulk solvent represented by a continuum model. Alternatively, starting from the VC-reduction intermediate, the ring opening of the EC moiety via an intramolecular electron-transfer transition state has also been located; its barrier lies between those of EC and VC (e.g., 17.2 kcal/mol for (EC)2Li+(VC)). On the basis of these results, we suggest the following explanation about the role that VC may play as additive in EC-based lithium-ion battery electrolytes; VC is initially reduced to a more stable intermediate than that from EC reduction. One possibility then is that the reduced VC decomposes to form a radical anion via a barrier of about 20 kcal/mol, which undergoes a series of reactions to give rise to more active film-forming products than those resulting from EC reduction, such as lithium divinylene dicarbonate, Li-C carbides, lithium vinylene dicarbonate, R-O-Li compound, and even oligomers with repeated vinylene and carbonate-vinylene units. Another possibility starting from the VC-reduction intermediate is that the ring opening occurs on the unreduced EC moiety instead of being on the reduced VC, via an intramolecular electron transfer transition state, the energy barrier of which is lower than that of the former, in which VC just helps the intermediate formation and is not consumed. The factors that determin...

The Role of Carbonate and Sulfite Additives in Propylene Carbonate-Based Electrolytes on the Formation of SEI Layers at Graphitic Li-Ion Battery Anodes

“…15 Enhanced chemical stability against electrolyte oxidation and reduction, high ionic conductivity, high boiling points, and low melting points are required, as well as the ability to solvate a wide range of lithium salts such as LiPF 6 , LiBF 4 or LiClO 4 , [16][17][18][19][20][21] in aprotic and organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), their mixtures, and ionic liquids. The decomposition mechanism of organic solvent and subsequent formation of SEI films near the graphite anode is a major research topic in lithium ion batteries from theoretical [22][23][24][25][26][27][28][29][30] and experimental [31][32][33][34][35][36][37][38][39] standpoints, and one of the least understood.…”

Anodic Polymerization of Vinyl Ethylene Carbonate in Li-Ion Battery Electrolyte

“…Examples are ethylene sulfite [4], and the unsaturated carbonates vinylene carbonate (VC) [5][6][7][8][9][10] and vinyl ethylene carbonate (VEC) [11][12]. The electrochemical reduction of both VC and VEC has been studied in some detail, including quantum chemical calculations of the energetics that identified the most probable reaction pathway [13][14]. Both VC and VEC appear to be reduced at potentials above 1.0 V (vs. Li/Li + ) and form a passivating film that prevents solvent cointercalation and exfoliation of the graphite at lower potentials [6][7]11].…”

Density Functional Studies of Ring-Opening Reactions of Li+-(ethylene carbonate) and Li+-(vinylene carbonate)

“…[13][14][15][16][17][18][19][20][21][22] It has been demonstrated that electrons would be initially transferred from the polarized electrode to the Li + -coordinated solvent (or additive) molecules, forming ion-pair intermediates. Then, a ring-opening would take place on the intermediates to generate radical anions, which participate in termination reactions resulting in proper products in the form of Li organic or inorganic salts, building up the SEI film.…”

“…Then, a ring-opening would take place on the intermediates to generate radical anions, which participate in termination reactions resulting in proper products in the form of Li organic or inorganic salts, building up the SEI film. [17][18][19][20][21][22] The elucidation of reaction mechanisms is a major challenge for theoretical studies. To date, many theoreticians have used quantum chemical methods to gain insights into the initial reactions at the microscopic level.…”

“…To date, many theoreticians have used quantum chemical methods to gain insights into the initial reactions at the microscopic level. [13][14][15][17][18][19][20][21][22] In the present work, Kohn-Sham density functional theory (DFT) calculations were used to investigate the reductive ringopening reactions of Li + -coordinated EC and vinylene carbonate (VC), in order to gain insights into the initial reactions. The molecular structures of EC and VC are depicted in Figure 1.…”