The SEI plays a crucial role in the stability and capability of a lithium-ion battery. While it is known that the quality of SEI films can be greatly improved by using additives substance, the reaction process in the formation of SEI films has not yet been clarified due to the difficulty in direct observation of the reactions experimentally, the authors noted.

In their study, they focused on both thermodynamics and kinetics of the possible processes and used density functional theory-based molecular dynamics (DFT-MD) with explicit solvent and blue-moon ensemble technique (introduced by researchers in 1989 to sample rare events that occur “once in a blue moon”) for the free energy change.

They considered Li⁺ in EC solvent only (EC system) as well as in EC solvent with a VC additive (EC/VC system) to elucidate the additive effects. Among their findings was a new mechanism involving the VC additive.

Contrary to the conventional scenario that VC additive is sacrificially reduced and makes a VC oligomer that seeds SEI formation, the present results provide a completely different mechanism: the VC additive preferentially reacts with the EC anion radical to suppress the 2e reduction of EC, the main initial stage of SEI formation in the VC-free EC electrolyte. Because this VC mechanism is realized via 1e reduction, the irreversible capacity at the SEI formation will decrease as well, which is also consistent with the experiments. These results not only reveal the primary role of the VC additive in the EC solvent but also provide a new fundamental perspective for the reductive decomposition of carbonate-based electrolyte near the negative electrode.

—Ushirogata et al.

Schematic summary of the reductive reactions of the EC and EC/VC systems. (a) EC only case: EC radical oligomerization following 2e reduction will be the source of SEI. (b) A conventional pathway for the EC/VC system, namely oligomerization of VC anion radical sacrificially reduced. (c) A new mechanism with 1e reduction only for the EC/VC system, proposed in this work. VC passivates the EC anion radical. (d) Possible case with reductively decomposed VC. The product dCO-VC− is not reactive so that it may need another electron or another reduced molecule for the subsequent reactions. Credit: ACS, Ushirogata et al. Click to enlarge.

Resources

• Keisuke Ushirogata, Keitaro Sodeyama, Yukihiro Okuno, and Yoshitaka Tateyama (2013) Additive Effect on Reductive Decomposition and Binding of Carbonate-Based Solvent toward Solid Electrolyte Interphase Formation in Lithium-Ion Battery. Journal of the American Chemical Society 135 (32), 11967-11974 doi: 10.1021/ja405079s