Japanese team elucidates effect of additives on reaction mechanisms for SEI formation in Li-ion batteries
A research group led by Dr. Yoshitaka Tateyama from the NIMS International Center for Materials Nanoarchitectonics (MANA), in collaboration with Fujifilm Corporation, has investigated the effect of adding vinylene carbonate (VC) to ethylene carbonate (EC) solvent (a typical electrolyte in Li-ion batteries, LIBs) on the reductive decomposition of the electrolyte to form the solid-electrolyte interphase (SEI) film. A paper on their work is published in the Journal of the American Chemical Society.
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.
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