The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and capability of a lithium-ion battery (LIB). Here we investigated the effects of adding vinylene carbonate (VC) to ethylene carbonate (EC) solvent, a typical electrolyte in LIBs, on the reductive decomposition. We focused on both thermodynamics and kinetics of the possible processes and used density functional theory-based molecular dynamics with explicit solvent and Blue-moon ensemble technique for the free energy change. We considered Li(+) in only EC solvent (EC system) and in EC solvent with a VC additive (EC/VC system) to elucidate the additive effects. In addition to clarifying the equilibrium properties, we evaluated the free energy changes along several EC or VC decomposition pathways under one-electron (1e) reduction condition. Two-electron (2e) reduction and attacks of anion radicals to intact molecules were also examined. The present results completely reproduce the gaseous products observed in the experiments. We also found a new mechanism involving the VC additive: the VC additive preferentially reacts with the EC anion radical to suppress the 2e reduction of EC and enhance the initial SEI formation, contrary to the conventional scenario in which VC additive is sacrificially reduced and its radical oligomerization becomes the source of SEI. Because our mechanism needs only 1e reduction, the irreversible capacity at the SEI formation will decrease, which is also consistent with the experimental observations. These results reveal the primary role of VC additive in the EC solvent.

Additives in the electrolyte solution of lithium-ion batteries (LIBs) have a large impact on the performance of the solid electrolyte interphase (SEI) that forms on the anode and is a key to the stability and durability of LIBs. We theoretically investigated effects of fluoroethylene carbonate (FEC), a representative additive, that has recently attracted considerable attention for the enhancement of cycling stability of silicon electrodes and the improvement of reversibility of sodium-ion batteries. First, we intensively examined the reductive decompositions by ring-opening, hydrogen fluoride (HF) elimination to form a vinylene carbonate (VC) additive and intermolecular chemical reactions of FEC in the ethylene carbonate (EC) electrolyte, by using density functional theory (DFT) based molecular dynamics and the blue-moon ensemble technique for the free energy profile. The results show that the most plausible product of the FEC reductive decomposition is lithium fluoride (LiF), and that the reactivity of FEC to anion radicals is found to be inert compared to the VC additive. We also investigated the effects of the generated LiF on the SEI by using two model systems; (1) LiF molecules distributed in a model aggregate of organic SEI film components (SFCs) and (2) a LiF aggregate interfaced with the SFC aggregate. DFT calculations of the former system show that F atoms form strong bindings with the Li atoms of multiple organic SFC molecules and play as a joint connecting them. In the latter interface system, the LiF aggregate adsorbs the organic SFCs through the F-Li bindings. These results suggest that LiF moieties play the role of glue in the organic SFC within the SEI film. We also examined the interface structure between a LiF aggregate and a lithiated silicon anode, and found that they are strongly bound. This strong binding is likely to be related to the effectiveness of the FEC additive in the electrolyte for the silicon anode.

To get insight of the formation mechanism of solid electrolyte interphase (SEI) film in Lithium-ion battery (LIB), we examine a probable scenario, referred to as "surface growth mechanism," for electrolyte involving ethylene carbonate (EC) solvent and vinylene carbonate (VC) additive by using density functional theory (DFT). We first extracted stable SEI film components (SFCs) for the EC/VC electrolyte and constructed probable SFC aggregates via DFT molecular dynamics. We then examined their solubility in the EC solution, their adhesion to a model graphite electrode, and the electronic properties. The results showed that the SFC aggregates are characterized by "unstable adhesion" to the graphite surface and "high electronic insulation" against the EC solution. These characteristics preclude explaining SEI growth up to a typical thickness of several tens of nanometers based on the surface growth mechanism. With the present results, we propose "near-shore aggregation" mechanism, where the SFCs formed at the electrode surface desorb into the near-shore region and form aggregates. The SFC aggregates coalesce and come into contact with the electrode to complete the SEI formation. The present model provides a novel perspective for the long-standing problem of SEI formation. Lithium-ion batteries (LIBs) have attracted considerable attention for use in larger power sources like electric vehicles and energy storage system because of their high energy densities.1,2 For such use, a higher degree of safety, a longer cycle life, higher voltage and capacity will be indispensable in the future. An important key of the stability and durability of the LIB is the solid electrolyte interphase (SEI) formed at the negative electrode-electrolyte interface.3,4 It is generally accepted that molecules in the electrolyte solution reductively decompose to form various SEI film components (SFCs), such as organic oligomers (e.g., (CH 2 OCO 2 Li) 2 , and ROCO 2 Li) and inorganic moieties (e.g., Li 2 CO 3 , LiF) at the first charging, 5 and that the SFCs precipitate on the electrode surface to form a stable SEI film with a thickness of several tens of nanometers. 6 The SEI hinders electron transport from the electrode to the electrolyte solution, preventing further electrolyte decomposition, while allowing Li + ion transport. This property decreases the irreversible capacity and improves the safety of LIBs. Additives to the electrolyte solution also have a large impact on SEI performance. Even a small amount of additive up to a few wt% significantly improves the irreversible capacity and cyclability.7 In general, the additive molecules modify the SFCs and lead to different properties of the SEI film. Despite the important roles of the SEI in LIB operation, the microscopic formation processes are still unclear because of the difficulty in operando observations of chemical reactions at the electrodeelectrolyte interfaces.The detailed formation processes of SEI have been typically assumed to involve "surface growth mechanism", mainly in the firs...