Additive Effect on Reductive Decomposition and Binding of Carbonate-Based Solvent toward Solid Electrolyte Interphase Formation in Lithium-Ion Battery

Abstract

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 for Functional Electrolytes of Li-Ion Batteries

“…Although the study on VC as an additive has more than 20 years of history, its working mechanism in batteries is still under debate. While some chemists believe VC is involved with radical/anionic polymerization, as there has been experimental proof of such a product [36], other chemists have proposed mechanisms that involve the interaction of VC with reduction products of EC, which agrees with the observation of gas products such as CO and CO 2 (Scheme 1) [126,135]. Electrochemical processes can be very complicated, and in batteries they can be even more convoluted due to the heterogeneous surface chemistry of electrode materials.…”

Surface Layer and Morphology of Lithium Metal Electrodes

“…The SEI components under the top polycarbonate layer are very similar to the SEI layer for the additive-free electrolyte. Thus we suspect that the EC is still the main source to form the SEI layer, considering a proposed SEI formation mechanism reported by Ushirogata et al 25 Furthermore, the top polycarbonate layer probably improves the mechanical properties of the SEI layer resulting in the delay of the dendrite formation and enhancement of the in-plane 2D growth of the lithium metal. Figure 7 shows the XPS spectra of the lithium metal immersed in the electrolyte solution containing 1 wt% LiBOB.…”

Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries

“…74,118 The ring-opening reaction of VC occurs via a homolytic C (carbonyl carbon)-O rupture first in the solvated complex 111 and is more likely to undergo the second electron reduction than EC or PC. 74 Ushirogata et al 119 considered the effect of VC from both thermodynamic and kinetic point of view via AIMD simulations. They found that by adding VC to the electrolyte, the normal twoelectron reduction of EC can be suppressed.…”

“…They found that by adding VC to the electrolyte, the normal twoelectron reduction of EC can be suppressed. 119 They argued that the VC additive protects the anode by reacting with the EC anion radical to suppress the two-electron reduction of EC rather than only being sacrificed to form the SEI components. 119 Tasaki 74 further argued that the reduced product Li 2 EDC of EC has higher density, stronger cohesive energy, and less solubility in the solvent than that of PC.…”

“…119 They argued that the VC additive protects the anode by reacting with the EC anion radical to suppress the two-electron reduction of EC rather than only being sacrificed to form the SEI components. 119 Tasaki 74 further argued that the reduced product Li 2 EDC of EC has higher density, stronger cohesive energy, and less solubility in the solvent than that of PC. Takenaka et al 108 demonstrated that SEI formed from PC-based electrolyte is sparser than that formed by EC, as shown in Fig.…”