Structure and Li⁺ ion transport in a mixed carbonate/LiPF₆ electrolyte near graphite electrode surfaces: a molecular dynamics study

Abstract

Electrolyte and electrode materials used in lithium-ion batteries have been studied separately to a great extent, however the structural and dynamical properties of the electrolyte-electrode interface still remain largely unexplored despite its critical role in governing battery performance. Using molecular dynamics simulations, we examine the structural reorganization of solvent molecules (cyclic ethylene carbonate : linear dimethyl carbonate 1 : 1 molar ratio doped with 1 M LiPF) in the vicinity of graphite electrodes with varying surface charge densities (σ). The interfacial structure is found to be sensitive to the molecular geometry and polarity of each solvent molecule as well as the surface structure and charge distribution of the negative electrode. We also evaluated the potential difference across the electrolyte-electrode interface, which exhibits a nearly linear variation with respect to σ up until the onset of Li ion accumulation onto the graphite edges from the electrolyte. In addition, well-tempered metadynamics simulations are employed to predict the free-energy barriers to Li ion transport through the relatively dense interfacial layer, along with analysis of the Li solvation sheath structure. Quantitative analysis of the molecular arrangements at the electrolyte-electrode interface will help better understand and describe electrolyte decomposition, especially in the early stages of solid-electrolyte-interphase (SEI) formation. Moreover, the computational framework presented in this work offers a means to explore the effects of solvent composition, electrode surface modification, and operating temperature on the interfacial structure and properties, which may further assist in efforts to engineer the electrolyte-electrode interface leading to a SEI layer that optimizes battery performance.

Lithium‐Ion Transfer at Cathode‐Electrolyte Interface in Diluted Electrolytes Using Electrochemical Impedance Spectroscopy

“…Furthermore, it was reported that the concentration of the electrolyte mixture (solvent, salt, and additives) and the applied voltage also affect the solvation structure near the surface, and subsequently impact the reduction of the electrolyte. [40][41][42][43][44][45][46][47][48] In particular, the reduction reactions of the SEI depend strongly on the composition of the electrolyte. [46] The ab initio molecular dynamics simulations for a 1 mol/dm 3 LiPF 6 /EC solution showed that the reaction depends on the distance between the anion and the active Li site at the surface because the reduction reaction usually occurs via electron transfer at the surface, although the salt would be expected to reduce first because of its higher reduction potential.…”

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

“…A later MD simulation by Borodin and Bedrov predicted that the activation energy for Li + de-solvation at the SEI/electrolyte interface is 0.42 (~42 kJ mol −1 ) and 0.46 eV (~47 kJ mol −1 ) for the Li 2 EDC/electrolyte and Li 2 BDC/electrolyte interfaces, respectively. 186 Boyer et al 93 performed MD simulations of this process and showed an accumulation of Li + on graphite surface under different charge densities. These large-scale MD simulations described the electrolyte structural changes under the applied electric field, but have not been able to predict the energetics of electrolyte reduction reactions or the charge transfer reaction, due to the force field limitation.…”

“…The composition and structure of the SEI layer evolve as a function of the applied potential in a working lithium half-cell. 91 Boyer et al 93 performed MD simulations with a classical force field and demonstrated the different rearrangements of EC and DMC on a graphite edge surface under the applied electric field. Thus, the electrolyte structure rearrangement near the electrode surface will certainly impact the onset of the reduction reactions and the Li + de-solvation process.…”

Electrode polarization effects on interfacial kinetics of ionic liquid at graphite surface: An extended lagrangian‐based constant potential molecular dynamics simulation study

“…This is because net charges opposite to ion are induced on the electrode surface even in the ΔΨ 0 = 0 V case. Although having said that, ions in the equilibrium state are unlikely to be on the electrode surface as shown in previous studies …”

“…Although having said that, ions in the equilibrium state are unlikely to be on the electrode surface as shown in previous studies. [33,55] Ion desorption rate by transition state theory. Transition state theory (TST) is another way to estimate a rate constant of many kinds of processes.…”