Ab initio molecular dynamics simulations of the initial stages of solid–electrolyte interphase formation on lithium ion battery graphitic anodes

Abstract

The decomposition of ethylene carbonate (EC) during the initial growth of solid-electrolyte interphase (SEI) films at the solvent-graphitic anode interface is critical to lithium ion battery operations. Ab initio molecular dynamics simulations of explicit liquid EC/graphite interfaces are conducted to study these electrochemical reactions. We show that carbon edge terminations are crucial at this stage, and that achievable experimental conditions can lead to surprisingly fast EC breakdown mechanisms, yielding decomposition products seen in experiments but not previously predicted.Improving the fundamental scientific understanding of lithium ion batteries 1-3 is critical for electric vehicles and efficient use of solar and wind energy. A key limitation in current batteries is their reliance on passivating solid electrolyte interphase (SEI) films on graphitic anode surfaces. 1-5 Upon first charging of a pristine battery, the large negative potential applied to induce Li + intercalation into graphite decomposes ethylene carbonate (EC, Fig. 1) molecules in the solvent, yielding a self-limiting, 30-50 nm thick, passivating SEI layer containing Li 2 CO 3 , lithium ethylene dicarbonate ((CH 2 CO 3 Li) 2 ), 2,4-6 and salt decomposition products. C 2 H 4 and CO gases have also been detected 7,8 and shown to come from EC. 9 Similar reactions occur during power cycling when the SEI film cracks and graphite is again exposed to EC. 2 If instead the solvent is pure propylene carbonate (PC), a stable SEI film does not materialize 1,2 and the battery fails. Our work shows that novel mechanisms for the initial stages of SEI-growth at electrode-electrolyte interfaces can be simulated within time scales accessible to ab initio molecular dynamics (AIMD), 10 which have successfully modelled liquid-solid interfaces. 11 AIMD is likely also applicable to shed light on cosolvent/additives which must decompose more readily than EC to alter and improve SEI structure, Li + transport, and passivating properties. 1,2 EC-decomposition mechanisms under electron-rich conditions have been proposed (e.g., Refs. 4,5) and investigated using gas cluster Density Functional Theory calculations with and without dielectric continuum approximation of the liquid environment. 12-15 Thus "EC − ", coordinated to Li + or otherwise, has been predicted to undergo ethylene carbon (C E )-oxygen (O 1 ) bond cleavage to form a more stable radical anion (Figs. 1a-b). The potential energy barrier involved is at least 0.33 eV. 12,14 Carbonyl carbon (C C )-O 1 bond-"breaking" (or elongation) in the gas phase EC − -Li + complex yields a lower barrier, but metastable products. 14 Unlike these previous work, AIMD simulations can include explicit liquid state environments and EC/graphite interfaces. Unlike classical force field-based simulations, 16,17 AIMD accounts for covalent bondbreaking. We apply the VASP code, 18,19 the PerdewBurke-Ernzerhof (PBE) functional, 20 Γ-point Brillouin zone sampling, 400 eV planewave energy cutoff, tritium masses for all protons to allow B...

The Role of Carbonate and Sulfite Additives in Propylene Carbonate-Based Electrolytes on the Formation of SEI Layers at Graphitic Li-Ion Battery Anodes

“…1, which has been confirmed in some specific density functional theory (DFT)-based ab initio molecular dynamics simulations. 55 Here, we first investigate the electronic structures of ternary graphite intercalation compounds (GICs), Li + (S) n = 1-4 C 14 (S = EC, PC) using density functional theory to determine the best choice of a range of carbonate and sulfite additives for PC-based electrolytes that promote stable SEI film formation at a graphite anode in Li-ion batteries. Quantum chemical computations give accurate energies of reactions and intermediates, including decomposition mechanisms that are key to SEI formation and stability in cycling for Li-ion batteries.…”

Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells

“…63 That is, the SEI works as a rectifier, passing Li + ions through while blocking the electrons. Second, while one-electron reduction of the solvent is a barrierless reaction, 74 the reduction of SEI can involve high reaction barriers even if the overall reaction is strongly exergonic. This applies, inter alia, to open chain carbonate anions ( Figure 13), which accounts for the occurrence of EDC 2− in the SEI.…”

First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces

“…EC configurations are pre-equilibrated using Monte Carlo simulations and simple molecular force fields, as described in an earlier work. 11 We also report AIMD simulations of hydroxylated MnO 2 surfaces in appendix B, using deuterium masses for H atoms and a 0.5 fs time step there.…”

“…[8][9][10][11][12][13] One may argue that its ubiquity and critical role make EC in LIB the battery equivalent of the H 2 O molecule in biology, geochemistry, and many solid-liquid interfacial science disciplines. The present work is indeed modeled after theoretical studies on water-on-mineral surfaces.…”