Lithium-electrolyte solvation and reaction in the electrolyte of a lithium ion battery: A ReaxFF reactive force field study

Abstract

In the electrode/electrolyte interface of a typical lithium-ion battery, a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from the anode to the electrolyte reduces the Li+-ion and makes it more reactive, resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange, and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended the existing ReaxFF reactive force field parameter sets to organic electrolyte species, such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate, and LiPF6 salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to the existing ReaxFF training data, and subsequently, we trained the ReaxFF parameters with the aim of finding the optimal reproduction of the DFT data. In order to discern the characteristics of the Li neutral and cation, we have introduced a second Li parameter set to describe the Li+-ion. ReaxFF is trained for Li-neutral and Li+-cation to have similar solvation energies, but unlike the neutral Li, Li+ will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li+-ions are reduced to Li-atoms, which can be simulated using a Monte Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li+-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of ethylene carbonate molecules solvating the Li-atom.

Modeling the Solid Electrolyte Interphase: Machine Learning as a Game Changer?

“…Similar as in hybrid MD/MC schemes (see below), the charge state of the lithium atoms can be altered in MC steps. [ 176 ] This is similar in spirit to simulations of a model battery cell without an explicit treatment of electron particles. [ 177 ] ML models that can universally track electron density and its transfer and modulation for large scale reactive simulations can usher in a new era (see Section 4.1).…”

“…Indeed, this framework could successfully describe the reduction of EC as well as the cascade of consecutive reactions [ 146 ] (left snapshot in Figure 3d), although these decomposition mechanisms have to be carefully parametrized. [ 176 ] Notably, the latter work revealed a notable impact of the lithium ion solvation shell on the EC degradation. Furthermore, the authors suggest to use two different parameter sets—one for metallic lithium and one for lithium ions—to incorporate distinct oxidation states.…”

Implications of the BATTERY 2030+ AI‐Assisted Toolkit on Future Low‐TRL Battery Discoveries and Chemistries

“…Given such findings, it is not surprising that continuous efforts during decades have been devoted to the development of polarisable and advanced force‐fields for application in the battery context, as exemplified here by a recent battery‐related interface study regarding solvent decomposition reactions at the anode/electrolyte interface using (≤2.5 ns) long MD simulations with the ReaxFF polarizable force field. [ 30 ] The considerable efforts spent in the literature on the development of polarizable interaction models for complex energy materials applications reflects both the great need for efficient force fields that allow large‐scale, long‐time computer simulations and the formidable challenge that it constitutes to generate force fields that accurately capture the electronic effects.…”

Atomistic-Scale Simulations on Graphene Bending Near a Copper Surface

“…The ReaxFF method has been extensively applied in battery applications, especially in the areas of anode/electrolyte interfaces [15], electrolytes [16][17][18], and cathode/electrolyte interfaces [19,20]. However, no ReaxFF study has been reported on the current collector/electrode interface.…”