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

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.