Fundamental Chemistry behind Rechargeable Lithium Ion Batteries: An Ab Inito Study

In recent years, due to increase in global energy demand, developing energy storage systems with higher energy densities are an urgent need. Rechargeable Li-ion batteries (LIBs) emerge as an important power sources for energy storage device for portable electronics and are being continuously investigated and developed for use in automobile electric vehicles (EV). However, during the cycling of LIBs, due to the decomposition of electrolyte, a passivation layer called solid electrolyte interphase (SEI) is formed on electrode surface. Although, SEI formation is vital in LIBs for long term performance, it leads to irreversible loss of lithium and thus, storage capacity of battery. While several studies have been performed in the past few decades on SEI, it is difficult to control its formation and growth, as the chemical composition, and stability of SEI depend on a list of factors. These factors include the electrode material, electrolyte composition, electrochemical conditions, and cell temperature. The perfect SEI would be a fast forming, flexible, stable, and contains insoluble species with low electronic and high ionic conductivity. Thus, the formation of SEI and the electrochemical stability of SEI components should be a primary topic of investigation in future development of LIBs. To understand the stability and chemistry of SEI layer, density function theory based molecular dynamics simulations were performed. We used metadynamics to understand various degradation pathways of electrolyte decomposition in LIBs in various reaction conditions as well as in presence of additives. Car-Parrinello molecular dynamics based simulations were performed to determine the solvation structure of Lithium ion in presence of additives. Fig (1) shows the structure of solvation shell of Lithium ion (Yellow color code) in Ethylene Carbonate (EC). EC molecules in the first solvation shell is shown by ball and stick model. For the purpose of clarity Hydrogen atoms are not shown in EC molecules. color code: Lithium-Yellow, Oxygen-Red, Carbon-Grey. Figure 1