| dc.description.abstract | Lithium-ion batteries (LIB) are the best option among batteries for portable electronic, power tools, and electric vehicles due to their higher energy storage, higher power, and lighter weight than other battery technologies such as Ni-based or lead acid. However, Li-ion batteries still face challenges such as safety, life, performance, and cost. One way to contribute to the solutions of these challenges and, consequently, improve the performance of Li-ion cells is to develop and design more stable passivation films at the electrode-electrolyte interface. Therefore, having a better understanding of the molecular processes that lead to the nucleation, growth, structure and morphology, as well as the electron and ion transport properties of the solid electrolyte interphase (SEI) is highly important for the development of new or improved lithium-ion batteries. In this work, computational methods, which allow studying phenomena not easily observable with experimental techniques, are used to study the electron transfer characteristics and the lithium ion diffusivity of the materials found in the SEI film formed in LIB with silicon anodes.
First, ab initio computational methods are used to study the electron transfer through selected finite models of SEI films formed at the anode-electrolyte interface. A combined ab initio density functional theory (DFT) and Green’s functions approach, as implemented in the Generalized Electron Nano-Interface Program (GENIP), is used to calculate the current-voltage characteristics of selected SEI configurations. The models studied consist of a LixSiy cluster, a SEI product (LiF, Li2O or Li2CO3), and an electrolyte component, ethylene carbonate (EC). Various parameters are considered in the investigation including: various lithiated states for the anode; several thicknesses and configurations for the SEI layer; and the presence of surface oxides (SiO2 and Li2Si2O5). The trend of conductance is found to be Li2O > SiO2 > LiF > Li2CO3 > Li2Si2O5, at the same applied voltage and anode configuration.
Then, lithium-ion diffusion is studied in the main components of the SEI layer using classical molecular dynamics (MD) simulations in order to provide insights and to calculate the diffusion coefficients of Li-ions at temperatures in the range of 250 K to 400 K. The compounds studied are lithium fluoride (LiF), lithium oxide (Li2O) and lithium carbonate (Li2CO3). A slight increase in the diffusivity as the temperature increases is found and since diffusion is noticeable at high temperatures, Li-ion diffusion in the range of 1300 to 1800 K is also studied and the diffusion mechanisms involved in each SEI compound are analyzed. The mechanisms of Li-ion diffusion observed include vacancy assisted and knock-off diffusion in LiF, direct exchange in Li2O, and vacancy and knock-off in Li2CO3. Moreover, the effect that an applied an electric field has in the diffusion of Li-ions at room temperature is also evaluated.
The long-term goal is to eventually have more control over interface parameters such as composition, structure, porosity and thickness, and thus accurately design SEI films and therefore better Li-ion batteries. This work is a step towards this ultimate goal. | en |
