(696g) Solubility of the Solid Electrolyte Interphase Layer Components

Most of modern technology requires the ability to store energy in a way that is portable, efficient and high performance. In practical use, these needs are met by the use of batteries of which the lithium ion battery is the current workhorse. The energy density of the lithium ion battery could be improved upon by the use of a lithium metal anode which would increase the energy density and thus the overall energy storage. However, the use of lithium metal presents many problems to its high reactivity and poor stability. These unfortunate properties lead to poor overall battery performance due to changes in the electrode morphology, passivity and the formation of lithium dendrites which can present serious safety concerns. Due to the nature of lithium, the formation of the SEI is unavoidable so in order to design a better battery the SEI must be engineered.

One of the issues facing the SEI is instability. The SEI is not a permanent stable layer which can lead to more electrolyte decomposition, allowing lithium dendrites to become nucleated and poor electrode mechanical properties. Understanding the stability of different SEI components will allow for intelligent engineering to create solutions to improve lithium metal anode performance.

We studied the solubility of different SEI components using first principles computational chemistry simulations to determine the SEI stability with regard to dissolution in the electrolyte. The use of computational chemistry allows for the investigation of properties and behaviors that are very difficult to screen experimentally. LiF, LiOH , Li2O and Li2CO3 were investigated to determine their structure in the electrolyte, the thermodynamics of dissolution and solvation, and their dissociation state. We found that that these simple components of the SEI do not have equal solubilities and different dissociation dynamics. Based on our first models, LiF is the most soluble while Li2O is the least soluble which indicates that a LiF SEI may be more unstable than a Li2O SEI. By exploring the dissociation states, the thermodynamic models and solvation dynamics can be expanded. We discovered that any dissociation of Li2O was highly unstable due to the reactiveness of the LiO anion and that LiF is thermodynamically favored to be a contact ion pair in the electrolyte. With these findings, we provide fundamental understanding of one crucial part of the stability of the SEI that can assist with future lithium battery design.