(Invited) Recent Progress in Understanding Battery Electrolyte Electrochemical Stability and Its Relationship with Electrolyte Structural Properties

Designing compatible electrochemical interfaces is paramount for enabling energy storage devices from batteries to electric double layer capacitors as electrolytes should be either electrochemically stable at the electrode and current collectors surfaces or form a stable electronically insulating but ionically conducting solid electrolyte interphase (SEI). Electrical double layer capacitors with narrow pores cannot rely on the SEI to extend their operating window placing more stringent requirements on electrolyte intrinsic electrochemical stability compared to batteries. In this presentation I will discuss a molecular scale insight into the initial mechanisms of battery electrolyte electrochemical degradation on the active and non-active electrodes obtained from a coordinated density functional theory (DFT) study and non-reactive molecular dynamics (MD) simulations of bulk and interfacial properties of electrolytes and the relationship between them. In the first part of the presentation, I will focus on understanding of electrolyte oxidation reactions on active and non-active electrodes. While HOMO screening for redox shuttle molecules was previously successful for quantitatively predicting trends, such screening is often not sufficient for traditional battery electrolyte solvents including cycling and linear carbonates, phosphates and sulfones.(1) Instead, quantum chemistry calculations have shown that the majority of battery solvents with the exception of the solvents with unsaturated functionalities such C=C double bonds do not undergo direct oxidation within typical operating range, instead their oxidation is coupled with the H-transfer to another solvent or oxygen of the electrode surface. Analogously, on the anode the F-transfer and LiF formation during anion and semi-fluorinated solvent reduction occurs at higher potentials compared to direct reduction of isolated anions and solvents.(2) Local ion and solvent environment influences both oxidation and reduction stability and initial decomposition reactions of electrolytes making electrolyte electrochemical stability dependent on the salt concentration and ion and solvent partitioning with the double layer that is in turn is dependent on the applied potential. Highly concentrated aqueous and non-aqueous electrolytes were found in MD simulations to exclude the solvent molecules from directly interacting with the positive electrode surface providing an additional mechanism for extending the electrolyte