Effect of Salt on Interfacial Structures of Highly Concentrated Acetonitrile Electrolytes

HCEs that contain acetonitrile (AN) are increasingly investigated for Li-ion and high-voltage Li-metal anode batteries due to the ability of HCEs to increase reductive stability of the AN solvent. However, chemical speciation at the interface and the impact of salt choice on these electrolytes are largely unknown. The impact of the anion on potential-dependent interfacial speciation within the electric double layer (EDL) of AN HCEs was first investigated through comparison of AN/LiTFSI and AN/LiFSI HCEs. Then, the impact of decreasing the ionic association by substituting the LiFSI salt with NaFSI and KFSI was investigated. SEIRAS was utilized to probe the potential dependence of surface concentration, ionic association, and conformation of the anions and AN molecule. FSI^–in the AN/LiFSI HCE exhibited a high degree of ionic association at the interface, which was not observed in the AN/LiTFSI, AN/NaFSI, or AN/KFSI HCEs, and was related to a change in anion conformation at the interface. Electrochemical impedance spectroscopy was used to estimate the differential capacitance in the electrolytes, which was higher and more potential dependent for the AN/LiFSI HCE than other HCEs investigated, which was attributed to increased ion pairing and reduced conformational shifts of the anion. This work shows that in these AN HCE systems, both anion and cation can have a large impact on the interrelated factors of interfacial ionic association and anion conformation.