Enhanced Electrochemical Properties of Electrolytes with Nitrile Functionalized Zwitterions for Lithium Ion Battery

Abstract

Nitrile-functionalized zwitterion such as 1-(propane nitrile)-2-methylimidazolium-3-(propyl sulfonate) (N I) is synthesized and investigated as an electrolyte additive toward the stabilization of the solid electrolyte interface (SEI) layer on the electrode surfaces and enhacement of ion conductivity, which can be crucial to improving the initial discharge capacity, C-rate performance andthe cyclic performance under room temperature. The methyl-functionalized zwitterion (DM) is used as compared with NI in order to explain the effect of the nitrile group. The synthesized zwitterions were characterized by 1H nuclear magnetic resonance (1H–NMR), elemental analysis (EA), and fast atom bombardment mass spectrometry (FAB–MS). The 1.0 M LiPF6 in EC/DEC (3/7 V/V) is used as a baseline liquid (E_0) electrolyte. Various amounts of the two types of synthesized zwitterion (0.02, 0.04, 0.08, 0.12, and 0.16 M) are added to E_0 (to produce the corresponding E_NI and E_DM). The electrolytes containing suitable contents of NI (E_NI), contrary to the electrolytes containing contents of DM (E_DM), is increased the ion conductivity to that of E_0. The ion conductivity gradually increases with increasing NI content. The ion conductivity of electrolytes containing 0.08 M of NI reaches a maximum value of 7.64 mS/cm. From the FT−IR spectroscopy, it is concluded that nitrile group and sulfonate group interacted with Li+ ion, so that these effects lead to enhancing lithium salt dissociation. The effects of both zwitterions on electrochemical performance are investigated via a combination of cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential capacity measurements and electrochemical impedance spectroscopy (EIS). The interfacial resistance of the lithium-ion battery containing E_NI is lower than that of E_0. It is confirmed that a NI can enhance the stability of SEI layer on the graphite electrode surface. These results are attributed to the electrochemical adsorption of NI on the electrode surface during aging process, which prevent electrolytes decomposition on electrode surface. However, although the ion conductivity of electrolytes containing 0.08 M of NI examine here have higher values than electrolytes containing 0.04 M of NI, the initial discharge capacity and C-rate performance display the inverse behavior. Accordingly, the ion conductivity is not crucial to determining the performance of the single full cell, at least over this ion conductivity range and in this system. The cell performance is found to depend significantly on the stability of the SEI layer, and one of the most stable SEI layers is produced by electrolytes containing 0.04 M of NIduring the initial cycle. Therefore, we conclude that the nitrile-functionalized zwitterions (NI) is expected to be a good candidate for an additive to high performance lithium-ion batteries.