Comparative Study on the Solid Electrolyte Interface Formation by the Reduction of Alkyl Carbonates in Lithium Ion Battery

Abstract

One of the major concerns in lithium ion battery is the composition of the electrolytes as it governs some of the vital physicochemical properties of the electrolytes such as viscosity, ionic conductivity, thermal stability and wettability. Most important properties of lithium ion batteries are related to the properties of the solid electrolyte interface (SEI) layer formed on the electrode surface. The SEI in turn is mainly dependent on the electrolyte composition [1]. Therefore, understanding the nature, mechanism and property of the SEI formation in different electrolyte composition is crucial in selecting electrolytes. In this work, we report a direct comparison of the reduction of propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC) as a single, binary and ternary solvent systems. Battery performance test, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscope (SEM) is employed to study the nature of the surface films formed. The reduction products are determined by using ex-situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), in the glove box, after employing linear sweep voltammetry (LSV) to certain potential regions. EC/PC/DEC based electrolyte shows better capacity retention and less impedance than the EC/PC system. In all systems, the FTIR results indicate the formation of (CH3CH2OCO2Li)2 and Li2CO3 due to the reduction of PC. EC is reduced to (CH2OCO2Li)2 and Li2CO3 in both the binary and ternary solvent system. The FTIR result at the potential where DEC reduction occurs shows the formation of (CH2=CH-OCO2CH2CH3)Li which is less stable and forms a less-passivating layer in the first cycle. However, this product can further react with another species in the subsequent cycles and form a stable SEI. [1] A.J. Gmitter, J. Gural, G.G. Amatucci, J. Power Sources, 217 (2012) 21-28.[2] K. Xu, J. Electrochem. Soc., 156 (2009) A751-A755.[3] G.V. Zhuang, H. Yang, B. Blizanac, P.N. Ross, Electrochem. Solid-State Lett, 8 (2005) A441-A445.