Esters As Cosolvents for Improving Low Temperature Cycling Performance of Ncm 523/Graphite Cells

Abstract

Lithium-ion batteries (LIB) are known to have high energy density and power density compared to other electrochemical storage devices. However, LIB’s applications and performances are vastly limited by the working temperature range of electrolyte formulation, particularly at low temperatures. In current state-of-the-art LIB electrolytes, low temperatures cause a drastic decrease in Li+ ion conductivity while increasing cell resistance which ultimately leads to a large capacity drop. However, using electrolyte solvents with a high dielectric constant and low melting points improves the ionic conductivity and gives a lower viscosity. Furthermore, incorporating film forming additives can improve the sluggish Li+ ion transfer kinetics at the electrode/electrolyte interphase by forming a thin and conductive solid electrolyte interphase (SEI). In this study, two ester cosolvents, namely methyl propionate (MP) and methyl acetate (MA), were used in a tertiary electrolyte formulation made of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) along with film forming additives, 2% fluoroethylene carbonate (FEC) or 2% vinylene carbonate (VC) in order to improve the low temperature performance in NCM 523/Graphite full cells at -20 °C. Ex-situ surface film analysis of extracted electrodes was carried out by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared (ATR-IR) spectroscopy to better understand the improved cycling performance seen with the ester formulations.