Propylene Carbonate-Based Electrolyte for Low Temperature Lithium Batteries

Considering the usage of smart phones, electrical vehicles, and power sources for grid storage application, lithium ion battery (LIB) operating under harsh circumstances have become a serious issue due to the sluggish ionic conductivity and increased viscosity of electrolyte at low temperatures. Ethylene carbonate (EC) is a widely used electrolyte component of commercial LIB owing to the suitable electrochemical stability window, high dielectric constant, and especially, its ability to form stable solid electrolyte interphase (SEI) on the graphite anode, which prevents continuous electrolyte decomposition and resulted in excellent cycling stability on graphite anode. However, its high freezing point (36℃), greatly increases electrolyte solidification temperature and hurts ionic conductivity at low temperatures, thereby leading to a narrow operation temperature range for LIB using EC-based electrolytes. Recently, plenty of efforts have been made to extend LIB operating temperature range, using techniques such as self-heating method with extra Ni-foil in specially designed cell configuration 1 , or electrolyte modifications by introducing low melting point solvents to increase ionic conductivity at low temperatures. Electrolyte modification is the most effective approach due to its low cost, high flexibility, and convenience. It can easily alter electrolyte physical and chemical properties, affect SEI chemical compositions, and result in positive impact on battery performance. Particularly, SEI is the interphase between anode and electrolyte, which allows Li ion transportation and insulates electron transfer, protects anode surface and prevents electrolyte decomposition. Many different research groups have made a lot of progress of electrolyte modification, including adjusting solvent salt combinations, fluorinating of solvents, and developing novel additives. 2 Nevertheless, there are still many challenges need to be addressed for stable low temperature operation. Herein, considering the research importance and current progress of low temperature electrolyte, we screened several organic compounds as co-solvents or additives to improve low temperature performance while keeping other properties of current LIB. For example, propylene carbonate (PC) has comparable dielectric constant and electrochemical window as EC, and much lower freezing point (-49℃), making it very promising candidate for low temperature electrolyte. 3 Unfortunately, PC cannot be used as single solvent for LIBs because it usually co-intercalates into graphite layer and cause graphite exfoliation. In order to prevent co-intercalation of PC, a stable SEI needs to be formed. We developed several new electrolytes using different salts, solvents and additives, which have higher reduction potential than PC intercalation voltage and enable stable SEI formation, making PC-based low temperature electrolyte application possible. In addition, advanced characterization techniques were used to characterize the interphase properties to reveal underlying mechanism of improved electrochemical performances by modifying electrolyte composition. These results will be presented at meeting. Reference C.-Y. Wang, Nature , 2016, 529 , 515. G. Zhu, Journal of Power Sources , 2015, 300 , 29-40. Q. Li, Applied Materials & Interfaces , 2017, 9 , 42761-42768. Acknowledgement The work carried out at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the US Department of Energy through the Applied Battery Research for Transportation (ABRT) program.