A Critical Investigation of the Viability of Fast Charging of Lithium-Metal Batteries in Glyme-Ether Electrolytes

Abstract

Widespread adoption of electric vehicles (EVs) largely depends on the development of efficient battery systems having high energy density with the capability of fast charging. In this regard, lithium (Li) metal-based batteries, such as Li-Oxygen and Li-Sulfur batteries, can provide much higher energy density compared to that of Li-ion batteries. However, low rate-capability of both Li-Oxygen and Li-Sulfur cells hinders their practical utilizations. It has been found that transport limitations in the electrolyte play a major role in affecting the rate-capability of the cells. Therefore, identifying a suitable electrolyte with high ionic conductivity is an essential requirement to improve the rate-capability of these batteries. Additionally, a desired electrolyte must also be stable toward the electrode processes in the cell. Glyme-ether based electrolytes, in these instances, are a preferred class of electrolytes for both Li-Oxygen and Li-Sulfur batteries due to their acceptable compatibility with Li metal as well as with the porous carbon electrodes. Nevertheless, the length of the glyme-ethers dramatically affects the ionic conductivities of the electrolytes and decides the rate-capability of the battery. Therefore, in this work we have investigated the viability of fast Li-plating in different glyme-ether electrolytes and critically analyzed the effects of ion-transport and also assessed the relative stabilities of the electrolytes during Li stripping/plating processes.The present study involves investigation of Li stripping/plating in six different electrolytes using three glyme-ethers, diglyme (G2), triglyme (G3) and tetraglyme (G4), as the electrolyte solvents with lithium nitrate (LiNO3) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the Li salts (1 M) and lithium bromide (LiBr) as an additive (0.05 M). It has been found that the ionic conductivity of the electrolyte increases as the glyme becomes smaller or highly associated LiNO3 is replaced with LiTFSI. As a result, both smaller glymes and LiTFSI help to achieve Li-plating at faster rates. However, unfortunately, stability of Li-plating in terms of Coulombic efficiency and overall cycle-life decreases in smaller glymes or in LiTFSI as the Li salt and detailed investigations reveal formation of unstable SEI to be responsible for lower stability of Li-plating in these electrolytes. Our results show that the consequences of increasing the ionic conductivity of the electrolyte by simply changing the glyme-ether solvent in Li-metal batteries are quite complex and warrant careful consideration of their relative stabilities with the electrode. All the results, in depth analyses and future perspectives of this work will be discussed during the presentation.