Fast-Charging Limitations of Advanced Electrolytes for Lithium Metal Batteries

Lithium metal batteries (LMBs) are being actively developed to meet the high-energy-density demand for electric vehicles (EVs). Fast charging is an important requirement for EV applications. While improving lithium metal Coulombic efficiency (CE) has been a focus for LMB electrolyte design, their performance under high current densities is less explored. Here, we evaluate the moderate-to-high-rate cycling stability of three recently developed advanced electrolytes, all of which are weakly solvating electrolytes with anion-derived solid electrolyte interfaces. All three electrolytes showed soft shorting behavior above various threshold current densities. Based on extensive characterizations, we propose a mechanism by which slow ion transport was the main factor that led to poor cycling stability due to concentration polarization, poor Li morphology, and closely packed residual solid electrolyte interphase (rSEI) structure. This work confirms the importance of fast ion transport for LMBs under moderate to fast charging conditions. Therefore, for electrolyte designs, improving CE must be accompanied by efficient ion transport in order to provide a viable solution to practical LMBs.