Tailoring Electrolyte Solvation for Practical Li Metal Batteries Cycled at Ultra-Low Temperature

Abstract

Lithium metal batteries (LMBs) hold the key to pushing cell level energy densities beyond 300 Wh kg-1, and remain one of the most promising technologies for the future of electronic devices operating at ultra-low temperatures (< -30 oC). However, current battery technologies generally experience significant reduction in performance at such temperatures, and are commonly paired with external warming devices in order to circumvent this issue. However, this solution is suboptimal due to the inherent inactive volume, mass, and power requirements of such warming devices. It follows that batteries capable of both charging and discharging at these temperature extremes are highly desirable due to their inherent reduction in external warming requirements. However, charging in ultra-low temperature LMBs has been under studied to this point, and thus little is known about the growth dynamics of Li metal under such conditions. Herein, we present evidence that proper design of the local solvation structure of the electrolyte is crucial for achieving high Li metal coulombic efficiency (CE) at ultra-low temperature, using two model systems which were investigated via molecular dynamics (MD) and quantum chemistry simulations. Specifically, the contact-ion pair (CIP) structure dominated 1 M lithium bis(fluorosulfonyl)imide (LiFSI) / diethyl ether (DEE) electrolyte was found to produce remarkable CEs of 99.0% and 98.4 % at -40 oC and -60 oC, respectively, which was in stark contrast to the commonly used 1 M LiFSI 1,3-dioxolane,/1,2-dimethoxyethane (DOL/DME) electrolyte, who’s solvent-separated ion pair (SSIP) structure was found to result in rampant soft shorting of the cells due to dendritic plating. These divergent outcomes were explained via a desolvation mechanism, in which the strong Li+/solvent binding of the SSIP structure promoted tip driven growth whereas the weak Li+/solvent interactions in the CIP electrolyte promoted homogenous deposition. These insights were applied to Li metal full cells, where a high-loading 3.5 mAh cm-2 sulfurized polyacrylonitrile (SPAN) cathode was paired with a one-fold excess Li metal anode, yielding unprecedented capacities of 519 mAh g-1 and 474 mAh g-1 when charged and discharged at -40 oC and -60 oC, respectively. We also put this system in historical context via cell-level energy density calculations, which reveal that our work stands alone, supplying projected energy densities of 218, 143, and 126 Wh kg-1 when charged and discharged at 23, -40, and -60 oC, respectively. This work provides crucial design criteria for the design of ultra-low temperature LMB electrolytes, in addition to providing a defining step for the performance of low-temperature batteries regardless of electrode chemistry. Figure 1