A Metal Organic Framework Electrolyte That Improves the High-Temperature Performance of Lithium Metal Batteries

Abstract

In recent years, Li secondary batteries have had a profound effect on daily life as the power sources for portable electronics and electric vehicles.[1] However, despite extensive exploration of potential anode materials, the rational design of Li metal anodes that provide high energy densities with a suitable degree of safety and outstanding high-temperature stability remains a challenge.[2] Presently, the majority of studies focus on improving the performance of Li metal batteries (LMBs) at ambient temperature by employing various electrolyte additives,[3, 4] artificial solid electrolyte interfaces[5] and Li metal hosts.[6] Such research has provided detailed insights into the feasibility of increasing the Coulombic efficiency of these devices while inhibiting dendrite growth at ambient temperature. However, the operation of LMBs at high temperatures (100–180 °C, as 180 °C is the melting point of Li metal) and high current densities has rarely been addressed. The rapid formation and growth of Li dendrites decreases the safety of these devices at high temperatures and also leads to low cycling efficiency during charging/discharging.[7] In addition, conventional organic electrolytes suffer from potential issues including leakage, volatilization, flammability and explosion potential,[8] and thus are not suitable for high-temperature LMBs. Herein, we report a novel ionogel (termed ILE@MOF) obtained by mixing imidazolate framework-67 (ZIF-67) particles with an ionic liquid electrolyte (ILE) via simple ball-milling, with the goal of employing these materials to make dendrite-free LMBs operable at high temperatures. The ILE used in this work was a mixture of N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([Py13][TFSI]) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). When encapsulated in the MOF 3D channels, this ILE does not lose its dynamic mobility and also maintains high ionic conductivity, even though the resulting ionogel has the appearance of a solid. Unlike Li metal at ambient temperature, Li at elevated temperatures is more susceptible to failure due to increased reactivity with electrolytes, which can lead to increases in the cell impedance. Our results demonstrate that the ILE@MOF effectively protects the Li metal anode by forming a particle-rich coating over the anode, and so markedly increases the anode stability at high temperature. When combined with this new electrolyte, the Li metal anode maintains a stable striping/plating voltage over 1200 h at 150 °C and a current density of 0.5 mA·cm-2. To the best of our knowledge, this is the first demonstration of a viable electrolyte that permits stable Li electrochemical striping/plating at 150 °C. Using the ILE@MOF electrolyte, Li/LiFePO4, Li/LiNi0.33Mn0.33Co0.33O2, Li/LiNi0.8Mn0.1Co0.1O2 and Li/Li4Ti5O12 cells were found to exhibit stable cycle performance over the range of 60–150 °C.