The demand for energy storages with high energy density and long lifetime is growing rapidly. Using lithium metal as an anode would maximize the energy density of lithium batteries by having an anode-free system in the discharged state. Lithium-ion batteries can undergo several thousand cycles using liquid electrolytes with low capacity losses and very low safety risks. Passivation of lithium metal and the uncontrolled formation of lithium dendrites has so far prevented the large-scale commercialization of lithium-metal batteries with liquid electrolytes. Understanding the passivation reactions and formation of the solid electrolyte interphase in of lithium metal in liquid electrolytes is crucial to increase the longevity and safety of this system. In this study, we present experimental results on the lithium metal liquid interfaces for next-generation lithium batteries utilizing isothermal microcalorimetry. The experimental setup involved the investigation of coin cells at a symmetrical lithium metal cell and lithium-copper (Li-Cu) half-cell level. As a baseline electrolyte, the carbonate-based LP57 was used (EC:EMC 3:7 wt%, 1M LiPF6) supplemented with fluoroethylene carbonate (FEC) and vinylene carbonate (VC) additives. As a comparison two ether-based- formulations were tested, a high concentration electrolyte (DME, 4M LiFSI) and a localized high concentration electrolyte (LiFSI:DME:TTE 1:1.2:3 molar).Those coincells were investigated at various current densities ranging from 0.5 to 5 mA/cm² at a constant cyclic capacity throughput of 5 mAh/cm².

Our results reveal insights into the thermodynamic behavior and stability of lithium metal interfaces indifferent electrolyte environments and for different current densities. The calorimetric measurements provide the additional information on the heat flow associated with plating and stripping processes, shedding light on the kinetics of lithium deposition and dissolution. Furthermore, we systematically investigated the influence of electrolyte composition, including the presence of FEC and VC additives, on the interfacial properties and passivation reactions of lithium metal. In symmetrical lithium metal cells, we observed distinct heat signatures corresponding to the coulombic efficiency (CE) in Li-Cu half cells. The addition of FEC and VC additives exhibited pronounced effects on the suppression of irreversible passivated lithium depositions and the enhancement of stability at high current rates.

Our results highlight the importance of electrolyte composition in the behavior of lithium metal/liquid electrolyte interfaces. The comprehensive understanding gained from these experiments provides a basis for testing new electrolyte compositions with additional insight over conventional testing methods.