Assessing Safety Properties of Lithium Metal Batteries

Currently, the so-called all-solid-state battery (ASSB) is getting enormous attention in academia and industry as a potential next-generation battery technology replacing the state-of-the-art lithium ion battery (LIB) based on a liquid electrolyte. Especially the high energy density (Wh/L) and specific energy (Wh/kg) are appealing [1] with regard to the application in electric vehicles and protable devices. However, the breakthrough of the ASSB technology is hindered by practical challenges in terms of processing, rate capability and cycle life. [2] Apart from high energy densities and a long cycle life, significantly enhanced safety properties are considered key for future battery technologies. Therein, the ASSB benefits from the absence of flammable liquid electrolyte as present in state-of-the-art LIBs. However, only very few studies were published showing the evaluation of the safety properties of lithium metal batteries. [3, 4] Therefore, this study aims on giving first insights into the factors influencing the thermal stability, hence the safety properties of different lithium metal battery (LMB) setups. As a baseline, lithium metal batteries based on liquid electrolyte were investigated in order to understand the influence of the lithium metal deposition behavior on the thermal stability of LMBs. Therein, during charging, lithium metal does not deposit homogeneously but forms dendritic or mossy structures often referred to as high surface area lithium (HSAL). [1, 5] At elevated temperatures, the high reactivity of HSAL can lead to fatal consequences, especailly in presence of liquid electrolyte (e.g. fire, explosion). Thus, the controlled deposition of lithium metal is crucial in order to guarantee safe operation of lithium metal batteries. This can be achieved by either modifying the charging process or the lithium metal surface (mechanically/chemically). Beyond that, lithium metal batteries based on either ceramic or polymer-based electrolytes are investigated. Therein, the cells are cycled to different sates of charge (SOC) and/or states of health (SOH) before analyzing the thermal stability by differential scanning calorimetry (DSC). This way, it is possible to determine distinct differences in the onset temperature for exothermic reactions and the evolving heat as a measure for the intensity of the reactions that might lead to a thermal runaway in large-scale cells. Moreover, the thermal stability of the cell components are characterized individually and in presence of each other du deduce interdependencies considering the chemical and thermal stability. In combination with comprehensive post-mortem analysis, it is possible to determine the factors influencing the thermal stability most. In summary, this study presents first results on the thermal stability of various LMB types to unravel the advantages/disadvantages compared to LIBs. [1] T. Placke, et al., J Solid State Electrochem 2017 , 21 , 1939-1964. [2] K. Kerman, et al., J Electrochem Soc 2017 , 164 , A1731-A1744. [3] T. Inoue, et al., ACS Appl Mater Interfaces 2017 , 9 , 1507-1515. [4] Uyama, et al., ACS Appl Energy Mater 2018 , 1 , 5712-5717. [5] M. Winter, et al., Adv Mater 1998 , 10 , 725-763.