Insights into the Safety Properties of Lithium Metal Batteries
Authors: Börner Markus , Stan Marian Cristian , Badillo Jimenez Juan Pablo , Imholt Laura , Beuse Thomas , Bieker Peter , Schappacher Falko , Winter Martin 
Published: February 2020

With regard to current trends, 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). Especially the high energy density (Wh/L) and specific energy (Wh/kg) are appealing properties of the ASSB [1] potentially enabling longer driving ranges for electric vehicles compared to the state-of-the-art lithium ion technology. However, the breakthrough of the ASSB technology is facing practical challenges in terms of processing, rate capability and cycle life. [2] In addition to high energy densities and a long cycle life, significantly enhanced safety properties are considered key for future battery technologies. Regarding the latter, the ASSB benefits from the absence of flammable liquid electrolyte compared to LIBs. However, only very few studies were published showing the evaluation of the safety properties of lithium metal batteries. [3] Therefore, this study aims on giving first insights into the factors influencing the thermal stability, hence the safety properties of different lithium metal battery setups.

As a starting point, 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 the cell. Therein, it has to be considered that during charging lithium metal does not deposit homogeneously but forms dendritic or mossy structures often referred to as high surface area lithium (HSAL). [1, 4] At elevated temperatures, the high reactivity of HSAL in presence of liquid electrolyte can lead to fatal consequences (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. 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 lithium metal battery types to unravel the advantages/disadvantages compared to LIBs.

The authors acknowledge the BMBF for funding the project “BCT” (03XP019I).

[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] M. Winter, et al., Adv Mater
1998, 10, 725-763.