Transfer of the conversion process of lead-acid batteries to lithium-sulfur batteries by verification by electrochemical impedance spectroscopy


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To reduce the carbon dioxide (CO2) emissions, the powertrain of the vehicle will be electrified. Furthermore, the electrification of utility vehicles, like long-distance vehicles, will play in future a significant role in achieving the world’s CO2 targets. In this context, the battery plays a key role in the electrification of the powertrain. Lithium-ion batteries are the first choice today, because they combine good performance in terms of price, safety and power capability but are limited in their specific energy density, which is a drawback for the utility transport. Therefore, a promising next generation of the lithium-ion technology is the lithium-sulfur battery (Li-S). This battery technology meets the demanding requirements of the electrification of the utility transport in terms of high theoretical energy density (2600 Wh/kg), capacity (1675 Ah/kg), and the reduction of battery costs by using the abundant sulfur as active material.
To use this battery technology in the automotive sector, a better understanding and description of the complex reaction mechanism within the conversion process is necessary. Therefore, a battery system with similar reaction processes has been identified. The internal reaction mechanism that takes place in the lead-acid battery (LAB) belongs to the conversion process, like the Li-S battery. In the conversion process, the materials react during oxidation and form completely new products. In the characterization of the internal processes, the internal reactions of the two battery technologies are subdivided in the three areas: electrochemical, chemical, and physical. The known reactions from the LAB are transferred to the Li-S batteries because the basic principle of the conversion process is the same. Nevertheless, a comparison of the three reaction mechanisms shows that these are differently pronounced in the two technologies. The transfer of the process properties shows that the conversion of elemental sulfur into lithium sulfide is only carried out via intermediate stages (polysulfide chains) and is not converted in one step as in the LAB. In the Li-S battery, the electrolyte system is also not part of the reaction, whereas this is the case within the LAB. Based on the comparison of the two battery technologies and the known electrical equivalent circuit (EEC) by M. Thele for the LAB, it is possible to derive an EEC for the Li-S battery. EEC consists of three RC-elements. The assumptions are verified by an electrochemical impedance spectroscopy on a prototype pouch cell at different state of charges and temperatures. Through the measurement, the three transferred process steps of the LAB can be verified by three depressed semicircles for the Li-S battery. The individual concentrated elements of the electrical equivalent circuit are parameterized and discussed in relation to the reactions within the Li-S cell.
Finally, a transfer between the reaction processes within the LAB to the Li-S battery could be shown. The knowledge of the transfer of the reaction processes allows a faster verification of the suitability of applications, such as long-distance vehicles concerning Li-S batteries.

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