How to Calculate Energy Contents of Different Cell Chemistries: Is There a Universal Factor between Theory and Practice?

The economical storage of electric energy at a large scale is one of the main technological challenges of this century. The state-of-the-art lithium ion batteries (LIBs) has several advantages as, e.g., high energy efficiency and safety. However, to improve the energy storage capabilities, the scientific community is intensively exploring various alternative rechargeable battery concepts as, e.g., lithium metal-based all-solid-state batteries (ASSBs), lithium/sulfur batteries (LSBs) or magnesium/sulfur batteries (MSBs) that could outperform LIBs in different aspects [1, 2]. This goes often along with the promise of a very high theoretical energy per mass or volume. The theoretical values, however, exclude numerous parameters relevant for the practical battery cell and, thus, might translate very differently into practically achievable energy values [3]. The discharge voltages achieved in practice and especially the required amount of inactive materials on stack level can vary substantially between the different cell chemistries. Consequently, the theoretical energy values, which are commonly quoted for emerging battery chemistries (mostly at the material level), could highly overestimate the realistic potential of these systems in comparison to established LIBs [3]. In this study, we calculated the energy for several battery technologies on stack and cell level, including LIBs, ASSBs, LSBs and MSBs to evaluate their practical specific energy (Wh/kg) and energy density (Wh/L) [3]. In order to provide a high comparability, the energy values are calculated in six successive steps, each adding further weight and volume of inactive material, like different amounts of electrolyte, inactive materials in the electrodes, separator, and an 18650 cell housing [3]. With a special emphasis on the amount of electrolyte and the volume expansion during charge and discharge, this study illustrates the impact these cell components have on the practical energy values in the individual cell chemistries. Thereby, the most critical cell components for achieving higher energy values are identified for each battery system [3]. By providing insights how to calculate the energy values of the different battery technologies with different necessary assumptions made evident, this study aims for more transparency and reliability in the comparison of the different cell chemistries. [1] J.W. Choi, D. Aurbach, Nat. Rev. Mater., 1 (2016) 16013. [2] T. Placke, R. Kloepsch, S. Duhnen, M. Winter, J. Solid State Electr., 21 (2017) 1939-1964. [3] J. Betz, G. Bieker, P. Meister, T. Placke, M. Winter, R. Schmuch, Adv. Energy Mater., (2018) 1803170.