Thermal Gradients through Sintered Solid State Electrolytes in Lithium-Ion Batteries

Recent advancements in research have made solid state electrolytes a strong contender for the previously common liquid electrolytes, as it enables fast charging at very high current densities without flammable liquids. High current in combination with higher ohmic resistance lead to much more heat being dissipated than in conventional Li-ion batteries. As a result, large amounts of ohmic heat are produced inside the battery. As dry electrochemical cells have much lower heat conducting capacity than liquid-containing ones, knowing the thermal conductivity of the cell components plays a crucial role in predicting the heat distribution in such a cell. Thermal conductivity was measured for three types of solid state electrolyte, Li 7 La 3 Zr 2 O 12 (LLZO), Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) at different compaction pressures. LAGP and LATP were measured in sintered condition, LLZO was measured before and after sintering the sample material. Thermal conductivity for the sintered electrolytes was measured to 0.470 ± 0.009 WK - 1 m - 1 , 0.5 ± 0.2 WK - 1 m - 1 and 0.49 ± 0.02 WK - 1 m - 1 at 3 bar compaction pressure for LLZO, LAGP and LATP respectively. Before sintering LLZO showed a thermal conductivity of 0.22 ± 0.02 WK - 1 m - 1 at 3 bar compaction pressure. A simple analytical model of a lithium-ion battery cell stack was constructed to show the impact of the results obtained through the thermal conductivity measurements. It is based on a work by Richter et al. [1]. The resulting temperature profiles are shown in Fig. 1. The temperature gradients inside the cell stacks need to be considered when assessing lifetime and degradation effects as seen in other electrochemical systems [2-4]. Figure 1: Temperature of the stack center compared to the outer boundary for a) a dry separator and b) a solid state electrolyte, each with two thicknesses. References [1] F. Richter, P. J. Vie, S. Kjelstrup, O. S. Burheim, Measurements of ageing and thermal conductivity in a secondary nmc-hard carbon li-ion battery and the impact on internal temperature profiles, Electrochimica Acta 250 (2017) 228 – 237. [2] R. Bock, A.D. Shum, X. Xiao, H. Karoliussen, F. Seland, I.V. Zenyuk, O.S. Burheim, Thermal conductivity and compaction of GDL-MPL interfacial composite material, J. Electrochem. Soc. 2018 165(7): F514-F525 [3] R. Bock, H. Karoliussen, B. G. Pollet, M. Secanell, F. Seland, D. Stanier, O. S. Burheim, The influence of graphitization on the thermal conductivity of catalyst layers and temperature gradients in proton exchange membrane fuel cells, International Journal of Hydrogen Energy, In-press 2018 [4] O.S.Burheim, M.A. Onsrud, J.G. Pharoah, F. Vullum-Bruer, P.J.S. Vie, Thermal conductivity, heat sources and temperature profiles of Li-ion batteries, ECS Trans., 58 (2013) 145-171. Figure 1