Abstract:
Cylindrical Lithium-ion secondary battery cells power various and numerous mobile and stationary devices nowadays. In many cases it is necessary to know its active and passive thermal behaviour to ensure proper and safe operation. Because of its form and setup, especially the passive properties like thermal conductivity are hard to measure. This work aims to determine this value for commercial 18650 cylindrical cells with two different approaches. First one is the introduction of a heating wire in combination with standard thermocouples as well as with optical fiber Bragg grating temperature sensors in the core of the cell, actively heat the cell with controlled power from the inside and compare the values of the cell with measurements on the outer case. Second one is to disassemble the cell and measure the thermal conductivity and thermal capacity of each cell component with laser flash and and differential scanning calorimetry. Together with the cell setup these methods are used to parameterize a finite element model of the cell and derive the thermal behaviour of the full cell from this simulation. For both methods the accuracy and the limitation are shown and compared to each other.

Conclusions:
The first method based on the pipe method showed lower values for the thermal conductivity compared to the second approach which determines the effective radial thermal conductivity by measuring each layer type of the
cell separately. The measurement results from the second method were used to build a FEM based simulation model which then was used to show possible measurement uncertainties when using the pipe method. The simulation results of the model show that the results of the pipe method can be heavily influenced by an unknown position of the temperature sensor and the heating wire position. Also, the hole tolerance and the possibility of missing or bad distributed thermal paste which results in a higher thermal resistance between the sensor and bulk surface can heavily influence the measurement results. The results also show that the separator has a major impact on the thermal conductivity. Furthermore, the effective radial thermal conductivity at
three different SOC levels was measured which increases slightly with increasing SOC. The authors of this work assume that due to the higher pressure in between the bulk material at higher SOC levels the thermal contact resistance between each layer gets lower which could result in a higher effective thermal conductivity. To further improve the thermal simulation model a thermal contact resistance between each layer should be considered which would lower the effective thermal conductivity of the cell resulting in a better agreement of both measurement methods shown in this work.