Non-Destructive Measurement Methods for Mechanical Parameters of Lithium-Ion Batteries

摘要

With increasing application of Lithium-Ion batteries (LIB) in various industrial fields, especially in electric vehicles, the mechanical properties and the structural integrity of LIB appear to play an increasingly important role for battery system designers, engineers, and the construction processes in question. Mechanical properties of LIB change with their State of Charge (SOC) due to internal changes in structure which are caused by the intercalation and deintercalation of the Lithium ions in the active material. Furthermore, the structure changes over the lifetime of the battery due to the degradation mechanisms. One of the main characteristics of a material is its elastic modulus or Young's modulus. Recent publications have shown that this mechanical property changes dependent on the lithium concentration in the electrodes; therefore, it cannot be taken as a constant value as in other materials and parts of a vehicle. Various mechanical tests have already been performed, but the goal of these was mostly to determine the failure behavior of the LIB cells for the determination of the elastic modulus of each component in the cell. Mostly this is done through destructive testing. Additionally, the Young's modulus of each separate component cannot be used to determine the overall Young's modulus of a LIB cell. This study focuses on a non-destructive estimation of the elastic modulus and its changes during the discharge and charge process of a full LIB pouch cell. This is realized with a 2 point bending test setup, which is equivalent to a cantilever beam. On the unclamped side of the beam, constant weights are placed, and the resulting deflection is recorded with a laser triangulation sensor. Additionally, the ultrasonic Time of Flight (ToF) is measured between an emitter and a receiver piezo element which are placed on the cell at a known distance. The time between the emitter sending the signal, which is propagated through the cell, and the receiver detecting the signal is measured as ToF. With both measurements the change of the mechanical and physical parameters caused by the intercalation process can be observed. The cell is charged and discharged several times at a constant temperature and the deflection and ToF measurements are recorded continuously. The Young's modulus and its dependency on SOC is calculated by using the differences in bending for different weights. Additionally, this result is correlated with the TOF to check the suitability of this method for inexpensive online monitoring of the mechanical properties of LIB pouch cells. The values are presented and compared to findings from literature which depend on conventional techniques.