Design for Safety: Characterization of Structural Impact on Lithium-ion Batteries

Abstract

The goal of this research is to develop a framework for design for safety of battery modules used in EV/HEV applications. Such a framework can enhance the crash safety of lithium-ion batteries by investigating prevailing failure mechanisms such as thermal runaway. The problem involves different scales from module level to laminate level. The multi-scale nature of the problem makes the analysis computationally complex and expensive. Furthermore, because of the risk of explosion, a battery testing apparatus needs special safety provisions making it difficult to test charged and ready-to-use battery cells. In this study, we have numerically investigated feasibility of using commercial explicit finite element code LS-DYNA for accurate modeling of impact on one cell so it can be used for an entire battery module. A jellyroll, the main component of a cylindrical cell, is a layered spiral structure which consists of thin layers of anode, cathode and separator. Because modeling hundreds of individual layers would be computationally expensive for a multi-cell model, two homogenization methods have been developed for the jellyroll in the lateral direction. Various types of experiments have been conducted on jellyroll layers to characterize their mechanical properties under dry and saturated conditions as well as their strain-rate dependence. For the cell level impact tests, cells without electrolyte have been used to eliminate the risk of fire. Two types of impact tests have been conducted on cylindrical cells using a drop cart in a custom-built drop test apparatus: lateral impact between flat plate and rod impact based on the UN DOT 38.3 requirements. Quasi-static indentation tests using a rigid rod was also performed on batteries with horizontal and vertical terminals orientations to characterize their behavior under steady loads. Mechanical deformations of the cells were studied experimentally using computed tomography scans and optical microscopy. Two homogeneous and heterogeneous finite element models were developed in LS-DYNA to simulate impact and indentation experiments. Simulations were validated by experimental results. A multi-scale framework was also proposed to simulate impact on PM12-JCI battery modules. The multi-scale process involved an initial impact simulation on battery module containing twelve homogeneous 6P-JCI cells and a subsequent simulation on a heterogeneous jellyroll model. The proposed multi-scale process can be developed further to simulate impacts on an entire battery pack.