Mechanical failure of lithium-ion batteries

Abstract

The commercialization of lithium-ion batteries has accelerated the electrification process of vehicles. In the past decade, one could see great advances in the life span, cost, performance, specific energy, and specific power of batteries. At the same time, the safety of batteries has not been adequately addressed by most stakeholders in the Electric Vehicle market. The present thesis systematically investigates the deformation mechanisms of the multi-layered structure of lithium-ion battery cells subjected to various loading conditions with particular emphasis on predicting the onset of the electrical short circuit. It starts with a comprehensive testing and modeling study of all the components of the cell, including the current collectors, the separator, the pouch/shell casing, and particularly, the coatings of electrodes.

A detailed computational model for quasi-static loading is subsequently established in Abaqus/explicit, which is very effective to predict the load-displacement response, peak load, displacement to fracture and short circuit, as well as the shear fracture phenomenon. The computational model is then extended to cover the effect of strain rate dependence by introducing the poro-mechanical theory. Darcy's law is used to describe the flow of the electrolyte inside the granular structure of the coating, and the Kozeny-Carman equation is adapted to calculate the permeability of the porous media of the battery cell. The model is shown to accurately predict the strengthening effect of the battery cell under low-speed dynamic loading, observed in experiments. The effect of mechanical deformations of a battery cell on its electrochemical performance is investigated next through a series of control tests on the coin-cell type batteries made of deformed electrodes.

The batteries are tested with ten cycles of charge-discharge, and a clear capacity fade in the damaged cells compared with the undamaged ones is observed. Electrochemical impedance spectroscopy tests are then performed, and the possible mechanism of the capacity fade is proposed. In the last part of the thesis, two applications of the developed computational modeling strategy are exhibited. One is the axial deformation of the 18650 cylindrical cells, and the other is the protective structural design of EV battery pack subjected to a "ground impact".