Modeling of Large-format Li-ion Cell Performance and Safety

Open Access
Author:
Zhao, Wei
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 05, 2013
Committee Members:
  • Chao Yang Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Chair
  • Donghai Wang, Committee Member
  • Christopher Rahn, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • Li-ion cell
  • battery
  • modeling
  • safety
Abstract:
Small Li-ion batteries have been widely used for consumer electronics due to their high power and energy density. Large-format Li-ion batteries are believed to be essential for vehicle- and grid- energy storage enabling a sustainable energy future. Today, scale-up of Li-ion cells has not maximized the potential of available battery materials, leading to much lower energy density than their coin cell benchmarks. How to unlock the potential of existing Li battery materials and scale up Li-ion cells to 10-100 Ah sizes without substantially lowering the cell’s energy density remains a key technological challenge. Safety has become another pressing issue with the increasing interest in large-format Li-ion batteries for automotive applications due to the high energy density of Li-ion batteries and wide-ranging working conditions for electric vehicles compared with electronic applications. The highest specific energy available in today’s commercial Li-ion rechargeable batteries is approximately 240 Wh/kg, almost 20% of the energy content of TNT at 4.61 MJ/kg. The release of the battery energy in an abnormal way could cause catastrophic consequences. Large-format Li-ion batteries are particularly vulnerable to abusive conditions because of their higher energy content. Experimental study of large-format Li-ion batteries performance and safety is more expensive and dangerous to perform than that of small batteries, making modeling a valuable tool for this purpose. In this work, a 3D, multiscale and electrochemical-thermal coupled model is introduced to study the performance and safety issues uniquely presented in large-format Li-ion cells. Firstly, we study the scale-up of Li-ion cells from coin cells to large-format cells. We show that significant performance penalty can be caused in large-format spirally wound cells if only one single pair of tabs is used for current collecting. The reason for the inferior performance is due to the in-plane electrons transport loss in the long current collector foils and non-uniform active material utilization. An effective design to mitigate the performance loss in large-format cells is to use multiple tabs minimizing the voltage loss and increasing the active material utilization uniformity. The effect of tab number and location is investigated using the 3D modeling tool. A quantitative relationship between the cell’s useable energy density and the current density non-uniformity is established, for the first time, in the literature. Secondly, we use the 3D electrochemical-thermal coupled model to study the full nail penetration process of a large format cell. It is found that the thermal response of cell is closely coupled with its electrochemical performance after penetration. Two heating regimes, namely global heating and local heating are identified. The effect of various parameters such as shorting contact resistance, nail diameter, nail thermal conductivity, cell capacity, etc. is investigated using modeling. Finally, we use the model to study the internal short-circuit of a large-format Li-ion cell by a metal particle. Compared with nail penetration, tab heating becomes an important mechanism in internal short-circuit. The cell electrochemical and thermal behavior changes significantly with the metal particle size and shorting resistance. The model results provide explanations for the poor reproducibility of the present nail penetration and internal short-circuit experimental methods.