FUNDAMENTAL MODELING THE PERFORMANCE AND DEGRADATION OF HEV LI-ION BATTERY

Open Access
Author:
Fang, Weifeng
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 20, 2010
Committee Members:
  • Chao Yang Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Chair
  • Long Qing Chen, Committee Member
  • Fan Bill B Cheung, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • Li-ion battery
  • HEV
  • degradation
  • modeling
Abstract:
Li-ion battery is now replacing nickel-metal hydride (NiMH) for hybrid electric vehicles (HEV). The advantages of Li-ion battery over NiMH are that it can provide longer life, higher cell voltage and higher energy density, etc. However, there are still some issues unsolved for Li-ion battery to fully satisfy the HEV requirement. At high temperature, thermal runaway may cause safety issues. At low temperature, however, its performance is dramatically reduced and also Li deposition may occur. Furthermore, degradation due to side reactions in the electrodes during cycling and storage results in capacity loss and impedance rise. An electrochemical-thermal coupled model is first used to predict performance of individual electrodes of Li-ion cells under HEV conditions that encompass a wide range of ambient temperatures. The model is validated against experimental data of not only the full cell but also individual electrodes and then used to study lithium deposition on the negative electrode during charging Li-ion battery at subzero temperature. The simulated property evolution, e.g. Li concentrations in electrode and electrolyte, shows that either low temperature or high charge rate may force Li insertion (into the negative carbon electrode) to occur in a narrow region near the separator. Therefore, Li deposition is mostly like to happen in this location. Modeling simulation shows that reduction of the negative electrode particle size can reduce Li deposition, which has same effect as improvement of the Li diffusion coefficient in the negative electrode. The model is also used to study charge protocols at subzero temperature. Model simulation shows that employing pulse current can improve cell temperature by the heat generated inside the cell, thus this designed charge protocol is able to reduce Li deposition and improve the charge efficiency as well. Individual aging mechanism is then implemented into each electrode to study Li-ion battery degradation during accelerated aging tests. The experimentally observed aging phenomena are interpreted using the degradation model. The simulated results show that the positive electrode active material loss is the main cause of capacity loss and impedance growth. And this is the key step for a model to well catch the experimentally observed aging phenomena in the two electrodes. In the future work, the degradation model will further help to prolong battery life through engineering and optimization in HEV applications.