Low-temperature Operation of Li-ion Batteries for Hybrid and Electric Vehicles

Open Access
Author:
Ji, Yan
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 03, 2013
Committee Members:
  • Chao Yang Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Chair
  • Daniel Connell Haworth, Committee Member
  • Fan Bill B Cheung, Committee Member
  • Michael John Janik, Committee Member
  • Karen Ann Thole, Special Member
Keywords:
  • lithium
  • battery
  • electrochemistry
  • modeling
  • low temperature
  • electric vehicles
Abstract:
Substantially reduced energy and power capabilities of lithium-ion cell operating at low temperatures pose a technical barrier for market penetration of hybrid electric vehicles (HEV) and pure electric vehicles (EV), which suffer from significant driving range loss as well as long charging time in subzero temperature environments. The present work delineates Li-ion cell behaviors at low temperatures by a combined experimental and modeling approach. An electrochemical-thermal coupled model, incorporating concentration- and temperature-dependent transport and kinetic properties, is applied and validated against 2.2Ah 18650 cylindrical cells over a wide range of temperatures (-20°C to 45°C) and discharge rates. Detailed resistance analysis indicates that performance limits at -20°C depend on not only discharge rates but also thermal conditions. The principal performance limitations are found to be Li+ diffusion in the electrolyte and solid-state Li diffusion in graphite particles. Simulation and experimental results demonstrate the dramatic effects of cell self-heating upon electrochemical performance. A nonisothermal Ragone plot accounting for these important thermal effects is proposed for the first time for Li-ion cells and more generally for thermally coupled batteries. To extend the driving range of EVs at low temperatures, preheating Li-ion batteries to room temperature is proposed as a practical method based on Li ion cell's strong self-heating effect. The present model is used to simulate the process of heating Li-ion batteries from subzero temperatures. Three heating strategies using battery power as heating source, namely self internal heating, convective heating and mutual pulse heating, are proposed and compared. Their advantages and disadvantages are discussed in terms of capacity loss, heating time, system durability, and cost. Model predictions reveal that Li-ion batteries can be heated from -20°C to 20°C at the expense of only 5% battery capacity loss within 2 minutes using mutual pulse heating with high-efficiency dc-dc converter, implying considerable potential for improved driving range of EVs in cold weather conditions. Whenever external power is available, high frequency AC signal with large amplitude is a preferred choice, offering benefits to both heating power and battery life. To reduce the charging time of EVs at low temperature, a new charging protocol CCCS, (constant current followed by constant lithium stoich) has been proposed. It consumes less time than the traditional CCCV method, and also avoids capacity loss due to lithium plating. Based on this new protocol, further attempts have been made to reduce the charging time by (1) heating batteries, (2) cell design and material property optimization. Model predictions demonstrate that the pre-heating-charging method is able to reduce the charging time from 3 hours to 18 minutes. Solid phase diffusion and electrode transport are identified to be the rate-limiting mechanisms for charging by both theoretical analysis and model prediction. This study also illustrates how these limitations are related to cell design parameters and material transport properties quantitatively. By alleviating these limitations, charging can be reduced to within 5 minutes, comparable to the time of refilling a fuel tank in gasoline vehicles.