Lithium-Ion batteries are being considered as a high-energy density replacement for Nickel Metal Hydride (NiMH) batteries in Hybrid Electric Vehicles (HEVs) and in the new Plug-In Hybrids (PHEVs). Although these cells can result in significant reduction in weight and volume, they have several safety related issues that still need to be addressed. We report here on the thermal response of Li-ion cells specifically assembled in our laboratory to test new materials, electrolytes and additives. Improvements in the thermal abuse tolerance of cells will be presented and discussed in terms of the need for overall battery system safety.
Thermal property measurements of 18650 cells for the Space Shuttle Orbiter Advanced Hydraulic Power System (AHPS, formerly known as EAPU) have been performed. Cells were measured from three commercial manufacturers: E-One MOLI (12 cells), Panasonic (5 cells) and Sanyo (5 cells). Thermal property measurements of the MOLI 18650 cells included measurements of specific heat, self discharge (microcalorimetry), overcharge response and thermal runaway by accelerating rate calorimetry (ARC). The Panasonic and Sanyo cells were measured only for thermal runaway response in the ARC at increasing states of charge (3.8V, 4.0V, 4.2V, 4.3V).
High-power 18650 Li-ion cells have been developed for hybrid electric vehicle applications as part of the DOE Advanced Technology Development (ATD) program. The thermal abuse response of two advanced chemistries (Gen1 and Gen2) were measured and compared with commercial Sony 18650 cells. Gen1 cells consisted of an MCMB graphite based anode and a LiNi0.85Co 0.15O2 cathode material while the Gen2 cells consisted of a MAG10 anode graphite and a LiNi0.80Co0.15 Al 0.05O2 cathode. Accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC) were used to measure the thermal response and properties of the cells and cell materials up to 400°C. The MCMB graphite was found to result in increased thermal stability of the cells due to more effective solid electrolyte interface (SEI) formation. The Al stabilized cathodes were seen to have higher peak reaction temperatures that also gave improved cell thermal response. The effects of accelerated aging on cell properties were also determined. Aging resulted in improved cell thermal stability with the anodes showing a rapid reduction in exothermic reactions while the cathodes only showed reduced reactions after more extended aging. Published by Elsevier B.V.
Li-ion cells are being developed for high-power applications in hybrid electric vehicles currently being designed for the FreedomCAR (Freedom Cooperative Automotive Research) program. These cells offer superior performance in terms of power and energy density over current cell chemistries. Cells using this chemistry are the basis of battery systems for both gasoline and fuel cell based hybrids. However, the safety of these cells needs to be understood and improved for eventual widespread commercial application in hybrid electric vehicles. The thermal behavior of commercial and prototype cells has been measured under varying conditions of cell composition, age and state-of-charge (SOC). The thermal runaway behavior of full cells has been measured along with the thermal properties of the cell components. We have also measured gas generation and gas composition over the temperature range corresponding to the thermal runaway regime. These studies have allowed characterization of cell thermal abuse tolerance and an understanding of the mechanisms that result in cell thermal runaway.
This document describes a general protocol (involving both experimental and data analytic aspects) that is designed to be a roadmap for rapidly obtaining a useful assessment of the average lifetime (at some specified use conditions) that might be expected from cells of a particular design. The proposed experimental protocol involves a series of accelerated degradation experiments. Through the acquisition of degradation data over time specified by the experimental protocol, an unambiguous assessment of the effects of accelerating factors (e.g., temperature and state of charge) on various measures of the health of a cell (e.g., power fade and capacity fade) will result. In order to assess cell lifetime, it is necessary to develop a model that accurately predicts degradation over a range of the experimental factors. In general, it is difficult to specify an appropriate model form without some preliminary analysis of the data. Nevertheless, assuming that the aging phenomenon relates to a chemical reaction with simple first-order rate kinetics, a data analysis protocol is also provided to construct a useful model that relates performance degradation to the levels of the accelerating factors. This model can then be used to make an accurate assessment of the average cell lifetime. The proposed experimental and data analysis protocols are illustrated with a case study involving the effects of accelerated aging on the power output from Gen-2 cells. For this case study, inadequacies of the simple first-order kinetics model were observed. However, a more complex model allowing for the effects of two concurrent mechanisms provided an accurate representation of the experimental data.