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Johnson – NETL, Sriramulu, Stringfellow – TIAX III.D.3 Internal Short Circuits in Lithium-Ion Cells for PHEVs their surroundings. Because a typical PHEV pack would be significantly larger than a typical laptop pack, the consequences of a field-failure in a PHEV pack could be far more severe than would be the case for a laptop pack, and may occur far more frequently. Although it is well-recognized that the commercial viability of Li-ion technology in PHEVs is dependent on avoiding spontaneous occurrence of such incidents on board vehicles, it is clear but less well-recognized that the safety technologies currently employed in commercial Li-ion batteries for portable electronic applications are inadequate with respect to such incidents. Furthermore, there are currently a variety of standard safety-related technologies to guard against abuse of the Li-ion battery. However, field-failures have occurred despite the presence of these technologies in cells and packs. There is also no adequate test for the type of field-failure that presents the basic safety issue for Li-ion. Given that field-failures occur in a manner that is not effectively addressed by any of the standard safety measures currently used in Li-ion batteries, and that there is no test currently available that can identify these cells before they undergo field-failure, it is clear that a fundamentally new approach is required to develop technologies that will prevent these rare but profoundly destructive safety incidents caused by internal short circuits in PHEV cells. Approach Our approach to developing guidelines for safe PHEV pack design is to employ an FEA model to determine the conditions under which thermal runaway of PHEV-size cells occurs and can be suppressed. The FEA model is first validated using thermal runaway data on 18650 cells. Thermal runaway was induced in an 18650 cell by introducing a miniature heater into the center of the cell through a hole drilled at the bottom of the can. This approach allowed us to simulate local, spot heating of the cell, akin to an internal short. These cells were custom-built on our cell prototyping line with a range of design variations (including active materials) and were tested under a range of external heat transfer conditions in a custom-built wind tunnel. Here we describe our work in validating the FEA model, as well as initial experimental data in suppressing thermal runaway. Results Cell thermal parameters. The heater method employed for estimating the thermal parameters for the 18650 cells is described in Figure III - 106. Temperature measurements inside the cell and at the cell surface were made for different heater powers (Figure III - 107). By fitting the model to these time-dependent data, we were able to estimate/verify the thermal parameters for the cell (Figure III - 107). 18650 cell Miniature heater designed and constructed at TIAX with a maximum power output rating of 30 W Thermocouple to measure outside temperature Thermocouple to measure inside temperature Figure III - 106: Schematic of experimental set-up to simulate internal short heat release in a cylindrical cell by using a miniature heater inserted in cell core. The heater power and external rate of heat transfer can be independently controlled 250 200 150 100 50 0 Model: Cell Surface Model: Inside cell Exp.: Inside Cell Exp.: Cell Surface Ambient 2W Pheater = 1 W 4W 3W 0 100 200 300 Time (min) Figure III - 107: Cell thermal properties, including specific heat and thermal conductivity were estimated/verified through heater experiments. Figure shows measured internal and cell surface temperatures for different heater power levels. Model fits are also shown. The cell was completely discharged prior to the experiment Heat release kinetics from active materials. Kinetics of heat release from the anode and cathode active materials are key inputs for any thermal runaway model of Li-ion cells. We developed representative models for heat release kinetics by fitting ex situ Differential Scanning Calorimeter (DSC) data for charged anode and cathode active materials used in our 18650 cells (Figure III - 108). These kinetic sub-models were then used in our FEA model for thermal runaway simulations. Thermal runaway of Li-ion cells. We induced thermal runaway by internally heating fully charged (4.2 FY 2013 Annual Progress Report 111 Energy Storage R&D Temperature (C)PDF Image | Advanced Battery Development
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