Energy Storage Thermal Safety
To guarantee electric-drive vehicle (EDV) safety on par with that of conventional petroleum-fueled vehicles, the automotive industry has turned to NREL to develop new materials, designs, control strategies, and evaluation protocols to safeguard drivers and passengers while optimizing battery performance and cost.
Although all car batteries are required to pass a wide variety of safety reviews and certifications, and more than 99% of the lithium-ion (Li-ion) devices used for EDV energy storage never exhibit problems, safety issues have presented impediments to mass-market adoption.
Every vehicle must operate safely under both routine and extreme conditions, including fluctuating temperatures, repeated charging and discharging, and a full range of driving cycles. Li-ion batteries are more sensitive to overheating, overcharging, and thermal runaway than the nickel-metal hydride (NiMH) technology found in conventional gasoline-powered vehicles. NREL's research helps regulate the thermal characteristics needed for safer and stronger performing EDVs.
Precise Tools Map Thermal Trouble Spots
NREL applies experimentation, modeling, analysis, simulation, and materials research capabilities in its quest for the safest possible EDV battery.
Battery Internal Short-Circuit Device
NREL has developed the Battery Internal Short-Circuit (ISC) Device to evaluate one of the most challenging Li-ion battery safety and reliability issues the internal short circuit. Short circuits typically surface with no previously detected malfunction, causing the temperature of battery cells to spike by hundreds of degrees and in extreme cases cells go into thermal runaway and cause fires.
The NREL-created Battery ISC Device does not rely on mechanically damaging the battery exterior to activate the short, as do most of the other evaluation methodologies, but instead triggers a true internal short. This makes it possible to accurately pinpoint and fix problems leading to malfunctions.
Other Safety-Related Tools
NREL's accelerating rate calorimeter measures the exothermic onset of reacting materials and batteries with very high sensitivity at various heating rates. Because the accelerating rate calorimeter evaluates how features incorporated into a battery cell affect the cell's overall safety performance, it is ideal to examine NREL's Battery Internal Short-Circuit device.
Also, NREL's R&D 100 Award-winning Isothermal Battery Calorimeters are the only instruments of their kind with the capacity and precision needed to evaluate thermal characteristics and related safety issues in cells, modules, sub-packs, and some full-size battery packs, as well as across energy systems.
Learn more about NREL's energy storage evaluation equipment including calorimeters.
Battery Safety Modeling
Abuse reaction/thermal runaway, internal short circuit, and electrical/chemical/thermal network models are used by NREL researchers to evaluate battery safety issues at all scales. At the particle scale, investigations of surface modifications help prevent electrolyte decomposition and subsequent gas generation. Electrode-scale simulations reveal the effect of microstructure on the short-circuit mechanism inside the cells. Pack-level modeling explores the propagation of stress build-up during abuse scenarios.
NREL's contributions to the U.S. Department of Energy's Computer-Aided Engineering of Batteries (CAEBAT) project largely focus on improving, validating, and incorporating mechanical-electrochemical-thermal (MECT) models into crush software tools. NREL's CAEBAT battery and abuse modeling is examining:
- Abuse reaction kinetics
- Internal short circuits
- Response to crash-induced crush
- Responses to nail penetration and cell structural deformation
- Cell-to-cell internal short circuit propagation in modules and packs.
A battery is typically still active when a fault is induced in a cell. Sometimes this is true through a thermal runaway event. NREL's electrochemical model makes it possible to predict a battery's dynamic response to a fault. The modular architecture of NREL's multi-scale multi-domain (MSMD) model facilitates flexible integration of the multi-physics components needed to accurately evaluate safety issues.
NREL's interdisciplinary constitutive models simulate the response of a battery developing a fault. For the induced short circuit, quantification of the fault discharge current and accompanying heat must take thermodynamic, kinetic, and geometric factors into consideration, while capturing the electronic pathways along intricate Li-ion battery geometries.
Chemical components in Li-ion batteries become thermally unstable when exposed to high temperatures. NREL's MSMD abuse reaction kinetics model predicts battery chemical response to various abuse behaviors. Once abuse reactions are induced, the reaction rates are determined by temperatures, chemical properties, and concentrations of involved chemical species.
Combined with the other sophisticated equipment of NREL's energy storage laboratories, these tools can map thermal trouble spots in batteries and across energy storage systems, providing designers and manufacturers with the information needed to create the safest possible batteries.
Faults leading to battery thermal runaway are believed to grow over time due to latent defects in cells. In large EDV battery packs, detecting a fault signal and confining the fault locally in a system are extremely challenging. NREL has developed an electrical topology, fault detection, and accommodation circuitry for large-capacity battery systems that allows a fault to be easily detected through measurements at the electrical terminals of a multi-cell module. The system also electrically isolates the faulty cell to prevent electrical energy from feeding into the faulted cell from its neighbors, increasing overall battery safety.
This patent-pending fail-safe design technology is available for licensing from NREL.