Energy Storage Safety for Electric Vehicles

To guarantee electric vehicle (EV) safety on par with that of conventional petroleum-fueled vehicles, NREL investigates the reaction mechanisms that lead to energy storage failure in lithium (Li)-ion batteries.

All car batteries undergo a wide variety of safety reviews and certifications to confirm they operate safely under both routine and extreme conditions, including fluctuating temperatures, repeated charging and discharging, and a full range of driving cycles. Although more than 99% of the Li-ion devices used for EV energy storage never exhibit problems, safety is an impediment to mass-market adoption. Li-ion batteries are more sensitive to overheating, overcharging, and thermal runaway than the nickel-metal hydride technology found in conventional gasoline-powered vehicles.

Battery Failure Characterization

NREL's research provides exhaustive characterization of failure characteristics to better regulate the thermal characteristics needed for safer and stronger performing EVs. The research looks beyond the pass/fail certification process of Li-ion batteries to determine how the crystal structure of a battery changes during failure. NREL’s Science of Safety Mechanical Characterization Laboratory is equipped with capabilities to look at what occurs within the liquid, solid, and gaseous components of a battery before, during, and after failure. Researchers use fault detection and isolation, gas chromatography-mass spectrometry, and X-ray diffraction computed tomography (XRD-CT) to characterize battery materials.

Advanced X-Ray Imaging of Battery Failure

NREL researchers are using XRD-CT to take a closer look at the chemical and structural changes in battery electrodes. These XRD-CT tests provide insight into the rapid structural dynamics that occur within the cell during thermal runaway, which allows internal events to be linked to the thermal response of the cell. An extensive library of data from the XRD-CT research, including experiment details and high-speed X-ray videos for over 200 abuse tests conducted on Li-ion batteries. Methods of abuse include nail penetration, thermal abuse, and internal short-circuiting.

Diagnostic Tools Advance Battery Modeling and Analysis

NREL applies experimentation, modeling, analysis, simulation, and materials research capabilities in its quest for the safest possible EV battery. Researchers use abuse reaction, thermal runaway, internal short circuit, and electrical/chemical/thermal network models 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-scale and system-level modeling explores the propagation of stress build-up during abuse scenarios, such as the impact from car accidents.

Battery Internal Short-Circuit Device

NREL has developed the Battery Internal Short-Circuit 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.

Isothermal Battery Calorimeter

Another NREL tool is the R&D 100 Award-winning Isothermal Battery Calorimeter, 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.

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