Recent battery fires have renewed investigation of the safety of Li-ion batteries. One possible path to battery safety is a solid-state battery that replaces the volatile and flammable liquid electrolyte with a non-flammable solid electrolyte. While the safety benefits of this solid electrolyte replacement are widely acknowledged, the broader safety of Li-metal anode solid-state batteries with high energy density has not been critically examined.

Now, researchers at Sandia National Laboratories and Lawrence Berkeley National Laboratory have presented the first thermodynamic models to evaluate solid-state and Li-ion battery heat release quantitatively under several failure scenarios. A paper on their work appears in Joule.

The researchers probed the upper bounds of heat release and temperature rise across several cell-level failure scenarios and battery configurations, including a direct comparison to Li-ion batteries.

They also evaluated the thermodynamic impact of liquid electrolyte inclusion in solid-state batteries—added at the cathode to reduce interfacial resistance—which may be a critical transition case on the path to all-solid-state batteries.

For their analysis, the team used an Li₇La₃Zr₂O₁₂ solid electrolyte.

The study found that short-circuited all-solid-state batteries can reach temperatures significantly higher than conventional Li-ion, which could lead to fire through flammable packaging and/or nearby materials.

Potential temperature rise based on cell format or increasing energy density. Bates et al.

It is often claimed that ASSBs [all solid-state batteries] are safer than LIBs [Li-ion batteries]. We show that while this is true under external heating failure scenarios, ASSBs are not necessarily safer than LIBs under short-circuit failure scenarios or if the SE integrity is compromised.

ASSBs are projected to experience higher temperature rises than LIBs in future high-energy-density configurations involving Li-metal anodes because the same amount of heat is generated over a smaller mass and volume. Short circuits are a common problem with ASSBs because Li dendrites can grow through the SE and reach the cathode. As SEs become thinner, driven by demands to increase energy density, the ability to prevent dendrite growth is typically reduced. Preventing Li-dendrite growth into SEs and ensuring reactive species do not cross SEs are critical safety problems to overcome before commercializing ASSBs.

The calculations provided above show expectations for potential temperature rise as ASSB and SSB concepts mature, although materials advances might lead to other paths. How-ever, the present work clearly shows that the evolution of energy density, SE thickness, and cell designs affect potential safety concerns. Therefore, we caution that safety testing on present-day and advanced formats may not be indicative of future SSB architectures as materials, components, and larger format cell development progresses.

—Bates et al.

Resources

• Bates et al. (2022) “Are solid-state batteries safer than lithium-ion batteries?” Joule doi: 10.1016/ j.joule.2022.02.007