A Comparative Safety Analysis of Lithium-Ion and Solid-State Lithium-Metal Pouch Cells

Solid-state batteries (SSBs) have become a promising candidate for the next step in lithium battery evolution. SSBs have the potential to provide larger gravimetric and volumetric energy densities, while many researchers believe they will also have an intrinsically better safety profile. As they lack a flammable liquid electrolyte, SSBs may not pose the same fire risk that conventional lithium-ion batteries (LIBs) present [1]. However, recent developments have called this common belief into question. Not only do solid-state electrolytes (SSEs) fail to prevent lithium filaments from propagating and causing short circuit events, but the higher energy density and presence of metallic lithium in SSBs create unique avenues for thermal runaway compared[JF1] to LIBs [2-3]. There exists no comprehensive assessment or framework to understand the overall safety of SSBs relative to current LIB technology.
This work presents an analysis of the comparative safety of LIBs and SSBs undergoing an open flame test. Using industry relevant combinations of LLZO garnet-type SSE, NMC811 active material, liquid catholyte, and PEO-LLZO composite polymer electrolyte, full pouch cells are subjected to a propane flame and allowed to burn to completion. Time-synchronized captures of open-circuit voltage, surface and gas temperatures, gas species identity, and high-speed imaging provide quantifiable metrics by which to compare the safety of SSBs against LIBs under identical conditions. Results indicate that while garnet-based all-solid-state cells may offer advantages in terms of safety, the risks of Li metal reaction and combustion cannot be ignored, and quasi-solid-state or “hybrid” cells present additional risks. Implications for future cell design will be discussed.
REFERENCES
1. J. Sastre et al., Commun Mater, 2 (2021) https://www.nature.com/articles/s43246-021-00177-4.
2. A. Manthiram, X. Yu, and S. Wang, Nat Rev Mater, 2 (2017) https://www.nature.com/articles/natrevmats2016103.
3. T. Inoue and K. Mukai, ACS Appl Mater Interfaces, 9 (2017) https://pubs.acs.org/doi/10.1021/acsami.6b13224.

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