(399b) Enhanced Thermal Safety of Li-Ion Batteries in Presence of Fire Retardants

The safety of lithium-ion batteries still remains as a major issue after decades of commercialization and accompanying research efforts.¹ When typical electrolytes composed of flammable organic liquid solvents vaporize and decompose exothermically during thermal runaway, they pose serious risk to human health and infrastructure.² Recent efforts in academia and industry alike have endeavored to replace volatile and flammable conventional liquid electrolytes (composed of classes of compounds such as carbonates or ethers) with intrinsically nonflammable liquid electrolytes (composed of fluorinated and phosphorus-containing organic solvents).^3–5 Of particular interest is the radical recombination of phosphorus compounds to nullify hydrocarbon oxidation and subsequent heat production. Simultaneously, solid-state electrolytes replace the liquid component of the cell to improve safety and also offer superior energy density, particularly in enabling the lithium metal anode.⁶

In this study, we demonstrate the synergy of the nonflammable and solid-state strategies in lending intrinsic safety to a system and elucidate the interfacial degradation pathways which act as a direct complement to the heat accumulation which may result in thermal runaway. A phosphate-integrated semisolid electrolyte is compared with a liquid nonflammable electrolyte to highlight a novel design strategy for fire retarding electrolyte systems. Through detailed multimodal calorimetry of selected electrolyte chemistries, characterization of the interfacial composition, and theoretical thermal modeling, this work answers lingering questions related to flammability as a downstream consequence of the interfacial reaction kinetics in lithium metal batteries, even in the presence of fire retarding compounds. In addition, fire retarding phosphate compounds are particularly incompatible with commercially used graphite anodes due to exfoliation of the graphite’s layered structure from co-intercalation of solvent molecules. We introduce an alternate strategy in the form of hard carbon, which utilizes a hybrid mechanism of lithiation which provides excellent synergy with the semisolid electrolyte’s encapsulation of phosphates as a means of electrode protection. We adapted the semisolid fire-retardant electrolyte system to both graphite and hard carbon anodes to not only greatly improve electrochemical cycling capacity and stability, but only to explore the fundamental mechanisms of interfacial passivation and protection when comparing two categories of anode lithiation mechanisms.

Overall, we propose and demonstrate strategies beyond materials-level design choices for improving “built-in” safety. These findings show highly safe fire retarding systems which allow for the simultaneous optimization of performance (in terms of energy density, cycling lifespan, and capacity retention) and thermal safety (in terms of reduced heat accumulation and mitigation of thermal runaway severity) in lithium-ion batteries.

(1) Feng, X.; Ren, D.; He, X.; Ouyang, M. Mitigating Thermal Runaway of Lithium-Ion Batteries. Joule 2020, 4 (4), 743–770. https://doi.org/10.1016/j.joule.2020.02.010.

(2) Wang, Q.; Jiang, L.; Yu, Y.; Sun, J. Progress of Enhancing the Safety of Lithium Ion Battery from the Electrolyte Aspect. Nano Energy 2019, 55, 93–114. https://doi.org/10.1016/j.nanoen.2018.10.035.

(3) Cao, X.; Xu, Y.; Zhang, L.; Engelhard, M. H.; Zhong, L.; Ren, X.; Jia, H.; Liu, B.; Niu, C.; Matthews, B. E.; Wu, H.; Arey, B. W.; Wang, C.; Zhang, J.-G.; Xu, W. Nonflammable Electrolytes for Lithium Ion Batteries Enabled by Ultraconformal Passivation Interphases. ACS Energy Lett. 2019. https://doi.org/10.1021/acsenergylett.9b01926.

(4) Wang, J.; Yamada, Y.; Sodeyama, K.; Watanabe, E.; Takada, K.; Tateyama, Y.; Yamada, A. Fire-Extinguishing Organic Electrolytes for Safe Batteries. Nat. Energy 2018, 3 (1), 22–29. https://doi.org/10.1038/s41560-017-0033-8.

(5) Zheng, Q.; Yamada, Y.; Shang, R.; Ko, S.; Lee, Y.-Y.; Kim, K.; Nakamura, E.; Yamada, A. A Cyclic Phosphate-Based Battery Electrolyte for High Voltage and Safe Operation. Nat. Energy 2020, 5 (4), 291–298. https://doi.org/10.1038/s41560-020-0567-z.

(6) Janek, J.; Zeier, W. G. A Solid Future for Battery Development. Nat. Energy 2016, 1 (9), 1–4. https://doi.org/10.1038/nenergy.2016.141.