With increasing demand for EV applications, assuring a sufficient level of safety has become one of the most important issues given the drastic increment of the energy density of LIBs and potentially high risk of battery explosion by car collisions. It is generally accepted that the safety of LIBs is closely associated with the internal presence of combustive components such as electrolytes and electrodes. Once ignition is initiated by internal/external electric shorts, the combustible ingredients (especially delithiated oxide-based cathodes and flammable electrolytes) facilitate thermal runaway, which is responsible for a rapid rise of internal temperature due to undesirable exothermic reactions, eventually leading to battery explosion.

… It should be noted that propagation of a fire in a LIB cell requires the existence of heat, fuel, and an oxidizing agent (fire-triangle) and thus at least one of the fire-triangle components has to be removed in order to stop a continuous combustion reaction. In this respect, researchers have made intensive efforts to exclude these components by (1) employing a nonflammable or extinguishing agent to the electrolytes or by (2) utilizing a nonflammable separator.

—Yim et al.

Although the first approach has been shown to be effective, “huge amounts” of additives are required to ensure a high level of cell safety, which consequently degrades the overall ionic conductivity of the electrolyte, resulting in poor electrochemical performance, the authors noted.

As for the second approach, adopting a nonflammable separator instead of a conventional poly(olefin)-based separator greatly inhibits the occurrence of internal shorts without significant electrochemical fading. However, the researchers observed, this cannot ultimately suppress serious thermal runaway because the highly combustible components, which still are present in the cell, can be ignited by an external short, and thus a fire can spread.

Their approach is to use the temperature-responsive “self-extinguishing” microcapsules to achieve highly reliable safety of the cell and also maintain the desired electrochemical performance.

Their extinguishing agent of choice was the commonly used 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)-pentane) (DMTP). With its endothermic properties, the agent can be vaporized in a timely manner by the absorption of external heat and finally extinguishes the fire before the cell reaches a serious thermal runaway state.

Schematic illustrations for the synthesis route of the microcapsules containing fire- suppression agent (DMTP). The DMTP droplets were encapsulated with a rigid PMMA shell via an oil-in-water emulsion-based polymerization reaction using MMA monomer, EGDMA as a cross-linking agent, and ADVN as a polymerization initiator. Credit: ACS, Yim et al. Click to enlarge.

Direct mixing of DMTP in an electrolyte is not really possible due to poor miscibility with conventional electrolytes. To get around this, the researchers encapsulated the DMTP with a temperature-responsive polymeric layer to avoid direct contact between DMTP and the electrolyte. This provides multiple advantages: excellent miscibility in the electrolyte; suppression of temperature rise; and reasonable electrochemical performance.

The microcapsules can be mixed in the electrolyte, or uniformly coated along with a binder polymer on the separator. The proposed encapsulation approach is highly scalable, convenient and inexpensive, the team said.

Temperature profiles during the nail penetration test of the full cell (graphite/LiNi0.5Co0.2Mn0.3O2(NCM523)). The cells were fully charged to 4.35 V before testing. Credit: ACS, Yim et al. Click to enlarge.

It also should be emphasized that the safety of LIBs can be considerably improved by employing the proposed microcapsules without any performance fading. Furthermore, the usage of these microcapsules is not limited to separators; it can be utilized with other cell components such as electrolytes or electrodes with optimized forms. Moreover, a variety of other extinguisher agents with different reaction temperatures and reaction kinetics also can be applied for tailor-made application of the microcapsules for various devices. The excellent scalability and cost-effectiveness of this approach may pave a new pathway for assuring personal safety during the use of high-power, high-density energy sources.Yim et al.

Resources

• Taeeun Yim, Min-Sik Park, Sang-Gil Woo, Hyuk-Kwon Kwon, Jung-Keun Yoo, Yeon Sik Jung, Ki Jae Kim, Ji-Sang Yu, and Young-Jun Kim (2015) “Self-Extinguishing Lithium Ion Batteries Based on Internally Embedded Fire-Extinguishing Microcapsules with Temperature-Responsiveness” Nano Letters doi: 10.1021/acs.nanolett.5b01167