Hygroscopic Chemistry Enables Fire‐Tolerant Supercapacitors with a Self‐Healable “Solute‐in‐Air” Electrolyte

Abstract

retardants such as triphenyl phosphate (TPP) [6] and triethyl phosphate (TEP) [7] can prevent thermal runaway, while materials that shutdown at high temperatures such as positive-temperature-coefficient (PTC) electrode [8] and polyethylene-coated separator [9] can stop the further thermal runaway. Extinguishing agents such as 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)-pentane (DMTP) [10] and heptafluoropropane [11] can put out fires caused by thermal runaway. While using nonflammable materials can prevent fires, [12] high temperatures still irreversibly damage the devices, resulting in a lack of reliability. Ideally, we need a device that is not only non-flammable at high temperatures to achieve high-safety but also maintains/recovers its performance after burning to achieve high-reliability, which we call "fire-tolerant" here. A typical supercapacitor has two electrodes, a separator, and the electrolyte. [13] While the electrode and separator can be replaced with non-flammable or even fire-tolerant materials, [14] there are currently no corresponding electrolytes that are stable at fire temperatures. [15] We propose using a self-healable electrolyte. A typical electrolyte consists of solute and solvent. [16] At high temperatures, though the solvent evaporates, the remained solute could be thermally stable. The electrolyte could be recovered either by refilling with fresh solvent or recapturing the evaporated solvent. For a self-healable electrolyte to recapture evaporated solvents from the environment, the solutes must be able to absorb water from the air (hygroscopic).Here, we report a high-reliability fire-tolerant supercapacitor that can restore itself after burning. The core is the hygroscopic CaCl 2 solute in the air, which spontaneously absorbs water from the air to form a CaCl 2 solution, used as a self-healable "solutein-air" electrolyte. The electrolyte evaporates at high temperatures and regenerates itself by spontaneously recapturing water (solvent) from the air at low temperatures (Scheme 1a), allowing the supercapacitor to revive its function after temporary failure from burning. To ensure the thermal stability of other components, thermally stable electrodes and separators were selected. To allow the CaCl 2 solute to contact air, we punched one hole on each side of a common coin cell case to form a semiopen supercapacitor. With the above changes to a conventional supercapacitor, a fire-tolerant supercapacitor is constructed with a reversible solvent evaporating/condensing High-temperature-induced fire is an extremely serious safety risk in energy storage devices; which can be avoided by replacing their components with nonflammable materials. However; these devices are still destroyed by the high-temperature decomposition; lacking reliability. Here, a fire-tolerant supercapacitor is further demonstrated that recovers after burning with a self-healable "solute-in-air" electrolyte. Using fire-tolerant electrodes and separator with a semiopen device configuration; hygroscopic CaCl 2...

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