7.8 brief comms MH

0.3 per million. The consequences of runaway reactions have so far focused mainly on the risks of fire, burns and the release of hydrogen fluoride. The compound 2-fluoroethanol is highly toxic (the 50%-lethal dose, LD50, in mice is 0.1 mg kg) and is readily formed by nucleophilic attack of fluoride ion on ethylene carbonate. Although no example of electrophilic fluorination using coordination anions such as LiPF6 is known, we investigated whether a similar reaction could occur by this route. Ethylene carbonate and LiPF6 alone do not form fluoro-organics, but these are generated in abundance when any of the positive-electrode materials (LixMO2, where M is Co, Ni or Mn) is also present. Gas chromatography with mass spectrometry (110 mass units) indicates that the fluoroethanol ether is formed under these conditions: two moles of ethylene carbonate react with one mole of LiPF6 to yield one mole each of (FCH2CH2)2O, LiF and volatile POF3 (boiling point, 139.7 °C), providing the driving force with the loss of two moles of CO2. Ethylene oxide (formed by the loss of CO2 from ethylene carbonate and catalysed by the transition metal) is a likely transient intermediate. The compound bis-(2-fluoroethyl)-ether, which has an LD100 in rats of 10 mg kg (estimated LD50 of 4 mg kg ; ref. 2), represents 80% of the total fluoro-chemicals in runs 2 and 4–6,which are summarized in Table 1. The carbon anion with covalently bonded fluorine (CF3-) is inert. Our experiments are ex situ and relate only to the reaction of electrolyte with the material of the positive electrode. However, the conditions triggering the ring-opening fluorination of ethylene carbonate could easily occur in large batteries (weighing more than 500 g), such as those proposed for use in electric vehicles, where a short-circuit might cause the temperature in the electrolyte to rise instantly above 250 °C. Although these batteries are in principle highly desirable for saving energy and preventing pollution and for CO2 abatement, their safety should be rigorously investigated under protected conditions. Further exploration of the chemicals evolved in runaway reactions should be undertaken with extreme caution and only where ad hoc facilities are available. Amer Hammami, Nathalie Raymond, Michel Armand Laboratoire International sur Les Matériaux Electro-actifs CNRS-UdM, UMR 2289, Université de Montréal, PO Box 6122, Montréal, Québec H3C3J7, Canada e-mail: [email protected]