Reactions at graphene | tetracyanoborate ionic liquid interface - New safety mechanisms for supercapacitors and batteries

•Graphene is stable in tetracyanoborate-based ionic liquid at − 2 < E < 2 V.•Graphene behaves reversibly as a bipolar transistor at − 2 < E < 2.2 V.•Graphene becomes irreversibly doped to a dielectric at E > 2.2 V.•Structure of dielectric polymer polycyanoborani was resolved.•Safe battery/supercapacitor concept was proposed.

If an electrochemical energy storage device, which contains tetracyanoborate anions in the electrolyte, is accidentally charged with much higher voltage, it won't explode, because of a self-blocking mechanism. Firstly, voltage (E) more positive than 2.2 V (vs. Ag | AgCl reference electrode) kills the conductivity of single layer graphene and electrochemical reactions stop. Secondly, at E > 2.2 V tetracyanoborate anion polymerizes to a dielectric polymer polycyanoborani, which electrically isolates graphite or porous carbon grains from current collectors.

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