An ionic liquid dual-ion battery with pure 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide as the electrolyte was developed, which exhibited a high discharge plateau initiated at 4.4 V, a well-defined capacity of 82 mA h g^?1, an ultrawide charge/discharge potential range of 1.0–5.0 V and a superior stability of ～100% capacity retention for 600 cycles.