Growth of dendrites, limited coulombic efficiency (CE), and the lack of high-voltage electrolytes restrict the commercialization of zinc batteries and capacitors. These issues are resolved by a new electrolyte, based on the zinc(II)–betaine complex [Zn(bet)2][NTf2]2. Solutions in acetonitrile (AN) avoid dendrite formation. A Zn||Zn cell operates stably over 10 110 h (5055 cycles) at 0.2 mA cm−2 or 110 h at 50 mA cm−2, and has an area capacity of 113 mAh cm−2 at 80% depth of discharge. A zinc–graphite battery performs at 2.6 V with a midpoint discharge-voltage of 2.4 V. The capacity-retention at 3 A g−1 (150 C) is 97% after 1000 cycles and 68% after 10 000 cycles. The charge/discharge time is about 24 s at 3.0 A g−1 with an energy density of 49 Wh kg−1 at a power density of 6864 W kg−1 based on the cathode. A zinc||activated-carbon ion-capacitor (coin cell) exhibits an operating-voltage window of 2.5 V, an energy density of 96 Wh kg−1 with a power density of 610 W kg−1 at 0.5 A g−1. At 12 A g−1, 36 Wh kg−1, and 13 600 W kg−1 are achieved with 90% capacity-retention and an average CE of 96% over 10 000 cycles. Quantum-chemical methods and vibrational spectroscopy reveal [Zn(bet)2(AN)2]2+ as the dominant complex in the electrolyte.