High-capacity aqueous imidazolium-ion batteries enabled by MMZ-H⁺/H⁺ co-intercalation in a near neutral electrolyte

Rechargeable aqueous batteries are regarded as promising candidates for large-scale energy storage with the advantages of cost-effectiveness, environmentally friendliness, and innate safety. However, to date, most of the aqueous ion batteries that have been reported are equipped with metal cation charge carriers and encounter either poor sustainability or low reaction activity. Here, we first reported an aqueous imidazolium-ion battery with MMZ-H⁺/H⁺ as co-intercalated ions. In detail, we configured an almost neutral electrolyte with a wide electrochemical window of 2.66 V by adding an appropriate amount of alkaline 1-methylimidazole (MMZ) to 0.5 M H₂SO₄, and named it 50M-10S electrolyte. Due to the strong binding energy between MMZ and H⁺, MMZ-H⁺ as an entire unit can be inserted into or extracted from the HATN-3CN (hexaazatrinaphthalene-2,8,14-tricarbonitrile) electrode. The MMZ-H⁺ and H⁺ co-insertion increases the capacity by 40% compared to pure H⁺ insertion in this proton battery (287.6 mAh g⁻¹ in 50M-10S electrolyte vs. 206.8 mAh g⁻¹ in 0.5 M H₂SO₄ electrolyte, 0.1 A g⁻¹). Theoretical calculations illustrated that the insertion of MMZ-H⁺ can further activate the unreacted N active sites due to their enhanced nucleophilicity derived from stronger electron-donating ability of ionized nitrogen sites than the protonated one. Moreover, the assembled full batteries also exhibit ultra-high specific capacity (266.6 mAh g⁻¹, 1 A g⁻¹) and ultra-slow degradation (capacity retention of 97%, 1 A g⁻¹, 1000 cycles). This research further enriches the library of inserted ions and will help to understand and enhance proton storage in near-neutral electrolytes and build new battery models.