MnO₂ superstructure cathode with boosted zinc ion intercalation for aqueous zinc ion batteries

The simultaneous intercalation of protons and Zn²⁺ ions in aqueous electrolytes presents a significant obstacle to the widespread adoption of aqueous zinc ion batteries (AZIBs) for large-scale use, a challenge that has yet to be overcome. To address this, we have developed a MnO₂/tetramethylammonium (TMA) superstructure with an enlarged interlayer spacing, designed specifically to control H⁺/Zn²⁺ co-intercalation in AZIBs. Within this superstructure, the pre-intercalated TMA⁺ ions work as spacers to stabilize the layered structure of MnO₂ cathodes and expand the interlayer spacing substantially by 28 % to 0.92 nm. Evidence from in operando pH measurements, in operando synchrotron X-ray diffraction, and X-ray absorption spectroscopy shows that the enlarged interlayer spacing facilitates the diffusion and intercalation of Zn²⁺ ions (which have a large ionic radius) into the MnO₂ cathodes. This spacing also helps suppress the competing H⁺ intercalation and the formation of detrimental Zn₄(OH)₆SO₄·5H₂O, thereby enhancing the structural stability of MnO₂. As a result, enhanced Zn²⁺ storage properties, including excellent capacity and long cycle stability, are achieved.