Ammonium-preintercalated layered manganese oxide with single-phase intercalation chemistry and enhanced two-electron reaction in aqueous zinc-ion batteries†

Rechargeable aqueous Zn–MnO₂ batteries are promising candidates for large-scale energy storage due to their low cost and high safety. However, the development of Zn–MnO₂ batteries with high capacity and good cycle stability has been constrained by cathode limitations. Herein, the electrochemical performance and charge storage mechanism of layered manganese oxides with preintercalated potassium (K-MO), ammonium (NH₄-MO), and tetramethylammonium ions (TMA-MO) are systematically investigated. The results reveal that the MnO₂ cathodes undergo both one-electron (1e) intercalation and two-electron (2e) dissolution/redeposition mechanisms. For the former, hexagonal NH₄-MO exhibits single-phase charge storage behavior with minimal lattice changes during the initial cycles, as characterized by in situ Raman microscopy and ex situ X-ray diffraction (XRD). Moreover, the low charge transfer resistance (R_ct) and high ion diffusivity make the NH₄-MO cathode more attractive. For the latter, the Mn⁴⁺/Mn²⁺ reaction chemistry takes place and appreciably contributes to the capacity of Zn–MnO₂ batteries. By detecting the Mn concentration in electrolyte, NH₄-MO is found to more effectively induce the dissolution of Mn²⁺ and deposition of Zn–Mn species, thus promoting the Mn⁴⁺/Mn²⁺ redox reaction. Benefiting from these features, NH₄-MO demonstrates appreciable discharge capacity (306 mA h g⁻¹ at 0.3 A g⁻¹), good rate capability (113 mA h g⁻¹ at 8 A g⁻¹), and excellent cycle performance (retaining 101 mA h g⁻¹ after 2000 cycles at 4 A g⁻¹). This study enriches cathode engineering methods for realizing an enhanced two-electron Mn⁴⁺/Mn²⁺ reaction in rechargeable aqueous Zn–MnO₂ batteries.