Abstract

Layered metal oxide cathodes for aqueous ammonium-ion hybrid supercapacitors (AHSCs) are held back by four interrelated deficiencies: sluggish reaction kinetics, rapid capacity fading, limited active sites, and poor electronic conductivity. To overcome these problems, herein, a CO₂ laser–irradiated, molten-salt-assisted synthesis was used to intercalate monovalent cations (Li⁺, Na⁺, and K⁺) into the δ-MnO₂ lattice, thereby tuning the structure and enabling high-active-surface electrochemistry. Cation pillaring widens the gallery, raises the Mn³⁺ fraction, and lowers the barrier to NH₄⁺ transport of the δ-MnO₂ cathodes. The structurally modified K⁺-pre-intercalated δ-MnO₂ (KIMO) cathode exhibits a high specific capacitance of 275 F g⁻¹ (at 0.5 A g⁻¹). Furthermore, potential-resolved in situ Raman, ex situ XPS and XRD, and first-principles modelling together resolve how charge is stored in the KIMO electrode. The assembled KIMO||Ti₃C₂T_x asymmetric AHSC, cycled to 1.8 V, delivers 36.39 Wh kg⁻¹ and maintains 89% of its initial capacitance over 5000 cycles. This strategy opens up a broad range of studies aimed at developing non-metallic charge-carrier NH₄⁺-based cathode materials for aqueous energy-storage applications.