Activation of the Pseudocapacitive Behavior of MXene/PANI for High-Performance Ammonium-Ion Batteries

The development of aqueous ammonium-ion batteries (AAIBs) requires electrode materials that combine high NH₄ ⁺ storage capacity with rapid and reversible ion transport. Herein, a metal-vacancy MXene/polyaniline (Mo_4/3CT_z/PANI) composite is reported, in which the pseudocapacitive response is synergistically activated by introducing 0.1 m H₂SO₄ into 1 m (NH₄)₂SO₄ electrolyte. This proton-assisted modulation enables rapid and reversible NH₄ ⁺/H₃O⁺ co-intercalation, in contrast to the negligible ion insertion observed in the absence of H₂SO₄. Combined experimental and density-functional theory (DFT) analyses reveal that proton doping significantly improves the electronic conductivity of PANI and induces a reversible Mo⁶⁺/Mo⁵⁺ redox transition during cycling, which dynamically modulates the NH₄ ⁺ adsorption energy (from -4.155 to -4.567 eV), thus facilitating both intercalation and deintercalation of NH₄ ⁺. As a result, the composite achieves a high specific capacity of 245 mAh g^-1 at 0.1 A g^-1, with excellent capacity retention of 84.2% after 11,000 cycles at 1.0 A g^-1. Furthermore, the MnO₂/CNTs||M:P = 5:1 full cell delivers a high energy density of 81.6 Wh kg^-1 and a power density of 16 000 W kg^-1. This work highlights a promising strategy for advancing MXene-based electrodes via proton-enhanced ion storage mechanisms, paving the way for high-performance AAIBs.