Al Pinning Effect in Birnessite for High-Performance Ammonium-Ion Storage

Layered birnessite has attracted considerable attention for its cathode potential in various aqueous energy storage devices owing to its two-electron transfer reaction (Mn²⁺/Mn⁴⁺), open diffusion channels, and tunable interlayer spacings. However, birnessite for reversible ammonium (NH₄⁺) ion storage generally suffers from irreversible structural collapse originated from Jahn–Teller (J–T) effect of Mn³⁺ and the intrinsic slow ionic diffusion kinetics. Herein, an Al pinning effect in birnessite is found to address these two issues simultaneously, which promoted enhanced structural stability and resulted in fast ionic diffusion kinetics for excellent high-rate capability. Strikingly, a robust cycling stability over 5, 000 cycles at 1.0 A g⁻¹ is achieved in the optimal Na_0.7Al_0.1Mn_0.9O₂, which surpasses that of most previously reported ammonium-ion batteries. Density functional theory calculations revealed that the pinned [Al³⁺O₆] octahedra not only decrease the Mn³⁺ content in birnessite, but also strengthen the covalency of Mn─O bonds to resist the collinear elongation/compression direction of the [Mn³⁺O₆] octahedra. Furthermore, Al pinning in birnessite can increase the interlayer spacing due to the regulation of Mn³⁺─O/Mn⁴⁺─O bond length and decrease the diffusion barrier for NH₄⁺ ion in the interlayer of birnessite. Thus, an accelerated NH₄⁺ ion diffusion coefficient of 1.58 × 10⁻⁹ cm² s⁻¹ has been achieved, which is ≈5 times higher than of the pristine one and also higher than that in other cathode materials. The findings demonstrate that layered Na_0.7Al_0.1Mn_0.9O₂ is a very promising cathode candidate for NH₄⁺ ion battery, and the Al pinning effect in birnessite can effectively suppress the J–T effect and enhance the NH₄⁺ ion diffusion kinetics simultaneously.