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

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Layered birnessite is a promising cathode material for aqueous energy storage due to its redox activity and ion diffusion channels.
• However, birnessite suffers from structural collapse and slow kinetics in ammonium ion (NH₄⁺) batteries, primarily due to the Jahn-Teller effect and diffusion limitations.

Purpose of the Study:

• To investigate the effect of aluminum (Al) pinning on the structural stability and NH₄⁺ ion storage performance of layered birnessite.
• To address the challenges of structural collapse and slow ionic diffusion kinetics in NH₄⁺ ion batteries.

Main Methods:

• Synthesis of Al-pinned birnessite (Na₀.₇Al₀.₁Mn₀.₉O₂).
• Electrochemical testing for cycling stability and rate capability.
• Density functional theory (DFT) calculations to elucidate the mechanism of Al pinning.

Main Results:

• Al pinning significantly enhances structural stability and suppresses the Jahn-Teller effect in birnessite.
• The optimal Na₀.₇Al₀.₁Mn₀.₉O₂ cathode exhibits remarkable cycling stability over 5,000 cycles at 1.0 A g⁻¹.
• Al pinning accelerates NH₄⁺ ion diffusion, achieving a diffusion coefficient of 1.58 × 10⁻⁹ cm² s⁻¹, approximately 5 times higher than pristine birnessite.

Conclusions:

• The Al pinning effect in birnessite is an effective strategy to overcome structural instability and kinetic limitations in NH₄⁺ ion batteries.
• Layered Na₀.₇Al₀.₁Mn₀.₉O₂ demonstrates excellent potential as a cathode material for high-performance aqueous NH₄⁺ ion batteries.
• This work provides insights into rational material design for advanced ion battery systems.