Water-Stable Cathode for High Rate Na-Ion Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Computational Materials Science

Background:

• Sodium-layered oxide cathodes often suffer from instability in humid environments.
• Element substitution in Na2/3Ni1/3Mn2/3O2 (NNM) improves sodium-ion diffusion but compromises water stability.
• Suppression of Na+ vacancy ordering in NNM is key for enhanced kinetics but often leads to structural degradation.

Purpose of the Study:

• To simultaneously address water instability and limited Na+ diffusion kinetics in sodium-layered oxide cathodes.
• To investigate the correlation between transition-metal layer design and cathode water stability.
• To optimize Na+ vacancy disordering for improved electrochemical performance.

Main Methods:

• Density functional theory (DFT) calculations to determine H2O adsorption energies on transition-metal (TM) layers.
• Rational design of the TM layer in Na2/3Ni1/3Mn2/3O2 (NNM) cathodes.
• Electrochemical testing of the designed cathode, including rate capability and cycling stability assessments after water soaking.

Main Results:

• DFT calculations revealed that Co/Mn and Fe/Mn units in the TM layer exhibit lower H2O adsorption energies than Li/Mn units, enhancing water stability.
• The Li/Mn unit effectively suppresses Na+ vacancy ordering, improving Na+ diffusion kinetics.
• The designed Na2/3Li1/9Ni5/18Mn2/3O2 cathode maintained structural integrity and electrochemical properties after water exposure, showing 78% capacity retention at 20C and 87% after 1000 cycles.

Conclusions:

• Rational design of the transition-metal layer is an effective strategy to achieve both water stability and improved Na+ diffusion kinetics in layered oxide cathodes.
• The developed Na2/3Li1/9Ni5/18Mn2/3O2 cathode demonstrates excellent rate capability and long-term cycling stability, overcoming previous limitations.
• This work provides a pathway for developing robust and high-performance sodium-ion battery cathodes.