# Slight compositional variation-induced structural disorder-to-order transition enables fast Na+ storage in layered transition metal oxides

## TL;DR

This paper shows that specific sodium and vacancy arrangements in a material improve sodium ion movement, leading to better battery performance.

## Contribution

The study reveals how zigzag sodium/vacancy ordering enhances ionic mobility in layered oxides for sodium-ion batteries.

## Key findings

- P2-Na2/3Ni1/3Mn2/3O2 shows higher Na+ diffusivity and lower activation energy than P3-type materials.
- Zigzag Na+/vacancy ordering increases the probability of concerted Na+ diffusion.
- The interplay between ordering and kinetics is clarified for P2-type layered cathodes.

## Abstract

The omnipresent Na+/vacancy orderings change substantially with the composition that inevitably actuate the ionic diffusion in rechargeable batteries. Therefore, it may hold the key to the electrode design with high rate capability. Herein, the influence of Na+/vacancy ordering on Na+ mobility is demonstrated firstly through a comparative investigation in P2-Na2/3Ni1/3Mn2/3O2 and P2-Na2/3Ni0.3Mn0.7O2. The large zigzag Na+/vacancy intralayer ordering is found to accelerate Na+ migration in P2-type Na2/3Ni1/3Mn2/3O2. By theoretical simulations, it is revealed that the Na+ ordering enables the P2-type Na2/3Ni1/3Mn2/3O2 with higher diffusivities and lower activation energies of 200 meV with respect to the P3 one. The quantifying diffusional analysis further prove that the higher probability of the concerted Na+ ionic diffusion occurs in P2-type Na2/3Ni1/3Mn2/3O2 due to the appropriate ratio of high energy ordered Na ions (Naf) occupation. As a result, the interplay between the Na+/vacancy ordering and Na+ kinetic is well understood in P2-type layered cathodes.

In layered transition metal oxides as cathode active materials for Na-ion batteries, Na+ diffusion kinetics are impacted by Na+/vacancy ordering. Here, the authors suggest that the P2-type Na2/3Ni1/3Mn2/3O2 with large zigzag ordering exhibits high Na+ mobility and thus superior rate capability.