Effect of Nanosizing on Reversible Sodium Storage in a NaCrO₂ Electrode

The effect of nanosizing on the sodium storage performance in NaCrO₂ is systematically examined. Cation-disordered rock-salt-type and nanosized NaCrO₂ is prepared by mechanical milling, and layered O3-type and nanosized NaCrO₂ is prepared by heat treatment of the rock-salt phase. The observation by high-resolution transmission electron microscopy reveals that secondary particles consist of highly crystalline and nanosized NaCrO₂ primary particles with enriched grain boundaries. Such morphological features influence the voltage profiles in sodium cells, leading to an S-shaped profile with a single-phase reaction even for layered NaCrO₂, in which a biphasic reaction dominates because of a large repulsive interaction between Na ions. Moreover, the O3-P3 phase transition is suppressed for the heat-treated sample with the presence of enriched grain boundaries. The suppression of the phase transition is proposed to be due to the cancellation of CrO₂ layers gliding for the incoherently aligned grain boundaries. Thus, good capacity retention as electrode materials is realized compared with as-prepared bulk O3 NaCrO₂. Nanotechnology potentially changes materials design strategies for sodium insertion materials, leading to the development of innovative rechargeable sodium batteries in the future.