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 03-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 03 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 O-3 NaCrO2. Nanotechnology potentially changes materials design strategies for sodium insertion materials, leading to the development of innovative rechargeable sodium batteries in the future.