Amorphization of Sodium Cobalt Oxide Active Materials for High-Capacity All-Solid-State Sodium Batteries

Amorphous Na0.7CoO2-NaxMOy (M = N, S, P, B, or C) positive electrode active materials were synthesized by a mechanochemical technique to achieve high capacities and improved cyclabilities owing to their open and random structures. As none of the X-ray diffraction peaks are attributable to the starting materials, it was clear that the reaction between Na0.7CoO2 and NaxMOy had been successful. The prepared Na0.76Co0.8N0.2O2.2(80Na(0.7)CoO(2)center dot 20NaNO(3) (mol %)) was easily densified by pressing at room temperature, and then applied as a positive electrode in bulk-type all-solid-state sodium cells (Na15Sn4/Na3PS4 glass-ceramic/Na0.7CoO2-NaxMOy). The cell based on the Na0.76Co0.8N0.2O2.2 active material without any conductive additives in an ultrathick positive electrode layer (similar to 50 mu m thickness) operated as a secondary battery at 25 degrees C. The average discharge voltage was 2.9 V, and the initial discharge capacity was 70 mAh g(-1) of the positive electrode. This cell exhibited a higher discharge voltage and a larger capacity than cells employing crystalline Na0.7CoO2 or milled Na0.7CoO2 as the positive electrode. The electrochemical properties of Na0.7CoO2 were therefore improved by amorphization with NaNO3. Furthermore, the cell with the composite electrode containing a conducting additive gave a discharge capacity of 170 mAh g(-1) of Na0.76Co0.8N0.2O2.2, which is the highest reported to date for all-solid-state sodium cells based on oxide positive electrodes. Therefore, the amorphization of layered transition-metal oxides with sodium oxy-acids is an effective way to achieve novel active materials with high capacities.