Promising Nanometric Spinel Cobalt Oxides for Electrochemical Energy Storage: Investigation of Li and H Environments by NMR

Spinel-type cobalt oxides with formula HxLiyCo3-delta O4 exhibit interesting properties for various electrochemical energy storage applications thanks to their attractive electronic properties, due to the presence of H and Li ions in their structure as well as their nanometric dimensions. The effect of temperature on the H and Li environments is studied by investigating materials heat-treated at temperatures ranging from 25 to 650 degrees C by means of NMR spectroscopy. Two types of proton are evidenced: one bonded to oxygen atoms belonging to the network (hydroxyl group) and the other one involved in the H2O molecule. This configuration is in agreement with IR spectroscopy measurements, revealing the absence of free -OH groups, which mean that protons in the structure are involved in hydrogen bonds. After heat treatments at increasing temperature, NMR confirms that hydrogen is released, which induces first the migration of Li ions beyond 200 degrees C (probably from the 8a to the 16c sites), followed by a progressive reorganization of the structure with formation of HT-LiCoO2 beyond 400 degrees C.