On the limited electroactivity of Li2NiTiO4 nanoparticles in lithium batteries

The electroactivity of the rock-salt structure Li2NiTiO4 in lithium batteries containing LiBOB as electrolyte salt was examined. The compound exhibits a random distribution of the metal ions in octahedral 4a positions. The combined use of electrochemical impedance spectroscopy, step potential spectroscopy and thermogravimetric analysis revealed that the electrode undergoes various changes during the charging process. Thus, Ni(II) is initially oxidised to Ni(IV) through Ni(III) as an intermediate state over the region from OCV to 4.6 V versus Li+/Li, which provides a high capacity (180 mAh g−1). Upon charging, however, highly charged Ni(IV) ions hinder charge carrier motion in both ions and electrons. In fact, the lithium diffusion coefficient was also very low (DLi+≈10−17 cm2 s−1DLi+≈10−17 cm2 s−1), which suggests a low ionic conductivity. From impedance studies, it seems that a quite stable polymer film is formed in the electrolyte/electrode interface during the first charge. The region between 4.6 and 4.8 V exhibited an additional capacity of 23 mAh g−1 that was assigned to the release of oxygen if the electrolyte oxidation at this stage is discarded. The electrode impedance increased three times after one cycle, which is consistent with irreversible rearrangement of lithium ions during the first oxidation process. Furthermore, a very resistive solid electrolyte interface is developed after a few cycles. Therefore, the capacity fading observed can be ascribed to deterioration of both, the electrolyte and the electrode conduction properties, by effect of hindered lithium and electron motion.