Chemical and structural evolution during solid-state synthesis of cobalt-free nickel-rich layered oxide cathode

In contrast to the surging need for the cobalt-free nickel-rich layered transition metal oxide cathodes in lithium-ion batteries, the chemical and structural change in the solid-state synthesis of such materials is still not clear enough. Herein, the solid-state synthetic process of LiNi0.94Mn0.04Al0.02O2 from room temperature to excessive temperature is systematically studied to present a detailed and full view of the structure transformation, valence variation, and impurity phase generation alongside the synthetic process. The results indicate three stages of the synthetic process, that is the decomposition-chemical combination stage below 500 °C, the regularization of R3¯m phase stage at 500∼800 °C, and the excessive calcining stage above 800 °C. Surprisingly, during the first stage below 500 °C, the precursor and alkali lithium start to react at a much lower temperature (e.g., 300 °C) than expected, accompanied by the integration of a considerable amount of Li element into the bulk of precursor and the oxidization of Ni. Specifically, the stoichiometric ratio of Li/TM in LixTMO2 is 37%, 55%, and ∼100% at 300, 400, and 500 °C, together with more than half of Ni oxidized to +3. However, on the third stage above 800 °C, R3¯m phase in LixTMO2 will become unstable and lead to the emergence of NiO-like rock-salt, accompanied by the reduction of a portion of Ni3+ and significant Li loss. The stoichiometric ratio of Li/TM in LixTMO2 is 79% at 1000 °C. These findings provide an insight into the solid-state synthesis of cobalt-free nickel-rich layered transition metal oxide cathodes.