Understanding the irreversible reaction pathway of the sacrificial cathode additive Li${}_6$CoO${}_4$

The use of a sacrificial cathode additive that contains a large amount of lithium is one potential solution to compensate for the irreversible capacity loss associated with next-generation anodes such as silicon. Antifluorite-type Li${}_6$CoO${}_4$ has attracted attention as a potential cathode additive owing to its remarkably high theoretical lithium extraction capacity. However, the complex mechanism of lithium extraction as well as the oxygen loss from Li${}_6$CoO${}_4$ is not well understood. A generalizable computational thermodynamics and experimental framework is presented to understand the lithium-extraction pathway of Li${}_6$CoO${}_4$. It is found that one lithium per formula unit can be topotactically extracted from Li${}_6$CoO${}_4$, followed by an irreversible and nontopotactic phase transformation to Li${}_2$CoO${}_3$ or LiCoO${}_2$ depending on the temperature. The results show that peroxide species may form to charge-compensate for Li extraction which is undesirable as this can lead to gas release during battery operation. It is suggested that charging Li${}_6$CoO${}_4$ at an elevated temperature that the electrolyte can withstand, redirects the reaction pathway and prevents the formation of intermediate peroxide species making it an effective and stable sacrificial cathode additive. (© 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH)