Understanding the Irreversible Reaction Pathway of the Sacrificial Cathode Additive Li6CoO4

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 Li₆CoO₄ 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 Li₆CoO₄ is not well understood. A generalizable computational thermodynamics and experimental framework is presented to understand the lithium-extraction pathway of Li₆CoO₄. It is found that one lithium per formula unit can be topotactically extracted from Li₆CoO₄, followed by an irreversible and nontopotactic phase transformation to Li₂CoO₃ or LiCoO₂ 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 Li₆CoO₄ 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.