Modulating anion redox reactions and structural evolution through Fe-substitution in Li${}_6$CoO${}_4$ hyper-lithiated sacrificial cathodes

Utilizing hyper-lithiated materials can offer a variety of options for designing high-energy lithium-ion batteries. As sacrificial cathodes, they compensate for the initial loss of Li${}^{+}$ at the anode. During the first delithiation process, a Fe-substituted Li${}_6$CoO${}_4$ (Li${}_{5.7}$Co${}_{0.7}$Fe${}_{0.3}$O${}_4$) supplies a large amount of Li${}^{+}$. Especially, the peroxide species formation and oxygen evolution are suppressed even though the charge compensation of oxygen is facilitated in Li${}_{5.7}$Co${}_{0.7}$Fe${}_{0.3}$O${}_4$. From a structural viewpoint, the anti-fluorite structure changes to defective disordered phases during the Li${}^{+}$ extraction, and the proportion of the electrochemical-inactive phase is more dominant in the case of Li${}_{5.7}$Co${}_{0.7}$Fe${}_{0.3}$O${}_4$ at the end of the charge. Consequently, the delithiated Li${}_x$Co${}_{0.7}$Fe${}_{0.3}$O${}_4$ is deactivated in subsequent cycles, reducing unexpected electrochemical reactions after the Li${}^{+}$ provision as sacrificial cathodes. These findings provide a comprehensive understanding of the reaction mechanism of hyper-lithiated materials and represent a significant step forward in developing high-performance sacrificial cathodes. (© 2023 Wiley‐VCH GmbH)