Effect of Cobalt Substitution on the Activation of the LiNiO₂ Discharge Reaction

Cobalt (Co) has been introduced to most of the practical Ni-rich layered positive electrode materials owing to its ability to stabilize the layered structure and lessen cation-mixing. However, it has been unclear whether a highly ordered structure is essential or Co addition itself has some effects. In this study, we synthesized Co-substituted LiNiO₂ (LNO) with and without the introduction of cation-mixing to investigate the detailed effects of Co on crystal/local structures and electrochemical properties. It was found that the charge-discharge reversibility of LNO was enhanced by Co substitution with an additional discharge capacity at around 3.5 V, showing that the reaction at the end of discharge was activated. This behavior was observed in Co-substituted LNO even if cation-mixing was largely introduced, implying the intrinsic effect of Co on reversibility. Solid-state NMR results showed that the local structure in LNO with cation-mixing significantly changed after charge-discharge, whereas that of Co-substituted LNO hardly changed even when cation-mixing was introduced, which seems to be responsible for better reversibility. Density functional theory calculation also supports the positive effect of Co on lithium transportation at the end of discharge.