Deciphering the contributing motifs of reconstructed cobalt (II) sulfides catalysts in Li-CO₂ batteries

Developing highly efficient catalysts is significant for Li-CO₂ batteries. However, understanding the exact structure of catalysts during battery operation remains a challenge, which hampers knowledge-driven optimization. Here we use X-ray absorption spectroscopy to probe the reconstruction of CoS_x (x = 8/9, 1.097, and 2) pre-catalysts and identify the local geometric ligand environment of cobalt during cycling in the Li-CO₂ batteries. We find that different oxidized states after reconstruction are decisive to battery performance. Specifically, complete oxidation on CoS_1.097 and Co₉S₈ leads to electrochemical performance deterioration, while oxidation on CoS₂ terminates with Co-S₄-O₂ motifs, leading to improved activity. Density functional theory calculations show that partial oxidation contributes to charge redistributions on cobalt and thus facilitates the catalytic ability. Together, the spectroscopic and electrochemical results provide valuable insight into the structural evolution during cycling and the structure-activity relationship in the electrocatalyst study of Li-CO₂ batteries.