Dynamic Ni-O Bonding Induced by Orbital Degeneracy Breaking for Efficient Li₂CO₃ Decomposition

Lithium carbonate, the primary discharge product in Li-CO₂ batteries with high thermodynamic stability and a wide band gap, leads to significant electrochemical inertness, limiting efficiency and cycle life. The strongly delocalized p_z orbital at a lower-energy HOMO level in Li₂CO₃ causes weak coupling with the O-p_z orbitals, increasing decomposition resistance owing to d-orbital degeneracy in high-local-symmetry catalysts. This study introduces metastable tetragonal-pyramidal nickel sulfide (tp-NiS) with low-symmetry NiS₅ coordination, breaking d-orbital degeneracy and bringing d_z ², d_xz, and d_yz orbitals closer to the Fermi level. Enhanced orbital overlap with Li₂CO₃ O-p_z orbitals facilitates robust Ni-O bond formation. In situ spectroscopy confirms reversible Ni-O bond formation during cycling, ensuring electron transfer and complete Li₂CO₃ decomposition. Conversely, weak interfacial interactions in octahedral NiS with highly symmetric local coordination only allow decomposition-resistant Li₂CO₃ and interface passivation. Consequently, tp-NiS exhibits superior electrochemical performance, with the best reported reversibility and stability, a charge potential below 4.0 V, and 92.03% capacity retention after 1800 h. This metal redox-driven mechanism establishes a reversible geometric conversion pathway, emphasizing the critical role of symmetry-engineered Ni-O interactions in bifunctional catalysts.