Design of Ligand-Nonbridging Sites in Metal-Organic Frameworks for Boosting Lithium Storage Capacity

Metal-organic frameworks (MOFs) are lagging in the use of lithium-ion batteries (LIBs), ascribing to full coordination between metal nodes and organic ligands, to a large extent. By integrating a modulator into a ligand with missing bridging functionality, this study elucidates the role of non-bridging defect sites in MOFs in tailoring lithium storage performance. A fully bridged pristine MOF (p-MOF) utilizing the meso-tetra(4-carboxylphenyl) porphyrin ligand is compared with a modified MOF containing non-bridging defects (d-MOF) introduced by a homologous ligand, tris(4-carboxyphenyl) porphyrin. Spectroscopic and cryogenic low-dose electron microscopy techniques verify the presence of non-bridging defect sites in the d-MOF and reveal their explicit local structure. Density functional theory calculations show significantly enhanced Li⁺ adsorption energies and reduced Li⁺ migration barriers at the non-bridging sites in the d-MOF compared to the fully bridging sites in the p-MOF. As a result, the d-MOF exhibits exceptional lithium storage performance, achieving a high capacity of 761 mAh g^-1 at 0.05 A g^-1 and superior rate performance of 203 mAh g^-1 at 5 A g^-1, which substantially outperform the p-MOF. This study highlights the potential of modulating MOFs with non-bridging defects to develop high-performance LIBs.