Ultrathin Manganese-Based Metal-Organic Framework Nanosheets: Low-Cost and Energy-Dense Lithium Storage Anodes with the Coexistence of Metal and Ligand Redox Activities

We herein demonstrate the fabrication of Mn- and Ni-based ultrathin metal-organic framework nanosheets with the same coordination mode (termed "Mn-UMOFNs" and "Ni-UMOFNs", respectively) through an expedient and versatile ultrasonic approach and scrutinize their electrochemical properties as anode materials for rechargeable lithium batteries for the first time. The obtained Mn-UMOFNs with structure advantages over Ni-UMOFNs (thinner nanosheets, smaller metal-ion radius, higher specific surface area) exhibit high reversible capacity (1187 mAh g^-1 at 100 mA g^-1 for 100 cycles), excellent rate capability (701 mAh g^-1 even at 2 A g^-1), rapid Li⁺ diffusion coefficient (2.48 × 10^-9 cm² s^-1), and a reasonable charge-discharge profile with low average operating potential at 0.4 V. On the grounds of the low-cost and environmental benignity of Mn metals and terephthalic acid linkers, our Mn-UMOFNs show alluring promise as a low-cost high-energy anode material for future LIBs. Furthermore, the lithiation-delithiation chemistry of Mn-UMOFNs was unequivocally studied by a combination of magnetic measurements, electron paramagnetic resonance, and synchrotron-based soft X-ray spectroscopy (O K-edge and Mn L-edge) experiments, the results of which substantiate that both the aromatic chelating ligands and the Mn²⁺ centers participate in lithium storage.