Lithium ion storage in 1D and 2D redox active metal-organic frameworks

The lithium ion storage properties of a series of metal-organic frameworks (MOFs) with formula {[M(L)(H2O)2]H2O}n and [M(CA)(Pyz)]n (where L refers to the tetraoxolene ligands: CA = chloranilate and DHBQ = dihydroxybenzoquinone; Pyz = pyrazine; M = Fe and Mn) and exhibiting a 1D and 2D structure, respectively, have been studied. The 1D MOFs ({[M(L)(H2O)2]H2O}n) show higher reversible capacity values for lithium ion insertion with respect to 2D structures containing two organic ligands (M(CA)(Pyz)]n). The gravimetric capacity for the 1D Fe-CA MOF is 75 mAh/g at 2.16 mA/g (∼ 1 lithium atom per formula unit) higher than for the Mn complexes which is 65 mAh/g at 2.12 mA/g, though isostructural. Lithium ion insertion in the 1D Mn-CA chains takes place at 2.4 V vs. Li+/Li which is ∼700 mV higher than what is recorded for the Fe analogue. This result is most probably due to much more stable d5 electronic configuration of Mn2+ than d6 of Fe2+ in its isostructural Fe-based framework analogue of the final reduced phases. The 1D Fe-DHBQ capacity is higher than its manganese analogue 75 mAh/g at 2.5 mA/g (0.8 lithiums) against 40 mAh/g. In general, the high voltages of reaction in these 1D MOFs suggest that they involve the participation of the ligand on the redox processes along with the reduction of the transition metal if any. In fact, the potential of ion insertion changed depending on the metal. This fact along with the absence of evidence of conversion reaction by x-ray diffraction of cycled electrodes suggests that the charge delocalization may be all along the metal-ligand molecular framework participating as a whole hybrid unit in the lithium storage.