Comparative analyses of MIL-88B(Fe) and MIL-100(Fe) metal organic frameworks as active anode materials for Li ion batteries

Two Fe based metal organic framework (MOF) nanorod structures, MIL-88B(Fe) and MIL-100(Fe), were syn-thesized and their performance as working electrodes for Li ion half-cell batteries was evaluated under identical testing conditions. It was found that MIL-88B(Fe) provides a charge / discharge capacity of 468 mAh/g at the current density of 100 mA/g, which is over 3 times higher as compared to that of MIL-100(Fe) under the same conditions. Nevertheless, after an impressive charge / discharge capacity in the first few cycles, a slow capacity degradation was noticed in the case of the MIL-88B(Fe) material. Energy dispersive spectroscopy (EDS) and Fourier transform infrared (FTIR) spectroscopy results suggest that capacity degradation may be due to strained metal coordination in the MIL-88B(Fe) network. In contrast, despite moderate charge / discharge capacity such issues are identified to be minimal in the case of MIL-100(Fe). Electrochemical impedance spectroscopy (EIS) analyses suggest that ohmic and polarization resistance are smaller for MIL-88B(Fe) as compared to MIL-100(Fe) (both cycle 1 and 100). This makes MIL-88B(Fe) an attractive anode over MIL-100(Fe) for Li ion batteries, but stability issues remain a concern with MIL-88(B)Fe.