Li–Mn Bimetallic Metal–Organic Framework and Its Derivative as a Cathode for Lithium-Ion Batteries

The bimetallic organic–inorganic hybrid complex [Li2Mn3(ipa)4(DMF)4]n (ipa = deprotonated 1,3-isophthalic acid, DMF = N,N′-dimethyl formamide) was synthesized via a solvothermal method and then further calcined at high temperature to prepare a spinel-type lithium manganate (LiMn2O4) cathode under different atmospheres with various calcination conditions. The structure of the complex [Li2Mn3(ipa)4(DMF)4]n was represented by single-crystal X-ray diffraction (XRD), powder XRD, and thermogravimetric (TG) analysis. The morphology and elements of LiMn2O4 were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of LiMn2O4 indicated that the direct calcination in an air atmosphere at 850 °C for 12 h was the optimal synthetic condition. The initial discharge specific capacity can reach 95.9 mA h g–1 with an open-circuit voltage of ca. 3.0 V and an upper cutoff voltage of ca. 4.3 V at 0.1 C. The initial discharge-specific capacity of 89.8 mA h g–1 at 1 C had a Coulombic efficiency of 95.3%. This was 73 mA h g–1 at a high rate of 5 C increasing to 91.6 mA h g–1 after returning to 0.1 C. After 500 cycles at 1 C, the system remained at 80.7 mA h g–1 with 89.9% of the initial discharge specific capacity. These features exhibit better stability than that of the reported LiCoO2 and LiNiO2 in battery material for LiMn2O4 enforcement.