Green and Rational Design of 3D Layer-by-Layer MnO_x Hierarchically Mesoporous Microcuboids from MOF Templates for High-Rate and Long-Life Li-Ion Batteries

Rational design and delicate control on the textural properties of metal-oxide materials for diverse structure-dependent applications still remain formidable challenges. Here, we present an eco-friendly and facile approach to smartly fabricate three-dimensional (3D) layer-by-layer manganese oxide (MnOx) hierarchical mesoporous microcuboids from a Mn-MOF-74-based template, using a one-step solution-phase reaction scheme at room temperature. Through the controlled exchange of metal–organic framework (MOF) ligand with OH– in alkaline aqueous solution and in situ oxidation of manganese hydroxide intermediate, the Mn-MOF-74 template/precursor was readily converted to Mn3O4 or δ-MnO2 counterpart consisting of primary nanoparticle and nanosheet building blocks, respectively, with well-retained morphology. By X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy, high-resolution TEM, N2 adsorption–desorption analysis and other techniques, their crystal structure, detailed morphology, and microstructure features were unambiguously revealed. Specifically, their electrochemical Li-ion insertion/extraction properties were well evaluated, and it turns out that these unique 3D microcuboids could achieve a sustained superior lithium-storage performance especially at high rates benefited from the well-orchestrated structural characteristics (Mn3O4 microcuboids: 890.7, 767.4, 560.1, and 437.1 mAh g–1 after 400 cycles at 0.2, 0.5, 1, and 2 A g–1, respectively; δ-MnO2 microcuboids: 991.5, 660.8, 504.4, and 362.1 mAh g–1 after 400 cycles at 0.2, 0.5, 1, and 2 A g–1, respectively). To our knowledge, this is the most durable high-rate capability as well as the highest reversible capacity ever reported for pure MnOx anodes, which even surpass most of their hybrids. This facile, green, and economical strategy renews the traditional MOF-derived synthesis for highly tailorable functional materials and opens up new opportunities for metal-oxide electrodes with high performance.