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 (MnO_x) 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 MnO_x 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.