Metal Organic Frameworks-Derived Electrodes For Lithium-Secondary Batteries

With the rapid development of the human society, the overuse of the traditional energy sources results in exhaustion of traditional energy sources and severe environment pollution, and it gets urgent to develope clean energy. The effective utilization of clean energy greatly depends on advanced electrochemical energy storage techniques. As a representative of the secondary batteries, Li-ion battery with a lot of advantages has been widely used in various fields. However, owing to the lower capacity of the commercial electrode materials, it’s of great urgency to develop high-energy-density electrode materials so as to meet the daily increasing demand. The current high-energy density electrode materials, including transitional metal oxides, sulfur, selenium, etc., often display some shortcomings when used as electrode materials for Li-secondary batteries, such as bad intrinsic electrical conductivity, tremendous volume changes, thus leading to bad electrochemical performance. In addition, sulfur and selenium also exhibit gradual loss due to the soluble intermediates. These shortcomings greatly hinder their commercialization. According to the scientific researches, the design of porous electrode materials, or fabricating composites with high conductive carbon materials are effective methods to improve the electrochemical performance of electrode materials. But the synthetic process is often complicated and uncontrollable. Metal organic frameworks (MOF), which consist of organic ligands and metal ions, can be transformed into porous metal species or porous carbons through proper methods through simple and controllable treatments. The researches on MOF-derived nanostructures and their applications as electrode materials or as hosts for S and Se are of great academic value and practical significance. The main research contents can be described as the following aspects:(1) Fe2O3/Co3O4 double-shelled hierarchical microcubes with hollow Fe2O3 as core and Co3O4 as shell, are synthesized by using Prussion Blue microcubes as templates. When tested as anode materials for Li-ion batteries, it exhibits excellent electrochemical performance, delivering a specific capacity of 500 mAh g-1 after 50 cycles at a current density of 100 mA g-1,3 times higher than that of pure Co3O4 nanoparticle sample. The good electrochemical performance can be attributed to the unique microstructure characteristics and synergistic effect between the inner shell of Fe2O3 and outer shell of Co3O4. The hollow structure can ensure the presence of additional free volume to alleviate the structural strain associated with repeated Li+-insertion/extraction processes, as well as a good contact between electrode and electrolyte. The robust Fe2O3 shell acts as a strong support for Co3O4 nanoparticles and efficiently prevents the aggregation of the Co3O4 nanoparticles, thus leading to enhanced cycle life.(2) A sandwich-like structure electrode RGO/ZnCo2O4-ZnO-C/Ni foam are synthesized by using a well-designed bimetallic MOF composite GO/Zn-Co-ZIF-0.68/Ni as precursor, and it is directly used as binder-free anode for Li-ion batteries. The MOF derived productions are composed of carbon-coated spinel ZnCo2O4-ZnO nanoparticle polyhedrons. The interconnected carbon layers together with the high conductive RGO and Ni substrate form a highly conductive network, acting as an unhindered highway for charge transfer. The open pores can serve as cushion space to alleviate volume changes. The RGO nanosheets also act as a flexible protector to firmly fix polyhedrons on the Ni foam to avoid their collapse.(3) Nitrogen-doped carbon (NDC) spheres with abundant mesopores and micropores are obtained by directly carbonization of ZIF-8 nanocrystals. After sulfur impregnation, the sulfur in 0.5 nm micropores can only exist in the form of S2-4 due to the space confinement. After a prolonged heat treatment of 300℃, the large S8 molecules in 22 nm mesopores can be removed, thus forming NDC/S2-4 hybrid, exhibiting excellent electrochemical performance when tested as cathode for Li-S batteries. The NDC/S2-4 hybrid cathode exhibits a reversible capacity of 936.5 mAh g-1 at 100th cycle with a Coulombic efficiency of 100% under a current density of 335 mA g-1. It displays a superior rate capability performance, delivering a capacity of 632 mAh g-1 at a high rate of 5 A g-1. The confinement of smaller S2-4 molecules in the micropores of NDS efficiently avoids the loss of active sulfur and formation of soluble high-order Li polysulfides. The abundant pore structure can buffer the volume expansion and contraction changes, promising a stable structure for cathode. Furthermore, N doping in MOF-derived carbon not only facilitates the fast charge transfer, but also is helpful in building a stronger interaction between carbon and sulfur, strengthening immobilization ability of S2-4 in micropores.(4) Reduced graphene oxide (RGO) wrapped metal-organic frameworks (MOFs) derived cobalt doped porous carbon polyhedrons were synthesized via simple carbonization of ZIF-67. After sulfur impregnation, the RGO/C-Co-S composite were obtained. The RGO/Co-C-S cathode exhibited excellent electrochemical performance when served as cathodes for lithium-sulfur (Li-S) batteries, exhibiting 949 mAh g-1 at 300th cycle at a current density 0.3 C. It can even exhibit 479 mAh g-1 at current density of 5 A g-1. The large-surface-area porous carbon matrix physically adsorb S/polysulfides to avoid its escaping. The chemical interaction between ultrafine Co and sulfur species exhibits further confinement of active materials. The tightly wrapped RGO nanosheets serve as barrier layer to further forbid polysulfides diffusion out of the hosts. The carbon matrix, Co and RGO together firmly confine sulfur, thus greatly inhibiting the shuttle effect, leading to excellent electrochemical performance.(5) Nitrogen-doped carbon sponges (NCS) are derived from rod-like Al-based MOF metal organic frameworks (MOFs) via carbonization at high temperatures under Ar and NH3 flow. The NCS are composed of interconnected carbon layers, with 0.4-0.55 nm micropores in the carbon layers and 3-10 nm mesopores between them. Then Se is impregnated into 0.4-0.55 nm micropores by melting-diffusion and infiltration methods. When serving as cathode materials for Li-Se batteries, the NCS/Se composite exhibits excellent electrochemical performance. The cathode can exhibit 443.2 mA h g-1 at the 200th cycle with a coulombic efficiency of up to 99.9% at 0.5 C, which leads to 0.031% capacity loss per cycle from 5th to 200th cycles. The confinement of Se within small-sized micropores of NCS efficiently prevents Se loss. The nitrogen-doped high conductive carbon matrix not only facilitates rapid charge transfer during electrochemical activities, its porous structure also provides buffer space for volume changes of active materials, thus leading to good stability of electrode materials.