Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage
Authors: Zou Feng , Zhu Yu , Chen Yu-ming , Liu Kewei , Liang Wenfeng 
Published: February 2020

Transitional metal oxides have been explored as next generation anode materials for lithium ion battery due to their much higher capacity (~1000 mAh/g) than graphite (372 mAh/g). However, the large volume expansion comes out of lithiation process as well as the relative low conductivity of metal oxides hinder the cycle and rate performance. It is believed these problems can be accommodated by the carbon-hybrid nanostructured metal oxide. Metal-organic framework (MOF) was recently used to prepare advanced transition metal oxide electrodes due to the capability of forming a well-organized nanostructure. By thermal annealing MOF materials, carbon-coated transition metal oxide with the desired structure can be achieved in a facile manner.
Here in this work, nickel based MOFs (Ni-MOFs) with hollow ball-in-ball structure were synthesized via a facile solvothermal reaction. Nickel nitrate hexahydrate, trimesic acid and polyvinylpyrrolidone (PVP) were used as metal ions source, organic ligand and stabilizing agent, respectively. A mixture of ethanol, N,N-dimethylformamide (DMF) and water was used as solvent. The resulting hollow structures were particularly interesting as they exhibited the excellent performance to mitigate the volume expansion. The resulting In order to convert the Ni-MOFs into conductive electrode materials, a two-step thermal annealing process was carried out and the hierarchical NiO/Ni/Graphene nanostructured materials were obtained: Firstly, graphene covered nickel nanoparticles were formed by annealing the Ni-MOFs samples under inert gas environment. During this procedure, the organic ligand can be carbonized and converted into graphene layers due to the catalysis of Ni nanoparticles. Subsequently, the nickel nanoparticles were converted into NiO/Ni complex nanoparticles by annealing the samples in air. The microspherical structure from the Ni-MOFs was intact throughout the annealing process. This hierarchical NiO/Ni/Graphene nanomaterial is an ideal anode material for the high-performance lithium-ion battery: it possesses a highly porous, hollow structure articulated with ultrafine transition metal oxide nanoparticles with conformal graphene coating. These features enable the electrode made from this material to have not only high specific capacity, but also stable SEI, excellent electrical conductivity and robust structure. As expected, The NiO/Ni/graphene anode exhibited high reversible specific capacity (1144 mAh/g), excellent cyclability (no significant capacity fading after 1000 cycles at 2 A/g) and ratability (805 mAh/g at 15 A/g) Additionally, these hierarchical nanomaterials were used in the anode of a sodium-ion battery (SIB), which is an attractive low-cost solution for a rechargeable electrochemical energy storage system. The initial investigation of the sodium ion battery in this work indicated that the hierarchical NiO/Ni/Graphene nanomaterial is also a promising anode for the sodium ion battery. The SIBs with NiO/Ni/graphene anode exhibited good cycle stability (0.2% specific capacity fading per cycle) and ratability (207 mAh/g at 2 A/g).

Figure 1