Preparation And Electrochemical Performance Of Sandwich-like Co-based Oxide/rGO As Anode Materials

Transition metal oxides（TMOs）with high theoretical capacities and good conductivity have been widely investigated and regarded as potential anode materials.Nonetheless,there are still some obstacles which needed to be overcame.Firstly,the extremely low electrical conductivity of TMOs may cause low rate capability.In the other hand,the significant volume change and severe particle aggregation of TMOs during Li+ insertion/desertion process deteriorate cycle performance and shorten lifetime.All of the above disadvantages limit the use of TMOs in lithium-ion batteries.Therefore,how to improve the capacity,rate performance and cycle stability of transition metal oxides is particularly important.In this paper,Co-glycolate was uniformly coated on the surface of rGO,obtaining Co-glycolate/rGO precursor.The porous Co₃O₄/rGO/Co₃O₄ sandwich structural material can be obtained by annealing the precursor.The strategy can be successfully applied to NiCo₂O₄ anode materials,preparing the sandwich-like porous NiCo₂O₄/rGO/NiCo₂O₄.Upon this,NiCo₂O₄/CNT/rGO composites were synthesized by a in-situ-catalytic method,using ferromagnetic particles as catalyst and dicyandiamide as carbon source.The prepared samples were investigated by XRD,XPS,Raman spectroscopy and BET.The morphologies and structures were characterized with FESEM and TEM.Electrochemical tests were performed using a multi-channel battery testing system（LAND）between 0.01 and 3V at room temperature.Cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）were tested with an electrochemical workstation.Co-glycolate/rGO precursor was successfully prepared by metal alkoxide method,using cobalt acetate as cobalt source,ethylene glycol as soft template and graphene as carrier.The porous Co₃O₄/rGO/Co₃O₄ sandwich structural material can be obtained by annealing the precursor in hydrogen and air atmosphere,sequentially.The porous Co₃O₄ particles are between 20-40 nm and can be uniformly coated on both sides of rGO sheets.The as-prepared Co₃O₄/rGO/Co₃O₄ composite electrode with porous sandwich-like nanostructure displays a high discharge specific capacity of 700.5mAh g-1 even after 500 cycles at a current of 2A g-1.The capacity and cycle stability were significantly improved compared to non-porous structural electrode materials（with a capacity close to 403.6mAh g-1 after 300 cycles at a current density of 0.5A g-1）.This unique architecture has a sufficient specific surface area to facilitate the interaction of the active materials with electrolyte and provide enough spaces to accommodate the serious volume change during Li+ insertion/extraction processes and maintain the mechanical integrity of the composite electrodesNiCo-glycolate/rGO precursor was successfully prepared by the addition of a certain amount of nickel acetate in the preparation of the precursor at a temperature of 170°C for 1h.Because of the Kirkhadar effect,the sandwich-like porous NiCo₂O₄/rGO/NiCo₂O₄ can be successfully prepared by calcining the precursor in a similar way of porous Co₃O₄/rGO/Co₃O₄.The as-prepared NiCo₂O₄/rGO/NiCo₂O₄ composite electrode with porous sandwich-like nanostructure displays a high discharge specific capacity of 1124.0mAh g-1 in the first cycle and 783.3mAh g-1 after 300 cycles at a current of 0.5A g-1.On the basis of the precursor-making method,NiCo₂O₄/CNT/rGO and Co₃O₄/CNT/rGO were successfully prepared by using a in-situ-catalytic synthesis method,using dicyanodiamine as carbon source and nitrogen source,metal ion as catalyst.Luckily,carbon nanotubes can be uniformly grew on the surface of graphene.The asprepared NiCo₂O₄/CNT/rGO composite electrode displays a high discharge specific capacity of 1500.1mAh g-1 and 966.2mAh g-1 in the first cycle at a current density of 0.5A g-1 and 2A g-1,respectively.Meanwhile,it can obtained 793.8mAh g-1 after 80 cycles and 493.9mAh g-1 after 250 cycles at a current of 0.5A g-1 and 2A g-1,respectively.Co₃O₄/CNT/rGO composites can maintain a capacity of 762.5mAh g-1 after 65 cycles at 0.5A g-1.Therefore,NiCo₂O₄/CNT/rGO composites have better rate performance and cycle stability than Co₃O₄/CNT/rGO composites.