An Investigation On The Energy Storage Properties Of Co-S Mateirals In Alkaline Solution

Recently, it is demonstrated that Co-based materials, such as Co-nonmetalcomposites （Co-B, Co-Si, Co-P）, CoO, Co₃O₄and Co（OH）₂possess excellent energystorage properties in alkaline solution. As negative electrode materials for alkalinerechargeable batteries, Co-based materials display high discharge capacity and goodcycle stability. However, the function mechanism of nonmetals on theelectrochemical performances of the Co electrode is rarely reported. Furthermore, theelectrochemical properties of Co-based materials are still needed to be improved.Cobalt oxide and cobalt sulfides are also widely investigated as the electrodematerials for supercapacitors, but the research is not enough in-depth. Similar to theabove Co-based materials, Co-S composites and CoS compound display goodelectrochemical behaviors. And it is easy to prepared the Co-S materials and controltheir morphologies. In view of the above question, Co-S materials are studies asnegative materials for alkaline rechargeable batteries and electrode materials forsupercapacitors.Part1Investigation of Co-S materials as negative materials for alkalinerechargeable batteriesKinds of Co-S composite and CoS compound are prepared by simple mixing, ballmilling and hydrothermal method, respectively. As negative electrode materials foralkaline rechargeable batteries, Co-S composite electrodes display high dischargecapacities of350mAh g1and good cycle stabilities. However, the CoS compoundelectrodes display quite low discharge capacity of40mAh g^-1. The differences of theelectrochemical properties are mainly due to the function of S in the electrodes: forCo-S electrodes, sulfur powders may improve the dispersion of Co particles and theyare easy to dissolve in alkaline aqueous during cycling, further enlarge the contactarea between Co and alkaline solution, which are favorable for improving thedischarge capacities. However, for CoS compound electrodes, the utilization of Co isdecreased because CoS is stable in alkaline solution, showing low discharge capacities.A series of novel Co-S composites composed of metallic Co coated with Co₉S₈areprepared via a facile hydrothermal method and investigated as negative electrodes forsecondary alkaline batteries. Compared with pure Co, the BET surface areas of Co-Scomposites are greatly increased. The maximum discharge capacity of the Co-Selectrode reaches420mAh g^-1and remains at410mAh g^-1after200cycles, while thedischarge capacity of pure Co electrode is only214.5mAh g^-1and rapidly decreases.The increase of the BET surface area and the coating structure of the Co-S compositedo favor for improving the usage of Co in the electrode and the cycling stability.Chainlike Co materials assembled by hierarchical nanostructures are synthesizedvia a facile hydrothermal and solvothermal method. Effects of reaction time,temperature and solvents on the morphologies are studied. The highest dischargecapacity of the as-prepared Co is about560mAh g^-1and it still remains480mAh g^-1after120cycles. The electrochemical properties are further improved by mixing withsulphur powder. The chainlike structures and the special morphology surface texturesare favourable for the improvement of electrochemical properties.Part2Investigation of CoS as electrode materials for supercapacitorsFlower-like CoS, CoS_1.097and CoS₂hollow spheres are prepared by a facilesolvothermal method using TAA, CS₂and S powder as sulfur sources, respectively.Effects of reaction time, temperature and solvents on the morphologies are studied.As the electrode materials for supercapacitors, the as-prepared CoS, CoS_1.097andCoS₂display typical pseudocapacitance performance in KOH aqueous solution.Co₃S₄hollow nanospheres and Co₃S₄/rGO hybrid are described by a noveltwo-step method. Electrochemical measurements reveal that the as-preparedCo₃S₄/rGO composite displays highest discharge capacitance of676.1F g^-1and it stillremains610.9F g^-1after1000cycles, with the capacitance retention rate of90%,which is better than the Co₃S₄hollow nanospheres. Several features make theCo₃S₄/rGO composite for high capacity, excellent rate capability and cycling stability.Firstly, the intimate interaction between the graphene substrates and the Co₃S₄directly anchored on them afford facile electron transport between individual hollowsphere and the conducting graphene network, which is the key to both of the high specific capacitance and the rate capability of Co₃S₄/rGO. The interaction also leadsto a good dispersion of the Co₃S₄grown on the rGO sheets to avoid aggregation,which is also helpful for the improvement of electrochemical properties.