Investigations On The Energy Storage Properties Of Co-and Ni-based Materials In Alkaline Solutions

A series of Co-X materials, such as Co-B, Co-Si, Co-P, Co-S, Co-BN and so on, are reported to demonstrate high discharge capacity and excellent cycle stability in alkaline rechargeable batteries. The inactive part can dissolve in alkaline aqueous, resulting in the creation of newly built interspaces among the Co particles. These newly built interspaces can increase the contact area between the active electrode materials of the electrodes and alkaline electrolyte. Thus the efficiency of electrochemical reactions is enhanced, resulting in better electrochemical performance than pure Co. As we know, Selenium can also dissolve in alkaline solutions, which is similar with B, Si, P and S. Besides, the electric conductivity of selenium is better than the above-mentioned elements. It is expected that Se has similar functions in electrochemical application. However, report on its application in alkaline rechargeable batteries has not been found.Co-Se composite is prepared through a chemical-reduction method and used as the negative electrode material of alkaline rechargeable batteries. The Co-Se composite shows a maximum discharge capacity of380mAh g-1at a current density of50mA g-1. After130cycles, the discharge capacity can still remain at356.5mAh g-1, with a retention rate of93.62%. As a comparison, the discharge capacity of pure Co is only130mAh g-1. This phenomenon can be explained as follows:the incorporation of Se improves the dispersibility of Co in the electrode. In addition, Se dissolves in the alkaline electrolytes, leading to more interspaces in the electrode, which is beneficial for increasing the contact area between the active materials and the electrolytes, thus enhances the efficiency of electrochemical reactions. Effect and function mechanism of Se in the electrochemical process are investigated in-depth.Co-Se-C composite is prepared through ball-milling method and used as the negative electrode material of alkaline rechargeable batteries. The discharge capacity of Co is obviously improved after the incorporation of Se and CNTs. The maximum discharge capacity of it can reach450mAh g-1at the discharge current density of50 mA g-1. Besides, the cycle number needed to reach the maximum discharge capacity is decreased. This can be attributed to the synergetic effect of Se and CNTs. The dissolution of Se enhances the effective contact between the active material and the electrolytes; whereas the introduction of CNTs improves the conductivity and toughness of the material.Co3O4is an important P-type semiconductor, in which the average valence of Co is8/3. So it is a potential multi-electron electrode material in alkaline rechargeable batteries. It is expected to display excellent performance as Co(OH)2and CoO.Co3O4with different morphologies are prepared by a solvothermal/calcination method. Their electrochemical performances as electrode materials of alkaline rechargeable batteries are investigated. The maximum discharge capacity of Co3O4polyhedrons can reach399.9mAh g-1at the discharge current density of100mA g-1. By lowering the calcination temperature, the BET surface area of the sample is enlarged and the electrochemical discharge capacity is increased. The maximum discharge capacity reaches490.2mAh g-1. And the cycle performance is excellent. Reaction mechanism of Co3O4during electrochemical performance is discussed in detail. Co3O4first changes to Co(OH)2in the charged state and then to Co. Faradaic reaction between Co and Co(OH)2accounts for most of the discharge capacity.Owing to their relative high specific capacitance, excellent reversible redox behavior and low cost, Cobalt/Nickel oxides and hydroxides have been considered as new types of supercapacitor electrode materials. Although many reports have been focused on them, preparation of materials with particular morphologies and excellent electrochemical performances are not systematic. In this paper, we will focus our eyes on Co3O4and a-Ni(OH)2. After consciously manipulating materials with3D hierarchical structures, electrochemical performances of them in supercapacitors are investigated systematically.Co3O4micro hollow spheres are prepared using carbon spheres as templates through a one-pot hydrothermal carbonization and calcination method. Carbonization process of the precursor at a low hydrothermal temperature of140℃is investigated by FTIR and Raman spectra. Effects of the concentration of the starting materials and hydrothermal reaction time on the size and thickness of carbon spheres are studied. When calcined at500℃in air, Co3O4hollow spheres with high purity are obtained. The role of BET surface area and pore diameter of Co3O4on their electrochemical performances are discussed. Owing to the relative high surface area and moderate pore diameter, the Co3O4micro hollow spheres display higher specific capacitance and excellent cycle performance.a-Ni(OH)2with different morphologies and BET surface areas are prepared through a solvothermal method using different alcohols and water as the mixing solvent. Effects of physical characters of alcohols on the morphologies of the product are studied. a-Ni(OH)2microspheres constructed by nanowires with a BET surface area of318.6m2g-1are obtained using triethylene glycol and water as the mixed solvent. Owing to the high BET surface area and large pore volume, this sample displays a high specific capacity of1788.9F g-1at a current density of0.5A g-1. Besides, rate performance of this sample is also excellent. This can be attributed to the small size of the nanowires constructing the microspheres, which can effectively shorten the proton diffusion distance and lower the ion diffusion resistance. Besides, the large BET surface area of the sample can ensure a sufficient contact between the active materials of the electrode and the electrolytes, making the chemically active materials almost100%usable for the redox reaction.