An Investigation On Electrochemical Properties Of New Type Co-X Electrode Material

In recent years, hydrogen storage alloys have been widely investigated as a high capacity negative material for nickel-metal hydride secondary batteries. AB5-type rare earth based hydrogen storage alloy has been most widely used as Ni/MH negative electrode materials for its good electrochemical performance and performance-price ratio. However, the actual discharge capacity (300～320 mAh g-1) is close to its theoretical capacity (～348 mAh g-1), it is difficult to further enhance the performance. It is the focus of the study to develop the novel anode materials with high discharge capacity, excellent discharge dynamic performance, low cost and pollution-free to replace the traditional anode material of LaNi5 based hydrogen storage alloy. Recently, some Co and Co-X materials are found to have high discharge capacity and excellent cycle stability. But the research is not enough in-depth, especially there are many disputes about the function mechanism of these metalloids on metallic Co electrode. In view of the above question, we choose the preparation simple and the performance outstanding of Co-B, Co(OH)2 and Co-S as an object of the study and investigate the effect of annealing treatment and composite on electrochemical properties of new type Co-X negative electrode material in this paper.Part 1 Effect of annealing treatment on the electrochemical properties of Co-BCo-B alloys is synthesized by chemical reduction and ball milling, and annealed at defferent temperatures. Effects of annealing treatment on electrochemical properties of the Co-B anode materials are investigated. Co-B alloys synthesized by chemical reduction are amorphous. After annealing treatment, the initial Co-B alloy decomposes to crystalline Co and B with a kind of coating sphere structure. Even after 80 cycles, the reversible capacity was still kept up at more than 350 mAh g-1. Co-B alloys synthesized by ball milling are also amorphous. After annealing treatment, the initial Co-B alloy decomposes to crystalline Co and B. The discharge capacity is improved further.Part 2 Preparation and electrochemical hydrogen storage performance of CoB-AB3 composites The La0.8Mg0.2Ni2.4Mn0.10Co0.55Al0.10 alloy is annealed and the effects of the annealing treatment on the structural and electrochemical properties are investigated systematically. It shows that the treatment reduces the segregation of the alloy and improves the discharge capacity and cycle life.In previous studies, a series of CoB-x wt.%AB3 (La0.80Mg0.20Ni2.4Mn0.10Co0.55Al0.10) composites were synthesized by simple mixing of CoB alloy powders and AB3 alloy powders based on the high discharge capacity of AB3 alloy. The electrochemical effect of AB3 alloy on CoB alloy in composite was also systematically investigated.It is found that AB3 alloy in composites improves the discharge capacity of CoB alloy. So the composites have the best electrochemical performance and cycle life compared with CoB alloy and AB3 alloy.Part 3 Preparation and electrochemical hydrogen storage performance of CoB-AB5 compositesCoB alloy and AB5 (MlNi4.0Al0.3Cu0.5Zn0.2) alloy were prepared by arc melting. CoB-AB5 composites were synthesized by simple mixing of CoB alloy powders and AB5 alloy powders, and their electrochemical hydrogen storage properties were studied as negative electrodes. The composites have the best electrochemical performance compared with CoB alloy and AB3 alloy. Both the faradaic redox reaction and hydrogen storage reaction occurr in composites during the charge-discharge process. AB5 alloy in composites improves the transformation utilization of Co(OH)2/Co on the surface of CoB alloy, so the discharge capacity is improved. In the composite, AB5 alloy acts not only as hydrogen storage alloy but also as a catalyst for the redox reaction of CoB alloy.Part 4 Effect and function mechanism of amorphous sulfur on the electrochemical properties of cobalt hydroxide electrodeS-Co(OH)2 composite is prepared via a facile co-precipitation method and investigated as negative electrode of Ni/Co battery. The addition of amorphous S improves the electrochemical properties of Co(OH)2 electrode. The discharge capacity of S-Co(OH)2 electrode can reach 413.2 mAh g-1 and still keep about 340 mAh g-1 after 300 cycles, which is much higher than that of S-free Co(OH)2 electrode. Amorphous S in S-Co(OH)2 electrode shows two functions during the charge-discharge process. One is that the addition of amorphous S with high specific surface area improves the dispersion of Co(OH)2 platelets. The other is that the dissolution of amorphous S in electrode brings the new interspaces among the Co(OH)2 platelets, these two factors largely increase the interspaces among Co(OH)2 platelets. More interspaces are correlated to larger contact area with alkaline solution, which is in favor of the surface electrochemical redox. Thus, the capacity utilization of Co(OH)2 is enhanced.Part 5 Electrochemical properties of Co-S negative electrode material via different methodsCrystalline hexagonal nest-like spheres and amorphous Co-S hollow sphere are synthesized by hydrothermal method and solvothermal method, respectively. Moreover, their formation mechanisms are also constructed. Co-S nest-like spheres electrode displays high reversible discharge capacity of 250 mAh g-1 and excellent cycle stability at current density 200 mA g-1...