| In this thesis, previous research work on V-based hydrogen storage electrode alloys were exhaustively reviewed first. On this basis, the research scheme for improving the phase structures and electrochemical performance of the ternary Ti-V-Ni alloys by means of multi-component alloying and anneal treatment was chosen and the method of step-by-step optimization was adopted. XRD analyses, SEM/EDS investigations and electrochemical measurements were used in this research.For ternary TiV21Nix (x=0.1-0.9) electrode alloys, the effects of Ni content on the phase structure and electrochemical properties were systemically investigated. It was found that all the as-cast alloys composed of a V-based solid solution main phase and a TiNi-based secondary phase. As the Ni content (x>0.3) increased, the amount of the secondary phase significantly increased and a three-dimensional network formed along the grain boundaries of the main phase. The electrochemical measurements showed that a little Ni content (x=0.3) was helpful to the activation property, and increasing the Ni content further can improve the high-rate dischargeability and the cycle stability of the alloys. After anneal treatment, there were little changes in the phase structure but a little Ti2Ni-based phase appeared in the alloys with Ni content x<0.5. Comparing with the as-cast alloys, the volume of unit cell and molar fraction of the main phase increased. It was found that anneal treatment can lead to an increase in discharge capacity but a decrease in high-rate dischargeability. The TiV2.1Ni0.5 alloy had a better activation property, a larger discharge capacity and a higher cycle stability. This alloy can be activated in 5 cycles and had a discharge capacity of 400mAh/g at a discharge current of 25mA/g. Furthermore, the TiV2..1Ni0.5 alloy showed a capacity retention rate of 70.60% after 30 charging-discharging cycles. However, its discharge capacity was only 73.28mAh/g at a higher discharge current of 500mA/g.On the basis of the above work, the quarternary alloys TiV2 ,NiOJHfx (x=0~0.25) was studied. It was found that the secondary phase changed from the TiNi-based phase to a C14 Laves phase after adding proper Hf (x>0.05) to the as-cast TiV2.1Ni0.5 alloy. The C14 Laves phase increased with the increase of Hf content and still precipitated as a three-dimensional network along the boundaries of the main phase. For the alloys with x < 0.05, both the activation property and high-rate dischargeability were improved with the increasing Hf content; And when the Hf content reached to 0.12, the alloy had the highest high-rate dischargeability but a worse activation property and a smaller discharge capacity; further increasing thecontent of Hf, the high-rate dischargeability decreased significantly. Additionally, adding Hf always led to a degradation in cycle stability of the alloys. After anneal treatment, some C14 laves phases also appeared in the alloy with x=0.025. Comparing with the as-cast alloys, the volume of the unit cell of the main phase contracted but that of the secondary phase expanded for the annealed alloys. At the same time, the amount of the secondary phase increased. As for the electrochemical performance, anneal treatment resulted in an increase in activation property and discharge capacity as x=0.025, and an improvement in high-rate dischargeability to some extension as x >0.05 (except the alloy with x=0.12). Among the alloys studied above, the as-cast TiV2 ,Ni0.5Hf0.05 alloy showed the highest discharge capacity of 444mAh/g (at 25mA/g) at the first cycle, but its high-rate dischargeability HRD400 was only 26.50% and its capacity retention was only 30.63% after 30 cycles.To improve the cycle stability of the. TiV2 ,Ni0.5Hf0.05 alloy, we chose Co, Cr, Nb and Ta as the adding elements and the TiV2 ,Ni0.5Hf0.05Mx (M=Co, Cr, Nb and Ta; x=0~0.192) were studied. It was found that the addition of Co or Cr or Nb and Ta gave rise to a decrease of the amount of the main phase and an increase of the amount of the secondary phase without change in t...
|
PDF Full Text Request