Effects of annealing treatment on phase composition and electrochemical properties of a V3TiNi0.56Co0.14Nb0.047Ta0.047 alloy

The effects of annealing on the phase composition and electrochemical properties of the V3TiNi0.56Co0.14Nb0.047Ta0.047 alloy have been studied. It was found that annealing at 1073 K leads to grain growth of the main phase and the decrease of the amount of the TiNi-based secondary phase. The volume fraction of the main phase increases up to annealing time of 24 h and remains almost constant when annealing exceeds 24 h. Annealing for t≥24 h results in the degradation of the network structure of the secondary phase and the formation of the tertiary Ti0.57V0.18Ni0.20Co0.05 phase with tetragonal structure. Moreover, the increase of the volume fraction of the main phase leads to the increase of the discharge capacity at a discharge current of 10 A kg−1. However, the decrease of the amount of the TiNi-based phase and the appearance of the Ti0.57V0.18Ni0.20Co0.05 phase cause the decrease of high rate dischargeability. Furthermore, the grain growth of the main phase and the degradation of the network structure of the secondary phase caused by annealing treatment worsen the cycle stability of the alloy electrode during charge–discharge cycling.