Charge—discharge characteristics and structural change in various niobium sulfide cathodes for lithium-nonaqueous secondary batteries

Electrochemical behaviors of niobium sulfides such as 3R-Nb_1+xS₂ (x = 0.00 ≈ 0.25), 2H-NbS₂ and NbS₃ for lithium batteries are investigated in 1 M LiClO₄-propylene carbonate solution. The cathodic reaction in the charge-discharge cycle of the disulfides is shown as follows:

X-ray diffraction measurement and open circuit potential data show that 3R-Nb_1.00S₂ gives continuous structural change along c-axis reversibly with cycling, where x takes from 0 to 1, whereas 2H-NbS₂ gives the stepwise change also reversibly with cycling, where x is at least 1. On the other hand, NbS₃ yields the ternary phase of Li_xNbS₃ giving an amorphous X-ray pattern and shows irreversible structural change with cycling. When the charge-discharge cyclings are carried out at a constant capacity of 0.50 equivalent mole^?1 of sulfide and a current density of 0.20 mA cm^?2, 3R-Nb_1.00S₂ can be cycled over 100 times without an increase in the charge potential, whereas in 2H-NbS₂ and NbS₃ the charge potentials rise to 4.6 V vs Li/Li⁺ with cycling, which limits the cycle number to 30 ≈ 50 times. This would be due to the fact that 3R-Nb_1.00S₂ shows the reversible structural change with keeping the original crystal lattice with cycling.