In this paper, in order to develop excellent composition ceramics for a piezoelectric energy- harvesting device, we synthesized 0.99(Na_0.52 K_0.443 Li_0.037)(Nb_0.883 Sb_0.08 Ta_0.037)O₃ + 0.01(Sr_0.95 Ca_0.05)TiO₃ + 0.3 wt% Bi₂O₃ + 0.3 wt% Fe₂O₃ + 0.3 wt% CuO (abbreviated as NKN-SCT) ceramics with different sintering times, using the ordinary solid-state reaction method. The effect of sintering time on the microstructure and piezoelectric properties was investigated. The ceramics with the sintering time of 7 h have the optimum values of the piezoelectric constant (d₃₃), piezoelectric voltage constant (g₃₃), planar piezoelectric coupling coefficient (kp), mechanical quality factor (Qm), and dielectric constant (εr): d₃₃= 314 [pC/N], g₃₃= 20.07 [10^-3 mV/N], kp = 0.442, Qm = 93, εr = 1,768, all being suitable for a piezoelectric energy-harvesting device.