Utilizing solid-phase sintering technology, lead-free piezoelectric ceramics (K_0.44Na_0.52Li_0.04) (Nb_0.9Ta_0.06Sb_0.04)O₃ with high density and superior electrical properties were successfully fabricated at temperatures ranging from 1 055 to 1 070 ℃. The impact of various crystal structures of niobium pentoxide on the sintering behavior, phase composition, microstructure, and electrical characteristics of potassium sodium niobate-based piezoelectric ceramics was investigated. Experimental results demonstrate that Nb₂O₅ with coexistence of orthorhombic and monoclinic crystal forms exhibits superior sintering characteristics and excellent electrical properties. The piezoelectric constant is d₃₃= 211 pC·N⁻¹, electromechanical coupling coefficient is k_p= 44%, and dielectric loss is tanδ= 0.032 4. This suggests that incorporating a minor amount of monoclinic Nb₂O₅ can optimize the diffusion kinetics and phase boundary control of Nb⁵⁺, thereby significantly improving the overall performance of the ceramics. This research provides a critical foundation for the selection of raw materials and structural design of high-performance lead-free piezoelectric ceramics.