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The apparent incompatibility between d33 and Qm in potassium sodium niobate-based lead-free ceramics is an obstacle to their applications. In this study, (K0.5Na0.5)0.98Li0.02Nb0.82Ta0.18O3 (LT-KNN) system near the orthorhombic–tetragonal (O–T) phase boundary but in the O phase region is selected as the matrix to ensure a relatively large d33 and high Tc, and help the CuO doping generate an effective "hardening" effect to improve Qm. The results show that adding proper amount of CuO into LT-KNN promotes the densification of ceramics and suppresses the abnormal grain growth, which helps to decrease the dielectric loss and obtain a high poling efficiency. Moreover, the Cu2+ doping in B-site (Nb/Ta)5+ generates oxygen vacancies ( V O ∙ ∙ ) and the Cu Nb / Ta ″ ′ - V O ∙ ∙ ′ defect dipoles could be formed, resulting an obvious "hardening" effect. With the introduction of 1 mol% CuO, the d33 increases from 82 to 180 pC/N and the Qm presents an abrupt rise from 136 to 665. More intriguingly, the addition of CuO delays the O–T phase transition while maintaining a high Tc of ~ 310 °C. Combined with the pinning effect of internal bias field, the d33 of 1 mol% CuO doped LT-KNN ceramics shows favorable stability in the wide temperature range of 20–280 °C, and a high Qm of larger than 250 can be achieved until 100 °C. Such a superior comprehensive electromechanical performance makes the ceramics promising in applications of high-power and high-frequency devices. [ABSTRACT FROM AUTHOR]