The effect of LiNbO₃ (LN) addition on the ferroelectric behavior and piezoelectric properties of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT20) lead-free piezoceramics were systematically investigated. Results showed that the LN substitution into BNKT20 induced a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to relaxor pseudocubic phases, which is accompanied by the significant disruption of ferroelectric order and with the shift of the ferroelectric-relaxor transition temperature T_F-R down to room temperature. Accordingly, a large accompanying normalized strain of ∼0.38% (corresponding to a large signal d₃₃^* of ∼475 pm/V) were obtained in BNKT20 with 2.5 mol. %LN addition near the phase boundary. Temperature-dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation. Moreover, an attractive property for application as high-temperature dielectrics was obtained in this system modified with 8 mol. %LN with a high permittivity of 1760 ± 15% from room temperature up to 500 °C, spanning a range of about 450 °C.