Bi 1/2 K 1/2 TiO 3 (BKT)-based solid solutions are promising lead-free relaxor ferroelectrics for electromechanical and energy storage applications. In this study, we investigate the structural, dielectric, and polarization properties of the ceramics of Bi 1/2 K 1/2 TiO 3 ‒Bi 1/2 Li 1/2 TiO 3 solid solutions [Bi (1+ δ )/2 K (1− x )(1−3 δ )/2 Li x (1−3 δ )/2 V δ TiO 3, BKT‒BLT] containing A-site vacancy ( V ) to elucidate the impacts of the Li + substitution for K + and the introduction of A-site vacancies on relaxor behavior. The temperature dependence of relative permittivity reveals that, upon cooling, the characteristic behavior of BKT, i.e., the temperature ( T )-induced spontaneous relaxor-to-ferroelectric phase transition, disappears in BKT‒BLT. Moreover, the E -induced irreversible relaxor-to-ferroelectric phase transition is hampered by increasing the contents of Li + ( x ) and A-site vacancies ( δ ). Interestingly, even a small amount of A-site vacancies can effectively reduce the freezing temperature ( T f ) and stabilize the ergodic relaxor phase with dynamic polar nanoregions (PNRs). The reduction of T f becomes more significant under the coexistence of Li + ions and A-site vacancies due to the simultaneous introduction of quenched random stress and electric fields arising from local structural distortions and local charge imbalances. As a result, BKT‒BLT exhibits slim P - E hysteresis loops with enhanced maximum polarization ( P max ), suppressed remanent polarization ( P r ), and superior energy storage properties compared with BKT. These findings establish a strategy for designing high-performance lead-free relaxor ferroelectrics by engineering quenched random fields in the BKT-based perovskites for energy storage applications.