Relaxor ferroelectric ceramics have attracted much attention for storing the electricity generated from clean and renewable energy sources due to their high permittivity and near-zero remnant polarization. The polarization of many relaxor based ceramics tends to saturate at high electric fields, however, which limits their energy storage performance. In this study, a lead-free Sn-modified (Na 0.5 Bi 0.5 )TiO 3 -SrTiO 3 system is investigated, where mitigated polarization saturation is observed with the addition of Sn 4+ , as a result of the different electronic configurations between d 10 Sn 4+ and d 0 Ti 4+ . As expected, high energy density of 3.4 J cm -3 and energy efficiency of 90% are simultaneously achieved in (Na 0.25 Bi 0.25 Sr 0.5 )(Ti 0.8 Sn 0.2 )O 3 ceramic. In addition, the ceramic exhibits good thermal stability, with the energy storage property variations below 5% over the temperature range of -20 °C to 150 °C, and satisfactory cycling stability with a variation of less than 8% over 10 5 cycles. All these merits demonstrate that the (Na 0.25 Bi 0.25 Sr 0.5 )(Ti 0.8 Sn 0.2 )O 3 ceramic has great potential for high power energy storage applications.