Dielectric ceramics capacitors are of great interest duo to the improved energy-storage performance and temperature stability. However, to lower the cost and achieve mass production, the co-sintering of dielectric ceramics with base metals (such as Ni, Cu) is a huge challenge, since the co-sintering requires a reducing atmosphere to prevent the oxidization of base metals. Here in this work, the multivalence element Mn is doped into a relaxor ferroelectric ceramic system, i.e. 0.85BaTiO3-0.15Bi(Mg1/2Zr1/2)O3 (BT-BMZ) ceramics through a conventional solid-state reaction method to realize the reducing atmosphere sintering. Effects of MnCO3 contents (0.05–0.25 wt%) on the energy-storage properties, temperature stabilities and electrical structures of the relaxor-ferroelectric BT-BMZ ceramics are investigated. The optimal energy-storage performance is found in 0.10 wt% MnCO3 doped BT-BMZ ceramics, with a high discharge energy density of 1.61 J/cm3 and an ultrahigh energy efficiency of 94.3% under the applied electric field of 230 kV/cm. Further, the variation in energy density is less than 15% over the temperature range from 25 to 140 °C under 120 kV/cm. Additionally, the impedance spectroscopy is used to analyze the insulation mechanism of these reducing-atmosphere-sintered Mn-doped BT-BMZ ceramics.