We report record energy storage density (>80 J·cm-3) in Pb-free relaxor ferroelectrics based on Mn-doped BiFeO3-BaTiO3 thin films. Rapid interval deposition was used to impose layer-by-layer growth improving crystallinity and lowering unwanted defects concentrations. The growth and Mn doping produced an order of magnitude lower leakage, with strongly reduced dielectric loss (from room temperature to >300 °C, and 100 Hz to 1 MHz), e.g. by a factor of 5 at 225 °C and 25 kHz. At room temperature (RT), the dielectric breakdown strength increased by a factor of 1.5 to >3000 kV·cm-1 while the dielectric constant remained flat, at ∼1000 from RT to 350 °C. The films perform better than competing materials (e.g. PZT and SrTiO3-based) while being Pb-free and while operating up to 350°C, which SrTiO3-based systems do not. Our work gives considerable promise for high energy and power density capacitors for harsh environments.We acknowledge funding support from EPSRC grants EP/N004272/1, EP/M000524/1, and EP/L011700/1, the Leverhulme Trust grant RPG-2015-017, the Winton Foundation and Deregallera Ltd., U.K
