Extending the frequency bandwidth (BW) of vibration energy harvesters (VEH) that power wireless sensor nodes is of scientific and industrial interest. In this aim, electrical methods to tune the resonant frequency of piezoelectric harvesters with strong electromechanical coupling coefficients have been developed. In this work, we provide guidelines for designing such strongly coupled VEH and present a broadband harvester with high normalized power density (NPD). Through an analytical model, we explain how the coupling coefficient k 2 and the quality factor Q m of a cantilever can be jointly maximized, thereby maximizing the figure of merit k e 2 Q m . The proposed cantilever prototype made of PZN-5.5PT and aluminum offers one of the best coupling coefficients among the state-of-the-art ( k 2 = 49.8%) and a high quality factor ( Q m = 140). Associated to an appropriate tunable electrical interface (short-circuit synchronous electric charge extraction (SECE) in our case), the prototype exhibits a NPD of 12.0 mW g −2 cm −3 and a frequency BW of 36.0% (56.5 Hz around 157 Hz) at 0.34 m s −2 with a tunable electrical interface: the short-circuit SECE. This represents the highest product NPW × BW from state of the art.